scholarly journals Vertically nested LES for high-resolution simulation of the surface layer in PALM (version 5.0)

2019 ◽  
Vol 12 (6) ◽  
pp. 2523-2538 ◽  
Author(s):  
Sadiq Huq ◽  
Frederik De Roo ◽  
Siegfried Raasch ◽  
Matthias Mauder
Keyword(s):  
Boundary Layer ◽  
Surface Layer ◽  
High Resolution ◽  
Large Eddy Simulation ◽  
Grid Resolution ◽  
Computational Power ◽  
Fine Grid ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Nesting Algorithm

Abstract. Large-eddy simulation (LES) has become a well-established tool in the atmospheric boundary layer research community to study turbulence. It allows three-dimensional realizations of the turbulent fields, which large-scale models and most experimental studies cannot yield. To resolve the largest eddies in the mixed layer, a moderate grid resolution in the range of 10 to 100 m is often sufficient, and these simulations can be run on a computing cluster with a few hundred processors or even on a workstation for simple configurations. The desired resolution is usually limited by the computational resources. However, to compare with tower measurements of turbulence and exchange fluxes in the surface layer, a much higher resolution is required. In spite of the growth in computational power, a high-resolution LES of the surface layer is often not feasible: to fully resolve the energy-containing eddies near the surface, a grid spacing of O(1 m) is required. One way to tackle this problem is to employ a vertical grid nesting technique, in which the surface is simulated at the necessary fine grid resolution, and it is coupled with a standard, coarse, LES that resolves the turbulence in the whole boundary layer. We modified the LES model PALM (Parallelized Large-eddy simulation Model) and implemented a two-way nesting technique, with coupling in both directions between the coarse and the fine grid. The coupling algorithm has to ensure correct boundary conditions for the fine grid. Our nesting algorithm is realized by modifying the standard third-order Runge–Kutta time stepping to allow communication of data between the two grids. The two grids are concurrently advanced in time while ensuring that the sum of resolved and sub-grid-scale kinetic energy is conserved. We design a validation test and show that the temporally averaged profiles from the fine grid agree well compared to the reference simulation with high resolution in the entire domain. The overall performance and scalability of the nesting algorithm is found to be satisfactory. Our nesting results in more than 80 % savings in computational power for 5 times higher resolution in each direction in the surface layer.

scholarly journals Vertically nested LES for high-resolution simulation of the surface layer in PALM (version 5.0)

2018 ◽  
Author(s):  
Sadiq Huq ◽  
Frederik De Roo ◽  
Siegfried Raasch ◽  
Matthias Mauder
Keyword(s):  
Boundary Layer ◽  
Surface Layer ◽  
High Resolution ◽  
Large Eddy Simulation ◽  
Computational Power ◽  
Fine Grid ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Nesting Algorithm ◽  
Resolution Simulation

Abstract. Large-eddy simulation (LES) has become a well-established tool in the atmospheric boundary-layer research community to study turbulence. It allows three-dimensional realizations of the turbulent fields, which large-scale models and most experimental studies cannot yield. To resolve the largest eddies in the mixed layer, a moderate grid resolution in the range of 10 to 100 m is often sufficient, and these simulations can be run on a computing cluster with few hundred processors, or even on a workstation for simple configurations. The desired resolution is usually limited by the computational resources. However, to compare with tower measurements of turbulence and exchange fluxes in the surface layer a much higher resolution is required. In spite of the growth in computational power, a high-resolution simulation LES of the surface layer is often not feasible: to fully resolve the energy containing eddies near the surface a grid spacing of O(1 m) is required. One way to tackle this problem is to employ a vertical grid nesting technique, where the surface is simulated at the necessary fine grid resolution, and it is coupled with a standard, coarse, LES that resolves the turbulence in the whole boundary-layer. We modified the LES model PALM (Parallelized Large-eddy simulation Model) and implemented a two-way nesting technique, with coupling in both directions between the coarse and the fine grid. The coupling algorithm has to ensure correct boundary conditions for the fine grid. Our nesting algorithm is realized by modifying the standard third order Runge-Kutta time stepping to allow communication of data between the two grids. The two grids are concurrently advanced in time while ensuring that the sum of resolved and subgrid-scale kinetic energy is conserved. We design a validation test and show that the temporal averaged profiles from the fine grid agree well compared to the reference simulation with high-resolution in the entire domain. The overall performance and scalability of the nesting algorithm is found to be satisfactory. Our nesting results in more than 80 percent savings in computational power for 5 times higher resolution in each direction in the surface layer.

Application of High-Resolution Large Eddy Simulation to Simplified Turbomachinery Flows

2016 ◽  
Author(s):  
Susumu Teramoto ◽  
Takuya Ouchi ◽  
Hiroki Sanada ◽  
Koji Okamoto
Keyword(s):  
Boundary Layer ◽  
High Resolution ◽  
Turbulent Boundary Layer ◽  
Large Eddy Simulation ◽  
Reynolds Stress ◽  
Sudden Expansion ◽  
Compressor Cascade ◽  
Rans Model ◽  
Eddy Simulation ◽  
Large Eddy

Fully resolved large eddy simulation (LES) is applied to two simple geometry flowfields with well-defined boundary conditions. The LES results are compared with simulations based on a Reynolds-averaged Navier-Stokes (RANS) model with turbulence, and pros and cons of using high-resolution LES for turbomachinery flows are discussed. One flow is a linear compressor cascade flow composed of the tip section of GE rotor B at Rec = 4 × 105 with a clearance, and the other is a Mach 1.76 supersonic turbulent boundary layer at Reδ = 5000 that laminerizes through a 12-degree expansion corner. The grids are prepared fine enough to resolve the turbulent boundary layer through a grid sensitivity study. The liner cascade result shows that all the turbulent shear layers and boundary layers including those in the small tip clearance are well resolved with 800 million grid points. The Reynolds stress derived from the LES results are compared directly with those predicted from the Spalart-Allmaras one-equation RANS turbulence model. The two results agreed qualitatively well except for the shear layer surrounding the tip leakage vortex, demonstrating that the RANS model performs well at least for flowfields near the design condition. From the simulation of the turbulent boundary layer experiencing sudden expansion, noticeable decreases of both Reynolds stress and local friction coefficient were observed, showing that the turbulent boundary layer has relaminarized through the sudden expansion. The boundary layer downstream of the expansion exhibits a nonequilibrium condition and was different from the laminar boundary layer.

Large Eddy Simulation of a Controlled-Diffusion Cascade Blade at Varying Flow Inlet Angles

2009 ◽  
Author(s):  
W. Andrew McMullan ◽  
Gary J. Page
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Leading Edge ◽  
Fine Grid ◽  
Controlled Diffusion ◽  
Eddy Simulation ◽  
Pressure Distributions ◽  
Wide Range ◽  
Large Eddy ◽  
Flow Features

A Controlled Diffusion cascade stator blade has been studied numerically using Large Eddy Simulation (LES). The aim of the study is to assess the performance of Large Eddy Simulation in predicting flow features on a highly-loaded blade, including leading-edge separation, transition and turbulent reattachment, particularly at off-design conditions. The need for LES to be performed on high resolution grids is highlighted by preliminary simulations on a mesh typically used in Reynolds-Averaged approaches. On a fine grid, the unsteady flow features captured by time-dependent simulation yield an improvement in surface pressure distributions and boundary layer profiles, although some weaknesses are apparent in the prediction of pressure-side boundary layer properties and wake profiles. The computed loss coefficients show potential for LES to be used to obtain loss-loop data over a wide range of incidence angles.

scholarly journals Sampling error in aircraft flux measurements based on a high-resolution large eddy simulation of the marine boundary layer

2021 ◽  
Vol 14 (3) ◽  
pp. 1959-1976
Author(s):  
Grant W. Petty
Keyword(s):  
Boundary Layer ◽  
High Resolution ◽  
Large Eddy Simulation ◽  
Random Error ◽  
Marine Boundary Layer ◽  
Sampling Error ◽  
Track Length ◽  
Length Scales ◽  
Eddy Simulation ◽  
Large Eddy

Abstract. A high-resolution (1.25 m) large eddy simulation (LES) of the nocturnal cloud-topped marine boundary layer is used to evaluate random error as a function of continuous track length L for virtual aircraft measurements of turbulent fluxes of sensible heat, latent heat, and horizontal momentum. Results are compared with the widely used formula of Lenschow and Stankov (1986). In support of these comparisons, the relevant integral length scales and correlations are evaluated and documented. It is shown that for heights up to approximately 100 m (z/zi=0.12), the length scales are accurately predicted by empirical expressions of the form If=Azb. The Lenschow and Stankov expression is found to be remarkably accurate at predicting the random error for shorter (7–10 km) flight tracks, but the empirically determined errors decay more rapidly with L than the L-1/2 relationship predicted from theory. Consistent with earlier findings, required track lengths to obtain useful precision increase sharply with altitude. In addition, an examination is undertaken of the role of uncertainties in empirically determined integral length scales and correlations in flux uncertainties as well as of the flux errors associated with crosswind and along-wind flight tracks. It is found that for 7.2 km flight tracks, flux errors are improved by factor of approximately 1.5 to 2 for most variables by making measurements in the crosswind direction.

scholarly journals A Nested Multi-Scale System Implemented in the Large-Eddy Simulation Model PALM model system 6.0

2020 ◽  
Author(s):  
Antti Hellsten ◽  
Klaus Ketelsen ◽  
Matthias Sühring ◽  
Mikko Auvinen ◽  
Björn Maronga ◽  
...  
Keyword(s):  
Boundary Layer ◽  
High Resolution ◽  
Large Eddy Simulation ◽  
Model System ◽  
Computational Domain ◽  
Convective Boundary ◽  
Large Eddy Simulation Model ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Parent Domain

Abstract. Large-eddy simulation provides a physically sound approach to study complex turbulent processes within the atmospheric boundary layer including urban boundary layer flows. However, such flow problems often involve a large separation of turbulent scales, requiring a large computational domain and very high grid resolution near the surface features, leading to prohibitive computational costs. To overcome this problem, an online LES-LES nesting scheme is implemented into the PALM model system 6.0. The hereby documented and evaluated nesting method is capable of supporting multiple child domains which can be nested within their parent domain either in a parallel or recursively cascading configuration. The nesting system is evaluated by simulating first a purely convective boundary layer flow system and then three different neutrally-stratified flow scenarios with increasing order of topographic complexity. The results of the nested runs are compared with corresponding non-nested high- and low-resolution results. The results reveal that the solution accuracy within the high-resolution nest domain is clearly improved as the solutions approach the non-nested high-resolution reference results. In obstacle-resolving LES, the two-way coupling becomes problematic as anterpolation introduces a regional discrepancy within the obstacle canopy of the parent domain. This is remedied by introducing canopy-restricted anterpolation where the operation is only performed above the obstacle canopy. The test simulations make evident that this approach is the most suitable coupling strategy for obstacle-resolving LES. The performed simulations testify that nesting can reduce the CPU time up to 80 % compared to the fine-resolution reference runs while the computational overhead from the nesting operations remained below 16 % for the two-way coupling approach and significantly less for the one-way alternative.

scholarly journals A nested multi-scale system implemented in the large-eddy simulation model PALM model system 6.0

2021 ◽  
Vol 14 (6) ◽  
pp. 3185-3214
Author(s):  
Antti Hellsten ◽  
Klaus Ketelsen ◽  
Matthias Sühring ◽  
Mikko Auvinen ◽  
Björn Maronga ◽  
...  
Keyword(s):  
Boundary Layer ◽  
High Resolution ◽  
Large Eddy Simulation ◽  
Model System ◽  
Computational Domain ◽  
Convective Boundary ◽  
Large Eddy Simulation Model ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Parent Domain

Abstract. Large-eddy simulation (LES) provides a physically sound approach to study complex turbulent processes within the atmospheric boundary layer including urban boundary layer flows. However, such flow problems often involve a large separation of turbulent scales, requiring a large computational domain and very high grid resolution near the surface features, leading to prohibitive computational costs. To overcome this problem, an online LES–LES nesting scheme is implemented into the PALM model system 6.0. The hereby documented and evaluated nesting method is capable of supporting multiple child domains, which can be nested within their parent domain either in a parallel or recursively cascading configuration. The nesting system is evaluated by first simulating a purely convective boundary layer flow system and then three different neutrally stratified flow scenarios with increasing order of topographic complexity. The results of the nested runs are compared with corresponding non-nested high- and low-resolution results. The results reveal that the solution accuracy within the high-resolution nest domain is clearly improved as the solutions approach the non-nested high-resolution reference results. In obstacle-resolving LES, the two-way coupling becomes problematic as anterpolation introduces a regional discrepancy within the obstacle canopy of the parent domain. This is remedied by introducing canopy-restricted anterpolation where the operation is only performed above the obstacle canopy. The test simulations make evident that this approach is the most suitable coupling strategy for obstacle-resolving LES. The performed simulations testify that nesting can reduce the CPU time up to 80 % compared to the fine-resolution reference runs, while the computational overhead from the nesting operations remained below 16 % for the two-way coupling approach and significantly less for the one-way alternative.

scholarly journals The Effect of Mesh Resolution on Convective Boundary Layer Statistics and Structures Generated by Large-Eddy Simulation

2011 ◽  
Vol 68 (10) ◽  
pp. 2395-2415 ◽  
Author(s):  
Peter P. Sullivan ◽  
Edward G. Patton
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Grid Resolution ◽  
Inertial Subrange ◽  
Entrainment Rate ◽  
Layer Height ◽  
Boundary Layer Height ◽  
Eddy Simulation ◽  
Solution Convergence ◽  
Large Eddy

Abstract A massively parallel large-eddy simulation (LES) code for planetary boundary layers (PBLs) that utilizes pseudospectral differencing in horizontal planes and solves an elliptic pressure equation is described. As an application, this code is used to examine the numerical convergence of the three-dimensional time-dependent simulations of a weakly sheared daytime convective PBL on meshes varying from 323 to 10243 grid points. Based on the variation of the second-order statistics, energy spectra, and entrainment statistics, LES solutions converge provided there is adequate separation between the energy-containing eddies and those near the filter cutoff scale. For the convective PBL studied, the majority of the low-order moment statistics (means, variances, and fluxes) become grid independent when the ratio zi/(CsΔf) > 310, where zi is the boundary layer height, Δf is the filter cutoff scale, and Cs is the Smagorinsky constant. In this regime, the spectra show clear Kolmogorov inertial subrange scaling. The bulk entrainment rate determined from the time variation of the boundary layer height we = dzi/dt is a sensitive measure of the LES solution convergence; we becomes grid independent when the vertical grid resolution is able to capture both the mean structure of the overlying inversion and the turbulence. For all mesh resolutions used, the vertical temperature flux profile varies linearly over the interior of the boundary layer and the minimum temperature flux is approximately −0.2 of the surface heat flux. Thus, these metrics are inadequate measures of solution convergence. The variation of the vertical velocity skewness and third-order moments expose the LES’s sensitivity to grid resolution.

Large Eddy Simulation of a Submerged Vortex in a Simplified Computational Model

2019 ◽  
Author(s):  
Yoshinobu Yamade ◽  
Chisachi Kato ◽  
Takahide Nagahara ◽  
Jun Matsui
Keyword(s):  
Large Eddy Simulation ◽  
Computational Model ◽  
Vortex Core ◽  
Static Pressure ◽  
Grid Resolution ◽  
Simplified Model ◽  
Fine Grid ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Pump Sump

Abstract The flow structures of a submerged vortex that appears in a model pump sump were numerically investigated by performing large eddy simulation (LES) of a model vortex in a simplified computational model with a sufficiently fine grid that could resolve the vortex core. The simplified model is designed to simulate the flow under the bellmouth in a model pump sump. The model pump sump is composed of a 2,500 mm-long water channel of rectangular cross section with a width of 300 mm, a water height of 150 mm and a vertical suction pipe with a diameter of 100 mm installed at its downstream end. Our previous large eddy simulations, which used approximately 2 billion grids and were applied to the model pump sump, have fully clarified the origin and formation mechanism of a submerged vortex. In these computations, however, the static pressure in the vortex core decreased only by as much as 4 kPa at a channel velocity of 0.37 m/s. The decrease in the static pressure was far smaller than the one for which one can expect initiation of cavitation in the vortex core. The static pressure drop was most likely to be underpredicted in our previous LES. Insufficient grid resolution was assumed to be one of the reasons for this underprediction. In the present study, LES with a sufficiently fine grid was applied to the simplified computational model that represents the stretch of a submerged vortex under a constant acceleration of the vertical velocity. Case studies for which the grid resolution was varied between 3.25 and 150 micrometres were performed while the size of the vortex core appeared in the simplified model was 500 micrometres. As a result, we confirmed the grid resolution finer than 15 micrometres is needed to resolve the vortex core with a diameter of 500 micrometres. Vertical and tangential velocities obtained by averaging those distributions of a submerged vortex that was computed in our previous LES were prescribed at the bottom wall of the computational domain as the inlet boundary conditions. In the present LES with the grid resolution finer than 15 micrometres, the static pressure decreased by more than 100 kPa. In addition, the parametric studies where the initial swirl numbers were changed have fully clarified the change in the dynamics of a submerged vortex. We found that a strong submerged vortex appears only at a relatively small range of the swirl-number from 1 to 3.

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