Graduate Seminar in Fluid Mechanics

September 14, 2018 @ 4:00 pm – 5:00 pm
132 Gilman Hall

4:10 pm Presentation

Displacement Thickness-Based Recycling Inflow Generation Method for Spatially Developing Turbulent Boundary Layer Simulations”

 Presented by SAMVIT KUMAR (Advisers: Profs. Meneveau & Mittal)

An improved method for generation of turbulent inflow for simulations of developing boundary layers is presented.  The approach is based on prior recycling methods for flow over smooth (Lund et al., 1998) and rough (Yang and Meneveau, 2015) surfaces. Both these methods rely on obtaining δ99 from the mean velocity profiles based on a velocity threshold. Since this value is heavily dependent on the shape of the profile, it can be very noisy and can suffer from large undesirable fluctuations, even when the profiles are time averaged. A profile-integrated quantity, such as the displacement thickness, can be used instead of δ99. In the recycling method, mean and fluctuation velocities on a sample plane are rescaled, combined and recycled back to the inlet, as the inflow velocity. A roughness-length related scale is chosen for rescaling of the inner layer, depending on the surface geometry and the displacement thickness is chosen instead of δ99 as the length scale to rescale the outer layer. The blending function, dependent on both the inner and the outer length scales, is used to combine the two profiles, to obtain the inflow velocity. Since the displacement thickness depends on the profile shape, an iterative scheme is implemented. This cushions the effect which an unusual mean velocity profile at the sampling plane may have on the value of the outer length scale and hence, on the rescaled velocity profile. Some applications and test cases are presented.

4:35 pm Presentation

Characterizing Energy Transfer in Restricted Nonlinear Wall-Bounded Turbulence”

 Presented by BENJAMIN MINNICK (Adviser: Prof. Gayme)

Experimental and numerical studies of wall-bounded turbulence have shown the presence of coherent structures elongated in the streamwise direction. Such structures have inspired the development of streamwise coherent models with the intention of reducing computational cost while retaining important characteristics of wall-bounded turbulence. The restricted nonlinear (RNL) model describes the evolution of a streamwise mean velocity field and perturbations which vary about that mean. By neglecting nonlinear interactions between nonzero streamwise Fourier modes, which influence the perturbation velocity field, the number of active streamwise modes is reduced leading to a lower order representation of the flow. Limiting the streamwise-varying modes of RNL turbulence has been shown to be self-sustaining and properly recover the momentum transfer mechanism. Although this approach shows promise, aspects of its dynamics are not yet understood. We focus on the production, dissipation, and transport of energy when limiting the RNL model to multiple streamwise-varying modes. Spanwise spectra and energy budgets predicted from the RNL model are compared to DNS data for low to moderate Reynolds numbers.

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