Cavitation for Dummies

When presenting the Syroco speedcraft concept, we talk a lot about cavitation, supercavitation, cavitating regimes… Most folks involved in naval architecture (and other fields of engineering) are familiar with these complex concepts, at least to a degree. For the benefit of others, we asked Catherine Ramirez Villalba and Michele Farnesi to explain as simply as possible what it is and why it matters for the Syroco speedcraft. 

Apologies to the experts, you may not learn a lot here - we just hope we did not oversimplify things. 

As a starting point, what is cavitation and what causes it? 

Catherine: The transformation of a liquid into vapor (called vaporization) is caused by a combination of temperature increase and pressure decrease. Most often, it happens when the temperature increases (boiling) but it can also be caused by a drop in the pressure. Cavitation is defined as the appearance of vapor in a fluid due to a decrease in pressure at constant temperature. This means that for given conditions, vapor bubbles appear whenever the local pressure drops below the saturation vapor pressure.

In the case of the foil we are designing for the Syroco speedcraft, it is the geometry, the speed and the angle of incidence of that foil in water which cause an extreme variation in the pressure field that surrounds the foil. As with any foil, there is a face working at high pressure (the intrados), and the opposite face is subject to a lower pressure (the extrados). The greater the speed and/or the angle of incidence, the lower the pressure on the extrados becomes. When this pressure gets low enough, cavitation (vapor formation) happens. 

When can cavitation become a challenge? 

Catherine: Depending on the pressure field around the foil surface, cavitation can occur in a steady or unsteady state. An unsteady state is characterised by changes in the pressure in regions near the surface of the foil. Due to this non-uniform pressure, the vapor bubbles that form in the front of the foil move downstream to regions of higher pressure where they collapse and detach. This creates shocks and vibrations - phenomenons we would rather avoid.

On the other hand, in a steady cavitation regime the pressure field is homogeneous in the regions near the surface of the foil. That allows the formation of a stable vapor pocket all along the foil's surface, with a closure (limit between the vapor and the liquid) taking place at least 1 to 2 chords behind the foil. This is known as supercavitation. When supercavitation occurs, the fluid streamlines are circulating around the vapor pocket as if it forms a solid shape, generating hydrodynamic lift.

The operating conditions of our speedcraft require not only high velocity (80 knots) but also the generation of sufficient lift by the foil. And all of this needs to happen at a depth of only 1 metre. We have the combo conditions that make lower pressures impossible to avoid… so we won’t even try to avoid cavitation. Instead we work to make it steady. We need to achieve a supercavitating regime.  

Cubit Innovation Labs is a key partner for Syroco, what kind of expertise are you guys bringing to the table? 

Michele: Cubit Innovation Labs has lots of experience in cavitation. Prof. Giovanni Lombardi, who founded the Fluid Dynamics Division of Cubit, is a world recognised expert in yacht design and hydrodynamics and has been working on the subject since his master thesis on racing sailboats. My own expertise resides in aerodynamics design and simulation (with my colleagues we have worked for example on exciting projects around fast Italian cars…) but not necessarily in cavitation. We have however a lot of experts we can rely on, and I have learnt very quickly (I did not have much of a choice!) 

How is Syroco partnering with Cubit’s numerical simulation expertise to optimise cavitation? 

Michele: We are using two types of simulation for this part of the project. The first one is CFD - Computational Fluid Dynamics. It creates a numerical model of the flow field based on the geometry of the foil and its parameters: speed, angle of incidence, etc. 

The second type is optimisation. We use tools developed at Cubit and leverage the immense expertise of the team to define the right optimisation workflows. 

Since we are dealing with a novel field, we started from standard cavitation foil profiles designed by NASA in the 1950s. And we set out to understand how changes to geometry would impact performance. So we set up procedures and workflows to search in the design space for the perfect shape in terms of profile and geometry of the entire foil.

All of this requires lots of computing power. At Cubit we have a HPC system with 9216 cores, that was developed especially for CFD and optimisation. In one of our latest workflows we ran simulation for 400 different designs, with approximately 8 hours of processing time for each!

How important is this foil optimisation? 

Michele: As I explained, we started with a generic profile of a cavitating foil. But this was only a starting point for the simulation. As we manually changed the geometry, based on our experience and supported by CFD simulations, we were able to improve its performance by 50% already. 

The next phase, the optimisation, still carries lots of improvement potential, we know we can reach better performance. We think that another 50% is within our reach. 

Catherine: All this optimisation that allows us to refine the geometry requires a significant effort in terms of high-performance calculations. But it’s worth the investment. And working with the team from our partner Cubit brings a very useful contribution, they have so much relevant expertise. 

At the end of the day, better efficiency means less drag for the same lift, and therefore more speed! Which is what the speed record is about… 

Catherine Ramirez Villalba holds a PhD in fluid mechanics with emphasis in fluid-structure interactions algorithms. She is in charge of Syroco’s research in cavitation and leads the startup’s numerical simulation programs. 

Michele Farnesi is an aerospace engineer specialised in aerodynamics design and simulation at Cubit Innovation Labs, a strategic partner of Syroco. In the Fluid Dynamics Division of Cubit, he is responsible for the partnership with Syroco.