Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session E21: Turbulence: Planetary Boundary Layers |
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Chair: James Brasseur, Penn. State University Room: 209 |
Sunday, November 22, 2015 4:50PM - 5:03PM |
E21.00001: Nonequilibrium Response of the Daytime Atmospheric Boundary Layer to Mesoscale Forcing James Brasseur, Bajali Jayaraman, Sue Haupt, Jared Lee The essential turbulence structure of the daytime atmospheric boundary layer (ABL) is driven by interactions between shear and buoyancy. A relatively strong inversion layer ``lid'' typically confines the ABL turbulence, whose height grows during the day with increasing surface heat flux ($Q_{0})$ to $\sim$ 1-2 km before collapsing with $Q_{0}$ towards the day's end. The 3D ``microscale'' ABL turbulence is forced largely in the horizontal by winds above the capping inversion at the ``mesoscale'' at the O(100) km scale. Whereas the ``canonical'' ABL is in equilibrium and quasi-stationary, quasi-2D weather dynamics at the mesoscale is typically nonstationary at sub-diurnal time scales. We study the consequences of nonstationarity in the quasi-2D mesoscale forcing in horizontal winds and solar heating on the dynamics of ABL turbulence and especially on the potential for significant deviations from the canonical equilibrium state. We apply high-fidelity LES of the dry cloudless ABL over Kansas in July forced at the mesoscale (WRF) with statistical homogeneity in the horizontal. We find significant deviations from equilibrium that appear in a variety of interesting ways. One of the more interesting results is that the changes in mesoscale wind direction at the diurnal time scale can destabilize the ABL and sometimes cause a transition in ABL eddy structure that are normally associated with increased surface heating. Supported by DOE. Computer resources by the Penn State ICS. [Preview Abstract] |
Sunday, November 22, 2015 5:03PM - 5:16PM |
E21.00002: Towards a Subgrid Model of Planetary Boundary Layers Based on Direct Statistical Simulation Joseph Skitka, Baylor Fox-Kemper, Brad Marston Reliable weather and climate modeling requires the accurate simulation of Earth's oceanic and atmospheric boundary layers. However, long duration turbulence-resolving simulation is centuries beyond the reach of present day computers, hence subgrid modeling is required. Direct statistical simulation (DSS) that is based upon expansion in equal-time cumulants offers the prospect of building improved subgrid schemes. In contrast to other earlier statistical approaches, DSS makes no unphysical assumptions about the homogeneity, isotropy, or locality of correlations. We investigate the feasibility of a second-order closure (CE2) by performing simulations of the ocean boundary layer in a quasi-linear approximation for which CE2 is exact. Wind-driven Langmuir turbulence and thermal convection are studied by comparison of the quasi-linear and fully nonlinear statistics.~ We also investigate whether or not basis reduction can be achieved by proper orthogonal decomposition (POD) of the second cumulant. [Preview Abstract] |
Sunday, November 22, 2015 5:16PM - 5:29PM |
E21.00003: Investigation of the pressure-strain-rate correlation in the convective atmospheric surface layer Khuong Nguyen, Shuaishuai Liu, Martin Otte, Chenning Tong Recent studies have identified the pressure-strain-rate correlation as the main cause of surface layer anisotropy in the convective atmospheric boundary layer. We decompose the pressure field into the rapid, slow, buoyancy, Coriolis, and harmonic parts using large-eddy simulation to investigate their contributions to the pressure-strain-rate correlation. In a strongly convective surface layer, the buoyancy contribution resulting from large-scale temperature fluctuations dominates. Contributions obtained by solving the free-space Poisson equation show the same trends. The buoyancy contribution is much larger than the harmonic part, indicating that the sources terms, rather than the boundary conditions for the pressure Poisson equation are the main cause of the observed behaviors of the pressure-strain-rate. The results have implications for modeling the pressure-strain-rate correlation. [Preview Abstract] |
Sunday, November 22, 2015 5:29PM - 5:42PM |
E21.00004: Adjustment of mean velocity and turbulence due to a finite-size wind farm in a neutral ABL - A LES study Varun Sharma, Marc B. Parlange, Marc Calaf Large-eddy simulation (LES) has become a well-established tool to simulate and understand the interaction between wind farms and the atmospheric boundary layer (ABL). A popular simulation technique considers wind turbines as actuator disks and simulates `infinite' wind farms due to periodic boundary conditions in the horizontal directions. These simulations have indicated the presence of a fully developed internal boundary layer (IBL) due to `wind farm roughness', which has been shown to have important implications, especially in stratified flow conditions. However, the relationship between the length of the wind farm and the resulting IBL vis-\`{a}-vis the asymptotic IBL and its relevance in real-world wind farms is not well understood at present. To address this issue, simulations of wind farms with different horizontal extents are performed in a neutral ABL using an extremely elongated computational domain. Results focus on identifying length scales defining the adjustment of the ABL to a new equilibrium within the wind farm and comparing it to the infinite wind farm case. Furthermore, analyses shall be extended upstream as well as downstream of the wind farm to determine the `impact' region and the `exit' region of the wind farm. [Preview Abstract] |
Sunday, November 22, 2015 5:42PM - 5:55PM |
E21.00005: Aerodynamic surface stress intermittency and conditionally averaged turbulence statistics William Anderson, David Lanigan Aeolian erosion is induced by aerodynamic stress imposed by atmospheric winds. Erosion models prescribe that sediment flux, $Q$, scales with aerodynamic stress raised to exponent, $n$, where $n > 1$. Since stress (in fully rough, inertia-dominated flows) scales with incoming velocity squared, $u^2$, it follows that $q \sim u^{2n}$ (where $u$ is some relevant component of the flow). Thus, even small (turbulent) deviations of u from its time-mean may be important for aeolian activity. This rationale is augmented given that surface layer turbulence exhibits maximum Reynolds stresses in the fluid immediately above the landscape. To illustrate the importance of stress intermittency, we have used conditional averaging predicated on stress during large-eddy simulation of atmospheric boundary layer flow over an arid, bare landscape. Conditional averaging provides an ensemble-mean visualization of flow structures responsible for erosion `eventsâ€™. Preliminary evidence indicates that surface stress peaks are associated with the passage of inclined, high-momentum regions flanked by adjacent low-momentum regions. We characterize geometric attributes of such structures and explore streamwise and vertical vorticity distribution within the conditionally averaged flow field. [Preview Abstract] |
Sunday, November 22, 2015 5:55PM - 6:08PM |
E21.00006: Large-eddy simulations of mean and turbulence dynamics in unsteady Ekman boundary layers Mostafa Momen, Elie Bou-Zeid Unsteady geostrophic forcing in the atmosphere or ocean not only influences the mean wind, but also affects the turbulent statistics. In order to see when turbulence is in quasi-equilibrium with the mean, one needs to understand how the turbulence decays or develops, and how do the turbulent production, transport and dissipation respond to changes in the imposed forcing. This helps us understand the underlying dynamics of the unsteady boundary layers and develop better turbulence closures for weather/climate models and engineering applications. The present study focuses on the unsteady Ekman boundary layer where pressure gradient, Coriolis, and friction forces interact but are not necessarily in equilibrium. Several cases are simulated using LES to examine how the turbulence and resolved TKE budget terms are modulated by the variability of the mean pressure gradient. We also examine the influence of the forcing variability time-scale on the turbulence equilibrium and TKE budget. It is shown that when the forcing time-scale is in the order of the turbulence characteristic time-scale, the turbulence is no longer in quasi-equilibrium due to highly nonlinear mean-turbulence interactions and hence the conventional log-law and turbulence closures are no longer valid. [Preview Abstract] |
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