Tuft Test Interpretation

[fbov]

Reading here, I understand that about the best you can do to reduce drag up front is block air inlets, and that the big drag reductions come at the rear of the vehicle. Having blocked off the front inlets, I recently did a tuft test to measure airflow across the back half of my car, in hopes of identifying some fruitful areas for modification and improvement.

I'd like your thoughts on the results.

I took video using a dashcam on a monopod tied to a chase car, and am working on editing out the interesting bits into postable size. In the meantime, I've attached a couple stills, and a verbal description of what I saw in the video.

General observations
Laminar flow is everywhere
- roof all the way back to the lip
- windows sweeping up toward the roof, right to the hatch glass
- door panels above rear wheel well, but ending under lights

Turbulent flow dominates the tailgate (lots of blurry tufts)
- lip under the hatch glass down to the top of the bumper
- flanks under rear lights sweeping forward to wheel wells

The big surprise is the rear window - near calm
- strings lay flat and don't move much
- pattern is downward with slight outward flare
- pattern extends to L/R edges of the glass, right next to high-speed laminar flow off the side windows

Granted, video will tell you more than stills, but what indications do you all see?

Have fun,
Frank

[aerohead]

*In elevation,Ford has done the K-form body,respecting boundary layer requirements for attached flow.
*In plan-view,they're borrowing from Paul Jaray's 1921 'Kombination-form' greenhouse.
*Both are truncated at a 'practical' length.
*You do have nice clean laminar flow up to where Ford exercised some aesthetics with the truncation.A trip down a dusty road would reveal your actual separation line.
*The wake is exactly as one would expect it should be.
*For lower drag you'd just elongate the body,picking up where Ford left off,extrapolating the curvatures along imaginary 'ideal' pathways.
*The effective-fineness ratio determines,more than any other factor,your potential for low drag.

[freebeard]

I recently did a tuft test ... in hopes of identifying some fruitful areas for modification and improvement.

That appears to be a Ford C-max. Google says 0.30 Cd. What target do you have, or conversely how much are you willing to push the (literal) envelope of it's aeroform?

I'm thinking a semipermeable box cavity.

[fbov]

Yes, I have a C-Max, and I'm aware that Ford did a decent job on the Cd, but it still punches a big hole in the air. I don't have a target Cd, but given I'm already running in the 2g/100m range for FC, I suspect the car will acrue more FC benefit from aero improvements than something less efficient. I've already done the front, and upped tire pressures... what's left? Beauty is not a driver, but will be considered...

And what is a semipermeable box cavity?

Have fun,
Frank

[fbov]

Aerohead, I didn't see your reply... and you've listed conclusions exactly describing the car, the results, and the opportunities I see as well. Had to look up fineness ratio...

I've already noticed what gets dirty/what stays clean (the back glass). The idea would be to extend the rear quarter panels into a truncated boat tail kind of structure?

My goal in this discussion is to understand the principles and optimum approaches, regardless what I actually do to the car, so whatever I end up with will be, to some extent, based on understanding of the issues as well as external factors (e.g. I plan to stay married).

thanks!
Frnak

[freebeard]

Had to look up fineness ratio...
...
My goal in this discussion is to understand the principles and optimum approaches, regardless...

You went right to private tutoring from aerohead. That's a good start. Have you looked at his public albums? Here's mine, some of the stuff he posted plus others of interest to me.
_________

semipermeable box cavity


You can think of a box cavity as a boattail that is cut off and not filled in. They tend to be short as in this example, but on truck trailers they will be 2ft deep. The interesting thing is that they can be inset or stepped in, instead of following the Template line as long as they terminate on that line. This could be helpful for clearing taillights.

The semipermeable refers to a perforated surface. This could mean 400 louvers on a '34 Ford roadster body, or the Mesh panels on the tail of a modern supercar. Here's an example of using a fabric mesh to affect turbulence and wind noise without adding drag:

[serialk11r]

How about going for the wheels? Mercedes has gotten below 0.26 with their vans and SUVs, and their bodies don't look so optimized. Maybe playing with the wheel air dams (using taller ones) could give a benefit?

[aerohead]

Originally Posted by fbov

Aerohead, I didn't see your reply... and you've listed conclusions exactly describing the car, the results, and the opportunities I see as well. Had to look up fineness ratio...

I've already noticed what gets dirty/what stays clean (the back glass). The idea would be to extend the rear quarter panels into a truncated boat tail kind of structure?

My goal in this discussion is to understand the principles and optimum approaches, regardless what I actually do to the car, so whatever I end up with will be, to some extent, based on understanding of the issues as well as external factors (e.g. I plan to stay married).

thanks!
Frnak

Yes,the extension of the body is the ticket.
Pressure drag dominates aerodynamic drag,and the base pressure behind the car is the culprit;which in turn is governed by the separation line and magnitude of the wake.
The premise of streamlining is to reduce or eliminate the separation,thereby reducing or eliminating the turbulent wake altogether.
Fachsenfeld and Kamm,together at the FKFS (of which Daimler-Benz now has Kamm's wind tunnel at Stuttgart) started with a complete streamline body of revolution half-body of Cd 0.12 and morphed it into a more rectangular body section where the occupants would be,and chopped the tail off to help with traffic and parking issues.
They settled upon a body chopped off where the wake had a cross-sectional area equal to 50% of the frontal area of the car (about 12.5 sq-ft for Kamm's K-form cars).
The best K-car ended up at Cd 0.23 (about twice the drag of the original half-body).
Koening-Fachsenfeld envisioned and patented an inflatable tail to get the boat-tailing back out closer to that of he original half-body.
This is what Hucho's been advocating that the automotive community reconsider, since the mid-1980s.
Hucho reports in his book,that you'll achieve your minimum drag when the car has a length-to-height ratio of 5:1.Or an effective fineness ratio of 2.5:1.
After that,you go after wheel/tire drag with a vengeance.
This is a long way of saying that your ultimate drag will come down to how much elongation you can personally stomach,bearing in mind that some of that length can be active aerodynamics.
Doing everything basjoos has done with his AeroCivic,ahead of the boat tail would buy you an iron-clad insurance policy for the aft-body mods.