Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session R36: Geophysical: Atmospheric IV |
Hide Abstracts |
Chair: Balaji Jayaraman, Pennsylvania State University Room: 407 |
Tuesday, November 26, 2013 1:05PM - 1:18PM |
R36.00001: Large-eddy simulations of the Ekman boundary layer over walls with variable temperatures: the complex implications of spatially-varying static stability Elie Bou-Zeid, Stimit Shah Understanding and parameterizing turbulent fluxes in statically-stable Ekman boundary layers (SABLs), where buoyant forces destroy turbulent kinetic energy, remains a challenging yet very important problem in geophysical fluid dynamics. The flow is further complicated when surface temperatures exhibit spatial variability leading to variation in buoyancy forces and to significant advection effects. A pertinent example is flow over polar leads and polynyas. To investigate the complex interactions of static stability and surface variability, large eddy simulations are performed over rough surface patches of different temperatures. Stable to more-stable, as well unstable-to-stable and stable-to-unstable transitions are simulated. Variability of surface temperature is shown to result in unexpected flow patterns: TKE is potentially higher under the more stable patches due to advection, and the subsidence and lofting of air over the different patches can counteract the effect of spatial TKE variability on the vertical fluxes. The TKE and flux budget development downstream of the surface temperature transition are investigates with the aim of developing models for upscaling the effect of surface heterogeneity under such conditions. [Preview Abstract] |
Tuesday, November 26, 2013 1:18PM - 1:31PM |
R36.00002: An LES Study of Transition in Atmospheric Boundary Layer Turbulence Structure from Neutral to Convective Stability States Balaji Jayaraman, James Brasseur The scales, strengths and detailed structure of atmospheric boundary layer (ABL) turbulence that affect wind turbine performance and reliability are strongly dependent on the relative contributions of buoyancy-driven vertical motions from surface heating and shear driven motions from geostrophic winds at the mesoscale, as characterized by the global stability state parameter --$z_{i}$/$L$. In the shear-dominant neutral limit, the ABL is characterized by streamwise-elongated coherent eddies of negative fluctuating horizontal velocity As surface heat flux is increased, buoyancy drives vertical fluctuations strongly correlated with shear-driven motions that eventually organize to generate streamwise rolls that couple upper with lower boundary layer. We use large-eddy simulation (LES) to study this transition between ``near neutral'' and ``moderately convective'' by quantifying correlations and integral scales as a function of -$z_{i}$/$L. $The interactions between outer and the surface layer eddies generate surprising turbulence dynamics that includes a special transitional stability state with unusually enhanced streamwise coherence. The transitional process includes a critical phenomenon with sudden dramatic change in ABL structure, and high sensitivity in horizontal fluctuations to surface heating at a low --$z_{i}$/$L.$ \textit{Supported by DOE}. [Preview Abstract] |
Tuesday, November 26, 2013 1:31PM - 1:44PM |
R36.00003: Buoyancy effects on the mean velocity profile in atmospheric surface layer flows Scott Salesky, Gabriel Katul, Marcelo Chamecki Within the diabatic atmospheric surface layer (ASL), the mean velocity profile deviates from its conventional logarithmic shape by a stability correction function $\phi_m(\zeta)$ that varies with the stability parameter $\zeta$. The $\zeta$ parameter measures the relative importance of mechanical to buoyant production or destruction of turbulent kinetic energy within the ASL. A theoretical framework is developed to link the coefficients in empirical curves for $\phi_m(\zeta)$ to stability-dependent properties of turbulence in the ASL including the variation with $\zeta$ of the integral lengthscales and anisotropy of momentum transporting eddies. Approximate asymptotic solutions for $\phi_m(\zeta)$ are derived for the slightly unstable and free convective limits in order to make an explicit link between $\phi_m(\zeta)$ and the stability variation of the integral lengthscale and eddy anisotropy. Analysis of data from the Advection Horizontal Array Turbulence Study indicates the theory is able to explain the behavior of $\phi_m(\zeta)$ accurately for slightly unstable and free convective conditions, but an explanation for the observed behavior of $\phi_m$ for stable conditions remains elusive. [Preview Abstract] |
Tuesday, November 26, 2013 1:44PM - 1:57PM |
R36.00004: Underactuated (bouyancy) control of sensor vehicle distributions in highly stratified flows Gianluca Meneghello, Thomas Bewley Balloons and drifters are useful tools in observation and monitoring of the atmosphere and the ocean. In their simpler configuration, data acquisition is performed while they are passively transported by the flow and no control on their distribution is possible. We present a control strategy employing vertical (buoyancy) actuation only to control both their vertical and horizontal distribution, with application to hurricane monitoring. The desired horizontal distribution is obtained leveraging knowledge of the stratified flow velocity field. The optimal control framework is employed to compute the buoyancy time sequence driving the vehicles to the desired spatial distribution. Uncertainties in both the flow field description and the vehicles position are accounted for. [Preview Abstract] |
Tuesday, November 26, 2013 1:57PM - 2:10PM |
R36.00005: Vertical concentration profiles of dust particles in the atmospheric surface layer Livia Souza Freire Grion, Marcelo Chamecki The study of the emission of dust particles from soil surfaces into the atmospheric boundary layer (ABL) has important applications to different environmental problems, from local air quality to large-scale aerosol transport and its interaction with climate. Due to the difficulty of measuring surface dust flux, a model relating it to the vertical profile of mean concentration is needed. In this study, we use Large-Eddy Simulation of the ABL to evaluate the effects of particle size and turbulence on the relationship between dust flux and concentration profiles. Results show that for very small particles (less than 5 micrometers) the settling velocity is usually negligible and the mean concentration displays a logarithmic profile. For large particles (more than 30 micrometers), there is an approximate balance between vertical turbulent diffusion and gravitational settling, so that Prandtl's power-law solution holds. However, a more general solution including non-zero net fluxes and gravitational settling exists, and it is in agreement with LES results for all particle sizes. Effects of atmospheric stability are also investigated. [Preview Abstract] |
Tuesday, November 26, 2013 2:10PM - 2:23PM |
R36.00006: ABSTRACT WITHDRAWN |
Tuesday, November 26, 2013 2:23PM - 2:36PM |
R36.00007: Modeling the structure and dynamics of a sea spray-stratified hurricane boundary layer Yevgenii Rastigejev, Sergey A. Suslov Accurate modeling of the interaction between sea spray and a turbulent hurricane boundary layer is important for correct forecasting of the hurricane path and intensity. Here we applied a variable density (non-Bousinessq) E-epsilon turbulence closure model to describe the sea spray-stratified hurricane boundary layer structure and dynamics. The model accounts for the effects of variation of turbulent energy and turbulent mixing length due to the sea spray presence, and the spray inertia. The obtained results confirm that the influence of such variations is significant over the complete range of possible spray concentration values. It is shown that when the spray concentration is large the inertia effect dominates the momentum exchange close to the sea surface decelerating the airflow relative to the reference logarithmic profile. However, at higher altitudes the flow acceleration caused by the turbulence suppression is always observed provided that the spray droplets are sufficiently small. It was also found that the variable density model predicts a faster decrease of the drag coefficient with the flow speed than the Boussinesq model. [Preview Abstract] |
Tuesday, November 26, 2013 2:36PM - 2:49PM |
R36.00008: On the Periodicity of Atmospheric von K\'{a}rm\'{a}n Vortex Streets Christopher Nunalee, Sukanta Basu For over one hundred years, a similarity relationship between Strouhal number ($Sr$), a non-dimensional metric for vortex shedding frequency ($N$), and Reynolds number ($Re$) has been aggressively pursued in the context of von K\'{a}rm\'{a}n vortex streets (VKVSs). In this study, we document the $Sr-Re$ relationship of atmospheric VKVSs (i.e., in the extremely high $Re$ regime) in order to gain new insight into a regime of the similarity theory which has never before been investigated. Through quasi-idealized numerical simulations of realistic atmospheric VKVS events, we observe a range of $Re$ in which mesoscale VKVSs are clearly present yet $Sr$ remains in a steady range of 0.15 - 0.22 (irrespective of $Re$). This relationship resembles what has been observed for VKVSs in the much lower $10^2 < Re < 10^4$ regime suggesting eddy viscosity as a proxy for molecular viscosity with regards to $Sr-Re$ similarity theory at high-$Re$. In addition, we find the dominant length scale dictating the $Sr-Re$ relationship in the atmosphere to be the cross-stream mountain diameter, specifically at the height of the boundary layer thermal inversion. [Preview Abstract] |
Tuesday, November 26, 2013 2:49PM - 3:02PM |
R36.00009: Vortex roll-up in a stratified fluid Surupa Shaw, John McHugh Recent simulations of a vortex pair in a stratified fluid show that for small Froude number $W/Nb$ the vortices disintegrate into internal waves, where $W$ is the vortex strength, $b$ is the vortex spacing, and $N$ is the buoyancy frequency. The kinetic energy loss from the vortex pair in this regime can be remarkably fast, essentially annihilating the coherent vortex pair before any noticeable propagation. If the Froude number is large the vortices remain coherent and propagate as they would in constant density flow. The transition in behavior occurs near a Froude number of unity, but is apparently not a sharp transition, as some wave-making appears to happen for Froude numbers above unity. Here we quantify the wave-making with an integral of the momentum flux around a sequence of circles centered on the vortex pair and moving with it. Numerical solutions are obtained using a spectral method, the flow is treated as Boussinesq and viscous, and the initial conditions are approximately the flow due to a line vortex. The results confirm that the transition is gradual, although the complexity of the wavy flow makes interpretation difficult. These results are related to vortex roll-up in a stratified fluid. [Preview Abstract] |
Tuesday, November 26, 2013 3:02PM - 3:15PM |
R36.00010: Finite-Amplitude Anelastic Internal Wave Transmission and Reflection in Non-uniform Flow and Stratification Lauren Eberly, Bruce Sutherland Linear theory predicts that vertically propagating internal waves in non-uniform background flow and stratification reflect where their Doppler-shifted frequency matches the buoyancy frequency at a particular height. If the height over which the waves are evanescent is small, the waves can tunnel, partially transmitting their energy above the reflection level. Furthermore, if the waves grow sufficiently in amplitude due to anelastic effects, weakly nonlinear effects (specifically, the transient acceleration of the mean winds by the wave-induced mean flow) can further enhance transmission. These dynamics are examined through fully nonlinear simulations of anelastic waves in retrograde shear and in non-uniform stratification. For non-hydrostatic waves that are modulationally unstable, we find that transmission is enhanced across a reflection level provided it is situated sufficiently high that weakly nonlinear effects become important but not so high that the waves overturn before reaching the reflection level. More hydrostatic, modulationally stable, waves have enhanced dispersion and so behave more like linear theory predictions except that their overturning heights can be many density scale heights above the predicted level. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700