Abstract
The surface heat flux on a 100 mm diameter hypersonic sphere was reduced through surface roughness on its forebody. The test model was subjected to a hypersonic freestream of Mach 8.8 and Reynolds number 1.98 million/m, in a shock tunnel. Forebody surface heat transfer rates measured on smooth and rough spheres, under the same free-stream conditions, were compared. The comparison of heat flux indicated an overall reduction in surface heating rates on the rough model, which could be attributed to the delayed nose tip transition. The surface roughness on the forebody of the model generated miniature cavities. Stability of the free shear layer over the miniature cavities and entrapment of the destabilizing vortices in the cavities, make the flow over the rough test model more stable than the attached boundary layer over the smooth model, under transitional conditions.
Highlights
Hypersonic reentry vehicles have large forebody bluntness to shield from high aerodynamic heating
The comparison of heat flux indicated an overall reduction in surface heating rates on the rough model, which could be attributed to the delayed nose tip transition
A considerable reduction in heat flux was observed in zone-2, on a rough surface, where the trend of distribution was leaning towards laminar
Summary
Hypersonic reentry vehicles have large forebody bluntness to shield from high aerodynamic heating. The large bluntness at hypersonic Mach numbers generates strong bow shock waves due to which the shock layers will have strong entropy layers, which are a region of strong vorticity [1]. Strong entropy layers destabilize the boundary layers, making them susceptible to even small perturbations in the free stream [2]. A large nose bluntness, which is required for thermal survival of reentry capsules, was found counterproductive as far as transition control was concerned, which was labelled “blunt nose paradox” in literature [1]. The large nose bluntness promoted transition prematurely, despite having a strong favourable pressure gradient around the nose region
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