Thread: Vortex Generators

Laminar flow will change to turbulent flow at particular values of Reynolds number. Reynolds number depends upon several factors such as viscosity, and also depends on length.

The length is an issue in testing scale models and the change in Reynolds number must be accounted for.

Compare insect wings (thin) compared to aircraft wings (thick). Both have very good aerodynamics, but an aircraft will not fly very well with airfoils like insect wings. This is because Reynolds number changes with length.

Also look at insect bodies. Those that fly at low speeds, have dumpy bodies, covered in hairs - not thin, streamlined and smooth. Again great aerodynamics because of Reynolds number. For the volume required the body has a smaller surface area and this is more important at low speed and Reynolds number.

The hairs create turbulent flow which is better for their body shape as it prevents air separation from the thick part of the body. This is the same principle as the dimples in golf balls - creating turbulent flow to prevent separation.

Now I haven't calculated Reynolds number for a Rover, but I expect that the flow will be turbulent at highway speeds because of the body length. This is the likely reason for not getting any advantage for turbulence stimulators. And as some else posted, you would need them up near the front to prevent air separation near the windscreen.

I would assume that the turbulence stimulators in the picture of the aircraft wing are to prevent separation at high angles of attack of the wings (pointing the nose too high). This will increase the angle at which the wing stalls and the aircraft drops from the sky if not corrected quickly by diving.

I think someones post was referring to winglets that are used increasingly on the tips of aircraft wings. The invention of these is attributed to Craig Whitcomb (spelling ??) from NASA. They are not turbulence stimulators. Their purpose is to tailor the flow at the wingtip (from the high pressure at the underside to the low pressure at the upper side). This improves the lift created by the wing.

BTW parabolic shaped (in plan) wings, like the Spitfire are most efficient because this plan shape reduces the flow around the tip. The disadvantage, and reason the shape is rarely used, is when the wing stalls (at high angle of attack) it stalls over the entire surface of the wing (very dangerous).

Thanks for alerting me to this thread Tombraider.
I do have these vortex generators fitted to my Disco2. As an engineer, I fitted them more in hope than anticipation and as far as fuel economy is concerned that has proven true. There has been so little change to fuel economy as to be unnoticeable. However there is definitely an improvement to the way dust, rain etc gets pulled in behind the vehicle and sticks to the rear window. That has been noticeably reduced.
When I was thinking about these things I did some research on the net and found a couple of articles from Mitsubishi and a Uni in USA regarding the effect of this type of vortex generator on fuel consumption. Both concluded that the effect exists but is small. The USA study on interstate trucks concluded that an underbody deflector to deflect air exiting under the rear of the vehicle up into the area immediately behind the vehicle had a much better effect. I haven't tried that yet.
Incidentally, I made some homemade vortex generators, (copied from the results of the Mitsubishi engineering article not Fuelsavers) and put them on my caravan. Again, I cant see any difference in fuel economy but the back of the caravan stays cleaner.
Russell