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Detached-Eddy Simulation of Shock Wave/Boundary-Layer Interactions in a Planar Transonic Nozzle
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Giacomo Della Posta, Università degli Studi di Roma La Sapienza, 2016-17
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The aim of this work is the characterization of the shock wave/boundary-layer interactions (SWBLI) in an overexpanded transonic planar nozzle, thanks to Delayed version of Detached-Eddy Simulation (DDES).
Shock wave/boundary-layer interactions are frequent problems in many applications in Aerospace field, as in inlets, supersonic nozzles, rockets during take-off and turbomachinery, and involve complex interactions of turbulent boundary layer with compression and expansion waves with unsteady features.
After a brief prospect of the SWBLI canonical configurations, a wider section is dedicated to the problem of overexpanded nozzles and damaging lateral forces ("side-loads") induced by several mechanisms. The difficulties of the experimental measurements of the unsteadiness exhibited, especially in the transient operational phases, brings the necessity of the validation of new numerical methodologies, starting from simpler cases such as the one studied.
The fundamental equations that steer fluid dynamics are reported, focusing on turbulence, its possible modelation and still open problems dictated by flow compressibility as well.
A survey of the available computational methodologies with their pros and cons focuses the attention on the DES approach, its different versions, its opportunities and especially its deficiencies that researchers have tried to solve with more or less success. Delayed version based on the Spalart-Allmaras model (SA-DDES) has been adopted for the planar transonic nozzle simulation that has been carried out, even if it is not the only possibility, owing to the no longer necessary link between DES and SA model.
Following a view of the details of the computational setup and the numerical solutions adopted, the used code (a finite volume solver for compressible Navier Stokes equations) has been validated through 2D RANS preliminary simulations, providing a sensitivity analysis varying the mesh resolution thanks to which a grid has been chosen and extruded for the 3D simulation.
The mean and instantaneous fields are able to define the flow and its pattern with 2D and 3D visualizations, that show shock oscillations, induced closed separation region and vortex shedding.
The pressure signals on the top wall, widely studied in literature both numerically and experimentally, have been analysed in their statistical and spectral features, estimating variance and fundamental spectra.
Continuous wavelet analysis, together with classical Fourier approach, on the pressure distribution and also on the evaluated shock position over time, allow to describe the modulations of the signals in frequency and their variations in time, characterizing the typical low-frequency unsteadiness of the shock system in SWBLI, also for the considered nozzle case.