LARGE-EDDY SIMULATION OF COMPRESSIBLE TURBULENT BOUNDARY LAYERS WITH HEAT ADDITION

Denman, Andrew Willem (2007). LARGE-EDDY SIMULATION OF COMPRESSIBLE TURBULENT BOUNDARY LAYERS WITH HEAT ADDITION PhD Thesis, School of Engineering , University of Queensland.

       
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Author Denman, Andrew Willem
Thesis Title LARGE-EDDY SIMULATION OF COMPRESSIBLE TURBULENT BOUNDARY LAYERS WITH HEAT ADDITION
School, Centre or Institute School of Engineering
Institution University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor Dr Peter Jacobs
Abstract/Summary Turbulent wall shear stress has a significant impact on high speed vehicle performance. Its large magnitude, relative to the thrust that can be produced by propulsion systems such as Scramjets, means that it is worthwhile to attempt to control the wall shear stress levels. Experimental and numerical investigations have revealed that combustion of hydrogen in the boundary layer provides a means for such control. When combustion occurs, changes in mean flow property profiles across the boundary layer have already been identified. These changes in mean flow properties have been used to hypothesize about changes occurring to the turbulent mechanisms of momentum transfer. The purpose of this thesis is to examine the changes that occur to the turbulent momentum transfer mechanisms for boundary layers that experience a wall shear stress reduction through chemical heat evolution from hydrogen combustion. The comparisons are made using large eddy simulation (LES) to simulate the turbulent transport processes within a fully developed turbulent boundary layer. With the advent of dynamic subgrid scale LESmodels that require no a priori specification of model coefficients, LES presents itself as a high fidelity means of studying turbulent processes without the computational expense of direct numerical simulation (DNS). Comparisons were made between two types of subgrid scale models, the eddy-viscosity/eddy-diffusivity model and the approximate deconvolution model (ADM). While the differences were minimal in most instances, the dynamically adjusting ADM procedure yielded better agreement with DNS and experimental datasets. The ADM subgrid scale model was able to clearly demonstrate the presence of the turbulent coherent structures found within boundary layers. The evolution of heat from the combustion of hydrogen within the boundary layer is included through two methods. Firstly, finite rate chemical kinetics are used to model the combustion reactions of hydrogen pre-mixed with the boundary layer flow. Secondly, the finite rate chemical kinetics approach is used to develop an energy source term to represent the heat evolution without the additional computational overheads of advancing the chemical reactions through time. The results from the numerical simulations with and without heat addition to turbulent boundary layers are presented and both methods of heat addition indicated reductions in wall shear stress. The analysis of the mechanisms of wall shear stress reduction used mean flow profiles, velocity fluctuation statistics, two-point spatial correlations, energy spectra, Reynolds stress transport budgets and instantaneous flow field visualizations. Changes to the processes of turbulent momentum transport were observed. The contribution of the Reynolds shear stress (pu'w') to the transport of momentum across the boundary layer was computed to be reduced by 40% when compared to the levels computed with no heat addition. Investigation of the Reynolds shear stress without density scaling (u'w') revealed that not all of the reduction is tied up in the reduced mean density profile. The observation of changes to the turbulent velocity field was confirmed by lower levels of the RMS of wall normal velocity fluctuations. An explanation for the reduced wall normal velocity fluctuation levels was provided by the heat addition process reducing the pressure-strain velocity fluctuation redistribution from the streamwise to wall normal direction. This redistribution process is the dominant mechanism responsible for the supply of the wall normal velocity fluctuations and hence momentum transport to the wall. The turbulent structures were observed to be altered significantly with the near wall streaks increasing their coherence length in the streamwise and spanwise directions. The increasing streamwise coherence revealed a possible mechanism of wall shear stress reduction with the streak termination event, near wall ejections, demonstrating a significantly reduced contribution to the Reynolds shear stress which it normally dominates. This work has shown that the changes that occur within the turbulent boundary layer can be captured using the LES technique. Additionaly, these changes that occur during the combustion process can be represented efficiently using a heat source approach. The hypothesis that the addition of heat to the boundary layer would not only induce a reduction in wall shear stress through mean property changes but also alter the turbulent coherent structures has been demonstrated.

 
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Created: Fri, 21 Nov 2008, 21:08:56 EST