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 R12: Vortex Dynamics and Vortex Flows X |
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Chair: Megan Leftwich, George Washington University Room: 336 |
Tuesday, November 26, 2013 1:05PM - 1:18PM |
R12.00001: Volumetric Velocity Fields Downstream of a 2-Bladed Turbine Daniel Troolin Tip vortices of axial-flow turbines are important in understanding the mean and turbulent characteristics of the wake. Volumetric 3-component velocimetry (V3V) was used to examine the flow downstream of a model two-bladed turbine in air. The turbine had a diameter of 177.8 mm and was powered by a motor operating at approximately 150 rpm. The measurement volume (50 x 50 x 20 mm) was positioned approximately 5 mm downstream of the blade tip, in order to examine the tip vortex structure. The V3V system utilized three 4MP cameras with 85mm lenses positioned in a fixed triangular frame located at a distance of 450 mm from the back of the measurement volume. The illumination source was a 200 mJ dual-head pulsed Nd:YAG laser operating at 7.25 Hz and illuminating 1 micron olive oil droplets as tracer particles. The particle images were then analyzed to produce volumetric vector fields. The focus was placed on visualizing the complex interaction between the turbine tip vortices. Insights on the tip vortex dynamics and three dimensional characteristics of the wake flow will be discussed. [Preview Abstract] |
Tuesday, November 26, 2013 1:18PM - 1:31PM |
R12.00002: Large-eddy simulations of a single vertical axis wind turbine Mahsa Rahromostaqim, Antonio Posa, Elias Balaras, Megan Leftwich Recently vertical axis wind turbines (VAWTs) have been receiving increased attention due to various potential advantages over the more common horizontal axis wind turbines. They can be placed for example in urban areas where space is limited, since they are moderately sized and virtually silent. In this study we will report large-eddy simulations (LES) of a Windspire VAWT. Computations will be conducted using an immersed boundary formulation, where the equations of motion are solved on a fixed Cartesian grid and the turbine blades rotate with a fixed tip speed ratio. The primary objective of this first series of LES is to understand the interaction between the wakes generated by the individual airfoils. To keep the computational cost low and increase the parametric regime we can examine, we will consider only part of the turbine hight and utilize periodic boundary conditions along the turbine axis. The computations will exactly mimic the conditions of closely coordinated experiments of a scaled down VAWT, which will enable us to access the impact of features that will not be captured, such as the tip vortices for example, on the results. Preliminary results reveal a complex interaction of the wakes created by the rotating airfoils and the boundary layer on the airfoils. [Preview Abstract] |
Tuesday, November 26, 2013 1:31PM - 1:44PM |
R12.00003: An Actuator Curve Embedding Method to Model Wind Turbine Wakes Pankaj Jha, Sven Schmitz The Actuator Line Method (ALM) is widely used in the wind energy community to model the complex interactions within large wind farms in large-eddy simulation (LES) of the atmospheric boundary layer (ABL) at various stability states. The state-of-the-art in ALM modeling is rooted in the work of Sorensen and Shen (2002). The major weakness of the ALM still remains in having the actuator line discretized as a superposition of individual spherically-spread body forces. The associated overlap of adjacent spherical force fields leads to a large sensitivity of computed blade loads to the way in which the spherical spreading radius is altered along the actuator line (Jha et al. 2013). An Actuator Curve Embedding (ACE) method is developed that considers a general actuator line in 3-D space where the force distribution along the actuator curve is embedded continuously onto the background mesh and without overlap. The ACE method thus is expected to show improved body-force discretization for wind turbine blades, in particular those subject to aeroelastic deformations. Some preliminary results contrasting the ALM and ACE methods are discussed. \textit{Support: DOE}. [Preview Abstract] |
Tuesday, November 26, 2013 1:44PM - 1:57PM |
R12.00004: Three-dimensional velocity measurements around a rotating vertical axis wind turbine Filippo Coletti, Kevin Ryan, John Dabiri, John Eaton Vertical axis wind turbines (VAWT) can be more closely spaced than conventional horizontal axis wind turbines (HAWT), which points to a potentially greater power that can be extracted from a given wind farm footprint. In order to optimize the inter-turbine spacing and to investigate the potential for constructive aerodynamic interactions, the complex dynamics of VAWT wakes need to be analyzed. To date, only single-point or at best two-dimensional measurements of such wakes have been documented. We have measured the full three-component mean velocity field around and downstream the scaled-down model of a rotating VAWT by Magnetic Resonance Velocimetry (MRV). The high spatial resolution allows to quantitatively explore the structure of the wake, its interaction with the floor, and its development. The flow is shown to be highly three-dimensional and asymmetric for the whole investigated region (up to 7 diameters downstream of the turbine). These results can inform low-order models to predict the performance of turbine arrays. [Preview Abstract] |
Tuesday, November 26, 2013 1:57PM - 2:10PM |
R12.00005: The wake of a single vertical axis wind turbine Danielle A. Barsky, Megan C. Leftwich The purpose of this study is to measure the wake of a Windspire vertical axis wind turbine (VAWT). In recent years, research on VAWTs has increased due to various potential advantages over the more common horizontal axis wind turbines (HAWTs). Unlike very large HAWTs, moderately sized--and virtually silent--VAWTs can be placed in urban and suburban regions where land space is limited. To date, many VAWT studies have assumed that the turbine has the same aerodynamic structure as a spinning cylinder despite a significant increase in geometric complexity. This experiment attempts to understand the fundamental wake structure of a single VAWT (and compare it to the wake structure of a spinning cylinder). In this experiment, a scaled-down VAWT is placed inside a wind tunnel under a controlled laboratory setting. A motor rotates the scale model at a constant angular speed. Stereo particle image velocimetry (PIV) is used to visualize the wake of the turbine and image processing techniques are used to quantify the velocity and vorticity of the wake. [Preview Abstract] |
Tuesday, November 26, 2013 2:10PM - 2:23PM |
R12.00006: Experimental investigation of the wake characteristics of flow-powered and motorized laboratory-scale wind turbines Daniel Araya, John Dabiri We present experimental data that compares the wake characteristics of a laboratory-scale vertical-axis turbine while it is either powered by the flow or by a DC motor. This distinction is relevant for laboratory experiments in which scale turbine models are used that require the use of a motor to spin the turbine blades. Particle image velocimetry is used to measure the velocity field in a two-dimensional plane normal to the axis of rotation. This velocity field is then used to compare time-averaged streamwise velocity, turbulence kinetic energy, and power of the two configurations. The results give insight into the kinematic effect of adding energy to the flow by way of the motor, and they suggest limits on the extrapolation of laboratory results to full-scale performance. [Preview Abstract] |
Tuesday, November 26, 2013 2:23PM - 2:36PM |
R12.00007: Experimental Study of Vortex Dynamics during Blade-Vortex Interactions Di Peng, James Gregory Vortices incident upon bodies, such as cylinders, airfoils, and rotor blades, can give rise to substantial unsteady loading, sound generation, and vibration in a variety of engineering applications. A comprehensive study on vortex dynamics during blade-vortex interaction (BVI) is performed in this work. Evidence has been found in previous studies that the vortex behavior during BVI varies with Reynolds number, but the effects are not clear. In the current study, the experiments are performed in a 3' $\times$ 5' low speed wind tunnel where the Reynolds number can be varied from 6 $\times$ 10$^{4}$ to 8 $\times$ 10$^{5}$ by adjusting freestream speed and airfoil size. The vortex is generated by the pitching motion of a wing, which is driven by an air cylinder. Another wing is placed downstream to initiate parallel interactions with the generated vortices. Smoke visualization is used originally to characterize the vortex. Then the BVI problem is studied in detail using time-resolved PIV and unsteady pressure measurements on the downstream target airfoil. The vortex behaviors at selected Reynolds numbers are investigated. The influence of other factors on vortex behavior, such as vortex strength and core size, is also discussed. [Preview Abstract] |
Tuesday, November 26, 2013 2:36PM - 2:49PM |
R12.00008: Combined Vorticity Confinement and Total Variation Diminishing Technique for Modeling of Blade Tip Vortex Alex Povitsky, Kristopher Pierson The Vorticity Confinement (VC) approach is combined with Total Variation Diminishing (TVD) technique to avoid over-confinement and divergence of upwind second-order of approximation schemes. The TVD schemes were combined with the first (constant confinement parameter $\varepsilon )$ and second (constant unit-less confinement parameter $c)$ VC formulations and with adoptive VC formulation by Hahn and Iaccarino. Combined VC/TVD techniques were first applied to convected Taylor vortex, which represent a model of wing tip vortex. For the former two VC methods combination of the second-order upwind discretization scheme with VC shows significant over-confinement of vortex whereas the first-order discretization scheme leads to strong dissipation of vortex. While the latter VC technique shows acceptable results for first-order upwind scheme, it either diverges or strongly over-confines when the second-order upwind discretization scheme is used. The VC/TVD techniques were tested with non-differentiable minmod and Van Leer flux limiters and with differentiable Van Albada limiter. The combination of VC and TVD with differential limiter computes most accurate vortex. The proposed technique is applied to tip vortex generated by rotating blade. Implementation of combined VC with TVD equipped with differential flux limiter to CFD code FLUENT shows much more close comparison to experimental results in terms of vortex velocity profile and size of vortex core compared to the same CFD code without VC approach. [Preview Abstract] |
Tuesday, November 26, 2013 2:49PM - 3:02PM |
R12.00009: ABSTRACT WITHDRAWN |
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