Dynamic calibration of transient sensors by spark generated cavity

The cavitation erosion in hydraulic machines is often due to repeated collapses of transient vortices shed by the main cavity attached to the leading edge of the blades. Vibratory approach in detection of cavitation erosion has shown that the cavitation induced noise may be used to measure the aggressiveness of a given cavity development. These measurements require the use of transient sensors such as pressure or acceleration sensors. Most of time, the frequency responses of such transducers are provided by the manufacturers but they are no longer valid because of their specific mounting. Thereby, one has to perform unsteady calibration on site. Thus, we propose to use an instrumented underwater spark generator which allows the generation of an exploding bubble. The resulting pressure impulse, similar to that induced by the cavitation collapse, is used to excite the transducers in a wide frequency band. Preliminary study of this new calibration system shows that the exploding bubble generates intense shock waves with resulting over pressure that may reach up to 1’700 MPa within a duration less than 5 μs. Validation of this new technique is performed in the IMHEF high speed cavitation tunnel. 30 piezoresistive pressure transducers mounted on a NACA009 hydrofoil are thus calibrated by introducing the spark generator in the test section.