I used to dream of building one of these in my room haha, but decided the lack of compressability would make it not so worth while?
Like Trebuchet03 said, the lack of compressability is no big thing. If you want to satisfy yourself, get an aeronautics textbook and see what kind of speeds they are talking about before they start talking about a compressability factor, or have some sort of integration applied to the density, or something like that.
I have used a Shop-Vac and 2" yarn tufts to visualize flow on a full size car.
The problem is the air become stagnant very quickly, so you cannot use it for areas larger than around a foot.
Testing on the front and sides of the car was useful, but there is not enough velocity to test the areas around the back of the car.
Tuft testing the car on the road showed that the ShopVac testing was accurate.
I have thought of creating a wind tunnel using a ShopVac and an enclosed length of PVC pipe.
The Shop Vac would be sealed so that one side of the pipe would blow , and the other suck ) I still have to find a model of my car ( A 1993 Civic hatchback ) that is a useable scale though. So far all that I have found is a 1:24 madel.
As far as using water as a visualization tool, why not just take a length of pipe and a model car into the pool with you the next time that you go swimming ?
If you unhook the pool pump and attach it so that it cycles the flow back and forth, it seems like it could be useful.
I've been trying to figure something out with my bathtub. I'm thinking a weight and pulley arrangement to draw a model through water a measured distance in a measured time would make for some useful data. Also might be possible to find some neutral buoyancy colored beads, so one could get some flow visualisation out of it too. Can't think of a convenient material to make the models in though... or scale, 1:43 is a tad small still, 1:20 would be more like it... but then there's issues with the bath being deep enough....
I for one won't worry tooooo much about the compressibilty of air compared to water until I have my speedo marked out in Mach numbers... (i.e. worry about that when you're planning 350mph and faster)
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice