PARNASSOS
Introduction
PARNASSOS is a dedicated CFD program to calculate the steady incompressible viscous flow around a ship hull, at model scale or full scale. It has been developed by MARIN jointly with Instituto Superior Tecnico (IST) in Lisbon. It has been used in practical applications since around 1995 already, and is a routinely used tool in MARIN consultancy today.

Computed wave pattern and longitudinal velocity distribution, for a trim wedge optimisation study
Computational approach
PARNASSOS solves the Reynolds-averaged Navier-Stokes (RANS) equations in a body-fitted curvilinear grid, using a finite-difference method. Various turbulence models of varying complexity are available. Multi-block structured grids are used. The near-wall flow is accurately resolved by using grid contraction to the hull surface and solving the RANS-equations up to that surface. Thus, PARNASSOS is one of the very few RANS codes that is able to predict the full-scale viscous flow without using approximate ‘wall functions’; thus removing one source of doubt on full-scale predictions.

The solution is obtained in a multiple-sweep marching process that takes best advantage of the properties of this class of flows. Full coupling of momentum and continuity equations is retained in the solution process based on preconditioned GMRES. The method is most accurate, as a result of its structured-grid formulation. But also, dense grids can be used (if needed, up to 20 million cells for a complete ship) to ensure a high numerical accuracy. Remarkably, such computations can be run on a standard PC in a very acceptable computation time, as PARNASSOS is an order of magnitude faster than other RANS codes.

The free surface can be incorporated in the computations, thus providing the total resistance and wave pattern including viscous effects. Here a free-surface fitting technique is used, with a special formulation that provides accuracy and efficiency.

Use of the program
A PARNASSOS computation provides detailed information on the velocity and pressure field around the hull, the wake field in the propeller plane, the possible occurrence of flow separation and the viscous resistance. It is an essential tool for efficiently analysing and optimising ship hull form designs for viscous resistance or propeller inflow, and it gives detailed and precise information on changes from model to full scale. Validations have confirmed the accuracy of all these results. The structured-grid formulation makes the code less practical for complicated appended hull computations, for which ReFRESCO is used instead.

The effect of the propeller on the flow can be modelled by a refined coupling with the propeller panel code PROCAL. PARNASSOS has also been embedded in a framework for evaluation of systematic hull form variations, ParnassosExplorer, which is being used in design practice already for optimisation of required power and wake field quality.

See the PARNASSOS publications for examples of applications for which both verification and validation studies have been performed.

Some recent research done using PARNASSOS:

  • Computational prediction of ship speed-power curve
  • Hull roughness effects on ship resistance and flow
  • Scale effects in transom stern wave making
  • Multi-objective hull form optimisation
  • Shallow-water effects on viscous resistance
Contact
For more information about PARNASSOS, contact our R&D Department at research@marin.nl.

A leaflet of the commercial available PARNASSOS version can be found here.

A leaflet of the full PARNASSOS functionality can be found here.

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