High enthalpy flow separation over bodies at hypersonic speeds continues to intrigue researchers, since considerable differences are observed between experiments and CFD. In the present study the separated flow field around a double cone has been investigated at nominal stagnation enthalpies of 4.2 MJ/kg and 1.6 MJ/kg. The flow around a double cone (first cone semi-apex angle = 25°; second cone semiapex angle=68°) has been visualized using highspeed image converter camera (IMACON) and double exposure holographic interferometry in the Shock Wave Research Center's (SWRC) free piston driven shock tunnel at Mach 6.99. Presence of a triple shock structure in front of the second cone, and non-linear unsteady shock structure oscillation in the flow field, are the significant results from visualization studies. Further surface convective heat transfer measurements have been carried out at a nominal Mach number of 5.75 in the Indian Institute of Science (USc), hypersonic shock tunnel HST-2. The surface heat transfer in the vicinity of transmitted shock impingement point on the second cone surface fluctuates between 100 W/cm 2-400 W/cm 2 (± 10 %) for nearly identical (± 8 %) free stream conditions, indicating the severe unsteadiness in the flow field. Similar unsteady fluctuations in the heat transfer and oscillatory shock structure in the flow field around the double cone are also observed in the numerical simulations carried out by solving the axi-symmetric Navier-Stokes equations.

The presence of an adverse pressure gradient, shock/shock interaction and shock wave/boundary layer interaction often induces flow separation around bodies. However, the effect of dissociated flow on separated flow characteristics, especially at hypersonic speeds, is still not clear, and considerable differences are observed between experiments and numerical simulations. In this investigation, the unsteady separated flow features around double cones are visualized in the Shock Wave Research Center (SWRC) free-piston driven shock tunnel at a nominal Mach Number of 6.99 using multiple optical techniques. The time resolved shock structure oscillations in the flow field around double cones (first cone, semi-apex angle = 25q; second cone, semi-apex angles=50q, 65q, 68q and 70q) have been visualized using a high-speed image converter camera (IMACON) at a nominal stagnation enthalpy of 4.8 MJ/kg. In addition, flow visualization studies around the double cone is also carried out using Schlieren and double exposure holographic interferometry in order to precisely locate the separation point and measure the separation length. The presence of a triple shock structure in front of the second cone and a non-linear unsteady shock structure oscillation in the flow field are the significant results from visualization studies on the 25q/65q, 25q/68qand 25q/70q double cones. On the other hand, the flow field around 25q/50q is relatively steady and Type V shock/shock interaction is observed. Illustrative numerical simulation studies are carried out by solving N-S equations to complement the experiments. The simulated flow features around a double cone agree well qualitatively with experiments.

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The coating of molten silicate glass on a porous carbon substrate was developed, without the formation of cristobalite at the carbon-glass layer interface, in order to improve the steam oxidation and thermal shock resistance. Initially, suitable conditions for coating were assumed from thermodynamic analysis. Based on these calculations, the wettability of the carbon to molten glass was modified by infiltration and pyrolysis of a Si-N precursor, and the coating with glass was carried out under higher N2 partial pressures. As a result, carbon substrates were completely sealed with glass, without the production of cristobalite at the interface, and the glass was infiltrated into the substrate. In contrast, coating with glass at lower N2 partial pressures, such as in Ar, were followed by the formation of cristobalite along with many pores at the interface. The structural changes occurring as a result of variation of the N2 partial pressure during sealing with glass are in good agreement with the thermodynamic analysis. The glass-coated carbon materials, which were fabricated at higher N2 partial pressure, possessed excellent steam oxidation resistance and thermal shock resistance.

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