Subgrid and hybrid RANS/LES models for scalar transport and boundary layer transition
Date
2022-08
Authors
Bader, Shujaut H
Major Professor
Advisor
Durbin, Paul A
Ward, Thomas
Sippel, Travis R
Leifsson, Leifur T
Simon, Jacob B
Luo, Songting
Committee Member
Journal Title
Journal ISSN
Volume Title
Publisher
Altmetrics
Research Projects
Journal Issue
Series
Department
Aerospace Engineering
Abstract
The present study consists of three campaigns - (i) on the development of a dynamic subgrid scale
(SGS) model for the turbulent flux of a passive scalar; (ii) on the formulation of a hybrid RANS/LES
model for transitional flows; and (iii) on the large eddy simulation (LES) of passive scalar transport
in transitional boundary layer over a flate plate and development toward an improved Higher Order
Generalized Gradient Diffusion Hypothesis (HOGGDH) model.
In the first project, a dynamic subgrid-scale (SGS) scalar-flux model, based on the exact rate
of production of turbulent scalar fluxes, is proposed. The model is derived from an assumption
that the pressure-scalar correlation in the equation for turbulent scalar flux is a vector that is
approximately aligned with the scalar flux itself. The formulation then yields a tensor diffusivity
which allows nonalignment of the SGS scalar fluxes with respect to the resolved scalar gradient. In
contrast to eddy diffusivity models and to general gradient diffusion hypothesis models, for which
the diffusivity tensor is symmetric, the present formulation produces an asymmetric diffusion tensor;
for theoretical and experimental reasons, that tensor is known to be very asymmetric. The model
contains a single coefficient, which is determined dynamically. The model is validated in fully
developed turbulent channel flow and in separated and reattaching flow over a backstep.
In the second project, a simple alternative to locally compute the model coefficient CDES in
the ℓ2 − ω Delayed Detached Eddy Simulation (DDES) model is presented. The formula for the
coefficient is derived from the structural function Bβ of Vreman. It, therefore, does not involve
explicit filtering or averaging procedures. By virtue of the variable coefficient being based on Bβ,
the model is expected to retain the property of relatively small dissipation in transitional and nearwall
regions. This property enables the present formulation to be an excellent candidate to predict
transitional flows. The formulation is validated in the canonical, fully developed turbulent channel flow and backward facing step flows, followed by simulations of the orderly, bypass and separation
induced laminar-to-turbulent transition in a spatially developing boundary layer over a flat plate.
In the third project, flat plate transition under freestream turbulence with heat transfer is
studied using subgrid dynamic scalar-flux models. The advantage of adopting tensorial subgrid
diffusivity is quantified. A set of Reynolds Averaged scalar-flux models is also evaluated using the
LES data. Based upon the observations, an improved Higher Order Generalized Gradient Diffusion
Hypothesis (HOGGDH) model is proposed with a modified, spatially varying model coefficient. The
model coefficient is made a function of the turbulent Reynolds number RT , which enables it to detect
the near wall region, thereby increasing the accuracy down to the wall. The mean temperature
predictions and the scalar fluxes in the wall-normal as well as the streamwise directions by the
improved version of the HOGGDH model are shown to be accurate compared to the HOGGDH
model with a constant coefficient and other considered models.