| 1. |
- Vesting, Florian, 1982, et al.
(författare)
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Development and application of optimisation algorithms for propeller design
- 2016
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Ingår i: Ship Technology Research. - : Informa UK Limited. - 0937-7255 .- 2056-7111. ; 63:1, s. 50-69
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Tidskriftsartikel (refereegranskat)abstract
- Propeller design is a comprehensive task in finding the best trade-off between competing objectives and constraints. It requires a multi-disciplinary evaluation of the propeller performance based on various input parameters. Thus optimisation algorithm applied to this type of problem require consideration of the problem to solve. The purpose of this paper is hence the improvement of commonly used population-based algorithms (NSGA-II and PSO) towards the application of marine propeller design. The extension to three algorithms are outlined utilising meta models, adapted constraints and modified constraints handling. The proposed algorithms are applied on a real-life marine propeller example. Results of 13 optimisations are compared in terms of optimisation convergence, constraints compliance and Pareto optimality and show advantageous performance of the developed cavitation constraints and the meta-model extended NSGA-II.
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| 2. |
- Abolfazl, Shiri, 1975, et al.
(författare)
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Hydrodynamics of a Displacement Air Cavity Ship
- 2012
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Ingår i: 29th Symposium on Naval Hydrodynamics, Gothenburg, Sweden, Aug. 2012.
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Konferensbidrag (refereegranskat)abstract
- To study the resistance reduction of an air cavity for adisplacement vessel, a simplified model of a single cavityis tested in a cavitation tunnel. The drag force actingon the cavity and the aft plate were measured andthe water-air interface was monitored in different conditions.Behavioural changes to the free surface wereobserved by changing the geometry and flow characteristicslike air pressure, air flow rate and water flow speed.Computational model for different cases with the samegeometry and conditions were simulated using computationalfluid dynamics(CFD).The length of the cavity was designed to include approximately2.5 times the expected wavelength of thewater-air interface with Froude number based on theship length of 0.19 which approximately represents adisplacement ship with multi-wave air cavity at its cruisingspeed.Parameters like cavity pressure play an important rolefor the wave’s shape and the stability of the free surfaceinside the cavity. Both computation and experimentshow that the amplitude of the wave is sensitive to the airpressure in the cavity and the re-attachment of the waterto the rear end of the cavity has a close correlationto the inlet air pressure and water velocity. The ultimategoal of this investigation is to minimize effective powerincluding air supply while reducing resistance in/aroundthe cavity, but equally important is it to gain amore basicunderstanding of the air-cavity’s behaviour and possibleadditional resistance components.
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| 3. |
- Abolfazl, Shiri, 1975, et al.
(författare)
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Study of an Air Cavity in Water Tunnel
- 2011
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Ingår i: Proceedings of the 14th Numerical Towing Tank Symposium, Poole, UK.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- To study the resistance reduction of an air cavity for a displacement vessels, a simplified model of a cavity is simulated using computational fluid dynamics(CFD) and model tests. Initial CFD computation have been performed to design the model tests and the model tests will subsequently be used to validate the CFD simulations. The length of the cavity is approximately 2.5 times the expected wavelength of the water-air interface and the Froude number based on the cavity length is 0.2. Parameters like pressure play important role in the wave’s shape and the stability of the water surface inside the cavity. Computation shows that the amplitude of the wave is sensitive to the air pressure in the cavity and the re-attachment of the water to the rear end of the cavity has a close correlation to the inlet air pressure. The ultimate goal of this investigation is to minimize air supply while reducing resistance in/around the cavity, but equally important is it to gain a more basic understanding of the air-cavity’s behavior and possible additional resistance components.
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| 4. |
- Ali, Md Ashim, et al.
(författare)
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Prediction of Propeller Tip Vortex Using OpenFOAM
- 2017
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Ingår i: Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE. - 9780791857731 ; 7A-2017
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Konferensbidrag (refereegranskat)abstract
- It is important to predict the propeller tip vortex flow and its effect on hull vibration and noise. In our previous work, the tip vortex flow of the David Taylor Model Basin (DTMB) 5168 propeller model has been studied based on the Reynolds Averaged Navier-Stokes equation (RANS) solution using various eddy viscosity and Reynolds Stress turbulence models. A set of structural grids were used, however, large Jacobian values of the structural grids around the propeller tip region led to the convergence problem and inaccurate solutions. In the present work, the numerical prediction of the same propeller model was improved by using a steady-state RANS solver simpleFoam in OpenFOAM with locally refined unstructured grid along the tip vortex trajectory. The computed thrust and torque coefficients and the velocity components across the vortex core are compared with experimental data and results in the previous studies. Improvement in the prediction of velocity components across the tip vortex core were achieved.
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| 5. |
- Alin, Niklas, 1963, et al.
(författare)
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3D Unsteady Computations for Submarine-Like Bodies
- 2005
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Ingår i: 43rd AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, Jan. 10-13, 2005. ; , s. 353-369
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Konferensbidrag (refereegranskat)abstract
- Results from a computational study using Unsteady Reynolds Averaged Navier Stokes (URANS) models and Large Eddy Simulation (LES) of flows past submarine-like bodies are here presented. The aims are to evaluate URANS and LES for high-Re number hydrodynamic flows, to investigate the influence of the turbulence and subgrid turbulence modeling, and to discuss some features of submarine hydrodynamics. For this purpose we have chosen to examine the flow past a prolate spheroid at 10° and 20° angle of attack at a body length Re number of 4-106, and the flow past the DARPA-2 Suboff bare hull and fully appended hull configurations at a body length Re number of 12-106. For both cases experimental data is available for comparison. One finite element and one finite volume flow solver has been used - both with the capability of employing a range of turbulence models and with the capacity of using unstructured and hybrid grids. Better agreement between predictions and experimental data is obtained with LES than with the URANS models, but at a considerably higher price, due to the finer grids and finer temporal resolution in LES.
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| 6. |
- Alin, Niklas, 1963, et al.
(författare)
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Current Capabilities of DES and LES for Submarines at Straight Course
- 2010
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Ingår i: Journal of Ship Research. - 1542-0604 .- 0022-4502. ; 54:3, s. 184-196
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Tidskriftsartikel (refereegranskat)abstract
- The flow around an axisymmetric hull, with and without appendages, is investigated using large eddy simulation (LES), detached eddy simulation (DES), and Reynolds averaged Navier Stokes (RANS) models. The main objectives of the study is to investigate the effect of the different simulation methods and to demonstrate the feasibility of using DES and LES on relatively coarse grids for submarine flows, but also to discuss some generic features of submarine hydrodynamics. For this purpose the DARPA Suboff configurations AFF1 (bare hull) and AFF8 (fully appended model) are used. The AFF1 case is interesting because it is highly demanding, in particular for LES and DES, due to the long midship section on which the boundary layer is developed. The AFF8 case represents the complex flow around a fully appended submarine with sail and aft rudders. An actuator disc model is used to emulate some of the effects of the propulsor for one of the AFF8 cases studied. Results for the AFF8 model are thus presented for both “towed” and “self-propelled” conditions, whereas for the bare hull, only a “towed” condition is considered. For the AFF1 and the “towed” AFF8 cases experimental data are available for comparison, and the results from both configurations show that all methods give good results for first-order statistical moments although LES gives a better representation of structures and second-order statistical moments in the complex flow in the AFF8 case.
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| 7. |
- Almeland, Silje Kreken, et al.
(författare)
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An improved air entrainment model for stepped spillways
- 2021
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Ingår i: Applied Mathematical Modelling. - : Elsevier BV. - 0307-904X. ; 100, s. 170-191
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Tidskriftsartikel (refereegranskat)abstract
- This work considers the application of the air entrainment model developed in [P. Lopes et al. Int. J. Nonlin. Sci. Num., vol. 18, pp. 559-574, 2017] to simulation of flows in stepped spillways. Four Froude numbers are considered for the flow, with parameters selected to match available experimental data. An extensive simulation campaign led to the identification of several deficiencies in the model, most importantly, its strong grid dependence. An improvement to the interface detection algorithm used in the model is proposed, and simulations reveal improved robustness with respect to the grid density. Furthermore, two criteria for predicting the inception point of air entrainment are tested, based on energy balance and boundary layer state, respectively. The boundary layer-based approach is recommended due to superior accuracy and robustness with respect to turbulence model selection.
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| 8. |
- Alves Lopes, Rui Miguel, 1992, et al.
(författare)
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A numerical study on the influence of crossflow transition on a marine propeller in open water
- 2024
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Ingår i: Ocean Engineering. - 0029-8018. ; 310
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Tidskriftsartikel (refereegranskat)abstract
- This work studies the influence of crossflow transition modelling on the performance and flow field of a controllable pitch propeller. The simulations are performed for two different crossflow terms, and baseline simulations without crossflow transition are performed as well. The results show that in the absence of a crossflow term, the flow over the propeller blades is almost fully laminar. When a crossflow term is included, a significant part of the flow becomes turbulent, thus causing a decrease in the thrust and torque coefficients. The change in the propeller performance is also due to the absence of laminar flow separation near the trailing edge, which is prevented when transition occurs upstream of that location due to crossflow. The comparison between the two crossflow terms shows that one always leads to a larger extent of turbulent flow and earlier transition than the other, although this not always translates in lower thrust and torque, depending on the considered advance coefficient. This illustrates the delicate balance in the effects taking place on the pressure and suction side of the propeller blades, and the importance of correctly including crossflow effects in simulations of model-scale propellers.
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| 9. |
- Alves Lopes, Rui Miguel, 1992, et al.
(författare)
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Numerical assessment of surface roughness on a full scale propeller
- 2024
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Ingår i: Proceedings of the 8th International Symposium on Marine Propulsors.
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Konferensbidrag (refereegranskat)abstract
- This work addresses the performance of a full scale propeller in an open water setup for varying roughness heights, obtained with a RANS solver and the k-omega SST turbulence model. The application of roughness is done with wall functions and by resolving the boundary-layer. Two cases are considered for the same propeller geometry, one with and another without the anti-singing edge on the propeller blades. Baseline simulations without roughness are performed as well, and grid refinement studies are carried out to estimate the numerical uncertainty. The results showed that the influence of roughness is weak if wall functions are not used, whereas a significant decrease in thrust and torque is obtained if roughness is applied in conjunction with wall functions. The inclusion of the anti-singing edge leads to an increase in thrust and torque, but decrease in efficiency for low advance coefficients. The region of separated flow near the trailing edge of the propeller caused by the anti-singing edge is influenced by the roughness height, and is absent in the geometry without the anti-singing edge.
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| 10. |
- Alves Lopes, Rui Miguel, 1992, et al.
(författare)
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Resistance Decomposition of a Self-propelled Ship in Full Scale
- 2023
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The simulation of ships at full scale includes several physically challenging aspects for Computational Fluid Dynamics such as wall roughness [1], free-surface effects, and interaction effects (e.g. between the hull and propeller) when considering self-propulsion scenarios where the propeller is included. On the other hand, the high Reynolds number (around 10^9) observed in these flows results in a high computational cost for CFD, due to the thin boundary-layer that develops along the hull, requiring an extremely large cell count if wall functions are not used. A third obstacle lies in the lack of measured data to compare the results against [2], which makes validation of the results impossible. In this paper, the flow around the JoRes 1 vessel is considered, for which measured data at full-scale is available through the efforts of the JoRes project [3]. Several sets of simulations with the Reynolds-Averaged Navier-Stokes equations are performed for this geometry, with a focus on the friction and pressure components of the resistance: a self-propulsion setup, fully resolving the propeller, a double-body setup with the propeller, to shed some light on the resistance due to the waves and a double-body setup without the propeller, which will be used in order to investigate the propeller-hull interaction. Different approaches for the surface roughness are considered as well, and grid refinement studies are performed in order to quantify the numerical error. The distinct simulation setups will assist in understanding the relative importance of each aspect, and provide a clearer picture in the comparison of the results obtained in the complete self-propulsion simulations with the sea trial data. REFERENCES [1] J. Anderson, D. R. Oliveira, I. Yeginbayeva, M. Leer-Andersen and R. Bensow, “Review and comparison of methods to model ship hull roughness”, Applied Ocean Research, Vol. 99, (2020). [2] H. Jasak, V. Vukcevic, I. Gatin and I. Lolovic, “CFD validation and grid sensitivity studies of full scale ship self propulsion”. International Journal of Naval Architecture and Ocean Engineering, Vol. 11, pp 33-43, (2019). [3] D. Ponkratov and G. D. Struijk, “JoRes JIP – a unique Joint Industry Project to close the knowledge gap on Ship Hydrodynamics”, Full Scale Ship Performance Conference, the Royal Institution of Naval Architect, London, UK, (October 2018).
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