Flow simulation over a Ahmed body using Ansys fluent

OBJECTIVE

          The aim of the project is to perform a flow analysis over an Ahmed body and perform a grid dependence test by varying the mesh around the Ahmed body.

Ahmed Body

          Ahmed body is a generic car body (a simplified car model) i.e the flow of air around the Ahmed body captures the essential flow features around an automobile. Ahmed body is used to validate their numerical model because the experimental data from wind tunnel testing is available for Ahmed body. Once the numerical model is validated, it is used to design new models of the car.

Three main features were seen in the wake:

1. The A recirculation region that is formed as the flow separates at the top of the vertical back surface of the model

2. The B recirculation region is formed due to the separation at the base of the model.

3. The c-pillar vortices that form as the vorticity in the side boundary layers roll up over the slant edges.

The wake was shown to be highly dependent on the slant angle. For slant angles less than 12°, the flow remains attached over the slant. The flow is essentially two-dimensional and has low drag. Between 12° and 30°, the flow becomes much more three-dimensional as the c-pillar vortices form. These reach maximum strength at 30°. The drag increases significantly as the low-pressure cores act on the rear surfaces. Past 30° the flow separates fully off the slant. This results in a sudden decrease in drag and weaker c-pillar vortices.

Effect of Aspect Ratio: The aspect ratio of the rear slant had a significant effect on the wake. The wider bodies ceased to reattach at slant angles of 25°, suggesting that the critical angle lowers as aspect ratio increases. They also provide a vortex core location from experimental data that can be used for validation.

PROCEDURE

The SpaceClaim is used to import the Ahmed body and unit system is changed from mm to m before importing the file.  

To perform a wind tunnel test an enclosure is created around the Ahmed body. Prepare --> Enclosure option is used to create a volume around the Ahmed body (i.e) it performs the boolean operation to create a single volume. Suppress the Ahmed body since flow over the body is only considered for the simulation. Design --> Plane option is used to check whether the enclosure created is correct or not.

The baseline mesh is created using the generate mesh option & create four named selection

i) Inlet - Front face, ii) Outlet - back face, iii) Symmetry - four walls, iv) Car wall - Ahmed body. 

The Fluent is opened to create the setup for the flow simulation. The density-based solver is used since the inlet velocity is set to 50 m/s the Mach no for the flow is greater than 0.3 so the compressibility of the fluid must be taken into account so this solver is chosen. The Absolute velocity is selected & steady-state method is selected since the fluid property during the simulation is not considered. For the viscous flow, the k-e turbulent model is chosen.

Boundary conditions

Inlet - V = 50 m/s.    Outlet - P = 0 gauge.    

          T = 300 k.                  T = 300 k.

The hybrid method solver is used & the simulation is initialized. The create a plane along z normal where the flow is symmetry by using the option create --> plane & select points & normal option. Points = x - 0, y - 0,z - 0 & Normal = x - 0, y - 0, z - 1, name it as z plane. Now create a velocity and pressure contour on the z plane & also create an animation for the velocity and pressure for the contour files.

The fluent results are used to plot the results of the various flow parameters by creating a plane along the XY plane & used the pressure & velocity contour to plot the data.

Pressure Contour

Velocity Contour

Vector Plot

From the vector plot, it is clear that the mesh is course so the data to capture the wake region at the rear of the vehicle is not possible.

The Location --> line option is used to create two lines along the XY plane at coordinates     2 0 0, 2 2 0 & 3 0 0, 3 2 0. The insert --> chart option is used plot the data for the lines,    x axis - Y coordinate & y axis - velocity u, similarly for the pressure.

Pressure chart

Velocity chart

CASE 2

The mesh is the previous case is to course so we couldn\'t capture the data around the Ahmed body properly so another enclosure is created around the Ahmed body. The intersection of the two enclosure is avoided by using the Prepare --> Intersection.  

The three different mesh is created first for the outer enclosure hex mesh of size 100mm & the max size of the mesh is set to 100mm. The inner enclosure is set to the size of 50 mm & for the car wall the mesh is set to 10mm. 

Section view of the mesh

The Fluent is opened to create the setup for the flow simulation. The density-based solver is used since the inlet velocity is set to 50 m/s the Mach no for the flow is greater than 0.3 so the compressibility of the fluid must be taken into account so this solver is chosen. The Absolute velocity is selected & steady-state method is selected since the fluid property during the simulation is not considered. For the viscous flow, the k-e turbulent model is chosen.

Boundary conditions

Inlet - V = 50 m/s.    Outlet - P = 0 gauge.    

          T = 300 k.                  T = 300 k.

The hybrid method solver is used & the simulation is initialized. The create a plane along z normal where the flow is symmetry by using the option create --> plane & select points & normal option. Points = x - 0, y - 0,z - 0 & Normal = x - 0, y - 0, z - 1, name it as z plane. Now create a velocity and pressure contour on the z plane & also create an animation for the velocity and pressure for the contour files.

The fluent results are used to plot the results of the various flow parameters by creating a plane along the XY plane & used the pressure & velocity contour to plot the data.

Pressure Contour

Velocity Contour

Vector Plot

The mesh is more refined than the previous case & it has more elements along the car wall to capture the physics properly.

The Location --> line option is used to create two lines along the XY plane at coordinates     2 0 0, 2 2 0 & 3 0 0, 3 2 0. The insert --> chart option is used plot the data for the lines,        x-axis - Y coordinate & y-axis - velocity u, similarly for the pressure.

Pressure chart

Velocity chart

CASE 3 :

The geometry of the model is not changed & only the mesh is further refined around the car wall.

An inflation layer is created around the car body of size 5 mm for 5 layers of growth size 1.2 to capture the physics more accurately around the mesh and also reduce the size of the cylinder at the bottom to 5 mm.

Inflation layer around the car wall

Pressure Contour

Velocity Contour

Vector Plot

The inflation layer has caused more elements to be formed around the car wall.

Velocity chart

Pressure chart

INFERENCE:

From the three cases, it is clear that mesh refinement produces better results and also the visualization of data. The baseline mesh is used to see the variation of the property of the fluid along the flow & then mesh is refined in the required area to obtain accurate results. The variation of the result is the last two cases are very less since we have just added inflation to capture the boundary property with more accuracy.

Variation in pressure at rear

The wake inside the boundary layer is the most important thing to analyze in order to determine the amount of drag (and other forces) experienced by the body.

A favorable pressure gradient is one in which the pressure decreases in the flow direction (i.e dp/dx < 0) it is called favorable because it tends to overcome the slowing of fluid particles caused by friction in the boundary layer. This pressure gradient arises when the freestream velocity U is increasing with x, for example, in the converging flow field in a nozzle. On the other hand, an adverse pressure gradient is one in which pressure increases in the flow direction (i.e dp/dx > 0) it is called adverse because it will cause fluid particles in the boundary-layer to slow down at a greater rate than that due to boundary-layer friction alone. The flow inside the boundary layer gets detached because of the adverse pressure gradient. This adverse pressure gradient (i.e. flow of fluid inside the boundary layer flowing opposite to the flow of fluid directed over the body because of the motion of the body) then creates a difference in pressure ahead of the body and behind the body which increases the overall drag. 

The adverse pressure gradient occurs at the rear of the Ahmed body which is the main reason for the negative which can avoided by varying the geometry.