hardingfv32 wrote: ↑
Mon Jun 04, 2018 5:12 pm
Obviously current wind tunnel usage yields very useful data.
How is that possible with such low air flow speeds? What is the science of scaling up the data so that it a useful representation of 2x-3x high air flow speeds?
Simplistically speaking, it comes down to matching similarity parameters which preserve similarity of viscosity and compressibility.
Reynolds Number is the similarity parameter for viscosity. It is defined as the ratio of the inertial forces (i.e. how resistant to some change in motion the flow is) vs. viscous forces (how sticky the flow is). You multiply the fluid density with its velocity and then some characteristic length (i.e. chord for a wing) and then divide all that by the flows viscosity coefficient. All the units cancel out and youre left with a dimensionless number. As you probably guessed, you can get this "constant" with any velocity and length you choose!! So if you are careful, you can scale up a wing x2 for example, and then as long as you 1/2 the velocity, you will obtain the same Reynolds Number. This means that you can treat the flows as somewhat identical. This is the easy part.
Where things get a bit tricky is that when you increase the velocity by enough (usually above 0.3x the speed of sound) air's compressible nature begins to have a fairly decent impact on the results. As your air moves over the part at lower speeds, the air is deflected and moved around the part. If your speed is above this ~0.3 Mach speed, then part of the energy that is in the flow is "lost" to being used to compress the air in regions where it first hits the goemetry (e.g. an aerofoils leading edge). Since there is a change in energy within the flow, thats where the tricky part comes in: measuring aerodynamic forces when there is a considerable amount of compressibility in either the scale or full-scale test will lead to different results if this similarity parameter isnt observed.
Manging both together is the tricky part since a decrease in scale leads to an increase in velocity in order to preserve Reynolds Number, which MAY lead to air exhibiting compressibility, which would influence your measured forces - making them less accurate. If Reynolds Number isn't kept constant, but Mach Number is, this can mean that the flow physics around a part aren't consistent, and yet again, give you incorrect results.