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Sedat Biringen - One of the best experts on this subject based on the ideXlab platform.

  • Modeling high-order statistics in the turbulent Ekman Layer
    Geophysical & Astrophysical Fluid Dynamics, 2016
    Co-Authors: Scott Waggy, Alan Hsieh, Sedat Biringen
    Abstract:

    Results from a direct numerical simulation (DNS) of the neutral and unstable turbulent Ekman Layer at a Reynolds number of 1000 were used to evaluate turbulence closure models. For the neutrally stratified Ekman Layer, the higher-order moments of velocity were examined and the accuracy of a kurtosis model was assessed. For the unstable Ekman Layer, the analysis of higher-order moments was extended to temperature-velocity correlations. Model coefficients were optimised using DNS data and it was shown that the optimised models accurately captured the distributions of all fourth-order moments. These low-Reynolds number results can be extrapolated to higher Reynolds numbers to parameterise turbulence in other flow fields with rotational effects such as the atmospheric boundary Layer.

  • PARALLEL IMPLEMENTATION OF A NAVIER–STOKES SOLVER: TURBULENT Ekman Layer DIRECT SIMULATION
    International Journal of Computational Methods, 2014
    Co-Authors: Scott B. Waggy, Alec Kucala, Sedat Biringen
    Abstract:

    A massively parallel direct numerical solution procedure for the turbulent Ekman Layer is presented. The simulations study the dynamics of turbulence in this flow by solving the incompressible Navier–Stokes equations with Coriolis and buoyancy terms. The governing equations are integrated via a semi-implicit time advancement algorithm which is massively parallelized using the Portable, Extensible Toolkit for Scientific Computation (PETSc) libraries. Accuracy of the numerical scheme was validated by comparisons of simulation results with the hydrodynamic linear stability theory for Poiseuille flow. Two cases are presented to demonstrate the capabilities of the code: (a) a neutrally stable case of Reynolds number, Re = 400 and (b) an unstably stratified case at Re = 1,000 requiring very high resolution in all coordinate directions. Results indicate that the scalability is not limited by the overall size of the problem, but rather by the number of mesh points per processor. Strong scaling is demonstrated for both cases with as few as 10,000 unknowns per processor.

  • parallel implementation of a navier stokes solver turbulent Ekman Layer direct simulation
    International Journal of Computational Methods, 2014
    Co-Authors: Scott Waggy, Alec Kucala, Sedat Biringen
    Abstract:

    A massively parallel direct numerical solution procedure for the turbulent Ekman Layer is presented. The simulations study the dynamics of turbulence in this flow by solving the incompressible Navier–Stokes equations with Coriolis and buoyancy terms. The governing equations are integrated via a semi-implicit time advancement algorithm which is massively parallelized using the Portable, Extensible Toolkit for Scientific Computation (PETSc) libraries. Accuracy of the numerical scheme was validated by comparisons of simulation results with the hydrodynamic linear stability theory for Poiseuille flow. Two cases are presented to demonstrate the capabilities of the code: (a) a neutrally stable case of Reynolds number, Re = 400 and (b) an unstably stratified case at Re = 1,000 requiring very high resolution in all coordinate directions. Results indicate that the scalability is not limited by the overall size of the problem, but rather by the number of mesh points per processor. Strong scaling is demonstrated for both cases with as few as 10,000 unknowns per processor.

  • Reply to ''Comments on 'Direct Numerical Simulation of the Turbulent Ekman Layer: Evaluation of Closure Models'''
    Journal of the Atmospheric Sciences, 2014
    Co-Authors: Stuart W. Marlatt, Scott Waggy, Sedat Biringen
    Abstract:

    In this note, we respond to J. C. Bergmann’s comments (Bergmann 2014) regarding our recent article assessing Ekman-Layer turbulence closure models using direct numerical simulation (DNS) results. As Bergmann notes in his introduction, many of the comments that he provides are not specifically related to our paper. While many of these issues are of interest for understanding the dynamicsoftheatmosphericboundaryLayer(ABL),wewill limit our responses primarily to those issues that directly relate to the objectives and the results of our paper. In the following reply, we first discuss of the applicabilityoftheneutral,turbulentEkmanLayerasamodelfor the ABL, and we then address some fundamental objections that Bergmann raises to the use of DNS. We also address Bergmann’s other concerns regarding turbulence models, momentum balance, and vertical exchange and eddy viscosity modeling assumptions.

  • Direct Numerical Simulation of the Turbulent Ekman Layer: Evaluation of Closure Models
    Journal of the Atmospheric Sciences, 2012
    Co-Authors: Stuart W. Marlatt, Scott Waggy, Sedat Biringen
    Abstract:

    AbstractA direct numerical simulation (DNS) at a Reynolds number of 1000 was performed for the neutral atmospheric boundary Layer (ABL) using the Ekman Layer approximation. The DNS results were used to evaluate several closure approximations that model the turbulent stresses in the Reynolds averaged momentum equations. Two first-order closure equations proposed by O’Brien and by Large, McWilliams, and Doney were tested; both models approximate the eddy diffusivity as a function of height using cubic polynomials. Of these two models, the O’Brien model, which requires data both at the surface Layer and at the top of the boundary Layer, proved superior. The higher-order k–e model also agreed well with DNS results and more accurately represented the eddy diffusivity in this rotational flow.

Scott Waggy - One of the best experts on this subject based on the ideXlab platform.

  • Modeling high-order statistics in the turbulent Ekman Layer
    Geophysical & Astrophysical Fluid Dynamics, 2016
    Co-Authors: Scott Waggy, Alan Hsieh, Sedat Biringen
    Abstract:

    Results from a direct numerical simulation (DNS) of the neutral and unstable turbulent Ekman Layer at a Reynolds number of 1000 were used to evaluate turbulence closure models. For the neutrally stratified Ekman Layer, the higher-order moments of velocity were examined and the accuracy of a kurtosis model was assessed. For the unstable Ekman Layer, the analysis of higher-order moments was extended to temperature-velocity correlations. Model coefficients were optimised using DNS data and it was shown that the optimised models accurately captured the distributions of all fourth-order moments. These low-Reynolds number results can be extrapolated to higher Reynolds numbers to parameterise turbulence in other flow fields with rotational effects such as the atmospheric boundary Layer.

  • parallel implementation of a navier stokes solver turbulent Ekman Layer direct simulation
    International Journal of Computational Methods, 2014
    Co-Authors: Scott Waggy, Alec Kucala, Sedat Biringen
    Abstract:

    A massively parallel direct numerical solution procedure for the turbulent Ekman Layer is presented. The simulations study the dynamics of turbulence in this flow by solving the incompressible Navier–Stokes equations with Coriolis and buoyancy terms. The governing equations are integrated via a semi-implicit time advancement algorithm which is massively parallelized using the Portable, Extensible Toolkit for Scientific Computation (PETSc) libraries. Accuracy of the numerical scheme was validated by comparisons of simulation results with the hydrodynamic linear stability theory for Poiseuille flow. Two cases are presented to demonstrate the capabilities of the code: (a) a neutrally stable case of Reynolds number, Re = 400 and (b) an unstably stratified case at Re = 1,000 requiring very high resolution in all coordinate directions. Results indicate that the scalability is not limited by the overall size of the problem, but rather by the number of mesh points per processor. Strong scaling is demonstrated for both cases with as few as 10,000 unknowns per processor.

  • Reply to ''Comments on 'Direct Numerical Simulation of the Turbulent Ekman Layer: Evaluation of Closure Models'''
    Journal of the Atmospheric Sciences, 2014
    Co-Authors: Stuart W. Marlatt, Scott Waggy, Sedat Biringen
    Abstract:

    In this note, we respond to J. C. Bergmann’s comments (Bergmann 2014) regarding our recent article assessing Ekman-Layer turbulence closure models using direct numerical simulation (DNS) results. As Bergmann notes in his introduction, many of the comments that he provides are not specifically related to our paper. While many of these issues are of interest for understanding the dynamicsoftheatmosphericboundaryLayer(ABL),wewill limit our responses primarily to those issues that directly relate to the objectives and the results of our paper. In the following reply, we first discuss of the applicabilityoftheneutral,turbulentEkmanLayerasamodelfor the ABL, and we then address some fundamental objections that Bergmann raises to the use of DNS. We also address Bergmann’s other concerns regarding turbulence models, momentum balance, and vertical exchange and eddy viscosity modeling assumptions.

  • Direct Numerical Simulation of the Turbulent Ekman Layer: Evaluation of Closure Models
    Journal of the Atmospheric Sciences, 2012
    Co-Authors: Stuart W. Marlatt, Scott Waggy, Sedat Biringen
    Abstract:

    AbstractA direct numerical simulation (DNS) at a Reynolds number of 1000 was performed for the neutral atmospheric boundary Layer (ABL) using the Ekman Layer approximation. The DNS results were used to evaluate several closure approximations that model the turbulent stresses in the Reynolds averaged momentum equations. Two first-order closure equations proposed by O’Brien and by Large, McWilliams, and Doney were tested; both models approximate the eddy diffusivity as a function of height using cubic polynomials. Of these two models, the O’Brien model, which requires data both at the surface Layer and at the top of the boundary Layer, proved superior. The higher-order k–e model also agreed well with DNS results and more accurately represented the eddy diffusivity in this rotational flow.

  • Parallel Implementation of a Navier-Stokes Solver: Turbulent Ekman Layer Direct Numerical Simulation
    50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2012
    Co-Authors: Scott Waggy, Alec Kucala, Sedat Biringen
    Abstract:

    A massively parallel direct numerical solution procedure for the turbulent Ekman Layer is presented. The simulation studies the dynamics of turbulence for this flow by solving the incompressible Navier-Stokes equations with Coriolis and buoyancy terms. The governing equations are integrated via a semi-implicit time advancement algorithm which is massively parallelized using the Portable, Extensible Toolkit for Scientific Computation (PETSc) libraries. Accuracy of the numerical scheme was validated by comparisons of simulation results with the hydrodynamic linear stability theory for Poiseuille flow. Two cases are presented to demonstrate the capabilities of the code: a) a neutrally stable case of Reynolds number, Re = 400 and b) an unstably stratified case at Re = 1000 requiring very high resolution in all coordinate directions. Results indicate that the scalability is not limited by the overall size of the problem, but rather by the number of mesh points per processor. Strong scaling is demonstrated for both cases with as few as 10,000 unknowns per processor.

Bin Liu - One of the best experts on this subject based on the ideXlab platform.

  • stokes drift induced and direct wind energy inputs into the Ekman Layer within the antarctic circumpolar current
    Journal of Geophysical Research, 2008
    Co-Authors: Bin Liu
    Abstract:

    [1] Theoretical analysis of energetics of the Ekman Layer by incorporating the CoriolisStokes forcing into the classical Ekman model shows that the wind energy input to the Ekman Layer has two components: the work done by the wind stress on the surface Ekman current and that done by the Coriolis-Stokes forcing on the whole body of water in the mixed Layer. Under the assumption of constant vertical diffusivity, analytical forms of the direct wind energy input and the Stokes drift–induced energy input are derived. Assessments of relative importance of surface waves are made by comparing the wind energy input into the Ekman Layer with and without wave-induced Stokes drift effects included. Using the European Centre for Medium-Range Weather Forecasts 40-year reanalysis wind stress and surface wave data sets, the total rate of wind energy input into the Ekman Layer within the Antarctic Circumpolar Current (ACC) is estimated to be 833 GW, in which the direct wind energy input is 650 GW (78%), and the Stokes drift– induced energy input is 183 GW (22%). The total mechanical energy input into the ACC due to wave effects is increased by approximately 4% (30 GW) compared to that into the classical Ekman Layer. Long-term variability of direct wind and Stokes drift–induced energy inputs to the ACC is also examined.

Wei Liu - One of the best experts on this subject based on the ideXlab platform.

  • Magnetized Ekman Layer and Stewartson Layer in a magnetized Taylor-Couette flow.
    Physical Review E, 2008
    Co-Authors: Wei Liu
    Abstract:

    In this paper we present axisymmetric nonlinear simulations of magnetized Ekman and Stewartson Layers in a magnetized Taylor-Couette flow with a centrifugally stable angular-momentum profile and with a magnetic Reynolds number below the threshold of magnetorotational instability. The magnetic field is found to inhibit the Ekman suction. The width of the Ekman Layer is reduced with increased magnetic field normal to the end plate. A uniformly rotating region forms near the outer cylinder. A strong magnetic field leads to a steady Stewartson Layer emanating from the junction between differentially rotating rings at the endcaps. The Stewartson Layer becomes thinner with larger Reynolds number and penetrates deeper into the bulk flow with stronger magnetic field and larger Reynolds number. However, at Reynolds number larger than a critical value $\ensuremath{\sim}600$, axisymmetric, and perhaps also nonaxisymmetric, instabilities occur and result in a less prominent Stewartson Layer that extends less far from the boundary.

  • Magnetized Ekman Layer and Stewartson Layer in a magnetized Taylor-Couette flow.
    Physical review. E Statistical nonlinear and soft matter physics, 2008
    Co-Authors: Wei Liu
    Abstract:

    In this paper we present axisymmetric nonlinear simulations of magnetized Ekman and Stewartson Layers in a magnetized Taylor-Couette flow with a centrifugally stable angular-momentum profile and with a magnetic Reynolds number below the threshold of magnetorotational instability. The magnetic field is found to inhibit the Ekman suction. The width of the Ekman Layer is reduced with increased magnetic field normal to the end plate. A uniformly rotating region forms near the outer cylinder. A strong magnetic field leads to a steady Stewartson Layer emanating from the junction between differentially rotating rings at the endcaps. The Stewartson Layer becomes thinner with larger Reynolds number and penetrates deeper into the bulk flow with stronger magnetic field and larger Reynolds number. However, at Reynolds number larger than a critical value approximately 600 , axisymmetric, and perhaps also nonaxisymmetric, instabilities occur and result in a less prominent Stewartson Layer that extends less far from the boundary.

Stuart W. Marlatt - One of the best experts on this subject based on the ideXlab platform.

  • Reply to ''Comments on 'Direct Numerical Simulation of the Turbulent Ekman Layer: Evaluation of Closure Models'''
    Journal of the Atmospheric Sciences, 2014
    Co-Authors: Stuart W. Marlatt, Scott Waggy, Sedat Biringen
    Abstract:

    In this note, we respond to J. C. Bergmann’s comments (Bergmann 2014) regarding our recent article assessing Ekman-Layer turbulence closure models using direct numerical simulation (DNS) results. As Bergmann notes in his introduction, many of the comments that he provides are not specifically related to our paper. While many of these issues are of interest for understanding the dynamicsoftheatmosphericboundaryLayer(ABL),wewill limit our responses primarily to those issues that directly relate to the objectives and the results of our paper. In the following reply, we first discuss of the applicabilityoftheneutral,turbulentEkmanLayerasamodelfor the ABL, and we then address some fundamental objections that Bergmann raises to the use of DNS. We also address Bergmann’s other concerns regarding turbulence models, momentum balance, and vertical exchange and eddy viscosity modeling assumptions.

  • Direct Numerical Simulation of the Turbulent Ekman Layer: Evaluation of Closure Models
    Journal of the Atmospheric Sciences, 2012
    Co-Authors: Stuart W. Marlatt, Scott Waggy, Sedat Biringen
    Abstract:

    AbstractA direct numerical simulation (DNS) at a Reynolds number of 1000 was performed for the neutral atmospheric boundary Layer (ABL) using the Ekman Layer approximation. The DNS results were used to evaluate several closure approximations that model the turbulent stresses in the Reynolds averaged momentum equations. Two first-order closure equations proposed by O’Brien and by Large, McWilliams, and Doney were tested; both models approximate the eddy diffusivity as a function of height using cubic polynomials. Of these two models, the O’Brien model, which requires data both at the surface Layer and at the top of the boundary Layer, proved superior. The higher-order k–e model also agreed well with DNS results and more accurately represented the eddy diffusivity in this rotational flow.

  • Direct Numerical Simulation of the Turbulent Ekman Layer: Instantaneous Flow Structures
    Journal of Thermophysics and Heat Transfer, 2011
    Co-Authors: Scott Waggy, Stuart W. Marlatt, Sedat Biringen
    Abstract:

    A direct numerical simulation of the turbulent Ekman Layer at a Reynolds number of 400 was performed. Twopoint velocity and pressure correlations were plotted to identify and estimate the average sizes and locations of instantaneous flow structures characteristic of turbulence. It was found that these structures are characterized by elongated eddies near the surface, which broaden away from the wall. The correlations roughly align with the mean sheardirectionnearthesurface;movingawayfromthewall,theyexhibittiltingandliftingofdownstreamsegments. The u 0 two-point correlation, in particular, showed significant tilting in the outer regions of the flow demonstrating that this is a significant deviation from typical nonrotating boundary-Layer behavior.

  • Instantaneous Turbulent Flow Structures of the Numerically Simulated Ekman Layer
    48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 2010
    Co-Authors: Stuart W. Marlatt, Scott Waggy
    Abstract:

    Results from a direct numerical simulation of the Ekman Layer at a Reynolds number of 400 have been evaluated to identify turbulent flow structures in the instantaneous velocity fields. Archetypal structures revealed by means of two dimensional two point velocity correlations show that the rotational boundary Layer is many respects similar to other turbulent flowfields, with the near wall region populated by streaky, elongated eddies, while the flowfield away from the wall shows marked spanwise broadening of the characteristic flow structures. As the eddies move away from the wall, tilting and lifting of downstream segments are observed, similar to observations made of non-rotational boundary Layers. Some structural differences are noted which may be a function of system rotation, especially in the spanwise sectional isocorrelation projections. Reversal of the observed tilting direction in the streamwise velocity two point correlations are attributed to vertical variations in the Reynolds stress and mean velocity gradient vectors.

  • Direct Numerical Simulation of the Turbulent Ekman Layer: Turbulent Energy Budgets
    Journal of Thermophysics and Heat Transfer, 2010
    Co-Authors: Stuart W. Marlatt, Scott Waggy, Sedat Biringen
    Abstract:

    Results from a direct numerical simulation of the Ekman Layer at a Reynolds number of 400 were analyzed to compute energy spectra and energy budgets for this flow. Energy budgets showed that the majority of the turbulent kinetic energy in the flow is produced by the coupling between the streamwise primary Reynolds shear stress and the streamwise velocity gradient. It was also found that the coupling of the vertical velocity variance with the spanwise directions allows transfer of kinetic energy by means of the return-to-isotropy pressure strain terms due to the ‘splatting effect’.