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.
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.
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.