Shear layer development over a NACA 0018 airfoil at a chord Reynolds number of 100 000 was investigated using a combination of flow visualization, velocity field mapping, surface pressure fluctuation measurements, and stability analysis. The results provide a detailed description of shear layer transition on an airfoil at low Reynolds numbers. An extensive comparison of measured surface pressure and velocity fluctuations demonstrated that time-resolved surface pressure sensor arrays can be used to identify the presence of flow separation, estimate the extent of the separated flow region, and measure disturbance growth rate spectra in significantly less time than is required by conventional techniques. Surface pressure sensor measurements of disturbance growth rate, wave number, and convection speed are found to compare well with predictions of linear stability theory, supporting the claim that convection speeds measured in separation bubbles over low Reynolds number airfoils are associated with wave packe...

Separated shear layer transition over an airfoil at a low Reynolds number

Luminescent molecular probes imbedded in a polymer binder form a temperature or pressure paint. On excitation by light of the proper wavelength, the luminescence, which is quenched either thermally or by oxygen, is detected by a camera or photodetector. From the detected luminescent intensity, temperature and pressure can be determined. The basic photophysics, calibration, accuracy and time response of a luminescent paints is described followed by applications in low speed, transonic, supersonic and cryogenic wind tunnels and in rotating machinery.

Boundary-Layer Transition Detection in a Cryogenic Wind Tunnel Using Luminescent Paint

The measurement of airframe noise on small-scale models is well known and common practice in conventional wind tunnels. Since conventional wind tunnels cannot generally achieve full-scale Reynolds numbers, measurements during the development process of modern aircraft are often performed in cryogenic and/or pressurized wind tunnels which are capable of higher Reynolds number flows. Thus, the characteristics of the moving fluid are better adapted to the scale model. At the DLR Institute of Aerodynamics and Flow Technology the microphone array measurement technique was further developed to perform measurements in a cryogenic wind tunnel at temperatures down to 100 K. A microphone array consisting of 144 microphones was designed and constructed for this purpose. In this paper, acoustic array measurements performed in a cryogenic wind tunnel are described for various Reynolds numbers using a 9.24% Dornier-728 half model. Additionally, the background noise of the empty test section was measured within the range of the measurements performed on the Dornier-728 half model. Our results seems to indicate a Reynolds number dependency of the measured sound power for various sources.

Aeroacoustic Measurements of a Scaled Half-Model at High Reynolds Numbers

Measurements on airfoils at low Reynolds numbers can have a strong dependence on the experimental setup as a result of the sensitivity of the transitioning separated shear layer that develops over the model. In this investigation, the effects of two aspects of the experimental setup, namely end plates and test-section blockage, on lowReynolds-numberairfoilexperiments areexploredthroughmeasurementsonaNACA0018airfoilmodelatachord Reynolds number of 100,000. The improvement in mean spanwise uniformity with end plates installed is quantified and demonstrates the importance of using end plates in two-dimensional airfoil experiments at low Reynolds numbers. Consistent with previous studies on the use of end plates on circular cylinder models, it is found that mean quantities measured on the center-span plane are least sensitive to end-plate spacing for spacings greater than roughly 7 times the projected model thickness. It is shown that the end-plate configuration affects vortex-shedding characteristics and disturbance amplification in the separated shear layer. Blockage effects are investigated by comparing measurements before andafter adaptive-wall test-section streamlining forsolid-blockage ratios between 4 and 8%. These blockage ratios are shown to cause errors in lift as high as 9% of the maximum lift and 3.5% in the wake vortex-shedding frequency. The results of this investigation can be used to estimate the effects of blockage on flow development in low-Reynolds-number airfoil experiments. It is demonstrated that a common blockagecorrectionmethodcanaccuratelycorrectliftmeasurementsformoderateblockagesinlow-Reynolds-numberairfoil experiments for conditions under which a separation bubble forms over the model.

Effects of End Plates and Blockage on Low-Reynolds-Number Flows Over Airfoils

A short history of the European Transonic Wind Tunnel ETW

Theoretical considerations indicate that cooling the wind-tunnel test gas to cryogenic temperatures will provide a large increase in test Reynolds number with no increase in dynamic pressure while reducing the tunnel drive-power requirements. Studies have been made to determine the expected variations of Reynolds number and other parameters over wide ranges of Mach number, pressure, and temperature with due regard to avoiding liquefaction and adverse real-gas effects. Practical operational procedures have been developed in a low-speed prototype cryogenic wind tunnel. Aerodynamic experiments in the facility have demonstrated the theoretically predicted variations in Reynolds number and drive power. Force and moment measurements on a wing model mounted on a water-jacketed strain-gage sting balance have demonstrated the feasibility of operation of such balances in a cryogenic environment.

High-Reynolds-Number Cryogenic Wind Tunnel

A circular cylinder was mounted horizontally across a low speed variable depth twodimensional test section having flexible top and bottom walls. The walls could be adjusted to quasi-streamlines using a set of screw jacks attached to ribs spaced out along their lengths. The aim was to determine the blockage limits, as indicated by mean drag and Strouhal number, beyond which wall interference could not be removed by streamlining the walls. Blockage has the normal definition of the ratio of cylinder diameter to test section depth at entry. Values between 25% and 100% were covered. Two flow types were observed, proving to be bi-stable using streamlined walls at 75% blockage and above, the direction of approach of the walls to streamlines substantially affecting their final contours and therefore cylinder performance. It was found that Strouhal number was lesser affected by variation of blockage. Interference effects on drag coefficient could not be removed beyond about 50% blockage. A flexible-walled test section operating at this blockage would increase Reynolds number by a factor of about 4 compared on an equal basis with a conventional solid-walled two-dimensional test section. The work may have relevance to general two-dimensional bluff body testing and to aerofoil testing at high incidence.

Michael J. Goodyer

Papers

High-Reynolds-Number Cryogenic Wind Tunnel

Blockage Limits in a Two-Dimensional Self-Streamlining Wind Tunnel