**4. Conclusion**

Steady and unsteady numerical simulations were carried out in many configurations: first, in a venturi duct, next in two blades cascades with different characteristics, and then, in three axial inducers.

In order to understand the cavitation behaviour in the inducers and to validate the steady results obtained numerically, many experimental tests were developed, in steady state, for the three studied inducers.

In general, a good agreement between experimental and predicted results was found for a range of flow rates and cavitation behaviour, i.e., overall performances, cavities sizes and cavities location, etc. This study shows that the optimal inducer design depends mainly on three criterions to choose the best design for the three studied inducers: The critical cavitation coefficients corresponding to 5% and 15% of head drop, the head and efficiency produced in cavitating and non-cavitating conditions, and the vibrations generated by the inducers operations.

The numerical results, in a simple geometry, suggest a strong interaction of the turbulence and the unsteady cavitation.

Cavitating flow in the blades cascades, for various values of σ and flow rates, predicted three types of cavitation behaviour on the blades cascade:


Symmetrical cavitations on all the blades were observed for high values of σ and high flow rates. Alternate blade cavitation was observed numerically for partial flow rates, when the *l/h* ratio was higher than about *65%*. This phenomenon was observed only on the two-blade inducer. Finally, the rotating cavitation was observed for lower flow rates, on both studied blades cascades. The calculations were carried out using RNG κ–ε model and RNG κ–ε modified model which provide different results.

Unsteady numerical results showed three different mechanisms of cavitation instabilities:



Finally, the unsteady cavitating calculations realized in three-dimensions for a two-blade inducer demonstrate the complexity to obtain and analyze the flow instabilities caused by the cavitation in these machines. The obtained results show that rotating cavitation appears for a partial flow rate, but it is less obvious in the inducer than in the blades cascade. It was noted that the shape and behaviour of cavitation is greatly disturbed by the tip radial clearance, which also modifies the torch which is formed upstream.
