Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session L23: Geophysical Fluid Dynamics: Boundary Layers |
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Chair: Eric Arobone, Stanford University Room: 2001 |
Monday, November 24, 2014 3:35PM - 3:48PM |
L23.00001: The near-wall structure of the vorticity field in atmospheric flows Curtis Hamman, Parviz Moin Prompted by suggestions that long-lived, coherent structures are regions of high helicity ($\vec{\omega} \cdot \vec{u}$) and low dissipation, Rogers \& Moin (1987, PoF) examined the helicity field in turbulent channel flow, and did not find evidence to support this helicity conjecture. They concluded, however, that buoyancy forces may preferentially and chronically concentrate cork-screw eddies in wall turbulence. We examine this hypothesis by studying numerical simulation databases of thermal convection with a mean flow in large-aspect ratio channels. Roll cells generated by buoyancy forces in the bulk are contrasted with near-wall, hairpin-like vortices sustained by mean shear. At moderate bulk Richardson numbers, near-wall helicity fluctuations increase showing strong peaks in relative helicity density pdfs but less so in regions of low dissipation. Transverse strain imposed by erupting and impinging thermal plumes embedded in the streamwise-aligned, large-scale circulation is found to tilt these hairpin packets in a herringbone pattern reducing the local turbulence production but increasing the local turbulent dissipation as in 3D turbulent boundary layers. Recent simulations and implications for understanding large-eddy structures in PBLs using LES are discussed. [Preview Abstract] |
Monday, November 24, 2014 3:48PM - 4:01PM |
L23.00002: Thermal transport processes in stable boundary layers Walter Gutierrez, Guillermo Araya, Praju Kiliyanpilakkil, Sukanta Basu, Arquimedes Ruiz-Columbie, Luciano Castillo Using the 200-m tower data (Reese, Texas), profiler and Mesonet data, and WRF runs, a 4-dim model is introduced which summarizes the main features of the Low Level Jet (LLJ) in stable boundary conditions over the aforementioned region and shows its patterns along the year. We also demonstrate the importance of LLJs for wind energy production. It has been observed that during a LLJ event the level of turbulence intensities and TKE are significantly much lower than those during unstable conditions. The major salient results from this study include: the vertical shears in the LLJ are very large at the current wind turbine heights, causing higher static and cyclical aerodynamic loads. The WRF model has accurately captured the beginning and end of the LLJ event; however, the local maximum wind speed at the LLJ ``nose'' has been under-predicted by approximately 15{\%}, which highlights the difficulties WRF still faces in predicting this phenomenon. Furthermore, power spectra and time-autocorrelations of thermal fluctuations will help us in the understanding of the thermal coherent structures involved in moderate and strong LLJ. [Preview Abstract] |
Monday, November 24, 2014 4:01PM - 4:14PM |
L23.00003: Non-equilibrium model of spray-stratified atmospheric boundary layer under high wind conditions Yevgenii Rastigejev, Sergey Suslov Tropical cyclone is a complex meteorological phenomenon which dynamics is defined by a wide variety of factors including exchange of momentum, heat and moisture between the atmosphere and the ocean. Ocean spray plays an important role in this air-sea interaction. Here we developed a two-temperature non-equilibrium variable density (non-Bousinessq) turbulence closure model to describe the ocean spray-stratified hurricane boundary layer structure and dynamics. The model consistently describes a two-way coupling between mechanical and thermodynamic influences of the ocean spray. The obtained results confirm that the impact of non-equilibrium effects is significant over the complete range of possible spray concentration values, therefore has to be included into a consistent parameterization of moisture, heat and momentum transfer process over the ocean under high wind condition of a hurricane. [Preview Abstract] |
Monday, November 24, 2014 4:14PM - 4:27PM |
L23.00004: Evaluation of the sphere anemometer for atmospheric wind measurements Hendrik Heisselmann, Joachim Peinke, Michael Hoelling Our contribution will compare the sphere anemometer and two standard sensors for wind energy and meteorology based on data from a near-shore measurement campaign. We will introduce the characteristics of the sphere anemometer - a drag-based sensor for simultaneous wind speed and direction measurements, which makes use of the highly resolving light pointer principle to detect the velocity-dependent deflection of sphere mounted on a flexible tube. Sphere anemometer, cup anemometer and 3D sonic anemometer were installed at near-shore site in the German Wadden Sea. A comparison of the anemometers was carried out based on several month of high frequency data obtained from this campaign. The measured wind speed and direction data were analyzed to evaluate the capability of the sphere anemometer under real operating conditions, while the sensor characteristics obtained from previous wind tunnel experiments under turbulent conditions served as a reference to assess the durability and to identify challenges of the new anemometer. A characterization of the atmospheric wind conditions at the test site is performed based on the recorded wind data. Wind speed and wind direction averages and turbulence intensities are analyzed as well as power spectra and probability density functions. [Preview Abstract] |
Monday, November 24, 2014 4:27PM - 4:40PM |
L23.00005: Effects of stratification on an ocean surface Ekman layer Hieu Pham, Sutanu Sarkar Large-eddy simulations are used to investigate the effects of stratification on structural and turbulent dynamics of an upper-ocean Ekman layer that is driven by a constant wind stress (friction velocity $u^*$) at low latitude with Coriolis parameter $f$. The surface layer evolves in the presence of interior stratification whose buoyancy frequency varies among cases, taking three values: $N/f = 19, 60$ and $192$. At quasi-steady state, a stratified turbulent Ekman layer forms with a surface current veering to the right of the wind direction. The thickness of the Ekman layer decreases with increasing $N$ and is found to scale with $u^*$, $f$, and $N$, similar to the neutral atmospheric boundary layer of Zilitinkevich \& Esau (2002) that is capped by a stratified layer with buoyancy frequency, $N$. As $N$ increases, the speed of the Ekman current increases but the Ekman transport is invariant. The surface veering angle also increases with larger $N$. The shear rate and buoyancy frequency are elevated at the base of the Ekman layer. The peak of down-wind Reynolds stress occurs near the surface and scales with $u^{*2}$ in all cases while the peak of cross-wind Reynolds stress occurs in the middle of the Ekman layer and decreases with increasing $N$. [Preview Abstract] |
Monday, November 24, 2014 4:40PM - 4:53PM |
L23.00006: A mass-spring-damper model for unsteady Ekman boundary layers Mostafa Momen, Elie Bou-Zeid The Ekman boundary layer is a central problem in geophysical fluid dynamics that emerges in atmospheric and oceanic boundary layers when pressure gradient forces, Coriolis forces, and molecular or turbulent friction forces interact in a flow. These boundary layers are dynamical systems; however, due to their inherent complexity most studies of these wall-bounded flows have focused on steady state conditions. The transient version of the problem, which occurs when these forces are not in equilibrium such as when the pressure gradients are changing in time, is solvable analytically only for a limited set of forcing variability modes, and the resulting solutions are intricate and difficult to interpret. In this study, we derive a simple physical model that reduces Navier-Stokes equations into a second-order ordinary differential equation that is very similar to the dynamical equation of a mass-spring-damper system. The validation of the proposed model is performed by comparing it to results from a suite of large-eddy simulations. The reduced model can be solved for a wider range of variable forcing conditions and serves to elucidate the physical origin of the inertia (mass), energy storage (spring), and energy dissipation (damper) attributes of the Ekman layer. [Preview Abstract] |
Monday, November 24, 2014 4:53PM - 5:06PM |
L23.00007: Effects of three-dimensionality on frontal instability Eric Arobone, Sutanu Sarkar The pure symmetric instability (SI) is a frontal instability that is independent of the along-front coordinate. Observational evidence suggests that along-front variability in vertical velocity and SST anomaly is not negligible (D'Asaro et al. Science. 2011 and Thomas et al. DSR2. 2013). We examine the three-dimensional evolution of frontal shear instabilities from both linear and non-linear perspectives. Linear stability suggests that significant growth rates are possible when along-front and across-front variabilities are comparable. Additionally, along-front variability results in misalignment of perturbations with respect to isopycnals. A suite of three-dimensional Direct Numerical Simulations (DNS) are performed exploring a horizontally homogeneous front with differing domain lengths in the along-front direction. For sufficiently large along-front domain lengths, the front develops along-front variation and a pure symmetric instability is not found in the DNS. The consequence of asymmetry in the instability on frontal evolution will be discussed. The effect of three-dimensionality of initial conditions will also be explored. [Preview Abstract] |
Monday, November 24, 2014 5:06PM - 5:19PM |
L23.00008: Instability map and transition characteristics of the bottom boundary layer under solitary wave Mahmoud Sadek, Luis Parras, Peter Diamessis, Philip Liu Transition prediction in the bottom boundary layer (BBL) flow driven by a soliton-like pressure gradient in an oscillating water tunnel (an approximation for the BBL under solitary waves) is investigated using hydrodynamic linear stability theory. The study of transition in such a flow is divided into two approaches. The first approach is associated with the classical transition resulting from the exponential growth of TS waves. The findings for this approach can be summarized in a map for the temporal instability of the base flow (BF). In this map, the connections between experimental observations, classical stability analysis and fully non-linear 2D numerical simulations have been established. The second approach deals with an alternative transition scenario for this BF due to the algebraic growth of the disturbance leading to the formation of turbulent spots (TS) as reported in laboratory experiments. In this regard, the stability analysis is reformulated in the non-modal framework for the purpose of finding the optimum disturbance characteristics leading to the formation of the observed TS waves. The results of the non-modal analysis are used as an input for 3D DNS of the BBL which aims to mimic the experimental observations and understand the different possible transition scenarios within the BBL under solitary waves. [Preview Abstract] |
Monday, November 24, 2014 5:19PM - 5:32PM |
L23.00009: Lagrangian Coherent Structures in an Unstable Bottom Boundary Layer Under a Solitary Wave Daniel Nelson, Gustaaf Jacobs, Mahmoud Sadek, Peter Diamessis The role of Lagrangian Coherent Structures (LCS) in fluid mixing is investigated in the unstable bottom boundary layer (BBL) under a solitary surface wave mimicked by a soliton-like pressure gradient driven flow in an oscillating water tunnel. The finite-time Lyapunov exponent (FTLE) field, both backward in time and forward in time, is determined for a two-dimensional direct numerical simulation (DNS) of the unstable BBL from the development of the instability through the growth of the large scale transport structures. Attracting LCS are identified trailing the primary vortices that form moving separation surfaces which pick up material from the boundary and transport it into the primary vortices. Weaker, secondary separation surfaces form beneath smaller, secondary vortices. At a later time, the secondary vortices are absorbed by the primary vortices and the separation surfaces from the smaller vortices merge with the separation surfaces from the larger vortices. The primary vortices are the most significant sources of mixing between the near wall and outside the boundary layer, implying that the primary vortices are the physical mechanism for particle resuspension. [Preview Abstract] |
Monday, November 24, 2014 5:32PM - 5:45PM |
L23.00010: Flow Structure and Turbulence Characteristics downstream of a Spanwise Suspended Linear Canopy through Laboratory Experiments Jundong Qiao, Sarah Delavan Laboratory experiments were conducted to explore the mean flow structure and turbulence properties downstream of a spanwise suspended linear canopy in a 2-D open channel flow using the Particle Tracking Velocimetry technique. This canopy simulated the effect of one long-line structure of a mussel farm. Four experimental scenarios with the approach velocities 50, 80, 110, and 140 mm s$^{\mathrm{-1}}$ were under investigation. Three sub-layers formed downstream of the canopy. An internal canopy layer, where the time-averaged velocity decreases linearly with increasing distance downstream, a canopy mixing layer increasing in vertical extent with increasing distance downstream of the canopy, and an external canopy layer with higher velocity under the canopy, which may bring nutrients from the local ambient environment into this layer. The canopy turbulence results in upward momentum transport downstream of the canopy within a distance of 0.60 of the canopy depth and downward momentum transport beyond 1.20 of it. In the scenarios with relatively lower approach velocities 50$^{\mathrm{\thinspace }}$and 80 mm s$^{\mathrm{-1}}$, the wake turbulence results in upward momentum transport. The broader goal of this study is to offer guidelines for the design and site selection of more productive mussel farms. The results suggest that distance interval between the parallel long-lines in a mussel farm should be less than 0.6 times the height of a long-line dropper. Also, potential farm locations that are characterized with current velocity from 50 to 80 mm s$^{\mathrm{-1\thinspace }}$are suggested. [Preview Abstract] |
Monday, November 24, 2014 5:45PM - 5:58PM |
L23.00011: Dynamic Exact Solutions For Stratified Wall Shear Flows George Harabin, Roberto Camassa, Tyler Kress, Grace McLaughlin, Richard McLaughlin An exact time dependent shear flow solution to the full Navier-Stokes equations under the Boussinesq approximation coupled to the advection-diffusion equation for density is investigated in semi-infinite domains with sloped wall boundaries. This solution extends the static solution found by O.M. Phillips in 1969 to include oscillatory time evolution. Long time asymptotics based on the analysis of the branch cut structures in the transform domain are derived and analyzed. Comparisons with preliminary experiments will be discussed. [Preview Abstract] |
Monday, November 24, 2014 5:58PM - 6:11PM |
L23.00012: Prandtl effects on mixing in nonlinear spinup Meline Baghdasarian, Arturo Pacheco-Vega, J. Rafael Pacheco, Roberto Verzicco Stratified spin-up experiments in enclosed cylinders have reported the presence of small pockets of well-mixed fluids; however, there have been shortfalls in terms of quantitative accounts of the mixedness of the fluid. Previous numerical studies reported in the literature have not been able to quantify these measurements either. Here we present a series of three-dimensional numerical simulations that address how the combined effect of spin-up and thermal boundary conditions for various Prandtl numbers enhances or hinders mixing of a fluid in a cylinder. Measurements of efficiency of mixing are based on the variance of temperature and explained in terms of the potential energy available. The numerical simulations of the Navier--Stokes equations for the problem with different sets of thermal boundary conditions at the horizontal walls and varying Prandtl number reported here have helped shed some light on the physical mechanisms of mixing, for which a clear explanation was lacking. [Preview Abstract] |
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