Project

External Aerodynamics of a Mercedes Truck After Performing Surface Wrapping in STAR-CCM+

MMohammad Saifuddin16 Feb 2020

Surface Wrapping

Surface wrapping is a method of extracting the desired surface with the desired volume of interest to perform CFD analysis. Surface wrapping does not incorporate all the minor details of the inside of the geometry, it only takes the outer detail of the geometry. When the CAD file contains a lot of errors and requires a lot of manual input to rectify all those errors, then to save time and energy we perform surface wrapping. In this project, I am performing external aerodynamics of a Mercedes truck. The CAD file contains a lot of errors, so I am performing a Surface wrap operation in STAR-CCM+. And then performing the external aerodynamics on this surface wrap.

CASE 1: External flow analysis of the truck with the trailer attached

1. 3D CAD

Surface repair

The first step is to import the CAD file. Now we can see that some surfaces are missing. We have to repair the CAD file before proceeding any further.
The geometry is missing the windows. We can select the edges and then use the fill hole feature to generate the window surface.

The full-scale model cannot be simulated on a laptop because of the limited computational power.
So, we have to scale down the geometry. The full-scale geometry provided is 15 x 2.5 x 4 m in dimension. According to laptop capability, we will scale it down 125 times smaller.
The new dimension of the truck: 12 x 2 x 3.2 cm. Now we can easily run the analysis on the laptop.
After the scale down process, we have to create a box for the airflow outside the truck.

Enclosure

Surface wrapping

Now we will perform the surface wrapping, as the geometry of the truck contains 637 pierced faces. The removal of all the pierced faces will require a lot of manual work.
Before surface wrapping, we have to name all the surface for the surface control. This will ensure a high-quality surface wrap.
Will will also define the contact prevention for many surfaces where we see the surface distortion after the first warp operation.

After surface wrapping, we will proceed to define a new automated mesh operation on this surface wrap.

2. Meshing

Now we will remesh this surface wrapper with the automated mesh operation.
We will use trimmer mesh in the meshing operation.

3. Physics setup

All the physics models used in this simulation are shown below.

Inlet - velocity inlet: 33.33 m/s.
Outlet - pressure outlet: 0 Pa (Gauge).
Road: wall
Remaining sides of the enclosure: symmetry plane.
Truck surface: wall.
After this, we will calculate the frontal area for calculating the drag and lift coefficient.
Now we will run the simulation for 750 iterations.

4. Postprocessing

Residuals

Drag coefficient

DC = 0.6989908511409354.

Lift coefficient

LC = 0.2746638400030229

Velocity contour

The section plane is filled with velocity values and the truck surface is colored with pressure values.

Pressure contour

Truck surface and the entire domain is colored with the pressure values.

Pressure values in different areas of truck surfaces

Velocity Streamlines

Streamlines are representing the velocity values and the truck surfaces are representing pressure values.

Velocity streamlines animation

From the graph of residuals, we can see that the solution has converged and reached a steady-state condition.
We can see that the graph of of drag coefficient has reached a steady-state condition. DC = 0.6989908511409354.
The lift coefficient graph has also reached a steady-state condition and the value of LC is 0.2746638400030229.
From the velocity contour, we can see that the velocity is high at the front upper edge of the truck and very low behind the trailer of the truck. The maximum velocity reached in during the analysis is 44.579 m/s.
From the pressure contour, we can see that the high value of pressure is at the front of the truck. The maximum pressure reached is 757.04 Pa.
We can also generate velocity streamlines to observe the fluid flow path around the truck. We can use the tracers to generate the animation.

CASE 2: External flow analysis of the truck without the trailer

Every setting will remain the same in case 2, the only change is that we are removing the trailer from the truck. After that, we have to update the surface wrapper and the automated mesh operation. Then run the simulation on this new geometry.

1. Geometry after removal of the trailer

2. Mesh

3. Physics Setup