Quiz: Qualitative Flow Analysis

[Vman455]

All right, all you armchair aerodynamicists of Ecomodder (alliterative and otherwise). Here's another quiz for you:

Below are three GIFs of tufts on the back window of a 2013 Prius liftback, taken on my cellphone taped to the trunk floor, looking up at the window. There is no antenna on the car (shark fin or wire), no washer nozzle, and no wiper. All three videos were taken at 55 mph indicated on the same section of road, northbound into a 15 mph NNE headwind.

The first GIF is your baseline. For the following two, I changed one thing--that is, there is exactly one change from the baseline configuration in each of those two videos. What are the changes?

Baseline:


Altered 1:


Altered 2:

[M_a_t_t]

Are you saying 2 and 3 had a piece put back on (as in return to OEM)? Or are we just guessing a random mod?

Also there is a poll option...

[JulianEdgar]

Love the challenge, and don't mind egg on my face if I am wrong.

For reference, but no antenna on the car, no washer nozzle, and no wiper:




Baseline: Very good flow, isn't it? I'd be pretty happy with that.

Altered 1: I'd be reasonably confident you've added a fence/lip (eg a 1-inch high vertical plate or similar) that causes separation from the top of the hatch.

Altered 2: A harder one. My wife (who has watched aero testing tufts for about 25 years) says the wiper has been put back on. I say the antenna has been put back on.

[Vman455]

Originally Posted by M_a_t_t

Are you saying 2 and 3 had a piece put back on (as in return to OEM)? Or are we just guessing a random mod?

Also there is a poll option...

Yep, just guessing a random mod based on tuft behavior. I will say that both mods are moderately-to-very common on Priuschat, but to my knowledge have never been measured by anyone using them (although Altered 1 is often claimed to "improve downforce" by people who fit it on their Prius).

Originally Posted by JulianEdgar

Baseline: Very good flow, isn't it? I'd be pretty happy with that.

I thought so too. I could see some of the movement of the outermost tufts that the camera didn't pick up so well, but I'll comment on what I think is happening after I pressure test (if that confirms what I suspect).

Originally Posted by JulianEdgar

Altered 1: I'd be reasonably confident you've added a fence/lip (eg a 1-inch high vertical plate or similar) that causes separation from the top of the hatch.

Bingo! Almost exactly right--I taped a 21mm high lip to the top edge of the window:

Originally Posted by JulianEdgar

Altered 2: A harder one. My wife (who has watched aero testing tufts for about 25 years) says the wiper has been put back on. I say the antenna has been put back on.

Yes, this one is more difficult. There's no disruption to the baseline flow pattern, but the tufts toward the trailing edge of the window are behaving slightly differently. (Actually, now that I look again the tufts at the leading edge have changed behavior slightly as well).

[JulianEdgar]

Originally Posted by Vman455

Yep, just guessing a random mod based on tuft behavior. I will say that both mods are moderately-to-very common on Priuschat, but to my knowledge have never been measured by anyone using them (although Altered 1 is often claimed to "improve downforce" by people who fit it on their Prius).



I thought so too. I could see some of the movement of the outermost tufts that the camera didn't pick up so well, but I'll comment on what I think is happening after I pressure test (if that confirms what I suspect).

Yes, could be some trailing vortex development down the pillars.

Bingo! Almost exactly right--I taped a 21mm high lip to the top edge of the window:

Glad I got it right! And it may well reduce lift - but at a penalty of lots of drag.

Yes, this one is more difficult. There's no disruption to the baseline flow pattern, but the tufts toward the trailing edge of the window are behaving slightly differently. (Actually, now that I look again the tufts at the leading edge have changed behavior slightly as well).

Now that I know it's a common mod, I'd suggest (un-needed) vortex generators have been added at the top of the window.

[Vman455]

Originally Posted by JulianEdgar

Yes, could be some trailing vortex development down the pillars.

That's what I suspect. The tufts on the edges of the window both a) fluttered at a higher frequency and b) were "tauter" than those in the middle of the window.

I may trial some fins along the hatch edges just to see what happens.

[jakobnev]

And it may well reduce lift - but at a penalty of lots of drag.

It may, but it is ruining the effectiveness of the stock spoiler so it's not certain at all.

[JulianEdgar]

Originally Posted by jakobnev

It may, but it is ruining the effectiveness of the stock spoiler so it's not certain at all.

Does the rear spoiler cause much of a change in angle to the rear hatch slope?



I've never looked at a Prius with this in mind, so I am not sure. If the spoiler is just a continuation of the hatch angle (or makes it only very slightly shallower) then it will do nothing / very little to reduce lift.

Of course, if you want to get rid of nearly all lift, place a fence across the mid-point of the roof!

[JulianEdgar]

Originally Posted by Vman455

That's what I suspect. The tufts on the edges of the window both a) fluttered at a higher frequency and b) were "tauter" than those in the middle of the window.

I may trial some fins along the hatch edges just to see what happens.

I'd be interested in your results.

Surely someone else has tried it and reported their results, but apart from my data, I've never seen anyone else reduce drag by added long fins down each side of a rear hatch that has attached flow (and I assume, therefore, trailing vortices).

[M_a_t_t]

Originally Posted by JulianEdgar

I'd be interested in your results.

Surely someone else has tried it and reported their results, but apart from my data, I've never seen anyone else reduce drag by added long fins down each side of a rear hatch that has attached flow (and I assume, therefore, trailing vortices).

What were you trying to do with the ones on your insight? Was the main goal just for something to mount the rear wing to?