Computational Fluid Dynamics Evaluation of Bleed Slot of Purdue Mach 6 Quiet Tunnel

Abstract

Introduction T HE Purdue Mach 6 quiet tunnel is a Ludwieg tube that was designed to study boundary-layer transition at hypersonic speeds.1 For high-quality transition research, the noise level in the test section should be comparable to flight and an order of magnitude lower than in conventional wind tunnels.2 To achieve these low “quiet” noise levels, laminar boundary layers must be maintained on the nozzle walls. NASA Langley Research Center pioneered the development of such quiet tunnels, with features that include highly polished nozzles and a bleed slot to remove the contractionwall boundary layer upstream of the throat.3 However, the tunnel (Fig. 1a), which has been operational since 2001, has achieved quiet flow at high Reynolds numbers only very recently. The probable cause for the lack of quiet flow during 2001–2005 is thought to be fluctuations generated at the nozzle throat as a result of flawed bleed slot design (as in Benay et al.4). Klebanoff and Tidstrom5 showed experimentally that if the Reynolds number is sufficiently small, the presence of a separation bubble does not alter the stability characteristics of the boundary layer downstream of the bubble; however, for sufficiently high Reynolds number, the presence of a separation bubble destabilizes the boundary layer downstream of reattachment, leading to an earlier transition to turbulence. Thus, the original design of the bleed slot has been modified, and altogether eight different bleed slot designs have been built and tested using the highly polished electroformed throat.6 The objective of the study presented in this Note is to simulate the flow in the bleed slot of the Purdue Mach 6 quiet wind tunnel. In the course of this computational study, the bleed slot design