Abstract
An experimental study has been undertaken to document compressibility effects in the annular shear layers of axisymmetric jets. Comparison is made of the measured flow development with the well-documented influence of compressibility in planar mixing layers. High Reynolds number (~106) and high Mach number jets issuing from a convergent nozzle at nozzle pressure ratios (NPRs) from 1.28 to 3.0 were measured using laser Doppler anemometry instrumentation. Detailed radial profile data are reported, particularly within the potential core region, for mean velocity, turbulence rms, and turbulence shear stress. For supercritical NPRs the presence of the pressure waves in the inviscid shock cell region as the jet expanded back to ambient pressure was found to exert a noticeable effect on shear layer location, causing this to shift radially outwards at high supercritical NPR conditions. After a boundary layer to free shear layer transition zone, the turbulence development displayed a short region of similarity before adjustment to near-field merged jet behaviour. Peak turbulence rms reduction due to compressibility was similar to that observed in planar layers with radial rms suppression much stronger than axial. Comparison of the compressibility-modified annular shear layer growth rate with planar shear layer data on the basis of the convective Mach number (M C) showed notable differences; in the annular shear layer, compressibility effects began at lower M C and displayed a stronger reduction in growth. For high Mach number aerospace propulsion applications involving round jets, the current measurements represent a new data set for the calibration/validation of compressibility-affected turbulence models.
Highlights
Introduction and backgroundThe primary engineering application motivating the current work is the increased interest over the last 10–15 years in novel methods for control of spreading rate and enhanced mixing of aeroengine exhaust nozzle jet plumes
laser Doppler anemometry (LDA) data were gathered for 10 nozzle pressure ratios (NPRs) values in the range 1.28–3.0 to cover both subcritical and supercritical regimes
This clearly identifies the expected classical behaviour in the potential core zone for subcritical (NPR = 1.45) and just choked (NPR = 1.89) jets, both producing a constant centreline velocity—or nearly so, a small vena contract effect is still observed in spite of the exit parallel extension
Summary
The primary engineering application motivating the current work is the increased interest over the last 10–15 years in novel methods for control of spreading rate and enhanced mixing of aeroengine exhaust nozzle jet plumes. Recent interest in higher cruise performance has extended the range of NPR to supercritical values (Long 2005), introducing the possibility of improperly expanded (overor under-expanded) jets, supersonic Mach numbers and additional shock cell noise. Supercritical and improperly expanded jet plumes are common, with the design objective for shear layer manipulation being enhanced mixing to achieve low observability (infrared signal signature reduction), leading to more aggressive manipulation devices such as tabs protruding into the exhaust stream (Reeder and Samimy 1997; Behrouzi and McGuirk 2009; Feng and McGuirk 2006)
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