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Detached-Eddy Simulation of Shock Wave/Boundary-Layer Interactions in a Planar Transonic Nozzle

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.

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Chapter 1 Introduction This study aims at better understanding a benchmark calculation on a transonic nozzle analysed both experimentally and numerically in a wide variety of papers available in literature, dating from theseventiesuptonowadays. Thefinaltargetofallthedifferentapproacheshasbeentocharacterize the low-frequency unsteadiness of SWBLI that takes place in the duct. A solid confidence with numerical methodologies capable of capturing such unsteady phenomena in both qualitative and quantitative ways is fundamental so as to have better knowledge in predicting possibly dangerous situations, e.g. the catastrophic generation of dynamic side loads that limit considerably the design of the expansion nozzles of liquid rocket engines, and so their performances. In particular, separation flows induced by shocks and subsequent reattachment are associated to many different kinds of flows, ranging from transonic airfoils, supersonic inlet, missile base flows to, for the final purpose of the case in analysis, overexpanded nozzles. These complex interactions of boundary layers with compression or expansion waves lead to flows that exhibit a low-frequency unsteadiness which still needs a univoque explanation, but that can cause buffeting, instability, thermal loadings, aerostructure fatigue with the coupling of the pressure oscillations with structures resonant frequencies. The fundamental problem is that a true knowledge of the physics of supersonic flows with a plethora of phenomena like shock reflection at wall, shock/shock and shock/boundary-layers interaction, with a deep understanding of the source or sources of low-frequency unsteadiness is still required. A possible approach is developing improved computational capabilities, able to investigate complex configurationsincluding3Dflows,andfosteringnon-standardsignalanalysisinordertoextractmore information about the mechanism at the origin of the shock movement joined with the shedding of vortical structures. Talking about overexpanded nozzles, the experimental analysis of separated flows for full-scale rocket nozzles is difficult and expensive, given the fact that the measurements of interest are relative to just few seconds or less. However, an accurate estimate of this very short transient period is crucial for the nozzle safe life since fluctuating pressure loads, consequence of shock wave trains or interactions oscillating in time, are so severe that the operation of the engine and launch vehicle are endangered irreparably (a series of practical examples of the consequences of uncontrolled and usteady off-axis forces is available in literature). 1

Tesi di Laurea Magistrale

Facoltà: Ingegneria

Autore: Giacomo Della Posta Contatta »

Composta da 179 pagine.


Questa tesi ha raggiunto 51 click dal 07/02/2018.

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