Would this test set-up be informative?

[j-c-c]

A lot of effort there, thanks for the contribution.

This did stand out as worthy consideration:

"'Streamlining' the grid elements would introduce a 2-D flow bias within a 3-D flow environment. One might get some of the grid properly addressed with respect to some of the oncoming flow, while the rest of the flow would be unaligned, introducing transverse contamination of the flow field of interest."

I'll dig deeper tonight when I have time.

[aerohead]

I had the driver's window rolled down ( which would destroy the local flow field ), trying to drive while holding on to the racquet.
If you could secure the racquet to something like the dual suction-cup holder, with safety lanyard, that CAR and DRIVER and other magazine road warriors use, attached to the door, downstream of the mirror, there'd be no 'harm' in 'looking' at whatever you saw. Or let a Go-Pro record while you concentrate on the road.
Information is information.

[j-c-c]

Originally Posted by aerohead

I did a rudimentary 'road test' of my tennis racquet. It's an adult, HEAD, Liquidmetal, Supreme. It's a light metal hoop / strut / handle, strung with 1mm-diameter Nylon. The mesh opening dimensions vary, depending upon position within the inner boundary of the distended hoop.
The 'most dense' pathways are near-center, creating a rectangular opening, 13mm by 10mm.
Due to flow separation, the wake, downstream of the 1mm string is 1.244mm, resulting in a vena-contracta-esque narrowing of the airway to 12.51mm by 9.51mm.
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1) Just 'swinging' the racquet produces an audible acoustic signature.
2) At 35-mph, it begins to 'sing' appreciably.
3) At 60-mph, it's 'roaring' significantly, and my wrist tired rapidly, attempting to steady the racquet in the oncoming airstream.
4) I've no means to parse out, and properly attribute the sound contribution and associated drag between the hoop and string grid.
5) I'm unwilling to invest the time to 'streamline' the hoop, and re-test.
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I suspect that there'd be little trouble coming to a consensus that, the noise was an artifact of separation, and any use 'upstream' of a road vehicle's body , or components would be unacceptable.
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Onset flow to side mirrors is typically 3-dimensional, making alignment of 'ALL' of the grid, normal to the flow downstream of a mirror problematic.
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I revisited Syed R. Ahmed's flow traverse research reported between 1979 and 1984. The 'densest' concentration of tuft intersections, in a half-body- width grid, was for the fastback body, with a Cartesian grid of eleven vertical columns, and thirty-two horizontal columns.
Based on the metrics he used for grid sizing, for a 2018 Nissan Leaf, imaging would require a grid of 73.6-inches height, by 42.3-inches width, with 352-tufts, spaced at around 3.5-inches, by 2.25-inches. Perhaps this would be a reasonable tuft population density.
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'Streamlining' the grid elements would introduce a 2-D flow bias within a 3-D flow environment. One might get some of the grid properly addressed with respect to some of the oncoming flow, while the rest of the flow would be unaligned, introducing transverse contamination of the flow field of interest.
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Tufting the side mirror itself would preserve fidelity of the actual flow.
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Since Hucho et al. published their shape optimization research in 1976, and new car production cycles back then ran about 48-months, it's hard to imagine a new production car manufactured after 1980, which would demonstrate significant flow issues for their forebody.
' Saturation' leading edge radii relationships were known by this time, and by 1969, the world community of auto designers were in possession of shape 'recipes' which would produce Cd 0.245, as of 1969.
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It seems to me that if everything published by 1976 is addressed, then lower drag will just come down to body elongation, along known streamlined contours/ silhouettes, also reported by 1969, if we ignore those known for over a hundred years now.

1. So your racket thread is slightly larger than my current grid solution, .030"
2. My grid openings are projected nearly 4 times larger.
3. Is the downstream "wake" measurement you note observed or measured or calculated? seems very acceptable for my use.
4. The acoustic conservation was what I was trying to reduce by dip epoxying the grid together rather than weave and post tension. Additionally, I thought the epoxy would act as a mass damper, and considered the residue drip while hardening might form a useful drag reducing shape. Bottom line silence is the goal, but not likely achievable. If it's simply an acoustic air resonance caused by simply pass thru a reduced opening, it will be what it is.
5. I appreciate the effort to do an initial test, I/we know more than we did at the start I suspect.
6. I am not clear how Ahemed arrive at his ideal grid sizing.
7. I already addressed the 2d flow influence prior and will just have to live with it.
8. My forebody testing was more to help out with intakes, inlets, winglets, etc which I think this rake/racket might help sort out.

Currently I'm at a standstill waiting on the aero shape tubing for the outer perimeter support.
Suction cups will be the go to mount, and a Hero11.

Thanks again

[aerohead]

Originally Posted by j-c-c

1. So your racket thread is slightly larger than my current grid solution, .030"
2. My grid openings are projected nearly 4 times larger.
3. Is the downstream "wake" measurement you note observed or measured or calculated? seems very acceptable for my use.
4. The acoustic conservation was what I was trying to reduce by dip epoxying the grid together rather than weave and post tension. Additionally, I thought the epoxy would act as a mass damper, and considered the residue drip while hardening might form a useful drag reducing shape. Bottom line silence is the goal, but not likely achievable. If it's simply an acoustic air resonance caused by simply pass thru a reduced opening, it will be what it is.
5. I appreciate the effort to do an initial test, I/we know more than we did at the start I suspect.
6. I am not clear how Ahemed arrive at his ideal grid sizing.
7. I already addressed the 2d flow influence prior and will just have to live with it.
8. My forebody testing was more to help out with intakes, inlets, winglets, etc which I think this rake/racket might help sort out.

Currently I'm at a standstill waiting on the aero shape tubing for the outer perimeter support.
Suction cups will be the go to mount, and a Hero11.

Thanks again

1) Yes, @ 1mm, the nylon webbing is 0.0393", with a wake of 0.0488, using Sighard Hoerner's schematic representation of the flow.
2) Your enlarged spacing would certainly be a plus.
3) Comparing actual 'known' wake images, to Hoerner's schematic representations, suggests that these line drawings have 100% fidelity, compared to actual flow imaging from which they're derived, and I believe that they can be used with a high degree of confidence.
4) The thing about the epoxy dipping would be the 'thickening' of each strand, which is already an issue, and as the coatings build up, and as they become 'boat-tailed' on their trailing edge, they could introduce a 'flow-straightener' effect, which could 'contaminate' the true orientation of the flow, frustrating your efforts to 'see' what's going on.
5) Yeah, information is what it is, and hopefully helps informs our next move.
6) I'm not even sure if Ahmed used a 'grid' at all. A bridge fiducial can be placed at a distance besides a wind tunnel test vehicle, with the 'tip' of a tuft wand placed successively within an imaginary grid, and photographically captured under a strobe light in a completely-dark wind tunnel, with an open shutter; and without advancing the film at all, 350-plus exposures can be captured on the same frame of film, building up a composite image from these stacked exposures. We did this in the physics lab for acceleration tests when I was at Texas Tech.
7) We do what we can.
8) Give it a go. We'll all learn something. I think it was Colin Chapman, of Lotus, who would strap himself to the top of a car, and while a test driver drove him around on a closed-course, would observe the behavior of tufts on some new prototype.
9) If the suction cups were to fail, some sort of safety lanyard backup would prevent the loss of your test rig.

[j-c-c]

All worthy points.
I like #6, very clever.
#8, I am too much of a blunt body to attempt such gymnastics
#9 extra safety, my other concern would be the sheet metal, paint, and potential glass damage that might occur as camera dangled around with its support mount at speed until car came to a rest, vs just flying off one time and replacing the camera.

[aerohead]

Yeah, it's spooky.
We used the Chrysler Proving Grounds in East Chelsea, Michigan for testing 'Spindletop.'
We were the only ones using the facilities that day, and completely out of reach of 'mortal man.'
It may not be long before a member uses a drone as a 'chase car,' to follow along a tufted car, video recording as it goes.
Hollywood, special effects, 'smoke cookies' could be detonated from a ten-foot 'Narwal 'tooth'', projecting forwards of the test vehicle, perhaps an advanced technique to what 'Think Flight' attempted with his Subaru Impreza out in the Mojave Desert.

[freebeard]

It may not be long before a member uses a drone as a 'chase car,' to follow along a tufted car, video recording as it goes.

I've had the thought that many drones are top speed limited; but with the car on a 100ft circle, the drone can sit at the center and spin. Then you get all crosswind conditions.

Actually, a tripod would work at that point.

[Piotrsko]

Drone speed is a function of prop pitch and motor power. Also most drones today have the prop tips going supersonic which is why they are as noisy as they are.

The other issue would be non autonomous control because a human reactions are slower than required for stable really fast flight. Auto pilot style controllers help here as would more stable planforms (which aren't as fun to fly)

[j-c-c]

The idea of a circular road tuff test with the drone flying on a reduced radius flight path to compensate for its limited air speed has merit IMO, but does not account for the constant yaw aero effect of the car body in still air, if that is significant at the suggested 100' radius? skid test pad.

[Piotrsko]

Adverse aero yaw and other forces are significant if the circle is small enough to present centripedal forces.

In freebeards defense, he was spitballing the solution he presented