Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session A1: Geophysical Flows: Atmospheric I |
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Chair: Philip Marcus, University of California, Berkeley Room: 301 |
Sunday, November 20, 2011 8:00AM - 8:13AM |
A1.00001: Vortex Roll Breakup in Three-Dimensional Turbulent Boundary Layers Curtis Hamman, Parviz Moin Large helical vortex rolls with axes in the general direction of the mean wind commonly appear in the unstably stratified atmospheric boundary layer. When a rapid shift in the mean wind direction occurs, the vertical transport of momentum and heat flux is sharply reduced compared to the equilibrium value. At long times, this non-equilibrium turbulent flow may develop back into a stable pattern of organized vortex rolls, now aligned with the new wind direction. This transition process is studied via direct numerical simulation of plane channel flow heated from below with impulsively started transverse pressure gradient ($Ri = -Ra/PrRe^2 = -0.25$, $Ra = 10^7$, and $Pr = 0.71$). The timescale for heat flux recovery is approximately the same for turning angles larger than 30 degrees. For higher turning angles, however, the Nusselt number will temporarily drop below one due to a significant reduction in vertical transport. Horizontal velocity and temperature spectra suggest that scale separation between large-scale, organized convective motions and turbulent eddies can prevent heat transfer reduction in transversely accelerated three-dimensional turbulent boundary layers. [Preview Abstract] |
Sunday, November 20, 2011 8:13AM - 8:26AM |
A1.00002: A vortex pair near a density gradient interface Surupa Shaw, Nick Jenkins, John McHugh The dynamics of a vortex pair in a stratified atmosphere near a density gradient interface is considered here using direct numerical simulations. A density-gradient interface has continuous density but discontinuous gradient of density, and is a common model of the tropopause. The vortex pair is released below the interface and allowed to propagate vertically toward the interface. The anelastic approximation of the Navier-Stokes equations are treated using a spectral method, and the initial vortex has a Gaussian distribution of vorticity. The results show that strong vortices propagate through the interface without much change in dynamics. Weaker vortices will dissipate energy when they reach the interface and although a remnant of the vortex pair transits the interface, it does not achieve the same altitude that it would have without the interface. Overall, the interface is not a barrier to vortex pairs, but would be expected to change the distribution of energy in more complicated flows. [Preview Abstract] |
Sunday, November 20, 2011 8:26AM - 8:39AM |
A1.00003: 3D Vortices in Stratified, Rotating Flows - Secondary Circulations and Changes in Aspect Radio Due to Dissipation Philip Marcus, Pedram Hassanzadeh The aspect ratio of a 3D vortex in a rotating, stratified flow is defined as the ratio of its vertical half-thickness $H$ to its horizontal scale $L$. We recently showed that due to hydrostatic and geo/cyclostrophic balance, an anticyclone has an equilibrium scaling law of $H/L = Ro (1-Ro) f/(\bar{N}-N_{in})$, where $Ro$ is the Rossby number of the vortex, $f$ is the Coriolis parameter, and $\bar{N}$ and $N_{in}$ are the Brunt-V\"ais\"al\"a frequencies of the local ambient fluid and of the vortex interior, respectively. Introduction of a viscous or thermal dissipation (the latter being much more rapid and therefore much more relevant in atmospheric, astrophysical, and planetary vortices) forces a vortex that was initially in equilibrium to decay through a series of quasi-stationary states. Both viscous and thermal dissipation rapidly induce secondary circulations within the vortex, but the circulations created by the two types of dissipation differ qualitatively from each other. Moreover, thermal dissipation rapidly changes the values of $Ro$ and $N_{in}$, so although the equilibrium scaling law above is still satisfied, the aspect ratio of the vortex changes rapidly. We show how the resulting aspect ratio of the vortex, and the magnitude and geometry of the secondary circulation are both strong functions of the vertical dependence of $\bar{N}$. [Preview Abstract] |
Sunday, November 20, 2011 8:39AM - 8:52AM |
A1.00004: Large-eddy simulation of the nighttime boundary layer over the US Great Plains Fotini Chow, Bowen Zhou The nighttime stable boundary layer (SBL) is associated with various atmospheric processes including low-level jets, internal gravity waves, Kelvin-Helmholtz shear instabilities, and turbulence events. These phenomena due to the strong stable stratification are quite difficult to represent in numerical models and often require very high grid resolution. In this study, nested large eddy-simulations (LES) are performed over the site where the Cooperative Atmospheric-Surface Exchange Study (CASES-99) field experiment took place, near Leon, Kansas. The night of Oct 5-6 (IOP2) is chosen to represent a typical intermittently turbulent night over the Great Plains. Two turbulent bursting events in the SBL are identified. Simulations are initialized with North American Regional Reanalysis (NARR) on 32 km grids, and one-way nested to a very fine grid with 16 m horizontal spacing. Results using the conventional TKE-1.5 and dynamic reconstruction turbulence models are compared with observations. While both turbulence closures predict the first event with great precision at 16 m scale, only the dynamic reconstruction model (DRM) is able to sustain intermittent turbulence and predict the second bursting event. [Preview Abstract] |
Sunday, November 20, 2011 8:52AM - 9:05AM |
A1.00005: Large-eddy simulation of stable atmospheric boundary layers to develop better turbulence closures for climate and weather models Elie Bou-Zeid, Jing Huang, Jean-Christophe Golaz A disconnect remains between our improved physical understanding of boundary layers stabilized by buoyancy and how we parameterize them in coarse atmospheric models. Most operational climate models require excessive turbulence mixing in such conditions to prevent decoupling of the atmospheric component from the land component, but the performance of such a model is unlikely to be satisfactory under weakly and moderately stable conditions. Using Large-eddy simulation, we revisit some of the basic challenges in parameterizing stable atmospheric boundary layers: eddy-viscosity closure is found to be more reliable due to an improved alignment of vertical Reynolds stresses and mean strains under stable conditions, but the dependence of the magnitude of the eddy viscosity on stability is not well represented by several models tested here. Thus, we propose a new closure that reproduces the different stability regimes better. Subsequently, tests of this model in the GFDL's single-column model (SCM) are found to yield good agreement with LES results in idealized steady-stability cases, as well as in cases with gradual and sharp changes of stability with time. [Preview Abstract] |
Sunday, November 20, 2011 9:05AM - 9:18AM |
A1.00006: Evening Transition of the Atmospheric Boundary Layer (ABL) H.J.S. Fernando Laboratory experiments and theoretical studies were conducted to understand the decay of convective turbulence, with applications to evening transition of ABL over flat terrain. Either a linearly stratified or a homogeneous fluid was used. Initial convection was established with a steady heat flux at the bottom, which was then subjected to gradual cooling. The convection starts to decay with decreasing heat flux, and in stratified runs convective turbulence strongly interacts with the stratification aloft, thus changing the decay behavior substantially depending on the bulk Richardson number. Surprisingly, the turbulence modification during decay is realized by the adjustments occurring in the $u$-component rather than the $w$-component as expected. It is shown that stratification near the bottom surface and strong buoyancy effects in the proximity of entraining inversion suppress vertical motions therein to the extent that horizontal layering and a ``jet-like'' flow structure form. Enhanced dissipation in these layers leads to rapid decay of turbulent kinetic energy, after a time of about $t$ = (3-4)$t*$, where $t*$ is the convective time scale at the onset of cooling. Oscillations of velocity and temperature fluctuations, development of layering, influence of overlying stratification and lack of agreement with previous numerical modeling were some of the salient features of experimental results. [Preview Abstract] |
Sunday, November 20, 2011 9:18AM - 9:31AM |
A1.00007: Experimental Characterization of Radiation Forcing due to Atmospheric Aerosols K.R. Sreenivas, D.K. Singh, V.K. Ponnulakshmi, G. Subramanian Micro-meteorological processes in the nocturnal atmospheric boundary layer (NBL) including the formation of radiation-fog and the development of inversion layers are controlled by heat transfer and the vertical temperature distribution close to the ground. In a recent study, it has been shown that the temperature profile close to the ground in stably-stratified, NBL is controlled by the radiative forcing due to suspended aerosols.~Estimating aerosol forcing is also important in geo-engineering applications to evaluate the use of aerosols to mitigate greenhouse effects. Modeling capability in the above scenarios is limited by our knowledge of this forcing. Here, the design of an experimental setup is presented which can be used for evaluating the IR-radiation forcing on aerosols under either Rayleigh-Benard condition or under conditions corresponding to the NBL. We present results indicating the effect of surface emissivities of the top and bottom boundaries and the aerosol concentration on the temperature profiles. In order to understand the observed enhancement of the convection-threshold, we have determined the conduction-radiation time constant of an aerosol laden air layer. Our results help to explain observed temperature profiles in the NBL, the apparent stability of such profiles and indicate the need to account for the effect of aerosols in climatic/weather models. [Preview Abstract] |
Sunday, November 20, 2011 9:31AM - 9:44AM |
A1.00008: Fluctuations in Atmospheric Boundary Layer Plumes Jay Boris, David Fyfe, MiYoung Obenschain, Keith Obenschain, Gopal Patnaik Pollution and short-term health considerations require the accurate prediction of airborne contaminant transport in cities. Even when a stationary source emits tracer gases continuously, the resulting plume fluctuates vigorously in the turbulence that results from air passing over any typical landscape. Computing this flow properly requires large-eddy simulations that resolve the vortices shed from buildings, trees, and terrain because these coherent effects govern the ``turbulent'' dispersion of pollutants, tracer gases, and potentially toxic agents. This paper uses long-time, high-resolution detailed studies of one urban configuration, computed with 5-meter spatial resolution and sub-second temporal resolution, to quantify the deviations of passive tracer plumes from steady state. Even when concentration values at a point are averaged over long times, as an accumulating sensor might do, the range of probable values spans orders of magnitude. At a 5-km scale, averaging tracer concentrations for as long as an hour still leaves likely sampling fluctuations of plus or minus a factor of ten from the long-time average. [Preview Abstract] |
Sunday, November 20, 2011 9:44AM - 9:57AM |
A1.00009: RANS modeling of scalar dispersion from localized sources within a simplified urban-area model Riccardo Rossi, Stefano Capra, Gianluca Iaccarino The dispersion of a passive scalar downstream a localized source within a simplified urban-like geometry is examined by means of RANS scalar flux models. The computations are conducted under conditions of neutral stability and for three different incoming wind directions (0$^{\circ}$, 45$^{\circ}$, 90$^{\circ}$) at a roughness Reynolds number of Ret = 391. A Reynolds stress transport model is used to close the flow governing equations whereas both the standard eddy-diffusivity closure and algebraic flux models are employed to close the transport equation for the passive scalar. The comparison with a DNS database shows improved reliability from algebraic scalar flux models towards predicting both the mean concentration and the plume structure. Since algebraic flux models do not increase substantially the computational effort, the results indicate that the use of tensorial-diffusivity can be promising tool for dispersion simulations for the urban environment. [Preview Abstract] |
Sunday, November 20, 2011 9:57AM - 10:10AM |
A1.00010: Dispersion of Particles Emitted from Area Sources into the Unstable Atmospheric Boundary Layer Ying Pan, Marcelo Chamecki, Scott Isard Dispersion of heavy particles emitted from area sources into the atmospheric boundary layer is of broad applications in diverse fields. Chamecki and Meneveau (JFM, 2011) used large-eddy-simulation (LES) to study properties of the particle plumes in the neutrally stratified atmosphere, providing theoretical expressions to predict mean particle concentration profiles, plume height, and horizontal transport above the source and ground deposition flux downwind. Here effects of unstable temperature stratification are considered. LES results suggest that particle plume heights, horizontal mass fluxes, and particle loadings downwind from the source increase as the atmospheric instability increases. The Obukhov length and the free-convection velocity are introduced to characterize the deposition fluxes in addition to friction and settling velocities used in the neutral case. The new theoretical predictions that consider the local atmospheric instability in the expression of the vertical velocity variance agree well with LES results when the particle plume is within the surface layer, where the assumption of self-preservation of concentration profiles holds. [Preview Abstract] |
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