Numerical and Experimental Analysis of Free Surface Flow in a 3D Non-Rotating Pelton Bucket Low frequency fluctuations often occur in hydraulic installations equipped with fixed bladed turbines or pumps operating under off design conditions. Passive measures, such as air admission or minor design modifications, are commonly taken as a last resort to try to reduce these phenomena. This article describes an active control approach to the problem, which is based upon a rotating valve exciter. The idea is to create an external excitation with the hydraulic exciter to diminish an undesired frequency component of the turbine’s natural excitation. One of the key elements required for this type of actuator is an adequate control strategy. It is based on a self-tuning extremum control technique, which implements an on-line identification of the optimal system parameters. The operation principles and performance issues are described and illustrated with experimental results obtained on a 90 kW Francis turbine model. The results demonstrate that the pressure fluctuations at a chosen frequency component can efficiently be reduced.