"uuid","repository link","title","author","contributor","publication year","abstract","subject topic","language","publication type","publisher","isbn","issn","patent","patent status","bibliographic note","access restriction","embargo date","faculty","department","research group","programme","project","coordinates"
"uuid:de6e82b5-28f1-4a7d-a073-032620a88b60","http://resolver.tudelft.nl/uuid:de6e82b5-28f1-4a7d-a073-032620a88b60","A Numerical trimvariation study for ships operating in off-design conditions","De Jong, R.H.","Veldhuis, H.J. (mentor)","2015","Fuel efficiency is an important factor for the shipping industry both regarding new build vessels and existing vessels. A method to reduce the fuel consumption of existing vessels operating in off design conditions is to trim a vessel in the most optimum trim condition. The current approach to determine the most efficient trim condition is to perform propulsion tests under different trim conditions in a towing tank. To determine the usability of the CFD program PARNASSOS a trim variation study is performed on a vessel that is already tested in one of the tanks of MARIN. The trend of the propulsion from the tank tests is used to validate the trend of the resistance from the CFD calculations. Before validating the trend of the resistance with the trend of the propulsion it was checked whether or not the change of the resistance is dominant over the change in propulsive efficiency. It turned out that the change of the resistance is dominant over the change of the propulsive efficiency and therefore the trend of the propulsive power can be used to validate the trend of the resistance. The uncertainty of the CFD calculations is determined using a grid refinement study according to the method developed by Luis Eça and Martin Hoekstra [Eça andHoekstra, 2014]. According to this method the uncertainty of the performed calculations is estimated at 3.5%. The uncertainty of the tank tests is estimated at 0.9% according to Martijn van Rijsbergen [van Rijsbergen, 2014]. Unfortunately the uncertainty value obtained using the current method for estimating the uncertainty is not small enough to validate the trend of the resistance with the trend of the propulsion from the towing tank tests. The fact that the uncertainty determined is not small enough to validate the trend obtained from the CFD calculations doesn’t necessarily invalidate the assumption that the results of the CFD calculations are correct. It is expected that the uncertainty of the difference is smaller than the relative uncertainty, an expectation that is supported by the fact that the fits of the power series estimation for the resistance coefficients of the even keel condition and the 1.5m aft trim condition show the same trend and the two fits do not cross each other. There are at least two ways to further reduce the estimate of the uncertainty. The first one is to use finer grids. The second one is to further optimize the method to determine the uncertainty of these CFD calculations and reduce the influence of data scatter and non-similarity of the grids on the estimate of the uncertainty. The results of the CFD calculations are analyzed as if the data is validated. Trimming the vessel aft resulted in an increase of the total resistance. Trimming the vessel forward reduced the total resistance. The total resistance, the frictional resistance and the hydrodynamic pressure resistance increase when the vessel is trimmed aft while the hydrostatic pressure resistance reduces. These results show that the change in frictional resistance and the hydrodynamic pressure resistance are dominant over the change in hydrostatic pressure resistance. For moderate changes of trim the change in wetted surface is dominant over the change of shear stress regarding the change in frictional resistance. For extreme changes in trim the cause of the increase of the frictional resistance is a sheet vortex developing at the bow of the vessel and running aft along the bilge of the vessel. This sheet vortex influences the local thickness of the boundary layer and therefore influences the local shear stress. At the transom of the vessel the local hydrodynamic pressure resistance is reduced caused by the submergence of the transom. Trimming the vessel aft reduced the pressure recovery at the stern of the vessel, which has a negative influence on the local hydrodynamic pressure resistance. At the forward shoulder of the vessel the hydrodynamic pressure resistance increases as well when the vessel is trimmed aft. Trimming the vessel aft resulted in an increase of the hydrodynamic pressure resistance, which shows that the effect of the pressure recovery at the stern of the vessel and the increase of hydrodynamic pressure at the forward shoulder of the vessel are dominant over the effect of the presence of a dead water zone behind the transom of the vessel.","CFD; Computational Fluid Dynamics; Trim Variation; Hull Flow; Physical explanation; Validation","en","master thesis","","","","","","","","","Mechanical, Maritime and Materials Engineering","Maritime & Transport Technology","","Ship Hydromechanics & Structures","",""
"uuid:4792857c-1c2e-4c05-bc0f-9ca1ece4499c","http://resolver.tudelft.nl/uuid:4792857c-1c2e-4c05-bc0f-9ca1ece4499c","Numerical modelling of bow thrusters at open quay structures","De Jong, J.","Vellinga, T. (mentor); Verheij, H.J. (mentor); Blokland, T. (mentor); De Koning Gans, H.J. (mentor); Labeur, R.J. (mentor)","2014","Introduction Bow thrusters are of great help for the navigation at quay walls, but the high and turbulent velocities can result in a bed load exceeding the strength of the bed or bed protection. To be able to design a stable bed the velocities at the bed need to be accurately determined. In design practise the velocities generated by a propeller are determined with formulae based on a mix of the momentum theory and measurements. The application of the formulae is often limited to cases for which measurements have been carried out and do not allow a secure design for more complicated structures and the different velocity field of a bow thruster. Scale model measurements To improve the calculation of velocities on a slope, a large number of measurements were done by Van Doorn [TU Delft, 2012] for several scenarios with and without piles and resulted in an amplification of the design formula for some of his scenarios. To also predict the velocities for other scenarios these measurements are used to build and calibrate a numerical model. Numerical bow thruster implementation The open source CFD package OPENFOAM is used for the construction of this numerical model. As the implementation of a rotating propeller in the mesh will result in high computational costs and to allow a fine calibration of the propeller efflux, the propeller is simplified to an actuator disc. At the actuator disc an axial and tangential body force, varying over the radius, are added to the momentum equations in the OPENFOAM solver. Functions for both a ducted and a free propeller are simulated and show comparable results, the free Goldstein propeller functions are further applied. The coefficients are estimated based on the measured thrust and torque and calibrated to achieve a good fit to the measured efflux. A local increase of the turbulence at the hub and the propeller tip is not implemented in the numerical computations. Results Comparing the calibrated model to the measured diffusion in axial direction, shows a very good agreement and the numerical model nearly exactly computes the distribution as derived by Blaauw and van der Kaa. When comparing the velocities at the slope to both theory and the scale model measurements, it shows an underestimation of the velocities at the toe for steeper slopes, which is explained by unexpected velocities in the wall boundary layer, as a result of the wall functions in OPENFOAM. The model is exerted on different geometries (with piles) to get insight in the velocities for quay structures.","OpenFOAM; CFD; Bow thrusters; Quay structures","en","master thesis","","","","","","","","2014-01-30","Civil Engineering and Geosciences","Hydraulic Engineering","","Ports and waterways","",""