Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session E32: Rough Wall Turbulent Boundary Layers - III |
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Chair: Mitul Luhar, University of Southern California Room: Oregon Ballroom 201 |
Sunday, November 20, 2016 5:37PM - 5:50PM |
E32.00001: A terrain-following modeling of wave boundary layers Jie Yu Applying the method of conformal transformation, we put forward a terrain-following modeling approach for Stokes boundary layer flows. This complements the recent new development of the exact Floquet theory of water waves, that gives a complete basis of solutions for time harmonic potential flows over an arbitrarily periodic seabed. The theory applies for any given frequency, including the resonant waves. For a non-steep seabed profile, but not necessarily small undulation height comparing with the water depth, the analytical solutions can be obtained for the boundary layer velocities, bed shear stresses and rate of viscous dissipation, explicitly showing the variations both across the boundary layer and along the bed. For a relatively steep bed profile, a remedy is proposed that allows the velocity profiles to be locally determined across the boundary layer avoiding solving the 2-D differential equation for the vorticity. The modeling methodology is presented here for a constant viscosity, including the case of constant eddy viscosity, but can be extended to the case of variable eddy viscosity to improve turbulence modeling. [Preview Abstract] |
Sunday, November 20, 2016 5:50PM - 6:03PM |
E32.00002: Energy amplification in turbulent flows over complex walls Mitul Luhar Many boundary layer flows in natural and manmade systems are characterized by the presence of complex walls (e.g. porous, rough, or patterned) that can substantially alter the near-wall turbulence. For example, the streaks and streamwise vortices prevalent in smooth-walled flows are often replaced by structures resembling Kelvin-Helmholtz vortices in flows over porous media and vegetation canopies. While stability analyses can reproduce some of these observations, they are limited in their ability to generate predictions for spectra and coherent structure in fully turbulent flows. The present effort seeks to address this limitation by extending the resolvent formulation to account for complex walls. Under the resolvent formulation, the turbulent velocity field is expressed as a linear superposition of propagating modes, identified via a gain-based decomposition of the Navier-Stokes equations. The presence of the complex substrate is modeled as a distributed body force, which alters the gain (i.e. energy amplification) and structure of the modes. Preliminary results show that this approach reproduces key observations from previous simulations and experiments of flow over porous media, vegetation canopies, as well as riblets with minimal computation. [Preview Abstract] |
Sunday, November 20, 2016 6:03PM - 6:16PM |
E32.00003: Experimental investigation of the effect of a singly-periodic perturbation on a rough-wall turbulent boundary layer Jonathan Morgan, Beverley McKeon A 3D printed surface which is singly periodic in the streamwise and spanwise directions was placed in a turbulent boundary layer facility. The zero-pressure gradient boundary layer which developed over this singly periodic roughness was characterized with hot-wire anemometry. Compared to a canonical smooth-wall flow, the periodic roughness introduces through its boundary condition a static, singly-periodic fluctuation in mean velocity. From this linear introduction of a single-mode perturbation into the flow, the nonlinear effects of the perturbation on travelling modes can be tracked through statistics, spectra, and mean flow quantities to establish a link between roughness geometry and flow physics. Variation of the velocity power spectrum within the rough boundary layer as well as variation between smooth- and rough-wall boundary layers show the effect of the roughness to be concentrated at wavenumbers which correspond to the roughness wavelength. The effects of the roughness ultimately manifest nonlinearly as an altered Reynolds-stress field which changes the mean velocity profile of the boundary layer. Implications for more general roughness are discussed. [Preview Abstract] |
Sunday, November 20, 2016 6:16PM - 6:29PM |
E32.00004: Study of transition mechanisms induced by an array of roughness elements Prakash Shrestha, Graham V. Candler We study transition mechanisms of a Mach 5.92 laminar boundary layer due to an array of prismatic roughness elements using large-scale direct numerical simulations (DNS). We simulate a boundary layer tripped by arrays of different numbers of roughness elements, corresponding to experiments conducted at the Texas A \& M University Actively Controlled Experimental (ACE) facility. We obtain solutions using a high-order, low-dissipation scheme for the convection terms in the Navier-Stokes equations. We perform separate 2D and 3D simulations. Flow parallel inflow acoustic disturbances are implemented in the 2D domain. We then interpolate spectral content obtained at 30 mm from the leading edge of the 2D domain to the inflow of the 3D domain. In the 3D domain, we compute optimal modes of pressure using dynamic mode decomposition (DMD). Using sparsity-promoting dynamic mode decomposition (SPDMD), we select the dominant modes to study the transition mechanisms. Recirculating vortices upstream and separated shear layers downstream of the roughness elements are observed to be the most dominant modes of transition. We compare streamwise mean mass flux and energy spectral densities at different streamwise locations to validate our simulations. [Preview Abstract] |
Sunday, November 20, 2016 6:29PM - 6:42PM |
E32.00005: Analysis of turbulent heat and momentum transfer in a transitionally rough turbulent boundary layer Ali Doosttalab, Suranga Dharmarathne, Murat Tutkun, Ronald Adrian, Luciano Castillo A zero-pressure-gradient (ZPG) turbulent boundary layer over a transitionally rough surface is studied using direct numerical simulation (DNS). The rough surface is modeled as 24-grit sandpaper which corresponds to $k^+ \approx 11$, where $k^+$ is roughness height. Reynolds number based on momentum thickness is approximately 2400. The walls are isothermal and turbulent flow Prandtl number is 0.71. We simulate temperature as passive scalar. We compute the inner product of net turbulent force $(d(u_1u_i)/dx_i)$ and net turbulent heat flux $(d(u_i\theta/dx_i))$ in order to investigate (i) the correlation between these vectorial quantities, (II) size of the projection of these fields on each other and (IIi) alignment of momentum and hear flux. The inner product in rough case results in larger projection and better alignment. In addition, our study on the vortices shows that surface roughness promotes production of vortical structures which affects the thermal transport near the wall. [Preview Abstract] |
Sunday, November 20, 2016 6:42PM - 6:55PM |
E32.00006: Internal interfaces over a step change in wall roughness R. Jason Hearst, Ronald E. Hanson, Bharathram Ganapathisubramani When flow encounters a step change in wall roughness, an internal boundary layer is formed near the wall. This internal layer grows with streamwise position and eventually dominates the entire boundary layer, returning it to equilibrium with the new boundary condition. It is well established that a canonical turbulent boundary layer is populated by patches of high and low velocity, referred to as uniform momentum zones (UMZs). The UMZs are separated by shear events. In this study, the characteristics of UMZs are examined as the flow transitions from one wall condition to another. Planar particle image velocimetry measurements were performed over both a rough-to-smooth (R$\rightarrow$S) and a smooth-to-rough (S$\rightarrow$R) step change in wall roughness. For the flow over a R$\rightarrow$S change, the maximum wall normal position of the high momentum UMZs that populate the outer region of the boundary layer moves outward towards the free-stream, while the low momentum UMZs that are situated near the wall move closer to the wall. The talk will discuss the implication of these results as well as the results for the flow over a S$\rightarrow$R step change in wall roughness. [Preview Abstract] |
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