AEE 558 COMPUTATIONAL FLUID DYNAMICS

Flow Over AHMED Configuration and itsAerodynamic Effects on Adjacent ModelsUsing ANSYS-FLUENT

MOHAMMED ZOHEB HUSSAIN4/23/2014

SUMMARY:

In this project we perform the investigation to predict the flowbehavior over the external surfaces. A simple car-like geometryis used in order to study the aerodynamic effects on it and thevehicle adjacent to it, by varying the spacing’s between them.The Ahmed model is a simple geometric body that retains the mainow features, specially the vortex wake ow where most part offl fl

the drag is concentrated and it is a good model to be used as abenchmark test as a vehicle body in automotive industry.Understanding this aerodynamic phenomenon helps us in reducingfuel consumption. The solver Used is FLUENT developed by ANSYS,with various turbulence models in order to reduce thecomputational time at high Reynolds number.

ANSYS is used to create the geometry and the Mesh and it isexported to FLUENT and then it is run to obtain the requiredsolutions. FLUENT converges the problem until the convergencelimit is met or the numbers of specified iterations are achieved.

The results are reported in the form of lift and drag coefficientvalues and flow field which includes pressure, velocity andturbulence contour and velocity vector fields. The results arecompared with those previously published actual wind-tunnelexperimental data.

In this work a large eddy simulation turbulence model isemployed. Moreover, the capabilities of FLUENT to predict theflow around the Ahmed Body has been analyzed by comparing theflow structure at wake region with experiments.

INTRODUCTION:

GEOMETRICAL DESCRIPTION:

We represent the vehicle body in two dimensions, and a flowdomain is created surrounding to it. The body geometry is shownin Fig.1. The characteristic length of the body is given asL=1.044m, the slant edge is at an angle of 30 degrees from to thetop edge and The size of the computation domain is 29.232m longand 4.126m high, with the down stream portion behind the rear ofthe Ahmed body is 19.836m. The projected surface area of theAhmed body in the mainstream direction is 0.112m2. The Ahmed bodyis suspended in the air with bottom surface is at 0.05m off theground.

BOUNDARY CONDITIONS:

The boundary conditions for the problem are given, with theReynolds number of 2.3. 106 the inlet velocity is calculated andset as 32.1 m/s. Air enters the computational domain at a freestream velocity u∞=32.1 m/s normal to the inlet surface. Thelower wall for the floor till the body is described by wallfunctions and is set as moving wall with velocity equal to thatof inlet so as to avoid boundary layer formation. In the realworld as we don’t see any boundary layer forming before themoving car as there is no velocity of the airflow, with the body

moving with the certain velocity. In this computational domain wetry to create the identical flow condition but instead of movingthe body the wall is moved with the velocity that of the freestream. So, we see that velocity of the flow in the domain beforeit hits the body is constant. Wall functions could also beapplied to the outer wall and the ceiling wall of the domain buthowever, their main effect is to keep the flow contained aroundthe body, so they are just set as wall. The ground and the bodysurfaces are treated as no-slip smooth walls. The temperature isassumed to be 293 K and the reference pressure is 1 atm. Heattransfer is not considered in this study. The Experiment isperformed in air so the density given is constant as 1.225kg/m3

and the viscosity is 0.000017894 kg/m-s. Outflow is used as theOutlet boundary.

INFLOW VELOCITY INLETOUTLET OUTFLOWBODY WALLLOWER WALL BEFORE BODY MOVING WALL=INLET

VELOCITYINTERIOR SURFACE BODY INTERIOR

TURBULENCE MODEL:

The Reynolds number with respect to the length of the body, Lwas given as 2.3*106 which means that the flow is turbulent. TheModel is then run on the FLUENT using different turbulent modelsso as to determine which model yields the satisfactory resultswhen compared to reference results. The k-ε turbulence model wasapplied at first to account for the turbulence. The K-omega,Spalart-Allmaras and the transition model are the other modelsthat were used to study the flow.

SINGLE MODEL:

The AHMED model is created in the ANSYS with the dimension that were provided which are identical with the original AHMED body. The domain is split near the body so as to study the flow more indepth. The Mesh is made very finer with a lot number of elements in this spilt region because the flow in this region is interesting and of our concern. The Mesh is kept coarser where there is no disturbance in the flow and is boring.

MESH:

The Geometry of the Model is made and then it is updated intothe Mesher. The number of elements in the split domain isincreased by increasing the number of divisions on the edges andthen giving sphere of influence on the split face. A fine mesh isgenerated and is shown in fig 2 with the total number of nodes inthe whole domain is 201223.

FIG 2 Mesh around the AHMED body in the Split Face.

Fig 3 Closer view of mesh on the edges of the model.

Different mesh densities have been used to understand the flowbehavior on the body clearly. Mapped face sizing was selectedfor the domain except the split region. The Inflation on all theedges of the AHMED body was given with first layer thickness of0.0001mm. The first layer thickness was decided based on wall y+.The desired Y plus value indicate the distance up to which theboundary layer effects will be pronounced. The main reason is tocapture the boundary layer on the edge.

CONVERGENECE HISTORY:

Fig 4 Residuals Vs Number of Iterations

The mesh generated is then exported to FLUENT. The input boundarycondition values are specified and the program is run for 10-5

number of residuals. The Fig 4 shows the convergence history forthe turbulent flow with free stream velocity as 32.1m/s and thefluid as air. The solution is converged after 2389 iterations.The Turbulence model used was SST k- Omega.

PRESSURE CONTOURS AROUND THE AHMED MODEL:

Fig 5 Contour of Pressure around the AHMED Body

The Fig 5 shows a contour plot of the pressure on the body. As we can see that the Pressure at the front face of the body is very high due to the obstruction of the flow. The sliced plot of the pressure shows that the flow for this Reynolds number is wellresolved except perhaps in the wake. We can Observe the wake in the below figure.

TURBULENCE CONTOURS:

Fig 6 Turbulence Contours showing wake

The Fig 6 shows the contours of Turbulence kinetic energy. Wecan see that there is a wake after the body. The wake is becauseof stagnation, boundary layer formation and flow separation. Themain Reynolds number effect in the wake is expected to be thelength of the recirculation. It is therefore probable that themesh resolution in the wake will suffice to converge a simulationat a higher Reynolds number too.

PATHLINES AND VELOCITY VECTORS:

Fig 7 Vel Path lines showing recirculation Bubbles Fig 8 Vel Vectors on the top surface of the Body

The figure 7 shows us the velocity path lines after the body. Themost notable feature of the flow field is an “empty” regionbehind the body. The region behind the slant surface, which isbounded by the separating and reattaching layer on the top and bythe lower wall on the bottom are the recirculation zones. Theflow gets re circulated in this region. We can see two bubblesformed in this region. The pressure difference can be observed inthis region. The thickness of the lines is given by the turbulentkinetic energy. The streamlines on the edge of the region arethick but with low velocity magnitude. This region is constitutedof the recirculation vortices visible in Figure 7. The regionends when vortices from the trailing edges of the body merge intotwo counter rotating vortices.

Figure 8 shows the Velocity vectors on the top edge of the theAHMED body. The vectors represent the positions and direction ofmotion of different single flow particles, with the length of thevector showing its magnitude. The velocity of a particle near the

wall is given by the small vector, which tells us that the flowis moving with lesser magnitude. However, the velocity of theflow increases as it moves away from the wall, the far theparticle is from the wall the less opposing force it experiencesand thus the faster it can move. The velocity vector shows thatflow is moving constant on the top surface of the body.

TWO MODELS:

Now we try to study the aerodynamic effects on the bodies when weplace a vehicle adjacent to the single model. The second model ismade just as identical to the first so as to understand theeffects clearly on such bluff bodies. The Body is created and isplaced adjacent to it with the spacing calculated with respect tothe length of the body, i.e. x/L. where x is distance between thetwo bodies. A complete investigation is performed on the effectsof the leading model and trailing model on each other.

MESH:

The Bodies are created in the using the ANSYS geometry option andthen it is imported to Mesher. The spacing’s between the twomodels are varied in order to get the idea and determine theideal case where there is very little effect of the trailingmodel on the leading model and vice versa. However in thisproject we have not gone much in depth to determine the idealcase. The Different families and different densities of mesheshave been created so as to capture the important and interestingbehaviors around the bodies.

Fig 9 Fine mesh of 2 bodies with 3 spacing Fig 10 closer view ofMesh near the edge

Figure 9 and 10 shows the Schematic representation of uniform 2-Dgrid structure, in the computational domain where the grids arequadrilateral in shape. Owing to the geometric shape and studyrequirements, the grid generated is made more dense near the bodywalls to capture the boundary layer. The Y+ value is used increating the geometry and the layer thickness provided was0.00001. The mesh generated consists of 382398 nodes in it.

CONTOURS OF PRESSURE VELOCITY AND TURBULENCE :

Fig 11 Pressure contours For 2 spacing model

Fig 12 Velocity Contours showing wake behind the leading model

Fig 13 Turbulence Kinetic energy contours showing interesting flow

MESH single L=1.044 L=2.088 L=2.61L=3.13

2Coarse 15619 26145 12985 16825 29978Medium 89348 105866 101366 132826 124154Fine 201223 247165 345873 264512 388398

The above table shows the different mesh densities tried on different Ahmed models with various spacing’s. This meshes yield us different results and that has been discussed below.

The Fig 11 shows us the pressure contours of the two Ahmed modelswith spacing 2 between them. It is seen from the figure that theleading model has lot of effect on the trailing model and thevortices created behind the first model acts as the inflow for the

second model. Thus, the second model should have comparativelyless drag when compared to that of leading model.

The Fig 12 shows us the velocity contours of the whole domainwhere the eddies behind the trailing model can be seen clearly.This eddies do not have constant shape and they change theirbehavior with space due to the pressure variation along thelength. This Vortices act as an inlet for the trailing model anddoes not get a free stream uniform flow.

The Fig 13 shows the turbulence kinetic energy contours. This clearly portrays the eddy vortices behind the model which get more worsen as they pass the second model and create a big large vortex behind the second model.

DRAG AND PRESSURE MEASUREMENTS

See below table 1 calculate the main drag values for both theleading and trailing surfaces of the Ahmed body. Here we canobserve that the contribution of the front part of the body isvery small compared with the total pressure drag. The interactionbetween the fore body and the rear end part is weak as aconsequence of the long distance between these two parts in termsof ow features. Therefore, the major contributions to theflpressure drag come from the slant and vertical base rear part. Weonly analyze the pressure drag contributions it should bementioned that the viscous effects add some drag mostly at thelateral parts of the body.

L=1.044 NODES MODEL

Iterations CL1 CL2 CD1 CD2

L=1.044

247165

K-eps enhanced 4245

0.6184 0.18 0.2075

0.2998

L=2.088 12985

K-Eps Enhanced 4126

0.6912 0.3412 0.3828

0.32707

L=2.088

101366

K-Eps Enhanced 6156

0.6102 0.29966 0.3817

0.3538

L=2.0 34587 K-eps 1955 0.60 0.3377 0.3359 0.316

88 3 enhanced 27 8 86L=3.132

382398

K-eps Enhanced 1752 0.57 0.376 0.3517

0.3696

L=3.132

382398

spalartaalmaras 1711

0.606 0.31 0.44

0.3606

The above table shows us the different cases run having differentmesh densities and different models. Also plotted is the drag andlift coefficient values for the leading and trailing models.

When a different Turbulence model like k- omega was used we observed that the flow got diverged where as the Spalart-Allmarasprovided the promising results. But, among all the models tried the k- epsilon has yielded the best satisfactory results when compared to the experimental data.

GRID CONVERGENCE STUDY:

Grid convergence study is performed for the AHMED model bytaking different mesh resolutions. The different densities ofmeshes are generated; by varying the mesh densities we obtaindifferent number of nodes (N) for each test and it also yield usdifferent drag values of the body. The sensitivity of differentdensities of meshes on coefficient of drag is studied.

The table below shows us different combinations of meshes triedon the single ahmed configuration run on different turbulencemodels and the model which produced the most satisfactory resultsis choosen and is plotted against the 1/ number of nodes.

Nodes

1/Nodes Lift

Drag

15619 6.4025E-05

1.07 0.8169

89348 1.1192E-05

0.868

0.45877

201223 4.9696E-06

0.6284

0.3619

0 0.00002 0.00004 0.00006 0.000080

0.20.40.60.81

1.2Lift CoefficientLinear (Lift Coefficient)Dag CoefficientLinear (Dag Coefficient)

1/Nodes

Fig 14. Plot between Coefficient of Lift and Drag Vs 1/N

The above plot shows the Coefficient of lift and drag values for their respected 1/ (number of nodes) N. Now, we draw a trend linefor the obtained curve and it is extended such that it touches the axis which yields us the value. This is the value at 1/N = 0 i.e. at N=infinity. So for zero mesh spacing the Coefficient of drag and Lift values obtained from the above curve are 0.32 and 0.65 resp. The values satisfactorily matches with the reference values of the experimental data.

Nodes 1/Nodes Lift Body1Drag Body1

Lift Body2

Drag Body 2

12985 7.7E-05 0.6912 0.3828 0.3412 0.32707

101366 9.87E-06 0.6102 0.3817 0.3328 0.3538

345873 2.89E-06 0.6027 0.33598 0.31686 0.3377

0 0.00005 0.00010

0.10.20.30.40.50.60.70.8

Lift Coeffi...

0 0.00005 0.00010

0.1

0.2

0.3

0.4

0.5

Lift Coefficient Body 2

Fig 15 Grid convergence for Body 1 Fig 16 Grid convergence for Body 2

The above figure shows us the Grid convergence histories for the two bodies at a spacing of 2 i.e. at x=2.61m between two bodies. The lift and drag values were calculated and are plotted against the 1/ number of nodes. The values for body 1&2 at zero mesh spacing is 0.34 and 0.35 resp. whereas the lift coefficient values are 0.59 for body 1 and 0.34 for body 2.

The values obtained in the grid convergence are in little disagreement with that of the values that were hoped to obtain based on the reference values. Overall, it can be said as the values of Body 2 can be said to be little different but are approximately near.

The Drag coefficient values for the single Ahmed model and twoadjacent Ahmed models at different Spacing for a finer meshresolution is tabulated below. The values obtained were byrunning the solver using k-epsilon enhanced model. TheCoefficient of drag values for the leading and the trailingmodels are calculated separately and then is plotted so as tocompare with that of the experimental values.

Distance

Spacing(x/L)

CD(single)

CD1

CD2

0.261 0.25 0.33194 0.2082 0.231.044 1 0.33194 0.2075 0.299

82.088 2 0.33194 0.3359

80.337

72.61 2.5 0.33194 0.3119

410.317

3.132 3 0.33194 0.3517 0.3696

0 0.5 1 1.5 2 2.5 3 3.50

0.050.10.150.20.250.30.350.4

Single Model Leading ModelTrailing Model

Spacing x/L

Fig 15: Drag coefficient for single Ahmed model and two Ahmed models at different vehicle spacing’s

The values obtained by doing the investigation are comparedagainst the experimental values. It is seen that the Drag valueslooks promising, despite of having so many un identicalparameters. In the real case the experimentalists might haveperformed the study under different conditions which maycompletely differ from those of this computational case. Namingfew of them, the model in this study was confined to 2 dimensionswhereas in real case it might have been a 3D. Similarly, in thisstudy the body is suspended in the air which is practicallyimpossible in real world and the supporter of the body alsoaccounts for drag.

One interesting observation that can be made from the abovefigures is that the drag coefficient of the leading model and thesingle model differ despite when in real world the leading modelis just practically same as the single model as there is no flowdisturbance before the body. But, However as the flow over theleading model after the body creates flow separation andrecirculation zone, and the flow re circulates behind the bodyits effects has an impact not only on the model trailing it butalso on itself.

Similarly, we perform comparison for lift coefficient with the experimental data and the results can be seen below.

Distance

Spacing(x/L)

CL(single)

CL1 CL2

0.261 0.25 0.6104 0.69 0.221.044 1 0.6104 0.385

70.18

2.088 2 0.6104 0.6027

0.31686

2.61 2.5 0.6104 0.614 0.3423.132 3 0.6104 0.57 0.3220

53

0 0.5 1 1.5 2 2.5 3 3.50

0.10.20.30.40.50.60.70.8

Single Model Leading ModelTrailing Model

Lift

Co

effi

cie

nt

Figure 16: Lift coefficient for single Ahmed model and two Ahmed models at different vehicle spacing’s

The Lift coefficient values are also compared and it gives ussatisfactory output considering the facts mentioned above. Thus,if all the conditions and parameters are made identical with thereal life case then it can be hoped that the computationalsimulation may also yield us the same results as obtained fromthe experiments.

CONCLUSION:

The flow field, lift and drag force of the flow over the Ahmedbodies are simulated by computational approach. Compared with the

SST K-omega, standard k-ε enhanced model and Spalart-allmarasmodels, k-ε performs better.

The spacing’s tried between the two bodies are varied and fordifferent families, densities of meshes are tried and are run fordifferent turbulence models and the better model from theobservance in accordance with the result is k-epsilon enhanced.

Computed results obtained on the Ahmed reference model usingFLUENT are in just little disagreement with experimental results.In particular, the drag coe cient values shows variance to thatffiof the experimentalist’s data. In contrast to it, the lift valuesobtained, absolutely match with that of the reference valuesexcept in few cases. Our results show numerical simulations to bea highly promising technique to research on physical phenomena inautomotive aerodynamics.

REFERENCES:

1. http://www.isca.in/rjrs/archive/v1i9/1.ISCA-RJRS-2012-139.pdf 2. http://venus.ceride.gov.ar/twiki/pub/Cimec/

AerodinamicaExteriorDeVehiculos/ahmed_gid.pdf 3. http://www.engineering.uiowa.edu/~me_160/Lab/Intermediate-2013/

Lab%204-Ahmed/Intermediate%20Lab%204%20Manual.pdf