Week 3 - External flow simulation over an Ahmed body.
Aim:
The aim of the challenge is to run the simulation by considering the given ahmed body.
Objective:
The main objective of the challenge is to describe the Ahmed's body and its importance, explaining the reason for the negative pressure in the wake region and explaining the significance of the point of seperation in the Ahmed's body. The expected results of this challenge is,
- Velocity contour and pressure contour.
- The drag coefficient plot & lift coefficient plot
- The vector plot clearly showing the wake region.
- Performing the grid independency test and providing the values of drag and lit with each case.
Describe Ahmed's body and its importance :
The Ahmed body is simplified car body used in 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 as well as there is a growing need for more insight into the physical features of these dynamical flows. The Ahmed Body is a simplified car, used in automotive field to investigate the flow analysis and find the wake flow around the body. Ahmed body is made up of round 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 which link 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. Through perceiving the mechanisms involved in creating drag one can be able to design a car to minimize drag and therefore reducing fuel consumption and maximize vehicle performance.
Explain the reason for the negative pressure in the wake region :
The vacuum is caused by the velocity difference applied at a certain Reynolds number and tail angle, which at low Reynolds number, when this pressure difference occurs, the resulting wake increases drag due to the pressure drop behind the Ahmed body, which at high Reynolds number, due to the vortices created, creates a small the amount of pressure pushing the body.
The air almost stagnates upon impact with Ahmed's body, resulting in the air exerting a very high pressure on the front of Ahmed's body. The air stream is then split between the upper and lower surfaces of Ahmed's body. The higher-pressure air on the front surface accelerates as it travels over the curved nose surface of Ahmed's body, causing a pressure drop. This lower pressure creates buoyancy over the surface of the roof as air passes over it. As the air continues to flow and make its way to the rear, the backslope flow separation creates a notch, leaving a vacuum or low-pressure space that the air is unable to properly fill. The resulting lower pressure creates lift and drag, which then acts on the backslope surface.
The meaning of the separation point :
Flow separation or boundary layer separation is the separation of the boundary layer from the surface into the wake. Separation occurs in a flow that decelerates with increasing pressure after passing through the thickest part of the streamline body or passing through an expanding passage.
The separation point defined by the Ahmed body and the flow, the angle alpha depending on which value is on it causes a change in flow direction, the forces required, the lift on the body, the induced drag due to the "tip vortex", at and the critical value of the flow at oblique stalls and reduced lift and drag reduction, all dependent on alpha and Reynolds number.
Geometry:
First, we need to set up the geometry of Ahmed's Body for our simulation.
Creating the enclosures around the Ahmed Body for baseline mesh generation and i given the value for ahmed body & enclosure.
we can also calculate the frontal area and length for reference in solver
Area = 0.06574 m^2
Length = 1.044 m
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.
Boundary Conditions:
Standard k-omega model with Inlet Velocity, V=25m/s, Density of air = 1.225 kg/m3, temperature = 300k and viscosity = 1.7894e-5 kg-s/m.
Boundary conditions are - uniform velocity at inlet, uniform pressure at outlet , symmetry at wall.
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 1:
where, 1st box domain = 100mm size of hexahedral mesh
2nd box domain = 50mm size of tetra mesh
and 4 inflation layer around the wall of ahmed body with max thickness of 11.94 mm.
SETUP :
Solver setup: pressure based
Viscous model: k-omega sst
Material properties: Air
Inlet Velocity, V=25m/s
Residuals:
Drag-coefficient :
Lift-coefficient :
Velocity contour plot:
Pressure contour plot:
Velocity vector plot:
Simulation was done for total no of mesh counts=90094
drag co-efficient(Cd) = 0.3353
Lift co-efficient(Cl) = 0.2283
CASE 2:
where, 1st box domain = 80mm size of hexahedral mesh
2nd box domain = 30mm size of tetra mesh
Residuals:
Drag-coefficient :
Lift-coefficient :
Velocity contour plot:
Pressure contour plot:
Velocity vector plot:
Simulation was done for total no of mesh counts=332289
drag co-efficient(Cd) = 0.31967
Lift co-efficient(Cl) = 0.23360
CASE 3:
where, 1st box domain = 70mm size of hexahedral mesh
2nd box domain = 30mm size of tetra mesh
Residuals:
Drag-coefficient :
Lift-coefficient :
Velocity contour plot:
Pressure contour plot:
Velocity vector plot:
Simulation was done for total no of mesh counts=337100
drag co-efficient(Cd) = 0.31851
Lift co-efficient(Cl) = 0.23186
Grid independence test:
Number of elements drag co-efficient(Cd) Lift co-efficient(Cl)
90094 0.3353 0.2283
332289 0.31967 0.23360
337100 0.31851 0.23186
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, 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.