Occurrence of cavitation in hydraulic machines is a challenging issue because it is often accompanied with loss of efficiency, noise emissions, vibrations, and erosion damages. Tip vortices, in particular, are an ideal site for the development of cavitation as the static pressure at their cores usually drops much below the freestream pressure. The first part of the present thesis is focused on the effect of dissolved gas content and other physical parameters on Tip Vortex Cavitation (TVC) trailing from
an elliptical hydrofoil. The inception and desinence thresholds measured at different flow conditions reveal that TVC often disappears at cavitation indices higher than the inception thresholds. The measurements show that TVC desinence pressure increases with the gas content and may even reach to atmospheric pressure. This is explained by the convective diffusion of dissolved gases from water into the cavity due to local supersaturation. The extent of the delay in desinence is, however, dictated by the bulk flow parameters. Owing to flow visualizations, it is asserted that the formation of a laminar separation bubble at the hydrofoil tip is a necessary condition for the delayed desinence. The separation bubble acts like a shelter and creates a relatively calm area at the vortex core.
The second part of the thesis is dedicated to TVC mitigation. First, the effectiveness of non-planar winglets in suppressing TVC is investigated. For this purpose, an elliptical hydrofoil is selected as the baseline and various winglets are realized by simply bending the last 5 or 10% of the span at ±45° and ±90° dihedral angles. TVC inception-desinence tests reveal undeniable advantages for the winglets while the hydrodynamic performances of the hydrofoils remain intact. It is observed that a longer winglet bent at a higher angle and toward the pressure side is more effective in TVC suppression. For instance, the 90°-bent-downward winglet reduces the TVC inception index from 2.5 for the baseline down to 0.8 at 15 m/s freestream velocity and 14° incidence angle. Stereo-PIV measurements show that for this winglet, the maximum tangential velocity of the tip vortex falls to almost half of the baseline and the axial velocity reduces tangibly at the vortex core. Second, the capacity of a flexible trailing thread in alleviating TVC is examined. To this end, one nylon thread with various diameters and lengths is attached to the tip of the elliptical hydrofoil. Under almost all the tested flow conditions, the thread experiences flutter due to hydro-elastic instabilities. Thereafter, the vortical flow forces the oscillating thread to coil around the vortex axis. This rotational motion decelerates the axial velocity at the vortex core due to increased drag and augmented turbulent mixing. A sufficiently thick thread may also be sucked into the vortex core under the effect of the pressure field. This results in the whipping motion, which consists of the quasi-periodic coincidence of a part of the thread and the tip vortex axis and is considerably superior to the rotational motion in TVC mitigation. It is shown analytically and confirmed experimentally that the whipping motion leads to viscous core thickening.
Altogether, the results presented in this thesis provide a better understanding of the physics of TVC and pave the way for future designs with less vulnerability to cavitation.
EPFL2020
