Effect of waves on cavitation and pressure pulses

Abstract

In view of environmental concerns, there is increasing demand to optimize the ships for the actual operating condition rather than for calm water. Now, in order to apply this for propeller design, a first step would be to study the effects of waves on propeller operation. Therefore, the aim of this paper is to identify and quantify the effect of various factors affecting the propeller in waves. The performance of KVLCC2 propeller in the presence of three different waves has been compared with calm water performance. Changes in performance in terms of cavitation, pressure pulses, and efficiency have been studied. Significant increase in pressure pulses has been observed due to wake change in waves even though cavitation did not show any significant change. An analysis using cavitation bucket diagram in different wave conditions indicates that a propeller optimized for calm water wake may perform much worse in presence of waves. Therefore, having wake variation at least in critical wave conditions (where the wavelength is close to ship length) in addition to calm water wake could be very useful to ensure that the propeller performs equally well in presence of waves.

Effect of waves on cavitation and pressure pulses of a tanker with twin podded propulsion

“…A notable increase in pressure pulses was observed in the analysis of KVLCC2 propeller in the presence of waves [11]. It was believed that the high wake of a full-bodied single-screw ship also lead to strong wake variations due waves.…”

“…In the analysis reported in [11], wake variation led to higher pressure pulses. However, change in maximum cavitation volume was small.…”

“…Therefore, considering the currently available tools, it is difficult to take into account the effect of wake variation in the propeller design. Significant change in the performance was observed both in this case and in the earlier investigations using a single screw ship [11]. Therefore, to limit the required computations while still capturing the main effects of waves in propeller performance, it is recommended to obtain wake variation at least in the critical wave condition (wavelength close to ship length) to have an idea about the possible performance variations due to waves.…”

“…Therefore, we decided to simulate the 8000 dwt chemical tanker in the presence of three different head waves at full-scale Reynolds number. Wavelengths were chosen such that different parts of the pitch RAO (response amplitude operator) can be covered, as wake is substantially affected by ship motions [11]. Simulations were performed at the design speed of the ship, which is 14 However, the pod was considered as a part of ship geometry and included in the computations, so that the effect of the pod on the wake variation was included.…”

“…However, it is also important to make sure that the boundaries of the domain are not so close that they can reflect back nonphysical waves into the domain. According to [11], it is recommended that the inlet boundary should be located 1-2 ship lengths upstream of the hull and outlet boundary should be 3-5 ship lengths downstream to avoid any reflections from the boundary walls. Domain size used for the simulations can be seen in Table 4.…”