# Lift of a Rotating Cylinder in Inviscid Flow

## Main Question or Discussion Point

Hi

I am wondering why a spinning cylinder will produce lift in an inviscid flow. From:
http://www.grc.nasa.gov/WWW/k-12/airplane/cyl.html

one of the mechanisms for lift generation was the sticking of fluid particles to the wall of the cylinder. I thought that the no slip condition only applies to viscous fluids so if the fluid was inviscid I don't see how lift can be generated. However, in the simulations in the link, the flow was simulated inviscid yet there is lift, why is this?

Thanks

## Answers and Replies

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In a truly inviscid flow it won't. When one analyzes a rotating cylinder with potential flow, one is essentially approximating the effect of viscosity through the use of a point vortex.

joshmccraney
In a truly inviscid flow it won't. When one analyzes a rotating cylinder with potential flow, one is essentially approximating the effect of viscosity through the use of a point vortex.
Hi

Thanks for the response.

Can you briefly describe what point vortex are? I was never introduced this.

And just to confirm, in the NASA simulation at http://www.grc.nasa.gov/WWW/k-12/airplane/cyl.html they state

This type of flow field is called an ideal flow field. It is produced by superimposing the flow field from an ideal vortex centered in the cylinder with a uniform free stream flow. There is no viscosity in this model (no boundary layer on the cylinder) even though this is the real origin of the circulating flow!
and is the reason that the simulation still shows lift when the cylinder spins because the supposed effect of viscosity is superimposed mathematically to the effect of the flow in the simulation that has no viscosity or boundary layer?

Thanks very much

You essentially insert manually the fluid rotation that viscosity would have caused. This breaks the symmetry of the flow, making flow on one side of the cylinder faster than the flow on the other one. Velocity is linked to pressure, which is what creates your lift.