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A spike attached to a hemispherical body drastically changes its flowfield and influences aerodynamic drag in a hypersonic flow. It is, therefore, a potential candidate for drag reduction of a future high-speed vehicle. The effect of the spike length, shape, spike nose configuration and angle-of-attack on the reduction of the drag is experimentally studied with use of hypersonic wind-tunnel at Mach 6. The effects of geometrical parameters of the spike and angle-of-attack on the aerodynamic coefficient are analysed using schlieren picture and measuring aerodynamic forces. These experiments show that the aerodisk is superior to the aerospike. The aerodisk of appropriate length, diameter and nose configuration may have the capability for the drag reduction. The inclusion of an aero disk at the leading edge of the spike has an advantage for the drag reduction mechanism if it is at an angle-of-attack, however consideration to be given for increased moment resulting from the spike is required.

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The effectiveness of micro jets to control base pressure in suddenly expanded axi-symmetric ducts has been presented in this paper. Four micro jets of 1 mm orifice diameter located at 90° intervals along a pitch circle diameter of 1.3 times the nozzle exit diameter in the base region were employed as active controls. The Mach numbers of the suddenly expanded flows were 1.25, 1.30. 1.48. 1.6. 1.8 and 2.0. The jets were expanded suddenly into an axi-symmetric tube with cross-sectional area 2.56, 3.24. 4.84, and 6.25 times that of nozzle exit area. The length-to-diameter ratio of the sudden expansion tube was varied from 10 to 1. The jets at all Mach numbers were correctly expanded. It is found that the micro jets can serve as active controllers for base pressure. Also, the wall pressure distribution is not adversely influenced by the micro jets.

The results of an experimental investigation carried out to control the base pressure in a suddenly expanded axisymmetric passage are presented in this paper. Active control in the form of micro-jets was employed to control the base pressure. Air injection at four locations at the base, symmetric to the nozzle axis, was used as the active control. The jet Mach numbers at the entry to the suddenly expanded duct studied were 1.87, 2.2 and 2.58. The length-to-diameter ratio of the suddenly expanded duct was varied from 10 to 1. Nozzles generating the above jet Mach numbers were operated with nozzle pressure ratio (NPR) in the range 3 to 11. In addition to base pressure, wall pressure field along the enlarged duct length was also studied. It is found that the active control in the form of blowing through small orifices (micro-jets) are effective in controlling the base pressure field. An increase as high as 95 per cent in base pressure was achieved for certain combinations of parameters of the present study. From the present investigation it is evident that for a given Mach number and NPR one can identify the L/D which will result in maximum increase/decrease of base pressure.