Wind Tunnel Techniques
3 0 0 3
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Wind tunnel techniques course depicts the types, working and characteristics of wind tunnels in the laboratory. The flow characteristics and flow visualization in the tunnel are recorded for further observations.
Objectives
The course should enable the students to:1) Understand the non-dimensional number by Buckingham theorem 2) Differentiate the wind tunnels on the basis of circuit, air flow and working.. 3) Know the calibration of a wind tunnel.
4) Understand the pressure and force measurements in wind tunnel 5) Deduce the flow visualization techniques used in the wind tunnel testing
Outcome
The students should be able to:1) Solve the Buckingham theorem to find the SI unit of a parameter
2) Clearly understand the working of blow down, in draft tunnels and their specifications 3) Know about horizontal buoyancy, flow angularities are checked while calibration
4) Know about component axis balance and internal balances are read and understood for the measurements in wind tunnel
5) Get a clear idea about the smoke and tuft flow visualization procedures in WT testing
Syllabus
Unit
Topic
No. of
Lectures
I Principles of Model TestingBuckingham theorem, Non-dimensional numbers, Scale effects, Types of similarities
6 II Wind tunnels
Classification, Special problems of testing in subsonic, transonic, supersonic and hypersonic speed regions, Layouts, Sizing and design considerations
8
III Calibration of Wind Tunnels
Test section speed, Horizontal buoyancy, Flow angularities, Turbulence measurements, Associated instrumentation, Calibration of supersonic tunnels
11
IV Wind Tunnel Measurements
Pressure and velocity measurements, Force measurements, Three and six component balances, Internal balances
Unit
Topic
No. of
Lectures
V Flow visualization techniquesSurface and turft flow visualization techniques, Dye injection techniques, Optical methods of flow visualization
8
References
1 Barlow, Jewel B. Rae, William H. and Pope, Alan “Low speed wind tunnel testing”, III Ed., ISBN 13: 9780471557746, ISBN 10: 0471557749, 1999 (Wiley India Edition, 2010, About Rs. 1,000).
2 Pope, Alan and Goin, Kenneth L, “High-Speed Wind Tunnel Testing”, ISBN-13: 978-0882757278; ISBN-10: 088275727X, 1978.
Instructor: Dr. N. Sitaram E_mail: [email protected]
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Test First Periodical Test* Second Periodical Test* Model Exam Seminar/ Assignment/Quiz Attendance End-semester Examination Weightage 10% 10% 20% 10% 10% 50%
Duration 2 periods 2 periods 3 hours - - 3 hours
Introduction
CFD vs. EFD
Computational Fluid Dynamics
Experimental Fluid Dynamics
Requirements High end computer, software, printers, plotters etc.
Experimental facility (Wind tunnel), Instrumentation etc.
Manpower One or two for small CFD work, A small team for large CFD work, each member specializing in grid developing, solver, post processing etc.
Usually requires moderate number of people for small EFD for preparing and installation of model, for measurements etc. Requires large number of people with different skills for large experiments. Initial Cost Moderate High
Time required Moderate for computation, but large for post processing
Large for preparation, small for data acquisition and post processing Information
acquired
Principles of Model Testing
Buckingham theorem
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
“Dimension” is characteristic of the object, condition, or event and is described quantitatively in terms of defined “units”.
A physical quantity is equal to the product of two elements: A quality or dimension
A quantity expressed in terms of “units”
Dimensions
Physical things are measurable in terms of three primitive qualities (Maxwell 1871) Mass (M)
Length (L) Time (T)
NOTE: Temperature, electrical charge, chemical quantity, and luminosity were added as “primitives” some years later.)
Principles of Model Testing
Buckingham theorem
Examples Length (L) Velocity (L/T) Force (ML/T2) Units: Measurements systems: CGS, MKS, SISI units are now the international standard (although many engineers continue to use Imperial or U.S.)
Buckingham theorem
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
SI Primitive Units
Dimension
Symbol
Unit
Symbol
Length L meter m
Mass M kilogram kg
Time T second s
Temperature T Kelvin K Elec. Current I Ampere A Luminous intensity C Candela cd Amount of substance N Mole mol
Principles of Model Testing
Buckingham theorem
SI Derived units
Description Derived unit Symbol Dimension
Force Newton N kg m/s2 Energy Joule J kg m2/s2 Pressure Pascal Pa kg/(ms2) Power Watt W kg m2/s3
Principles of Model Testing
Buckingham theorem
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Fundamental rules:
All terms in an equation must reduce to identical primitive dimensions Dimensions can be algebraically manipulated.
Uses:
Check consistency of equations
Deduce expression for physical phenomenon
Principles of Model Testing
Buckingham theorem
Simple Example: Drag on a Sphere
Drag depends on FOUR parameters:
sphere size (D); velocity (V); fluid density (
); fluid viscosity (
) Difficult to know how to set up experiments to determine dependencies
Buckingham theorem
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Only one dependent and one independent variable
Easy to set up experiments to determine dependency
Easy to present results (one graph)
F
ρV D
=
Simple Example: Drag on a Sphere
Principles of Model Testing
Buckingham theorem
Simple Example: Drag on a Sphere
Principles of Model Testing
Buckingham theorem
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Step 1:
List all the parameters involved Let
n
be the number of parametersExample: For drag on a sphere, F, V, D,
Hencen
= 5Step 2:
Select a set of primary dimensions For example M (kg), L (m), T (s).
Example: For drag on a sphere, choose MLT
Principles of Model Testing
Buckingham theorem
Step 3
List the dimensions of all parameters Let r be the number of primary dimensions Example: For drag on a sphere r = 3
Parameter:
F
V
D
Unit:
ML/T
2L/T
2L
M/L
3M/LT
Step 4
Select a set of m dimensional parameters that includes all the primary dimensions Example: For drag on a sphere (m = r = 3) select ϱ, V, D
Buckingham theorem
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Step 5
Set up dimensionless groups
p
s There will be (n – m)=2
equations Example: For drag on a sphereM L T =
ML
T
M
L
L
T
L
Π = Fϱ V D
Exponents to be determined to satisfy dimensional homogeneity Equating exponents of M, L and T on left and right sides
For M: 0=1+
a
Hencea
=-1 For T: 0=-2-b Henceb
=-2For L: 0=1-3
a+b+c
0=1+3-2+c
Hencec
=-2Hence the first dimensionless number can be written as
=
F
ρV D
Principles of Model Testing
Buckingham theorem
Step 6
Check to see that each group obtained is dimensionless Example: For drag on a sphere
ML
T
L
M
T
L
1
L
Π =
F
ρV D
Π =
=
ρ
ν
M
L
L
T
L
LT
M
Principles of Model Testing
Buckingham theorem
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Direct Rationalization of Dimensionless Groups
Obtain the Πterms by simple reasoning:1 Determine their number from Buckingham theorem. Make sure the appropriate number of groups is obtained.
2 Identify variables (
g
etc.) that contain force quantities and formulate the corresponding force. For example, F=
V2L2 and F=
VL. Then take ratios of these forces (F/F=
VL/
) to get dimensionless groups.3 If the force groups do not comprise the number of dimensionless groups sought, then look for length type terms, velocity type terms, and/ or time type variables which can be divided to give dimensionless groups.
Principles of Model Testing
Buckingham theorem
Direct Rationalization of Dimensionless Groups
There may be more than one length in the problem, i. e. L and D which give the dimensionless group, L/D. Length type variables can also include area and volume, so That A/L2and V/L3are dimensionless groups.Velocity type terms include velocity, V, angular velocity,
w
, and volume flow rate, Q, which yield dimensionless groups,w
r
/V and Q/VL2.Time type terms consist of period,
, of the motion and convective time scale, L/V, which give the dimensionless group,
V/LTypes of forces encountered in fluid phenomenon
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Inertia Force, Fi: = mass X acceleration in the flowing fluid.
Viscous Force, Fv: = shear stress due to viscosity X surface area of flow. Gravity Force, Fg: = mass X acceleration due to gravity.
Pressure Force, Fp: = pressure intensity X C.S. area of flowing fluid.
Principles of Model Testing
Significant Dimensionless Groups in Fluid Mechanics
These are numbers which are obtained by dividing the inertia force by viscous force or gravity force or pressure force or surface tension force or elastic force.
As this is ratio of once force to other, it will be a dimensionless number. These are also called nondimensional parameters.
The following are most important dimensionless numbers. Reynolds Number
Froude Number Euler Number Mach Number
Principles of Model Testing
Significant Dimensionless Groups in Fluid Mechanics
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Reynolds Number, Re:
It is the ratio of inertia force to the viscous force of flowing fluid..
.
Re
.
.
.
.
.
.
.
.
Velocity
Volume
Mass
Velocity
Fi
Time
Time
Fv
Shear Stress Area
Shear Stress Area
Q V
AV V
AV V
VL
VL
du
V
A
A
A
dy
L
Principles of Model Testing
Significant Dimensionless Groups in Fluid Mechanics
Euler Number, Eu:
It is the ratio of inertia force to the pressure force of flowing fluid. 2.
.
Pr
.
Pr
.
.
.
.
.
/
/
uVelocity
Volume
Mass
Velocity
Fi
Time
Time
E
Fp
essure Area
essure Area
Q V
AV V
V
V
P A
P A
P
P
Significant Dimensionless Groups in Fluid Mechanics
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Froude Number, Fe:
It is the ratio of inertia force to the gravity force of flowing fluid. 2.
.
.
.
.
.
.
.
Velocity
Volume
Mass
Velocity
Fi
Time
Time
Fe
Fg
Mass Gavitational Acceleraion
Mass Gavitational Acceleraion
Q V
AV V
V
V
Volume g
AL g
gL
gL
Principles of Model Testing
Significant Dimensionless Groups in Fluid Mechanics
Mach Number, M:
It is the ratio of inertia force to the elastic force of flowing fluid. 2 2 2.
.
.
.
.
.
.
.
/
:
/
Velocity
Volume
Mass
Velocity
Fi
Time
Time
M
Fe
Elastic Stress Area
Elastic Stress Area
Q V
AV V
L V
V
V
K A
K A
KL
K
C
Where C
K
Principles of Model Testing
Dimensionless numbers for force quantities
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Force intensities, pressure and shear stresses, are non-dimensionalized with the dynamic pressure
Pressure coefficient:
=
Local shear stress coefficient:
=
Resultant forces, such as lift and drag, are non-dimensionalized with the dynamic pressure force,
ρ V
A
, where A is some characteristic area.Lift coefficient,
C =
and
Drag coefficient,
C =
Principles of Model Testing
Dimensionless numbers for force quantities
Torque, T, is force F times a moment arm, L, which is non dimensionalized by multiplying the dynamic pressure force by L to give torque coefficient.
Torque coefficient:
C =
Power is the rate of doing work, FL/T or FV, which requires the dynamic pressure force be multiplied by V to give power coefficient.
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Elaborate Example: Power required to rotate a propeller
The power, P required to rotate a propeller depends on the fluid density,
, the propeller diameter, D, the rotational speed, w, the velocity, V, of the fluid approaching thepropeller, the speed of the sound, C, and the angle,
a
, of a propeller blade.P = f (
, D,
w
, V, C,
a
)
Seven variables (
n
=7) give four dimensionless groups (m
=n-r
=4). Two force like terms yield one dimensionless group.The dynamic pressure, can be used to non-dimensionalized forces. As power is F V,
Π =
1
P
2
ρV A
Principles of Model Testing
Elaborate Example: Power required to rotate a propeller
For a rotating propeller, L=D and V=
w
D to obtainThere are three velocity variables:
w
, V and C, which give two dimensionless numbers.Π = C =
P
1
2
ρω D
Π
= Advace ratio =
V
ωD
Π
= Mach number =
V
C
Principles of Model Testing
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Elaborate Example: Power required to rotate a propeller
Need one more dimensionless group, but have not yet considered the angle, a. An angle can be defined in terms of its tangent which is the ratio of two lengths and is dimensionless. That is
a
is a Π term. Then orΠ = α
C =
P
1
2
ρω D
=
V
ωD
,
V
C
, α
Π =
Π , Π , Π , Π
Principles of Model Testing
Dimensional Analysis
Definition: Dimensional analysis is a process of formulating fluid mechanics problems in terms of non-dimensional variables and parameters.
Why is it used :
•Reduction in variables [ If F (A1, A2, … , An) = 0, then f (P1, P2, … Pr < n) = 0, where, F = functional form, Ai = dimensional variables, Pj = non-dimensional
parameters, m = number of important dimensions, n = number of dimensional variables,
r=n–m
). Thereby the number of experiments required to determine f vs. F is reduced.•
Helps in understanding physics•
Useful in data analysis and modelingDimensional Analysis: Example
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Drag = f (V, L,
, C, t, e, T, etc.) From dimensional analysis,Vortex shedding behind cylinder
Principles of Model Testing
Similarity and Model Testing
Definition : Flow conditions for a model test are completely similar if all relevant
dimensionless parameters have the same corresponding values for model and prototype.
•
i model=
i prototypefor
i
= 1 to
n
•
Enables extrapolation from model to full scale•
However, complete similarity usually not possible. Therefore, often it is necessary to use Re, or Fr, or Ma scaling, i.e., select most important parameter andPrinciples of Model Testing
Dimensional Analysis and Similarity
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Geometric Similarity:
The model must be the same shape as the prototype. Each dimension must be scaled by the same factor.Kinematic Similarity:
Velocity as any point in the model must be proportionalDynamic Similarity:
All forces in the model flow scale by a constant factor tocorresponding forces in the prototype flow.
Complete Similarity:
is achieved only if all above three conditions are met.Principles of Model Testing
Dimensional Analysis and Similarity
Complete similarity is ensured if all independent groups are the
same between model and prototype.
What is ?
We let uppercase Greek letter denote a nondimensional parameter,
e.g., Reynolds number Re, Froude number Fr, Drag coefficient, C
D, etc.
•Consider automobile experiment •Drag force is F = f (V,
,
L)Flow Similarity and Model Studies
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Example: Drag on a Sphere
Principles of Model Testing
Flow Similarity and Model Studies
Example: Drag on a Sphere
For dynamic similarity …
Principles of Model Testing
Scaling with Multiple Dependent Parameters
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Example: Centrifugal Pump
Pump head:
h=g
1(Q, wD, )
Pump power:
P=g
2(Q, wD, )
Principles of Model Testing
Similitude-Type of Similarities
Geometric Similarity: is the similarity of shape.
Where: L
p, B
pand D
pare Length, Breadth, and Diameter of Prototype
and L
m, B
m, D
mare Length, Breadth, and Diameter of Model.
L
R= Scale ratio
p p p r m m mL
B
D
L
L
B
D
Similitude-Type of Similarities
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Kinematic Similarity: is the similarity of motion.
Where: v
p1& v
p2and a
p1& a
p2are velocity and accelerations at point 1
& 2 in prototype and v
m1& v
m2and a
m1& a
m2are velocity and
accelerations at point 1 & 2 in model.
V
rand a
rare the velocity ratio and acceleration ratio
1 2 1 2 1 2 1 2
;
p p p p r r m m m mV
V
a
a
V
a
V
V
a
a
Principles of Model Testing
Similitude-Type of Similarities
Dynamic Similarity: is the similarity of forces.
Where:
(F
i)
p,
(F
v)
pand
(F
g)
pare
Inertia,
Viscous
and
Gravitational Forces in Prototype and (F
i)
m, (F
v)
mAnd (F
g)
mare
Inertia, Viscous and Gravitational Forces in Model.
F
rIs The Force Ratio
g i p v p p r i m v m g m F F F F F F F Principles of Model Testing
Flow Similarity and Model Studies
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Scaling with Multiple Dependent Parameters
Example: Centrifugal Pump
Head coefficient:
=
,
Power coefficient:
Principles of Model Testing
Flow Similarity and Model Studies
Scaling with Multiple Dependent Parameters
Example: Centrifugal Pump (Negligible viscous effects)
If then neglecting Reynolds number (viscous effects)
then
gh
and
ω D
=
gh
ω D
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Consider an aircraft flying in still or almost still atmosphere.
U
It is possible to get the aircraft performance from flight tests, but complicated and resource (time, money, human resources etc.) consuming. Also only limited information can be obtained. Alternatively a still aircraft model can be tested in a wind tunnel with air flowing at a speed of U. This reduces the resources required and more information can be obtained.
A wind tunnel can be used to test models of aircraft and other vehicles and components of aircraft (ex. wings).
Alternative testing resources: Whirling arms (no longer used), Water tunnels Can CFD replace EFD (wind tunnels)?
CFD vs. EFD
Computational Fluid Dynamics
Experimental Fluid Dynamics
Requirements
High end computer, software, printers,plotters etc.
Experimental facility (Wind tunnel), Instrumentation etc.
Manpower
One or two for small CFD work, A small team for large CFD work, each member specializing in grid developing, solver, post processing etc.Usually requires moderate number of people for small EFD for preparing and installation of model, for measurements etc. Requires large number of people with different skills for large experiments.
Initial Cost
Moderate HighTime required
Moderate for computation, but large for post processingLarge for preparation, small for data acquisition and post processing
National Aerospace Laboratory Trisonic Wind Tunnel Facility
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Test section: 1.2mx1.2m Operation: Intermittent blowdown Test duration: 40 secs (Typical) Mach Number Range: 0.2 to 4.0 Reynolds number: 8 to 60x106per meter
Lockheed Martin High Speed Wind Tunnel Circuit Layout
Mach Number Range:
Transonic range: 0.3 to 1.8 Supersonic range: 1.6 to 4.8
Reynolds Number Range
4 to 34x106/foot
Equivalent Airspeed
150 to 1,200 knot (280 to 2200 kmph) (144 to 1132 mps)
World largest wind tunnel
Test sections: 40’x80’& 80’x120’
Speeds: 300 knots 100 knots 556 kmph 185 kmph 154 mps 51 mps80’x120’ test section can test full size Boeing 737 aircraft at velocities up to unit Reynolds numbers of 1.1x106/ft.
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Classification of Wind Tunnels
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
I
Based on Size:
Small: Test section
˜
0.3 mx0.3 m, mainly for college instructional purposes Medium: Test section˜
1 mx1 m, used for university research purposesLarge: Test section
˜
2 mx2 m, used for testing industries and research laboratoriesVery Large: Test section
˜
4 mx4 m, used for testing large models in industries and research laboratories (for ex: NASA Ames Research Laboratory)II
Based on Wind Velocity:
Low velocity: Velocity less than 100 m/s, incompressible flow used in instructional and university research purposes
Subsonic velocity: Compressible flow (M<0.8) used for university, industry and research laboratory research purposes. No shock waves occur.
Transonic velocity: Compressible flow (M
˜
0.8 to 1.2) used for university, industry and research laboratory research purposes. Shock waves occur.Supersonic velocity: Compressible flow (M=1.2 to 5) used for university, industry and research laboratory research purposes. Shock waves occur.
Hypersonic velocity: Very high Mach numbers (M>5) used for industry and research laboratory research purposes. Rarefied gas with very low density.
Classification of Wind Tunnels
III Based on Test Section Configuration:
Open Circuit: Used mainly for small and intermediate tunnels. Test section may be without solid boundaries (open jet) or with solid boundaries (closed jet)
Closed Circuit: Used mainly for large and very large tunnels, may have closed or open (Fluid Mechanics Laboratory, Department of Applied Mechanics, IIT Madras). May have single return, double return or annular return.
IV Based on Operation:
Continuous: Most of the tunnels operate continuously.
Short Duration: These tunnels usually operate for very small time (Intermittent: A few minutes, Blow down: a few seconds) These types of tunnels are used in high speed turbomachinery testing. Saves enormous amount of energy for testing.
V Special Tunnels:
High Reynolds Number Tunnels
Low Turbulence Tunnels: The tunnel test section usually have a turbulence level of 1%. Certain applications require very low turbulence levels
˜
0.1% (MTL wind-tunnel at the Department of Mechanics, KTH, Sweden)Variable Density Tunnels: Independent variation of Mach and Reynolds numbers. Cryogenic tunnels.
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
I
Aeronautical applications
Most of wind tunnels are used for aeronautical applications for instructional purpose, for research carried out at universities, industries and research laboratories. The research carried out may be for fundamental understanding of fluid flow phenomena or for
developmental testing of components for aircrafts. Even moderate sized models of actual aircraft are tested in large wind tunnels.
Aeronautical wind tunnels can be further classified as high Reynolds number wind tunnels, V/STOL wind tunnels, Free-flight wind tunnels, Spin tunnels or vertical wind tunnels, Stability tunnels, Propeller tunnels, Propulsion tunnels, Icing tunnels, Low turbulence tunnels, Two-dimensional tunnels.
II
Smoke tunnels
III Automobile wind tunnels
IV Aeroacoustic wind tunnels (Anechoic tunnels)
Applications of Wind Tunnels
V Water tunnels
Used for flow visualization studies, underwater vehicle development and for cavitation studies. Usually small size and works at low velocities to obtain realistic Reynolds numbers.
III General purpose wind tunnels
Used for study of people drag (bike racers, skiers etc.), birds and insects, wind power devices solar collectors, radar antennas and satellite television receivers, sails and above-water parts of ships, bridges, etc.
IV Environmental or meteorological wind tunnels
These tunnels are designed to simulate Earth’s natural boundary layer, which typically has a thickness of 300 to 600 m. They are used for study of static loads and associated experiments on buildings, dynamic loads on buildings, unsteady aerodynamics in wind engineering, agricultural/wind breaks, agricultural/agronomy, agricultural/soil erosion
Applications of Wind Tunnels
Snowdrift study for lodge, near building in photo, at a ski resort. Note the effect of tower at left rear, which provides self-removal
of snow from entrance with prevailing from left. Skier in wind tunnel
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Applications of Wind Tunnels
Effects of wind barriers on solar collectors
being simulated in a wind tunnel Above: Efflux velocity is equal to wind speed and slack height is 1.5 times building height. Building is downstream of stack.
Below: Same conditions as above except building is upstream of the stack simulated in a wind tunnel
Open Circuit Wind Tunnel
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Plan view of an open circuit wind tunnel (Daimler-Benz Aerospace Airbus, Bremen, Germany)
Wind Tunnel Layouts
Open Circuit Wind Tunnel
The following are the advantages and disadvantages of an open circuit wind tunnel
Advantages
1) Construction cost is typically much less.
2) If one intends to run internal combustion engines or do extensive flow visualization via. smoke, there is no purging problem provided both inlet and exhaust are open to the atmosphere.
Wind Tunnel Layouts
Open Circuit Wind Tunnel
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Disadvantages
1) If located in a room, depending on the size of the tunnel to the room size, it may require extensive screening at the inlet to get high quality flow. The same may be true, if the inlet and/ or exhaust is open to the atmosphere, when wind and cold/hot weather can effect operation. 2) For a given size and speed, the tunnel will require more power to operate. This is usually a
factor only if used for development experiments, where the tunnel has high utilization rate. 3) In general, open circuit wind tunnels tend to be noisy. For larger tunnels (test sections of
6 m2and more), noise may cause environ mental problems, limit hours of operation, and/or require extensive noise treatment of the tunnel and surrounding room.
Because of low initial cost, an open circuit wind tunnel is often ideal colleges and universities, where a tunnel is required for class room instruction purposes and research and high utilization Is not required. Open circuit design are also frequently used by science fair participants who build their own wind tunnels.
Wind Tunnel Layouts
Closed Circuit Wind Tunnel
Plan view of a closed circuit wind tunnel (Defense Establishment Research Agency,
DERA, 13x9 ft (3.9x2.7 m=10.9 m
2) tunnel in Bedford, England
Closed Circuit Wind Tunnel
Closed circuit (return flow) open test section wind tunnel,
Fluid Mechanics Laboratory, Department of Applied Mechanics, IIT Madras
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Open test
section: 1 m dia.
Instrumentation
stand
Wind Tunnel Layouts
Closed Circuit Wind Tunnel
The following are the advantages and disadvantages of a closed circuit wind tunnel
Advantages
1) Through the use of corner turning vanes and screens, the quality of the flow can be well controlled and most important will be independent of other activities and in the building and weather conditions.
2) Less energy is required for a given test-section size and velocity. This can be important for a tunnel used for developmental experiments with high utilization (two or three shifts,
five to six days a week).
3) There is less environmental noise when operating.
Disadvantages
1) The initial cost is higher due to return ducts and corner vanes.
2) If used extensively for smoke visualization experiments or running of internal combustion engines, there must be a way to purge tunnel.
Wind Tunnel Layouts
Open or Closed Test Section?
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
An open test section in conjunction with an open circuit wind tunnel requires an enclosure around the test section to prevent air being drawn into the tunnel from the test section rather than the tunnel inlet.
For closed return wind tunnels of large size with an external balance, the open test section tends to have one solid boundary, since the balance must be shielded from the wind. This is an anomaly for aircraft experiments other than takeoff and landing, but it is a natural
condition for experiments on automobiles or surface-borne marine vehicles.
Many open test-section, closed circuit wind tunnels have experienced severe flow fluctuation problems that require extensive post construction diagnostics and corrective actions.
One of the tunnels currently acknowledged to be one of the most useful tunnels in existence nevertheless has had substantial difficulties with unsteady flow and noise when running in the open test-section configuration.
Wind Tunnel Layouts
Open or Closed Test Section?
The most common geometry is a closed test section, but a wide range of tunnel geometries have provided good experimental conditions, once the tunnel idiosyncrasies have become known to the operators and users.
Slotted wall test sections are becoming more common as are test sections that can be converted among two or more configurations.
A rectangular test section is preferable in larger size wind tunnels, as it is easier to change a model when working off a flat surface.
Further, if automobile or other ground vehicle experiments are to be conducted, a flat floor is a requirement.
General Layout: Open Circuit and Blower Tunnel
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Open Circuit:
This type of tunnel is widely used for instructional purposes and for investigations of fundamental flow phenomena. The test section is closed type (a).Blower Tunnel:
This type of tunnel also is widely used for instructional purposes and for calibration of flow devices. A characteristic design problem for these facilities is choice of details of a wide-angle diffuser between the fan or blower and the settling chamber before the contraction. The test section is open type (a).Wind Tunnel Layouts
Test Section Size
In general it may be expected that the test section should have as large a cross-sectional area as possible.
Ideally, a tunnel would be large enough to handle a full-scale vehicle (aircraft, car etc.).
In fact, several tunnels were built in 1920s and through the 1940s to achieve this goal for aircraft. However, since World War II era, and presumably in the future, the size of the aircraft have
become such that wind tunnels to accommodate full-scale vehicles are not practical. If one uses the rule of the thumb that the model span should be less than 0.8 of the tunnel
width, then Howard Hughes’ Hercules, or as more popularly known, the “Spruce Goose”, which was designed and built in the 1940s with a 320 ft wing span, would require a test section 400 ft wide.
Wind Tunnel Layouts
Test Section Size
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The cost of building a model, transporting it, and erecting in the tunnel, as well as making changes during an experimental program, would also be an interesting, albeit expensive task. Thus, it is apparent for the larger of modern aircraft, the concept of a tunnel to accommodate
full-size aircraft is out of question based on costs and practical difficulties.
Recalling from the earlier discussions of flow similarity, it is more important to seek to obtain Reynolds numbers for the model experiments that are as nearly as possible to the full-scale values than to be concerned with size alone.
In practice most development are done in tunnels with widths from 10 to 20 ft (3 to 6 m).
Wind Tunnel Design
Basic Considerations
The first step in the design of a wind tunnel is to determine the size (small, intermediate, large or very large) and shape of the test section (square, octagonal, circular or
rectangular), based on the intended uses of the tunnel.
The details given address tunnels for which the primary use is vehicle and vehicle components.
A major part of the testing will be force testing, where information is sought for performance, fuel efficiency, stability or control of a vehicle that may be an aircraft, an automobile, a racing car, a submarine, a racing yacht, or possibly others..
The cross-sectional area of the test section basically determines the overall size of the wind tunnel.
The size of the wind tunnel will be the primary factor in determining the structural or shell costs, and the power and operating hours will determine the energy portion of the operational cost.
Basic Considerations
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The major cost of operation is the salaries of the tunnel personnel.
But the electrical energy cost to run the tunnel and its auxiliaries is not an insignificant cost and it will increases in the long run.
The details given address tunnels for which the primary use is vehicle and vehicle components.
There must be a balance between initial costs and operating cost.
In the past many tunnels have been built with short diffusers and related features to allow short circuit length to hold down initial cost while accepting higher energy costs of operation.
This trade-off should be carefully examined with due consideration given to anticipated energy costs, which are likely to increase.
Wind Tunnel Design
Overall Aerodynamic Objective
The overall aerodynamic objective of most wind tunnels is to obtain a flow in the test section that is as near as possible to a parallel steady flow with uniform velocity throughout the test section.
Perfection is not possible so every design is bounded by constraints that include maximum cost, available space, available time, and available knowledge. It is almost always desired to obtain the largest size of the test section and the
highest speed for the available funds.
Wind Tunnel Design
Basic Decisions: Unit Reynolds Number
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A central issue in the sizing of a low-speed wind tunnel is the achievable Reynolds numbers for the models that can be accommodated.
The same question arise for vehicle developers who must select from available wind tunnels one which to carry out the tests in a development program.
Reynolds number, Re=
r
∞
V∞
L/m
∞
. A maximum Mach number of 0.3 is chosen as the upper limit of Mach number for the free stream beyond which the effect of Mach number are to be considered.
Considering sea level standard atmospheric conditions, the maximum V
∞
will be about 100m/s and the unit Reynolds number (for unit length) will be about 2.1x106ft-1(7x106m-1).
These numbers and the appropriate characteristic length of the test article (or tunnel width) give a good approximation to the available Reynolds number in an atmospheric wind tunnel. Hence there has been and continues to be much attention focused on obtaining effective
results with less than full-scale articles or with various separate components tests.
An important example of the contribution of component testing is the development of airfoil profiles for various profiles that are then incorporated into three-dimensional wing designs.
Wind Tunnel Design
Basic Decisions: Unit Reynolds Number
Another widely used method is to test half models since many vehicles have a plane of symmetry.
Most important of all is the careful study of aerodynamic phenomena as they are affected by variation of Reynolds number so that useful conclusions can be obtained from tests that do not duplicate the operating Reynolds number.
For many studies it is not necessary to produce the full-scale Reynolds number, but it must be of a “reasonable” value.
For vehicles including aircrafts and racing automobiles that can operate at speeds of Mach 0.3 or greater in the atmosphere, test articles would have to be atleast full scale to achieve operational Reynolds numbers in an atmospheric wind tunnel.
This is impossible or very costly for many vehicles.
Much low speed testing involves aircraft takeoff and landing configurations, where the Mach number is typically in 0.15 to 0.3 range.
Both the lift curve slope and maximum lift coefficient are affected by Mach numbers as low as 0.2.
Basic Decisions: Unit Reynolds Number
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This tends to require a tunnel speed approximately equal to the full scale landing speed. In an unpressurized tunnel using air, this means that the Reynolds number ratio of model to
full scale is approximately equal to the size ratio between the scale model and the aircraft. A primary decision is the choice of the minimum acceptable value of Reynolds number. Because much of low-speed testing is at high-lift conditions, the effect of Reynolds number
on the airfoils at high lift must be considered.
Maximum lift and lift curve shape near stall for single-element airfoils vary considerably with Reynolds number up to at least one million.
For multi element airfoils, this range is much greater.
The continuing need for testing facilities that allow near-full-Reynolds number of transport aircraft high-lift systems is a primary reason for serious consideration of construction of major new wind tunnels in the mid-1990s.
In any case, the lower boundary for testing airfoils and wings for vehicles that will operate at higher scale values is a Reynolds number in the range of 1 to 1.5 millions based on chord.
Wind Tunnel Design
Basic Decisions: Unit Reynolds Number
At these values of Reynolds number, the model is likely to have an extensive region of laminar flow, and the possibility exists of poor simulation owing to separation of the nodel’s laminar boundary layer.
It is assumed that laminar separations are likely to occur at full scale in the normal operating conditions.
Therefore, flow similar to full scale is more likely to be achieved by the transition location on the model.
If the Mach number is taken as 0.2, then the tunnel velocity is about 240 kmph (70 m/s) For this speed, the unit Reynolds number is a little less than 1,500,000 ft-1(450,000 m-1).
Although the minimum Reynolds number can not be rigidly defined, the above rationale has been used to define a minimum Reynolds number of between 1,500,000 and 2,500,000 based on wing chord for low-speed tunnels to be used for aeronautical development testing.
Wind Tunnel Design
Basic Decisions: Unit Reynolds Number
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There are also an increasing number of small low-speed unmanned vehicles (MAVs and UAVs).
Operational Reynolds numbers for these aircraft are often obtainable in a medium-sized wind tunnel.
For vehicles that operate in the atmosphere at speeds such that the Mach number is less than 0.3, the operational Reynolds number can be duplicated in an atmospheric wind tunnel with a scaled model.
Consider a production automobile at 60 mph (30 m/s).
The operating Reynolds number can be obtained using a three-eighth scale model with an atmospheric tunnel test speed of 160 mph (80 m/s)
Wind Tunnel Design
Basic Decisions: Test Section Size
This is commonly the starting point in the design of a wind tunnel.
The choice will follow from considerations of the desired Reynolds number capability, the budget for tunnel construction, and the costs for tunnel operation and tunnel users as reflected particularly in required model characteristics.
Basic Decisions: Test Section Size
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Airfoil models:
For a mean geometric chord of 0.3 m and aspect ratio of 8-9 for an aircraft wing, the span is about 2.5 m. The maximum span should be less than about 0.8 of the tunnel width due to effects of tunnel walls on the flow, which leads to a width of about 3 m. For rectangular solid-wall tunnels, wall correction factors for a small wing will be minimum for a width-to-height ratio of about 1.5. These considerations indicate why so many tunnels have been build in the 7x10 to 8x12 ft (2x3 to 2.4x3.6 m) size range with the maximum speed in the range of 200-300 knots (370 to 550 kmph or 100-150 m/s). In addition to the consideration of flow properties, a model for a tunnel of this size is large enough so that the smaller parts are relatively easy to fabricate.V/STOL Aircraft:
For V/STOL models in a STOL descent case, the speed will be near 70 knots due to model power limits or tip Mach number on propellers and rotors. The reduction in in test speed will require a larger model to maintain reasonable Reynolds numbers. Tominimize the wall corrections due to large downwash angles from these models, the model-span-to-tunnel-width ratio must be smaller, typically between 0.3 and 0,5. Thus the V/STOL tunnels built in the 1960s have test section that are 20-30 ft (6 to 9 m) wide.
Wind Tunnel Design
Basic Decisions: Test Section Size
Automobile:
A key issue for automobile tunnels is the blockage based on frontal area. The flow around automobiles is often more characteristic of “bluff bodies” than of “streamlined bodies”. This means there is almost a sizable region of separated flow. The wind tunnel test sections needs to be sufficiently long so that e separated flow regions “close”, before end of the test section and the entry of the diffuser. Otherwise the pressure in the separated region will not be correct and a large influence on drag will exist. In addition, the length-to-width ratio of automobiles is greater than for aircraft while the width-to-height ratio is much less. A wind tunnel test section sized for automobiles is therefore typically longer than a test section sized for aircraft and the width-to-height ratio approximates the width-to-height ratio of standard automobile. Ideally the blockage, the ratio of model frontal area to test-section area, will be about 5% or less.Keels, Submarines and Sails:
Submarines and surface ships have large length-to-width ratios. Their operational “leeway” is smaller than that for angle of attack or side slip for aircraft. AWind Tunnel Design
Basic Decisions: Test Section Size
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Instructional and Other Small Tunnels:
For small research tunnels and student tunnels at universities and elsewhere, the prospect of achieving or even approaching operational Reynolds numbers is usually beyond the available budget. In many instances the problem of building models accurately may be a critical factor. Assuming that students can hold an airfoil dimension of 0.25 mm and that it is sufficient to hold the model to 1-2% tolerance, the following results can be reached. For a 12% thick airfoil with 2% tolerance, the maximum thickness equals 1.2 cm and the chord is 10 cm. Using a mean chord of 10 cm and aspect ratio of 8. the span is 0.8 m. AS the maximum span is 0.8 of the tunnel width, the width is 1 m. Using a width-to-height ratio of 1.5 for a minimum wall correction factor, the height is 0.7 m. The cross sectional area is then 0.7 m2. A minimum test velocity would be about 30 m/s or a dynamic pressure of about 50 mm of water (or 50 kgf/m2). Because of lower speeds and corresponding dynamic pressures, instrumentation sensitivity is very low. These instruments may not be available or very costly.Many demonstration tunnels and calibration tunnels have test sections areas of 0.01 m2or less. These tunnels are very useful for observation of basic flow phenomena and calibration of instrumentation probes. However, they are of little use for doing vehicular component studies except in cases for which Reynolds number are quite low (ex: MAV and UAV).
Wind Tunnel Design
Basic Decisions: Test Section Size
For a rectangular tunnel, the width determine the model size and the Reynolds number at affixed speed. The cost of the tunnel shell and its required power tend to vary with the square of the test section width. Since funds for a tunnel are usually fixed, the largest tunnel that the funds will buy is generally built.
The size of smaller tunnel is frequently determined in the final analysis by the size of the room that will house the tunnel.
Special tunnels to increase unit Reynolds number:
The unit Reynolds number can be increased by building either a pressure tunnel or a cryogenic tunnel using a cold gas such as nitogen. Other working fluids such as Freon and sulfur hexafluoride have beeb used to obtain desired conditions. These are special purpose tunnels, and the need for their special capabilities must justify the cost as with any other tunnel. The time required for model changes will be long unless special provisions are made because the test section must be isolated before workers can enter to work on the model. Test productivity, as well as flow characteristics, s an important characteristic of a wind tunnel.Open or Closed Return
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Another basic design consideration is whether the tunnel will be of return or non return (open circuit) type.
Almost all of the small research tunnels are of the non return type, usually because of the lower construction cost.
Power consumption for such tunnels is usually not a significant factor in overall cost. Although there exists some larger tunnels of non return design, most of the larger
tunnels are of the return types, the majority being single return. A few of the earlier tunnels are of double return type.
However, it has been more than 70 years since the double return design has ben chosen for a new wind tunnel.
Wind Tunnel Design
Open or Closed Test Section
Open jet tunnels will have a lower energy ratio than a closed jet wind tunnel owing to the jet entraining stagnant air as it passes from the contraction cone exit to the collector inlet. If the tunnel has an external balance, the balance usually has to be shielded from the air
jet and one of the boundaries tends to be closed.
Open throats do not work for an open circuit tunnel with a propeller in the diffuser unless the test section region is enclosed in an air tight plenum or room.
Open throat tunnels offer suffer from pulsations similar to vibrations in organ pipes. An open throat gives easy access to the model in small tunnels.
In large tunnels scaffold of some type is required to gain access to the model. The setting up and removal of the scaffold require additional model change time.
Since the jet length is usually kept short to reduce losses, there is the possibility that high-lift models may deflect the wake enough to miss the collector or that the wake of a bluff
Wind Tunnel Design
Open or Closed Test Section
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In general, the advantages appear to be with the closed throat tunnel for aeronautical testing.
However, considerations of bluff body aerodynamics and overall size requirements have led to a number of open, partially open or slotted wall, and convertible jet tunnels built by automobile companies and others who engage in automobile test work or V/STOL aircraft development.
These arrangements provide greater flexibility in uses of a tunnel provided the staff are sufficiently knowledgeable about the variety of wall effects that must be understood. Closed thrat tunnels that are vented to the atmosphere not at the test section but at
another location will have the test section below atmospheric pressure.
Thus they can suffer from leaks either through holes cut in walls for probes, wires, pipes, and so on, or through the struts required to mount the model.
These tunnels usually have a sealed room or plenum around the test section. Most small open circuit tunnels are not built this way and suffer from leaks.
This makes wood an ideal material for such tunnels because it is easy to patch.
Wind Tunnel Design
General Layout: Closed Return
The general layout for closed return tunnels has reached a form generally agreed upon for reasons of construction economy and tunnel efficiency.
Starting with the test section and going down the common configuration includes the following elements:
a) The test section, which may be closed, open, partially open or convertible.The test-section-length-to-hydraulic-diameter ratio may typically be chosen to be 2 or more, in contrast to the shorter test sections of earlier era tunnels.
b) A diffuser of at least three or four test section lengths.The typical equivalent cone angle is in the range of 2-3.5O
with the smaller angles being more desirable. The area ratio is typically 2-3, again with the smaller values being more desirable.
c) “First corner” incorporating turning vanes.
d) Second leg that may continue the diffuser or may be constant area.
e) Safety screen to prevent parts of failed models or other unintended flying objects reaching the fan.This screen is usually just ahead of the second-corner turning vanes. f) “Second corner” incorporating turning vanes that may be essentially copies of the first
General Layout: Closed Return
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g) Transition from rectangular to circular cross section to take flow into the fan.
h) Fan and straightener section. Other drive devices such as ejector have also been used. i) Return or second diffuser. This will commonly incorporate a transition back to rectangular
from the circular cross section at the fan exit. The transition will likely begun in the
straightner section. The second diffuser should follow similar design guides as the diffuser. j) “Third corner” incorporating turning vanes.
k) Third leg that may be constant area. l) Heat exchanger.
m) “Fourth corner” incorporating turning vanes that may be copies of the third corner vanes. n) Wide angle diffuser with separation control screens. Typical properties are angles of about
45O
and area ratios of 2-4. o) Settling area.
p) Flow conditioners typically including flow straighteners and turbulence control screens. q) Contaction or nozzle.Typically area ratios are in the range of 7-12, although lower and
higher values are not uncommon.
Wind Tunnel Design
General Layout: Closed Return
Wind Tunnel Design
General Layout: Closed Return
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The plane of the return passage is almost always horizontal to save cost and make the return passage easier to access.
A vertical return is justified only when space is at a premium and has only been used for small sized tunnels.
A concept for obtaining economics in wind tunnel construction by producing a series of various sizes for which there are many common components was developed and used. The first- and second-corner assemblies for one size tunnel would be the same as the
third- and fourth-corner assemblies for the next smaller size tunnel.
The fan diffuser for one size tunnel would be the same as the test section diffuser for the next smaller size tunnel.
The engineering and construction drawings would be the same for all sizes except for the specified scale for each instance.
There are many innovative configurations to accommodate particular needs that are quite different from
Wind Tunnel Design
General Layout: Open Circuit and Blower Tunnel
Open Circuit:
This type oftunnel is widely used for instructional purposes and for investigations of fundamental flow phenomena.
Blower Tunnel:
This type of tunnel also is widely used for instructional purposes and for calibration of flow devices. A characteristic design problem for these facilities is choice of details of a wide-angle diffuser between the fan or blower and the settling chamber before the contraction.Definition of High Speed
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When compressibility effects are pre dominant the flow is generally said to
be of high speed.
A lower limit is approximately M=0.5. Power requirements vary as cube of
velocity in the wind tunnel. This does not hold into the high speed regime
exactly.
Because of large power requirements, high speed wind tunnels are usually
of the intermittent type.
This material is taken from the course notes of Experimental Aero (Gas) dynamics by Prof. Job Kurian of Department of Aerospace Engineering, IIT Madras
High Speed Wind Tunnels
Types of High Speed Wind Tunnels
1 Continuous (for all speed ranges)
2 Intermittent
2.1 Blowdown: M > 0.5 < 5.0
2.2 Indraft
High Speed Wind Tunnels
Pressure Driven Blow Down Wind Tunnel
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High Speed Wind Tunnels
Comparison between Indraft and Pressure Driven Wind Tunnels
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Indraft wind tunnels
Pressure driven wind tunnels
Stagnation temperature at supply condition is constant during a run.
So also is total pressure. No fluctuations as those generated by a pressure regulator.
Reynolds number can be varied at a particular Mach no.
No possible contamination such as that due to oil.
Cost is much less than of an indraft tunnel. Vacuum is safer to handle than pressure.
Pressure regulators are not needed.
The wind tunnels described above can be converted as continuous tunnels.
The comparison between blow down and continuous wind tunnels are given in next slide.
High Speed Wind Tunnels
Comparison between Intermittent and Continuous Wind Tunnels
Intermittent (blow down) wind tunnels
Continuous wind tunnels
Simple to design and less costly More in control of conditions and return to a given test condition with more accuracy. A single drive may run several tunnels Check points are easily obtained
No panic of rapid testing
Model testing is more convenient Test conditions can be held constant for a longer time.
Extra power is available to start
High Speed Wind Tunnels
Supersonic Wind Tunnels: Introduction
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The nozzle regulates the speed of air entering the test section of the wind tunnel so that the desired Mach number is established. Mach number is uniquely determined by the area ratio of the nozzle. A well designed nozzle makes the flow parameters uniform across the cross section. The design of a suitably shaped nozzle contour to obtain the desired uniform flow at the nozzle exit is based on the method of characteristics.
High Speed Wind Tunnels
Supersonic Wind Tunnels: Test Section Parameters
Dynamic pressure:
The local dynamic pressure 1/2ρv
2can be related to the local Mach number and static pressure.Mass flow rate:
The mass flow rate is given asm
=
ρv
A=
ρ*a*
A*
where * represents choked or sonic conditions (M=1)Supersonic Wind Tunnels: Test Section Parameters
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High Speed Wind Tunnels
Supersonic Wind Tunnels: Test Section Parameters
Writing in terms of stagnation conditions
High Speed Wind Tunnels
Supersonic Wind Tunnels: Test Section Parameters
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Mass flow density is expressed as a function of stagnation conditions and the area ratio. For isentropic flow, A* is a constant and A/A* is a unique function of local Mach number.
High Speed Wind Tunnels
Supersonic Wind Tunnels: Test Section Parameters
.
Test section velocity;
v
=
a
M=
Maximum velocity;
is maximum whenc
pT=0c
p-c
v=Rc
p/c
v=g
c
p-c
p/g
=c
p(1-1/g
)=(c
pg-c
p)/g
=c
p(g-1)/g
=RSupersonic Wind Tunnels: Test Section Parameters
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The test section flow velocity
v
for a given stagnation temperature TOapproaches the maximum valuev
maxat relatively low supersonic Mach numbers.For example, in the case of TO= 300 K R = 287J/kgK and a test section Mach number of 5.0, the ratio of
v/v
maxcan be calculated to see that it is equal to 0.913.This means at ordinary stagnation temperatures, the velocity in the test section reaches 91% of the maximum possible velocity corresponding to the total energy of the fluid. The stagnation temperature T0 rather than the Mach number which is important to attain high velocities.
High Speed Wind Tunnels
Supersonic Wind Tunnels: Test Section Parameters
Free stream Reynolds Number (Re):
Re=ρv
L/μ
Experimental observation is that
μ
is independent of pressure in the range of 0.001 to 20 atmospheres.High Speed Wind Tunnels
Supersonic Wind Tunnels: Test Section Parameters
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Free stream Reynolds Number (Re):
Re=ρv
L/μ
If this relation is assumed then the free stream Re can be expressed as a function of M1, the test section Mach number and of the stagnation parameters, Reynolds number per unit length Re/L=
ρv/μ
are expressed in stagnation quantities as given below.M=
v/a v
=a
M=a
High Speed Wind Tunnels
Supersonic Wind Tunnels: Test Section Parameters
.
n= 0.768 for air Simplifying
Both
a
oandμ
oare functions of stagnation temperature. Both increase with temperature.Hence, appreciable changes in free stream Re/unit length for a given M can be obtained only by varying stagnation density.
Components of a Supersonic Wind Tunnel
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Air storage tanks:
Size of the storage will be dependent on the mass flows required and the frequency of runs.Pressure storage tanks are available on the shelf basis – They are mounted horizontally or vertically. Tanks are painted black to absorb heat.
They are provided with safety disk or pressure relief valve.
As air is drawn from the storage, polytropic expansion takes place within the tank. This results in drop of reservoir temperature which is very bothersome.
Fall of stagnation temperature causes resultant change in the stream temperature for a given Mach number. Change in temperature results in the change of viscosity which in turn affects the boundary layer thickness. Changes in Reynolds number and Mach number during a run are thus consequential to the fall in reservoir temperature. To maintain constancy of stagnation temperature, it is a practice to stack the reservoir volume with empty metallic cans. They serve as heat storing matrix during compression and release heat during the expansion process. Another way to maintain the constant stagnation temperature is by providing heater units in the reservoir.
