CFD Analysis of Air Lubrication Effects on Ship Propulsive Performance

Abstract

Shipping is one of the most cost-effective
and environmentally sustainable modes of transportation. Given that
approximately 60% of a typical ship’s propulsive power is used to overcome
frictional drag, implementing practices to reduce this resistance stands to
yield substantial economic and environmental benefits, (Larsson & Raven,
2010). A promising drag reduction technique for a ship is air lubrication. Damen
Shipyards Group is currently making this technology commercially available as
the Damen Air Cavity System (DACS). The system reduces the frictional
resistance of a ship by creating stable air cavities on the bottom hull of a
ship. The air cavities cause a reduction in friction drag by decreasing the
wetted area of a ship’s bottom. During the development of the DACS system, it
was observed that air cavities also change the inflow into the propeller. Both
the changed inflow and the frictional drag reduction affect the propulsive
efficiency and required propulsive power of the vessel. This thesis aims to
provide a better understanding of how the propulsive performance of a ship is
affected by air cavities. Additionally, a key application for air lubrication
systems is on inland waterway vessels, which frequently operate in shallow
waters. However, the impact of shallow water conditions on the performance of
the air cavity system is currently unknown. The research goals of this thesis
are investigated with the help of computational fluid dynamics (CFD). A
literature review identified the most feasible method to model a ship with air
cavities i.e. representing the cavities as surfaces with a slip boundary
condition. The influence of the air cavities on propulsive performance is
investigated by studying the change in nominal wake field, thrust deduction,
and propeller efficiency. Three ships were investigated: a cargo ship, a cruise
ship, and an inland ship. The CFD results of the cargo ship were compared to
sea trial data. Model test data was available for comparison of the cruise ship
results. First, a grid convergence study and a sensitivity analysis were performed
to investigate the accuracy of the numerical simulations and the power
predictions. The biggest source of numerical uncertainty arose from the
pressure drag. From the sensitivity analysis, it was found that the uncertainty
of the power prediction is mainly affected by the uncertainty of the thrust
prediction, followed by the uncertain propeller geometry for the cargo ship. For
the cargo ship, it was found that the air cavities cause a significant
frictional drag reduction. It was also found that the air cavities caused a
decrease in pressure drag because they decreased flow separation at the stern.
Furthermore, a strong decrease of the nominal wake fraction was observed for
this ship, because the air cavities change the boundary layer on the bottom of
the ship. A change in propeller efficiency was also observed because the
propeller working point changes. The magnitude of the change was larger than
for the other ships because this ship has a controllable pitch propeller running
at a fixed rpm. When comparing sea trial data to the CFD results, it was found
that CFD underpredicts the power, especially at higher speeds. Next to this,
CFD predicted a larger reduction in power than measured during the trials. The
prediction of the cavity length was identified as the most likely cause for the
difference. A good comparison between model tests and CFD results was found for
the cruise ship. It was found that the drag reduction and change in propulsive
performance could be predicted reasonably accurately by modeling the air
cavities as surfaces with a slip boundary condition. Furthermore, it was
observed that the air cavities caused little change in propulsive efficiency on
this ship. This is because the propellers of the cruise ship are located
further away from the boundary layer of the ship and the wake field is
therefore only slightly affected by the air cavities. On the inland ship, it
was observed that the pressure drag and flow separation at the stern were influenced
due to the air cavities. However, no comprehensive conclusions could be made
due to scatter in the data. This is most likely due to the uncertainty present
when modeling flow separation. Furthermore also for this ship, a decrease in
propulsive efficiency was found because the air cavities decreased the wake
fraction of the ship. Additionally, CFD simulations were conducted for the
inland ship at varying water depths to assess how shallow water conditions
impact the performance of the air cavity system. Since the ship’s frictional
drag increased in shallow water, the total drag reduction from the air cavities
also increased. Next to this, a change between pressure and flow separation
when comparing air on and air off was observed. Also here, no strong conclusion
could be made due to scatter in the data. Lastly, it was found that the change
in wake field caused by the air cavities is larger in shallow water than in
deep water. ii Based on the results of the three ships studied it can be
concluded that an air cavity system affects the propulsive performance of a
ship. It was found that an air cavity system reduces the wake fraction of a
ship because it limits the growth of the boundary layer on the bottom of a
ship. The reduction increases for ships with a high block coefficient and a
large air-covered area. For a twin screw ship, the change of the wake field is
less significant. The change in propeller efficiency depends on the resistance
reduction, possible change of the wake field, and the original working point of
the propeller. Furthermore, it was found that the thrust deduction effect is
not affected by the air cavity system. It can also be concluded from the
results of the cruise ship that the flow around a ship with air cavities can
modeled reasonably accurately provided that the shape of the air layer under
the ship is known. More research is recommended on how air cavities change the
flow separation and pressure drag of a ship.