The Hamburg Ship Model Basin

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CFD Seakeeping

HSVA’s CFD seakeeping predictions focus on complex, non-linear analysis mostly in regular waves. While simpler, efficient potential flow models based on strip theory or panel codes can determine a range of typical seakeeping problems with sufficient accuracy, they typically fail to predict more complex non-linear phenomena. Here, more powerful CFD tools such as the in-house RANS code FreSCo+ can provide answers.

Roll damping

Ship roll motions are highly non-linear and dominated by viscous effects. Calculations with classical strip methods are typically enhanced using empirical estimates for damping coefficients, e.g. from BLUME or IKEDA. Still these predictions tend to fail near the resonance point.

Simulating the roll damping with FreSCo+ and using tailored roll damping coefficients shows a much improved behaviour in comparison with experimental results. The example below shows Response Amplitude Operators (RAOs) for the roll motion of a small, blunt tug.

CFD Seakeeping 01

Response Amplitude Operator (RAO) of Roll Motion of a Tug Boat in Regular Waves

Added resistance in regular waves

Additional resistance forces (compared with the calm water resistance) are an unwelcome by-product of real-life ship operation. While in the past often generous surcharges were made to compensate for speed losses in waves, today’s quest for improved energy efficiency and the introduction of formal power limits in form of the EEDI call for a more precise analysis of the resistance and powering requirements in a seaway. As an alternative to model tests, a range of these predictions can be done numerically. The following example shows a comparison of FreSCo+ model scale results with experimental data for a bulk carrier in calm water and two different head waves.

CFD Seakeeping 02

CFD Seakeeping 03

Slamming loads

Large ship motions often lead to increased slamming loads with modern hullforms sporting large bow flare and a flat bottom at the stern. CFD can help to predict slamming loads which in turn can be used for subsequent structural analysis. The following example shows a comparison of predicted loads on a containership model with experiments performed in the large towing tank at HSVA.

CFD Seakeeping 04

The video below shows the ship motion of an open top container vessel in regular head waves (ξ = 12 m / full scale) in the towing tank and the corresponding CFD simulation.