Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session D1: Geophysical: Atmospheric I |
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Chair: Jan Kleissl, University of California, San Diego Room: 22 |
Sunday, November 18, 2012 2:15PM - 2:28PM |
D1.00001: Velocity statistics and spectra over a forested site measured with a tall mast Antonio Segalini, Henrik Alfredsson, Ebba Dellwik, Johan Arnqvist, Hans Bergstr\"om In the large expansion of wind power it becomes necessary to use also non-ideal sites for the placement of turbines. Such sites may have a complex terrain in terms of surface elevation as well as being forested. The atmospheric boundary layer is assumed to be severely different as compared to the one over flat, low-vegetation areas, which changes the mean velocity distribution as well as the turbulence intensity, thereby negatively affecting both the power production and loads on the turbines. In this study we use data from a 140 m tall mast in a forest in South-Eastern Sweden, where a unique measurement campaign with sonic anemometers has been running since November 2010 for 16 months. The sonic anemometers give the three velocity components with a frequency resolution of about 10 Hz. The site is covered by approximately 20 m high trees and a 40 degree sector, representative of an approximately homogeneous forest flow, is selected for the analysis of the velocity statistics. The screening of the results indicates the presence of a constant stress layer up to 3-5 canopy heights from the ground. An evaluation of the turbulence statistics in this layer and the levels above is presented. In addition, the spectra are evaluated and compared with the commonly used turbulence models. [Preview Abstract] |
Sunday, November 18, 2012 2:28PM - 2:41PM |
D1.00002: Large eddy simulation analysis of thermally stratified atmospheric boundary layer interacting with large wind farms Adrian Sescu, Charles Meneveau Based on a series of large eddy simulation (LES) studies of interactions between the atmospheric boundary layer (ABL) in neutral conditions and infinitely large arrays of wind turbines, new models for the effective roughness length have been developed (Calaf et al. 2010, PoF). Here we consider stratified ABL interacting with large wind turbine arrays. However, in the case of non-neutral conditions, achieving statistically stationary conditions is challenging since, for example, the heat flux at the ground causes vertical profiles of mean temperature to vary in time. To achieve statistical stationarity we use an artificial source of heat, in a region located above ABL, using a PI controller. Another controller is used to drive the flow within ABL, causing the mean velocity to achieve a prescribed direction at a specified height. A suite of simulations at various resolutions and levels of thermal stratification are presented, and the profiles of horizontally averaged velocity, temperature and turbulent fluxes, with and without wind turbines, are compared with each other. In stable conditions the turbulent heat flux increases when wind turbines are included, but in unstable conditions the turbulent heat flux decreases with increasing stratification. [Preview Abstract] |
Sunday, November 18, 2012 2:41PM - 2:54PM |
D1.00003: Collective phenomena in large-eddy simulations of extended wind farms Richard Stevens, Charles Meneveau A major issue with respect to the incorporation of large wind farms in power grids is that their power output strongly fluctuates over time. Understanding these fluctuations, especially its spatio-temporal characteristics, is important for the design of the backup power that must be available. The power fluctuations of the turbines depend on the effect of the wakes, created by a prior row of turbines, on the operation of the turbines, the inter-turbine correlations, and the interaction between the turbines and the atmospheric boundary layer (ABL). We analyze the power fluctuations in large eddy simulations of extended wind-parks in the ABL. We consider various aggregates of wind turbines such as the total average power signal, or sub-averages within the wind farm. In particular, we find that the power variations of the total wind park decreases more than one would expect if one assumes the power output of the turbines to be uncorrelated. The non-trivial correlations are due to the interactions between turbines placed down-stream from each other. Surprisingly, the frequency spectra of the total wind-farm output show a decay that follows approximately a -5/3 power-law scaling regime, qualitatively consistent with observations made in field-scale operational wind parks (Apt, 2007). [Preview Abstract] |
Sunday, November 18, 2012 2:54PM - 3:07PM |
D1.00004: Wind farms and scalar fluxes over a farmland, a Large Eddy Simulation study Marc Calaf, Chad Higgins, Marc Parlange Recent numerical studies have shown that when the horizontal dimension of the wind farms exceeds the height of the atmospheric boundary layer by a factor of ten or more, a fully developed Wind Farm Array Boundary Layer (WFABL) develops, and scalar fluxes beneath the wind turbines are increased by about 10{\%}. Additionally, in situ measurements have shown a variation in scalar concentrations close to the surface below large wind farms. Therefore, it seems clear that large wind farms do actively interact with and change the local boundary layer, and the consequence (on scalar flux and concentrations) of this interaction might be non-negligible. Are increases in irrigation required to offset an increase in evapo-transpiration? A geo-spatial analysis of the placement of wind farms relative to irrigated lands revealed that 26{\%} of all wind farms in the US are located above some irrigated agriculture, but this overlap only represents 1{\%} of the total wind farm footprint, thus irrigated lands below wind turbines are highly fragmented and variable. Therefore, a new set of Large Eddy Simulations (LES) with variable surface boundary conditions designed to replicate the simplified agricultural landscape below a typical large wind farm were performed. Wind turbines were modeled using the standard actuator disk approach. Results showing the breakup of the internal farmland boundary layer due to the presence of a large wind turbine array will be presented. The role that spatial variability plays in the scalar transport below and above the wind turbines is explored. [Preview Abstract] |
Sunday, November 18, 2012 3:07PM - 3:20PM |
D1.00005: Energetic Large-Scale Structures found in Urban-type Boundary Layers Bruno Monnier, Candace Wark, Brian Shaw, Maria Cortes, Sriharsha Kandala, Dietmar Rempfer Flow through a complex environment typical of that found in urban boundary layers is investigated. MAVs maneuvering through an urban field are particularly susceptible to the turbulence encountered in these complex flows. We will present results describing the energetic large-scale coherent structures that are of interest to the aerodynamics community for the purpose of designing the next generation of MAVs. Experimentally we have used SPIV to fully characterize the mean 3D structures as well as investigate various flow regimes encountered in typical urban environments. Various coherent structures that are a function of the geometry of the modeled urban area as well as the incidence angle of the incoming wind direction will be presented and compared with the numerical simulations shown by Kandala et al. An attempt to quantify more accurately the size, location and strength of these structures will be presented. [Preview Abstract] |
Sunday, November 18, 2012 3:20PM - 3:33PM |
D1.00006: Insight on Gust/Turbulence Characteristics in an Urban-type Boundary Layer Candace Wark, Bruno Monnier, Maria Cortes, Brian Shaw, Sriharsha Kandala, Dietmar Rempfer There is lack of accurate data regarding wind gust profiles in complex environments typical of urban boundary layers. SPIV measurements are used to obtain a 3D map of the mean and turbulent flow encountered in a simplified urban array in a modeled atmospheric boundary layer facility. Gust intensity as a function of space in addition to the gust correlations will be shown and compared with the numerical simulations presented in Kandala et al. The dependence of the gust characteristics on the incidence angle of the incoming wind direction into the urban array is also investigated. These results are of importance to MAV applications as it directly impacts their flying characteristics and can be directly related to surging/plunging/pitching motion studies. The gust characteristics are used to build histograms of the angle of attack that an MAV would encounter while maneuvering through an urban array. [Preview Abstract] |
Sunday, November 18, 2012 3:33PM - 3:46PM |
D1.00007: Numerical Simulations of the Urban-Type Boundary Layer Experiment Sriharsha Kandala, Dietmar Rempfer, Monnier Bruno, Candace Wark Due to their small size, limited power and relatively low speeds compared to the magnitude of typical velocity fluctuations in the atmospheric boundary layer, MAVs are highly susceptible to gusts encountered in complex urban environments. As such, an understanding of the spatial and temporal characteristics of these flow fields can play an important role in the design of these MAVs. In this talk, we present the results of numerical simulation of an urban-type boundary layer described in the talk by Monnier, Wark \emph{et al.} Specsolve, a parallel spectral element solver, is used for these simulations. POD is used to generate a low dimensional representation of the velocity field from the simulation data. Gust statistics and various structures relevant to MAV navigation are presented. These results are compared with SPIV data from the experiment. [Preview Abstract] |
Sunday, November 18, 2012 3:46PM - 3:59PM |
D1.00008: An Improved Simulation of the Diurnally Varying Street Canyon Flow Neda Yaghoobian, Jan Kleissl, Kyaw Tha Paw U The impact of diurnal variation of temperature distribution over building and ground surfaces on the wind flow and scalar transport in street canyons is numerically investigated using the PArallelized LES Model (PALM). The Temperature of Urban Facets Indoor-Outdoor Building Energy Simulator (TUF-IOBES) is used for predicting urban surface heat fluxes as boundary conditions for a modified version of PALM. TUF-IOBES dynamically simulates indoor and outdoor building surface temperatures and heat fluxes in an urban area taking into account weather conditions, indoor heat sources, building and urban material properties, composition of the building envelope (e.g. windows, insulation), and HVAC equipment. Temperature (and heat flux) distribution over urban surfaces of the 3-D raster-type geometry of TUF-IOBES makes it possible to provide realistic, high resolution boundary conditions for the numerical simulation of flow and scalar transport in an urban canopy. Compared to some previous analyses using uniformly distributed thermal forcing associated with urban surfaces, the present analysis shows that resolving non-uniform thermal forcings can provide more detailed and realistic patterns of the local air flow and pollutant dispersion in urban canyons. [Preview Abstract] |
Sunday, November 18, 2012 3:59PM - 4:12PM |
D1.00009: Flushing of a dense pollutant from a square 2D street canyon Nigel Kaye, Zahra Baratian Experimental results are presented the rate of flushing of a dense pollutant from a square street canyon. Both finite and continuous release source conditions are considered. For Richardson numbers $Ri$ greater than 1 a clear two-layer stratification is observed, whereas for small $Ri$ the canyon remains well mixed. For the finite release experiments, the initial rate of removal of pollutant is well described with an exponential decay function with the non-dimensional decay rate $k$ being a function of $Ri$. Steady-state measurements from continuous release experiments for $Ri>1$ showed that the rate of entrainment across the density interface (directly analogous to the exponential decay rate) is almost exactly the same function of $Ri$, provided the Richardson number is based on the buoyancy of the lower layer and the canyon height. An energy argument can be used to relate $k(Ri)$ to the mixing efficiency of the turbulent shear flow. Our results indicate that there is a peak in the mixing efficiency at $Ri=1$ above which the mixing efficiency decreases with increasing $Ri$ and a two-layer stratification is observed. [Preview Abstract] |
Sunday, November 18, 2012 4:12PM - 4:25PM |
D1.00010: Experimental investigation of the flow over periodic hills Christian Cierpka, Sven Scharnowski, Matthias Ehlert, Michael Manhart, Christian J. K\"{a}hler The flow over periodic hills is a common geometry for the validation of numerical flow simulations and an ERCOFTAC test case (Nr. 81). The numerical prediction is quite difficult, since flow separation and reattachment are not fixed in space and time due to the smooth geometry (Fr\"{o}hlich et al., J Fluid Mech 526, 2005). Furthermore, the separated and fully three-dimensional flow from the previous hill impinges the next hill and results in very complex flow features including turbulent splashing, Taylor-G\"{o}rtler vortices and a very thin shear layer with developing Kelvin-Helmholtz instabilities (Rapp, Manhart, Exp Fluids 51, 2011). To examine the characteristics of the flow features even close to the wall an experimental PIV investigation with up to single pixel resolution was performed at the existing experimental facility at the TU Munich. To further enhance spatial resolution and measurement precision also advanced particle tracking methods were applied to evaluate the data. The dynamics of the flow features was measured by using time-resolved sampling of the flow. The analysis of the data shows that all relevant scales can be resolved. Therefore a better data set for the validation of numerical simulation is now available. [Preview Abstract] |
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