In the investigation described, the base pressure resulting from the abrupt expansion of an air jet from a circular nozzle into a concentric cylindrical duct or shroud has been measured. Stagnation pressure ratios of the forcing jet to atmospheric of up to six were used with shrouds of various lengths and diameters. As the primary or forcing jet pressure is increased and then decreased, the jet flow attaches and separates from the shroud wall and a hysteresis effect is exhibited by the pressure at the base of the shroud. With an attached flow, the base pressure attains a minimum value which depends mainly on the duct to nozzle area ratio and on the geometry of the nozzle, lower base pressures being obtained with convergent-divergent nozzles. When the jet pressure was increased beyond that required to attain the minimum value of the base pressure, it was observed that the ratio of the forcing jet pressure to base pressure remained constant.Noise measurements of the flow from the nozzle and shroud are prese...

Base Pressure and Noise Produced by the Abrupt Expansion of Air in a Cylindrical Duct

This paper presents the results of heat transfer measurements taken on a two-dimensional supersonic parallel diffuser. The wall static pressure distributions and the corresponding heat transfer coefficients and fluxes have been measured for a range of initial total pressures. The effects of varying the area of the diffuser cross-section for the same upstream generating nozzle have also been studied. Mach number profiles measured at sections along the diffuser show that in the presence of shock waves and a positive pressure gradient the flow is very much underdeveloped. In general, the mean level of heat transfer is found to be much greater than that predicted by conventional empirical equations for subsonic pipe flows with zero pressure gradient. Further, on comparison between normal and oblique shock diffusion the former is found to give the higher level of heat transfer.

Heat Transfer in a Supersonic Parallel Diffuser

Following the work of Baker and Martin (1), this paper provides further information about static pressure recovery in axi-symmetric supersonic parallel diffusers of fixed length and the same upstream generating nozzle when the diffuser cross-sectional area is varied over a wide range. Correlations based on these and associated experiments by Martin and Baker (2) indicate an area ratio for maximum possible static pressure recovery. At breakdown of the single shock, the diffuser stagnation pressure ratio corresponds to that for normal shock pressure recovery, while the outlet Mach number becomes independent of area ratio as the latter increases. The factors which influence the development and stability of the single shock regime are considered in some detail, from which the role of the boundary layer is shown to be predominant.

Factors Affecting the Performance of the Supersonic Parallel Diffuser

The reattachment of a supersonic jet with a turbulent separating boundary layer abruptly expanding into a two-dimensional parallel diffuser has been experimentally investigated using a surface-flow technique. The reattachment criterion proposed by Mukerjee & Martin (1969) for axisymmetric confined and unconfined flows is found to correlate equally well similar two-dimensional flow measurements in terms of the free-stream Mach number after separation.

A note on turbulent shear-layer reattachment downstream of a backward-facing step in confined supersonic two-dimensional flow

Paper 22: The Performance of a Supersonic Parallel Diffuser:

This paper extends the investigation of the supersonic parallel diffuser previously reported by Martin and Baker (I)‡ and Baker (2) to cover the case of the axi-symmetric cross-section. The flow patterns and the wall static pressure distributions appear broadly to correspond to those which occur in the parallel diffuser of rectangular cross-section. But the axi-symmetric diffuser is found to have a wider operating range (in terms of overall blowing pressure ratio) before the single shock gives way to an oblique shock system, and this permits higher optimum recovery of static pressure. In the axi-symmetric configuration, only very short lengths of duct are necessary to achieve full static pressure recovery in both the single shock and the oblique shock regimes. The use of short diffusers would lessen the cooling problems of large-scale jet engine test facilities, in which exhaust jets must be recompressed at elevated temperatures.

B. W. Martin

Papers

Some Operating Characteristics of the Supersonic Axi-Symmetric Parallel Diffuser