Presentation Dr. Greenblatt

Some Active Flow Control Challenges for CFD
Dr. David Greenblatt
Hermann Foettinger Institute fuer Stroemungsmechanik, TU Berlin

The reemergence of active flow control as a technology with potential application in aerodynamics has spawned an urgent need to develop CFD methods with a predictive capability. Separation control is particularly effective when applied in a nominally two-dimensional manner, for example, at the leading-edge of a wing or at the shoulder of a deflected flap. Despite intuitive understanding of control effects, at present there are no theoretical or computational models capable of adequately predicting the flow or of describing the effects of the leading parameters. This difficulty stems partly from the superposition of coherent structures and incoherent turbulence where the former are usually driven by at least one instability mechanism.
To address this problem, low speed flow separation control over a wall-mounted hump model was studied experimentally in order to generate a data set for the development and evaluation of computational methods. The investigation formed part of a CFD validation workshop sponsored jointly by NASA Langley Research Center, ERCOFTAC, US AFOSR, IAHR, QNET-CFD, and the NIA. To assist and facilitate CFD predictions, three primary cases were considered: a ``baseline'' (no control) case considering only the separated flow; a steady suction control case; and a zero mass-flux control case. The baseline and controlled data sets comprised time-mean surface pressure, phase-averaged unsteady surface pressures and PIV flowfield measurements (2D and stereo) and wall shear stress obtained via oil-film interferometry. Additional surface pressures were acquired for a wide variety of control conditions in order to facilitate CFD parametric studies.
Stereoscopic PIV, surface pressures, and oil-film flow visualization indicated that the baseline time-averaged separated flow field was two-dimensional. With the application of suction, mild three-dimensionality was evident in the spanwise variation of pressure recovery, reattachment location and spanwise pressure fluctuations. For zero mass-flux control, triple-decomposition of the fluctuating velocity and pressure fields was employed for presenting and analyzing the experimental data. This facilitated an assessment of the mechanism of separation control and the quantification of the coherent and turbulent surface pressures, Reynolds stresses and energy fluxes. Spanwise surface pressures and phase-averaged stereoscopic PIV data revealed an effectively two-dimensional flowfield despite highly three-dimensional instantaneous flow structures. Due consideration was also given to characterizing the flow in the vicinity of the control slot, with and without external flow, and to perturbation two-dimensionality.
Despite time-averaged two-dimensionality, the zero mass-flux test case is expected to be particularly challenging for CFD codes because counteracting mechanisms dominate the separated flowfield during different parts of the control cycle.
(This work was performed while the author held a National Research Council - NASA Langley Research Center Associateship.)
Ort: FZR, Gebaeude 101.2, Seminarraum
Zeit: 30.05.2006, 14.00 Uhr