Ahmed Body Challenge

Aim :

1.Describe Ahmed's body and its importance.

2.Explain the reason for the negative pressure in the wake region.

3.Explain the significance of the point of separation.

Solution:

We will be doing the CFD analysis of above component.

 

Ahmed Body : 

The Ahmed body is a generic car body (a simplified vehicle model). The airflow around the Ahmed body captures the essential flow features around an automobile and was first defined and characterized in the experimental work of Ahmed [1]. Although it has a very simple shape, Ahmed body allows us to capture characteristic features that are relevant to bodies in the automobile industry.

This model is also used to describe the turbulent flow field around a car-like geometry. Once the numerical model in validated, it is used to design new models of the car.

Complexly shaped, cars are very challenging to model and it’s difficult to quantify the aerodynamic drag computationally. The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools. The Ahmed body shape is simple enough to model, while maintaining car-like geometry features.

Importance of Ahmed Body :

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.

There are certain ways to modify a car to optimize its aerodynamics and decrease the drag coefficient. To streamline the exterior, you can remove certain aftermarket items such as the roof rack, mud flaps, spoilers, and radio antenna. Professional car racers also take off their windshield wipers and side mirrors, but this isn’t recommended for the average driver! You can also add wheel covers, a partial grille block, an under tray, fender skirts, and a modified front bumper to improve the drag coefficient and make your car stand out in the process.

 

Analysis:

 

                                              Creation of the Enclosure                                                     

 

                                                Creation of a Cut View 

 

 

Meshing Of Ahmed Body :

 

As you can see when we use the default meshing setting ,meshing is also done inside the ahmed body hence we have to remove this meshing as the analysis which is being carried out is on the exterior surfaces of the body.

After using the supress phyiscs option on the ahmed body which was creating the body twice during meshing we get the desired results.

 

 

 

 

Now we have used the following specifications for meshing :

 

Element size = 0.1 m

Element Type = Tetrahedron 3D element

Since there is an abrupt change on the walls of ahmed body ,we have used inflation with growth rate of 1.2 and no of layers as 5 .We get the following mesh around the ahmed body:

 

As you can see 5 layers are made around the edges which make sure the meshing are smooth around it and we get accurate results.

 

Mesh Quality :

 

 

Most of the element are in the range of 70-90  % and hence they are good mesh.

 

Improvisation : 

 

As the body is perfectly symmetric, we can run the simulation by considering the only half body. This is the best practice where you can save on the number of cells and get the results faster. We can use the 'split body' command in SpaceClaim to perform the operation and then you can use the symmetry boundary condition in fluent to perform the simulation. 

 

 

The body has been split into two equal halves as shown and the part has been suppresed for physics .After this we will create two enclosures ,the first one will be bigger compared to the smaller and have coarse mesh while the smaller one will take care of the ahmed body and will have fine mesh so that better results are achieved .

 

 

As you can see the section of the two enclosures are interfering and hence they will create overlapping while meshing.

Using the interference command we have solved this problem we have separated the two enclosures.

 

Refined Meshing :

 

]

 

Y+

Now we will be discussing an important term known as Y+ .

 

The behaviour of the flow near the wall is a complicated phenomenon and to distinguish the different regions near the wall the concept of wall  has been formulated. Thus  is a dimensionless quantity, and is distance from the wall measured in terms of viscous lengths.

 

One of the reasons for the need of  is to distinguish different regions near the wall or in the viscous region, however how exactly it helps in turbulence modelling or in general CFD modelling need to be well understood. Let us try to understand this with an example. A fisherman uses fishing net, a grid kind of structure to trap the fishes. If he is trying to catch medium to big sized fishes the grids in the net he uses is somewhat big, but if he is trying to trap even small sized fishes then the grid size of the net should be small enough to capture them. In this case even the large fishes are also captured. Similarly coming back to our case if we intend to resolve the effects near the wall i.e., in the viscous sub layer then the size of the mesh size should be small and dense enough near the wall so that almost all the effects are captured. But in some cases if the wall effects are negligible then there is option of including semi-empirical formulae to bridge between the viscosity affected region and fully turbulent region and in this case the mesh need not to be dense or small near the wall i.e., coarse mesh would work.

Considering the first case i.e., near wall modelling it is well-known that the mesh size should be small enough, however then the question follows is how small ? Thus, here comes the concept of , and based on the  value the first cell height can be calculated. The near wall region is meshed using the calculated first cell height value with gradual growth in the mesh so that the effects are captured and avoiding overall heavy mesh count. Let us now look at the  concept i.e., what exactly it is ? What is its mathematical form? How different regions are distinguished in viscous sub layer based on different  values? But before that let us refresh the concept of boundary layer especially turbulent boundary layer.

 

 

 

From the above image we need to be careful to ensure that our  values are not so large that the first node falls outside the boundary layer region. If this happens, then the Wall Functions used by our turbulence model may incorrectly calculate the flow properties at this first calculation point which will introduce errors into our pressure drop and velocity results.

Let us see how to calculate first cell height. Firstly, we should calculate the Reynolds number for our model based on the characteristic scales of our geometry.

From the definition of , we know: 
where  is shear velocity. The target  value and fluid properties are known a priori, so we need to calculate the frictional velocity as given above.
The wall shear stress, can be calculated from skin friction coefficient,  is such that: 

Thus to calculate we need to know, there are empirical formulae to calculate  which are given as:

For internal flows - 
For external flows - 

 

Now we know that 

Velocity at inlet = 25 m/s 

Reynolds Number =1,759,696

 Desired Y+ value = 300

Substituting the value we get the wall thickness  = 3 mm 

 

 

 

 

 

 

 

Density Based Solver or Pressure Based Solver :

The pressure-based solver traditionally has been used for incompressible and mildly compressible flows. The density-based approach, on the other hand, was originally designed for high-speed compressible flows. Both approaches are now applicable to a broad range of flows (from incompressible to highly compressible), but the origins of the density-based formulation may give it an accuracy (i.e. shock resolution) advantage over the pressure-based solver for high-speed compressible flows.

Speed of Air at = 25 m/s 

Speed of Sound = 344 m/s

Mach Number = 25 / 344 = 0.07 

Since the mach number is less than 0.3 we will be using the pressure based solver since it an incompressible flow .

Reynolds Number =1,759,696

Therefore the flow is turbulent .

 

 

Case 1 : For velocity of 25m/sec and Refined Meshing

 

 

 

 

 

 

 

 

 

Case 2 : Vector plot clearly showing the wake region 

 

 

Case 3 : Grid Independency Test 

Grid independent test appears to mean the same thing but the name is non-sensical since no, or at least very,very few, numerical entities are ever grid independent. *Grid convergence is the term used to describe the improvement of results by using successively smaller cell sizes for the calculations.

a. No elements = 181539 ,Inner enclosure Mesh size = 50 mm , Car wall mesh size = 10 mm

 

 

 

 

b. No elements =2307249  ,Inner enclosure Mesh size = 25  mm , Car wall mesh size = 5  mm

 

 

 

 

 

Conclusion

1.The coefficient of drag is 0.14 and the coefficient of lift is 0.09

 

2. The following is the reason for the negative pressure in the wake region:

Wake is the region of recirculating flow immediately behind a moving or stationary blunt body, caused by viscosity, which may be accompanied by flow separation.

As the air hits the frontal face of the body the maximum pressure is been created and the flow gets separated into two regions, above the body and lower the body. Ahmed body is more of a rectangular shape and hence the separated air along the body travelling to the rear end doesn't contact the body surface. This causes the void developing the negative pressure where the pressure even drops further at the corners of the Ahmed body which we see in above contours.

3. Significance of the point of separation

Boundary layer separation occurs due to adverse pressure gradient which means the flow takes place from low pressure area to high pressure area. Due to viscosity the velocity of fluid particles reduces. A point comes when the velocity of fluid particles first reduces to zero at the wall (also known as separation point) and then to negative or the direction of fluid particle changes at the wall and the areas close to the wall while the layers of fluid above them continues to flow in same direction. This leads to boundary layer separation