scholarly journals Statistical properties of a stochastic model of eddy hopping

2021 ◽  
Author(s):  
Izumi Saito ◽  
Takeshi Watanabe ◽  
Toshiyuki Gotoh
Keyword(s):  
Stochastic Model ◽  
Reference Data ◽  
Cloud Model ◽  
Computational Cost ◽  
Statistical Properties ◽  
Large Eddy Simulations ◽  
Cloud Droplets ◽  
Large Eddy ◽  
Typical Parameter ◽  
Proper Scaling

Abstract. Statistical properties are investigated for the stochastic model of eddy hopping, which is a novel cloud microphysical model that accounts for the effect of the supersaturation fluctuation at unresolved scales on the growth of cloud droplets and on spectral broadening. It is shown that the model fails to reproduce a proper scaling for a certain range of parameters, resulting in a deviation of the model prediction from the reference data taken from direct numerical simulations and large-eddy simulations (LESs). Corrections to the model are introduced so that the corrected model can accurately reproduce the reference data with the proper scaling. In addition, a possible simplification of the model is discussed, which may contribute to a reduction in computational cost while keeping the statistical properties almost unchanged in the typical parameter range for the model implementation in the LES Lagrangian cloud model.

scholarly journals Statistical properties of a stochastic model of eddy hopping

2021 ◽  
Vol 21 (17) ◽  
pp. 13119-13130
Author(s):  
Izumi Saito ◽  
Takeshi Watanabe ◽  
Toshiyuki Gotoh
Keyword(s):  
Stochastic Model ◽  
Reference Data ◽  
Cloud Model ◽  
Statistical Properties ◽  
Large Eddy Simulations ◽  
Original Version ◽  
Cloud Droplets ◽  
Large Eddy ◽  
Typical Parameter ◽  
Proper Scaling

Abstract. Statistical properties are investigated for the stochastic model of eddy hopping, which is a novel cloud microphysical model that accounts for the effect of the supersaturation fluctuation at unresolved scales on the growth of cloud droplets and on spectral broadening. Two versions of the model, the original version by Grabowski and Abade (2017) and the second version by Abade et al. (2018), are considered and validated against the reference data taken from direct numerical simulations and large-eddy simulations (LESs). It is shown that the original version fails to reproduce a proper scaling for a certain range of parameters, resulting in a deviation of the model prediction from the reference data, while the second version successfully reproduces the proper scaling. In addition, a possible simplification of the model is discussed, which reduces the number of model variables while keeping the statistical properties almost unchanged in the typical parameter range for the model implementation in the LES Lagrangian cloud model.

scholarly journals On the limits of Köhler activation theory: how do collision and coalescence affect the activation of aerosols?

2017 ◽  
Vol 17 (13) ◽  
pp. 8343-8356 ◽  
Author(s):  
Fabian Hoffmann
Keyword(s):  
Critical Radius ◽  
Cloud Model ◽  
Cloud Droplet ◽  
Aerosol Concentration ◽  
Cloud Base ◽  
Cloud Droplets ◽  
Activation Theory ◽  
Large Eddy ◽  
Kohler Theory ◽  
Additional Process

Abstract. Activation is necessary to form a cloud droplet from an aerosol, and it is widely accepted that it occurs as soon as a wetted aerosol grows beyond its critical radius. Traditional Köhler theory assumes that this growth is driven by the diffusion of water vapor. However, if the wetted aerosols are large enough, the coalescence of two or more particles is an additional process for accumulating sufficient water for activation. This transition from diffusional to collectional growth marks the limit of traditional Köhler theory and it is studied using a Lagrangian cloud model in which aerosols and cloud droplets are represented by individually simulated particles within large-eddy simulations of shallow cumuli. It is shown that the activation of aerosols larger than 0. 1 µm in dry radius can be affected by collision and coalescence, and its contribution increases with a power-law relation toward larger radii and becomes the only process for the activation of aerosols larger than 0. 4–0. 8 µm depending on aerosol concentration. Due to the natural scarcity of the affected aerosols, the amount of aerosols that are activated by collection is small, with a maximum of 1 in 10 000 activations. The fraction increases as the aerosol concentration increases, but decreases again as the number of aerosols becomes too high and the particles too small to cause collections. Moreover, activation by collection is found to affect primarily aerosols that have been entrained above the cloud base.

Large-Eddy Simulations of Submarine Propellers

2015 ◽  
Vol 59 (04) ◽  
pp. 227-237
Author(s):  
Elias Balaras ◽  
Seth Schroeder ◽  
Antonio Posa
Keyword(s):  
Computational Cost ◽  
Open Water ◽  
The Body ◽  
Large Eddy Simulations ◽  
Immersed Boundary ◽  
Tip Vortices ◽  
Marine Propellers ◽  
Acoustic Signature ◽  
Large Eddy ◽  
Water Conditions

High-fidelity, eddy-resolving, simulations of marine propellers are challenging due to the coexistence of moving and stationary elements within the computational box, as well as the need to accurately resolve the dynamics of wake structures such as the tip and hub vortices, which have an effect on the acoustic signature of underwater vehicles. Although an isolated propeller in open-water conditions can be simulated in a rotating reference frame, in a computation involving the body of an appended submarine, e.g., the relative motion needs to be properly treated. This increases the computational cost and reduces the accuracy/robustness of typical body-fitted approaches. In this work, an immersed boundary formulation is utilized to perform large-eddy simulations of a propeller in open-water conditions and in the presence of an upstream appendage at zero incidence. In such case, the requirement for the grid to conform to the moving body is relaxed—solution is locally reconstructed to satisfy boundary conditions—and efficient, conservative structured solvers can be used. This enables us to capture the detailed dynamics of the tip vortices and their footprint on the statistics of the wake. The influence of the upstream appendage is also assessed.

Corner Stall Prediction in a Compressor Linear Cascade Using Very Large Eddy Simulation Lattice-Boltzmann Method

2020 ◽  
Vol 142 (7) ◽  
Author(s):  
Antoine Maros ◽  
Benoît Bonnal ◽  
Ignacio Gonzalez-Martino ◽  
James Kopriva ◽  
Francesco Polidoro
Keyword(s):  
Lattice Boltzmann Method ◽  
Lattice Boltzmann ◽  
Computational Cost ◽  
Large Eddy Simulations ◽  
Navier Stokes ◽  
Compressor Cascade ◽  
Numerical Work ◽  
Large Eddy ◽  
Boltzmann Method ◽  
Linear Compressor Cascade

Abstract Compressor corner stall is a phenomenon difficult to predict with numerical tools but essential to the design of axial compressors. Predictive methods are beneficial early in the design process to understand design and off-design limitations. Prior numerical work using Navier–Stokes computational methods has assessed the prediction capability for corner stall. Reynolds-averaged Navier–Stokes (RANS) simulations using several turbulence models have shown to over-predict the region of corner hub stall where large eddy simulations (LES) and detached eddy simulations (DES) approaches improved the airfoil surface and wake pressure loss prediction. A linear compressor cascade designed and tested at Ecole Centrale de Lyon provides a good benchmark for the evaluation of the accuracy of numerical methods for corner stall. This paper presents results obtained with Lattice-Boltzmann method (LBM) coupled with very large-eddy simulations (VLES) approach of PowerFLOW and compares them with the experimental and numerical work from Ecole Centrale de Lyon. The ability to achieve equivalent accuracy at a lower computational cost compared to LES scale resolving methods can enable multi-stage design and off-design compressor predictions. A methodical approach is taken by first accurately simulating the upstream flow conditions. Geometric trips are modeled upstream on the endwalls to match both the mean and fluctuating inflow boundary layer conditions. These conditions were then applied to the simulation of the linear compressor cascade. The benchline experimental study includes trips on both the pressure and suction of the airfoil. These trips are also included for the current simulation. The significance of capturing both inflow conditions and including trips on the airfoil is assessed. Detailed comparisons are then made to airfoil loading and downstream losses between experiment and previous RANS and LES simulations. LBM-VLES corner stall results of pitchwise averaged total pressure match LES agreement relative to experimental data at 50 times lower computational cost.

scholarly journals Modelling Small-Scale Drifting Snow with a Lagrangian Stochastic Model Based on Large-Eddy Simulations

2014 ◽  
Vol 153 (1) ◽  
pp. 117-139 ◽  
Author(s):  
C. D. Groot Zwaaftink ◽  
M. Diebold ◽  
S. Horender ◽  
J. Overney ◽  
G. Lieberherr ◽  
...  
Keyword(s):  
Stochastic Model ◽  
Large Eddy Simulations ◽  
Small Scale ◽  
Lagrangian Stochastic Model ◽  
Model Based ◽  
Drifting Snow ◽  
Large Eddy

scholarly journals An Anisotropic Subgrid-Scale Parameterization for Large-Eddy Simulations of Stratified Turbulence

2020 ◽  
Vol 148 (10) ◽  
pp. 4299-4311
Author(s):  
Sina Khani ◽  
Michael L. Waite
Keyword(s):  
Equations Of Motion ◽  
Computational Cost ◽  
Large Eddy Simulations ◽  
Subgrid Scale ◽  
Stratified Turbulence ◽  
Grid Spacing ◽  
Large Eddy ◽  
Vertical Grid ◽  
Ocean Models ◽  
Les Model

AbstractSubgrid-scale (SGS) parameterizations in atmosphere and ocean models are often defined independently in the horizontal and vertical directions because the grid spacing is not the same in these directions (anisotropic grids). In this paper, we introduce a new anisotropic SGS model in large-eddy simulations (LES) of stratified turbulence based on horizontal filtering of the equations of motion. Unlike the common horizontal SGS parameterizations in atmosphere and ocean models, the vertical derivatives of the horizontal SGS fluxes are included in our anisotropic SGS scheme, and therefore the horizontal and vertical SGS dissipation mechanisms are not disconnected in the newly developed model. Our model is tested with two vertical grid spacings and various horizontal resolutions, where the horizontal grid spacing is comparatively larger than that in the vertical. Our anisotropic LES model can successfully reproduce the results of direct numerical simulations, while the computational cost is significantly reduced in the LES. We suggest the new anisotropic SGS model as an alternative to current SGS parameterizations in atmosphere and ocean models, in which the schemes for horizontal and vertical scales are often decoupled. The new SGS scheme may improve the dissipative performance of atmosphere and ocean models without adding any backscatter or other energizing terms at small horizontal scales.

Assessment of Wall Modelling for Large Eddy Simulations of Turbomachinery

2018 ◽  
Author(s):  
Jagadeesh Movva ◽  
Dimitrios Papadogiannis ◽  
Stéphane Hiernaux
Keyword(s):  
Computational Cost ◽  
Large Eddy Simulations ◽  
Compressor Cascade ◽  
Potential Cost ◽  
Wall Model ◽  
Model Interaction ◽  
Large Eddy ◽  
Rans Simulations ◽  
The Impact ◽  
Near Wall

The design process of turbomachinery components relies heavily on Reynods Averaged Navier Stokes (RANS) simulations. This approach is well suited for steady simulations and comes with a low computational cost. However, turbomachinery flows are complex and difficult to predict accurately with RANS computations. Large Eddy Simulations (LES), capable of resolving the larger scales of turbulence, are a promising way to improve the predictive capability of numerical simulations. The main drawback of LES for wall bounded flows is its high computational cost, scaling with Re1.86 [1]. Turbomachinery components are characterized by Re ≈ 105–6, implying simulations with several billions of cells, with most allocated to resolve the turbulent scales inside the boundary layers. A potential cost-reducing approach is to introduce wall modelling. However, several questions remain, notably the wall model interaction with the laminar-to-turbulent transition and the impact of grid resolution. To clarify these points we investigate the flow across a linear compressor cascade with Wall Resolved LES (WRLES) and Wall Modelled LES (WMLES) simulations. Various near-wall resolutions are tested at on and off-design conditions to characterize the impact of the wall model on the flow field and the aerodynamic losses. RANS simulations complement the analysis. The results indicate that the WRLES agree the closest with experimental measurements. WMLES with relatively high near-wall resolution capture most of the flow physics while allowing a significant speed-up. However, reducing the resolution further leads to unphysical flow separations, despite staying well in the range of wall model validity.

scholarly journals On the Limits of Köhler Activation Theory: How do Collision and Coalescence Affect the Activation of Aerosols?

2017 ◽  
Author(s):  
Fabian Hoffmann
Keyword(s):  
Critical Radius ◽  
Cloud Model ◽  
Cloud Droplet ◽  
Aerosol Concentration ◽  
Cloud Base ◽  
Cloud Droplets ◽  
Activation Theory ◽  
Large Eddy ◽  
Kohler Theory ◽  
Additional Process

Abstract. Activation is necessary to form a cloud droplet from an aerosol, and it occurs as soon as a wetted aerosol grows beyond its critical radius. Traditional Köhler theory assumes that this growth is driven by the diffusion of water vapor. However, if the wetted aerosols are large enough, the coalescence of two or more particles is an additional process for accumulating sufficient water for activation. This transition from diffusional to collectional growth marks the limit of traditional Köhler theory and it is studied using a Lagrangian cloud model in which aerosols and cloud droplets are represented by individually simulated particles within large-eddy simulations of shallow cumuli. It is shown that the activation of aerosols larger than 0.1 μm in dry radius can be affected by collision and coalescence, and its contribution increases with a power-law relation toward larger radii and becomes the only process for the activation of aerosols larger than 0.4–0.8 μm depending on aerosol concentration. Due to the natural scarcity of the affected aerosols, the amount of aerosols that are activated by collection is small with a maximum of 1 in 10 000 activations. The fraction increases as the aerosol concentration increases, but decreases again as the number of aerosols becomes too high and the particles too small to cause collections. Moreover, activation by collection is found to affect primarily aerosols that have been entrained above the cloud base.

Corner Stall Prediction in a Compressor Linear Cascade Using Very Large Eddy Simulation (VLES) Lattice-Boltzmann Method

2019 ◽  
Author(s):  
Antoine Maros ◽  
Benoît Bonnal ◽  
Ignacio Gonzalez-Martino ◽  
James Kopriva ◽  
Francesco Polidoro
Keyword(s):  
Lattice Boltzmann Method ◽  
Lattice Boltzmann ◽  
Computational Cost ◽  
Large Eddy Simulations ◽  
Navier Stokes ◽  
Compressor Cascade ◽  
Numerical Work ◽  
Large Eddy ◽  
Boltzmann Method ◽  
Linear Compressor Cascade

Abstract Compressor corner stall is a phenomenon difficult to predict with numerical tools but essential to the design of axial compressors. Predictive methods are beneficial early in the design process to understand design and off-design limitations. Prior numerical work using Navier-Stokes computational methods have assessed the prediction capability for corner stall. Reynolds Averaged Navier-Stokes (RANS) simulations using several turbulence models [1] have shown to over-predict the region of corner hub stall where Large Eddy Simulations (LES) and Detached Eddy Simulations (DES) approaches improved the airfoil surface and wake pressure loss prediction [2, 3]. A linear compressor cascade designed and tested at Ecole Centrale de Lyon [3,4] provides a good benchmark for the evaluation of the accuracy of numerical methods for corner stall. This paper presents results obtained with Lattice-Boltzmann Method (LBM) coupled with Very Large-Eddy Simulations (VLES) approach of PowerFLOW and compares them with the experimental and numerical work from Ecole Centrale de Lyon. The ability to achieve equivalent accuracy at a lower computational cost compared to LES scale resolving methods can enable multi-stage design and off-design compressor predictions. A methodical approach is taken by first accurately simulating the upstream flow conditions. Geometric trips are modeled upstream on the endwalls to match both the mean and fluctuating inflow boundary layer conditions. These conditions were then applied to the simulation of the linear compressor cascade. The benchline experimental study includes trips on both the pressure and suction of the airfoil. These trips are also included for the current simulation. The significance of capturing both inflow conditions and including trips on the airfoil are assessed. Detailed comparisons are then made to airfoil loading and downstream losses between experiment and previous RANS and LES simulations. LBM-VLES corner stall results of pitchwise averaged total pressure match LES agreement relative to experimental data at 50 times lower computational cost.

scholarly journals Stochastic Subgrid Parameterizations for Simulations of Atmospheric Baroclinic Flows

2009 ◽  
Vol 66 (9) ◽  
pp. 2844-2858 ◽  
Author(s):  
Meelis J. Zidikheri ◽  
Jorgen S. Frederiksen
Keyword(s):  
Numerical Simulation ◽  
Stochastic Model ◽  
Large Scale ◽  
Large Eddy Simulations ◽  
Energy Spectra ◽  
Subgrid Scale ◽  
Turbulent Eddies ◽  
Large Eddy ◽  
Level Model ◽  
Two Level Model

Abstract A stochastic subgrid modeling method is used to parameterize horizontal and vertical subgrid-scale transfers in large-eddy simulations (LESs) of baroclinic flows with large-scale jets and energy spectra typical of the atmosphere. The approach represents the subgrid-scale eddies for LES (at resolutions of T63 and T31) by a stochastic model that takes into account the memory effects of turbulent eddies. The statistics of the model are determined from a higher-resolution (T126) direct numerical simulation (DNS). The simulations use a quasigeostrophic two-level model and the subgrid terms are inhomogeneous in the vertical and anisotropic in the horizontal and are represented by 2 × 2 matrices at each wavenumber. The parameterizations have the largest magnitudes at a cusp near the largest total wavenumbers of the truncations. At T63 the off-diagonal elements of the matrices are negligible (corresponding to effectively decoupled levels) and the diagonal elements are almost isotropic. At the lower resolution of T31 the off-diagonal elements are more important and even the diagonal elements are more anisotropic. At both resolutions, and for anisotropic or isotropized subgrid terms, LESs are in excellent agreement with higher-resolution DNS.

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