I think I've figured out "Step 1" of understand flow separation, now I'm trying to work on "Step 2." It's akin to learning how to put a computer together, then trying to tackle building one completely from scratch...many orders of magnitude greater in difficulty.
It still boggles my mind to think of this phenomenon at the molecular level. If I can't visualize little atoms passing through the flow, then I don't really believe I understand the concept. In any case, here are a few diagrams I created, borrowed, or assembled from the internet to aid my learning.
Sphere - Laminar Flow
The flow around this sphere is completely laminar (note the low Re #). Essentially, the viscosity of the fluid is dominating the flow pattern. Those bubbles you see separate off are called the "Von Karman Vortex Street" since it was Von Karman who first studied the phenomenon. In reality, DaVinci observed them first...
In any case, I've tried paying particular attention to the bubble separation point. It helped me visualize the high pressure, low pressure interaction that occurs during flow separation. To me, it almost seems like high pressure air is finding any weakness to fill the wake bubble. This was a
MAJOR pain in the ass to create, by the way.
Airfoil - Upper Surface Pressure Distribution
The shaded graph only represents pressure over the upper surface of the airfoil (which is symmetric). What boggles my mind here is how rapidly high pressure air transforms into low pressure air. It is nearly instantaneous in position. You can also note the re-emergence of high pressure air at the tail section.
CFD Pressure Distribution
I found this graph immensely helpful because it showed the pressure distribution around the airfoil, rather than just at its surface. Seeing the gradient from low to high has helped me visualize the total effect of the airfoil on the free airstream. I think its important to understand the action of molecules in the total fluid flow, not just the surface.
Brain Storming
This is just an image I created while brainstorming different ideas. I figured I might as well include it. The upper left was my attempt at understanding the total pressure distribution before I found the CFD plot. The right is my attempt at visualizing molecular changes in pressure. I'm having a hard time separating pressure from density at the molecular level. Lastly, the bottom right was another attempt of mine at developing a total pressure distribution. Those dots are supposed to be molecules, which I intended to use to help me visualize changes in pressure.
Actual Sphere Drag
This graph goes alongside the one presented in the explanation. This is a generic flow pattern that experiences viscosity. What I found interesting was the pressure distribution around the sphere. The trailing pressure seems directly related to the magnitude of the low pressure.
The lower the pressure the sphere experiences, due to its curvature, the greater its trailing pressure drag will be. It seems that by killing "lift," you also decrease the potential for pressure drag. From an airfoil perspective, it's interesting to see that the wake actually kills potential lift. It should also help some visualize the repressurization of the flow as it slows down.
Good look in your learning endeavors.
- LostCause