CFD Simulation for the Analysis of Ships Operating in Extreme Trim
For the marine industry as well as for the marine R&D departments, Computational Fluid Dynamics (CFD) is a key method to predict the performance of flow-exposed geometry on the basis that it is possible to analyse and to improve the geometry design in terms of energy efficiency and safety.
In order to analyse the performance of a ship in extreme trim conditions, three types of CFD simulations are required. Similar to the procedure of experimental ship model tests, the project also requires simulations of all three conditions; propeller open water, ship hull towing and self-propulsion. Without the computational power provided by facilities such as ARCHIE-WeSt HPCs this research wouldn’t be possible as the numerical simulations require both fine numerical meshes and long simulation times.
The first stage of the project focuses on the performance prediction of a marine propeller in open water conditions. For an extreme trim case the propeller works in off-design conditions which could cause flow phenomena such as cavitation. Most of todays’ CFD solvers are able to predict flow physics fairly well. Nevertheless, there are limitations related to predicting turbulent flow in very small scales of space and time. Cavitation, the formation and destruction of vapour cavities in water, is a flow phenomenon which highly depends on those small scales. Predicting details of cavitational flow becomes crucial for running comprehensive hydrodynamic form optimisation in marine industrial applications, especially for appendages geometry such as propeller and rudder. Different numerical CFD methods (e.g. solver and turbulent models) offer different levels of accuracy and not all are suitable for predicting cavitation.
In order to compare the capability of two CFD models, the commonly used Unsteady Reynolds-Averaged Navier Stokes Equations (URANSE) simulations and the more complex Detached-Eddy Simulation (DES), two well-known experimental test cases were chosen to simulate three-dimensional cavitational flow.
Figure 1 shows the comparison of cavitational flow around the Twisted Delft Hydrofoil using the URANSE and the DES model. Both simulations gave good results (e.g. lift and drag forces) when compared to the experiments; however the URANSE model was not able to predict detached cavitation. The DES model, on the other hand, was able to predict more details that also affect the downstream flow field substantially.
As a more complex test case the Potsdam Propeller Test Case (PPTC) was chosen. Again the results for both models (e.g. thrust- and torque coefficients) were in good agreement with the experiments, but the URANSE simulation did not predict detached cavitating flow (Figure 2).
Figure 2. Three-dimensional cavitating flow around the Potsdam Propeller Test Case (PPTC).
The comparison proved that both models were able to fairly predict performance quantities such as lift and thrust but that only the DES model was able to properly predict the formation of cavitation and its influence on the flow field. The latter becomes important when structures located downstream of a cavitating device loose performance due to a disturbed flow field. The next stage of the project will include more complex cases, such as propeller-rudder interaction in cavitating flow and a fully appended ship hull in self-propulsive conditions.
For more information about the project contact Osman Turan (firstname.lastname@example.org), Professor at the Department of Naval Architecture, Ocean and Marine Engineering at the University of Strathclyde.
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