Measurements of cavitation inception in a turbulent shear layer

The mechanisms that govern the location and frequency of cavitation inception events in turbulent shear flows have not been well characterized. Water tunnel experiments in the shear layer generated by a 10 mm (h) backward facing step at speeds of 5.3, 10.5 and 16 m/s examine the mean flow, Reynolds stresses, and the onset of cavitation. High-resolution 2D PIV confirm that the separating boundary layer is turbulent with Re_τ=807, 1504 and 2345, respectively. The profiles of mean velocity as well as the normal and shear Reynolds stresses collapse when scaled by the freestream velocity and reattachment length. The latter decreases with increasing Reynolds number, varying between 5.3h to 6.2h. Highspeed silhouette imaging in perpendicular views captures early phases of cavitation events, corresponding to cavitation indices in the 0.45 to 0.55 range, primary at the higher two speeds. Inception occurs in the quasi-streamwise braids developing between the spanwise vortices shortly after they roll up in the near field of the shear layer. Hence, the intermittently forming cavities appear as inclined elongated strings. The transition from a microscopic nucleus to a fully formed 5-7 mm strings occur in less than 250 μs, and they break up and collapse in about 1ms.