Aristeu da Silveira Neto

Computations of the Flow Past a Still Sphere at Moderate Reynolds Numbers Using an Immersed Boundary Method

This paper presents an immersed boundary formulation for three-dimensional incompressible flows that uses the momentum equation to calculate the Lagrangian force field indirectly imposing the no-slip condition on solid interfaces. In order to test the performance of this methodology the flow past a sphere for Reynolds numbers up to 1,000 have been calculated. Results are compared with numerical data from other authors and empirical correlations available in the literature. The agreement is found to be very good.

Numerical simulation of the flow through a compressor-valve model using an immersed-boundary method

ABSTRACT Hermetic reciprocating compressors are widely used in small- and medium-size refrigeration systems based on the vapor-compression cycle. One of the main parts of this type of compressor is the automatic valve system used to control the suction and discharge processes. As the suction and discharge losses represent a large amount of the total thermodynamic losses (47%), a small improvement in the suction and discharge processes can produce expressive increases in the thermodynamic efficiency of the compressor. In this work, a new numerical methodology is applied to solve the flow through reed-type valves. The numerical results were experimentally validated through the pressure distribution acting on the frontal disk of a radial diffuser, which is a geometry usually used to model this type of valve. The numerical results for the velocity and pressure fields were comprehensively explored during the opening and closing movement imposed to the reed. The good quality of these results show that the numerical methodology is very promising in terms of solving the flow in the actual dynamics of reed-type valves.

Large Eddy Simulations of a Hydrocyclone

Subgrid-scale modeling, which characterizes Large Eddy Simulation (LES), has been used to predict the behavior of a water-fed hydrocyclone operating without an air core. The governing equations were solved by a fractional step method on a staggered grid. The Smagorinsky subgrid-scale model was employed to account for turbulent effects. Numerical results actually capture the main features of the flow pattern and agree reasonably well with experiments, suggesting that LES represents an interesting alternative to classical turbulence models when applied to the numerical solution of fluid flows within hydrocyclones.Copyright

Mathematical and Numerical Modeling of Turbulent Flows

The present work is devoted to the development and implementation of a computational framework to perform numerical simulations of low Mach number turbulent flows over complex geometries. The algorithm under consideration is based on a classical predictor-corrector time integration scheme that employs a projection method for the momentum equations. The domain decomposition strategy is adopted for distributed computing, displaying very satisfactory levels of speed-up and efficiency. The Immersed Boundary Methodology is used to characterize the presence of a complex geometry. Such method demands two separate grids: An Eulerian, where the transport equations are solved with a Finite Volume, second order discretization and a Lagrangian domain, represented by a non-structured shell grid representing the immersed geometry. The in-house code developed was fully verified by the Method of Manufactured Solutions, in both Eulerian and Lagrangian domains. The capabilities of the resulting computational framework are illustrated on four distinct cases: a turbulent jet, the Poiseuille flow, as a matter of validation of the implemented Immersed Boundary methodology, the flow over a sphere covering a wide range of Reynolds numbers, and finally, with the intention of demonstrating the applicability of Large Eddy Simulations - LES - in an industrial problem, the turbulent flow inside an industrial fan.

Numerical simulations of flows over a rotating circular cylinder using the immersed boundary method

In this paper, numerical simulations of incompressible flows around rotating circular cylinders have been performed. The two-dimensional Navier-Stokes equations are solved by using a Cartesian non-uniform grid. The Immersed Boundary Method (IBM) with the Virtual Physical Model (VPM) was used in order to model the presence of the circular cylinder in the flow. The fractional time step method was used to coupling the pressure and velocity fields. The simulations were carried out for Reynolds numbers equals to 60, 100 and 200 for different specific rotations. The effects of rotation on flow characteristics and fluctuating forces were investigated. The Strouhal number, obtained by performing the Fast Fourier Transform (FFT) of the temporal distribution of the lift coefficient, and the pressure coefficients, were also been calculated. Vorticity contours are presented considering different values of the Reynolds number and specific rotation. The numerical results obtained are compared to those obtained by other authors and the usefulness of the numerical methodology composed by the combination of the IBM with the VPM to simulate flows in the presence of mobile bodies is highlighted.

A novel immersed boundary/Fourier pseudospectral method for flows with thermal effects

ABSTRACT A novel immersed boundary method (IBM) for flows with thermal effects is proposed, combining high accuracy and low computational cost, provided by the Fourier pseudospectral method (FPSM), for the possibility of handling complex and nonperiodical geometries using the IBM. With focus on incompressible flow problems modeled by Navier-Stokes, mass, and energy equations, the method of manufactured solutions is used for the numerical verification of Dirichlet boundary conditions imposed via the IBM. Then, the proposed method is applied on two different 2-D cases: (1) energy transfer due to natural convection in a square cavity, and (2) an annulus between horizontal concentric cylinders nonuniformly heated. Good agreement with available data in the literature has been achieved.

Fourier pseudospectral method for nonperiodical problems: A general immersed boundary method for three types of thermal boundary conditions

ABSTRACT In the present paper a general scheme for the three types of thermal boundary conditions is proposed and applied to natural convection and diffusion problems. The numerical algorithm, denominated thermal IMERSPEC, consists of the application of the Fourier pseudospectral method, where Dirichlet, Neumann, or Robin boundary conditions are modeled through immersed boundary method (IBM). The methodology is to impose the boundary conditions on the interface and transmit through distribution functions. Source terms are added to the two-dimensional Navier-Stokes and energy equations on the Cartesian mesh. Manufactured solutions are used for the numerical verification of Dirichlet, Neumann, and Robin boundary conditions, imposed through IBM. The proposed method is applied for solving problems involving thermal energy transfer for natural convection in an annulus between horizontal concentric cylinders. Results for these applications using the thermal IMERSPEC and the traditional finite volume method are compared and a good agreement is obtained for both methodologies.

Modelling and simulation of a compression cylinder-piston system with combustion reaction

In flammable gases, due to the energy stored in its molecular bonds, the construction of systems to transform the chemical energy into mechanical energy was possible. These systems were named motor. With the combustion reaction, a high rate of energy is supplied to the system in the form of heat. Expanding it suddenly and due to the action of an external force against this expansion, the movement becomes oscillatory. Within this context, the present paper contributes to the computational simulation and study of sensitivity of the velocity of the piston, the temperature and the density of the gas in the combustion process, in an integral formulation, given equations for mass, energy, momentum and molar concentration balance. The Arrhenius’ equation, numerical methods for solving differentials equations system and integral formulation using Reynolds Transport Theorem (RTT) are also used. The study is done gradually in order to understand in detail all the variables from ideal conditions. The gas is assumed to be ideal, the cylinder is perfectly sealed and the flow is assumed to be instantaneously homogeneous. The system works as a set containing gas, which is compressed by an external force. Initially, there is no friction and the system is considered adiabatic. So it was possible to understand the variation of the parameters in a model that does not lose energy. Conversely, it is conservative. The final system has friction between the piston and the cylinder wall, change of heat through the wall of the system and combustion reaction, then comparison and analysis were done. The first model response is an oscillatory system with perpetual motion, because there is no dissipation and energy is always constant. The last model response is also oscillatory, but the system tends to be static, due to the non conservation of energy. Heat transfer through the cylinder walls was observed and the temperature oscillates, tending to the prescribed temperature. As a response the density decreases, showing that the volume of the system increases, characterizing the behavior of the pressure and temperature of an ideal gas. When the chemical reaction of combustion is added, there is a very sudden increase of the temperature and volume of the system, because the rate of energy was very high.

Parametric Analysis of Different Nacelle Positions in the DLR-F6 Model by Means of the CFD++ Code

The integration of engines into the aircraft involves many important aspects. These aspects can be investigated by means of wind tunnel experiments or numerical simulations. The main goal of this study is to analyze the effects of different nacelle positions on the aerodynamic coefficients and pressure distributions through numerical simulations. The aircraft geometry investigated is the DLR-F6 configuration. The nacelle is changed from the original configuration to four different positions, which are characterized by distances in the longitudinal and vertical directions. High-quality hexahedral grids are used in the CFD calculations. In order to evaluate the effects of the nacelle shift, all simulations are carried out in the same conditions and the drag, lift and pressure coefficients are evaluated numerically. Interesting results are obtained. The lift coefficient is shown to be more sensitive to variations of nacelle position than the drag coefficient. Based on the simulation results, it can be inferred that the optimum position for the nacelle, among those investigated, is the most forward one.