|
A word of thanks
Hi Teri, I hope you will not mind if I use your thread to congratulate Ironside on his sterling efforts to quantify the side-wind stability effects of the stall strip. The two videos posted in his post #82 are the most detailed public domain research that I have seen.
Prior to this work, Craig Vetter’s tuft test photographs from an accompanying vehicle were about the best stuff out there. However, Ironside has trumped Craig for two reasons, firstly by choosing video footage which imparts far greater information than a single still image, and secondly because he has focussed on sidewind stability as a specific issue rather than the general drag reduction which was the point of Craig’s image.
So I think due recognition for this important step is warranted.
But, and I hate that there is a “but” , but the videos are of limited value as Ironside himself pointed out "To be honest, I can't see any discernible difference, but then again I don't know what I'm looking for."
I have taken some time to study the videos and have some experience of tuft testing myself, so here are my observations, which I hope will be seen as positive and helpful
Point 1 – Poor quality air – if you pause the first (no stall strip) video and look at the tufts in the first row (lets call it row A) at the centreline of the fairing, you will see that most tufts exhibit rapid direction change in a random sequence. Some side-by-side tufts have almost opposite flow direction, even nearest the centre where flow is greater. Compare this with tufts on an undisturbed area of a car body, which would appear almost motionless, and the problem is clear. What is needed is a “flow straightening device” to eradicate the turbulence created by the fans own blades. You could make such a device quite cheaply using the tube stacking method, but before you do see point 2 (below)
Point 2 – Insufficient air - Although the fan is not shown, my guess is that its only about 12” diameter ducted fan (single phase). From the video it can be observed that it produces a strong central flow and a weaker peripheral flow. This shows up as a divergent angle between the tufts at the extremities. A similar effect is produced by the rounded body profile but the tell-tale in this case is the energy level between the central and edge tufts. My guess, and it is a guess, is that you would need approximately ten times the mass airfow to cover the relative frontal area for this test
Ironside, please don’t give up, you are almost there. I would suggest real world riding rather than fan tests. I would keep the tufts on and take a ride with a couple of friends in a car alongside. One driver and one camera operator (simple handycam will do, phone footage will not work at suitable clearance distance) who can shoot you from slightly ahead and off to side, in windy and still conditions. Compare and contrast the footage with and without the strip(s) attached, and you will own this subject area.
An alternative method would be to fit a GoPro camera on a suitable mount and do it all yourself. You could also use a “flow visualisation grid” with this method, located at a critical point it will show more than just the boundary layer flow.
I hope you get to do more testing and I look forward to seeing the results. It would seem that your test rides have shown some promise but I hope you can find a scientific method for accurate measurement of results.
But heh, a genuine world leader!
|