External flow simulation over an Ahmed body.
External flow simulation over an Ahmed body.
Aim:
Q1. Describe Ahmed's body and its importance.
Q2. Explain the reason for the negative pressure in the wake region.
Q3. Explain the significance of the point of separation.
Expected Results:
1. Velocity and pressure contours.
2. The drag coefficient plot for a refined case. ( For velocity of 25m/sec, the drag coefficient should be around 0.33).
3. The vector plot clearly showing the wake region.
4. Perform the grid independency test and provide the values of drag and lift with each case.
Explanation:
1. 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.
AHMED's BODY
Geometry:
2.Reason for the negative pressure in the wake region
When fluid velocity increases, the pressure decrease. The wake region is formed due to adverse pressure in the rear end causes the boundary layer to separate out from the body. The wall shear stress and velocity becomes zero and wake formation starts. These wakes are nothing but small vortices that create negative pressure. These create circulation of velocity and induces drag forces.
3.The significance of the point of separation.
point of separation.
Separation occurs due to an adverse pressure gradient encountered as the flow expands, causing an extended region of separated flow. The part of the flow that separates the recirculating flow and the flow through the central region is called the dividing streamline. The point where the dividing streamline attaches to the wall again is called the reattachment point. As the flow goes farther downstream it eventually achieves an equilibrium state and has no reverse flow.When the boundary layer separates, its remnants form a shear layer and the presence of a separated flow region between the shear layer and surface modifies the outside potential flow and pressure field. In the case of airfoils, the pressure field modification results in an increase in pressure drag.
POINT OF SEPARATION
significance
The flow around the Ahmed reference body is complicated. Due to the slant in the rear of the vehicle,flow separation and counter rotating vortices are generated at the slant edges. The drag forces of Ahmed Body reaches at maximum when the slant angle is 30 deg. For slant angles higher than this value, the adverse pressure gradient in between the slant and the roof is so intense that the flow fully detaches over the slant. Below this critical slant angles (30 degree), the flow still separates but the pressure difference between the slant region and the side walls is still sturdy enough to generate substantial stream-wise vortices at the lateral slant edges. These prompt a downward motion over the slant, mainly in the downstream part. As a result, the flow separating at the upstream end of the slant can coupled further to the downstream. The flow around the Ahmed body has several flow separations from the front to rear of the vehicle. The flow recirculation caused by these flow detachment contributes the vehicle’s drag. The location point at which the flow separates determines the size of the separation zone, and accordingly the drag force, thus an exact simulation of the wake flow and of the separation process is essential for the accurate result of drag predictions.
Now to get the expected results we have to go through the following steps:
The process of discretizing our domain of interest i.e. the geometry into smaller pieces is called mesh generation. Ansys Fluent uses a technique called the "bounding box" method to generate a body-fitted mesh. This is done by creating the smallest size hypothetical box possible that can be used to fit the entire geometry into it. We will be running a total of 3 case setup & each of the cases will have varying mesh refinement, therefore each case will have varying "total number of cells" & "total number of nodes". This is done to perform a Grid independence test & compare the simulation results.
step-1:
For Case 1, we will be using a tetrahedral mesh with default size, which generates a total cell count of 84053 & no additional layers of mesh will be added. This will be used as our base mesh & the mesh will be evenly spread around the Ahmed body wall & the wind tunnel section
Importing Geometry and Editing:
We start by importing the pre-modeled Ahmed body geometry to Spaceclaim, where we will be editing our geometry according to our simulation requirements. For any external aerodynamic simulation, the first step of the editing process involves designing a virtual wind tunnel. The dimensions of the wind tunnel vary according to the geometry of the object that is to be simulated. For our case, our Ahmed body geometry is of length 1.04m & our wind tunnel will have dimensions of 2m upstream of the Ahmed body & 5m downstream with a height of 1m from the origin (bottom of the Ahmed body). The designing of the wind tunnel geometry is carried out using the "Enclosure" feature available in Spaceclaim software. The "Enclosure" feature allows users to create an enclosure around the geometry, the enclosure can be a box, cylinder, or sphere. For our purpose, we used a "Box Enclosure" .
First open the file where Ahmed body file is and drag it to spaceClaim Geometry.
Then Go to prepare-Enlouser-select Ahmed body-uncheck the symmetry option-ctrl+6 to see the front view and change the geometry as described above -ctrl+4 to see the side view and change the sides as 0.5 each and then click on the green button and ctrl+0 to see the isometric view and click escape.
Front view:
side view:
Isometric view:
Then uncheck the "Ahmed body 20" portion on the left corner and ansys automatically delete the portion of ahmed body into the wind tunnel . we want that to see the mesh inside that portion. Then close Geometry and open mesh.
In mesh first generate mesh -then go to section plane and from top view cut the enclouser -we want to see the portion of mesh where the Ahmed body is and we see it coarse mesh and so we go to Geometry portion and suppress for physics the "Ahmed body 20" portion ,as here we can see two ahmed body and close it and come to mesh - generate mesh-update. Here we can see the cut portion of the wind tunnel mesh.
Then we have to name the boundaries of the body so first select face selection -then select the left portion as inlet and type 'n' on keyboard and name selection board will poop up. Like this select right portion as outlet and others as symmetry(while selecting others select one wall then before selecting other walls press ctrl and select otherwise it will be treated as single select)
Now we have to select the car wall so that we can name it .so we have to hide the wind tunnel so that we can select the car wall.so select all portion of the wind tunnel and right click-hide(we can type F8 after selecting walls instead).
After hiding select car-wall by selecting the select-mode as box selection mode and name it as car-wall.
After that generate the mesh-update and close the mesh portion . Now open set up (use double precision).In the mesh portion we are using the default mesh and not refining any mesh.
In set-up If mesh is not displaying select the marked portion and display edge and faces following below
Then go to physics,choose reference value at the left corner and select following
we choose k-epsilon ,standard model for viscous portion.
We are using pressure-based solver ,absolute, stedy state.
Tick the energy portion as we are considering air temperature.
In boundaries portion choose inlet-edit-velocity-25m/s,outlet-gauge pressure-0 pa
In solution go to method and select pressure-velocity coupling-coupled
Why we are using pressure based solver ?
Because-pressure based coupled model is a pseudo transient model and its accuracy is more than pressure based SIMPLE model as it solve the equation at each timestep.Also in pressure based model drag coefficient curve become constant into several iteration but density based solver case the drag coeffient rise ups and downs and after a large iteration become constant.
In solution-Defination-New -Force report-both drag and lift one by one
Then for initilization -Hybrid, initialize it , choose iteration no -150 then calculate.save the plots by clicking on files-then save picture.
Then close set up and go to results.
In the results portion we plot a xy plane by insert-Location-plane.We can change its Geometry to xy plane and z=0,choose its colour as variable and can select pressure/velocity.
WE can also choose vector from insert option and plot it to this plane or any plane . Like this we can also plot streamlines.
RESULTS:
pressure contour:
velocity contour:
drag coefficient:
lift coefficient:
drag force:
lift force:
Step-2:
we will be adding a volume of "Box Enclosure" near the Ahmed body geometry with dimensions of 0.5m upstream, 1m downstream & 0.5m height. This region of enclosure is added to provide a more refined mesh near the Ahmed body where the wake region will be more prominent. We will be making use of the "multi-zone" feature available in ANSYS, the MultiZone mesh method provides a combination of pure hexahedral mesh wherever possible & tetrahedral mesh near the geometry region. For our case, the hexahedral mesh will dominate the entire "outer box" or the wind tunnel region with a mesh element size of 80mm i.e. will have a coarse mesh. The inner volume or the "smaller box" will have a mesh element size of 40mm i.e. has a finer mesh
In geometry first uncheck the first encloser then select the second enclouser as described previously .
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 and also share the boundaroies for the enclouser.Here shows the image below.
First go to Design-create-plane- then click at the centre of x-y-z , escape-ctrl+6 , In the mode-check section mode, before that uncheck all planes and check only the first plane , then go to prepare-in the remove portion go to inteference- select green tick-escape.
Then in The top left portion select FFF-3 then in the bottom of that share topology option-select share.Then close it and open mesh.
In mesh right click -then insert method -In method select multizone-In mesh select element size 80 mm and in sizing select max size 80mm (Select outer wind tunnel as geometry).
Then mesh-right click-insert-sizing and select the inner enclouser , keep element size 40mm and generate mesh. But we can see that legs of the Ahmeds body is not round so we have to do sizing again .
We select face sizing of the legs and keep element size 5mm.Then it seems round.
Next to capture the flow over the body we make the inflation layer which helps us in caputuring the flow very finer way and the mesh will also be capture along the body perfectly.We take the number of inflation layers as 6, Total thickness value as 3.6mm.
Here go to inflation - Then use Automatic inflation-All faces in chosen name,
Named selection-car-wall,Inflation option-First layer thickness, First layer Hight-3.6mm,Maximum layers-5
For getting the total thickness we are taking the Y+ = 100.
Some of the calculations needed to find these values,
where,
ρ , µ are fluid density and viscosity ; v is the free stream velocity ; L is characteristic length ; Uf is frictional velocity ; Tw is wall stress , Cf is skin friction coefficient
In set up portion viscous model will only be changed - k-omega SST
Rest are same as described above.
Results portion also be edited as described above.
RESULTS:
meshmetric:
As seen, most elements have their quality >0.6, hence it can be said that the mesh quality is satisfactory.
pressure contour:
velocity contour:
drag coefficient:
drag force:
lift coefficient:
lift force:
case-3:
In the case-3 the outer box has element size as 80mm and inner box has element size of 32.58mm and the Geometry is same as before.In case of set up it is
pressure based solver
velocity at inlet 25m/s
outlet gauge pressure 0 pa
Reference values:
RESULTS:
pressure contour:
Velocity contour:
Velocity contour with mesh:
Velocity contour with vector:
velocity contour at YZ plane in different position of wake region:
at x=1.05m
at x=1.1
at x=1.3
at x=1.5
show planes:
Drag coefficient:
Drag Force:
Lift coefficient:
Lift force:
Data Table:
| Drag coefficient | Drag Force | Lift coefficient | Lift Force | Element | Nodes | mesh size at small enclouser | mesh size at large wind tunnel | |
| case-1 | .7267 | 18.92 | .4135 | 10.83 | 78181 | 45970 | 120 | 163 |
| case-2 | 0.4 | 18.33 | .3267 | 14.80 | 264422 | 80294 | 40 | 80 |
| case-3 | .3287 | 17.93 | .2792 | 14.75 | 432132 | 110469 | 32.58 | 80 |
Conclusion
- From running a baseline simulation, the post CFD results gave inaccurate results at the region of mesh overlapping.
- The grid independency test shows best results close to the experimental data. With refining the mesh size with y+ factor, the error is significantly reduced.
- The y+ is chosen in between a stable range to give a correct prediction of velocity and pressure contour plots.
- The flow seperation and appearence of vortices gets more prominent as y+ is increased. As seen in baseline meshing no wake formation is captured properly, on refining the mesh, the flow is captured properly.
REFERENCES:
ANIMATION OF WAKE REGION:
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