Document Type

Dissertation

Date of Degree

Fall 2014

Degree Name

PhD (Doctor of Philosophy)

Degree In

Mechanical Engineering

First Advisor

Frederick Stern

Second Advisor

Jianming Yang

Abstract

Large-eddy simulations of turbulent flows past a circular cylinder have been performed at sub-, critical and super-critical Re using an orthogonal curvilinear grid solver, CFDship-Iowa version 6.2. An extensive verification and validation study has been carried out. Various aspects of the flow field have been investigated.

The aspect ratio of the computational domain has major effects on the results. In general, large aspect ratio produced best results for the sub-critical Re. Small dependency on both aspect ratio and grid resolution was observed for the critical Re. Small aspect ratio and conservative scheme produced best results for the super-critical Re.

Overall flow features and the drag crisis phenomenon have been correctly predicted. A lot of experimental and numerical studies of flow past a circular cylinder were collected and used for the validation of the present LES study. Integral and local variables were in fairly good agreement for the sub-critical Re. Sharp behavior including drag crisis was predicted for the critical Re. Although some discrepancy including early formation of turbulent separation was observed, local flow structures including separation bubble were observed for the super-critical Re.

The formation of secondary vortex near the cylinder wall and its evolution into separation bubble were observed. The spectral analysis showed that the separation bubble had the instabilities close to the shear layer frequency. The proximity of shear layer to the cylinder enhanced the mixing process of boundary layer and shear layer and led to the formation of separation bubble. A snapshot POD method was used to extract flow structures in the boundary layer, shear layer and wake. In the boundary layer, the secondary vortices and separation bubble were successfully extracted. Due to the weak TKE distribution, specific flow structures were hard to find in the shear layer. Large two-dimensional flow structures representing the Karman shedding vortices were extracted for the sub- and super-critical Re.

Public Abstract

Large-eddy simulations of turbulent flows past a circular cylinder have been performed at sub-, critical and super-critical Re using an orthogonal curvilinear grid solver, CFDship-Iowa version 6.2. An extensive verification and validation study has been carried out. Various aspects of the flow field have been investigated.

The aspect ratio of the computational domain has major effects on the results. In general, large aspect ratio produced best results for the sub-critical Re. Small dependency on both aspect ratio and grid resolution was observed for the critical Re. Small aspect ratio and conservative scheme produced best results for the super-critical Re.

Overall flow features and the drag crisis phenomenon have been correctly predicted. A lot of experimental and numerical studies of flow past a circular cylinder were collected and used for the validation of the present LES study. Integral and local variables were in fairly good agreement for the sub-critical Re. Sharp behavior including significant drag reduction was predicted for the critical Re. Although some discrepancy including early formation of turbulent separation was observed, local flow structures including separation bubble were observed for the super-critical Re.

The formation of secondary vortex near the body and its evolution into separation bubble were observed. The proximity of shear layer to the cylinder enhanced the mixing process of boundary layer and shear layer and led to the formation of separation bubble. A snapshot POD method was used to extract flow structures in the boundary layer, shear layer and wake.

Keywords

publicabstract, Cylinder, LES, Proper Orthogonal Method, Shear layer instability

Pages

xiii, 140 pages

Copyright

Copyright 2014 Seong Mo Yeon

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