Experimental and numerical research into propeller thrust breakdown due to ventilation

Abstract

When
propellers are operating near the free surface, 
phenomenon called ventilation might occur. Due to insufficient
immersion and high thrust loading, the propeller draws air, resulting in a
reduced thrust. Reduced thrust may have consequences such as loss of propulsive
power, control and steerability, and may therefore be leading to safety issues.
Long time exposure to ventilation’s unsteady torque loading can also lead to
propulsive unit malfunctioning. As propeller diameters tend to grow bigger,
free surface clearance decreases and room is left for air to be drawn. To
increase understanding of the phenomenon, current experimental and numerical
research was executed.  The used
propeller was a Wageningen C4.55-propeller, an in design condition
lightly-loaded propeller with low blade area, fitting to the trend of increasing
diameters. The research was bound by perfect conditions to capture ventilation
in the purest form; no influences of wake, waves and ship motion were taken
into account. Experimental research showed that free surface ventilation
appeared to be the most stable and predictable ventilation regime. Inception
through free surface breaking mainly depends on the pressure gradient between
the propeller tip and free surface, the tip immersion rate and the ability to
draw the free surface. Increased ventilation thrust breakdown showed to be
influenced by the local velocity on the blade, which mainly depends on the
propeller rotation rate. Vortex ventilation was the most unstable regime in the
experiments. Inception seems to be independent of the propeller loading, but
influenced by local flow phenomena in the area above the propeller and
propeller characteristics . It is believed that vortex inception, shape and
wash-out resembles the appearance of the cavitating propeller-hull vortex.
Vortex ventilation showed a bistability effect. Experimental results were
obtained using a statistical research planning/Design of Experiments, such that
polynomial models could be constructed. The model fitted the data well,
demonstrated by the fitting coefficient r2 exceeding 0.9. Structural
shortcomings were found in capturing the highly unsteady vortex ventilation,
variations in mixed ventilation and increased thrust breakdown in free surface
ventilating. Numerically, ventilation was simulated using the incompressible
VoF-solver ReFRESCO. Vortex ventilation inception was not found, even when a
scale resolving simulation was conducted. This is ascribed to insufficient
application of the SRS-model in the near blade area, due to insufficient
convergence of the omega-equation. Also application of Boussinesqs assumption
in the k-equation might be stringent. Free surface ventilation inception was
accurately found, both in simulations with a for ventilation adapted actuator
disk model and with the propeller. Thrust breakdown was underestimated by CFD.
Only breakdown due to surface piercing was found. Underestimation is ascribed
to the absence of air entrainment. Application of TNT-EARSM-model (which is not
using Boussinesqs assumption) and application of free-slip boundary conditions
did not improve the shortcoming. As in literature, other free surface
discretization schemes showed the same lack of air engtrainment, the origin
might be in the VoF-assumption, being the increased interpolated density used
in the momentum equation which prevents air to be convected.