Before the now common use of hot-wire anemometry, a turbulence sphere was the primary way to measure the relative t b l f i d t l It i
turbulence of a wind tunnel. It remains a very useful and easy way to characterize the turbulent environment in a tunnel and to check if there is an indicated effect following changes in the tunnel configuration or special installations that might affect the flow quality. The configuration of a turbulence sphere is sho n in Fig 4 The critical Re nolds
Fig. 4 Turbulence sphere
0.3 0.4 0.5 c ie n t of S phere
shown in Fig. 4. The critical Reynolds number for the sphere can be measured in two ways. One method is to plot the
measured CD based on cross-sectional area
versus Reynolds number, as shown in Fig. 5. Fig. 5 Variation of drag coefficient for a turbulence sphere as a function of Reynolds number
2x105 3x105 4x105 0.1 0.2 D rag Co eff ic
Reynolds Number of Sphere
Critical Reynolds Number=336,000
Associated Instrumentation
Before the now common use of hot-wire anemometry, a turbulence sphere was the primary way to measure the relative turbulence of a wind tunnel. It remains a very useful and easy way to characterize the turbulent environment in a tunnel and to check if th i i di t d ff t f ll i h i th t l fi ti i l if there is an indicated effect following changes in the tunnel configuration or special installations that might affect the flow quality. The configuration of a turbulence sphere is shown in Fig. 4. The critical Reynolds number for the sphere can be measured in two ways. One method is to plot the measured CD based on cross-sectional area versus Reynolds number, as shown in Fig. 5. From the plot the Reynolds number in the tunnel for CD = 0.3 is read. The second method is to take the average of the four pressures on the aft surface of the sphere and subtract this value from the stagnation value at the leading edge of the sphere ieldingP A plot of P/q ers s Re nolds n mber is
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
leading edge of the sphere, yielding P. A plot of P/q versus Reynolds number is prepared for the sphere and the Reynolds number determined for P/q = 1.22, as indicated in Fig. 6. The pressure method has certain advantages. It needs no drag balance with the associated balance calibration and no evaluation of support tares for the portion of the support sting exposed to the airstream. Also, the sphere support sting can be stiffer as no deflection is needed by the drag balance.
Calibration of Wind Tunnels
Associated Instrumentation
From the plot the Reynolds number in the tunnel for CD = 0.3 is read. The second method is to take the average of the four pressures on the aft surface of the sphere and subtract this 1 2
1.4 P/q=1.22 ent, P/
q
value from the stagnation value at the leading edge of the sphere, yieldingP.
A plot of P/q versus Reynolds number is prepared for the sphere and the Reynolds number determined forP/q=1.22, as indicated in Fig. 6. The pressure method has certain advantages. It needs no drag balance with the
i t d b l lib ti d 1 5x100.8 5 2 0x105 2 5x105 3 0x105 3 5x105 1.0 1.2 ressur e Loss C oeff ici Critical Reynolds Number=299,000
associated balance calibration and no evaluation of support tares for the portion of the
support sting exposed to the airstream. Also, Fig. 6 Variation of pressure coefficient
for a turbulence sphere as a 1.5x10 2.0x10 2.5x10 3.0x10 3.5x10
P
r
Calibration of Wind Tunnels
Associated Instrumentation
The critical Reynolds number as defined by either force or pressure measurements is then used to define a turbulence factor for the tunnel by comparing the tunnel's critical Reynolds number to the atmospheric free-air Reynolds number:
TF 385 000/R TF=385,000/Retunnel
Then the effective test Reynolds number is defined by Reeffective=TFXRNtest The use of a turbulence sphere yields what may be thought of as an average value of tunnel turbulence. It does not give any information on the magnitude of
turbulence in either the axial or lateral direction. The use of a turbulence sphere may, however, prove to be a simple method of monitoring any change in tunnel turbulence. Its use requires no prior calibrations and the installation and running in
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
q p g
a runnel can be designed to be simple and quick.
The relation between the critical Reynolds number of a sphere and turbulence intensity as measured by a hot wire is shown in Fig. 7.
Calibration of Wind Tunnels
Associated Instrumentation
A brief examination of Fig. 7 might lead to the conclusion th t th hi h th t b l th b tt th t l th
Turbulence spheres can be made from cue, duck, and bowling balls. Several sizes are needed to enable the turbulence factor to be measured over a range of tunnel air speeds.
2.4
that the higher the turbulence, the better the tunnel, as the effective Reynolds number of the test would be higher, This correction is not exact and if the tunnel has excessive turbulence, the model may have a premature transition from laminar to turbulent flow, which can be critical for laminar flow airfoils. However, low-speed models are often equipped with trip strips that fix the transition point on the model and may reduce the requirement for extremely low
0.8 1.2 1.6 2.0 b ulence Inten s ity, TI (% )
model and may reduce the requirement for extremely low turbulence. The need for low test-section turbulence is not as severe for small student tunnels as it would be for larger tunnels that are used for research and development tests. The screens and honeycomb elements are effective
for reducing turbulence in wind runnels. Fig. 7 Variation of turbulence factor with turbulence intensity from hot-wire probe measurements 1.0 1.4 1.8 2.2 2.6 3.0 0.0 0.4 Tur b Turbulence Factor, TF