openfoam simulation of flow over ahmed body using visual cfd software

Srinivas Nag H.V QA &Central Support - CFD June16th, 2016

Simulation of Flow over Ahmed Body using OpenFoam -

Visual CFD software

Introduction

The aim of this study is to validate the Drag Coefficient of steady-state incompressible turbulent flow around the Ahmed Body

The aerodynamic behavior of the Ahmed body with slant angle 250 is investigated

numerically.

The Reynolds number Re of the flow is 2.8*106

The open source CFD tool OpenFoam 2.3.1 is tested for its ability to reproduce the

aerodynamic drag coefficient of the body.

External aero case setup is done using ESI’s software Visual-CFD 12.0 Drag coefficient is compared with the experimental results from literature.

Geometry

The generic Ahmed Body reference model has been chosen as the benchmark for

carrying out computations for studying of the aerodynamic parameters. The body was first proposed by Ahmed et al., (1984).The Ahmed body is a very simple bluff

body which has its shape simple enough to allow for accurate flow simulation but retains some important practical features relevant to automobile bodies. It has a slant on its rear end, whose angle can be manipulated and the corresponding drag

and lift coefficients calculated.


Geometry of Ahmed Body

Wind Tunnel STL format of 3D CAD model of Ahmed Body geometry with 250 slant angle is used for the analysis. Geometry is placed in wind tunnel of dimensions

(15mX1.4mX1.87m) with respect to X, Y and Z directions respectively.

Front space length is kept equal to 3 times the length of geometry and rear space length is kept 6 times length of geometry.

Wind tunnel has 6 faces namely XMin, XMax, YMin, YMax, ZMin and ZMax.

Geometry is located at ground YMin.

Geometry is mounted on 4 stilts. Projected area of geometry along positive X direction including stilts is 0.115 m2


Wind Tunnel of dimensions (15mX1.4mX1.87m)

Flow Physics Fluid Properties:

Fluid is Air

Density of the fluid rho = 1.225 kg/m3

Dynamic Viscosity of fluid µ = 1.84*10-5 Ns/m2

Kinematic Viscosity = Dynamic Viscosity/Density = 1.502*10-5 m2/s

Reynolds number of the flow Re = U*L/ = 2.8*106


Boundary Conditions:

Inlet (XMin): Fluid Velocity, U = 40 m/s

Outlet (XMax): No external pressure applied with zeroGradient Velocity

Wall: Geometry Surfaces and Ground (YMin)

Symmetry: ZMin, ZMax and YMax

Time dependence: Steady State analysis.

Turbulence Modelling

Turbulence quantities can be calculated by following formula

Turbulent Kinetic Energy k = 1.5(UI) 2

I is turbulent intensity in percentage

U is free stream velocity

Energy Dissipation omega= k0.5/ (Cu

0.25*l)

Where Cu is model constant = 0.09

l = length scale which can be estimated as 0.07L. L is characteristic length

Turbulence quantities used for study are mentioned below

k = 0.287 m2/s2, omega = 0.215 s-1

Turbulence model used is kOmegaSST

Spalding Wall functions (nutUSpaldingWallFunction in OpenFoam) is used on Walls.

Y-plus, First cell height and Boundary layers

The near wall region is meshed using the calculated first cell height value with gradual growth in the mesh so that the viscous effects are captured and avoiding overall heavy mesh count.


Yplus value should be in range 30-300 when Reynolds Average Stress turbulence (RAS) models are used. There are various Yplus online calculators available. We

have used the one from CFD Online

Mesh definition in Visual CFD

YPlus of 50 was chosen for this study and obtained first cell height (FCH) of 0.478 mm (~0.5mm). This is minimum cell size to be created near walls to accurately capture viscous effects. This given initial guess for user about the smallest cell size

to be created in computational domain to fairly capture the effects of fluid flow.


Boundary layer thickness can be calculated from below formula

δ = x*0.382/Rex0.2, where x is the distance in downstream where turbulent boundary

layer starts. Transition to turbulence will be at Re = 10*105. We have taken total

length of the geometry L = 1.044m and calculated total boundary layer thickness equal to 15mm.

So, in order to capture turbulent flow accurately, FCH should be within δ=15mm.

Meshing Base cell size (bsc) is set to 100mm.

‘blockMesh’ utility of OpenFOAM will create a structured hexahedral grid of 100 mm

(bsc) throughout wind tunnel. Output of blockMesh is input to ‘snappyHexMesh’ utility. This creates unstructured polyhedral meshes.

snappyHexMesh works in 3 stages.

1. Snapping: In this stage, fluid mesh is attached to geometry surfaces.

2. Castellation: In this stage, grid refinement occurs. 3. Layer addition: Snapped and Castellated mesh will be projected outwards

from geometry and boundary layers are created near walls.

Total drag force is the summation of Pressure force and Viscous force.

To capture drag due to viscous forces, we have to resolve by creating fine meshes and boundary layers near walls.

To capture drag due to pressure forces, we have to refine fluid cells in front and

rear part of the geometry. This is done by creating refinement regions Boundary layers creation

Surface mesh size on walls are set to 3.125mm. FCH in OpenFOAM is based on

surface mesh size. With relative mesh size on, FCH is set to 0.08. This means the first cell height will be equal to 0.08*3.125 = 0.25mm. Final layer thickness is 1.85mm

12 boundary layers are created with expansion ratio of 1.2. Total boundary

thickness is calculated from geometry progression and equal to 9.89mm. This covers around 66% of total boundary layer thickness. This should be sufficient to capture near wall viscous effects.

Another important parameter in creating boundary layers correctly is final layer

ratio which is equal to final layer thickness/surface cell size. This ratio is equal to


0.592 in this case. For proper layers to be created, this ratio should be within range of 0.2 to 0.6.

Boundary layers are created as shown below

Boundary layers generated from snappyHexMesh

Refinement Regions

Cell sizes at a given refinement level ‘n’ is obtained from following equation written below

CellSize@nthlevel = bsc/2n

Say n = 4, then cell size will be equal to 100/24 = 6.25mm

Four refinement regions are created with different levels of refinement as shown in figure below


Refinement Regions


Meshing started with 12 cores, 32GB RAM machine.

snappyHexMesh completed meshing within 10 minutes by generating ~4.7 million

cells.

OpenFoam mesh quality criteria is cleared and grid points are transformed to m.

Mesh renumbering is done to reduce band with. This will lead to faster calculation.

Numerical Solver Attributes

For velocity Gauss linearUpwindV scheme is used. For the rest of the variables

upwind schemes are used. All other attributes are kept with default value of Visual CFD.

Automatic switching of first order schemes to second order is used. Switching is

done after 200 iterations. First order schemes are numerically stable but less accurate. Second order schemes

are numerically unstable but more accurate. Hence first 200 iterations are run with first order and switched to second order schemes.


Numerical Solver attributes

Force Coefficients Calculation Force coefficients are calculated by formula

Cd = 2Fd/(rho*A*U2), where Fd is Drag force and Cd is drag coefficient

Cl= 2Fl/(rho*A*U2), where Fl is Lift force and Cl is lift coefficient

A is projected area along flow direction.

Force Coefficients inputs


Simulation

Flow is initialized using potentialFlow solution. This is done by executing potentialFoam solver.

simpleFoam solver is run for 5000 iterations.

Results

Velocity Contour


Velocity at x=1.244 m (behind slant)


Pressure Contour

Total Pressure Coefficient (Cp) distribution in flow field


Pressure Contour at x=1.244 m (behind slant)

Pressure Coefficient distribution at x=1.244m (behind slant)


Pressure Distribution on Surfaces

Pressure Coefficient Cp distribution on surfaces.


Cp = 1 indicates point of stagnation pressure

Cp = 0 indicates that pressure at a point is same as free stream pressure

Cp <1 indicates negative pressure or vacuum

Drag Coefficient (Cd)

Average Drag Coefficient value of last 500 iterations is 0.309.

Experimental Cd value = 0.300.

3% variation is found between experimental Cd and simulation Cd.

Drag Forces

Total force acting on geometry = 34.94 N

Drag Force due to Pressure forces = 29.41 N

Drag Force due to Viscous forces = 5.53 N From above values is it estimated that approximately 84.17% of total drag forces are

contributed by Pressure Forces and 15.83% of total drag forces are contributed by Viscous Forces.


References

https://www.learncax.com/knowledge-base/blog/by-category/cfd/basics-of-

y-plus-boundary-layer-and-wall-function-in-turbulent-flows

http://www.iosrjournals.org/iosr-jmce/papers/vol12-issue4/Version-

3/M012438794.pdf

http://www.computationalfluiddynamics.com.au/tag/wall-functions/

http://www.engr.uconn.edu/~wchiu/ME3250FluidDynamicsI/lecture%20notes/ch09.pdf

http://www.cfd-online.com/Tools/yplus.php

http://www.cfd-online.com/Wiki/Y_plus_wall_distance_estimation

https://en.wikipedia.org/wiki/Boundary_layer_thickness

https://www.simscale.com/docs/content/validation/AhmedBody/AhmedBody.

html#id7

CFD Simulation of Flow around External Vehicle: Ahmed Body Saurabh Banga1, Md. Zunaid2*, Naushad Ahmad Ansari3, Sagar Sharma4, Rohit Singh Dungriyal5

Pressure and Viscous force contribution on Total Drag

Pressure forces

Viscous Forces

https://www.learncax.com/knowledge-base/blog/by-category/cfd/basics-of-y-plus-boundary-layer-and-wall-function-in-turbulent-flows

https://www.learncax.com/knowledge-base/blog/by-category/cfd/basics-of-y-plus-boundary-layer-and-wall-function-in-turbulent-flows

http://www.iosrjournals.org/iosr-jmce/papers/vol12-issue4/Version-3/M012438794.pdf

http://www.iosrjournals.org/iosr-jmce/papers/vol12-issue4/Version-3/M012438794.pdf

http://www.computationalfluiddynamics.com.au/tag/wall-functions/



http://www.cfd-online.com/Tools/yplus.php


https://en.wikipedia.org/wiki/Boundary_layer_thickness

https://www.simscale.com/docs/content/validation/AhmedBody/AhmedBody.html#id7

https://www.simscale.com/docs/content/validation/AhmedBody/AhmedBody.html#id7


https://en.wikipedia.org/wiki/Turbulence_kinetic_energy

http://www.cd-

adapco.com/sites/default/files/conference_proceeding/pdf/Ahmed%20Body-50thAIAA.pdf

http://www.ijettjournal.org/volume-18/number-7/IJETT-V18P262.pdf

ERCOFTAC - Ahmed Body experimental values

http://cfd.mace.manchester.ac.uk/cgi-bin/cfddb/prpage.cgi?82&EXP&database/cases/case82/Case_data&database/cases/case82&cas82_head.html&cas82_desc.html&cas82_meth.html&cas82_

data.html&cas82_refs.html&cas82_rsol.html&1&0&0&0&0

http://cfd.mace.manchester.ac.uk/cgi-bin/cfddb/prpage.cgi?82&EXP&database/cases/case82/Case_data&database/

cases/case82&cas82_head.html&cas82_desc.html&cas82_meth.html&cas82_data.html&cas82_refs.html&cas82_rsol.html&1&1&1&1&0&unknown

http://user.engineering.uiowa.edu/~me_160/lab/cfdefdahmed.pdf

http://www.taoxing.net/web_documents/cfd_lab4.pdf

https://www.researchgate.net/publication/282272911_OpenFOAM_step_by_

step_tutorial

http://cfd.mace.manchester.ac.uk/twiki/pub/Main/TimCraftNotes_All_Access/advtm-wallfns.pdf


https://en.wikipedia.org/wiki/Turbulence_kinetic_energy

http://www.cd-adapco.com/sites/default/files/conference_proceeding/pdf/Ahmed%20Body-50thAIAA.pdf



http://www.ijettjournal.org/volume-18/number-7/IJETT-V18P262.pdf

http://cfd.mace.manchester.ac.uk/cgi-bin/cfddb/prpage.cgi?82&EXP&database/cases/case82/Case_data&database/cases/case82&cas82_head.html&cas82_desc.html&cas82_meth.html&cas82_data.html&cas82_refs.html&cas82_rsol.html&1&0&0&0&0




http://cfd.mace.manchester.ac.uk/cgi-bin/cfddb/prpage.cgi?82&EXP&database/cases/case82/Case_data&database/cases/case82&cas82_head.html&cas82_desc.html&cas82_meth.html&cas82_data.html&cas82_refs.html&cas82_rsol.html&1&1&1&1&0&unknown




http://user.engineering.uiowa.edu/~me_160/lab/cfdefdahmed.pdf

http://www.taoxing.net/web_documents/cfd_lab4.pdf

https://www.researchgate.net/publication/282272911_OpenFOAM_step_by_step_tutorial

https://www.researchgate.net/publication/282272911_OpenFOAM_step_by_step_tutorial




openfoam simulation of flow over ahmed body using visual cfd software

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