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It might be worth reviewing what’s happening in the domain. When driving a car with no wind, the air is mostly stationary and the car is moving at a velocity u. (Not all the air is stationary though, as some air is displaced by the car’s motion.)

However, you may have noticed that I talk about the air moving, rather than the car moving. When simulating something like this, it’s common to use the car as your frame of reference. This means that the car will have a velocity of 0, while the air (and the road) will be travelling with a velocity of -u.

Boundary Conditions

Boundary conditions are conditions that stay constant during the simulation. Every surface needs a boundary condition to solve the system, so we’ll create 6. Create a Velocity inlet, a Pressure outlet, 3 Walls, and a Symmetry condition.

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Now add a simulation run. If you anticipate running multiple simulation runs, try to give each run a descriptive name. The simulation usually takes a few hours to complete.

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Now how do you know if a simulation is complete? It doesn’t really matter how long a simulation has been running or how many iterations it has completed. What matters is the convergence of the simulation. A common way to track convergence is to plot the simulation residuals at each iteration. SimScale automatically generates this plot and it can be found in the navigation panel, under the run that you started, then under Convergence plots. The goal is to have residuals that are as close to zero as possible. They will not actually reach zero though, so you have to end the simulation when you've achieved sufficient convergence. At most, your residuals should be 1e-3 to have any confidence that your results are realistic, ideally they should be in the 1e-4 to 1e-6 range. At residuals of 1e-5 to 1e-6 you can have moderate confidence that your predicted drag and lift values will match the real values (Assuming the rest of the simulation was set up appropriately).

SimScale also has several other convergence plots for different regions of the simulation (domain, inlet, outlet, and walls), good convergence is indicated in these plots by achieving stable values that don’t change very much.

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title Convergence Plots
Figure 3.1: A residual plot, the main tool for determining convergence. Figure 3.2: A wall convergence plot, SimScale also creates a plot for the domain, inlet, and outlet.

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