6th Gen Accord Aerodynamics

[GasSavers_brick]

Yeah, I'm not going to lie. I do like the way the car looks with the spoiler, which is precisely why Honda produced it. But what I'm really trying to figure out is what impact it really has on the aerodynamics of the vehicle.

The spoiler is shaped a little like an upside-down airfoil, with the top being relatively flat and the bottom relatively rounded (blunter side to the rear.) Assuming that this shape is viable, it would tend to accelerate the air between the trunk surface and the spoiler, and directing it upward right as it exits the trunk lid surface. **BUT** That's the behavior that it would exibit in free stream air. More than likely the airfoil shape is ineffective due to flow separation and turbulence so close to the surface and relatively close to the rear window. That window is well raked, but I don't think it looks like a sufficient angle as to prevent detachment. There's also the fact that the auxiliary third brake light is a very dirty feature and the word "Accord" is written in large letters that are recessed 4-5 millimeters into the entire bottom surface of the wing. So, yeah. How about that air foil?

I also found a flow-vis image of a similar wing as used on an Infiniti G20 in that SAE publication I have. That particular wing produces down force in roughly the manner described above, but also produces a much larger wake behind the vehicle, indicating increased drag as a side effect.

[MetroMPG]

Here's what to do. I'm sure you're familiar with tuft testing.

Stick some rows of yarn on the trunk lid and figure out some way to be able to record their behaviour or have someone else drive the car and you watch it. If the yarns at the back of the lid are streaming freely back, you've done a good thing.

I think it's likely, as you say, that flow separates on the rear window. But I'd be surprised if it isn't reattatched by the end of the trunk lid. Tuft testing will tell you that for certain, and then you can feel good without doubt about taking the spoiler off.




Tuft testing a Lexus LS 400 - note separation at the forward end of the lid (tuft is lifted in turbulence) and reattached at trailing edge (tufts streaming back).

[Mighty Mira]

Originally Posted by MetroMPG

Here's what to do. I'm sure you're familiar with tuft testing.

Could you go over the theory behind tuft testing and what exactly to look for?

I figure that my main modus operandi once I start aero modding will be to do a combination of electronic timing assisted coast down testing (via a microcontroller with VSS signal input), and instantaneous FE metering.

Since we are modifying the Cd or Cd*A I figure that the best place to measure improvement is that particular spec. But if the tuft testing can provide a quick and dirty method of testing, so much the better.

Particularly, I'm imagining manipulating a coroplast boattail until tuft testing indicates that the flow is detached.

[GasSavers_brick]

I am familiar with tuft testing, but haven't tried it yet due to the fact that I really don't have many options for an assistant around here. (If only I were still in school...) I could probably get a good idea of what's going on at the rear glass by taking some over-the-shoulder stills with the digicam. Better yet, I just had a brainstorm about that digital camera. Supposedly it can double as a webcam, and could stream a live feed to my laptop. I'm thinking mount the camera somewhere on the back shelf of the car, stream the video to the lappy 386, check de email (strongbad fans?), capture, host, party like a bunch of physicists at a start trek convention. It might not be as good as a view directly from the rear or side, but could be about good enough, yeah? That would make it really easy to do runs with and without the wing to see what's going on with the air flow. The other reason I've wanted to do this is to see what kind of turbulent low pressure region exists on the rear windscreen, leading to possible VG experimentation.

[MetroMPG]

Originally Posted by Mighty Mira

Could you go over the theory behind tuft testing and what exactly to look for?

I started a new thread on the topic, since it's relevant beyond just 6th gen. Accords.

> Tuft testing

[MetroMPG]

Originally Posted by brick

stream a live feed to my laptop. I'm thinking mount the camera somewhere on the back shelf of the car, stream the video to the lappy 386

That's a great idea! And you can post clips too.

[GasSavers_brick]

Tuft testing pics are in! Not the greatest but not too terrible considering that I did this solo. All I did was rig the camera high in the rear window, connect it to the laptop via USB, and set it to capture at the highest frame rate available of one every 5s. (It's not exactly a new camera.)

First pic showing the setup: two tufts along the edge of the trunk lid, one about in the middle, and two that are very close to the glass.



Second pic showing the tufts at 60mph:


And another at 60mph:



Hooray for attached flow! You can see that the tuft in the middle of the lid pointed pretty much straight back, and it looks like the two at the edge are straight back and wrapped around as expected. The two by the glass are clearly in turbulent flow, indicating a low pressure region.

[MetroMPG]

That's awesome!! You get my vote for best post of the month.

And congratulations on the attached flow at the lip.

Lip extension, anyone?

Seriously, good work.

[SVOboy]

Dudeness, that's good stuff. I'll second darin's vote!

Get yourself a camera with crappy video, I got mine for 20 3 years ago,

[GasSavers_brick]

My camera does have crappy video, but for some reason I can't get the remote capture software to grab it via the USB link. If you click through all of the frames really fast it looks like a crappy video but I guess that's not the same .

Glad you guys are getting a kick out of this, because I sure am. The next time I do this will probably be an attempt to assess the low pressure region on the rear glass. (And obviously another trial with a lip spoiler once I make it.) Anybody want to see what vortex generators really do?