Bleed Lip Geometry Effects on the Flow in a Hypersonic Wind Tunnel

L AMINAR-turbulent transition in boundary layers is important for the prediction and control of skin friction, heat transfer, and other boundary layer properties. Therefore, it is important to have reliable capabilities in predicting boundary layer transition to optimize hypersonic vehicle performance [1]. The mechanisms leading to transition are poorly understood [2]. Transition experiments have been carried out in conventional ground testing facilities for many decades. However, most of the experimental data obtained from these facilities are not reliable because they have much higher disturbance levels compared with actual flight conditions [3]. Quiet-flow wind tunnels are intended to replicate the low noise conditions of actual flight at hypersonic speed. Reaching quiet flow requires the maintenance of a laminar boundary layer on the nozzle wall to avoid acoustic fluctuations generated by boundary layer turbulence. One method to reduce noise is to delay boundary layer transition using a bleed slot before the nozzle throat. The Boeing/Air Force Office of Scientific Research (AFOSR)Mach 6 wind tunnel at Purdue University has been designed as a quiet tunnel with a bleed slot for which the noise level is an order of magnitude lower than that in conventional wind tunnels. It is a Ludwieg tube that is a long pipe having a converging-diverging nozzle followed by a test section as shown in Fig. 1. A close-up view of the tunnel geometry around the bleed slot lip is shown in Fig. 2. However, the tunnel (which has been operational since 2001) is not yet quiet for the desired range of stagnation pressures of up to 150 psi. Two different nozzles have been fabricated and tested. The tunnel is quiet up to a stagnation pressure of 8 psi with the original electroformed nozzle. The original design of the outer surface of the bleed slot has been modified, and eight different bleed slot designs together with the original one have been tested [4]. A second nozzle throat has been fabricated from aluminum [5]. The tests on the tunnel with this aluminum surrogate throat show that the tunnel is quiet up to a stagnation pressure of 93 psi. Early transition of the nozzle wall boundary layer has been identified as the cause of the test section noise for the tunnel at Purdue University. Separation bubbles on the bleed lip and associated fluctuations induced near the bleed lip were identified as the most likely cause of early transition [4]. The experimental study of Klebanoff and Tidstrom [6] showed that the presence of a separation bubble of sufficient size destabilizes the laminar boundary layer downstream of reattachment thereby leading to an earlier transition to turbulence, i.e., the location of transition moves upstream relative to where it would occur without the separation bubble. This hypothesis regarding a separation bubble was supported by the measurements showing an increase in quiet-flow stagnation pressure from 8 to 93 psia when the electroformed nozzle throat was replaced with the aluminum throat [5]. The bleed lip of the electroformed throat has a 0.001 in. kink that is not present in the aluminum throat, and it appears that the kink in the electroformed throat exacerbates a natural tendency to form a separation bubble near the lip. This separation bubble is highly unsteady and can lead to early transition downstream [7]. However, the separation bubble apparently still exists even at 93 psia, according to the computations presented herein. To achieve quiet flow above 93 psia, and to make the quiet flow less sensitive to the exact shape of the bleed lip, it is desirable to eliminate the separation bubble completely. The situation in the hypersonicwind tunnel at PurdueUniversity is an example illustrating the importance of the bleed lip geometry and the effects of separation bubbles that form around the bleed lip on the quality of the flow at the test section. The objective of this study is to demonstrate the effect of separation bubbles on flow structure by numerically investigating the existence of steady and unsteady separation bubbles on the main-flow or the bleed-flow side of the nozzle lip of the Boeing/AFOSR Mach 6 wind tunnel at Purdue University, and to design a new geometry to eliminate or reduce the size of the separation bubbles.