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Determination of Turbulence Level in the UC DavisAeronautical Wind Tunnel

J. Phoreman, S. Saephan, J.C. Vander Kam

AbstractTurbulence spheres were used to determine the turbulence level in the UC DavisAeronautical Wind Tunnel. Three different tunnel velocities were evaluated through theuse of three different diameter spheres. Results confirmed that turbulence levels are onthe order of 0.1% of freestream throughout the majority of tunnel operating speeds.Additionally, the method of turbulence spheres was evaluated and found to be useful andrelatively easy to implement.

NomenclatureTF = turbulence factor RN e = effective Reynolds number RN test = test Reynolds number RN tunnel = critical Reynolds number for UC Davis AWTRN C = critical Reynolds number CD = drag coefficient based on cross sectional areaVC = critical velocity = freestream air density = freestream air viscosityd = sphere diameter

IntroductionVariations in flow quality between two different wind tunnels will cause variations

between the results obtained from the two tunnels when like experiments are performedfor the same Reynolds number. One of these flow quality parameters is turbulence level.An understanding of turbulence level is helpful in determining an effective Reynoldsnumber (RN e) for a given configuration and flow condition.

Spheres are known to have a distinct critical Reynolds number above which the flow onthe upstream face of the sphere is fully turbulent causing the drag coefficient to dropdramatically. This is because the turbulent boundary layer results in separation further aftthan a laminar boundary layer, thus producing a smaller wake. The Reynolds number atwhich this transition occurs is strongly dependent on the degree of turbulence in the windtunnel.

In this experiment, 3.5in, 5.5in, and 9in turbulence spheres were used to determine thelevel of turbulence and resultant turbulence factor for the 33.6in 48in 12ft test sectionof the UC Davis Aeronautical Wind Tunnel. The critical Reynolds number for the threespheres was determined by examining the measured drag coefficient C D (based on cross-sectional area) as a function of Reynolds number. The subsequent sections explain thetest set-up, the results obtained from this experiment, and a discussion of the results.


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Test SetupThe turbulence sphere tests were conducted in the Aeronautical Wind Tunnel 1 (AWT) atthe University of California, Davis. The AWT is an open circuit tunnel with acontraction ratio of 7.5:1. Inside the contraction section is one honeycomb screen

followed by four anti-turbulence screens. The closed test section measures 33.6in 48in 12ft and has solid walls with tapered fillet corners. Forces and moments are measured by a pyramidal balance system installed beneath the test section. The balance canmeasure up to 50 pounds of drag with a load dependent accuracy which is typicallywithin 5%. Raw data is acquired through a 16-bit acquisition system tied to LabView.The 125 horsepower motor can obtain a maximum tunnel flow speed of 165 mph.

The turbulence spheres were mounted onto a custom built mount shown in Figure 1. The

mount is made of two solid cylindrical stock pieces of 6061 T6 aluminum joined at aright angle. The threaded end of the vertical piece is screwed directly into the tunnel

balance. A hole is drilled through the other end, perpendicular to the axis of the piece.One end of the horizontal piece is inserted through this hole and tightened with a set-screw. The free end of the horizontal piece is inserted into a hole in the spheres. 85% of

the vertical mount was faired with a symmetric airfoil shaped fairing.

The 3.5in and 5.5in spheres were actually hollow Christmas ornaments. The ornamentswere neither perfectly smooth nor spherical, though close enough for the purpose of thisstudy. A slight parting line (parallel to the flow direction) and finishing blemishes werenoticeable, but were assumed to have little impact on the overall results. The 9in spherewas borrowed from NASA Langley. It was produced using stereolithography and is verynearly spherical with a less smooth, but more uniform surface than the ornaments.

Title:

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Figure 1


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Test ProceduresPrior to testing, theoretical values were calculated for the predicted transition velocitiesand the drag forces expected from the experiment. These are presented here in Figure 2as drag force versus freestream velocity for the conditions of the experiment. The

horizontal line in this plot indicates the predicted drag force at the theoretical transition

Reynolds number. For a given Reynolds number, the drag force is independent of spherediameter:

Thus, we see a constant drag force for all sphere diameters at the critical Reynoldsnumber.

Data AcquisitionPrior to taking data, a zero point is taken with the sphere mounted and the wind off. Theairspeed is then increased to level about 20 miles below the turbulence transitionthreshold. Data are acquired at 5 mph increments up to and following transition. Near the region of transition, data are acquired at smaller airspeed intervals. A secondcalibration point is taken at the end of each test run. The data at each airspeed are theaverage of five separate data samples and then averaged to arrive at a single data point for each tunnel speed. Data gathered from the force balance were corrected through the use

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2222 RN C d C V D D D ==

Drag Force vs. Freestream Velocity

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0 20 40 60 80 100 120 140 160 180

Velocity [mph]

F o r c e [ l b f ]

9in. Sphere

5.5in. Sphere

3.5in. Sphere

Transition RN=385,000 @ (72 F, 29.92 inHg)

Figure 2


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of a calibration matrix developed by Yen and Bruchle. 2 A correction was also included based on the wind off zero taken before and after each test run. This allowed the data to be shifted to correspond with the zero-drag signal from the force balance. Data wascollected using a 16-bit acquisition system tied to the LabView software package.

Data ReductionOnce the raw data were obtained it became clear that the contribution of the mountingstructure was not negligible for the smaller sphere sizes. To account for this, a correctionwas made based on the assumption that the influence of the mount in the 9in sphere casewas negligible. This assumption is based on the fact that the mounting structure did not

protrude beyond the frontal projected area of the sphere. An average was then taken of the drag coefficient values obtained for the 9in diameter sphere prior to the drag drop due

to transition. An average of the pre-drop values was taken for the other two size spheresas well. The entire data set for the 3.5in and 5.5in spheres was then shifted an amountequal to the difference between their pre-drop averages and the pre-drop average of the9in. sphere case. The resulting data is presented in Figure 3. This shifting process resultsin curves that are aligned about a drag coefficient 0.6 prior to the drag drop due totransition. This allows the method outlined in Barlow, Rae & Pope to be used. 3 FromFigure 3 it is clear that a fairly sharp drop in the drag coefficient is seen at the onset of

boundary layer transition. During the testing, it was also observed that the amount of vibration in the mounting structure decreased dramatically as the transitional region wassurpassed. This is due to the fact that the separation characteristics of the sphere go fromlarge-scale vortex shedding to a more chaotic turbulent wake. The turbulent separation

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150000 200000 250000 300000 350000 400000 450000

RN

C d

9in. Sphere

5.5in. Sphere

3.5in. Sphere

RN = 385,000

Figure 3


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results in smaller, less coherent vortices than the laminarly separated wake. Thus there isless strong cyclic excitation of the mounting structure.

The method presented by Barlow et al. relates the turbulence level in a wind tunnel to theReynolds number at which the drop in drag of a sphere is observed. It is known that the

drop in drag due to transition for a perfectly smooth sphere in an atmospheric flow willoccur at a Reynolds number of 385,000. 3 The turbulence factor for a wind tunnel maythen be found from:

where TF is the turbulence factor and RN C is the Reynolds number at which themeasured drag coefficient passes through 0.3 during transition from laminar to turbulent

boundary layer flow. The turbulence factor is then related to the tunnel turbulence levelusing the relation obtained with hot-wire anemometry seen in Figure 4. 4

Using this correlation, the turbulence level in the wind tunnel may be deduced from theobserved Reynolds number at which the drag of a sphere drops due to the transition of the

boundary layer. Using this method, the following turbulence levels are found:

Sphere Diameter (in.) RN c Vc (mph) TF Turbulence Level3.5 345,276 132.5 1.12 0.1%5.5 367,658 89.8 1.05 0.1%9.0 328,342 49.0 1.17 0.1%

C RN TF

000,385=

Figure 4


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Clearly, the turbulence level is quite low. These findings are encouraging as low levels of turbulence were found over a large range of operating test-section velocities.

DiscussionThe results presented in the previous section are very encouraging as they demonstrate

the low turbulence level of the Aeronautical Wind Tunnel at UC Davis. There are somefactors, however, that may have been contributors to uncertainty in these findings. Themost significant of these was the intermittent vibration seen in the mounting structure andsphere prior to transition. The 9in sphere weighed significantly more than the other twoand experienced greater vibration amplitudes especially in the transitional range.Vibrations were present in the 3.5in and 5.5in spheres as well, but were of higher frequency and lower amplitude. These vibrations may have affected the nature of theflow over the spheres or the data obtained from the force balance. The dragmeasurements obtained for the 9in sphere contained a great deal more scatter than thesmaller spheres. This phenomenon may be part of the reason that the transitional regionseen in the 9in sphere spans a larger range of Reynolds number than the two smaller

spheres. Though these vibrations appeared significant from observations taken at the timeof testing, the resulting data indicates that the actual impact of these vibrations is withinacceptable limits A stiffer mounting solution may yield results with less scatter.

Alignment of the spheres with the freestream and their position in the tunnel test sectionis ruled out as a possible source of uncertainty due to the uniformity of the sphericalshapes. The position in the test section was kept as close to the centerline as possible andis also thought to be negligible as a source of uncertainty.

ConclusionTurbulence spheres were used to obtain drag coefficient measurements for a range of Reynolds number flows. The critical Reynolds number for freestream atmospherictransition was taken to be 385,000 and was divided by the measured critical Reynoldsnumber to obtain a turbulence factor. The turbulence factor was then used to referencehistorical data taken with a hot-wire anemometer. This comparison yielded the turbulencelevel as a percentage of free stream velocity The results indicate that the UC DavisAeronautical Wind Tunnel has a good quality of flow with freestream turbulence levelson the order of 0.1% and a turbulence factor of less than 1.2.

The data obtained from this study serves to illustrate both the usefulness of turbulencespheres in determining turbulence levels as well as the notable flow quality in theAeronautical Wind Tunnel at UC Davis. The method of turbulence spheres is relativelysimple and provides a good, if somewhat qualitative, understanding of the turbulencelevels in a wind tunnel. Based on these findings, it is clear that the AWT at UC Davis

produces excellent flow quality throughout a range of test section velocities.


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References1. UC Davis Aeronautical Wind Tunnel website, http://windtunnel.engr.ucdavis.edu

2. Bruchle, F., Yen, D.T., Calibration and Uncertainty Analysis for the UC Davis

Wind Tunnel Facility, UC Davis MAE Department Report, May 2000.

3. Barlow, J.B., Rae Jr., W.H., Pope, A., Low-Speed Wind Tunnel Testing, Wiley& Sons, Inc., New York, 1999. pp 147-150.

4. Dryden, H.L., Keuthe, A.M., Effect of Turbulence in Wind TunnelMeasurements, NACA Report 342, 1929.

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