STABILITY OF FLOW IN AIR-INDUCTION SYSTEMS FOR BOUNDARY-LAYER SUCTION

Abstract

Abstract : Theoretical and experimental studies of the stability of flow in air induction systems with high pressure recoveries for boundary-layer suction have been carried out. The boundary- layer flow entering such systems may have the characteristic of increasing its total head with increasing flow rate for part of the operating range. This characteristic produces both static and dynamic instability. The static instability is evidenced by the appearance of unequal flows in ostensibly identical branches of a system. The dynamic instability occurs in the form of regular oscillations of the flow. These oscillations take place at the characteristic frequencies of the duct and plenum chamber system. In simple cases, the measured frequency shows good agreement with that calculated on the basis of the acoustic theory of the Helmuholtz resonator. In the experimental investigation, it was shown that the installation of splitter plates in a system with a wide slot which exhibited both static and dynamic instability eliminated the latter but not the former. Both types of instability could be eliminated by introduction of high slot and duct losses into the system or by ingesting a flow quantity into the system sufficient to resolve almost the entire boundary layer ahead of all slots. It should be pointed out that a model setup simulating an airfoil boundary layer control system would not exhibit either of the instabilities. This was thought to be possible due to a plenum volume scale effect and/or to the relatively higher losses caused by the model scale effect.