Gas Injection into Second Mode Instability on a 7 degree Cone at Mach 7

Abstract

The influence of injection gas type on second mode instabilities is researched on a 7 • half-angle cone at Mach 7. The wind tunnel model of 594.5 mm length with a sharp nosetip is fitted with a porous aluminium patch that spans 60 • in azimuth and 25 mm in axial length. Four different gases are injected into the boundary layer flow, namely nitrogen, carbon dioxide, helium and argon. Three different tunnel test conditions with different Re u provide a variety of amplification levels of the second mode instability at the injection location. Frequency data is obtained using PCB sensors and the density boundary layer thickness is inferred from high speed z-type schlieren images. Early analysis of initial data shows a drop in second mode frequency and second mode power behind the injector. If transition is not caused the effect weakens further downstream. Higher blowing ratios caused a stronger decrease in frequency and also reduced the power in the frequency band more. Larger blowing ratios are required at lower Re u to achieve the same effect. The observed boundary layer thickness, as inferred from schlieren images, followed comparable dynamics. A larger blowing ratio of the same coolant gas lead to more thickening, while Helium had a significantly stronger total effect than carbon dioxide. Preliminary results suggest that helium injection at a moderate rate causes the second mode peak to be disproportionally damped and its bandpower to be significantly reduced. A strongly simplified analytical model suggest that the mechanical increase in boundary layer thickness alone may not be sufficient to explain all observed outcomes.

Aerodynamic Effects and Heat Flux Augmentation of a Transpiration Cooled Hypersonic Sharp Leading Edge

“…Transition onset for the transpiration cooling of cylindrically or spherically blunted bodies is an area of active research. Recent preliminary studies have showed that that mass injection on an axisymmetric nosetip may delay transition with moderate mass fluxes and light gases [34].…”

“…The only recent work on leading edges with distributed outflow have used slots [32], variable permeability [33], or multiple plenums and injectors [34,35]. These studies did not consider leading edge-only injection, and only [34] was carried out in a hypersonic facility, and took heat flux augmentation measurements. The rest were carried out in heated subsonic facilities, or in plasma facilities [36], taking temperature measurements only.…”

Displacement of hypersonic boundary layer instability and turbulence through transpiration cooling

“…1, has been previously employed in the same configuration and injector by Kerth. 41,42 The model has a straight, sharp nose tip with a nose radius smaller than 0:05 mm as given by the manufacturer and is instrumented with high frequency differential pressure sensors (PCB132A31 and PCB132B38) and in-house manufactured thin film gauges (TFG). 43,44 To achieve a smooth surface finish, outer radii of the model were machined with a 0.02 mm tolerance, and individual segments of the model were lathed into the outer conical shape after they were assembled.…”

“…Their method to infer transition onset is based on comparing theoretically obtained laminar heat flux levels to averaged thermocouple data. 14 Work by Kerth et al 34 has dealt with a similar gas injection scenario, wherein transition was inferred from schlieren images and thin film gauges. Hereby, transition took place far away from the injection location.…”

“…Accordingly, schlieren, surface pressure, and heat flux measurements were all in good agreement with respect to boundary layer stability results. 34 It is however assumed that the contrasting injector location in the work presented below-a porous segment within the cone frustum near the location of natural transition-introduces a significantly more complex flowfield. This makes it harder to obtain physical quantities to infer any interpretation about the local stability state of the boundary layer.…”