May 27, 2021 at 6:16 pm

peteroznewman

Subscriber

I found an interesting page on scaling fluid flow models using Reynolds Number. https://physics.stackexchange.com/questions/313685/understanding-the-reynolds-number-scaling-problem

You revised the Reynolds number to 10^7 at 130 m/s so I estimated a 1.2 m chord for the real airplane.

If the physical prototype is a 1/6 scale model with a chord of 0.2 m then the velocity has to increase to 780 m/s to keep the Reynold's number the same, assuming the pressure and fluid stay the same.

However, the speed of sound is 343 m/s and supersonic flow is a different physics, so you can't just increase the velocity. The article mentions raising the pressure to get the same flow, so a 1/6 scale model can be tested in air at 6 atmospheres of pressure. The other way to test the scale model is to use water instead of air and adjust the velocity downward.

Emma, if you just want to compare a simulation result of a 0.2 m chord model with an experimental result of a 0.2 m chord prototype, then use 100 m/s and don't worry about scaling it up to a full size result.

if I understand Y+ correctly, the first cell height should be equal to the Estimated wall distance shown below. I choose 100 m/s because lower velocities increase the wall distance. Y+ of 1.0 is desirable to get a good estimate of the drag coefficient when using the SST k-w turbulence model and the mesh should have at least 10-15 layers in the boundary layer.

You revised the Reynolds number to 10^7 at 130 m/s so I estimated a 1.2 m chord for the real airplane.

If the physical prototype is a 1/6 scale model with a chord of 0.2 m then the velocity has to increase to 780 m/s to keep the Reynold's number the same, assuming the pressure and fluid stay the same.

However, the speed of sound is 343 m/s and supersonic flow is a different physics, so you can't just increase the velocity. The article mentions raising the pressure to get the same flow, so a 1/6 scale model can be tested in air at 6 atmospheres of pressure. The other way to test the scale model is to use water instead of air and adjust the velocity downward.

Emma, if you just want to compare a simulation result of a 0.2 m chord model with an experimental result of a 0.2 m chord prototype, then use 100 m/s and don't worry about scaling it up to a full size result.

if I understand Y+ correctly, the first cell height should be equal to the Estimated wall distance shown below. I choose 100 m/s because lower velocities increase the wall distance. Y+ of 1.0 is desirable to get a good estimate of the drag coefficient when using the SST k-w turbulence model and the mesh should have at least 10-15 layers in the boundary layer.