External flow simulation over Ahmed body
Objective: To perform external flow simulation over an Ahmed body.
Ahmed Body:
The Ahmed Body was first created by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. Since then, it has become a benchmark for aerodynamic simulation tools.
Ahmed body is a bluff body that represents a generic car body. It has a simplistic shape and has the basic aerodynamic properties of a vehicle and has been used as a reference in many aerodynamic studies. so, Ahmed body shape is simple enough to model.
Importance of Ahmed body:
The three-dimensional flow around a vehicle has become a subject of significant importance in the automotive industry. One apparent technique of improving the fuel efficiency of a vehicles is to reduce aerodynamic drag force by optimizing the body shape. Execution of fine aerodynamic design under stylistic constraints requires an immense understanding of the flow pattern phenomena and, especially, how the aerodynamics are impacted by changes in body shape. The flow area which is presents the major contribution to a cars drag is the wake flow backing the vehicle. The point at which the flow separates directs the size of the separation zone, and as a result the drag force. Clearly, a more accurate simulation of the wake flow
Most cars have an average drag coefficient of between .30 and .35. Boxy cars have a higher number, like the HUMMER® H2 vehicle at .57, while more streamlined and agile cars have a lower number, like the Mercedes-Benz® C-Class® vehicle at .24. Again, this is just an average measurement. The exact drag coefficient of a car varies with the Reynolds number and various other factors.
Drag Coefficient:
The drag coefficient quantifies the resistance of an object in a fluid environment. It is not an absolute constant for a body’s shape because it varies with the speed and direction of flow, object shape and size, and the density and viscosity of the fluid.
The lower the drag coefficient of an object, the less aerodynamic or hydrodynamic drag occurs. In terms of a car, the lower the drag coefficient, the more efficient the car is. As well as affecting the top speed of a vehicle, the drag coefficient also affects the handling. Cars with a low drag coefficient are sought after, but decreasing the drag drastically can reduce the downforce and lead to loss in road traction and a higher chance of car accidents.
`Cd= (2*Fd)/(ρ*V^2*A)`
where, Cd= Drag coefficient
Fd= Drag force
ρ= Density
A= Area
V= Velocity
Negative Pressure at wake:
When the vehicle is moving at a certain velocity, the viscous effects in the fluid are restricted to a thin layer called boundary layer (Outside the boundary layer is the inviscid flow). As thousands and even millions of air molecules draw near the car’s front, they start compressing, thus raising the air pressure on the front of the vehicle. Likewise, the air molecules that are travelling along the car’s sides are at atmospheric pressure. This means the pressure is lower than the molecules on the vehicles front. i.e. the fluid flow imposes pressure force on the boundary layer. When the air reaches the rear part of the vehicle, the fluid gets detached. Within the boundary layer, the movement of the fluid is totally governed by the viscous effects of the fluid. The fluid moving around the vehicle is dependent on the shape of the vehicle and the Reynolds number.
There is another important phenomenon, which affects the flow of the car and the performance of the vehicle. This phenomenon is commonly known as Wake of the vehicle. When the air moving over the vehicle is separated at the rear end, it leaves a large low-pressure turbulent region (vacuum) behind the vehicle known as the wake. This wake contributes to the formation of pressure drag, which is eventually reduces the vehicle performance.
Flow Seperation :
Whenever there is relative movement between a fluid and a solid surface, whether externally round a body, or internally in an enclosed passage, a boundary layer exists with viscous forces present in the layer of fluid close to the surface. Boundary layers can be either laminar or turbulent. A reasonable assessment of whether the boundary layer will be laminar or turbulent can be made by calculating the Reynolds number of the local flow conditions.
Flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake. Separation occurs in flow that is slowing down, with pressure increasing, after passing the thickest part of a streamline body or passing through a widening passage, for example. Flowing against an increasing pressure is known as flowing in an adverse pressure gradient. The boundary layer separates when it has travelled far enough in an adverse pressure gradient that the speed of the boundary layer relative to the surface has stopped and reversed direction. The flow becomes detached from the surface, and instead takes the forms of eddies and vortices.
Y+ Concept :
It's important that your mesh near the wall is properly sized to ensure accurate simulation of the flow field. The y+ value is a non-dimensional term that can be used to understand how coarse or fine our grid is.
with a desired y+ value, we can calculate the wall distance or first cell height (y).
`Re = (ρ*V*L)/μ`
`Cf = 0.026/(Re^(1/7`
`Tw = (Cf*ρ*V^2)/2`
`Uf = sqrt((Twall)/ρ`
`y=(y^+*μ)/(Uf*ρ`
Geometry of Ahmed Body:
The simple geometrical shape has a length of 1.044 meters, height of 0.288 meters, and a width of 0.389 meters. It also has 0.5-meter cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 40 degrees.
Procedure:
Step1: Geometry
The computational domain for the given simulation is prepared using space claim. The Enclose Option is created and the dimensions of the cuboidal domain is set accordingly.
A smaller enclosure is used around the Ahmed body in order to capture the wake region correctly with a refined mesh in that region.
As the Ahmed body is symmetrical along its length, the body can be split and only one half of it is used for analysis in order to limit the number of computational cells. The inside of the body is not a part of the analysis and hence the Ahmed body is suppressed for physics.
Step2: Meshing
Different mesh size is used for each enclosure and for Ahmed body wall to get accurate Drag values.
And set the named selections accordingly.
Step3: Setup & Solution
*Solver type: Pressure based
*Check the Energy equation
*Model: K-Epsilon - Standard (Range of Y+ value is 30 - 300)
*Material: Air
*Inlet Boundary Condition : 25m/s
*Solution method scheme: Coupled
*Reference Values:
*Create a surface plane to see the contours
*Create a Cd plot for Ahmed wall
we shall consider 2 cases for grid independency test
case A: Same Y+ value with different mesh size
case B: Different Y+ value with same mesh size
Step3: Result
CASE A: Same Y+ value with different mesh size
Y+ = 30
First layer thickness: 0.43
CASE 1 CASE 2
Cd Plot
Velocity Contour
Pressure Contour
Velocity Vector Plot
Pressure Vector Plot
CASE B: Different Y+ value with same mesh size
Outer enclosure: 100 ; Inner enclosure: 30 ; Ahmed Body faces (2- stand): 5
case1
case2
case3
Conclusion:
Refined mesh with appropriate y+ value results in accurate Cd value. In my case i got the approximate Cd value as 0.3787