Leandro Franco de Souza

Numerical investigation of the three-dimensional secondary instabilities in the time-developing compressible mixing layer

Mixing layers are present in very different types of physical situations such as atmospheric flows, aerodynamics and combustion. It is, therefore, a well researched subject, but there are aspects that require further studies. Here the instability of two-and three-dimensional perturbations in the compressible mixing layer was investigated by numerical simulations. In the numerical code, the derivatives were discretized using high-order compact finite-difference schemes. A stretching in the normal direction was implemented with both the objective of reducing the sound waves generated by the shear region and improving the resolution near the center. The compact schemes were modified to work with non-uniform grids. Numerical tests started with an analysis of the growth rate in the linear regime to verify the code implementation. Tests were also performed in the non-linear regime and it was possible to reproduce the vortex roll-up and pairing, both in two-and three-dimensional situations. Amplification rate analysis was also performed for the secondary instability of this flow. It was found that, for essentially incompressible flow, maximum growth rates occurred for a spanwise wavelength of approximately 2/3 of the streamwise spacing of the vortices. The result demonstrated the applicability of the theory developed by Pierrehumbet and Widnall. Compressibility effects were then considered and the maximum growth rates obtained for relatively high Mach numbers (typically under 0.8) were also presented.

Numerical study of synthetic jet actuator effects in boundary layers

This work has as a fundamental objective the numerical study of the effects of synthetic jet actuators on the flow of the boundary layer developed on a flat plate and on a hypothetical airfoil. The aim is to obtain computational data to indicate how these effects may be used as a means of flow control, describing the dynamics of the synthetic jet in the presence of external flow. The present paper uses a spatial Direct Numerical Simulation (DNS) to solve the incompressible Navier-Stokes equations, written in vorticity-velocity formulation. The spatial derivatives are discretized with a sixth order compact finite difference scheme. The Poisson equation for the normal velocity component is solved by an iterative Line Successive Over Relaxation Method and uses a multigrid Full Approximation Scheme to accelerate the convergence. The results of simulations with different values of frequency, amplitude and slot length were analyzed through a temporal Fourier analysis. Through this analysis the decision as to which are the better parameters to delay the separation of the boundary layer is examined.

An Eulerian Immersed Boundary Method for flow simulations over stationary and moving rigid bodies

The Computational Fluid Dynamics (CFD) research area increases every day. The main reason is the development in the processing and storage capacities of the computers. However, the main studies in this area are restricted to simple geometries. The numerical studies of flow over bodies with complex geometries, stationary or not, require a mesh and numerical code that is able to reproduce the physics of the flow. Normally the type of meshes adopted coincides with the boundaries of the body. An alternative method is the use of approximations where the boundaries of the body do not need to coincide with the computational mesh, allowing the use of a Cartesian grid. The great challenge of these methods is the use of approximations that assure both accuracy and numerical efficiency.

La Influencia de la Altura de la Capa Límite Oceánica en la Región del Centro de Lanzamientos de Alcántara en Brasil

The influence of the height of the ocean boundary layer in the region where the Alcântara Space Center is located, was studied. The topography of the region is very complex and it includes a 50 m coastal cliff. A 2D numerical code was used, where the equations were written using the vorticityvelocity formulation. High order compact finite differences schemes were used for the spatial derivatives and the time integration was performed with a fourth-order Runge-Kutta scheme. The coastal cliff was specified through the immersed boundary method. The numerical results showed that the height of the ocean boundary layer influences the formation of the height of the internal boundary layer of the cliff, increasing the turbulence in the region.

Heat transfer enhancement via Görtler flow with spatial numerical simulation

Purpose The centrifugal instability mechanism of boundary layers over concave surfaces is responsible for the development of quasi-periodic, counter-rotating vortices aligned in a streamwise direction known as Gortler vortices. By distorting the boundary layer structure in both the spanwise and the wall-normal directions, Gortler vortices may modify heat transfer rates. The purpose of this study is to conduct spatial numerical simulation experiments based on a vorticity–velocity formulation of the incompressible Navier–Stokes system of equations to quantify the role of the transition in the heat transfer process. Design/methodology/approach Experiments are conducted using an in-house, parallel, message-passing code. Compact finite difference approximations and a spectral method are used to approximate spatial derivatives. A fourth-order Runge–Kutta method is adopted for time integration. The Poisson equation is solved using a geometric multigrid method. Findings Results show that the numerical method can capture the physics of transitional flows over concave geometries. They also show that the heat transfer rates in the late stages of the transition may be greater than those for either laminar or turbulent ones. Originality/value The numerical method can be considered as a robust alternative to investigate heat transfer properties in transitional boundary layer flows over concave surfaces.

Analysis of the Ocean Boundary Layer influence on the Mobile Tower Integration at the Alcantara Space Center

This work presents a numerical and experimental analysis of the Ocean Boundary Layer (OBL) influence on the formation of the Internal Boundary Layer (IBL) at the Alcantara Space Center (ASC), from where the Brazilian rockets are launched. The numerical simulation was carried out considering a two-dimensional flow with velocity-vorticity formulation for Navier Stokes equations. A 6th order compact finite differences scheme was used for spatial discretization and a 4th order Runge-Kutta scheme with temporal integration. The coastal cliff along the shoreline in the ASC neighborhood was specified trough the immersed boundary technique. The experimental analysis was carried out in a subsonic wind tunnel using a twodimensional Particle Image Velocimetry (PIV) system. The measurements were carried out considering the irregularity of the coastal cliff. The results confirmed that OBL height influences the height of Internal Boundary Layer that reaches the Mobile Integration Tower (MIT).

Investigação da Estabilidade em Escoamento Bidimensional para o Fluido Oldroyd-B

Diversos escoamentos de interesse pratico sao de fluidos nao-Newtonianos e uma classe destes fluidos e classificada como fluidos viscoelasticos. Muitas vezes e desejavel saber se estes escoamentos propagam-se no estado laminar ou no turbulento. Para isto, utilizase a teoria de estabilidade linear (LST) para verificar a estabilidade dos escoamentos a perturbacoes nao estacionarias de fluidos viscoelasticos. O modelo matematico adotado para o fluido nao-Newtoniano do tensor extra-tensao e o Oldroyd-B. Neste trabalho, a teoria de estabilidade linear foi utilizada para investigar a conveccao das ondas de Tollmien-Schlichting no escoamento de Poiseuille para o modelo de fluido Oldroyd-B. A fim de avaliar as curvas neutras de estabilidade, diferentes valores de parâmetros adimensionais sao testados para os escoamentos de fluidos Newtoniano e no-Newtoniano. Os resultados numericos obtidos sao comparados com resultados da literatura.

Estudo da instabilidade secundária de uma bolha de separação laminar tridimensional.

As mudancas topologicas associadas com a tridimensionalidade das bolhas de separacao laminares podem iniciar o processo de instabilidades naquelas bolhas em que o pico de escoamento reverso seja fraco demais para sustentar uma instabilidade absoluta bidimensional. O objetivo e investigar a rota precisa das instabilidades e a existencia de instabilidade secundarias de escoamentos 3D bifurcados. Uma analise da instabilidade secundaria das bolhas de separacao laminar tridimensionais e estacionatias que aparecem como consequencia da bifurcacao do escoamento primaria foi realizada. Uma nova metodologia foi proposta para esta analise e se baseia na analise fracamente nao-paralela para escoamento 3D. Os resultados foram validados com os resultados obtidos pela analise de autovalor 3D. Os resultados obtidos indicam que a tridimensionalidade da bolha de separacao originada pela instabilidade primaria desempenha um papel fundamental na origem da instabilidade autossustentada das bolhas de separacao na ausencia de excitacoes externas.

Método de Interface Imersa de Alta Ordem para o Cálculo de Derivadas de Funções Descontínuas

Os metodos chamados de interface imersa tem tido grande estudo nos ultimos tempos, por se tratar de metodos eficientes para a resolucao de problemas de escoamentos multifasicos ou com descontinuidades nas interfaces. Nesses metodos, as equacoes a serem resolvidas sao discretizadas numa grade cartesiana fixa. Como resultado, a fronteira do dominio nem sempre esta em conformidade com a fronteira do dominio computacional. Isto da origem as descontinuidades no interior do dominio computacional. Na presente investigacao, usamos o metodo de interfaces imersas para resolver derivadas de funcoes descontinuas com salto em algum ponto do dominio. Foram realizadas analises dos resultados obtidos, usando um esquema de discretizacao de diferencas finitas compactas de quarta ordem.

Resolução numérica das equações de Navier-Stokes usando o método das interfaces imersas com correção do salto implícita em malha deslocada

O metodo das interfaces imersas (MII) tem sido utilizado para simular problemas de escoamento de fluidos em geometrias complexas. Uma das dificuldades do MII e calcular os saltos nos valores da funcao e em suas derivadas, pois nem sempre esses valores sao conhecidos analiticamente. Em [4], foi desenvolvido um MII que calcula numericamente os saltos, por meio de um processo iterativo explicito que utiliza os pontos vizinhos da descontinuidade. Seguindo essa ideia, foi desenvolvida em [5] uma versao implicita nao-iterativa desse metodo para resolver uma equacao de Poisson. O presente trabalho apresenta a resolucao das equacoes de NavierStokes, pelo metodo da projecao, usando o MII desenvolvido em [5] em variaveis primitivas e em uma malha deslocada.