Scaling of heat transfer augmentation due to mechanical distortions in hypervelocity boundary layers

Abstract

We examine the response of hypervelocity boundary layers to global mechanical distortions due to concave surface curvature. Surface heat transfer and visual boundary layer thickness data are obtained for a suite of models with different concave surface geometries. Results are compared to predictions using existing approximate methods. Near the leading edge, good agreement is observed, but at larger pressure gradients, predictions diverge significantly from the experimental data. Up to a factor of five underprediction is reported in regions with greatest distortion. Curve fits to the experimental data are compared with surface equations. We demonstrate that reasonable estimates of the laminar heat flux augmentation may be obtained as a function of the local turning angle for all model geometries, even at the conditions of greatest distortion. This scaling may be explained by the application of Lees similarity. As a means of introducing additional local distortions, vortex generators are used to impose streamwise structures into the boundary layer. The response of the large scale vortices to an adverse pressure gradient is investigated. Surface streak evolution is visualized over the different surface geometries using fast response pressure sensitive paint. For a flat plate baseline case, heat transfer augmentation at similar levels to turbulent flow is measured. For the concave geometries, increases in heat transfer by factors up to 2.6 are measured over the laminar values. The scaling of heat transfer with turning angle that is identified for the laminar boundary layer response is found to be robust even in the presence of the imposed vortex structures.

Turbulence structures and statistics of a supersonic turbulent boundary layer subjected to concave surface curvature

“…It can be seen that the density on the concave surface increases in the main stream and in the boundary layer downstream of the turning point, and also that compression waves become evident as curvature radius decreases. As measured in previous studies (Fernando & Smits (2006), Donovan et al (1994), Spina et al (1994), Flaherty & Austin (2013a), Wang et al (2016b)), the boundary layer thickness decreases on a concave wall and on a flat plate with adverse pressure gradient in a supersonic flow. In contrast to this, recent by nano-particle planar laser scattering visualizations of the instantaneous two-dimensional flow Wang & Wang (2016) observed that the concave boundary layer becomes thicker.…”

“…Figure 25(b) shows the inner region flow interaction with adverse pressure gradient, which also represents the boundary layer interaction with the adverse pressure gradient without centrifugal effects. As given in the previous studies (Fernando & Smits 1990;Donovan et al 1994;Flaherty & Austin 2013b;Wang et al 2016a), the boundary layer thickness on the flat plate decreases when subject to adverse pressure gradient. Comparing figure 25(a)-(b), the prominent difference is that the Görtler vortices induce stretching, twisting and distortion of the density gradient surfaces .…”

“…Smith & Smits (1994 did experiments to compare the Reynolds stresses of the curved flow with a flat plate flow which has an equal adverse pressure gradient. Flaherty & Austin (2013a) also demonstrated that heat transfer was also significantly enhanced on concave surfaces. Spina et al (1994) used two parameters to characterize the dilatation/compression and the streamline curvature.…”

“…It has been suggested that the increase in large-scale activity leads to an overall increase of the turbulent intensity (Harun et al 2013). Several experiments have tried to visualize the flow structures of Görtler vortices, including Luca et al (1993) for a Mach 7 flow, Ciolkosz & Spina (2006) at low supersonic Mach numbers varying from 1.06 to 2.87 and Flaherty (2013) at Mach 5.12. Recently, Wang & Wang (2016) studied the response of turbulent structures in a Ma = 2.95 supersonic boundary layer to concave curvature experimentally and found that the large scale vortices formed in the flat plate region break down into smaller ones immediately after being convected into the concave region.…”

The amplification of large-scale motion in a supersonic concave turbulent boundary layer and its impact on the mean and statistical properties

“…Wang et al (2016a) found that the principal strain rate had different streamwise varying trends at different wall-normal locations. Flaherty & Austin (2013) examined the surface heat transfer of a hypersonic concave boundary layer with Mach number up to 7.45. They found significant augmentation in the surface heat transfer over the baseline flat plate.…”