Study Question Flashcards
Describe, in words, the two free surface boundary conditions for free surface gravity waves.
Dfsbc states a constant pressure on the free-surface. It is derived from the Bernoulli equation.
Kfsbc states that a particle on the free-surface will remain on the free surface which means that no flow is allowed through the free-surface.
Describe the physical effects behind the three forces that occur for forced heave in calm water.
Added mass: occurs due to acceleration of the water surrounding the ship.
Damping: occurs due to radiation of waves from the oscillating ship.
Restoring: occurs due to buoyancy effects when the ship is moved from its equilibrium position.
Why is it more difficult to estimate roll motion in heave and pitch?
The viscous contribution to roll damping is important for roll motion while it can be negotiated for heave and pitch. The magnitude of the viscous roll damping cannot be computed by simple potential flow computations.
Describe one passive and one active roll damping device.
Passive anti-roll tank: the anti-roll yank is designed to give a motion of the water that is out of phase with the motion of the ship thereby generates a roll moment that counteracts the roll motion of the ship.
Active roll damping device: active fin stabilization. The angle of attack of the fins is controlled to give a roll moment that counteracts the roll motion of the ship.
Describe the assumptions that must be valid in order to analyze ship motions ij the frequency domain.
The wave system can be approximated by a sum of sine and cosine waves.
Small wave amplitudes.
Small response amplitudes.
The response can be computed for one wave component at the time.
The total response can be obtained summarizing the responses from the wave components
Describe the strip theory approach for solving the Seakeeping problem.
The ship divided into 2-dimensional strips. The added mass, damping, and restoring coefficients are computed for each strip. It is assumed that total added mass, damping, and restoring coeff can be obtained by summation of the contributions from the strips. The equation of motion is solved using the total coefficients.
