Again, I am no ultimate authority on this, but shouldn't drag be in large part the vector sum of pressures on the car? I.e. a bubble, even if enclosed in laminar flow, is still a low pressure region
You're right to call me on that one, Mira. I'm no expert either (as I often preface my aero comments...).
I can tell you where my thinking that led to the "entrained bubble" comment came from though: I was thinking about Kammback shapes.
Kamm showed that an abbreviated teardrop actually worked better; the air still flowed as if the entire teardrop were still there, but without the surface drag of the long point. - Wikipedia
I'm probably mis-applying that concept to the rear window bubble issue on Brick's car:
But I saw a similarity in Kamm's "entrained bubble" of lower pressure at the rear of a truncated shape being no worse off aerodynamically (better, actually) than a fully tapered shape escorting attached airflow all the way home.
The fact is, I don't know for sure, and I'm glad you called me on it.
I've been slacking since last week, but I'm back on track after priming & painting one side of the spoiler, with the underside to come tomorrow. I took care of all of the sharp edges by rolling the leading edge under, and by applying foil tape to the trailing edge since I didn't leave enough material to make a good seam. It's not perfect because you can see where the tape is, but it isn't terrible, either. After all, it's a prototype! That just leaves fastening & round 2 of tuft testing, which could happen later this week or early next week. Pics to follow tomorrow, probably.
Alright, the spoiler is on the car. (Will get pics, really.) And you know what? It sucks! If anything it's increasing drag vs. no spoiler at all, not reducing it. Coasting down the hills that are very marginal to begin with are still very marginal or, if anything, I lose more speed than before.
I'm pretty sure I know what the problem is. First, it doesn't quite sit flat against the trunk lid so there are places where air can get under it rather than being forced up and over. Second, the straight edges mean that the edge of the spoiler varies in distance from the edge of the trunk lid, and I don't think it really has a chance to do what it's supposed to do. I still need to tuft test this weekend, but I'm willing to bet I know how it's going to look: Lots of turbulent flow over the entire thing, and the string probably pointed straight down at least in some places. I'm probably wasting energy on generating big vortices at the edges, too.
Oh noes! But at least we know something more than we did before, eh? The creative process is much more interesting to other people than "hey look at this, I did a lot of revision and this works, bai." So yeah, , I've been looking at lots of spoilers in parking lots and on honda-tech recently!
What I meant to assert is that I definitely did not notice a big change for the better, and that the effects are probably slight or nothing at all, and that if there really is a change it's more likely to be for the worse than the better.
Tuft testing this weekend will tell the story better than my "seat of the pants" assessment. If I get ambitious I might try some timed coast-down runs. I'm just saying I don't expect much.
And to answer the question directly: No, I did not notice much of anything when I took off the original spoiler, nor did I attempt to quantify its effects in a way that anyone would consider accurate. I expect that the best I could hope for would be a CD improvement in the .003-.005 range (based on some reading about the Accord Hybrid's aerodynamics) which will be small enough as to be very hard to quantify. In a way, I have to accept the fact that I may not be able to measure results directly. That means coming up with something that should work, where the tuft testing points to improvement, and go with it. Actually seeing improvement can be a bonus if it happens.