Week 3 - External flow simulation over an Ahmed body.

AIM: External flow simulation over an Ahmed body.

OBJECTIVES:

• Describe Ahmed's body and its importance.
• Explain the reason for the negative pressure in the wake region.
• Explain the significance of the point of separation.
• Run the simulation for the velocity of 25 m/sec with the default air properties in fluent.
• Perform the grid independency test and provide the values of drag and lift with each case.

THEORY:

The Ahmed body is a simplified car body used in the automotive field to study the impact of the flow pattern on the drag. The
external aerodynamics of the car defines many major traits of an automobile like stability, comfort, and fuel consumption at
high speeds. The flow around the vehicle is characterized by high turbulent and three-dimensional flow separations. There is a growing need for more insight into the physical features of these dynamic flows. The Ahmed Body is a simplified car,
used in the automotive field to investigate the flow analysis and find the wake flow around the body. Ahmed's body is made up
of around the front part, a moveable z slant plane in the rear of the body to study the detachment phenomena at different angles,and a rectangular box that links the front part and the rear slant plane. The principal objective to study such a simplified car body is to tackle the flow processes involved in drag production. By perceiving the mechanisms involved in creating drag, one can design a car to minimize drag and reduce fuel consumption and maximize vehicle performance.

Importantance of Ahmed body:

An Ahmed body represents a simplified vehicle volume which helps understand the fundamental physics defined by the
engineers the lift and drag forces and coefficients at certain characteristically velocities, pressure differences, and seeing and
analyzing the vortices, turbulence, and Reynolds number obtained. ANSYS is a general-purpose FEA and CFD simulation
software. It gives a numerical prediction of the behavior of a system.These numerical predictions need to be validated by experimental data. People working in vehicle aerodynamics use Ahmed body to validate their numerical model because the experimental data from wind tunnel testing is available for Ahmed body. Once the numerical model is invalidated, it is used to design new models of the car. Its shape is simple enough to allow for accurate flow simulation but retains some important practical features relevant to automobile bodies. This model describes how to calculate the turbulent flow field around a simple car-like geometry using the Turbulent Flow, k-epsilon, and k-omega interface. Later, either experimentally or computationally it can be recreated all the same but with the desired geometry, new to come up to the market or research.

The Ahmed body was described originally by S. R. Ahmed in 1984 [1]. 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 edge

The wake was shown to be highly dependent on 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.

Reason for negative pressure in the wake region: The path lines showing the exact circular motion or vorticity generation just at the rear end of the car. The separation takes two regions of distribution. One at the upper part of the rear and the other at the lower part of the rear end. The path lines show the nature of the fluid interaction with the rear end of the Ahmed body which here shows that the fluid just after leaving the contact from the slope still possess the momentum in it and the fluid particles near to the surface almost show the no-slip region which means the very least possible velocity of the fluid particles is in contact with the slope and base of the rear part. Thus the fluid particles away from the rear region having the higher pressure beside the wake region try to approach from the higher pressure region to the lower pressure region thus creating a circularity exactly in the wake region created. The body penetrating the fluid creates a region of emptiness just beside its end due to the nature of the geometry of the body here which can be seen least in the case of a perfectly streamlined-shaped body. This emptiness created is a low-pressure region towards which the fluid column exactly behind the wake region which of atmospheric pressure tries to fill the space thus dragging the fluid column with it thus creating the region of vorticity. This also results in the huge pressure gradient in the region exactly rear of the body creating the –ve pressure at the end. Also, this is the layman’s reason why there is negative gauge pressure in the wake region.

Significance of the point of separation:
The fluid separation point can be defined as a point where the dv/dy i.e. gradient of velocity goes zero along with the dp/dx = 0, for the specific point. In a more simplified term we can say that at the point where the velocity gradient goes zero and the pressure gradient along the x-direction goes zero the point of separations starts occurring or the negative pressure gradient starts forming after that point. The point of separation cannot be understood just by a single contour in a hard and fast way. It can be supportively studied with the help of the dp/dx plots or velocity plots w.r.t the Y direction and other such plots in which we can see the nature of such physical properties like dp/dx=0, and dv/dy=0 experiencing in a particular region of fluid and body interaction.

Modeling approach

Baseline simulation approach using k-epsilon model.

• Ahmed's body is loaded into the Spaceclaim.
• A single enclosure is drawn across Ahmed's body.
• Now the geometry is loaded for meshing.
• A baseline simulation is set up with the default element size of Ansys (413.6 millimeters).
• A named selection is created for the enclosure viz; inlet, outlet, symmetry.
• To create the named selection for the car the enclosure faces are hidden and box select is selected from the meshing toolbar and (car wall) name is given to it.
• Ansys Fluent window is opened and the geometry is loaded.
• Steady-state, the density-based solver is selected.
• Reference values are updated as per the prescribed conditions.
• In the inlet boundary condition, inlet velocity is 25 m/sec and 300 K temperature are set.
• Drag-coefficient plot and lift-coefficient plot is created by setting the reference as (car walls).
• Hybrid initialization is selected.
• The solution is initialized by hitting t=0t=0.
• The cut-plane-z is created along Ahmed's body.
• The velocity contour, pressure contour is created along the cut-plane-z.
• The animation video is created for the velocity contour.
• The simulation is executed for 300 iteration.

CASE 1:

RESULTS:

Residual plot:

Coefficient of Drag:

Coefficient of Lift:

Pressure Contour:

Velocity Contour:

Velocity vector:

Inner Enclosure:

• Right face is located at a distance of 1 m from the right side face of Ahmed body along the X axis.
• Left face is located at a distance of 0.5 m from the left side face of Ahmed body along the X axis.
• Top face of the encloser is at a distance of 0.25m from the top side face of the Ahmed body along the Y axis.
• The bottom face of the encloser is attached to the bottom face of the Ahmed body along the YZ plane.
• The front and back face of the encloser is at a distance of 0.26 m along the Z axis.

Interference Region:

Initially there was a huge interference between the Outer and Inner encloser so that the mesh gets overlapped which gets not acceptable so to avoid this we made share topology and get deleted the Interference region that by using the interference command after this we get a good interaction between the Inner and Outer enclosers so that we can get the mesh properly.

Meshing:

The Ahmed body is surrounded by a very fine hexahedral grid to record flow structures and inner enclosure is surrounded by tetrahedral grid and inflation layers are created along the walls of ahmed body to accurately capture the flow separation from its rear.

CASE 2:

Simulation was done for total no of mesh counts = 1,43,751

where,

1st box domain = 100mm size of hexahedral mesh

2nd box domain = 30mm size of tetra mesh

and 3 inflation layer around the wall of ahmed body with max thickness of 12.4mm.

Pre-processing and solver setting

The next step in the simulation after generating the meshing is to set up the physical properties for the case.

• Type of solver: Density-based
• Simulation method: Steady-state simulation
• Turbulent Model: k-epsilon standard
• Material properties of the fluid (Air)

Density: 1.225 kg/m3

Cp (Specific heat): 1006.43 J/kg-k

Thermal Conductivity: 0.0242 w/m-k

Viscosity: 1.7894e-05 kg/m-s

• Inlet velocity: 25 m/s
• Inlet Temperature: 300 K
• Outlet: Pressure type, 0-gauge pressure.
• Walls: wall type with no-slip

RESULTS:

Residual Plots:

Coefficient of Drag:

Coefficient of Lift:

Pressure Contour:

Velocity Contour:

Velocity Streamline:

Velocity Vector:

CASE 3:

The simulation was done for a total no of mesh counts = 3,87,319

where 1st box domain = 35mm size of hexahedral mesh

2nd box domain = 30mm size of tetra mesh

Pre-processing and solver setting

The next step in the simulation after generating the meshing is to set up the physical properties for the case.

• Type of solver: Density-based
• Simulation method: Steady-state simulation
• Turbulent Model: k-epsilon standard
• Material properties of the fluid (Air)

Density: 1.225 kg/m3

Cp (Specific heat): 1006.43 J/kg-k

Thermal Conductivity: 0.0242 w/m-k

Viscosity: 1.7894e-05 kg/m-s

• Inlet velocity: 25 m/s
• Inlet Temperature: 300 K
• Outlet: Pressure type, 0-gauge pressure.
• Walls: wall type with no-slip

RESULTS:

Residual Plot:

Coefficient of Drag:

Coefficient of Lift:

Pressure Contour:

Velocity Contour:

Velocity Vector:

Grid independence test:

Number of elements drag co-efficient(Cd) Lift co-efficient(Cl) No. of Iterations to Converged

847048 32.431343 18.579436 165

1,43,751 0.048867 -0.006838 71

3,87,319 0.04720825 0.0021214886 81

Result and Conclusion:

The baseline mesh gave a poor result, it wasn't able to capture the flow around the Ahmed body and the wake structure, due to coarse mesh.

The refined mesh gave better results. The plots show the wake formation accurately and the flow around the Ahmed body was also captured pretty well.

There is a variation of pressure across the Ahmed body. The pressure in front of the body is positive because that's the location where the flow first comes in contact with the body and comes to rest. At the corner, the flow accelerates, thereby decreasing the pressure.

The reason behind negative pressure in the wake region is due to flow separation, and recirculating flow. It's a phenomenon that generally happens as the air moving past the body isn't able to pass through the immediate backside of the body creating a region, where there is no movement of air and thereby creating a pressure gradient.

As the wake flow is the main contributor to the drag force(pressure drag), the point of separation is an important aspect of the flow. The more attached the flow is to the body the recirculation zone will be smaller and hence reduce the adverse effect of pressure drag, improving the fuel efficiency of the body.