Lee Jay - 25/6/2007 5:54 AM
terynd - 24/6/2007 10:44 PM
Good point. We'll assume at the moment that we don't have any optical illusions/effects going on here. If that rainbow-colored cloud was the actual shock, then I dont believe that you would see it through condensation because the pressure increases through the shock (pressure has to drop to support condensation). If there's a rainbow, then there must be condensation or water vapor droplets to create the rainbow.
I don't really find this argument persuasive since there are other ways besides droplets to get scattering.
There are other ways to get scattering (dust, ice, etc) but since the scattering is localized in this case, only condensate droplets are reasonable.
This condensation is only created by a drop in local pressure. So, all this doesn't support that we are seeing the shock in this image. Plus we don't see the bow shock which would also be seen if there was a Schlieren effect here.
On the other hand, I find this argument very persuasive. If this is a shock, where is the bow shock! Case closed in my mind - P-G singularity caused the rainbow - makes more sense to me.
As far as I am aware, I don't think there is any natural "Schlieren effect." You can certainly visualize shocks (and other density gradients {technically the second derivative of density for shadowgraphs and the first derivative for Schlieren}) through the shadowgraph technique using the sun as a light source, however. Maybe this is what you are referring to.
My take on the origins of the various vapor clouds:
The shuttle is transonic in this image, meaning that the local flow over the shuttle is supersonic in some regions and subsonic in others, but the vehicle is traveling slower than the speed of sound. Because the vehicle is subsonic, there is no bow shock being generated, so even if there were some way to tell it was there, you wouldn't find one (shocks arent normally visible). Amplification of the low pressure regions (behind the cockpit windows, behind the rudder, and behind the SRB nosecones) by the Prandtl-Glauert singularity causes the condensation clouds to appear there.
The rainbow cloud is similarly formed. The subsonic/supersonic interplay generates a number of fairly weak shock waves originating at various locations on the vehicle. These shocks do induce a local rise in temperature and pressure, but because of the inertia of the gas, there is a rarefaction wave behind the shock (the rarefaction wave is necessary to cancel the shock at a finite distance from the vehicle since the vehicle is subsonic overall) that actually pulls the local pressure (and temperature) below the ambient values over a small region, before they rise back to ambient. When this low pressure is amplified by the Prandtl-Glauert singularity, it can induce condensation as is seen here, although you can compare the density of the condesation clouds behind the shock and behind the other cloud generating areas (e.g. cockpit) to get an idea of the relative pressure drop in each location and see that the pressure drop behind the shock is pretty low. I suspect the reason the fainter shock cloud is producing the visible rainbow is just that it is sparse enough to allow most of the light through and it is far enough from the vehicle to have direct illumination at the necessary angle.