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“Scalar Transport in Restricted Nonlinear Wall-Turbulence”
Presented by BENJAMIN MINNICK
(Adviser: Prof. Gayme)
Streamwise coherent structures are inherent in wall-bounded turbulence and their study has given insight into structural features and dynamics of the flow. For example, streamwise vortices have been attributed with redistributing the momentum and developing the blunted mean velocity profile. Numerical and experimental studies of scalars, such as heat or chemical species, transported by wall-bounded turbulent flow have shown scalar fluctuations are highly correlated to streamwise velocity fluctuations, therefore it is likely that the dynamics of streamwise coherent structures also play a key role in scalar transport. We test this hypothesis by simulating scalar transport in the restricted nonlinear (RNL) model, a quasi-two-dimensional representation of wall-turbulence shown to accurately predict low-order statistics and cross-plane features of the momentum field. We have found that restricting both the momentum and scalar fields to streamwise constant mean dynamics interacting with a single non-zero wavenumber perturbation is capable of accurately predicting the expected time-averaged scalar log-law profile for a range of Prandtl numbers. Furthermore, this RNL model is shown to predict similar cross-plane structures in the scalar field to direct numerical simulation (DNS), suggesting streamwise vortices are indeed the structures largely responsible for mixing scalars in wall-bounded turbulent flow. After applying this model to study passive scalar transport, we conclude by investigating buoyancy effects in stably-stratified turbulence.
“Towards a Reduced-Order Model for Finite-Sized Bubbles in Turbulence”
Presented by ASHIK U. M. MASUK
(Adviser: Prof. Ni)
In both natural and industrial turbulent multiphase flows, bubbles are often present as the dispersed second phase controlling many important processes such as ocean-atmosphere gas exchange, heat transfer, and mixing in chemical reactors. For better predictive modeling of such processes, it is essential to understand the contribution of different flow components in determining the shape evolution of bubbles. The shape and relative orientation of a bubble directly control the forces that it experiences in a flow field which eventually determines its dynamics in the flow. In this work, from our unique simultaneous 3D measurements of bubbles and their surrounding flow, we identify the mechanisms that are responsible for the deformation of finite-size bubbles; and based on these mechanisms, we develop a phenomenological model to predict the deformation and orientation of bubbles in turbulence. Finally, the model predictions and experimental measurements are compared.