Abstract
Detailed knowledge of jet plume development in the near-field (the first 10–15 nozzle exit diameters for a round jet) is important in aero-engine propulsion system design, e.g., for jet noise and plume infrared (IR) signature assessment. Nozzle exit Mach numbers are often high subsonic but improperly expanded (e.g., shock-containing) plumes also occur; high Reynolds numbers (O (106)) are typical. The near-field is obviously influenced by nozzle exit conditions (velocity/turbulence profiles) so knowledge of exit boundary layer characteristics is desirable. Therefore, an experimental study was carried out to provide detailed data on nozzle inlet and exit conditions and near-field development for convergent round nozzles operated at Nozzle Pressure Ratios (NPRs) corresponding to high subsonic and supersonic (underexpanded) jet plumes. Both pneumatic probe and Laser Doppler Anemometry (LDA) measurements were made. The data revealed that internal nozzle acceleration led to a dramatic reduction in wall boundary layer thickness and a more laminar-like profile shape. The addition of a parallel wall extension to the end of the nozzle allowed the boundary layer to return to a turbulent state, increasing its thickness, and removing vena contracta effects. Differences in nozzle exit boundary layers exerted a noticeable influence but only in the first few diameters of plume development. The addition of the exit extension removed the vena contracta effects of the convergence only design. At underexpanded NPRs, this change to nozzle geometry modified the shock cell pattern and shortened the potential core length of the jet.
Highlights
Aero-engine manufacturers require a detailed understanding of propulsion jet flow characteristics for both civil and military applications, driven by design objectives of low jet noise and a reduced infrared (IR) signature
Lepicovsky et al [15]), and to provide specific inlet conditions for Computational Fluid Dynamics (CFD) prediction of the combined nozzle/plume flow, pneumatic probe measurements captured the boundary layer characteristics over the full range of Nozzle Pressure Ratios (NPRs) tested (1.3–2.4) and both nozzle sizes (Note—it was not expected that the addition of the exit extension would influence inlet profiles so data was collected for the ‘clean’ nozzles only)
Reθi showed a monotonic increase with NPR with all values indicating a fully turbulent boundary layer
Summary
Aero-engine manufacturers require a detailed understanding of propulsion jet flow characteristics for both civil and military applications, driven by design objectives of low jet noise (civil) and a reduced infrared (IR) signature (military). In this context, it is jet/ambient near-field mixing which is primarily of interest (i.e., approximately the first 10–15 nozzle exit diameters) rather than the far-field, where jet development is observed to obey self-similarity laws. It is jet/ambient near-field mixing which is primarily of interest (i.e., approximately the first 10–15 nozzle exit diameters) rather than the far-field, where jet development is observed to obey self-similarity laws This increases the technical challenge considerably because developing near-field turbulence is more complex. Near-field jet aerodynamic and aeroacoustic characteristics depend strongly on the near-field turbulent structures which control temperature reduction in hot jets and are the primary source of broadband noise in subsonic jets
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