When Tesla launched their Cybertruck pickup model in November, I was intrigued by its unique shape. Given the very angular, wedge-like styling, the first thought that came to mind was that I could have designed the exterior shape in SOLIDWORKS in 5 minutes with a few extrudes, cuts and draft features. I have no doubt that I could have saved Tesla millions of dollars spent on their exterior stylists and designers! I also pondered whether this shape performed well aerodynamically by achieving low drag. Given the sleek shape of the popular Tesla Model S, it certainly appears that Tesla’s engineers value low drag body shapes. I was curious as to whether the Cybertruck upheld that ethos when compared to other competitors in this sector of the automobile industry.
I set about creating a ‘best guestimate’ 3D model of the Cybertruck in SOLIDWORKS. I started with a profile view photo of the Cybertuck which I imported into SOLIDWORKS using the ‘Sketch Picture’ command. I scaled the picture to the right length and height based on published data of the Cybertruck’s principal dimensions. I was able to create the basic shape very quickly using extrudes, cuts and drafts. My 5-minute estimate to model the basic shape of the car was not far off! That said once I was fully engaged in the modeling exercise I decided to include more of the details such as the front and back bumpers, windows, wheel well protectors, wheel spoke details and step feature. This obviously took more time. All told my modeling effort took about 12 hours.
I created two configurations, one with an open truck bed and the other with a fully enclosed bed. I thought it would be interesting to compare the aerodynamic performance of these different configurations.
Here’s what the resulting ‘open bed’ configuration looks like.
Here’s what the resulting ‘closed bed’ configuration looks like.
With the CAD modeling complete I then turned to SOLIDWORKS Flow Simulation to study the aerodynamics.
Taking advantage of geometric symmetry, by making use of symmetry boundary conditions in SOLIDWORKS Flow Simulation is a highly recommended best-practice technique for speeding up your flow analysis runs. Given the longitudinal plane of symmetry that exists on the car’s center plane, the Cybertruck presented a perfect test case to quantify the time saving benefits that result when a symmetry boundary condition is activated.
My computational domain extended 8m upstream of the front of the car, 20m downstream of the front and 4m above ground. For the case where I didn’t use the symmetry boundary condition, my domain extended 4m on either side of the center plane. For the case where I activated the symmetry boundary condition, the domain extended 4m out from the center plane on the driver’s side.
Symmetry can be activated by first right clicking on ‘Computational Domain’ in the design tree and then left clicking on ‘Edit Definition’.
Once inside the ‘Computational Domain’ property manager, symmetry is activated by left clicking on the ‘Boundary Condition’ drop down menu to the right of the domain ‘Size and Condition’ fields.
Once symmetry has been activated, SOLIDWORKS Flow Simulation gives excellent visual feedback to the user by including the mirror image of the domain as a dashed line.
I chose to solve both the open bed and closed bed configurations, with and without symmetry, while monitoring the number of cells, drag force, drag coefficient and CPU time. Creating multiple studies is easy in SOLIDWORKS Flow Simulation using the ‘Clone’ command.
A result plot of the open bed case without symmetry, showing relative pressure contours on the body of the car and velocity streamlines in the flow field is shown below.
The tabulated results for the solved SOLIDWORKS Flow Simulation projects are shown below.
In each case, there are only 56% as many cells in the domain when symmetry is activated. The closed bed case with symmetry activated only takes 39% of the CPU time compared to the case without symmetry. The open bed case with symmetry activated does even better and only takes 33% of the CPU time compared to the case without symmetry. The drag is 5% lower when symmetry is activated in the closed bed case, and 6% lower in the open bed case.
I found these results to be interesting. With only 56% as many cells in the domain, I would have expected something like 56% of the CPU time to solve the problem. However, the CPU time savings were even better than that. I think the reason for this is that the center plane symmetry boundary condition ensures that the flow has no Z component of velocity at this boundary. Without any crossflow to account for, the solver converges quickly. Without enforced symmetry, turbulence in the downstream wake of the truck causes small amounts of crossflow (i.e. flow actually does cross the center plane despite the geometry being totally symmetric about the center plane) which causes the solver to take longer to converge.
This lack of crossflow also explains why the drag is lower when the symmetry boundary condition is activated. The symmetry constraint is enforcing flow that is uniform and parallel to the center plane (i.e. no Z component of velocity), which reduces the energy loss due to vortices in the flow and results in a lower drag.
One might ask, which is approach (with or without symmetry) is correct? In my opinion, it depends on what the end goal of your analysis is. If your end goal is to evaluate different exterior shape permutations and compare them against each other for the purposes of a preliminary design study, then activating symmetry is highly recommended because the time savings are substantial. If the end goal is to validate a final design with the most accurate computation that captures the physics as closely as possible, then solving the full domain is what I would recommend.
In term of drag coefficient and focusing on the no symmetry cases since these more closely reflect reality, the Tesla Cybertruck achieves a drag coefficient of 0.38 with a closed bed configuration and 0.41 with an open bed configuration. I believe from the literature on the subject that this is low compared to other pickup trucks. That said, in my assessment, the engineers at Tesla have a lot of work to do to achieve a drag coefficient of 0.3 for the Cybertruck, which is what Elon Musk recently claimed was their goal.
Of course, my geometry is only approximate and it sounds like the final shape is still in development. Perhaps the final shape will include some modifications that provide aerodynamic benefits that I didn’t capture when I modeled the Tesla Cybertruck.
I hope you have found this article interesting and it has provided some guidance as to the use of symmetry boundary conditions in SOLIDWORKS Flow Simulation. In my next blog I plan to expand on this topic further by exploring some nuances related to the correct setup of a moving vehicle in a CFD ‘virtual wind tunnel’. I also hope to get my hands on a CAD model of a more conventional pickup truck (like a Ford F-150) so I can compare it’s drag performance to the Tesla Cybertruck.
Now go design some innovative products with the help of the comprehensive analysis tools available in SOLIDWORKS Flow Simulation.
Simulation Product Specialist
Computer Aided Technology, LLC