Frequently asked questions (by experts) on the Supramar Anticavitation Groove
last update 11.12.02: corrections faq 7
1. Is the pressure on the body in any place reduced below water vapor pressure ?
No, but instead of a void at vapour pressure, the two-phase mixed flow of the mixing layer fills this void with the consequence, that the body surface and the outer void boundary are coupled by a fluid with high compressibility, specific mass and kinematic viscosity. That means, that a direct collapse of such a void filled as in ordinary cavitation with two-phase medium is not any more dynamically possible. Observed consequence of such a mixing layer present is a static pressure recovery after the point of minimum static pressure at vapour pressure, which attains a substantial part of the value reached without any cavitation. At speeds higher than sonic speed in the two-phase mixing layer this phenomenon is very much enhanced.
2. What is the range of angles of attack you have tested ?
At present the experience available covers angle of attack ranges of a few degrees (+- 3.5 degrees) and profiles with thickness up to 12% at 15% chord.
3. Is your effect occuring at near zero-incidence and very thin profiles only ?
Of course we started doing tests with very thin profiles around zero degree angle of attack. The test purpose was checking lift variation with airfeed. Status is profiles 12% thick at +- 3.5 degrees.
4. What is the length of cavity with your groove applied ?
With a real two-phase mixing layer there is no such thing as a cavity length. The mixing layer extends either clearly beyond trailing edge or dissolves without collapse before the trailing edge (if it is a mixing of water vapour bubbles and water only)
5. can it be combined with airfeed ?
Yes, with air injected the mixing layer is thicker and more stable, which will enhance its effects. Dissolution of the mixing layer before reaching trailing edge does no more occur.
Another very important consequence of a mixing layer combined with airfeed is that in contrast to recent published work (Fast 2001 Conference), where by air injected on the profiles suction side a cavity is produced, we get a displacement thickness change of the attached boundary layer causing the lift variation.The corresponding time constants for change of the effect are thereby massively reduced and contrary to a lift control with cavity size no hysteresis is caused. As tested in the live 1967 Skagerrak tests on a Supramar Hydrofoil PT50 and later operationally demonstrated by several types of Supramar hydrofoil vessels, servo response time of airfed foils for lift variation is reaching consistently more than 20 cycles (compared to about 1 cycle reported in the actual FAST 2001 paper on using air cavities). This is the order of servobandwidth that was proven to support effective ride control in presence of more than one surface wave system. The Supramar mixing layer has no speed limits (tested above 100+ kts for low sigma values). To the past experience of Supramar there are two conditions to be satisfied for dependable fast response lift control by airfeed:
Usually a variation of lift coefficient of some 0.15 Cl is largely sufficient considering the important dynamic head at fast speeds in water.
6. what is the range of configurations and speeds you already have tested ?
Testing took place at subsonic, sonic and supersonic conditions (up to 45 m/s). The most recent confirmation tests were done at subsonic speeds (e.g. 11 m/s) with no air and deaireated water, which represent the most adverse conditions for creation of real mixing layers.
7. what happens in applications to rotational symmetric bodies ?
The same as on profiles: no cavitation and no supercavitation. A dynamic coupling of body surface static pressures and outer void surface leading to almost normal flow around a body. This would also allow conventional steering methods. Drag will be however bigger compared to a supercavitation configuration because we will have base drag with mixing layer.
8. what is the difference of cavitation and a mixing layer consisting of water & bubbles ?
" I am wondering if the mixture you create with the groove (without air supply) is not really cavitation". If by cavitation we define that vapour bubbles (whatever their respective size) are present, the above cited statement holds. If however as we do cavitation or supercavitation means a sizable void (at vapor pressure), which may collapse, either before or after the trailing edge, then a boundary layer consisting of an intense two-phase mixture of water and bubbles(vapour and/or air) is to our interpretation of the releveant observations something totally different: A Two Phase Mixing Layer (having a distinct low sonic speed property).
9. what about a conventional propeller with groove, is not a surface piercing propeller or a waterjet-system anyhow a superior solution for high vessel speeds ?
Applying a groove to the blades (together with a small blade profile correction) on a classic submersed propeller will have two main effects:
This would do away with two main limits classical submersed propellers at normal ship drought depths have:
Of course application of a groove with mixing layer on propeller blades will cause additional profile drag, which has however to be weighted against additional drag or efficiency losses of the surface piercing propeller, waterjet propulsion systems and supercavitating submersed propellers.
10. why is a groove superior in creating a mixing layer ?
Of course for creating mixing with ordinary means you could do so by increasing surface friction between a body surface and the fluid, e.g. by applying increased surface roughness. By this however also the boundary layer thickness is increased, the velocity gradient at the wall is decreased consequently, what as a result limits the mixing achievable.
Using a groove the boundary layer above the groove changes from a wall boundary layer to a free jet layer. Free jet layers have at high enough Re-numbers a transverse momentum transport, which is more than 10 times the values in a wall boundary layer. This combined with the reattachment of the free jet layer at the wall at the downstream end of the groove produces a maximum mixing intensity combined with a full, almost rectangular velocity profile of the resulting mixing layer boundary. It is to our understanding also the reason, why a mixing layer produced by a groove reattaches itself after the groove even in positive pressure gradients.