Dynamic pressure on underside of the hull.

Cost effective CFD-analyses of ships

Dynamic pressure on underside of the hull.

cDynamics is now using the CFD software by Cloud Towing Tank, and can provide very cost effective simulations.

cDynamics has now added the computational fluid dynamics (CFD) software by Cloud Towing Tank to our tool kit. This software is an automated and modified version of OpenFOAM, tailored specifically for marine applications. We also have access to their high-performance computation (HPC) facilities, and can therefore now utilise a streamlined approach for resistance studies of ship hulls, in both calm water and with incoming waves. A large number of other marine problems are also possible to analyse without adding too much extra work.

The following project was done for our client Marine Design & Consulting. A wind assisted tanker for the ship owner Seatrans, called Trans Tind, was to be studied. Special emphasis was put on the centre of pressure on the submerged part of the hull for different drift angles. The wind (rotor) sails will also exercise sideways forces on the ship, making it to crab and follow a more slanted trajectory. This leads to some interesting hydrodynamic phenomena.

Geometry of the ship hull.

The mesh consists of approximately 10 million cells:

Computational mesh.

The ship hull was free to move in heave and pitch, and three velocities with three different angles of attack were simulated.  

Definition of the drift angle.

The peculiar result from these simulations with drift angles is that the centre of pressure, i.e. the position where an applied force would yield zero moment, is ahead of the bow of the vessel. The reason for this big moment is the so called ‘Munk moment’, which arises when there is an angle between the current acting on a body and its longitudinal axis. The physical cause for this moment is the asymmetric location of the stagnation points. The high-pressure stagnation points in the bow and stern will exert a moment on the hull.

The formal expression of this moment is the following: 1/2 Uc2(A22- A11) sin2β,

where Uc is the current velocity, A11 and A22 is the added mass in surge and sway respectively, and β is the drift angle. This is a purely potential flow effect and is therefore in addition to any other moment from viscous sources.

A sphere would not experience any Munk moment in an inviscid flow, but all other bodies will. This moment is always destabilising, meaning that it constantly wants to turn the hull perpendicular to the flow. It is therefore important to be able to counteract this turning moment. In the case of this particular project, placement of the sails towards the bow and proper design of the rudder is important.

Dynamic pressure on underside of the hull at drift angle 10°.

Velocity streamlines.

Wave elevation.

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