Cavitation propeller and hydrofoils or foils

cavitation forming just after the leading edge

Cavitation is primarily an ebullition of water:

When the ambient pressure becomes lower than the saturated vapor pressure appears cavitation. Cavitation can be caused by a warming of the water or by a pressure drop. Cavitation observed on propellers or hydrofoils is mainly caused by the pressure drop of water in areas where localized pressure drop is observed.

Viewing the surface pressure of marine turbine in the software Heliciel:blade cavitation detection

What resources do we have to avoid cavitation? : See more closely how the cavitation appears and how to detect when designing a propeller:

Detailed results of pressure distribution of the blades elements in heliciel software

pressure on the blade

  1. The lift applied to the upper surface is the result of the vacuum created by the profile shape. If this depression reduces the ambient pressure below the saturation vapor pressure, the blade cavite. This cavitation forms a gas pocket which covers the maximum point of depression concentrated around the center of lift of the profile. cavitation If left on the blade without localized beyond the trailing edge, the lift is s' not necessarily collapse because depression of the pocket continues to suck the upper surface, but the elasticity gas pocket prevents the increase the lift as effectively as in the liquid. Any increase in lift will be transformed into gas expansion. It follows an increase in the volume of the pocket and its surface in contact with the upper surface. This increase in surface depression further increases lift, but when the surface of the pocket reaches the trailing edge and covers the entire width of the profile, the performance 's collapse. Some research leads to the conclusion that when the cavitation is moderate it can even increase performance because it reduces friction. But the constantly changing operating conditions propellers make this range very complex to master, and we will prefer not to risk a total loss of performance, so we keeping our propeller as far as possible from the cavitation zone.
  2. The drag of the profile can generate a negative pressure, shown on the rear of the profile. If streaks applies over an area equivalent to the frontal surface (super cavitating profiles). this lowers the ambient pressure below the saturated vapor pressure, and the blade may cavitate.
It is therefore essential to assess whether cavitation is concentrated around the point of lift or if it applies to the entire surface of the profile. To better assess the risk of loss of performance, thus Heliciel differentiates three types of depressions on the propeller blade:
  1. Depression concentrated (lift applied symmetrically around the point of lift)
  2. Depression distributed over the surface (lift applied over the entire surface)
  3. Depression drag applied on an area equal to the front surface
Evolution curves risk of cavitations drag, lift concentrated and distributed according to the rotational speed in Tab heliciel optimize / (multiple analysis operating points) :

Evolution of the depressions on the blades as a function of the speed of rotation of a propeller with software HELICIEL

Viewing the risk of cavitation (drag, lift distributed, concentrated lift) for each element from the foot to the end of the blade.

graphic cavitation

How The profile shape manages the distribution of depression on the upper surface so the tendency to cavitation Profile?
the lift applied to the surface of the upper surface of blade gives us the depression due to the lift. But as we have seen earlier, this lift is not uniformly applied on the upper surface, in fact, the point of application of the lift on the profiles is usually located around 0.25 times the profile chord starting from the leading edge.(Heliciel software uses data profiles to find the point of lift).

extract from the database HELICIEL

profile aerodynamic hydrodynamics (extract from the database héliciel)

Depression is concentrated around the point of lift. We can roughly estimate that the depression will be equal to the lift applied symmetrically around the point of lift. For a point lift at 0.25 times the rope: 0.25 front and 0.25 rear, which reduce the surface action 0.5 times the upper surface of the lift if the point is 0.25 times the rope. Depression will be max: Depression = (Lift force N) / (0.5 X surface_extrados). If the point of application of the lift of the profile is located at of the leading edge 0.35, héliciel considers that the suction zone is concentrated (0.35 X 2) = 0.7 times the area of ​​the upper surface. We just imagine that a profile generating its lift closest to the center of the rope, (0.5) distribute its lift on the entire surface of the upper, so the depression will be low. Profiles with a lift point near the center are less prone to cavitation!
Several theories explain the phenomenon of lift: ome expressed that the phenomenon is not sufficient to explain all the lift forces.

 

In any case, one thing is certain is that a vacuum is generated on the upper surface and pressure is generated on the underside. (If the angle of incidence generates lift). The fact is that the lift is concentrated in the area between the edge (just after) and the point where the fluid is no longer forced to change direction. The area where the fluid is no longer obliged to change direction, it is the area of ​​maximum thickness if the incidence (attack angle) is zero.

 

 

The suction zone is distributed, from leading edge (next) to the point where the upper surface is parallel to the fluid stream. The lift point is in the middle of this area. The cavitation zone is therefore around the point of lift:

 

In the curve of lift coefficient on a profile in the software Heliciel (below), The pink curve shows the distribution of depression on the upper surface. Point 0 of the x-axis corresponds to the leading edge and point 1 corresponds to the trailing edge. The blue curve represents the intrados. (The y-axis is reversed: positive values under the x-axis)

 

If one angle of incidence is given to the profile, the point where the suction becomes parallel to the fluid, approaches of the leading edge and reduces the area where the lift is concentrated. Thus depression focuses and cavitation will appear more elift curveasily. If you look at a profile, and its upper surface, you know that the point of lift is approximately halfway between the leading edge and the area parallel to the flow:

.cavitation bubbles

More the area of max thickness is moved back, the more particles continue to accelerate a long time, thus distributing their area of ​​depression over a larger area. The same lift force spread over a large surface generates less depression concentrated. Profiles of the propeller blades operating in the water should have a max thickness rearmost possible so as not cavitate too soon.

 

Below is a profile lift of high concentration that might cavitate easily :

depression concentrated front

Below is a profile of low concentration but the rear lift may generate a lot of drag because of its thickness! This profile may be not cavitate because of the lift but because of its drag.

depression Tail profile

Below is a profile which spread the best its lift without creating too much drag:

depression distributed cavitation

These forms are taken as an example and are not qualified for a propeller Here is a more classic marine profile

(extracted from the database Heliciel software):

non-cavitating profile

Répartion de portance d'un profil dans Heliciel . Forme classique de profil aérien(extrait de la base de données héliciel):

lift coefficient curve

Photo of a foil with a thickness max on the rear distributing its lift. better than the rudder (vertical) which supports it. It is a good example of the importance of the choice of profilescavitation profile
Cavitation is a complex topic that is approached only superficially on this page. For further reading I invite you to consult the thesis on cavitation from Mr Surasak PHOEMSAPTHAWEE which is in my humble opinion a useful reference.