The Entropy Wave Generator (EWG): A reference case on entropy noise

The entropy noise mechanism was investigated both experimentally and numerically on a generic test case. The model experiment features electrical heating to generate non-isentropic perturbations in a spatially varying average flow field. This flow field is set by the geometrical boundary conditions of an axisymmetric convergent–divergent nozzle in an otherwise straight tube. The considered flow conditions range from low subsonic to a transonic chocked base flow. The general response of the system to an abrupt heating pulse is studied experimentally and numerically. Furthermore, for two specific cases a detailed investigation of the pressure response in the outlet section is provided. Comprehensive experimental data are provided for the validation of numerical methods with respect to entropy noise. The numerical investigations use a commercial available computational fluid dynamics (CFD) method with partially and non-reflective boundary conditions for unsteady compressible simulations on one hand and a high-order CAA method with a time-domain impedance model on the other hand. It is found that the determination of reflections from the downstream and/or upstream open ends of the test configuration are necessary for the correct prediction of the experiment. The results of both methods are analyzed for the presence of acoustic sources considering the source term of an acoustic analogy and the acoustic intensity, respectively. Strong sources are found in the convergent/divergent nozzle by both methods. These sources show a much larger source strength than the direct sources due to the unsteady heat input. A saturation of the peak pressure response with a increasing Mach number in the nozzle throat above 0.8 is attributed to a phase shift of the source contributions between nozzle and diffuser. The presented results enable a deep understanding of the entropy noise phenomenon especially due to the combination of experiments and two fairly different numerical approaches. However, even in spite of the simplified model case the investigated entropy noise mechanism still appears in a comprehensive complexity. Therefore, and because of the increasing relevance as an aero-engine noise source, further research on entropy noise, also under application of the presented reference cases, should be performed.