Ben
Cavitation, ventilation and vortex shedding are different.
Cavitation is the result of the water boiling under low pressure usually at the leading edge of a foil. It can also occur around the outside edge as well. It is related to the severity of curvature in the foil and speed. It can be violent if it is collapsing and reduces effectiveness.
Ventilation is just air getting down the suction face of the blade. It reduces effectiveness. I have seen some of the foiling tris using fences on their surface piercing foils to to cut off the air supply.
Vortex shedding is due to flow separation. There are three zones of flow over a blade - laminar, turbulent and separated in that sequence. The transitions depend on the Re# viscosity and pressure gradient. When the flow is separated there are back eddies or vortices that cause fluctuating pressure. You can get violent shedding the tunes to structures. If in tune it can be damaging to the structure. It is the reason flags flutter.
Generally thinner foils will have lower pressure coefficients and that is going to be better for conditions where cavitation is likely. However in doing this rudder exercise I found this interesting original work on the NACA07 series:
I have been using the 16 series for my prop blades for quite a while now as It has some nice features. I was not aware of the 07 series until I started this rudder exercise. It is probably a better section for use on a water prop than the 16 series but I am yet to confirm that - an interesting aside for me to play with tomorrow.
Rick
On 04/08/2010, at 10:59 PM, bjarthur123 wrote:
thanks for the confirmation guys. the last time i heard the humming i was experiencing quite a bit of lee helm. perhaps the rudder was just stalling b/c of the AOA required to counteract it. i did not take the time to figure out whether it was the rudder or daggerboard though. i'll experiment with balancing the centers of effort better next time and see if it goes away.
class rules prohibit any modifications to the foils of course. the trailing edge is already quite sharp though.
just out of curiosity: high speed foils are generally thinner, right? precisely to avoid such cavitation/ventilation/vortex shedding/whatever it's called? the tradeoff is that there's less lift at lower speeds?
ben arthur
weta #358, "gray matter"
ithaca, new york
> It is almost certainly the trailing edge of your rudder/daggerboard shedding
> vortices and it slows you down. Make the trailing edge a 45 degree angle
> instead of a flat and it will disappear.
> Probably vortex shedding off a bluntish trailing edge or flow
> detachment on a control surface.
> The vibration results from the supporting structure getting in tune
> with the shedding. So you have to avoid the vortices or stiffen the
> structure to eliminate the vibration.
Rick Willoughby
03 9796 2415
0419 104 821