Virtual wind tunnel · STEM Racing & rockets
DesignClass CFD comparison score. Use it to compare designs in this tool only. It is not a real-world Cd and won't match a wind tunnel or professional CFD. Why?
Your wind tunnel is empty.
Upload a STEP file of a STEM Racing car or a model rocket.
A STEP file stores the exact curved surfaces of your design. The OpenCascade CAD engine (the same one inside FreeCAD) turns each surface into thousands of small flat triangles. Mesh detail controls how closely the triangles follow the curves.
Your model is placed in a virtual box of air, split into tiny cubes called voxels (like 3D pixels). Voxels inside the model become solid. There is space in front, lots of space behind for the wake, and room at the sides. The sides and roof are open: air pushed aside by your model can flow out, just like in open air, so the tunnel walls don't squeeze the flow.
Each voxel holds 19 "packets" of air moving in different directions. Every time-step, packets hop to the next voxel (streaming), bounce off your model's surface, then mix together (collision). The bounce uses the exact position of the surface inside each voxel, so curves aren't treated like Lego blocks. Repeat this thousands of times and realistic airflow appears. Your graphics card does millions of these updates every second using WebGPU.
Where air slows down against the nose, pressure rises (red). Where it speeds up around curves, pressure drops (blue). Every packet that bounces off your model gives it a tiny push. Adding up all those pushes gives the drag force (this is called momentum exchange). Dividing by the wind's "push" and the frontal area gives the drag coefficient Cd. The split into pressure drag and friction drag comes from reading the pressure and the air speed just next to every triangle.
Real airflow depends on the Reynolds number (speed × size ÷ air's viscosity). A real STEM Racing car is around 250,000. To run on a school computer, this tool uses a coarse grid, which can only simulate a few hundred to a few thousand. That's like testing your car in syrup: the airflow pattern is similar, but friction is far more important than in real life, and the flow stays smooth (laminar) instead of turbulent. So the numbers are useful for ranking your own designs against each other but not as real-world values.