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Old 01-11-2013, 10:35 AM   #37 (permalink)
ryannoe
Not Ordinary Engineering
 
Join Date: May 2011
Location: Alabama
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Quote:
Originally Posted by kennybobby View Post
i have some more questions, but i saw a late model toyota camry up close today at rideout and martin road--the headlights have some sort of VG molded into the top outer corner that even has some sort of zigzag-stair-step pattern on the leading edge. Thanks to this thread i had a clue...lol

Ryan,
When you say that your rear window has separated flow, do you mean there is a laminar flow boundary layer close to the glass and there is also a flow stream higher up above the surface of the glass that has detached from the upper edge of the boundary layer? And this separated flow layer makes the rear spoiler ineffective? Is the separated layer (always) turbulent or possibly laminar/free-stream in character? How far above the glass do you think this separated flow begins (at the top edge of a very thick boundary layer)? Maybe some redundancy in the questions here, so feel free to summarize it if you can explain all this flow stuff to some of us aero novi...
KennyBobby - I didn't realize how close you are. We should grab lunch... I'm on the airfield off Hale Rd (the first light as you come in Gate 9 going south on Rideout).

As for the questions, I'll just go down through the para.

Q1: When you say that your rear window has separated flow, do you mean there is a laminar flow boundary layer close to the glass and there is also a flow stream higher up above the surface of the glass that has detached from the upper edge of the boundary layer?
A1: No, this isn't possible to my knowledge. -Boundary layers are a result of the shear stress of the object on the air. It starts smoothly, trips to turbulence, and then eventually if the object has adverse curvature (curving away from the flow), the flow detaches and there is a stagnation point... if the detached flow is large enough and an adverse pressure gradient is present, you'll have reversed flow.

Q2: And this separated flow layer makes the rear spoiler ineffective?
A2: Yes, the spoiler requires air to move across it to produce it's downforce. If the flow across the spoiler is moving very slowly as a result of being in the detached flow (the stagnant or reverse flow area), no air is really pushing across it rendering it ineffective. It doesn't produce drag! But it doesn't produce downforce...

Q3: Is the separated layer (always) turbulent or possibly laminar/free-stream in character?
A3: Separated flow is flow that is no longer part of the freestream. This is a hard question to answer because stagnant air has neither turbulent nor laminar characteristics. The separated "area" can changes characteristics based on your velocity. It can start out as just a stagnant spot with very small eddies and as you go faster, can result in the oscillating vortices that you experience behind the big rigs on the interstate.

Q4: How far above the glass do you think this separated flow begins (at the top edge of a very thick boundary layer)?
A4: On my Prius, it looks to separate just behind the antenna. It appears the separation caused by the antenna acts like a catalyst to break up the flow further down the back glass.

-Ryan
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