OU BLISS sponsors seminars generally every other Friday from 2-3 pm in room 5600 of the National Weather Center. Members of BLISS, as well as outside guests, will speak on a variety of topics related to boundary layer research, including fieldwork, laboratory simulations, and more. All are welcome.

Sept 8 2015: 2:00PM Room 5600

William Anderson : Large-eddy simulation of rough wall turbulence: effects of complex topography, evidence of inner-outer effects, and the role of turbulence in aeolian systems

High Reynolds number rough wall turbulent flows are ubiquitous in engineering and geophysical flows. Turbulent momentum transport influences the aero-/hydro-dynamic signature of bluff bodies and the performance of vapor power systems; in geophysical flows, turbulent mixing impacts urban dispersion, the hydrologic cycle, and sedimentary processes in fluvial/aeolian systems. Recently, it has been shown that spanwise topographic heterogeneity can induce a mean domain-scale (δ) circulation. We demonstrate that these circulations are Prandtl’s Secondary Flow of the Second Kind: sustained and driven by spanwise—wall-normal heterogeneity in the Reynolds stresses (all of which vanish in the absence of spanwise topographic heterogeneity). These findings are supported by large-eddy simulation (channel flow: Anderson et al., 2015: J. Fluid Mech.) and experimental measurement (boundary layer: Barros and Christensen, 2014: J. Fluid Mech.) Mejia-Alvarez and Christensen, 2013: Phys. Fluids termed the resulting heterogeneity in spanwise—wall-normal streamwise velocity low- and high-momentum pathways (in order to draw distinction against low- and high-momentum regions – LMR, HMR – which are a spatially meandering, transient feature of wall turbulence). This work has prompted closer inspection on how mean secondary flows alter the structural attributes of LMRs and HMRs. Results have demonstrated that the inclination angle of coherent structures is steepened within high-momentum pathways (i.e., the hairpin packet paradigm is preserved, but altered, due to turbulent secondary flows). We have also investigated how spanwise spacing, s, between topographic heterogeneities influences turbulent secondary flows, finding that s/δ > 2 is a necessary condition for formation of δ-scale mean circulations (i.e., δ-scale circulations can be attenuated by interaction with adjacent circulations). In other work, we have explored the presence of an “amplitude modulation” effect of the roughness sublayer by inertial layer LMRs and HMRs; we have shown that periods of momentum excess(deficit) in the inertial layer precede periods of elevated(depressed) streamwise—wall-normal Reynolds shearing stress in the roughness sublayer. This work is inspired by Marusic et al., 2010: Science, who showed that LMRs and HMRs in the logarithmic region of smooth wall turbulent boundary layers exhibit an amplitude modulation of the viscous wall region. A decoupling procedure presented by Mathis et al., 2009: J. Fluid Mech. is used to illustrate that an amplitude modulation effect is indeed present for rough wall flows. Finally, we present results from LES of neutrally stratified atmospheric boundary layer flow over a sparsely vegetated, arid landscape. On such landscapes, aeolian erosion is induced (and sustained) by kinetic energy fluxes in the aloft surface layer. Conceptual models typically indicate that sediment flux, q (via saltation or drift), scales with imposed aerodynamic stress raised to some exponent, n, where n > 1. Since aerodynamic stress (in fully rough, inertia-dominated flows) scales with incoming velocity squared, u2, it follows that q ~ u2n (where u is some relevant component of the flow, u(x,t)). Thus, even small (turbulent) deviations of u from its time-averaged value may play an enormously important role in aeolian activity. In order to illustrate the importance of surface stress intermittency, we have used conditional averaging predicated on aerodynamic surface stress during LES (where threshold selection is guided by probability density functions of local surface stress). This averaging procedure provides an ensemble-mean visualization of flow structures responsible for erosion “events”. Preliminary evidence indicates that surface stress peaks are associated with the passage of inclined, high-momentum regions flanked by adjacent low-momentum regions.

Other upcoming seminars:

TBD

06/26/15 - Cedrick Ansorge: Conditional analyses of the very stable boundary layer

05/06/15 - Paul Flanagan: The Dryline, Convective Initiation, and Rapid Evolution of Drought in Oklahoma During 2011

05/01/15 - William (Greg) Blumberg: Ground-Based Infrared Thermodynamic Sounders: A Fresh Look at the Enivornments Supporting Deep Convection

04/24/15 - Ethan Cook: A very basic model for velocity statistics of an Ekman-like neutral turbulent layer

04/10/15 - Jinxin Wang: Urban heat island of Dallas Fort-Worth during July 2011 North-Central Texas drought: WRF-SLUCM model verification and sensitivity study of UHI’s dependence on land-use category.

03/14/15 - Larissa Reames: Boundary Layer and Urban Scheme Performance for a nested 500m WRF simulation

03/07/15 - Jennifer Newman: Optimizing Lidar Scanning Strategies for Wind Energy Turbulence Measurements

01/16/15 - Dr. Alan Shapiro: A Unified Theory for the Great Plains Nocturnal Low-Level Jet

12/12/14 - Chiel van Heerwaarden: A comparison of heterogeneously heated convective boundary layers with fixed flux and fixed temperature boundary conditions

11/21/14 - Cedrick Ansorge: Study of Stabily Stratified Bounday Layers using Turbulent Ekman Flow

10/31/14 - Tim Bonin: Thermal and Turbulence Characteristics of the Southern Great Plains Nocturnal Boundary Layer

10/24/14 - Mike Buban: The formation of small-scale atmospheric vortices via horizontal shearing instability

10/17/14 - Dr. Alan Shapiro: On the lower boundary condition for pressure in numerical simulations of boundary layer flows driven by surface buoyancy variations

09/26/14 - Dr. Jeremy Gibbs: Step Brothers 2: Revisiting Surface Heat-Flux and Temperature Boundary Conditions in Models of Stably Stratified Boundary-Layer Flows

04/25/14 - Stephen Castleberry: Evaluation of a Microwave Radiometer Thermodynamic Retrieval Algorithm

04/18/14 - Charlotte Wainwright: Sampling the boundary layer with a sodar simulator

04/11/14 - Jinxin Wang: Urban effects on precipitation: a literature review and climatology study for Dallas-Fort Worth

04/04/14 - Kerstin Schaefer: Validation of the mesoscale transport and fluid model METRAS for Berlin based on a measurement campaign in summer 2012

03/28/14 - Nathan Anderson: Vertical Velocity Turbulence Profiles Measured by Two Horizontally Separated Doppler Lidars

02/21/14 - Larissa Reames: The WRF-simulated effects of the Oklahoma City urban area on its environment on the dynamics of a simulated supercell thunderstorm

02/14/14 - Dr. Alan Shapiro: New similarity model solutions for boundary-layer flows

01/31/14 - Dr. Petra Klein: Scaling of Mean Flow and Turbulence in the Urban Canopy Layer

01/17/14 - Jennifer Newman: Optimizing Lidar Scanning Strategies for Turbulence Measurements