EXTERNAL AIRFLOW SIMULATION AROUND MB-ACTROS TRUCK USING STAR CCM+

OBJECTIVE:

The aim of this project is to simulate an external air flow over a Mercedes Benz Actros truck in STAR CCM+. It'll be done with two configurations, one being the whole truck along with the trailer and container(oriented in a way during cornering) and the other being only the truck alone. For the mentioned configurations, the drag coefficient and lift coefficient will be calculated, compared and inferred. Also, streamlines are used to visualize the path of the flow.

PROCEDURE FOR THE TRUCK WITH THE TRAILER AND CONTAINER:

CAD import and virtual wind tunnel creation:

The first step is import the Actros truck CAD model and a virtual wind tunnel is created around the truck with appropriate dimensions. The truck is as follows.

Fig-1:Truck CAD model

Fig-2:Truck with virtual wind tunnel

During the extrusion of the wind tunnel sketch, the parts are merged. It'll be done after the new parts creation.

Surface repair and boolean operation:

After this, a new part is created from the 3D CAD model. Then a boolean operation is carried out. The truck body group is subtracted from the wind tunnel body. Once subtracted, the next step is repair the surface. The first thing to do in the surface repair tool is to close the truck windshield openings using the fill hole option. It is very essential to perform an external flow analysis.

Fig-3:Surface repair window

Scaling down the model:

The truck model we have is a full scaled truck. The computational cost will be very high if we simulate the same. So it is scaled down to 1/32 times of the original model and the simulation is carried on.

Surface wrapping and Boundary condition:

As can be seen from the above picture, the number of surface issues is very high in such a way that it is a tedious process to correct them manually. In these kind of situations, it is efficient to wrap the whole surface using the surface wrapper tool. It is somewhat similar to a surface mesh creation, in which all the surfaces of the body are captured and recreated using triangular elements. Custom controls can be implemented for the areas which require precision. For that surfaces selected and appropriate names are given for them.

Fig-4:Surface wrapped truck body

Once the surface is wrapped, it should be checked for leak detection. And then regions are created and boundaries for each part surface. The boundary condition is as follows.

1. Inlet - Velocity inlet
2. Outlet - Pressure outlet
3. Sides of the wind tunnel - Symmetry
4. Truck - Wall

Mesh generation:

Once the boundary conditions are set, it's time to mesh the model. The following parameters and specifications are made.

Meshers:

1. Surface remesher
2. Automated surfair repair
3. Trimmed cell mesher (Best suited for external aerodynamics)
4. Prism layer mesher ( Necessary to capture the properties at wall)

Default controls:

1. Base size : 40 mm
2. Minimum surface size : 12 mm
3. Maximum surface size : 200 mm

Custom surface control for Truck:

1. Target surface size : 2 mm
2. Minimum surface size : 2 mm
3. Number of prism layers : 6
4. Prism layer total thickness : 10 mm

Custom surface control for Wind tunnel:

1. Target surface size : 60 mm
2. Minimum surface size : 60 mm

Fig-5:Mesh model

Fig-6:Mesh scene in a plane section

Fig-7:Mesh scene with prism layers

Physics continua :

Selected physics models:

1. Space : 3-Dimensional
2. Time : Steady
3. Fluid : Gas
4. Flow : Segregated (No coupling in this velocity range)
5. Equation of state : Constant density (Incompressible flow)
6. Viscous regime : Turbulent
7. RANS model : Realizable K-Epsilon (Effective and cost efficient for this particular setup as there is no adverse pressure gradients)
8. Wall treatment : All y+ (Works well irrespective of the placement of the first node from the wall)

Physics values in the boundaries:

1. Inlet velocity : 33.33 m/s (120 kmph which is common for trucks in highways. The same is used for the scaled down model also, because, beyond a Reynolds number of 1e5, the flow becomes Dynamically similar)

2. Outlet pressure : 0 Pa (Gauge pressure)

Reports, plots and derived parts:

Reports for Drag coefficient of the truck, Lift coefficient of the truck and the velocity at the outlet are created with appropriate reference values. Then separate monitors and plots are also created for the same.

A plane which is derived part is already created. After that, Streamlines are created which is very much important to visualize the flow around the truck.

Calculation :

Once all the necessary post processing tools are created, the stopping criteria is set as 500 and the simulation is made to run.

Fig-8:Residuals-Simulation of truck with trailer

Fig-9:Velocity at the outlet - Simulation of truck with trailer

Convergence:

From the above two plots, it is clearly seen that the solution has converged.

RESULTS:

Fig-10:Drag coefficient - Simulation of truck with trailer

`C_d` = 0.705

Fig-11:Lift coefficient - Simulation of truck with trailer

`C_l` = 0.53

Fig-12:Velocity contour around the truck with trailer

Fig-13:Pressure contour

Fig-14:Streamlines around the truck with trailer

Animation-1 : Streamlines with truck coloured by pressure

Animation-2 : Streamlines around the truck with trailer

Inference from the contours and animations:

1. From the contours, it can be clearly seen that the velocity is at the lowest at the rear side of the container. It is because of the recirculation region also known as the wake region. It's where the region, the flow reversal happens.

2. And due to the flow reversal, the difference in velocities between layers(also the wall) is high. This caused the adverse pressure gradient which produces recirculation region. So the velocity is less in that region.

3. The pressure has it's highest value at the front face due to the direct impact and obstructing the flow and it is the lowest at the rear end because of flow reversal. This pressure difference in turn causes induced drag.

4. The streamlines show the path of the air particles and how they are trapped in the wake region.

PROCEDURE FOR THE TRUCK WITHOUT THE TRAILER AND CONTAINER:

Once the trailer and the container is removed, the truck will be like as below.

Fig-15:3D CAD model of the truck without the container

Similar procedure for surface wrapping and meshing:

The same parameters and specifications are set for this configuration also. With the same values, the wrapping is done and mesh is generated.

Fig-16:Surface wrap of the truck

Fig-17:Mesh model of the truck

Fig-18:Mesh model in Plane section view

Fig-19:Plan section view with prism layers

Continua, Physics values and calculation:

The same physical models are selected and the inlet velocity is set as 33.33 m/s and the calculation is done.

Fig-20:Residuals - Simulation of truck without the trailer

Fig-21:Velocity at the outlet - Simulation of truck with trailer

Convergence:

From the above two plots, it is clearly seen that the solution has converged.

RESULTS:

Fig-22:Drag coefficient - Simulation of truck without trailer

`C_d` = 0.221

Fig-23:Lift coefficient - Simulation of truck without trailer

`C_l` = 0.23

Fig-24:Velocity contour of the truck without trailer

Fig-25:Pressure contour of the truck without trailer

Fig-26:Velocity Streamlines of the truck without trailer

Animation-3: Streamlines with truck coloured by pressure

Animation-4: Streamlines around the truck without container

Comparison of streamlines:

Animation-5 : Streamlines comparison between the configurations

Inference from the contours and animations:

1. First thing is that without the extra components, the form of the whole body simplified(low frontal area) which heavily reduces drag.

2. From the streamlines, we can see that the flow separation is severe in the case with the trailer and container, but mostly attached in the second configuration. This also reduces drag.

3. This is due to the absence of gap between the truck and the container. In the first configuration, the flow hits the truck and it immediately separates and then have to flow around the container. So the separation is more.

Comparison of force coeffcients between the configurations:

	Drag coefficient	Lift coefficient
With container	0.705	0.53
Without container	0.221	0.23

CONCLUSION:

It can be concluded that without the trailer and container, the drag is reduced upto 68% of the original drag especially during cornering. The velocity streamlines behaves more or less similar in both the configurations. The pressure difference between the rear and front end is higher in the configuration with the trailer and container which means that the induced drag is also high in that configuration.