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Papers by Carlos JUNQUEIRA JUNIOR

Journal of the Brazilian Society of Mechanical Sciences and Engineering, 2019

Acoustics loads are rocket design constraints which push researches and engineers to invest effor... more Acoustics loads are rocket design constraints which push researches and engineers to invest efforts in the aeroacoustics phenomena which is present on launch vehicles. Therefore, an in-house computational fluid dynamics tool is developed in order to reproduce high-fidelity results of supersonic jet flows for aeroacoustic analogy applications. The solver is written using the large eddy simulation formulation that is discretized using a finite-difference approach and an explicit time integration. Numerical simulations of supersonic jet flows are very expensive and demand efficient high-performance computing. Therefore, non-blocking message passage interface protocols and parallel input/output features are implemented into the code in order to perform simulations which demand up to one billion degrees of freedom. The present work evaluates the parallel efficiency of the solver when running on a supercomputer with a maximum theoretical peak of 127.4 TFLOPS. Speedup curves are generated using nine different workloads. Moreover, the validation results of a realistic flow condition are also presented in the current work.

The present work is concerned with a study of large eddy simulations (LES) of unsteady turbulent ... more The present work is concerned with a study of large eddy simulations (LES) of unsteady turbulent jet flows. In particular, the present analysis is focused on the effects of the subgrid-scale modeling used when a second-order spatial discretization methodology is employed for the numerical simulations. The present effort addresses perfectly expanded supersonic jets, because the authors want to emphasize the effects of the jet mixing phenomena. The LES formulation is discretized using the finite difference approach, after the equations are rewritten in a generalized coordinate system. Both space and time discretizations are second-order accurate and an explicit time march is adopted. Special care is dedicated to the discretization of the energy equation to appropriately model the set of filtered equations appearing in the LES formulation. The classical Smagorinsky, the dynamic Smagorinsky and the Vreman models are the subgrid-scale closures selected for the present work. The computational results are compared to data in the literature to validate the present simulation tool. Results indicate that the characteristics of numerical discretization can be as important as the effects of the subgrid-scale models for such low-order spatial discretization schemes. A detailed analysis is presented for the performance of each subgrid closure in the numerical context here considered.

The work is a study of conservation on linearization techniques of time-marching schemes for the ... more The work is a study of conservation on linearization techniques of time-marching schemes for the unstructured finite volume Reynolds-averaged Navier-Stokes formulation. The solver used in this work calculates the numerical flux applying an upwind discretization based on the flux vector splitting scheme. This numerical treatment results in a very large sparse linear system. The direct solution of this full implicit linear system is very expensive and, in most cases, impractical. There are several numerical approaches which are commonly used by the scientific community to treat sparse linear systems, and the point-implicit integration was selected in the present case. However, numerical approaches to solve implicit linear systems can be non-conservative in time, even for formulations which are conservative by construction, as the finite volume techniques. Moreover, there are physical problems which strongly demand conservative schemes in order to achieve the correct numerical solution. The work presents results of numerical simulations to evaluate the conservation of implicit and explicit time-marching methods and discusses numerical requirements that can help avoiding such non-conservation issues.

The present work deals with the calculation of wall heat fluxes over the small ballistic reentry ... more The present work deals with the calculation of wall heat fluxes over the small ballistic reentry Brazilian vehicle SARA (acronym for Satélite de Reentrada Atmosférica). The results of the present investigation will be used in the future to size the thermal protection material that will have to be added to the SARA configuration in order to guarantee the integrity of the vehicle during its flight. Experimental data such as altitude and velocity of the vehicle were calculated at different positions of the launch trajectory at Instituto de Aeronáutica e Espaço (IAE). Numerical simulations of the compressible flow over the ballistic vehicle are performed in order to correctly compute the heat transfer at the wall. The flight trajectory data are used as boundary conditions for the numerical simulations. During the portion of the trajectory of interest here, the Mach number ranges from 0.5 to 7.8, and the altitude from 0.5 km to 56 km. For all simulations performed in the present work, the dimensionless distance of the wall of the first mesh cell is less than one. Therefore, one mesh was created for each position along the launch trajectory, because the mesh is dependent on the Reynolds number. The numerical results are used as a reference to validade engineering calculations of the SARA heat transfer rates. At this point, the plans for the sub-orbital SARA capsule flight are such that it will fly without any wind tunnel validation of the heat fluxes here determined.

Este artigo tem como objetivo apresentar a necessidade de computação de alto desempenho (HPC) par... more Este artigo tem como objetivo apresentar a necessidade de computação de alto desempenho (HPC) para a Mecânica dos Fluidos Computacional. O autor mostra curvas de Speed-Up obtidas para o solver de Navier-Stokes utilizado no Instituto de Aeronáutica e Espaço (IAE) chamado de Le-MANS.

The present work is primarily concerned with studying the effects of artificial dissipation and o... more The present work is primarily concerned with studying the effects of artificial dissipation and of certain diffusive terms in the turbulence model formulation on the capability of representing turbulent boundary layer flows. The flows of interest in the present case are assumed to be adequately represented by the compressible Reynolds-averaged Navier-Stokes equations, and the Spalart-Allmaras eddy viscosity model is used for turbulence closure. The equations are discretized in the context of a general purpose, density-based, unstructured grid finite volume method. Spatial discretization is based on the Steger-Warming flux vector splitting scheme and temporal discretization uses a backward Euler point-implicit integration. The work discusses in detail the theoretical and numerical formulations of the selected model. The computational studies consider the turbulent flow over a flat plate at 0.3 freestream Mach number. The paper demonstrates that the excessive artificial dissipation automatically generated by the original spatial discretization scheme can deteriorate boundary layer predictions. Moreover, the results also show that the inclusion of Spalart-Allmaras model cross-diffusion terms is primarily important in the viscous sublayer region of the boundary
layer. Finally, the paper also demonstrates how the spatial discretization scheme can be selectively modified to correctly control the artificial dissipation such that the flow simulation tool remains robust for high-speed applications at the same time that it can accurately compute turbulent boundary layers.

The work is an study of conservation on linearization techniques of time-marching schemes for uns... more The work is an study of conservation on linearization techniques of time-marching schemes for unstructured finite volume Reynolds-averaged Navier-Stokes formulation. The solver used in this work calculates the numerical flux applying an upwind discretization based on the flux vector splitting scheme. This numerical treatment results in a very large sparse linear system. The direct solution of this full implicit linear system is very expensive and, in most cases, impractical. There are several numerical approaches which are commonly used by the scientific community to treat sparse linear systems, and the point-implicit integration was selected in the present case. However, numerical approaches to solve implicit linear systems can be non-conservative in time, even for formulations which are conservative by construction, as the finite volume techniques. Moreover, there are physical problems which strongly demand conservative schemes in order to achieve the correct numerical solution. The work presents results of numerical simulations to evaluate the conservation of implicit and explicit time-marching methods and discusses numerical requirements that can help avoiding such non-conservation issues.

Journal of the Brazilian Society of Mechanical Sciences and Engineering, 2019

Acoustics loads are rocket design constraints which push researches and engineers to invest effor... more Acoustics loads are rocket design constraints which push researches and engineers to invest efforts in the aeroacoustics phenomena which is present on launch vehicles. Therefore, an in-house computational fluid dynamics tool is developed in order to reproduce high-fidelity results of supersonic jet flows for aeroacoustic analogy applications. The solver is written using the large eddy simulation formulation that is discretized using a finite-difference approach and an explicit time integration. Numerical simulations of supersonic jet flows are very expensive and demand efficient high-performance computing. Therefore, non-blocking message passage interface protocols and parallel input/output features are implemented into the code in order to perform simulations which demand up to one billion degrees of freedom. The present work evaluates the parallel efficiency of the solver when running on a supercomputer with a maximum theoretical peak of 127.4 TFLOPS. Speedup curves are generated using nine different workloads. Moreover, the validation results of a realistic flow condition are also presented in the current work.

The present work is concerned with a study of large eddy simulations (LES) of unsteady turbulent ... more The present work is concerned with a study of large eddy simulations (LES) of unsteady turbulent jet flows. In particular, the present analysis is focused on the effects of the subgrid-scale modeling used when a second-order spatial discretization methodology is employed for the numerical simulations. The present effort addresses perfectly expanded supersonic jets, because the authors want to emphasize the effects of the jet mixing phenomena. The LES formulation is discretized using the finite difference approach, after the equations are rewritten in a generalized coordinate system. Both space and time discretizations are second-order accurate and an explicit time march is adopted. Special care is dedicated to the discretization of the energy equation to appropriately model the set of filtered equations appearing in the LES formulation. The classical Smagorinsky, the dynamic Smagorinsky and the Vreman models are the subgrid-scale closures selected for the present work. The computational results are compared to data in the literature to validate the present simulation tool. Results indicate that the characteristics of numerical discretization can be as important as the effects of the subgrid-scale models for such low-order spatial discretization schemes. A detailed analysis is presented for the performance of each subgrid closure in the numerical context here considered.

The work is a study of conservation on linearization techniques of time-marching schemes for the ... more The work is a study of conservation on linearization techniques of time-marching schemes for the unstructured finite volume Reynolds-averaged Navier-Stokes formulation. The solver used in this work calculates the numerical flux applying an upwind discretization based on the flux vector splitting scheme. This numerical treatment results in a very large sparse linear system. The direct solution of this full implicit linear system is very expensive and, in most cases, impractical. There are several numerical approaches which are commonly used by the scientific community to treat sparse linear systems, and the point-implicit integration was selected in the present case. However, numerical approaches to solve implicit linear systems can be non-conservative in time, even for formulations which are conservative by construction, as the finite volume techniques. Moreover, there are physical problems which strongly demand conservative schemes in order to achieve the correct numerical solution. The work presents results of numerical simulations to evaluate the conservation of implicit and explicit time-marching methods and discusses numerical requirements that can help avoiding such non-conservation issues.

The present work deals with the calculation of wall heat fluxes over the small ballistic reentry ... more The present work deals with the calculation of wall heat fluxes over the small ballistic reentry Brazilian vehicle SARA (acronym for Satélite de Reentrada Atmosférica). The results of the present investigation will be used in the future to size the thermal protection material that will have to be added to the SARA configuration in order to guarantee the integrity of the vehicle during its flight. Experimental data such as altitude and velocity of the vehicle were calculated at different positions of the launch trajectory at Instituto de Aeronáutica e Espaço (IAE). Numerical simulations of the compressible flow over the ballistic vehicle are performed in order to correctly compute the heat transfer at the wall. The flight trajectory data are used as boundary conditions for the numerical simulations. During the portion of the trajectory of interest here, the Mach number ranges from 0.5 to 7.8, and the altitude from 0.5 km to 56 km. For all simulations performed in the present work, the dimensionless distance of the wall of the first mesh cell is less than one. Therefore, one mesh was created for each position along the launch trajectory, because the mesh is dependent on the Reynolds number. The numerical results are used as a reference to validade engineering calculations of the SARA heat transfer rates. At this point, the plans for the sub-orbital SARA capsule flight are such that it will fly without any wind tunnel validation of the heat fluxes here determined.

Este artigo tem como objetivo apresentar a necessidade de computação de alto desempenho (HPC) par... more Este artigo tem como objetivo apresentar a necessidade de computação de alto desempenho (HPC) para a Mecânica dos Fluidos Computacional. O autor mostra curvas de Speed-Up obtidas para o solver de Navier-Stokes utilizado no Instituto de Aeronáutica e Espaço (IAE) chamado de Le-MANS.

The present work is primarily concerned with studying the effects of artificial dissipation and o... more The present work is primarily concerned with studying the effects of artificial dissipation and of certain diffusive terms in the turbulence model formulation on the capability of representing turbulent boundary layer flows. The flows of interest in the present case are assumed to be adequately represented by the compressible Reynolds-averaged Navier-Stokes equations, and the Spalart-Allmaras eddy viscosity model is used for turbulence closure. The equations are discretized in the context of a general purpose, density-based, unstructured grid finite volume method. Spatial discretization is based on the Steger-Warming flux vector splitting scheme and temporal discretization uses a backward Euler point-implicit integration. The work discusses in detail the theoretical and numerical formulations of the selected model. The computational studies consider the turbulent flow over a flat plate at 0.3 freestream Mach number. The paper demonstrates that the excessive artificial dissipation automatically generated by the original spatial discretization scheme can deteriorate boundary layer predictions. Moreover, the results also show that the inclusion of Spalart-Allmaras model cross-diffusion terms is primarily important in the viscous sublayer region of the boundary
layer. Finally, the paper also demonstrates how the spatial discretization scheme can be selectively modified to correctly control the artificial dissipation such that the flow simulation tool remains robust for high-speed applications at the same time that it can accurately compute turbulent boundary layers.

The work is an study of conservation on linearization techniques of time-marching schemes for uns... more The work is an study of conservation on linearization techniques of time-marching schemes for unstructured finite volume Reynolds-averaged Navier-Stokes formulation. The solver used in this work calculates the numerical flux applying an upwind discretization based on the flux vector splitting scheme. This numerical treatment results in a very large sparse linear system. The direct solution of this full implicit linear system is very expensive and, in most cases, impractical. There are several numerical approaches which are commonly used by the scientific community to treat sparse linear systems, and the point-implicit integration was selected in the present case. However, numerical approaches to solve implicit linear systems can be non-conservative in time, even for formulations which are conservative by construction, as the finite volume techniques. Moreover, there are physical problems which strongly demand conservative schemes in order to achieve the correct numerical solution. The work presents results of numerical simulations to evaluate the conservation of implicit and explicit time-marching methods and discusses numerical requirements that can help avoiding such non-conservation issues.