The shipbuilding industry and market are interested in designing hull forms that run faster, generate less noise and are more energy-efficient. A critical factor in predicting the performance of new hull designs is the ability to accurately simulate wave patterns around the hull form, as the waves have a major impact on the forces acting on the hull. In order to minimize those forces, hull shapes need to be studied and optimized in detail.
The small margins for improvement that are usually targeted in hull design nowadays require precision tools capable of discerning small differences between alternative designs. Custom modelling tools based on simplified numerical methods and assumptions cannot provide the required accuracy. For this reason, viscous computational methods are playing an increasingly important role in ship design, as they are capable of simulating turbulent flow around hulls, featuring thick boundary layers under the influence of strong cross-flow, pressure gradients, streamline curvatures, and stream-wise vortices. For this purpose, MARINTEK uses two different commercial RANSe (Reynolds Averaged Navier-Stokes equations) solvers, STAR-CCM+ and Fine/MARINE, to solve the equations of continuity and momentum of viscous flow around the hull. Both codes include state-of-the-art turbulence models suitable for high Reynolds number flow fields, and they have been thoroughly validated for hull resistance prediction and free-surface modelling. They use VOF (Volume of Fluid) formulations which are capable of accurately capturing the sharp water-air interface.
CFD offers many inherent advantages that make it particularly suitable for an effective hull-lines optimisation process. It can handle arbitrary geometries, viscous flow and compressible flow. Detailed 3D descriptions of the flow conditions provide the information needed to minimize hull resistance in calm water at various speeds and under different operating conditions, which are factors of vital importance for the competitiveness of new designs. Changes in the design drawings can therefore be easily evaluated in terms of their hydrodynamic performance.
A major advantage of using CFD in the design optimisation process is the detailed level of information of the flow field. Based on the results of a bare hull CFD study valuable insight in the optimum position of appendages such as brackets, bilge keels and headbox, can be given. By using CFD, problematic details of the hull design are also easily identified, which can include flow phenomena such as separated flow areas, local pressure variations, cross stream over sharp edges and areas with free surface spray or breaking waves.
Listed below are some examples of results that MARINTEK generally provides, based on the analysis of the computed flow field.
- Hull resistance
- Dynamic position, motion and acceleration
- Pressure distribution on the hull
- Free-surface wave pattern
- Limiting streamlines on the hull
- Local flow field in vicinity to appendages
- Wake field in the propeller plane.
Given on its long experience of hull design, MARINTEK can provide recommendations for the improvement of the design. By an iteration process alternating between CFD and revised drawings, the performances of several designs can be compared, thus gradually bringing the client toward the optimal solution. Because CFD provides answers more rapidly than building models and performing experiments, this process enables the designer to consider a much greater number of possible solutions than could ever be made in the past.
While calculations are especially performed for optimization of hulls and/or appendages during the early stages of the design process, model trials are usually carried out in order to evaluate the final design. This approach provides an opportunity to compare computed values of mean forces, as well as dynamic position, with experimental data, for the final design. The free-surface elevation at the bow and stern can be compared qualitatively with photographs taken during model testing in the towing tank.
This unique strategy whereby CFD is fully integrated into the design process can be applied to a very wide range of vessel types.
The figures shown here have been taken from the hull lines optimization process of a 72.900 DWT Shallow Draught Product Carrier designed by the Brazilian design company PROJEMAR; the CFD computations were performed last summer. Since 2004, PROJEMAR has utilised MARINTEK’s expertise in hull lines optimization based on CFD analysis for several ship designs, including tankers, ore carriers and container ships, with excellent results.
To see the figures in this article, please open the pdf version of the article in MARINTEK Review No. 2-12