Mass Flow Performance for Large Aspect Ratio Supersonic Boundary Layer Bleed Holes

Accurate estimation of the bleed orifice flow coefficient, which relates bleed plenum pressure to mass flow removed, is important to predicting inlet performance, as well as, estimating bleed drag. Much of the flow coefficient data at conditions of interest to inlet designers is based on bleed plates with multiple rows of holes. The flow coefficient for these plates is typically presented as a function of bleed plenum pressure normalized by the freestream total pressure. Numerical simulations of the flowfield at the entrance of the bleed hole show that the flow is complex, especially for supersonic free stream flow, whereby an alternating expansion/compression wave pattern initiates at the porous bleed surface as the flow turns to enter the hole. This implies that a significant portion of the tangential flow total pressure is given up upon entering a 90° hole. For large aspect ratio (length-to-diameter ratio) bleed holes the effect of the frictional pressure drop is to lower the required plenum pressure to achieve a given mass flow. Conversely, the mass flow will be reduced due to the higher pressure at the start of the duct. Empirical data show that the flow coefficient for supersonic boundary layer bleed holes stops increasing as the plenum pressure to total pressure ratio continues to decrease, indicating that the flow becomes choked. Thus the chocked flow condition helps to make the bleed hole mass flow under these conditions less sensitive or insensitive to the effects of friction caused by the extended hole length. The extent to which this happens is the focus of the current effort, with the paper reporting on experimental and numerical results on flow characteristics and mass flow performance of supersonic bleed holes featuring a range of aspect ratios beyond what has been reported in the past.