Experimental and Numerical simulation of the Translational Downburst using impinging jet model

Experimental and Numerical simulation of

the Translational Downburst using

impinging jet model

K.K.Das1*, A.K.Ghosh2,K.P.Sinhamahapatra3

1*

Department of Mechanical Engineering, Assam Engineering College,Guwahati, INDIA

2 3

Department of Aerospace Engineering, Indian Institute of Technology Kharagpur, INDIA

*

Corresponding Author: e-mail: [email protected], Tel +91-09864405087

Abstract

Severe thunderstorms are important weather phenomenon which impact on various facets of national activity

like civil and defense operation, particularly aviation, space vehicle launching, agriculture in addition to its

damage potential to life and properties. Experimental and numerical simulation studies on thunderstorm

downburst have been reported by many researchers during the past two decades. Most of the numerical studies

are based on stationary downburst. Translational downburst occur more frequently than stationary downburst

due to the presence of ambient boundary layer winds. In the present work a axisymmetric numerical code has

been developed to simulate the translational downburst using vorticity-stream formulation, with LES model for

the turbulence. In addition a microburst simulator has been fabricated with a 165 mm diameter nozzle to

generate experimental data for the translational downburst.

Keywords: Microburst, Experimental simulation, Ring vortex, Macro-flow dynamics.

1.0 Introduction

The famous atmospheric scientist Fujita(1981) has observed and studied the flow due to downburst impacting

on the ground and spreading outward in the different directions. He classified downburst as either microburst or

macroburst depending on their horizontal extent of damage. For the complexity of the full scale phenomenon,

the physical simulation of the downburst is confined to the generic experiments of density currents impinging on

numerical code to simulate the translational downburst and also to fabricate a physical simulator for validating

the code.

2.0 Numerical Simulation

The two-dimensional incompressible Navier-Stokes equations in stream function-vorticity form are solved

numerically to simulate the axisymmetric impinging jet downburst problem. The LES technique is adopted to

model the turbulence. The Poisson equation for the stream function ψ is given by

V2ψ=-ξ ---- (1)

the vorticity transport equation in non-dimensional form is

( ) ( )

2

2 Re

2

2 2 2

2 2

u v

t x y

u v u v

y _x x _y x y x y

sgs

sgs

sgs

sgs

ν ν ξ ξ ξ _ξ ν ν ν ν ν ν ν ν ν ν   +   ∂ ∂ ∂ _{+ + = ∇   }     ∂ ∂ ∂ _{   } _{∂ ∂  +  ∂ ∂  +  ∂ ∂  ∂  + }  _{     } +  _{ }+ _{ }− _ − _{  } ∂ ∂ ∂ ∂ ∂ ∂  _{∂   ∂  }   _{ }  

The vorticity transport equation (2) is normalized with the jet exit parameters. The unsteady vorticity transport

equation is parabolic, and is solved using the implicit ADI technique, whereas the Poisson equation for stream

function is elliptic and is solved by successive over-relaxation (SOR) method with a relaxation factor (ω) of

1.85. It is further assumed that the flow enters the computational domain with the jet exit velocity, where the

fluid is stationary at t = 0. The Smagorinsky constant (Cs) is taken as 0.15 for this CFD simulation.

2.1 Boundary Conditions

The impermeable no-slip boundary condition is imposed for the solid surface BC as shown in figure 1.The

outflow boundary condition is imposed for surfaces CD, DE and AH. Free slip condition is imposed for EF and

HG. Logarithmic velocity profile is considered for the surface AB.

Figure 1: Computational Domain for the translational downburst

3.0 Physical modeling

Physical simulation of the translational dry microbursts is done using the impinging jet model with a 165 mm

exit diameter pipe. Two 1.5 HP centrifugal blowers are used to generate the impinging jet.. The translational

velocity (Vtrans) of the jet is kept fixed at 5 m/s for the entire experimentation. The dimension of the wooden

platform on which the jet impinges is 2.0 m

×

the impinging platform (H) can be varied between 125 mm and 400 mm using an adjustable frame to change the

value of H/Djet. The H/Djet ratios considered for this work are 1.0, 1.5, 2.0. Three jet velocities (Vjet) of 10 m/s,

15 m/s and 20 m/s are used in the experiments. Experimental setup is shown in figure 2. Velocity is measured

using DANTEC 56C17 CTA probe with traverse system and CTA software.

.

Figure 2 Physical simulator of the thunderstorm microburst fabricated at IIT Kharagpur

Figure 3 Flow visualization of the simulated travelling downburst from the physical simulator

The primary, secondary and tertiary vortices are observed in the flow visualization photographs shown in figure

3. The initial trailing and the intermediate vortices can also be seen in figure 3. Contrary to the primary and

secondary vortices, the tertiary vortex is attached to the impinging plate. Hence, it generates high wind shear

3.1 Velocity and Pressure measurements

A DANTEC 56C17 hot wire anemometer system is used to measure the velocity in the flow field. In addition,

a vane type digital anemometer is also used to measure the velocity at some locations in the flow field. To

determine the velocity profiles in the radial and axial directions, hot wire anemometer probe is placed in the

DANTEC traversing system as shown in figure 4. Pressure is measured using a PDCR23 pressure transducer

system with a scanivalve. To estimate the pressure on the impinging platform 300 pressure taps are placed on

the platform. Pressure taps are connected to the scanivalve through 1 mm diameter PVC tubing. A multi tube

manometer is also used to verify the pressure readings of the PDCR23 pressure transducer system.

Figure 4 DANTEC CTA Probe with the Traversing system and the controller

3.2 Flow visualization

Flow visualisation of the impinging jet is done using a smoke generator and high speed cameras. Smoke

generator is connected to the inlet of the blower. Flow patterns at different jet velocities and plate locations are

photographed.

(b) (c)

Figure 5 Impinging jet used for the physical simulation.

4.0 Results and Discussions

Some assumptions are made in the present numerical and experimental simulations of the dry downburst. It is

assumed that the buoyant acceleration characteristics of a natural downburst can be modeled by impinging jet.

Cross jets are used to generate secondary injection, which reduces the boundary layer effect of the jet wall,

which gives a better representation of the density driven flow in the natural downburst.

The spatial scale of the simulation is estimated based on the observations related to stationary microbursts made

by Hjelmfelt (1988). Hjelmfelt observed that the microburst typically had a diameter of 1.8 km and that the

maximum outflow winds occurred at approximately 1.5 km from the center of the descending column of air.

Based on this observation the geometric scaling factor in this study is about 1/10000. Also, it is found from the

preliminary tests performed in the physical simulation that the model produces peak radial wind of nearly 32m/s

compared to a maximum velocity of nearly 60m/s in natural downburst (Fujita, 1981). Therefore the velocity

scaling factor is 1/2. Combining the geometric and velocity scaling factors lead to the time scale of 1/5000 for

this study.

Four distinct major vortices are observed within the downburst flow field: a primary, an intermediate, a

trailing and a counter-rotating secondary vortex as seen in figure 6. Several transient vortices are also observed

near the test surface immediately after the impact of the jet on the test surface as seen in figure 4. The secondary

vortex is generated at x = 1.8 – 3.5. From these figures it is observed that the axial location of the primary vortex

The primary and secondary vortices form at a larger radial distance from the point of impact when the plate

separation ratio is smaller. Similar results are also observed for other plate separations and Reynolds numbers

Figure 6 Vortex formations in the travelling downburst from the numerical simulation

Figure 7 shows the velocity fields of the travelling downburst from the numerical simulation. Figures 8 shows

the computed temporal variation of vorticity at three radial stations, x=1.0, 1.2 and 1.5 at Reynolds number of

1.1×105 and plate separation ratio of 1.0 and 1.5. The point of interest at each radial station is the point of

maximum radial velocity, which is about 5-8% of Djet. In each case the vorticity shows considerable fluctuation.

It is further observed that for a particular value of Reynolds number and plate separation ratio the fluctuations in

vorticity at x=1.2 and 1.5 are almost identical with the time-mean nearly constant. At x=1.0 the fluctuation

Figure 8 Temporal variation of vorticity

In order to investigate the second order statistics, radial profiles of the turbulence intensity within the range of x

= 0.4 – 1.5 are presented in figure 9. Identical turbulence intensity profiles are observed for x = 0.75 – 1.5 but

the turbulence intensity profiles for x = 0.4 – 0.6 are different as the primary vortex gains momentum in the

radial direction beyond x=0.75.

The computed and experimental radial velocity profiles from the present simulations are compared with the

field observation data from the project NIMROD (Fujita, 1981) and empirical profiles due to Rajaratnam (1976)

and Wood et al. (2001). The comparison is presented in figure 10. The radial velocity in the figure is normalized

with the maximum radial velocity and the height is normalized with respect to the height at which radial

velocity falls to 50% of the maximum. The experimental and numerical results shown in figure 8 are for jet

velocity 30 m/s and H/Djet =1.0. The computed radial velocity profile matches closely with the experimental

data. The two-dimensional computation too agrees reasonably well. Very good agreement is also observed with

the full scale data from NIMROD (Fujita, 1981) as shown in figure 10.

Figure 9 Turbulence intensity profiles

5.0 Conclusion

ground and about 1.5Djet away from the jet axis.

c. Ground pressure coefficient distribution is independent of Reynolds number and jet separation ratio.

d. The magnitude of the maximum radial velocity increases for decreasing value of surface slip but the

maximum velocity occurs closer to the surface for higher value of slip.

e. The secondary vortex reduces the velocity decay rate beyond 1.5Djet and hence increases the radial extent

of the downburst.

f. Vertical location of the primary vortex is independent of Reynolds number and jet separation but the

horizontal location depends on these parameters. For lower Reynolds number and higher jet separation

ratio, primary vortex forms closer to the point of impact.

g. The primary vortex separates from the ground at x = 2.0 - 2.5Djetfor all Reynolds number and cloud height

Figure 8 Comparison with full scale data

6.0 Nomenclature

x Radial direction

y Axial direction

Vjet Jet velocity

u Radial velocity

v Axial velocity

Djet Diameter of the jet

H Distance of the jet from the impinging plate.

H/Djet Plate separation ratio, Cloud height for the full scale downburst.

7.0 References

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[3] Cooper, D., Jackson, D.C., Launder, B.E., Liao, G.X., 1993. Impinging jet studies for turbulence model [4] Assessment-I. Flow-field experiments. Int. J. Heat Mass Transfer 36 (10), 2675–2684.

[5] Craft, T.J., Graham, L.J.W., Launder, B.E., 1993. Impinging jet studies for turbulence model Assessment-II: an ex amination of the performance of four turbulence models. Int. J. Heat Mass Transfer 36 (10), 2685–2697.

[6] Das K.K, Ghosh A.K., Sinhamahapatra K.P.,2010, Investigation of the axisymmetric microburst flow field, Journal of Wind and Engg., Vol. 7 no. 1, Jan 2010, pp 1-15.

[7] Didden, N., Ho, C.M., 1985. Unsteady separation in a boundary layer produced by an impinging jet. J. Fluid [8] Mech. 160, 235–256.

[9] Fujita, T.T., 1981. Tornadoes and downbursts in the context of generalized planetary scales. J. Atmos. Sci. 38, 1511–1534. [10] Fletcher C.A.J 1987 Computational Techniques for the Fluid Dynamics(vol. 2) Springer-Verlag Publication

[11] Hjelmfelt, M.R., 1988. Structure and life cycle of micoburst outflows observed in Colorado. J. Appl. Met. 27, 1988, 900-927 [12] Holmes, J.D., Oliver, S.E., 2000. An empirical model of a downburst. Eng. Struct. 22, 1167–1172.

[13] Kim, J., Hangan, H., 2007. Numerical simulation of impinging jets with application to downbursts. J. Wind Eng. Ind. Aerodyn. 95, 279–298.