Date of Award

Fall 2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Steven H. Frankel

Committee Chair

Steven H. Frankel

Committee Member 1

Jun Chen

Committee Member 2

Carl r. Wassgren

Committee Member 3

Jie Shen

Abstract

Buoyancy effects due to small density differences commonly exist in turbulent fluid flows occurring in nature and in engineering applications. The large eddy simulation (LES) technique, which is being increasingly used for simulating buoyant turbulent flows, requires accurate modeling of the subgrid sclae (SGS) momentum and buoyancy fluxes. This thesis presents a series of LES and direct numerical simulation (DNS) studies towards a priori and a posteriori evaluation of existing SGS models, and development of new SGS models for the buoyancy flux. This thesis also presents the application of LES, in elucidating qualitative physical features and accurate measures of important quantities such as turbulence budgets, in a simplified flow configuration involving buoyancy effects on a turbulent flow.

Three existing LES SGS models for buoyant turbulent flows are assessed by performing LES and comparing the results to DNS data found in the literature. The test problem for which the accuracy of these existing LES SGS models is studied is the flow in a three-dimensional thermal-driven cavity. In addition to serving as an excellent test case for SGS models, this problem demonstrates interesting phenomena related to the interaction between buoyancy and wall-bounded turbulence. Particularly, the effect of buoyancy on the vertical wall boundary layer is studied.

One drawback of existing LES models is that the SGS diffusivity is dependent solely on the velocity field. A new model for the SGS diffusivity is developed, which is expected to represent the buoyancy flux more accurately. Improvements are also made to the dynamic procedure for determining the model coefficient by taking into account the contribution of the buoyant force.

DNS of a number of homogeneous non-buoyant and buoyant flow configurations is carried out using a pseudo-spectral method, which is free from numerical errors, and artifacts such as the effect of artificial boundary conditions. A well-validated and accurate DNS database is generated, and employed for a priori evaluation of various SGS models. The ability of different models to predict the orientations and magnitudes of buoyancy fluxes is evaluated, and four new models are proposed. Preliminary a posteriori evaluation indicates that these models show an improvement over existing models.

Finally, the horizontal buoyant jet configuration, which is an example of a free-shear turbulent flow affected by buoyancy, is studied using LES. A numerical investigation of this novel flow configuration aids in outlining the physical mechanisms leading to suppression and enhancement of turbulent mixing in different regions of the flow field. Qualitative and quantitative comparisons to previous experimental results are also made, demonstrating the ability of the LES technique to accurately simulate buoyancy and stratification effects in turbulent flows.

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