Shock Train/Boundary-Layer Interaction in Rectangular Isolators

Shock trains in rectangular isolators with inflow Mach number of 2.5 and ARs of 3.0 and 6.0 are investigated. Knowledge gained is used to propose a rectangular isolator modification to existing empirical shock train length relations based on circular ducts. A novel, multiplane shadowgraph concept allows simultaneous flow visualization from multiple perspectives. Boundary-layer separation in the low-momentum corner flow regions is observed to occur upstream of the center-flow field by approximately one duct height. The leading edge shock train structure is composed of a hybrid oblique-normal shock front, with oblique shocks spawning from the corner flow separation transforming into a normal shock front near the centerline. Dynamic wall pressure measurements made along the duct major axis allow for direct comparison between the outboard and centerline sensor's ability to detect incipient unstart, with the outboard sensor detecting the separation-induced pressure rise well ahead of centerline measurements. A modified shock train length relation for rectangular isolators accounts for the effects of potential boundary-layer momentum thickness asymmetry, upstream corner flow separation length scale, and AR magnitude. Improvements in correlation strength are discussed for both ARs, with R-2 coefficient of determination and root mean square error improvements on the order of 60% observed for the highest AR tested.