I am working on my final year project, and am at a point which requires us to deflate a tyre in 0.8 seconds. This needs to be simulated using CFD. We have drilled a hole into the rim and attached a valve which will be opened with a button. Currently, we need to determine how many holes are required for the system to deflate within 0.8 seconds. The tyre is 8bar and we have a straight connector joining the rim and the valve. The current issue is that we are unsure of how to determine the rate of the deflation using CFD ie. how to get the correct graphs that show us the pressure drop over time. We are currently working on an inlet condition of 8bar and outlet being 1 bar, and wanting to determine how long it will take for the inlet pressure to reduce to about 1 bar. We are using StarCCM+, Ansys, and Flownex, but are open to using other software that may be more suitable.

  • $\begingroup$ You can do it 0.8 seconds - just use a big hole, seen commercial tires explode… $\endgroup$
    – Solar Mike
    May 10, 2022 at 6:02

1 Answer 1


I don't think CFD is the right tool. You can use an orifice equation, say from ASME, to calculate flow rate from inner and outer pressure. This takes the form of:

$$\dot{m}=\frac{EC_D}{\sqrt{1-\beta^4}}A\sqrt{2\rho\Delta P}$$

You can choose a starting pressure, find your instantaneous flow rate, then use Ideal Gas Law to step the pressure forward in time and recalculate the flow rate until the tire is empty. Then you can tweak the hole diameter and number of holes until you get what you need.

Predicting orifice flow relies heavily on empirical data, and I doubt it can be accurately modeled in CFD simply by solving the Navier-Stokes equations, or even using turbulence methods.


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