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# Checking the flow type inside an empty cavity

## Checking the flow type inside an empty cavity

(OP)
Hello everyone,

In the system I am working on, I have an empty cavity that has static air inside. One of the walls of the cavity moves following the operation of the system and actually causes the air to move inside the cavity.
For a particular test, I need to check if the flow inside the cavity is laminar flow. And for that from what I understood I need to calculate the Reynolds number of the air in cavity.

The equation:
Re=(u*L*density)/(d. viscosity)

The typical length in the equation, is the moving wall length?
And how do I know what ranges give me a laminar flow in the cavity? of course, I know the speed of the wall.

thanks for the help.

### RE: Checking the flow type inside an empty cavity

Probably the average of width and height, or the equivalent diameter, like you would calculate for flow in a rectangular duct. I'd stay with that duct model for the laminar / turbulent boundary condition too.

### RE: Checking the flow type inside an empty cavity

If you are taking about a piston compressing air in a cylinder I find it difficult to imagine having turbulent flow. The air is compressing and while doing so, it is moving a little, but not really flowing. Even in pipe flow the transition from laminar flow to turbulent flow takes some time.

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