Semester

Fall

Date of Graduation

2001

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Ismail Celik.

Abstract

The objective of the present study is to improve our understanding of turbulent wake flows. Large-Eddy Simulation (LES) technique is applied for this purpose. A readily available code was used with necessary modifications. Three dimensional incompressible Navier-Stokes equations are solved in non-orthogonal curvilinear coordinates. Finite-volume approach is implemented on a non-staggered grid. The core of the numerical scheme is a fractional step method. The overall accuracy of the method is second order in both space and time.;The LES approach has been validated for four cases: channel flow, flow past a square body, a shear layer flow, and open channel flow. Different subgrid-scale models and numerical schemes have been tested for these benchmarks. Comparisons between the simulations and experiments show the capability of this LES method.;An efficient and accurate Random Flow Generation (RFG) approach has been improved to provide turbulent initial and inflow conditions for developing wake flow calculations. The RFG method can handle anisotropy and inhomogeneity, and it satisfies the instantaneous continuity equations. This approach has been verified by reproducing a turbulent channel flow based on a Direct Numerical Simulation (DNS). Perfect agreements have been obtained.;A typical one equation sub-grid scale model has been selected and modified to include the backscatter of energy by applying the RFG algorithm. Meanwhile, an attempt was made to implement the one equation sub-grid scale model dynamically. Comparisons have been made between the experiments and the simulation results using different one equation sub-grid scale models.;The RFG approach along with the LES technique has been applied to the wake behind a flat plate. Effects of grid resolution and SGS models on the turbulent flow field have been investigated. Good results are obtained as compared to experiments.;Wake flows behind a Naval ship model (5415) have been studied in details by applying the combined LES-RFG method. The calculation domain starts from a plane behind the ship model. Because of the lack of experimental data, Reynolds Averaged Navier-Stokes (RANS) calculations are used to provide the RFG procedure with the information on the inflow boundary. The ship wake flow with an impose surface wave has been studied. The result shows that the wave surface has a significant influence on the turbulent kinetic energy distribution. Finally, part of the ship hull is included in the simulation of the ship wake with a static wave surface (Fr = 0.28) to investigate the difference between RANS and LES results in the near wake of a ship model. The overall quality of the LES calculations is found to be very good. In particular the large coherent structures with significant vorticity concentration and gradients could be captured in the wake of a ship model which is not possible to accomplished by RANS. These structures have significant importance in the transplant of entrained air bubbles.

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