Would this test set-up be informative?
 

It's not something I have seen or read about. It seems wool tuff testing is useful but often seems to be mainly 2d in its results. I'm wondering if a say 1/2" thin wire but rigid multicolored tuff grid (tuffs attached at nodes on the grind), say 24" square, mounted perpendicular to air flow might give a clearer picture of airflow around say a door mounted mirror if placed slightly down wind, or say mounted just in front of the leading edge of the grille intake/hood/bumper area, or any location to gather a clearer picture of air flow, short of a computer..
My concern is, how much the thin wire grid would distort the takeaway.
Worth a try?

It's been done. Maybe someone will come along with an example or results.

Well, if it has been done, it must not be very useful or informative, and I have never heard it referred to on this site, which should speak volumes?

Try 1-2 thousandths of an inch.

I've had some time to search be not much success:

https://www.researchgate.net/profile...he-area-of.png
www.researchgate.net/figure/Tuft-flow-visualization-in-the-wind-tunnel-experimental-vein-highlighting-the-area-of_fig3_292208380

This was in a wind tunnel. I know I've seen a picture of a 6x6ft grid hitch-mounted behind a moving vehicle, but I can't prove it.

Cumbersome, only works trailing the vehicle; it may show the size of the wake but not the extent.

I like the idea of tiny, smoke-filled soap bubbles.

No success yet on finding any prefab grid mesh of that gauge, especially in purchased useful sizes ( ie 100ft min).
I was thinking of more like .030" SS Tig welding rod, dipped into epoxy (for node connections) with a 3/4"? square opening, attached to a 4 sided aero shaped tube now rectangular frame, with attaching points for suction cup mounts, etc.
I like the TIG rod solution as its straight, rigid, and I can oven burn off the tuffs and their glues when grid needs to be re-tuffed.
So my main concern, will .030 be useful here, or do I need a better solution? I get smaller is better.
But it does seem the individual tuffs will far exceed the FA than the wire mounting will.

Is my assumption you feel that any upwind testing as I noted has little merit with this proposed rig?

It's all speculation, of course. Have you ever been in a wind tunnel? They (Darko) use[d] a fabric mesh to calm the air at the throat of the tunnel.

https://ecomodder.com/forum/member-f...6-100-1191.jpg

Tufts on a grid on a mesh might work. A circular hoop strung like a badminton racket might do better.Who knows?

Well, the idea here is to visualize disturbed air, not to calm it, and not to further disturb it, I believe.
I really do like the hand held badminton wool tuff concept. Kind of like a basic multi point additive smoke wand. Interesting.
And yes, I have visited two in use wind tunnels, at GT and Windshear.

Lightyear One engineers used a custom rake with pressure probes to measure the wake. They also installed one near the rear quarter.

https://www.youtube.com/watch?v=CI8BkK3ycYo

A bit harder to visualize real time and way above my budget.

At least it's real world and not in a wind tunnel.

'wire grid'
 

Wire has a drag coefficient of Cd 1.17, higher than a sheet of plywood, crossways in the wind. If you did the grid you'd want to use as 'small' a wire as you could find. Like a 0.008 #1 guitar string or piano wire.
Long ago, the bi-planes used 'streamline' wire for all the tension bracing between the two wings, and it was designed for low-drag, with a 'teardrop' cross-section. Wicks, or one of the other catalog stores for experimental aircraft supplies might be a source. Don't know.
A problem with it is that, if the airflow is not 'parallel' to it's symmetrical streamlined chord profile, you'll get separated flow, and the turbulence will affect what the tufts are 'telling' you.
For the mirrors, the 'tennis racket' is a very good idea. If the wire is properly 'stretched', where they cross, will not require bonding, as in wire fabric.
In my wind tunnel, I use hollow aluminum radio-controlled aircraft wing strut from hobby shops to attach my yarn to, and then rotate it in the airstream to see at what angle of attack the flow is actually moving, then I know that I'm getting as good a flow representation as I'm likely to get.
For the front of the car, you could create a 'holder', which is mounted outboard of the body, like is done with 'Go-Pros', and extend the strut 'into' the footprint of the nose from there, taking care to 'aim' it.
In a wind tunnel, you'd just use a propylene glycol smoke generator and wand ( a vey large 'vape' ).
For the back of the car, do as was already mentioned about Lightyear.

Not clear what the difference here is in concept between my original idea and the tennis racket that you and I both really like, except the racket has unique real time positioning attributes.

In my test setup my intention was not to weave the solid wire grid, ala a tennis racket, and the epoxy dip glue process intended was to prevent/reduce wire vibrations by connecting the cross wires and maybe improve the cd as the epoxy hardened as it drains off, improving the trailing edge shape, even if
I have to dip/coat it a few times.

Regardless, I'll try a 9? tuff 3d mini square with wire and see how it looks before I proceed. I was also thinking with the diameter of the wire vs that of the cross-sectional size of the tuff, and the length of the tuff, wire induced turbulence would minimally effect the visualization results, but that would be an unknown until tested in my mind.

Badmiton > tennis > crochet hoop

A circular frame can be tensioned, where a square frame will warp into a saddle shape.

True, but in this context, that matters how?

I wonder if I could use .030 alum welding tig rod and run it thru a bead roller with angled rollers to seek a teardrop approx/improved shape?

That would end the luxury of burning off old tuffs with high heat/flame when time calls for their replacement when not using ss for the grid.

High dynamic loads in a public roadway?

Let's compromise -- a triangle, else two diagonals or four corner brackets for the square.

Why are you resistant to reducing the cross section? I suggested 1-2 thou, aerohead 0.008 #1 guitar string or piano wire.

Good luck with that. They will still need to be in tension and cross each other.

I am not concerned that the loads will be high, nor will I be undertaking any testing with nearby traffic, albeit stll on a public road.

No need to compromise, the thin wire is likely the best option, except I am concerned with the amount time/complexity involved in the small wire tensioning (tig rod requires no tension), and the cumulative forces on a rectangle frame from that tensioning already noted, and lastly, is there bang for buck with the small wire solution. One little mentioned aspect so far, any attachment of a wool tuff of a size large enough to be videotaped and analyzed by the naked eye, will likely require an up wind glue glob attachment many times the size of any small wire solution, and the glue being centered axially with the tuff and hard to minimize, will almost in my suspicion completely negate any improvements a small wire solution might provide regarding induced turbulence..

What sort of inter-tuft spacing and tuft length do you contemplate?

Tie the tuft in a knot and snip off the un-free end.

Multi color by row, 1'Sq, assuming a 24"x12" frame, with 1.25" gap on each side to reduce frame interference, grid spacing allows 94% opening per square with .03" wire/rod.

I was also considering adding a spot of glue to the tuff ends to see if that might mass dampen/steady the tuff fluctuations.

Frame is at this point streamline tubing .45"x .27"

'wire grid tuft flow imagin'
 

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.
--------------------------------------------------------------------------------------
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.
------------------------------------------------------------------------------------
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.
-------------------------------------------------------------------------------------
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.
--------------------------------------------------------------------------------------
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.
--------------------------------------------------------------------------------------
'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.
-------------------------------------------------------------------------------------
Tufting the side mirror itself would preserve fidelity of the actual flow.
-------------------------------------------------------------------------------------
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.
------------------------------------------------------------------------------------
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.

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.

'racquet testing'
 

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.

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

'racquet'
 

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.

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.

'might occur'
 

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.:p

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.

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)

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.

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

As always. It got a better reception than last time.

I see no need to defend another, and I totally mentioned Freebeard's suggestion had merit.

Besides, I thought this whole forum was nothing but a spitballing session. Was I wrong?:confused:

But I do try and not get hit.:D

What radius would obviate the 'adverse aero yaw and other forces'? 100ft is an industry standard. A quarter mile would be possible on a salt or dirt flat.

Also high speed drones change things. 40MPH used to be a stretch.

I have no idea, but not to lose sight of the filming problem of using a drone unable to keep pace with a straight-line high speed tufted vehicle, using two different radii for the vehicle and the drone IMO helps to alleviate that potential speed differential issue, but adds the unknown aero yaw factor.

The suggested 1/4+ mile radius would likely alleviate the yaw issue within test limits, but the drone camera shot/quality/resolution at the needed reduced radius with conventional drones would rather be suspect.

drone speed
 

Our vehicles are at super-critical Reynolds number by 20-mph ( 32.2-km/h ). That's all a drone would have to do in order to 'observe' tufts or smoke.

Then why are most tuff tests at multiple speeds faster on the road and/or in a tunnel?

It's IMO a lot easier to securely fasten a camera for 20? mph tests than messing with a drone and nearby traffic.

Signal to noise. Without still air, crosswinds are amplified.

Traffic.

With tuff testing we only need effectively a brief snapshot of the tuffs to gain insight, the problem is with a drone, straight-line testing on the open road, it only takes one wayward drone encounter with another vehicle to at the minimum ruin one or more's whole day, and still outdoor air testing is possible. "Crosswinds" on a circular skid pad are a 100% given because of yaw effects, albeit at least consistent when in still air, and reduceable by enlarging the testing radius.
All of the above is overcomable, and circling back to testing it only needs to be at 20+ mph as claimed?

Well to shoot that down: disregards the reynolds theory particularly for critical and transition speeds where stuff can be turbulent, laminar, or "we dontknow" which will affect flow and subsequently drag.

The only scenerio I see with a cross traffic is being high enough to avoid the disruption a high profile vehicle moving at speed makes to the local area. Could tumble a 'quad. Now you get into focus and adequate resolution.

'speeds faster'
 

The Cd will be 'fixed' at around 20-mph.
A laminar boundary layer becomes unstable at Rn 500,000.
Turbulent boundary layer is firmly established by Rn 1,000,000.
The Cd 0.495 VW Beetle, reported by Hucho/ Hoerner, Figure 4.119, Page-200, Hucho, 2nd-Edition, presents the VW at Rn 11,000,000, which is associated with 85-mph ( 137 km/h )
The Beetle achieves supercritical Rn at 7.738-mph ( 32.2-km/h ).
--------------------------------------------------------------------------------------
Higher tunnel and CFD velocities are associated with pitch, yaw, and roll moment magnitude, and the lift and lift coefficients for each axle, all of which come into effect at higher speed. Overkill for Cd measurement.
--------------------------------------------------------------------------------------
Everyone here at the Aero Forum would benefit from knowing how to calculate Reynolds number, and why it exists.
--------------------------------------------------------------------------------------
A Go-Pro at 20-mph is just fine! Just practice situational awareness when you're testing. Calm conditions away from traffic is your friend.