Feature 1 | CFD AND HYDRODYNAMICS Al-FreSCo+ airs complex RANS code
To face the ever increasing challenges in hydrodynamic analysis, Hamburgische Schifau Versuchsanstalt GmbH (HSVA) and the Technical University in Hamburg-Harburg have joined forces to develop a dedicated RANS code for maritime applications. Marco Schneider, Dieke Hafermann and Jochen Marzi explain the developments of FreSCo+
. O
riginating from an earlier cooperation of the two, the “Free Surface Code” FreSCo+
was
further developed during the European VIRTUE project. Having matured over the last year, the code is now the main backbone of HSVA’s computational fluid dynamics (CFD) services. Based on a modern computational approach, the code’s versatility will allow users to address a broad span of typical maritime applications covering resistance, wake field, propeller and cavitation predictions, as well as dynamic simulations for manoeuvring or seakeeping analysis. Te following examples highlight some typical design – analysis problems encountered in the routine of a ship model basin which have been solved using FreSCo+
.
Numerical model The FreSCo+
Figure. 1: Surface grid for twin screw vessel. code solves the
incompressible, unsteady Navier- Stokes-equations (RANS). Te transport equations are discretised with the cell-centred finite volume method. Using a face-based approach, the method is then applied to fully unstructured grids using arbitrary polyhedral cells. Terefore, the code can use grids from different grid generators like the fully unstructured, automatic grid generator HEXPRESS. This reduces the time needed for
generating typical grids from several days to hours. The code is efficiently parallelised in space using the message passing interface (MPI). A wide range of different 1- and 2-equations turbulence models have been implemented to account for turbulent flows.
Propeller study Cavitation is an imminent problem of marine propulsion, often caused by inappropriate propeller designs
38 The Naval Architect July/August 2010
especially for high speed vessels. The accurate prediction of these phenomena in the early design process can help the shipyards, propeller manufacturers and ship operators to adopt design solutions avoiding or at least reducing the associated risks of erosion and vibration excitations. In the context of the VIRTUE project, a common test case based on a four bladed
fixed pitch propeller from INSEAN has been selected to test and further enhance cavitation prediction capabilities of RANS codes. Available Model scale experiments from towing tank and cavitation tunnel for this propeller were compared with numerical results. The FreSCo+
results
shown in figure (leſt) indicate a realistic prediction of cavitation extension and
Figure 2: Vapour volume fraction iso-surface for sv = 0.5, Computations, Experiments [1].
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