Flow-Quality Measurements and Qualification of the Pennsylvania State University Low-Speed, Low-Turbulence Wind Tunnel

Transcription

1 50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition January 2012, Nashville, Tennessee AIAA Flow-Quality Measurements and Qualification of the Pennsylvania State University Low-Speed, Low-Turbulence Wind Tunnel Amandeep Premi 1 and Mark D. Maughmer 2 The Pennsylvania State University, University Park, PA, Christopher Brophy 3, Naval Postgraduate School, Monterey, CA This study aims at documenting the flow characteristics of the Pennsylvania State University Low-Speed, Low-Turbulence Wind Tunnel. The spatial uniformity, turbulence intensity and spectra were measured and are presented here. The airfoil measurements made in the tunnel are shown to have good agreement with those made at Low-Turbulence Pressure Tunnel (LTPT) at the NASA Langley Research Center and the Low- Speed Wind Tunnel at Delft University of Technology in The Netherlands. The flow quality is concluded to be high and well suited for reliable two-dimensional airfoil characteristics measurements. c c d c l c m C P R Nomenclature = airfoil chord = profile-drag coefficient = section lift coefficient = section pitching-moment coefficient about the quarter-chord point = pressure coefficient = Reynolds number based on free-stream conditions and airfoil chord = Angle of attack relative to x axis, deg I. Introduction IND tunnels have been, and will continue to be important facilities for studying aerodynamics. While wind Wtunnel measurements once were the basis for all aerodynamic understanding and improvements, they now find major importance in calibration of computational results. Nevertheless, to ensure that the airfoil performs as intended, wind-tunnel experiments are routinely used to obtain airfoil-performance measurements representative of those expected in flight. Thus, it is imperative that accurate and reliable data be obtained from wind tunnels. Freestream disturbances such as turbulence, acoustic noise levels, and spatial non uniformity of the mean velocity greatly affects the general flowfield and the laminar-turbulent transition process 1 and therefore the aerodynamics forces experienced by the body in the flow. Spatial-uniformity and low-turbulence levels are necessary for simulating normal atmospheric conditions experienced by the full-scale body, and especially for the study of laminar flow airfoils. This puts forth the need to document the flowfield characteristics of a wind tunnel to have confidence in the results it produces. It is of prime importance for an experimentalist to realize this responsibility of publishing accurate data. The Pennsylvania State University Low-Speed, Low-Turbulence(LSLT) Wind Tunnel is a low turbulenceintensity wind tunnel, frequently used to test two-dimensional airfoil sections. Often, the airfoils tested have significant amounts of laminar flow, making it imperative to have a quiet tunnel with low freestream-turbulence levels and a uniform spectrum. Transition measurements made at the facility can be considered reliable only after the flow quality of the wind-tunnel has been carefully evaluated and the results have been compared with benchmark tests done at other highly regarded facilities. This paper presents the flow characteristics of the Penn State LSLT Wind Tunnel. The Penn State LSLT Wind Tunnel began operation in It was designed especially for the testing of laminar-flow airfoils in the range of Reynolds numbers from 6.0 x 10 4 to 2.0 x The flow-quality measurements made during the design phase of the 1 Graduate Assistant, Department of Aerospace Engineering, 229 Hammond Bldg, Student Member AIAA. 2 Professor, Department of Aerospace Engineering, 229 Hammond Bldg, Associate Fellow AIAA. 3 Associate Professor, Mechanical and Aerospace Engineering Department, Associate Fellow AIAA. 1 Copyright 2012 by the, Inc. All rights reserved.

2 tunnel, 2 which document the benefits/drawbacks of introducing additional turbulence management devices, are presented in this paper. These measurements include the turbulence intensity, flow uniformity and angularity in the test section for different design configurations of the wind tunnel. The different design configurations are listed in Table 1, with configuration 5 being the final one. Also presented are the turbulence spectra measurements, which were taken more recently to confirm the transition measurements made at the tunnel, along with the comparison of benchmark airfoil measurements done at Low-Turbulence Pressure Tunnel (LTPT) at the NASA Langley Research Center and the Low-Speed Wind Tunnel at Delft University of Technology in The Netherlands with those taken the Penn State LSLT Wind Tunnel. II. Experimental Methods A. Facility Description The Penn State Low-Speed, Low-Turbulence Wind Tunnel, shown in Fig. 1, is a closed-throat, single-return atmospheric facility. It is primarily constructed of wood, with only the fan and the diffuser between the second and third corners being constructed of steel. The overall dimensions of the facility are 23 m (75 feet) by 7.6 m (25 feet). The test section is rectangular, 1 m (39.9 in) high and 1.5 m (59 in) wide with a length of 1.85 m (6 feet and 1 inch). The corners of the test section are filleted. The maximum velocity in the test-section is 67 m/s (220 ft/s). Airfoil models are mounted vertically in the test section and attached to computer-controlled turntables that allow the angle of attack to be set. The turntables are flush with the floor and ceiling and rotate with the model. The axis of rotation is between the quarter- and half- chord locations of the model. The gaps between the model and the turntables are sealed to prevent leaks. The wind tunnel is powered by a three-phase, three hundred horsepower motor connected directly to the fan. Wind speed is controlled via electronic rpm control of the motor with an accuracy of 0.1 rpm. The fan has eight adjustable-pitch blades, and thirteen straightener-vanes are located immediately downstream of the blades, after which the flow enters the main diffuser section to the third corner of the wind tunnel. The wind tunnel has a constant cross section from the third to the fourth corner. Both corners contain double-wall turning vanes with six inch chords and have a 0.5 chord spacing. After the fourth corner the flow passes through a fine mesh (46M) dust screen and into a 2:1 expansion section. Two thirds of the way through this expansion, a perforated plate is used to mitigate wall separation. The entrance to the settling chamber contains a honeycomb section having a length to cell ratio of 16 and a cell size of 3/8 inch. The honeycomb is designed to eliminate secondary flows (swirl) and reduce cross stream fluctuations. After passing through the honeycomb, the flow passes through a series of screens to reduce the freestream turbulence. The first screen is a 16 mesh screen placed 1.5 cell diameters downstream of the honeycomb exit. Two fine mesh screens, the first being a 43 mesh and second being a 46 mesh, are downstream. A spacing of 100 mesh lengths was maintained for all screens, allowing adequate time for turbulence decay before the next screen is encountered. 3 Figure 2 shows the final configuration of the turbulence managements devices installed. The test section is rectangular with tapered fillets which begin at the entrance of the contraction section and increase in size up to the test section, where they are 6 inches by 6 inches. The floor and ceiling of the test section diffuse slightly to offset the effects of growing boundary layer. A 2.5 inches breather slot is located at the end of the test section to ensure atmospheric static pressure at that location. A catch net is located at the entrance of the first corner as a safety precaution. The first and the second corners contain single wall circular arc turning vanes which have chords of 8 inches and are spaced at 0.25 chord. Due to physical constraints, the section that connects these corners diffuses slightly in order to reach the necessary crosssectional area at the fan inlet. After leaving the second corner, the flow then re-enters the fan. B. Models The airfoil models used in the experiments are mounted vertically in the wind tunnel and completely span the height of the test section. They are generally constructed of solid aluminum or composite materials. The aluminum models and the molds for the composite models are produced using a numerically-controlled milling machine. The remaining models were produced from solid aluminum using a numerically-controlled milling machine. Each model has approximately 33 pressure orifices on the upper surface and roughly the same number on the lower surface. The orifices have a diameter of 0.51 mm (0.020 in) and are drilled perpendicular to the surface. The orifice locations are staggered in the spanwise direction to minimize the influence of an orifice on those downstream. 2

3 C. Data Acquisition System for Airfoil Measurements The wind-tunnel pressures are measured using VALIDYNE DP-15, differential pressure-sensing diaphragm transducers. The pressures on the model are measured by a ScaniValve automatic pressure-scanning system with similar pressure transducers. Data from the transducers are amplified/filtered by TECHKOR MEPTS 9000 and Validyne MC 1-10 rack mounted modules and then fed into the computer. To obtain drag measurements a wake-traversing, Pitot-static pressure probe is mounted from the ceiling of the tunnel. A traversing mechanism incrementally positions the probe across the wake, which automatically aligns with the local wake-centerline streamline as the angle of attack changes. The surface pressures measured on the models are reduced to standard pressure coefficients and numerically integrated to obtain section normal- and chord-force coefficients, as well as the section pitching-moment coefficient about the quarter-chord point. Section profile-drag coefficients are computed from the wake total and static pressures using standard procedures 4. Wake surveys are not performed, however, at most post-stall angles of attack, in which case the profile drag coefficients are computed from normal- and chord-force coefficients as obtained from pressure integration. Low-speed wind-tunnel boundary corrections are applied to the data 5. A total-pressure-tube displacement correction, although quite small, is also applied to the wake-survey probe 4. As is clear from the applying the procedures prescribed in Ref. 6, the uncertainty of a measured force or moment coefficient depends on the operating conditions and generally increases with increasing angles of attack. In the higher lift regions, for which the uncertainty is the greatest, the measured lift coefficients have an uncertainty of c l = ± The uncertainty of drag coefficients measured in low-drag range is c d = ± while, as the angle of attack approaches stall, this increases to c d = ± The pitching-moment coefficients have an uncertainty of c m = D. Instrumentation for Turbulence Measurements For turbulence intensity measurements, both single element and cross wire hot-wires were used with DISA 55M01 constant temperature anemometers, then filtered and amplified by Krohn Hite 3320 filters. A single element TSI hot-film probe with an overheat ratio of 1.75 was used with the DISA 55M01 Constant Temperature Anemometer (CTA) system for turbulence spectra measurements. The idea is to capture any frequency peaks that might be present in the freestream spectrum. Even though they may not contribute to the overall turbulence intensity, these peaks may still cause premature transition if one happens to have sufficient amplitude and be of the similar frequency as one of the critical instabilities of the profile being tested. Hot wire signals to measure the turbulence intensities of different configurations were acquired at 8 khz per channel and bandpassed on the Krohn Hite filters from 1 Hz to 4000 Hz. The hot-film signals for spectra measurements of the final configuration were acquired at 12 khz and bandpassed from 1 Hz to 6000 Hz digitally on MATLAB. The inbuilt fft command in MATLAB was used for the fourier analyses Different windowing techniques were considered to avoid errors due to spectral leakage and to provide better frequency resolution. Fig. 3 shows the comparison between simple fft, Hann and flattop windowed signal responses. The default constants for the window functions in MATLAB were used. From the responses in Fig. 3, it can be seen that Hann window works to resolve the peak better but also enhances the noise peaks. The flattop response is a bit better in terms of lower noise peaks but the aerodynamic peak also gets less defined. The frequency resolution is more important in the present research than amplitude accuracy; hence, Hann windowing appears to be a better choice. A Hann window function with 50 percent overlap is applied in the spectra presented here. III. Results A. Characterization of Tunnel Turbulence The turbulence intensity, flow-uniformity and flow-angularity measurements were done during the fabrication of the wind tunnel. Different configurations, as listed in Table 1, were tested to obtain a characterization of the turbulence management system. All the turning vanes were properly aligned for a test section velocity of 38 m/s (125 ft/s) before the tests and were generally observed to operate adequately at off-design velocities. The turning vanes at the first and second corners were observed to be insensitive to tunnel velocity; however, the turning vanes in the third and fourth corner did show a dependence on tunnel velocity, thereby requiring a particular angle for a given flow speed for optimum performance. The flow angularity in the test section had a center of rotation around the centerline of the test section. The addition of honeycomb in the final configuration reduced the flow angularity from ± 2 degrees in configuration 4 to the accuracy of the probe for angularity measurements i.e. ± 0.25 degrees. 3

4 The final configuration included the installation of a m x m (6.0 inches x 3/8 inch) hexagonal cell honeycomb section, m (1inch) from the inlet of the settling chamber and m (½ inch) upstream of the 16 mesh screen. The location of the 16 mesh screen 1.5 cell diameters downstream of the honeycomb is recommended by Nagib and Loerhke 7 as an effective distance to disrupt the exiting shear layers from the honeycomb, thereby reducing time for the flow to become isotropic and also resulting in a faster transfer of energy to more dissipative scales. The results of the final configuration are presented in Fig. 4(a-e) along with a comparison with other configurations. Results are presented at a flow velocity of 150 ft/s but data was taken at other velocities too. The measurements for spatial behavior were taken with the hot-wire mounted in the middle of the test-section with respect to the height and length of the test-section. Figures 4a and 4b show the spatial behavior of the streamwise turbulence intensity across the width of the tunnel for different configurations. Figure 4c presents the spatial distribution of velocity at two different tunnel velocities. Figure 4d shows the spatial behavior of turbulence intensity of the final configuration. As can be seen, the subsequent addition of these turbulent devices at the correct cell diameters distance to each other, resulted in effective decrease in turbulence levels with less penalty in power increase as can be seen from Fig. 5 which shows the centerline test section velocity at different fan rpm for different configurations. A single element hot wire was used to take data for streamwise fluctuations at various velocities and the spectrum was inspected for any frequency spikes. The idea behind this recent test was that even though the turbulence intensity levels are low at a certain velocity, if the freestream has a spike in the amplitude of a certain frequency falling in the critical Tollmien-Schlichting range for a certain profile being tested, the transition characteristics can change. To measure the contribution of electronic noise and ambient disturbances on the measurements, the wind tunnel fan was shut down and measurements were taken. All of these cases had no wind blowing in the tunnel and the test-section window was closed to avoid any ambient gusts. Figure 6 represents the case where the CTA equipment was put on standby mode (not on operate) and all the lights were shut down to see the influence of the CTA, wires, fluorescent-tube lights and probe on the noise levels. As can seen from the spectrum in Fig. 6, the 2750 Hz peak was still present and is, therefore, attributed to the data acquisition card connected to the computer. A set of symmetrical peaks around the 2570 Hz peak are also observed. Fig. 7 shows the spectrum for the case where all the lights around the data acquisition station were switched off but the CTA equipment was running. This case surprisingly shows a decrease in the magnitude of white noise as compared to the previous case. It can be concluded from this Fig. that the CTA equipment itself did not contribute to any frequency peaks and works to decrease the background noise, when operating. Fig. 8 shows the case were the lights are switched on and the rest of the conditions are similar to the last case. It is observed that the fluorescent light tubes are adding noise in the signals acquired from the CTA equipment. This case was run several times, switching different lights on and off, but the essential characteristics were unchanged. This noise signature was seen in the actual data too since the light was switched on while taking the data but it was of low amplitude and can be attributed to non aerodynamic source. The turbulence spectra for different tunnel velocities are presented in Figs. 9(a-d). Although many more were taken, only few are presented here. The measurements were taken at a sample rate of 12 khz for a 10 second interval. There are certain frequency peaks seen all through the range of tunnel velocities. However, most of the frequency peaks seem to be the impression of the noise from the lights observed in the noise estimation study. 8 To ensure their electronic origin the experimental transition location on three airfoils was compared to the theoretically predicted transition location. 8 Since, the theory did not take into account the effects of receptivity and was based on boundary-layer theory and linear-stability theory, the agreement with theory showed the independence of the spectral peaks and the experimental results. The airfoils results were matched for cases where the frequency peaks in the tunnel spectra (whose origins are believed to be mostly electronic) were coincident on the theoretical critical frequencies for that airfoil but no anomalies in the transition location were found. Since, the theory itself is not at a completely mature stage and the value of n could be altered in the e n analysis to better match the experimental results, the trends of the movement of the transition location with change in angle of attack were compared than actual exact values. The trends show good agreement and were shown to contain no anomalies. The details of the study can be found in Ref. 8. Even more confidence in the results can be put from the fact that the airfoil measurements show very good agreement with measurements taken in other highly regarded tunnels (which may not have the same frequency spectra), as is presented in the next section. B. Airfoil Measurements While the attainment of high flow quality is certainly a requisite for measuring meaningful airfoil aerodynamic characteristics, additional confidence in the measurements made in the Penn State facility is gained by making comparisons with airfoil measurements taken elsewhere. For this purpose, perhaps the two most highly regarded 4

5 two-dimensional, low-speed wind tunnels are the Low-Turbulence Pressure Tunnel (LTPT) at the NASA Langley Research Center 9 and the Low-Speed Wind Tunnel at Delft University of Technology in The Netherlands. 10 For low Reynolds number airfoil aerodynamics, a benchmark set of data is that obtained with the Eppler E387 airfoil in LTPT 11. In Fig. 9 these results were compared with those obtained in the Penn State tunnel for R = 4.6 x Except for post-stall aerodynamics, which are highly three-dimensional and not all that meaningful with respect to two-dimensional measurements, the agreement of the data from LTPT with that of the Penn State facility at all Reynolds numbers tested is excellent. Equivalent agreement is found in comparisons down to R = 0.6 x In addition to the force and moment data comparisons, pressure distributions and transition locations measured at Penn State also show excellent agreement with those obtained in the Langley facility. 11 In Fig. 10, Penn State tunnel measurements made on the laminar-flow S805 wind turbine airfoil 13 are compared with those obtained using the same wind-tunnel model at Delft. 10 These data, at Reynolds numbers from R = 5.0 x 10 5 to 1.5 x 10 6, also demonstrate excellent agreement. Although not all the comparisons are presented here, pressure distributions and measured transition locations obtained at Penn State also show excellent agreement with those of the Delft experiments. 10 IV. Conclusion The Penn State Low-Speed, Low-Turbulence Wind Tunnel has been designed to maintain high flow quality and low turbulence levels for high quality data acquiring capabilities. The combination of the turbulence management devices installed results in a low freestream turbulence intensity 2 with clean spectra, 8 which ensures the high quality of aerodynamic measurements taken at the facility. The spectra have been shown to have no high amplitude frequency peaks 8 in the flow to cause any anomalies in the measurements made in the tunnel. Furthermore, the measurements taken at the facility agree well with those taken in LTPT NASA Langley 9 and in the low speed tunnel at Delft University of Technology, Netherlands. 10 This bends a high degree of confidence that the Penn State Low-Speed, Low-Turbulence Wind Tunnel is a well-designed, high flow-quality facility, well suited for reliable two-dimensional airfoil characteristics measurements. References 1 Michel, U., and Froebel, E., Aerodynamic Data Accuracy and Quality: Requirements and Capabilities in Wind-Tunnel Testing AGARD Conference Proceedings No Brophy, C.M., Turbulence Management and Flow Qualification of The Pennsylvania State University Low Turbulence, Low Speed, Closed Circuit Wind Tunnel, M.S. Thesis, Department of Aerospace Engineering, Penn State University, University Park, PA, J. Tan-Atichat, Loehrke, R. I., and Nagib, H. M., Interaction of free-stream turbulence with screens and grids: a balance between turbulence scales 4 Prankhurst, R.C. and Holder, D.W., Wind-Tunnel Technique, Sir Isaac Pitman & Sons, Ltd, London, Allen, H.J., and Vincenti, W.G., Wall Interference in a Two-Dimensional-Flow Wind Tunnel, With Consideration of the Effect of Compressibility, NACA Report 782, Assessment of Experimental Uncertainty with Application to Wind Tunnel Testing, AIAA Standard S-071A-1999, Revision A of the Standard, AIAA, Reston, VA, Loehrke, R. I., and Nagib, H. M., Control of Freestream Turbulence by Means of Honeycombs: A Balance Between Suppression and Generation, Journal of Fluids Engineering, Sept. 1976, pp Premi, A., Qualification of Airfoil Transition Measurements Taken in the Penn State Low-Speed, Low-Turbulence Wind Tunnel, M.S. Thesis, Department of Aerospace Engineering, Penn State University, University Park, PA, McGhee, R.J., Beasley, W.D., and Foster, J.M., Recent Modifications and Calibration of the Langley Low-Turbulence Pressure Tunnel, NASA TP-2328, van Ingen, J.L., Boermans, L.M.M., and Blom, J.J.H., Low-Speed Airfoil Section Research at Delft University of Technology, ICAS , Munich, October McGhee, R.J., Walker, B.S., and Millard, B.F., Experimental Results for the Eppler 387 Airfoil at Low Reynolds Numbers in the Langley Low-Turbulence Pressure Tunnel, NASA Technical Memorandum 4062, October Somers, D.M. and Maughmer, M.D., Experimental Results for the E 387 Airfoil at Low Reynolds Numbers in the Pennsylvania State University Low-Speed, Low-Turbulence Wind Tunnel, U.S. Army Research, Development and Engineering Command TR 07-D-32, May Somers, D.M., Design and Experimental Results for the S805 Airfoil, National Renewable Energy Laboratory, NREL Report No. SR , October

6 Figure 1. Layout of The Pennsylvania State University Low Speed, Low Turbulence Wind Tunnel 2. Figure 2. Layout of the turbulence management devices installed 2. 6

7 Amplitude (ft/s) Amplitude (ft/s) Amplitude (ft/s) Mean velocity = ft/s = m/s Hanning Windowed Flattop Windowed Figure 3. Comparison of different windowing techniques for a tunnel velocity of 187 ft/s. a) Turbulence intensity profiles for configurations 1 & 2, here W TS =width of the test-section, Y=width axis of the test-section with 0 at the center of the tunnel. Figure 4. Turbulence characteristics of the wind tunnel. 7

8 b) Turbulence intensity profiles for configurations 3,4 & 5. c) Spatial behavior of mean velocity of the final configuration in the test section. Figure 4. Continued. 8

9 d) Cross-wire measurements in test-section for configuration 5. e) Streamwise turbulence intensity variation with test-section mean velocity. Figure 4. Concluded. 9

10 Amplitude Figure 5. Test-section velocity vs fan RPM. 3.5 x Figure 6. Noise spectra with CTA equipment setup on standby, lights and the tunnel shut off. 10

11 Amplitude Amplitude 2.5 x Figure 7. Noise spectra with CTA equipment setup running and the lights and tunnel shut off. 2.5 x Figure 8. Noise spectra with CTA equipment setup and lights running and the tunnel shut off. 11

12 Amplitude (ft/s) Amplitude (ft/s) a) 9 8 x 10-3 Mean velocity = ft/s = m/s Mean velocity = ft/s = m/s b) Figure 9. Frequency spectrum at different tunnel speeds. 12

13 Amplitude (ft/s) Amplitude (ft/s) c) Mean velocity = ft/s = m/s Mean velocity = ft/s = m/s d) Figure 9. Concluded. 13

14 E-387 a) R = 460,000. E-387 b) R = 300,000. Figure 10. Aerodynamic characteristics of the E 387 airfoil measured at Penn State compared with those measured at LTPT, NASA Langley Research Center. 14

15 c) Pressure distribution for R = 300,000 at =4⁰. d) Pressure distribution for R = 300,000 at =10⁰. Figure 10. Concluded. 15

16 a) R = 1,500,000. b) R = 1,000,000. Figure 11. Aerodynamic characteristics of the S805 airfoil measured at Penn State compared with those measured at Delft. 16

17 c) Pressure distribution comparison at R = 500,000. d) Pressure distribution comparison at R = 1,500,000. Figure 11. Concluded. 17

18 Turbulence management devices Configuration installed One Two Three Four Five Perforated Plate Dust Screen (46 Mesh) 43 mesh screen 46mesh screen 16 mesh screen Honeycomb Table 1. Different configurations Device Fan Motor Turning Vanes First and Second corners Third and Fourth corners Dust Screen Perforated Plate Honeycomb Settling Chamber Screens First Second Third Rapid Expansion Contraction Section Test Section First Diffuser Return Diffuser Description 8 blades and 13 stators, max RPM phase, 300hp Single Wall, 8" chord, 3/16" thick, 0.25 chord apart Double Wall, 6" chord, 9/16" max thickness, 0.5 chord apart 46 Mesh, " wire diameter, β= /8" holes, 3/32" thickness, β=0.62 Cell size =3/8", length to cell ratio=16:1 16 Mesh, 0.009" wire diameter, β= Mesh, 0.005" wire diameter, β= Mesh, " wire diameter, β= :1 Area ratio 9.3:1 Area ratio, tapered corner fillets Closed section with fillets, 6'1" length: 3.25' x5' Average cross section: max speed, 220 ft/s 1.9:1 Area ratio 2.2:1 Area ratio Table 2. Wind tunnel device description 18