Prius Aerodynamics tuft test vids (no rain on rear glass = flow separation?)

[F8L]

What tires do you have and at what pressure do you run them? The Prius comes equipped with 3 possible tire options; the Fuel Max, AVID S33D or Ecopia EP20.

[ryannoe]

F8L - AVID S33D

The rest, I forgot to account for fuel onboard. Adding it to my calculator has changed the FPD to 0.6958 (or Cd to 0.2911). Sorry for the slip.

-Ryan

[F8L]

Originally Posted by ryannoe

F8L - AVID S33D

The rest, I forgot to account for fuel onboard. Adding it to my calculator has changed the FPD to 0.6958 (or Cd to 0.2911). Sorry for the slip.

-Ryan

Thanks, Ryan.

I'll be watching your threads with interest. Especially as the seasons change. In this cold (30F-55F & dry) weather I notice my coast downs (glides) are MUCH reduced compared to 95F-110F temps using the same tire pressure.

Maybe you can do future testing with known very LRR tires like the Energy Saver A/S and Ecopia EP422.

[ryannoe]

Originally Posted by F8L

Thanks, Ryan.

I'll be watching your threads with interest. Especially as the seasons change. In this cold (30F-55F & dry) weather I notice my coast downs (glides) are MUCH reduced compared to 95F-110F temps using the same tire pressure.

Maybe you can do future testing with known very LRR tires like the Energy Saver A/S and Ecopia EP422.

If you provide the tires, I'll provide the gas.

-Ryan

[F8L]

Originally Posted by ryannoe

If you provide the tires, I'll provide the gas.

-Ryan

If only you lived closer. I go through tires/wheels like a girl goes through shoes.

I've had 4 sets of wheels and 7 sets of tires on my car and I've only had it since March. I love testing tire/wheel combos.

[MetroMPG]

Originally Posted by kennybobby

Hey Metro,

Would you please explain the meaning of the presence or absence of snow patches on the probe?

There's a discussion in the Probe thread: http://ecomodder.com/forum/showthrea...tml#post346077

Assuming the snow was dry & light (ie. not "sticking" to the bodywork), it seems fair to say that where there was snow left behind, the airflow was slower, turbulent or separated.

[ryannoe]

It's hard finding time to do some testing right now... but I made a trip out tonight.

Here's a teaser.

It might be hard from the picture to tell but look at the luft in the yarn near the center of the window. The top of the window has a tighter yarn and the yarn near the C-pillar is curled outward stuck in a vortex.

I have video I might edit tonight/tomorrow. I want my friend to see the video first but he's presenting a paper in Dallas so you guys might get a sneak peak.

-Ryan

[ryannoe]

Hey all,

I have a video I uploaded for you to see. I cut a makeshift vortex generator (not sized, angled, or located properly). The footage taken wasn't placed in the same position for a good comparison. I take a lot of angles to see what is happening with the tufts and I ran out of time (because of the rain) to view the video and reposition the camera for comparison footage.

Remember the vehicle is traveling at 60 MPH in BOTH videos. The tufts are NOT moving at 60 MPH in the first video... in the second video, look for the vortex generator and the swirling air directly behind it. Then look further downstream. Perfection ... Then look to the right which is the the same as the first video.

Also... stick with the video as long as you can. During the VG portion I stop and turn around. You can see the flow attaching fast the rear.

Enjoy!

http://www.youtube.com/watch?v=6N7S5...ature=youtu.be

[kennybobby]

What does all this mean? It looks like tufts are shaking and moving in the non-vg portion. In vg they are jumping up and shaking more. Don't you want smooth laminar flow at or near the surface? Would turbulent flow at the surface be a "good thing"? Do you not want a boundary layer at the surface? Please explain.

[ryannoe]

Originally Posted by kennybobby

What does all this mean? It looks like tufts are shaking and moving in the non-vg portion. In vg they are jumping up and shaking more. Don't you want smooth laminar flow at or near the surface? Would turbulent flow at the surface be a "good thing"? Do you not want a boundary layer at the surface? Please explain.

Good questions. I'm having a hard time with flowing one thought to another in this explanation, I'm sorry. Please try to follow as best you can.

Thought 1: The pressure gradient across a boundary layer is constant (from the surface to the top of the boundary layer). That means the pressure at the surface is equal to the static pressure of the streamtube adjacent at that location.

Thought 2: The flow at the surface NOT behind a VG is NOT flowing at the desired 60+ mph. I say 60+ because 60 should be the minimum velocity unless you are pulling the flow along with you. If the flow behind your vehicle is slower than freestream, you are grabbing that air and sucking it along with you (we don't want that).

Thought 3: Think back to thought 1. Pressure at the surface is equal to pressure at the streamtube adjacent. If we have pulled the flow down to the surface to test velocity, we have also pulled the streamtube to the surface. This has a positive effect by either: stretching the streamtube at that location (top location stays put and the bottom comes down) which would increase static pressure, or bring the entire streamtube down which will stretch adjacent streamtubes which will have relatively equivalent effect.

Thought 4: BAD - More skin friction... but should be relatively small.


Bringing it together:

Pulling the air down to the surface to as close to the freestream will increase the pressure at the surface. This is good for two reasons, it increases the pressure on the glass, but it also increases the pressure in the separated wake - a synergy effect.

MAKESHIFT VG DATA:
FPD change 0.6405 to 0.6286 (Cd change of 0.268 to 0.263, not TOO much) with makeshift VGs that were "eyeball'd" relative to tuft streamlines. Size, shape, location, and AoA were ALL eyeballed with a possible positive difference being realized. However please note these numbers are not validated with statistical analysis. I have some work to do.


-Ryan