Glossary
Lift and Drag coefficients :
Aerodynamic forces can be expressed as a coefficient that depends neither on the size of the system, nor the apparent wind speed seen by the system. These coefficients, which depend mainly on the type of velic propulsion used, are characteristic of the aerodynamic performance of these solutions.
The lift coefficient is related to the lift force, and the drag coefficient to the drag force.
Average power savings :
SimWAPS calculates the average power savings, at the main engine propeller shaft, due to the wind-assisted-propulsion.
In case of active systems, this is the net energy savings. The WAPS consumption has already been deduced.
The average power saving highly depends on the statistical weather conditions encountered on the selected route.
The average power saving can be expressed in kW for the whole system or in kW per square meters (kW/m2) which is the ratio of the average power saving per unit of area.
Average fuel saving :
This value is deduced from average power savings based on a specific fuel oil consumption (SFOC) of 180g.kWh-1.
CO2 emission savings :
The reduction in CO2 emissions provide by SimWAPS is counted from "tank to wake".
This figure is calculated using a typical fuel carbon content of 85%, hence the combustion of 1 tonne of fuel oil rejects 3.1 tonnes of carbon into the atmosphere.
Lift to Drag ratio :
A high lift to drag ratio indicates that the profile is able to develop lift with few drag.
This ratio is particularly important when the apparent wind angle is low, i.e. when the vessel is sailing close to the wind.
Propulsive force :
The propulsive force generated by the WAPS is the component of the aerodynamic force directed in the ship's foward direction.
This force reduces the propulsion required from the propeller of the main engine.
Side force, drift, heel :
The aerodynamic force generated by the velic propulsor has a forward, propulsive, component that reduces the demand on the vessel's main engine, and a side force, which must be balanced by hydrodynamics.
The ship compensates for the side force by drifting and/or using anti-drift appendages. The result is an increase in forward resistance. SimWAPS estimates the corresponding power loss.
Criteria are integrated into SimWAPS to detect potential excessive heel or drift. In this case, a complementary study is required to better define the potential of velic propulsion. Please contact us.
Aerodynamic interactions :
Wind propulsion systems modify the wind, mainly in direction, around their upstream and downstream positions. This flow modification causes the different wings to interact with each other.
These interactions increases with the reduction of distance between wings and with the increase of the wings area and lift.
CRAIN has tools that model wing interactions using a vortex method, taking into account the real lift to drag ratio of the wings. CRAIN has dran on its expertise with this type of tool to propose an estimate of the interactions between the wings. To achieve this, SimWAPS takes into account :
- Up to 8 systems,
- The distance between the wings,
- The apparent wind angle,
- The area of the wings,
- Wings lift.
The interactions applies to all the propulsion solutions proposed in SimWAPS, including rotors and suction wings, with the exception of dynamic kites.
Lift :
Lift is the component of aerodynamic force that is perpendicular to the flow, in this case to the apparent wind. The name comes from the early days of aeronautics, as it is the component that makes aircraft fly.
In the case of sail propulsion, lift is necessary for the sail system to be able to develop a propulsive force when the apparent wind is ahead of the beam, which is the vast majority of conditions encontered.
For most apparent wind angles, tje optimum operating point for wind propulsion systems is the one with maximum lift. The maximum lift of a system is therefore an essential characteristic of wind propulsion systems.
One way to increase the maximum lift of a profile is to introduce camber, i.e. to move away from the symmetrical profile. This asymmetry will increase flow deflection and raise the profile's lift, until it stalls. When the airfoil is stalled, lift falls sharply.
Maximum lift, CLmax :
The maximum lift coefficient expresses the maximum force per unit area that the propulsion system can develop. A high CLmax value allows reducing the area and the footprint of the system.
For the vast majority of apparent wind angles at which the propulsion system operates, this is the point on the aerodynamic polar curve at which propulsive force is maximized, and therefore energy saving.
Shaft power :
This is the power required by the drive shaft, which supports the ship propeller, to move the ship foward under the desired conditions. The power required by the drive shaft is reduced by the use of vellic propulsion. The ship propulsion consumption is directly related to this power.
Ship Lenght :
Overall lenght of ship.
This parameter is mainly used to evaluate the distance between the various units on the deck. This parameter is also used to define the value of other parameters.
Ship Speed :
Expressed in knots. Doesn't take into account drift or current.
This parameter has a major influence on energy production for wind propulsion, as it greatly influences the apparent wind.
Propulsive power :
Propulsive power is the product of the propulsive force of the ship propulsion system and the ship speed. The power required by the engine to generate this propulsive power is obtained by dividing by the propeller efficiency.
Drag :
Drag is the component of aerodynamic force along the flow axis. When the apparent wind is ahead of the beam, which is the vast majority of operating points, drag reduces the propulsive force generated by the sail propulsion system.
Area :
The aerodynamic area of a WAPS is the projected, or planar, area of the surface. For a rectangular wing, this is the product of the span (height) and the chord (width). Aerodynamic forces are proportional to area.
Span :
For a WAPS, the span is about the height of the lifting device. For a given area, a higher span leads to a higher aspect
ratio, leading to better aerodynamic performance
Active vs. Passive systems :
Passive systems require no energy input other than wind power to operate (apart from control systems, whose consumption is assumed to be low).
Active systems require energy input. These are the Suction Wing, which has an internal fan, and the Rotor which rotates permanently at high speed.
It is the energy input in active system that make it possible to achieve lift coefficient far superior to those of passive systems. The power balance must obvioulsy take this consumption into account, as is the case in SimWAPS.
Apparent Wind :
The apparent wind is that which is seen by the wind propulsion system. It is the combination of true wind and ship speed.
In the case of wind propulsion, it is important to note that :
- The apparent wind angle decrease as the ship speed increases,
- For the same ship speed and wind speed, the apparent wind speed is much higher when the vessel is upwind than when it is downwind,
- True wind speed increase with altitude, so a sail placed higher up on the ship deck will benefit from a higher true wind speed.
WAPS :
English acronym for Wind Assisted Propulsion System.
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