Trans RINA, Vol 153, Part A1, Intl J Maritime Eng, 2011 Jan-Mar between 0.41 and 0.7 for the one and same valve,
depending on the height of the water levels on both sides of the valve. The discusser is not convinced about the correctness of this evaluation. Care has to be taken to choose correct values for hA and hB in Eq. (3), when calculating the discharge Q.
It would be of great interest to see the discharge
coefficient Cd derived from the experimental data, as a function of time or as a function of the height difference between the water levels in the two tanks. The use of two separate values is of course a coarse simplification.
Any further analysis of the discharge through real openings in the ship structures is for modeling purposes of great relevance. Not much information is available on the discharge coefficients to be used when modeling the flow through e.g. a collision damage in the steel structure at the ship side, or through a typical grounding damage, which are not as idealized in shape as the man made openings inside the ship.
The true challenge in simulating or modeling the
behavior of a damaged ship in seaway lays somewhere else than in the discharge coefficients. The authors’ work in confirming generally used modeling assumptions is, however, a very welcome contribution is this field. The discusser hopes that the authors continue their work towards simulating more real cases, what comes to the openings in the ship and the behavior of the damaged ship in seaway.
K McTaggart, Defence R&D Canada – Atlantic, Canada
The authors have done excellent work, and the full-scale validation is a significant contribution to the field of damage stability.
The paper includes time series of heel angle for the damaged ship. Were any experiments conducted to examine the roll damping characteristics of the intact ship?
Section 1 ‘intermediate phases’ – If the phases are defined as transient, progressive and equilibrium then I think that all these phases can be addressed by time simulation tools.
Section 2.3 What can you tell about the
measurement accuracy? (levels, pressures, flow, vessel attitude) And could these influence the conclusions you have drawn?
Section. 3.2(a) Discharge coefficients for the valve & pipe two values are used, one for fully submerged conditions, one for one side submergence. What about the intermediate stages? There must be some transition effect since the Cd value will not from 0.41 to 0.70.
jump
Section. 3.2(b) Permeabilities; What is included in the permeability values listed in table 3? Do these take into account all the machinery inside the compartments or was this compensated for before the permeability was determined (e.g. by subtracting the volume of ‘large’ equipment)?
Section 3.3 Timestep & Level Criterion; How sensitive were the simulations for the size of the applied timestep and for the criterion of 0.01 mm? Did a larger timestep or criterion give significantly different results?
Section.3.3 Simulated water level heights; The levels in the plots are presented wrt to the ‘bottom’ of each compartment – thus the local compartment-level (or it seems like that).
Section.4.2 Air pressure peak; The measured peak is higher than
the simulated peak.
Prediction of roll damping remains a significant challenge for seakeeping analysis of intact ships.
It is encouraging to see an efficient and robust method for computing flow within damaged vessels. Do the computations run faster than real-time?
Have the
computations been compared with other methods, such as Reynolds-averaged
Navier-Stokes (RANS) approaches for computational fluid dynamics (CFD)?
The numerical approach assumes that no sloshing occurs, which is likely a valid assumption for naval and other vessels with small internal compartments.
Have the
authors considered how sloshing might be added to their numerical model while still maintaining efficiency and robustness?
A damage opening with a diameter of 250 mm was modelled. Have the authors had any experience with
Effects that
contribute to this difference are a faster rise of the level in the model tests and/or a slower escape of air. When I look at the measurements of the flow velocity of water (through the damage opening) than the measured velocity is higher than the simulated velocity, the same applies to the measured air velocity. The last might be a secondary effect caused by the higher pressure, the first might however be caused by Cd values which were too low for the inlet and too high for the outlet openings. The difference in flow velocity is not
reflected in the level
measurement. What is the influence of measurement (in)accuracy – especially on the flow measurements?
Section. 4.1 The comparison between measured and simulated levels, heel and trim is excellent (almost scary). It seems that the mismatch in air pressure does not really influence this. What if compressibility is completely ignored
the air in the
simulations? How does that influence the comparison? – This applies only to the first case.
larger damage sizes, which could give more pronounced transient effects?
E Ypma, MARIN, Netherlands
Overall a very good paper, describing very interesting model tests. Herewith my (detailed) comments:
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©2011: The Royal Institution of Naval Architects
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