Laminar flow control of subsonic boundary layers by suction and heat-transfer strips

Abstract

The effects of suction and heat-transfer strips on the stability of subsonic boundary layers over flat plates are investigated. The mean flows are calculated by using interacting boundary layers. Then the linear two-dimensional quasiparallel spatial stability theory of compressible flows is used to calculate the growth rates and hence the amplification factors. Using the eN criterion, the optimal location of a suction strip for delaying transition is investigated. Moreover, using the fact that the incremental growth rates due to suction in subsonic flows vary linearly with suction velocity, influence coefficients are calculated that can be used to design ‘‘smart’’ suction configurations. A major finding of the present investigation is the reversal of the effect of heating by strips compared with uniform heating. The present results show that a heating strip located near branch I of the neutral stability curve very much stabilizes the flow, in contrast with uniform heating, which destabilizes the flow. On the other hand, a cooling strip located near branch I of the neutral stability curve destabilizes the flow, in contrast with uniform cooling, which stabilizes the flow. It is also found that a properly placed cooling strip can delay transition in subsonic boundary layers. The optimal locations of heating and cooling strips are determined. The present findings may have important implications for the design of laminar flow control surfaces.