Title page for ETD etd-03092005-094123


Type of Document Dissertation
Author El-Okda, Yasser Mohamed
Author's Email Address yelokda@vt.edu
URN etd-03092005-094123
Title Experimental and Numerical Investigations of the Effects of Incident Turbulence on the Flow Over a Surface-Mounted Prism
Degree PhD
Department Engineering Science and Mechanics
Advisory Committee
Advisor Name Title
Hajj, Muhammad R. Committee Co-Chair
Ragab, Saad A. Committee Co-Chair
Devenport, William J. Committee Member
Masoud, Ziyad N. Committee Member
Morris, Don H. Committee Member
Keywords
  • Low-Rise Structures
  • Surface-Mounted Prism
  • Compact Upwind Schemes
  • Large Eddy Simulation
  • Particle Image Velocimetry
Date of Defense 2005-02-11
Availability unrestricted
Abstract
The issue of the effects of free stream turbulence on the flow field over a surface-mounted prism is examined through experimental and numerical investigations. In the experimental studies, particle image velocimetry measurements are conducted in the ESM water tunnel at Reynolds number of $9,600$ and under two cases of turbulent inflow conditions. The results show that the mean flow separation, reattachment and parameters such as mean velocity, root mean square, Reynolds stresses and turbulent kinetic energy are affected by the turbulence characteristics of the incident flow. The instantaneous dynamics of the interactions between the separating shear layer and the solid wall and between the shear layer and the turbulence in the incident flow are detailed.

In the numerical studies, large eddy simulations of the flow over a surface-mounted prism under two inflow conditions, namely, smooth inflow and isotropic homogeneous turbulence inflow, are performed. The use of a fifth-order scheme (CUD-II-5), which is a member of a family of Compact Upwind Difference schemes, in large eddy simulations of this flow is assessed. The performance of this scheme is validated by comparing the rate of temporal decay of isotropic turbulence with available experimental measurements for grid-generated turbulence. The results show that the spectra are sensitive to the method of flux vector splitting needed for the implementation of the upwind scheme. With van Leer splitting, the CUD-II-5 scheme is found to be too dissipative. On the other hand, using the Lax-Friedrichs vector splitting yields good agreement with experiments by controlling the level of artificial dissipation. This led us to recommend a new procedure, we denote by C6CUD5 scheme, that combines a compact sixth-order scheme with the CUD-II-5 scheme for large eddy simulation of complex flows. The simulation results, including flow patterns, pressure fields and turbulence statistics show that the CUD-II-5 scheme, with Lax-Friedricks flux vector splitting, provides high resolution of local flow structures. The results present new physical aspects of the flow topology over surface-mounted prisms. The effects of the incident homogeneous turbulence on the size of the separation region and suction pressures are determined by pointing out differences in the flow topologies between the two incident flow cases.

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