The Experts below are selected from a list of 1950 Experts worldwide ranked by ideXlab platform

Charles Meneveau - One of the best experts on this subject based on the ideXlab platform.

  • Filtered actuator disks: Theory and application to wind turbine models in large eddy simulation
    Wind Energy, 2019
    Co-Authors: Carl R. Shapiro, Dennice F. Gayme, Charles Meneveau
    Abstract:

    The actuator disk model (ADM) continues to be a popular wind turbine representation in large eddy simulations (LES) of large wind farms. Computational restrictions typically limit the number of grid points across the rotor of each actuator disk and require spatial filtering to smoothly distribute the applied force distribution on discrete grid points. At typical grid resolutions, simulations cannot capture all of the Vorticity Shed behind the disk and subsequently over-predict power by upwards of 10%. To correct these modeling errors, we propose a vortex cylinder model to quantify the Shed Vorticity when a filtered force distribution is applied at the actuator disk. This model is then used to derive a correction factor for numerical simulations that collapses the power curve for simulations at various filter widths and grid resolutions onto the curve obtained using axial momentum theory. The proposed correction therefore facilitates accurate power measurements in LES without resorting to highly refined numerical grids.

  • filtered lifting line theory and application to the actuator line model
    Journal of Fluid Mechanics, 2019
    Co-Authors: Luis A Martineztossas, Charles Meneveau
    Abstract:

    Lifting line theory describes the cumulative effect of Shed Vorticity from finite span lifting surfaces. In this work, the theory is reformulated to improve the accuracy of the actuator line model (ALM). This model is a computational tool used to represent lifting surfaces, such as wind-turbine blades in computational fluid dynamics. In ALM, blade segments are represented by means of a Gaussian body force distribution with a prescribed kernel size. Prior analysis has shown that a representation of the blade using an optimal kernel width of approximately one quarter of the chord size results in accurate predictions of the velocity field and loads along the blades. Also, simulations have shown that use of the optimal kernel size yields accurate representation of the tip-vortex size and the associated downwash resulting in accurate predictions of the tip losses. In this work, we address the issue of how to represent the effects of finite span wings and tip vortices when using Gaussian body forces with a kernel size larger than the optimal value. This question is relevant in the context of coarse-scale large-eddy simulations that cannot afford the fine resolutions required to resolve the optimal kernel size. For this purpose, we present a filtered lifting line theory for a Gaussian force distribution. Based on the streamwise component of the Vorticity transport equation, we develop an analytical model for the induced velocity resulting from the spanwise changes in lift force for an arbitrary kernel scale. The results are used to derive a subfilter-scale velocity model that is used to correct the velocity along the blade when using kernel sizes larger than . Tests are performed in large-eddy simulation of flow over fixed wings with constant and elliptic chord distributions using various kernel sizes. Results show that by using the proposed subfilter velocity model, kernel-size independent predictions of lift coefficient and total lift forces agree with those obtained with the optimal kernel size.

  • filtered lifting line theory and application to the actuator line model
    Journal of Fluid Mechanics, 2019
    Co-Authors: Luis A Martineztossas, Charles Meneveau
    Abstract:

    Lifting line theory describes the cumulative effect of Shed Vorticity from finite span lifting surfaces. In this work, the theory is reformulated to improve the accuracy of the actuator line model (ALM). This model is a computational tool used to represent lifting surfaces, such as wind-turbine blades in computational fluid dynamics. In ALM, blade segments are represented by means of a Gaussian body force distribution with a prescribed kernel size. Prior analysis has shown that a representation of the blade using an optimal kernel width . Tests are performed in large-eddy simulation of flow over fixed wings with constant and elliptic chord distributions using various kernel sizes. Results show that by using the proposed subfilter velocity model, kernel-size independent predictions of lift coefficient and total lift forces agree with those obtained with the optimal kernel size.

Luis A Martineztossas - One of the best experts on this subject based on the ideXlab platform.

  • filtered lifting line theory and application to the actuator line model
    Journal of Fluid Mechanics, 2019
    Co-Authors: Luis A Martineztossas, Charles Meneveau
    Abstract:

    Lifting line theory describes the cumulative effect of Shed Vorticity from finite span lifting surfaces. In this work, the theory is reformulated to improve the accuracy of the actuator line model (ALM). This model is a computational tool used to represent lifting surfaces, such as wind-turbine blades in computational fluid dynamics. In ALM, blade segments are represented by means of a Gaussian body force distribution with a prescribed kernel size. Prior analysis has shown that a representation of the blade using an optimal kernel width of approximately one quarter of the chord size results in accurate predictions of the velocity field and loads along the blades. Also, simulations have shown that use of the optimal kernel size yields accurate representation of the tip-vortex size and the associated downwash resulting in accurate predictions of the tip losses. In this work, we address the issue of how to represent the effects of finite span wings and tip vortices when using Gaussian body forces with a kernel size larger than the optimal value. This question is relevant in the context of coarse-scale large-eddy simulations that cannot afford the fine resolutions required to resolve the optimal kernel size. For this purpose, we present a filtered lifting line theory for a Gaussian force distribution. Based on the streamwise component of the Vorticity transport equation, we develop an analytical model for the induced velocity resulting from the spanwise changes in lift force for an arbitrary kernel scale. The results are used to derive a subfilter-scale velocity model that is used to correct the velocity along the blade when using kernel sizes larger than . Tests are performed in large-eddy simulation of flow over fixed wings with constant and elliptic chord distributions using various kernel sizes. Results show that by using the proposed subfilter velocity model, kernel-size independent predictions of lift coefficient and total lift forces agree with those obtained with the optimal kernel size.

  • filtered lifting line theory and application to the actuator line model
    Journal of Fluid Mechanics, 2019
    Co-Authors: Luis A Martineztossas, Charles Meneveau
    Abstract:

    Lifting line theory describes the cumulative effect of Shed Vorticity from finite span lifting surfaces. In this work, the theory is reformulated to improve the accuracy of the actuator line model (ALM). This model is a computational tool used to represent lifting surfaces, such as wind-turbine blades in computational fluid dynamics. In ALM, blade segments are represented by means of a Gaussian body force distribution with a prescribed kernel size. Prior analysis has shown that a representation of the blade using an optimal kernel width . Tests are performed in large-eddy simulation of flow over fixed wings with constant and elliptic chord distributions using various kernel sizes. Results show that by using the proposed subfilter velocity model, kernel-size independent predictions of lift coefficient and total lift forces agree with those obtained with the optimal kernel size.

Carl R. Shapiro - One of the best experts on this subject based on the ideXlab platform.

  • Filtered actuator disks: Theory and application to wind turbine models in large eddy simulation
    Wind Energy, 2019
    Co-Authors: Carl R. Shapiro, Dennice F. Gayme, Charles Meneveau
    Abstract:

    The actuator disk model (ADM) continues to be a popular wind turbine representation in large eddy simulations (LES) of large wind farms. Computational restrictions typically limit the number of grid points across the rotor of each actuator disk and require spatial filtering to smoothly distribute the applied force distribution on discrete grid points. At typical grid resolutions, simulations cannot capture all of the Vorticity Shed behind the disk and subsequently over-predict power by upwards of 10%. To correct these modeling errors, we propose a vortex cylinder model to quantify the Shed Vorticity when a filtered force distribution is applied at the actuator disk. This model is then used to derive a correction factor for numerical simulations that collapses the power curve for simulations at various filter widths and grid resolutions onto the curve obtained using axial momentum theory. The proposed correction therefore facilitates accurate power measurements in LES without resorting to highly refined numerical grids.

Flemming Rasmussen - One of the best experts on this subject based on the ideXlab platform.

  • a near wake model for trailing Vorticity compared with the blade element momentum theory
    Wind Energy, 2004
    Co-Authors: Helge Aagaard Madsen, Flemming Rasmussen
    Abstract:

    A near wake model for trailing Vorticity originally proposed by Beddoes for high-resolution helicopter blade vortex interaction computations has been implemented and compared with the usual blade element momentum models used for wind turbine calculations. The model is in principle a lifting line model for the rotating blade, where only a quarter revolution of the wake system behind the blade is taken into account. This simplification of the wake enables a fast computation of the downwash from the trailed vortex system along the blade using the indicial function method and thus makes it realistic to use the model in aeroelastic time simulations. The downwash from the Shed Vorticity is also computed with a fast indicial function algorithm. In particular the model is investigated for use in calculations of aerodynamic damping for the different mode shapes of an operating wind turbine. Numerical results for the downwash of a wing in straight flow with elliptical circulation are compared with analytical results. Further, the downwash distribution of a 40 m long rotating blade is computed. Aerodynamic damping of the blade in axial harmonic translation and in the first flapwise mode is computed with the near wake model and compared with the results of a standard momentum model including a model for dynamic inflow. Copyright © 2004 John Wiley & Sons, Ltd.

Nr Mcdonald - One of the best experts on this subject based on the ideXlab platform.

  • a simple model for Sheddies ocean eddies formed from Shed Vorticity
    Journal of Physical Oceanography, 2016
    Co-Authors: Or Southwick, Er Johnson, Nr Mcdonald
    Abstract:

    AbstractRecent studies show that vertical eddy diffusivity is sufficient on its own to introduce intense horizontal shear layers at sloping ocean margins (Molemaker et al.; Gula et al.; Dewar et al.). These layers influence mesoscale energy and potential Vorticity budgets but cannot be fully represented in models without sloping boundaries, no-slip boundary conditions, and sufficiently high resolution. This paper investigates the detachment of these shear layers and their subsequent rolling up into concentrated eddies. These Shed eddies, or “Sheddies,” may have significant oceanographic impacts. Their growth is considered using a simple point vortex model that adapts the Brown–Michael model of vortex Shedding to quasigeostrophic flow and allows detailed consideration of the Vorticity fluxes. The model shows good qualitative agreement with observations and experimental and numerical results. It is applied to a number of examples of well-known cases of Sheddy formation, including the Agulhas cyclones, Calif...

  • A Simple Model for "Sheddies" – Ocean Eddies Formed from Shed Vorticity
    'American Meteorological Society', 2016
    Co-Authors: Or Southwick, Er Johnson, Nr Mcdonald
    Abstract:

    Recent studies show that vertical eddy diffusivity is sufficient on its own to introduce intense horizontal shear layers at sloping ocean margins (Molemaker et al. 2015; Gula et al. 2015; Dewar et al. 2015). These layers influence mesoscale energy and potential Vorticity budgets but cannot be fully represented in models without sloping boundaries, no slip boundary conditions and sufficiently high resolution. This paper investigates the detachment of these shear layers and their subsequent rolling up into concentrated eddies. These Shed eddies, or “Sheddies”, may have significant oceanographic impacts. Their growth is considered using a simple point vortex model that adapts the Brown–Michael model of vortex Shedding to quasigeostrophic flow and allows detailed consideration of the Vorticity fluxes. The model shows good qualitative agreement with observations and experimental and numerical results. It is applied to a number of examples of well known cases of Sheddy formation, including the Agulhas Cyclones, California Undercurrent and Canary Eddy Corridor, and also to investigate the effects of Shed Vorticity in the growth of the Cook Strait Eddy and the interaction of the North Brazil Current Rings with the islands of the Lesser Antilles