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 D1: Geophysical: Atmospheric II |
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Chair: Michele Guala, University of Minnesota Room: 323 |
Sunday, November 24, 2013 2:15PM - 2:28PM |
D1.00001: Influence of inflow condition on wind turbine operation and wake unsteadiness Kevin Howard, Leonardo P. Chamorro, Michele Guala A model wind turbine was tested in a closed-circuit wind tunnel under three different inflow conditions, (i) smooth wall turbulent boundary layer, (ii) preceding turbine wake and (iii) three dimensional sinusoidal hill wake, and three thermal stratifications. Two particle image velocimetry (PIV) fields were taken simultaneously upwind and downwind of the turbine along with the turbine voltage, which quantifies rotor fluctuations. Both wall-normal PIV fields were oriented on the centerline of the turbine and captured flow data in a window of approximately 1.1D by 1.1D, where D is the rotor diameter of the turbine. The upwind PIV measured the changing inflow conditions while both the voltage and downwind PIV field provided data that describes the response of the turbine and near wake to the inflow, respectively. Changes occurring in the inflow, whether upwind perturbation or thermal stability related, were statistically linked to the turbine voltage production and wake unsteadiness, as shown by turbulence intensity and swirling strength contours. A laboratory to field scale comparison is completed by inspecting light detection and ranging (Lidar) data taken upwind of the EOLOS utility scale, 2.5 MW wind turbine in conjunction with the turbine power production time signals. [Preview Abstract] |
Sunday, November 24, 2013 2:28PM - 2:41PM |
D1.00002: Stratificaiton Effects on wake of large wind turbines in wind farm Kiran Bhaganagar, Mithu Debnath The focus of the present talk is to demonstrate the interplay of the complex interactions between the wind turbulence and the wake turbulence under different stratification conditions. Large eddy simulation (LES) has been used to simulate flow over multi mega-watt wind turbines. The results have revealed different empirical relations for the mean velocity deficit decay and turbulence kinetic energy decay rates in the wake region of the wind turbine. The simulation for wind farm has revealed the wake decay rates as a function of the radial and streamwise distance from the upstream wind turbine. Vertical mixing plays a major role in altering the flow dynamics in the wake region. [Preview Abstract] |
Sunday, November 24, 2013 2:41PM - 2:54PM |
D1.00003: Wind Tunnel Simulation of the Atmospheric Boundary Layer Tristen Hohman, Alexander Smits, Luigi Martinelli To simulate the interaction of large Vertical Axis Wind Turbines (VAWT) with the Atmospheric Boundary Layer (ABL) in the laboratory, we implement a variant of Counihan's technique [Counihan 1969] in which a combination of a castellated barrier, elliptical vortex generators, and floor roughness elements is used to create an artificial ABL profile in a standard closed loop wind tunnel. To examine the development and formation of the artificial ABL hotwire and SPIV measurements were taken at various downstream locations with changes in wall roughness, wall type, and vortex generator arrangements. It was found possible to generate a boundary layer at $Re_{\theta} \sim 10^{6}$, with a mean velocity that followed the 1/7 power law of a neutral ABL over rural terrain and longitudinal turbulence intensities and power spectra that compare well with the data obtained for high Reynolds number flat plate turbulent boundary layers [Hultmark et al. 2010]. [Preview Abstract] |
Sunday, November 24, 2013 2:54PM - 3:07PM |
D1.00004: Experimental investigations of a sphere anemometer: Wind tunnel and field tests Hendrik Heisselmann, Joachim Peinke, Michael Hoelling In our contribution we will compare the sphere anemometer and two standard sensors for wind energy and meteorology based on results from laboratory and atmospheric measurements. The sphere anemometer is a drag-based sensor for simultaneous wind speed and direction measurements. The new anemometer makes use of the velocity-dependent deflection of a lightweight sphere mounted on top of a flexible tube. The deflection of the sphere is detected by means of a highly sensitive light pointer, as used in atomic force microscopy. This allows for the detection of very small displacements and thus enables a high sensor resolution. In wind tunnel experiments the sphere anemometer, a 3D sonic anemometer and a standard cup anemometer were exposed to a turbulent wind field generated with a so-called active grid. All acquired data was compared to those of a highly resolving hot-wire probe. Moreover, the sphere anemometer and the two reference sensors were installed on two near-shore sites in the German Wadden Sea. Several month of data from these campaigns were analyzed regarding wind speed and direction measurements as well as durability and stability of the new anemometer. [Preview Abstract] |
Sunday, November 24, 2013 3:07PM - 3:20PM |
D1.00005: MATERHORN Field Campaigns: An Overview Harindra Fernando, Eric Pardyjak Emerging important applications have resuscitated scientific and societal interests in mountain terrain flows. Funded by a MURI grant from the Office of Naval Research in 2011, the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program has achieved several important scientific milestones, which will be outlined in this presentation. MATERHORN has four principal thrusts - Modeling, Experimental, Technology and Parameterizations - that are symbiotically directed toward identifying model deficiencies and knowledge gaps, conducting process studies, and developing knowledge and tools for model improvements. Two comprehensive field studies were conducted during September 25 to October 31, 2012 (focusing on quiescent fair weather; wind speeds \textless\ 4 m/s) and May 1 to May 30, 2013 dealing with synoptic influence, moister surface conditions, and moderate (5 to 10 m/s) and strong (\textgreater\ 10m/s) wind periods. An unprecedented suite of high-end instrumentation was used, allowing investigations from tens of km to millimeters and hours to second scales. A host of phenomena that signifies flow interactions across a range of scales were identified, and the dynamics of a selected few will be discussed. [Preview Abstract] |
Sunday, November 24, 2013 3:20PM - 3:33PM |
D1.00006: Slope and Valley Flow Interactions in MATERHORN-1 Christopher M. Hocut, R. Dimitrova, Z. Silver, S. Di Sabatino, L.S. Leo, S.W. Hoch, Y. Wang, E.R. Pardyjak, H.J.S. Fernando In the fall 2012, the \textbf{M}ount\textbf{a}in \textbf{Ter}rain Atmosp\textbf{h}eric M\textbf{o}deling and Obse\textbf{r}vatio\textbf{n}s Program (MATERHORN) conducted its first extensive field experiment at the Granite Mountain Atmospheric Science Testbed (GMAST), US Army Dugway Proving Grounds (DPG), Utah. Of particular interest to MATERHORN-1 were the complex multi-scale interactions between thermally driven meso-scale up/down valley flows and up/downslope flows. To capture these phenomena, a suite of advanced instrumentation was used, which could identify and educe salient physical processes. LiDAR observations were particularly useful, showing the collision of the downslope flow with the valley flow, forming intense turbulent regions, intrusions and instabilities. To further investigate these intriguing interactions and identify meso-scale model shortcomings, WRF simulations have been conducted. In addition to the field measurements and computations, slope and valley flow interactions are the focus of an on-going laboratory study. The goal is to determine the nature of the interactions, determine if there are flow instabilities, examine the turbulence near the region of interaction, and develop a simple scaling in the flow destruction region. [Preview Abstract] |
Sunday, November 24, 2013 3:33PM - 3:46PM |
D1.00007: Effects of complex terrain on atmospheric flow: dividing streamline observations and quantification Michael Thompson, Harindra Fernando, Silvana Di Sabatino, Laura Leo As part of the MATERHORN field campaign on atmospheric flow in mountainous terrain, the dividing streamline concept for stratified flow over obstacles was investigated using smoke flow visualization and meteorological measurements. At small Froude numbers (\textit{Fr} \textless\ 1), a stratified flow approaching a mountain either possesses enough kinetic energy to pass over the summit or else flow around the sides, with dividing streamlines separating the two scenarios. An isolated northwestern peak of the Granite Mountain, approximately 60$m$ in height, was used for the study. Incoming flow velocities and temperature profiles were measured upstream using sonic anemometers and thermocouples mounted on a 32$m$ tower, while onsite measurements were taken with portable weather stations. Sufficiently strong stratification was developed around 3:00AM GMT, with Froude numbers in the range for dividing streamlines to exist. In the first trial, suitably placed red smoke releases were used and in another trial white smoke was released from a 25$m$ crane. In both cases well-defined dividing streamlines were observed and its vertical location was at a height about half of the mountain height, which is consistent with theoretical results based on Shepard's formula. [Preview Abstract] |
Sunday, November 24, 2013 3:46PM - 3:59PM |
D1.00008: The critical slope for orographic rain Robert Breidenthal, Nedjeljka Zagar Krishnamurti has shown that orographic rain depends on the slope of the windward terrain rather than just the total elevation gain. A simple physical model is proposed to account for the effect of slope. Based on the inhibiting effect of vortex (rotational) acceleration on entrainment, a critical slope angle is derived. If the rate of orographic lifting is sufficiently large, the enhanced buoyancy from latent heat release increases the acceleration parameter. As a consequence, the entrainment rate of under-saturated air is reduced. The critical slope corresponds to the situation where the rate of condensation in a rising adiabatic parcel just equals the rate of evaporation due to the entrainment of under-saturated air. The model is also applied to the trigger conditions for towering cumulus in general. [Preview Abstract] |
Sunday, November 24, 2013 3:59PM - 4:12PM |
D1.00009: A numerical study of turbulent flow over complex aeolian dune fields: the White Sands National Monument William Anderson, Marcelo Chamecki, Gary Kocurek, David Mohrig The structure and dynamics of fully-developed turbulent flows responding to aeolian dune fields are studied using large-eddy simulation with an immersed boundary method. An aspect of particular importance in these flows is the downwind migration of coherent motions associated with Kelvin-Helmholtz instabilities which originate at the dune crests. These instabilities are responsible for enhanced downward transport of high momentum fluid via the so-called turbulent sweep mechanism. However, the presence of such structures and their role in determining the bulk characteristics of fully developed dune field sublayer aerodynamics has received relatively limited attention. Moreover, many existing studies address mostly symmetric or mildly asymmetric dune forms. The White Sands National Monument is a field of aeolian gypsum sand dunes in southern New Mexico. In the dune field sublayer, the flow statistics resemble a mixing layer: at approximately the dune crest height, vertical profiles of streamwise velocity exhibit an inflection and turbulent Reynolds stresses are maximum; below this, the streamwise and vertical velocity fluctuations are positively and negatively skewed, respectively. We evaluate the spatial structure of Kelvin-Helmholtz instabilities present in the dune field sublayer -- shear length, Ls, and vortex spacing, Lambda\textunderscore x -- and show that Ls $=$ m Lambda\textunderscore x, where m is approximately 8 in the different sections considered. [Preview Abstract] |
Sunday, November 24, 2013 4:12PM - 4:25PM |
D1.00010: Three-dimensional Quasi-Geostrophic Convection in the Rotating Cylindrical Annulus with Steeply Sloping Endwalls Keith Julien, Michael Calkins, Philippe Marti The rotating cylindrical annulus geometry was first developed by Busse (JFM 1970) as a simplified analogue for studying convection in rapidly rotating spherical geometries. Though it has provided a more tractable two-dimensional model than the sphere, it is formally limited to asymptotically small slopes and thus weak velocities in the direction parallel to the rotation axis. We present an asymptotically reduced three-dimensional equation set to model quasi-geostrophic convection in the annulus geometry where order one slopes are permissible; this model provides a closer analogue to quasi-geostrophic convection in spheres and spherical shells where steeply sloping boundaries are present. A linear stability analysis of the reduced equations shows that a new class of three-dimensional, convectively-driven Rossby waves is present in this system. The gravest modes exhibit strong axial variations as the slope of the boundaries becomes large. Additionally, higher-order eigenmodes showing increasingly complex axial dependence are found that possess critical Rayleigh numbers close to that of the gravest mode. [Preview Abstract] |
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