mig wrote: i read a review of the latest racing wheels idea in the comic a few weeks ago. usual ridiculous babble about this and that with the added comment that a pattern on the rim was an echo of shark's skin (or whale or something) which made that more hydro-dynamic. er...okay...so your new £3k+ wheels are good a cutting through water?
ehhh when i were a lad...
There is indeed a lot of technobabble and outright BS in the world of bike magazines. I have not bought a bike magazine for about a decade. About five years ago I picked up one to leaf through whilst waiting for a train and just spending a couple of minutes reading bits of it reduced me to alternate fits of apoplexy and laughing out loud. A load of nonsensical hackneyed drivel for the most part.
Regarding the science, there is an effect that it may be possible to exploit to make a marginal gain. The main phenomenon is that when an object is either 'blunt' or presented into the airflow unfavourably, the flow tends to detach on the lee side and this creates more drag. However if the boundary layer is made turbulent (under conditions where it would otherwise be laminar) then it tends to adhere better and the onset of flow detachment is delayed somewhat. Note that if the flow was going to stay attached anyway, there is a small penalty for turning the boundary layer flow turbulent; this creates a little bit more drag, all the time. But if it prevents flow detachment even for 10% of the time, there will likely be a net gain overall. There is also a second effect whereby the skin drag is reduced (all over) if certain textures are employed. This effect (widely exploited in nature) is less well understood by fluid dynamicists.
Examples where this kind of effect is used practically include;
- reducing the stall speed of light aircraft; Flow detachment is synonymous with 'stalling' in aircraft wings when flying at low speed. Quite a few models of light aircraft have been retrofitted with 'turbulators' just aft of the fattest part of the chord section near the wing root; this can delay the onset of a stall by 5-10 knots in some cases, and in some cases the stall is less abrupt too, which all means that the plane is safer and easier to fly.
- golf balls; the dimpled surface causes the boundary layer flow to be turbulent, the flow remains better attached, the ball sees less drag, and flies further than if it were not dimpled. Golf balls used to be made smooth at one time; golfers soon noticed that worn balls (with rough surfaces) usually flew further than new (smooth) ones.
- shark skins; these are made of tiny bones called 'denticles'. Their shape is convoluted and mysterious; it is thought to possess the flow detachment properties created by dimples. In addition the shape is thought to modify/control the boundary layer flow in such a way that skin friction is reduced even if the flow is not just about to detach.
-use of 'riblets' on aircraft and wind turbine blades. About 15 years ago 3M made some tape that could be applied to the outside skin of an aircraft and this was trialled on long-haul aircraft crossing the pacific. There was a small but significant fuel saving, however the surface tended to stop working if it became dirty. There is a summary of the work to 2002 herehttp://inter.action.free.fr/faq/riblets.pdf
More recently research has looked at wind turbine efficiency http://web.archive.org/web/20150226044952/http://m-selig.ae.illinois.edu:80/pubs/SareenDetersHenrySelig-2011-AIAA-2011-558-WindTurbineAirfoilsRiblets.pdf
again showing an effect but one that isn't well understood, or always of benefit, even.
Regarding bike wheels, it is questionable if the Reynolds number is in the right range to do any good. It is also questionable whether the flow over the wheel rim would be susceptible to being controlled in this fashion (the wheel is turning, and except for the very front of the front wheel, the flow is already 'mucky' anyway). Doubtless one could devise a test (probably not representative of real conditions) that appears to show a drag reduction using dimples, in an attempt to flog your new rims. Whether it is likely to make any real
difference or not is quite another matter....