WAKE REGION ANALYSIS FOR AHMED BODY AND GRID DEPENDENCE TEST

Ahmed vehicle model and its salient features:

• Wakes of ground vehicles are the important study for vehicle aerodynamics ,as the impact of aerodynamic drags on the power consumption of vehicle is highly unpredictable.
• As the complexity of flow around the ground vehicles is huge and also turbulent, evaluation of essential flow features around them is always debated and troublesome.
• The scientific endeavors on this particular area are enormous and continously changing over the years the whole essence on understanding these concepts.
• Ahmed body is a vehicle model originated from the study and experimental works of ahmed,which posses a great deal of essential flow features along the real vehicle and also exibhits the 3d flow seperation along the rear end of vehicle and large coherent structures and unsteadiness.
• Ahmed results indicated that almost 85% of total aerodynamic resistance is contributed by pressure drag which is generated at the rear end.

Schematics of the Ahmed vehicle model:

In terms of ratio , the model's length:width:height is 3.36:1.37:1.

Computational domain :

• A wind tunnel is built around the ahmed body using space claim's enclosure option .
• In this case, grid refinement is required on the surface of ahmed body and the near wake regions predominantly which led to building a second enclosure inside the first enclosure.
• Topology is set to share as it is the only way to bring in a conformal mesh where bodies meet, and is the only way to be sure that intersection of bodies is meshed aptly.
• Section plane tool ensures the topology sharing proximity by displaying the hatching intersection between the bodies.

Figures are as follows,

Computational Domain for the Ahmed Body simulation

section view (ensurement of zero overlapping in future meshing practices)

Boundary conditions:

• The BCs for the problem are symmetry for upper and sides of the computational domain.
• For the lower face and the surface of ahmed body is no slip condition.
• At the inlet a uniform velocity of 55 m/s and guage pressure is used as the outlet boundary condition.

Setup:

Solver:

Model:

Materials: air

Number of iterations: 600 - 900 (varies for 3 different setups but produces closer results)

Analytical value:

Reynolds number = velocity * characterstic length / kinematic viscosity

= 55*0.288/(15*10^-6) (at 300 k)

= 1056000

Baseline-mesh setup simulation:

To have a hassle free simulation for the different grids and also to understand the wake region extent , a baseline mesh setup simulation is done.

Mesh:

Slice through the midline of domain showing the distribution of grid

Details of mesh:

Mesh stats:

Results

Velocity contour :

The stream wise velocity around the body from a cut across the centreline of geometry

The stream wise velocity around the body from a cut across the centreline of geometry with meshlines

A contour plot of pressure field on the ahmed body:

Vector plots to visualise the direction of fluid particles(vortice shedding) in the wake region:

Velocity plots in near after rear end regions:

The outer body is meshed with different element sizes for the following 3 mesh refinements using the multizone method

Refinement 1:

Mesh :

Slice through the midline of domain showing the distribution of grid and near distribution mesh near the body

Details of mesh:

Body details:

Face details:

Face sizing details 2:

Mesh stats:

Results

Velocity contour :

The stream wise velocity around the body from a cut across the centreline of geometry

The stream wise velocity around the body from a cut across the centreline of geometry with mesh lines

A contour plot of pressure field on the ahmed body:

Vector plots to visualise the direction of fluid particles(vortice shedding) in the wake region:

Velocity plots in the Rear end:

Refinement 2:

Mesh

Slice through the midline of domain showing the distribution of grid

The stream wise velocity around the body from a cut across the centreline of geometry with meshlines.

Mesh details:

Body sizing:

Face sizing:

Mesh stats:

Results

Velocity contour :

The stream wise velocity around the body from a cut across the centreline of geometry

A contour plot of pressure field on the ahmed body:

Vector plots to visualise the direction of fluid particles(vortice shedding) in the wake region:

Velocity plots in the Rear end:

Refinement 3:

Mesh Generation:

Slice through the midline of domain showing the distribution of grid

Mesh details:

Body sizing:

Face sizing:

Mesh stats:

Results

Velocity contour :

The stream wise velocity around the body from a cut across the centreline of geometry

The stream wise velocity around the body from a cut across the centreline of geometry with meshlines

A contour plot of pressure field on the ahmed body:

Vector plots to visualise the direction of fluid particles(vortice shedding) in the wake region:

Velocity plots in the Rear end:

1)Grid independence test or Mesh convegence study is done using three different element sizes in different regions of domain and from the charts it is observed that there is minimal difference between the values. And the charts of refinement 2 and refinement 3 are outputting the really closer equal values. Therefore , we can conclude that refinement 3 is the mesh that produces the desired results in this ahmed model simulation.

2)As the simulation was performed using an academic version of ANSYS WORKBENCH , the element number cannot be increased more than 512k.In accordance to this limitation, the simulation is pushed to be performed with maximum number of permissible elements in the domain.

Heuristically speaking on the negative pressure regions:

Pressure contour of refinement 2 (a midline section of the domain)

• Automobiles are designed in such a way that their speed increase must not produce observable lifts .Therefore, inducing downforce using the slanted rear ends and reduced pressure beneath of a car surface.
• Downforce is always accompanied by dragforce and the rear end slant angle determines the amount of downforce and drag that comes along with it.
• Air becomes almost stagnant at the front end of ahmed body which results in air exerting higher pressure in that region.
• At the nose , the air starts drifting off which leads in pressure reduction causing lifts on the top surface of car and as it reaches the initial section of the rear slant again the air pressure increases which offers balance for a speeding car and reduce the tendency of lifts.
• As the dramatic shift in geometry occurs after the rear end , the air starts accelerating out leaving from different regions of car body leading to a negative pressure or adverse pressure gradient in the wake flow region
• The wake area is low pressure region of recirculating air which is caused due to flow separation as the flow encounters changes in curvature and shape at the rear end of ahmed body.