lift and drag changes at the onset of cavitation

[boogie]

hi there,

i'm working on some high speed windsurfer fins and quite a few riders [fortunately not the ones on my fins ] seem to be hitting a "wall" around 43/44kn peak speeds.

where would i find some information on how the lift and drag changes on a vertical foil at the onset of cavitation.
is it more of a sudden loss of lift / increase in drag or is it more of a gradual steady process?
what would be the critical CPmin values to look out for in Xfoil that actually give you the incipient speed for cavitation?

ventilation does not seem to be a problem.

thnx
boogie

[jehardiman]

There is not much available work on symetric foils at or above the critical cavitation number, which is what I bet is happening here, as it is a condition you tend to design to avoid or in the case of power vessels just power over. What happens is face cavitation occurs at the point of minimum pressure. This tends to cause a reduction in the Cl-alpha slope at the condition of inception. Additionally, there is a dramatic rise in the turbulent drag, especially for laminar sections if they have not already tripped due to inflow conditions (see Hoerner, FDD, chapt 10).

[Tim B]

OpenFOAM is capable of simulating dispersed gas in a fluid. Depending on how much detail you want to know, I suggest you have a look at doing some CFD. You'll need a powerful machine running Linux, Open-FOAM and Gmsh at the very least.

Oh, and a fair bit of know-how and patience to get it running.

This is not a simple job by any means.

Tim B.

[tspeer]

Originally Posted by boogie

where would i find some information on how the lift and drag changes on a vertical foil at the onset of cavitation.

There are lots of different types of cavitation. The loss of lift is predictable with the right codes, but it's going to depend on where and how the cavitation develops, and it's probably better to avoid cavitation to begin with. Or else go with sections that induce cavitation in a controlled manner.

what would be the critical CPmin values to look out for in Xfoil that actually give you the incipient speed for cavitation?

It depends on the speed. This chart shows the Cpmin from XFOIL (converted to maximum velocity ratio for better resolution at high speeds) for several sections, along with a grid showing the
values for incipient cavitation. At 45 kt, you're looking at a Cpmin around -1.6 for incipient cavitation. If you're trying to go 50 kt, you only have a Cpmin of -0.7 to work with.

A problem area is the intersection at the root of the foil. The perturbation velocitis of the fin and board add in this region, so the local velocities can lead to cavitation at the intersection before it begins on the rest of the fin.