Separation control at flight Reynolds numbers - Lessons learned and future directions

Avi Seifert* and LaTunia G. Pack*NASA Langley Research Center and *TeI-Aviv UniversityAbstractActive separation control, using periodic excitation, wasstudied experimentally at high Reynolds numbers. Theeffects of compressibility, mild sweep, location ofexcitation slot and steady momentum transfer on theefficacy of the method were identified. Tests conductedat chord Reynolds numbers as high as 40xl06demonstrated that active control using oscillatory flowexcitation can effectively delay flow separation from,and reattach separated flow to aerodynamic surfaces atflight conditions.The effective frequencies generate one to four vorticesover the controlled region at all times, regardless of theReynolds number. The vortices are initially amplified bythe separated shear-layer, and after initiatingreattachment, the strength of the vortices decay as theyare convected downstream. Large amplitude, lowfrequency vortices break down to smaller ones uponintroduction at the excitation slot. The effects of steadymass transfer were compared to those of periodicexcitation. It was lound that steady blowing issignificantly inferior to periodic excitation in terms ofperformance benefits and that the response to steadyblowing is abrupt, and therefore undesirable from acontrol point of view. Steady suction and periodicexcitation are comparable in effectiveness and bothexhibit a gradual response to changes in the magnitudeof the control input. The combination of weak steadysuction and periodic excitation is extremely effectivewhile the addition of steady blowing could bedetrimental.Compressibility effects are weak as long as separation isnot caused by a shock-wave/boundary-layer interaction.The undesirable effects of the shock-induced separationcould be alleviated by the introduction of periodicexcitation upstream of the shock wave, inside the regionof supersonic flow.The effccts of mild sweep were also studied and periodicexcitation was found to be very effective in reattachingthree-dimensional separated flow. Scaling laws thatcorrelate 2D and 3D controlled flows were tested andverified.Several performance benefits could be gained byapplying the method to existing configurations, but it isexpected that the full potential of the method can only berealized through the design of new configurations. Acomprehensive, fully turbulent, database was generatedin order to guide the development, and enable validation,of candidate unsteady CFD design tools.