Aerodynamic flow simulation on Ahmed Body in ANSYS fluent

Aim:-> To perform aerodynamic flow simulation on Ahmed body.

Theory:-

Ahmed body:- The Ahmed body is a bluff body or rather a generic car body (a simplified vehicle model). The air flow around the Ahmed Body captures the essential flow features around an automobile and was first defined and characterized in the experimental work of Ahmed. Although it has a very simple shape, Ahmed Body allows us to capture characteristic features that are relevant to bodies in automobile industry. This model is also used to describe the turbulent flow field around a car like geometry. Once the numerical model is validated, it is used to design the new models of the car.

The Ahmed Body is a geometric shape first proposed by S.R Ahmed and Ramm in their research, "Some Salient Features of the Time Averaged Ground Vehicle Wake" in 1984. The purpose behind design of this Ahmed Body is to study geometric effect on the wakes of ground vehicles.

In todays scenario, cars are complex in shape hence are challenging to design and difficult to compute aerodynamic drag. So Ahmed body has become a benchmark for aerodynamic simulation tool. The shape is simple enough to allow accurate flow simulations.

Fig: Ahmed body geometry

In above model, the curved face at front side is usually referred as nose of car body. The flat rear vertical part is back body of car. The slant part on back side of body is 'Slope' whose angle can be vary to compare velocity and simulation at wake region. The roof of body is perfectly horizontal and plain. And the body is symmetrical along its length.

The Ahmed body shape has 1044 mm length, height of 288 mm and 389 mm width. The body has slant of 40 Degrees.

As we know when a vehicle moves in an environment (on road) it experience a resisting force due to air, that resisting force results poor vehicle efficiency. So automotive industries compete to increase vehicle efficiency by optimizing shape, size and weight. And to lower down resisting force, automotive companies improves shape by reducing drag coefficient. The drag force is not constant for body because it varies with body shape, speed, direction of flow, density and viscosity of fluid. The lower drag coefficient results low resisting force hence, improves efficiency and speed of vehicle.

Pre-processing:-

Geometry:-

Below figure shows car model inside enclosure. The blue face on left side defines inlet face and red face on right side defines outlet.

As we know for computational fluid dynamics we need to mesh fluid volume. So we have prepared an enclosure in space claim about an Ahmed body.

Mesh:-

Fig: Meshed geometry

As we know as mesh is low it gives better result but simultaneously it also increases computational time and computational memory also. So instead of refining mesh for whole enclosure we have provided another small enclosure inside bigger enclosure. So refining mesh in this small enclosure results better visualization around body wall. And also easy to and time saving to find accurate result.

The y+ value is a non-dimensional distance (based on local cell fluid velocity) from the wall to the first mesh node, as shown in below figure. To use a wall function approach for a particular turbulence model with confidence, we need to ensure that our y+ values are within a certain range.

Considering above figure its important to ensure y+ values are not so large that the first node falls outside the boundary layer region. If this happens, then the Wall Functions used by turbulence model may incorrectly calculate the flow properties at this first calculation point which will introduce errors into pressure drop and velocity results.

Reynolds number, `Re=(rhoVL)/nu`

As we know inlet velocity V = 25m/s and material is air with density 1.225.

`:.Re=(1.225xx25xx1.044)/(1.7894xx10^(-4))`

`:.Re=1786772.1`

`Cf=0.0576/(Re)^(1/5) =0.003236`

`tau_w=(Cfxxrhoxxv^2)/2`

`:.tau_w=(0.003236xx1.225xx25^2)/2`

`:.tau_w=1.23878 N/m^2`

`u_f=sqrt(tau_w/rho)`

`:.u_f=sqrt(1.23878/1.225)=1.00561 m/s`

As we considered y+ value 80,

hence first layer thickness,

`y_1=(nuy^+)/(rhou_f)`

`y_1=(1.7894xx10^(-5)xx80)/(1.225xx1.00561)`

`y_1=1.1621 mm`

Solver & inlet condition:-

We have used `k-epsilon` solver for project. y+ value range for this type of solver is 30 to 300. After iterating we have selected y+ value as 80.

Inlet velocity taken as 25 m/s and as we are performing aerodynamic simulation so fluid is considered as air with density of 1.225 Kg/`m^3` and kinematic viscosity `(nu)` of 1.7894`e^(-5)` kg/m-s.

For calculations the reference values are taken from inlet as shown in following figure.

Post processing:-

~Case A:

Number of nodes = 23076

Number of elements = 73412

Drag coefficient (Cd) = 0.4065582

Lift coefficient (Cl) = 0.29368639

Fig: Residual plot

Fig a: Velocity contour Fig b: Pressure contour

Fig a: Drag plot Fig b: Lift plot

Fig: Vortex creation

Fig: Wall shear

~Case B:

Number of nodes = 52611

Number of elements = 191250

Drag coefficient (Cd) = 0.37568842

Lift coefficient (Cl) = 0.36497976

Fig: Residual plot

Fig a: Velocity contour Fig b: Pressure contour

Fig a: Drag plot Fig b: Lift plot

Fig: Vortex creation

Fig: Wall shear

~Case C:

Number of nodes = 123032

Number of elements = 500620

Drag coefficient (Cd) = 0.31976994

Lift coefficient (Cl) = 0.22288089

Fig: Residual plot

Fig a: Velocity contour Fig b: Pressure contour

Fig a: Drag plot Fig b: Lift plot

Fig: Vortex creation

Fig: Wall shear

As the air from inlet hits front face of body has maximum pressure and flow gets separated into two regions, above and below of body. As Ahmed body is in cuboidal shape so separated air travelling along body merely contact to rear side of body. This phenomenon creates void at rear which results negative pressure at wake region.

As we know boundary layer separation occurs due to adverse pressure gradient which means fluid flows from low pressure area to high pressure area and due to viscosity velocity of fluid reduces at wall. At some point velocity of fluid becomes to zero and then to negative or the direction of fluid changes, that point is considered as point of fluid separation.

Animation:

Fig: Velocity flow animation

As seen in all of the above residual plots, the similar pattern is repeating, hence it can be shown the solution is converged.

The velocity and pressure contour figures also shows detachment and recirculation of flow at rear part of Ahmed body.

Conclusion:- By observing above simulation data and plots we can conclude that-

1. The baseline mesh gives poor results.
2. As mesh size decreases quality of mesh increases as well as result also become more precise and accurate.
3. With decreased in mesh size, computational time increases.
4. The drag coefficient for Ahmed body is 0.32 which is near to 0.33 (theoretical) for fine meshing. Hence refined mesh gives better result.
5. Changing y+ value results in variable drag coefficient values.
6. Providing inflation layer capture flow better around the body.
7. Location of flow separation point is very important as flow starts recirculating from that point.
8. The negative pressure is created due to flow separation and recirculation of flow.