Studies on twin non-parallel unventilated high-speed jets

Abstract

The interaction of twin high-speed jets is an important problem which is most relevant to aircraft engine exhaust systems. Many studies have been carried out on twin parallel high-speed jets, but similar studies for non-parallel configurations are almost absent. Such an investigation would be useful in understanding jet flow fields associated with V/STOL aircraft. The present work is an attempt to investigate experimentally the interaction of two high-speed axisymmetric jets obliquely oriented towards each other. Twin nozzles of exit diameter ( D) 10 mm are set on a common end wall with their axes intersecting each other at half-angles ( α) of 0, 5, 10 and 15°. The centre-to-centre spacings ( S) of the nozzles, non-dimensionalized as S/D, are varied as 3.1, 4 and 5. The nozzles are operated at fully expanded sonic and underexpanded exit conditions to yield fully expanded Mach numbers Mj of 1, 1.19, 1.41 and 1.52. Some of the conclusions arrived at were based on the experimental results and are given below. In the case of sonic jets, the variation of mean velocity along the nozzle block centre-line can be correlated with empirical equations based on the geometric intersection distance and the maximum centre- line velocity. In general, at a particular nozzle spacing and angle of interception, the merging distance decreases with increasing fully expanded Mach numbers. In most of the cases, the complete combining of the twin jets into a single jet occurs downstream of the geometric intersection point, the location of the combining point being strongly influenced by the fully expanded Mach number.