scholarly journals Importance of Sub-Grid Scale Modeling for Accurate Aerodynamic Simulations

2020 ◽  
Vol 143 (1) ◽  
Author(s):  
Petter Ekman ◽  
James Venning ◽  
Torbjörn Virdung ◽  
Matts Karlsson
Keyword(s):  
The Body ◽  
Small Scale ◽  
Piv Measurements ◽  
Vortical Structures ◽  
Scale Modeling ◽  
Simple Geometry ◽  
Image Velocimetry ◽  
Large Eddy ◽  
Near Wall ◽  
Ahmed Body

Abstract The Ahmed body is one of the most well-investigated vehicle bodies for aerodynamic purposes. Despite its simple geometry, the flow around the body, especially at the rear, is very complex as it is dominated by a large wake with strong interaction between vortical structures. In this study, the flow around the 25 deg Ahmed body has been investigated using large eddy simulations and compared to high-resolution particle image velocimetry (PIV) measurements. Special emphasis was put on studying three commonly used sub-grid scale (SGS) models and their ability to capture vortical structures around the Ahmed body. The ability of the SGS models to capture the near-wall behavior and small-scale dissipation is crucial for capturing the correct flow field. Very good agreement between simulations and PIV measurements were seen when using the dynamic Smagorinsky-Lilly and the wall-adopting local eddy-viscosity SGS models, respectively. However, the standard Smagorinsky-Lilly model was not able to capture the flow patterns when compared to the PIV measurements due to shortcomings in the near-wall modeling in the standard Smagorinsky-Lilly model, resulting in overpredicted separation.

scholarly journals Vertically localised equilibrium solutions in large-eddy simulations of homogeneous shear flow

2017 ◽  
Vol 827 ◽  
pp. 225-249 ◽  
Author(s):  
Atsushi Sekimoto ◽  
Javier Jiménez
Keyword(s):  
Shear Flow ◽  
Vertical Direction ◽  
Large Eddy Simulations ◽  
Small Scale ◽  
Box Dimension ◽  
Equilibrium Solutions ◽  
Vortical Structures ◽  
Homogeneous Shear ◽  
Large Eddy ◽  
Homogeneous Shear Flow

Unstable equilibrium solutions in a homogeneous shear flow with sinuous (streamwise-shift-reflection and spanwise-shift-rotation) symmetry are numerically found in large-eddy simulations (LES) with no kinetic viscosity. The small-scale properties are determined by the mixing length scale $l_{S}$ used to define eddy viscosity, and the large-scale motion is induced by the mean shear at the integral scale, which is limited by the spanwise box dimension $L_{z}$. The fraction $R_{S}=L_{z}/l_{S}$, which plays the role of a Reynolds number, is used as a numerical continuation parameter. It is shown that equilibrium solutions appear by a saddle-node bifurcation as $R_{S}$ increases, and that the flow structures resemble those in plane Couette flow with the same sinuous symmetry. The vortical structures of both lower- and upper-branch solutions become spontaneously localised in the vertical direction. The lower-branch solution is an edge state at low $R_{S}$, and takes the form of a thin critical layer as $R_{S}$ increases, as in the asymptotic theory of generic shear flow at high Reynolds numbers. On the other hand, the upper-branch solutions are characterised by a tall velocity streak with multiscale multiple vortical structures. At the higher end of $R_{S}$, an incipient multiscale structure is found. The LES turbulence occasionally visits vertically localised states whose vortical structure resembles the present vertically localised LES equilibria.

Wave-in-Deck Impact Loads in Relation With Wave Kinematics

2017 ◽  
Author(s):  
Jule Scharnke ◽  
Rene Lindeboom ◽  
Bulent Duz
Keyword(s):  
Particle Image Velocimetry ◽  
Particle Image ◽  
Impact Load ◽  
Breaking Waves ◽  
Offshore Structures ◽  
Small Scale ◽  
Impact Loads ◽  
Piv Measurements ◽  
Wave Kinematics ◽  
Image Velocimetry

Breaking waves have been studied for many decades and are still of interest as these waves contribute significantly to the dynamics and loading of offshore structures. In current MARIN research this awareness has led to the setup of an experiment to determine the kinematics of breaking waves using Particle Image Velocimetry (PIV). The purpose of the measurement campaign is to determine the evolution of the kinematics of breaking focussed waves. In addition to the PIV measurements in waves, small scale wave-in-deck impact load measurements on a fixed deck box were carried out in the same wave conditions. To investigate the link between wave kinematics and wave-in-deck impact loads, simplified loading models for estimating horizontal deck impact loads were applied and compared to the measured impact loads. In this paper, the comparison of the model test data to estimated loads is presented.

Flow Regime Characteristics in Porous Media Flows at High Reynolds Numbers

2012 ◽  
Author(s):  
Vishal A. Patil ◽  
James A. Liburdy
Keyword(s):  
Turbulent Flow ◽  
Flow Characteristics ◽  
Reynolds Numbers ◽  
Small Scale ◽  
Mean Velocity ◽  
Time Resolved ◽  
Vortical Structures ◽  
Large Eddy ◽  
High Reynolds ◽  
Media Flows

An experimental study on the turbulent flow characteristics in a randomly packed porous bed is presented and discussed. Time resolved PIV measurements, taken in specific pore spaces are used to evaluate transitional and developed turbulent flow statistics for pore Reynolds numbers from 54 to 3964. Three different regimes of steady laminar, transitional and turbulent flow are presented. Small scale coherent vortical structures are examined, using large eddy scale (LES) decomposition, for pore Reynolds number of greater than 1000. Integral length scales were found to reach asymptotic values of approximately 0.1 times the hydraulic diameter of the bed. The integral Eulerian time scales are found to reach an asymptotic value of approximately 0.3 times the convective time scale in the bed. Mean velocity vector maps show flattening of the velocity distribution due to increased momentum mixing. Turbulent stresses show increasing level of homogeneity at higher pore Reynolds numbers.

Turbulent Flow Around a Semi-Submerged Rectangular Cylinder

2013 ◽  
Vol 135 (4) ◽  
Author(s):  
Tufan Arslan ◽  
Stefano Malavasi ◽  
Bjørnar Pettersen ◽  
Helge I. Andersson
Keyword(s):  
Three Dimensional ◽  
Separated Flow ◽  
Piv Measurements ◽  
Eddy Simulation ◽  
Rectangular Cylinder ◽  
Image Velocimetry ◽  
Large Eddy ◽  
3D Unsteady Flow ◽  
Computational Fluid Dynamics Cfd

The present work is motivated by phenomena occurring in the flow field around structures partly submerged in water. A three-dimensional (3D) unsteady flow around a rectangular cylinder is studied for four different submergence ratios by using computational fluid dynamics (CFD) tools with the large eddy simulation (LES) turbulence model. The simulation results are compared to particle image velocimetry (PIV) measurements at the Reynolds number Re = 12,100 and the Froude number Fr = 0.26. The focus in our investigation is on the characterization of the behavior of vortex structures generated by separated flow. Another target in the study is to obtain a better knowledge of the hydrodynamic forces acting on a semi-submerged structure. The computed force coefficients are compared with experimental measurements.

Subgrid-Scale Modeling of Turbulent Convection Using Truncated Navier-Stokes Dynamics

2002 ◽  
Vol 124 (4) ◽  
pp. 823-828 ◽  
Author(s):  
J. A. Domaradzki ◽  
S. Radhakrishnan
Keyword(s):  
Stokes Equations ◽  
Navier Stokes ◽  
Small Scale ◽  
Subgrid Scale ◽  
Navier Stokes Equation ◽  
Navier Stokes Equations ◽  
Scale Modeling ◽  
Large Eddy ◽  
Mesh Resolution ◽  
Subgrid Scale Modeling

Using concepts from the subgrid-scale estimation modeling we develop a procedure for large-eddy simulations which employs Navier-Stokes equations truncated to an available mesh resolution. Operationally the procedure consists of numerically solving the truncated Navier-Stokes equation and a periodic processing of the small scale component of its solution. The modeling procedure is applied to simulate turbulent Rayleigh-Be´nard convection.

scholarly journals The nature of the vortical structures in the near wake of the Ahmed body

2017 ◽  
Vol 231 (9) ◽  
pp. 1239-1244 ◽  
Author(s):  
James Venning ◽  
David Lo Jacono ◽  
David Burton ◽  
Mark C Thompson ◽  
John Sheridan
Keyword(s):  
Water Channel ◽  
Field Measurements ◽  
The Body ◽  
Wake Flow ◽  
Generic Model ◽  
Toroidal Vortex ◽  
Near Wake ◽  
Slant Angle ◽  
Vortical Structures ◽  
Ahmed Body

This study presents the results from high-spatial-resolution water-channel velocity-field measurements behind an Ahmed body with 25° rear slant angle. The Ahmed body represents a simplified generic model of a hatchback automobile that has been widely used to study near-wake flow dynamics. The results help clarify the unresolved question of whether the time-mean near-wake flow structure is topologically equivalent to a toroidal vortex or better described by a pair of horizontally aligned horseshoe vortices, with their legs pointing downstream. The velocimetry data presented allows the tracking of the vortical structures throughout the near wake through a set of orthogonal planes, as well as the measurement of their circulation. The spanwise vortices that form as the flow separates from the top and bottom rear edges are shown to tilt downstream at the sides of the body, while no evidence is found of a time-mean attached toroidal vortex, at least for the Reynolds number (based on the square root of the frontal area) of [Formula: see text] under consideration.

Experimental Measurements in Near-Wall Regions by Particle Image Velocimetry (PIV)

2014 ◽  
Author(s):  
Puxuan Li ◽  
Steve J. Eckels ◽  
Garrett W. Mann ◽  
Ning Zhang
Keyword(s):  
Particle Image Velocimetry ◽  
Particle Image ◽  
Particle Density ◽  
Piv Measurements ◽  
Experimental Fluid Mechanics ◽  
Micro Piv ◽  
Piv Measurement ◽  
Image Velocimetry ◽  
Eddy Structures ◽  
Near Wall

The current study investigates the flow field near a surface with a micro-PIV system using a square tube to enhance optical access. Measurements of velocity fields and eddy structures near the wall of tubes are important to the design of in-tube surface geometries. In experimental fluid mechanics, particle image velocimetry (PIV) is now a common way to measure velocity. However, PIV measurements near walls require efforts to deal with low particle density, high shear gradient and wall reflection. The current paper discusses a PIV measurement technique utilized to observe flow dynamics in near-wall regions. PIV uncertainty analysis is discussed in this study. The experimental results are compared with previous results for validation.

High-Cycle Thermal Fatigue in Mixing Tees: Large-Eddy Simulations Compared to a New Validation Experiment

2008 ◽  
Author(s):  
Johan Westin ◽  
Pascal Veber ◽  
Lars Andersson ◽  
Carsten ’t Mannetje ◽  
Urban Andersson ◽  
...  
Keyword(s):  
Experimental Data ◽  
Large Scale ◽  
Turbulent Viscosity ◽  
Large Eddy Simulations ◽  
Small Scale ◽  
Wall Velocity ◽  
Fine Mesh ◽  
Validation Experiment ◽  
Large Eddy ◽  
Near Wall

The present paper describes new experimental data of thermal mixing in a T-junction compared with results from Large-Eddy Simulations (LES) and Detached Eddy Simulations (DES). The experimental setup was designed in order to provide data suitable for validation of CFD-calculations. The data is obtained from temperature measurements with thermocouples located near the pipe wall, velocity measurements with Laser Doppler Velocimetry (LDV) as well as single-point concentration measurements with Laser Induced Fluorescence (LIF). The LES showed good agreement with the experimental data also when fairly coarse computational meshes were used. However, grid refinement studies revealed a fairly strong sensitivity to the grid resolution, and a simulation using a fine mesh with nearly 10 million cells significantly improved the results in the entire flow domain. The sensitivity to different unsteady inlet boundary conditions was however small, which shows that the strong large-scale instabilities that are present in the mixing region are triggered independent of the applied inlet perturbations. A shortcoming in the performed simulations is insufficient near-wall resolution, which resulted in poor predictions of the near-wall mean velocity profiles and the wall-shear stress. Simulations using DES improved the near-wall velocity predictions, but failed to predict the temperature fluctuations due to high levels of modeled turbulent viscosity that restrained the formation of small scale turbulence.

Large Eddy Simulations of Static and Rotating Ribbed Channels in Adiabatic and Isothermal Conditions

2017 ◽  
Author(s):  
Thomas Grosnickel ◽  
Florent Duchaine ◽  
Laurent Y. M. Gicquel ◽  
Charlie Koupper
Keyword(s):  
Flow Direction ◽  
Secondary Flows ◽  
Flow Topology ◽  
Time Resolved ◽  
Piv Measurements ◽  
Eddy Simulation ◽  
Image Velocimetry ◽  
Large Eddy ◽  
Rib Turbulators ◽  
The Impact

In an attempt to better understand spatially developing rotating cooling flows, the present study focuses on a computational investigation of a straight, rotating rib roughened cooling channel initially numerically studied by Fransen et al. [1]. The configuration consists of a squared channel equipped with 8 rib turbulators placed with an angle of 90 degrees with respect to the flow direction. The rib pitch-to-height (p/h) ratio is 10 and the height-to-hydraulic diameter (h/Dh) ratio is 0.1. The simulations are based on a case where time resolved two-dimensional Particle Image Velocimetry (PIV) measurements have been performed at the Von Karman Institute (VKI) in a near gas turbine operating condition: the Reynolds number (Re) and the rotation number (Ro) are around 15000 and ± 0.38 respectively. Adiabatic as well as anisothermal conditions have been investigated to evaluate the impact of the wall temperature on the flow, especially in the rotating configurations. Static as well as both positive and negative rotating channels are compared with experimental data. In each case, either an adiabatic or an isothermal wall boundary condition can be computed. In this work, Large Eddy Simulation (LES) results show that the high fidelity CFD model manages very well the turbulence increase (decrease) around the rib in destabilizing (stabilizing) rotation of the ribbed channels. Thanks to the full spatial and temporal description produced by LES, the spatial development of secondary flows are found to be at the origine of observed differences with experimental measurements. Finally, the model is also able to reproduce the differences induced by buoyancy on the flow topology in the near rib region and resulting from an anisothermal flow in rotation.

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