Hybrid RANS/LES transition modeling
In this project, a simple method to locally compute the model coefficient CDES in Reddy et al. (Int J Heat Fluid Flow 50:103-113, 2014. https://doi.org/10.1016/j.ijheatfluidflow.2014.06.002) is presented. The formula for the coefficient is derived from the structural function Bβ of Vreman (Phys Fluids 16(10):3670-3681, 2004. https://doi.org/10.1063/1.1785131). 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 near-wall regions. This property enables the present formulation to be a reasonable candidate to predict transitional flows. The formulation is validated in the canonical, fully developed turbulent channel and backward facing step flows, followed by simulations of orderly, bypass and separation induced laminar-to-turbulent transition in a spatially developing boundary layer over a flat plate.
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Dynamic Subgrid-scale scalar-flux modeling
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.
View Publication #2
LES of scalar transport in transitional boundary layer and improved HOGGDH model
In this project, the experiment of flat plate transition under freestream turbulence with heat transfer conducted by Blair (1983) is reproduced using subgrid dynamic scalar-flux models (Bader and Durbin, 2020; Moin et al., 1991). 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, 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.
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Development, design and RANS simulations of novel flow control devices for wind turbine performance enhancement
Wind turbine farms suffer from wake losses, where the downstream turbines generate less power, and/or the leading turbines are throttled to reduce the downstream power losses. In this paper, we focus on possible external modifications that can enhance the wind turbines' performance when they are operating in a farm environment. In particular, this study is interested in enhancing the performance of the downstream turbines in wind farms. The idea is to move each turbine's wake down and away from subsequent turbines. This goal is achieved by using stationary external airfoils that are placed in proximity to the rotating blades. A number of different designs are tested and the design concepts are tested using Reynolds-Averaged Navier-Stokes simulations of an aligned array of 2 wind turbines. The turbines are modeled as actuator disks with axial induction and are placed in a velocity field that is modeled as a turbulent atmospheric boundary layer. It is found that fixed external airfoils can enable partial or full power recovery at turbine separations of as small as 3 rotor diameters downstream. We will also demonstrate that some devices can also improve the performance of the upstream turbine. The physical reasons for these power recovery phenomena are discussed.
View Publication #1 and Patent #1