| 1. |
- Malmek, Karolina, 1990, et al.
(författare)
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Rapid aerodynamic method for predicting the performance of interacting wing sails
- 2024
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Ingår i: Ocean Engineering. - : Elsevier Ltd. - 0029-8018 .- 1873-5258. ; 293
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Tidskriftsartikel (refereegranskat)abstract
- Rapid performance prediction tools are required for the evaluation, optimization, and comparison of different wind propulsion systems (WPSs). These tools should capture viscous aerodynamic flow effects in 3D, particularly the maximum propulsion force, stall angles, and interaction effects between the lift-generating units. This paper presents a rapid aerodynamic calculation method for wing sails that combines a semiempirical lifting line model with a potential flow-based interaction model to account for 3D interaction effects. The method was applied to a WPS that consisted of several wing sails with considerable interaction effects. The results were compared to CFD RANS simulations in 2D and in 3D. For the evaluated validation cases, the interaction model improved the prediction considerably compared to when the interaction was not accounted for. The method provided acceptable driving force, moments, and stall predictions, with negligible computational cost compared to 3D CFD simulations.
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| 2. |
- Xu, Kewei, 1992, et al.
(författare)
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Drag reduction of ship airflow using steady Coanda effect
- 2022
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Ingår i: Ocean Engineering. - : Elsevier BV. - 0029-8018. ; 266
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Tidskriftsartikel (refereegranskat)abstract
- This paper studies the steady Coanda effect for reducing the aerodynamic drag of the Chalmers ship model (CSM) using Large Eddy Simulation (LES) with Wall-Adapting Local-Eddy Viscosity (WALE) model. The flow control mechanism is explored, and the analysis of energy efficiency is conducted to evaluate the net benefit of the flow control. Validating the numerical methods, the predicted aerodynamic drag of the ship and pressure coefficients distribution on the baseline CSM agree well with the experimental measurements and the maximum discrepancy is 4.2%. In creating the flow control models, the hanger base of the baseline CSM is modified with a Coanda surface and two different sizes of jet-blowing slots, 1%h (hanger height) and 2%h, respectively. A drag reduction of 5.34% is achieved by the 1%h slot-size case. The 2%h slot-size case further increases the drag reduction to 6.22% but has doubled power consumption. It is found that vectoring vorticity towards the low-speed area on deck is effective for enhancing the energization. Finally, the analysis of energy efficiency indicates that the net benefit is achieved in both flow control cases, and the case with the 1%h slot size is 11.9% more efficient due to a stronger Coanda effect.
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| 3. |
- Svennberg, Urban, 1967, et al.
(författare)
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Experimental analysis of tip vortex cavitation mitigation by controlled surface roughness
- 2020
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Ingår i: Journal of Hydrodynamics. - : Springer Science and Business Media LLC. - 1878-0342 .- 1001-6058. ; 32:73
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Tidskriftsartikel (refereegranskat)abstract
- This study presents the results of experiments where roughness applications are evaluated in delaying the tip vortex cavitation inception of an elliptical foil. High-speed video recordings and laser doppler velocimetry (LDV) measurements are employed to provide further details on the cavitation behavior and tip vortex flow properties in different roughness pattern configurations. The angular momentum measurements of the vortex core region at one chord length downstream of the tip indicate that roughness leads to a lower angular momentum compared with the smooth foil condition while the vortex core radius remains similar in the smooth and roughened conditions. The observations show that the cavitation number for tip vortex cavitation inception is reduced by 33% in the optimized roughness pattern compared with the smooth foil condition where the drag force increase is observed to be around 2%. During the tests, no obvious differences in the cavitation inception properties of uniform and non-uniform roughness distributions are observed. However, the drag force is found to be higher with a non-uniform roughness distribution.
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| 4. |
- Andersson, Jennie, 1986, et al.
(författare)
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Review and comparison of methods to model ship hull roughness
- 2020
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Ingår i: Applied Ocean Research. - : Elsevier Ltd. - 0141-1187 .- 1879-1549. ; 99
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Tidskriftsartikel (refereegranskat)abstract
- There is a large body of research available focusing on how ship hull conditions, including various hull coatings, coating defects, and biofouling, influence the boundary layer, and hence resistance and wake field of a ship. Despite this there seems to be little consensus or established best practice within the ship design community on how to model hull roughness for ship-scale CFD. This study reviews and compares proposed methods to model hull roughness, to support its use in the ship design community. The impact of various types of roughness on additional resistance and wake fields are computed and presented for the well-established test case KVLCC2. The surfaces included in the review are divided into three groups: 1) high quality, newly painted surfaces, 2) surfaces with different extent of poor paint application and/or hull coating damages; and 3) surfaces covered with light slime layers. The review shows the use of a variety of roughness functions, both Colebrook-type and inflectional with three distinct flow regimes, as well as a variety of strategies to obtain the roughness length scales. We do not observe any convergence within the research community towards specific roughness functions or methods to obtain the roughness length scales. The comparison using KVLCC2 clearly illustrates that disparities in surface texture cause large differences in additional resistance, and consequently no strong correlation to a single parameter, e.g. AHR (Average Hull Roughness). This implies that, to be able to select a suitable hull roughness model for a CFD-setup, more details of the surface characteristics are required, such as hydrodynamic characterization of hull coating and expected fouling. © 2020 The Authors
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| 5. |
- Chernoray, Valery, 1975, et al.
(författare)
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Flow pass a rotating cylinder with end disk: PIV measurements and RANS computations
- 2012
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Ingår i: EFMC 2012, European Fluid Mechanics Conference.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Because of the increasing attention to fuel consumption and anticipated legislation, the Flettner rotor has attained renewed interest recently as an auxiliary ship propulsion device. To improve the understanding of the flow involved, the flow structure around a Flettner rotor is studied by both three-dimensional PIV measurements and RANS simulation for a case with Re equal to 2.0 104, based on the inlet velocity (U) and cylinder diameter (D) for the aspect ratio 6, and spin ratio equal to 4.3.A test model of Flettner rotor is installed in the wind tunnel. A thin polymethyl methacrylate disk with diameter of 2D is installed at the top the cylinder following Prandtl1. The horizontal measurement plane is perpendicular to the axis of cylinder as shown in figure 1. To build up the fully three dimensional measurement data, 62 sequences of two-dimensional three components measurements at different axial position are made. The steady RANS simulation is done with SST k –ω model. The geometry of Fletter rotor as in wind tunnel measurement is used. The size of computation domain is chosen according to the one in the literature2. The domain is discretized by a hexahedral mesh, with the carefully designed boundary layer mesh around the rotor (see Figure 1). A uniform velocity inlet boundary condition is used for the inlet and slip boundary condition is chosen for the sides.Both PIV measurement and CFD show similar vortices around the rotor but with some differences as indicated in Figure 2: the simulation cannot well predict the flow at the middle plane, which important for the lift prediction. Further verification and validation will be done with force measurement.
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| 6. |
- Irannezhad, Mohsen, 1990, et al.
(författare)
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Comprehensive computational analysis of the impact of regular head waves on ship bare hull performance
- 2023
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Ingår i: Ocean Engineering. - : Elsevier Ltd. - 0029-8018 .- 1873-5258. ; 288
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Tidskriftsartikel (refereegranskat)abstract
- This paper focuses on investigating the impact of waves on ship hydrodynamic performance, enhancing our understanding of seakeeping characteristics and contributing to advanced ship and propeller design. It examines the resistance, motions, and nominal wake of the KVLCC2 bare hull, which is free to surge, heave, and pitch, in both calm water and regular head waves using a RANS approach. The research reveals a substantial dependency of the wake on grid resolution, particularly in calm water and shorter waves, while motions and resistance display a weaker dependency. The computed nominal wake is compared against towing tank SPIV measurements. Utilizing Fourier analyses and reconstructed time series, the study examines correlations among various factors influencing the bare hull's performance in waves. The axial velocity component of the wake in waves demonstrates significant time variations, mainly driven by higher harmonic amplitudes. This dynamic wake is influenced by instantaneous propeller disk velocities due to hull motions, orbital wave velocities, boundary layer contraction/expansion, bilge vortex and shaft vortex dynamics. The wake distribution at the propeller plane not only differs significantly from the calm water wake in longer waves but also exhibits notably larger time-averaged values (up to 21%).
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| 7. |
- Irannezhad, Mohsen, 1990, et al.
(författare)
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Experimental and numerical investigations of propeller open water characteristics in calm water and regular head waves
- 2024
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Ingår i: Ocean Engineering. - : Elsevier Ltd. - 0029-8018 .- 1873-5258. ; 302
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Tidskriftsartikel (refereegranskat)abstract
- Propeller Open Water (POW) performance of a non-ventilating and fully-submerged propeller in model-scale is investigated in calm water and regular head waves using experimental tests (EFD) and Computational Fluid Dynamics (CFD). Laminar flow dominance is observed in calm water, particularly at higher advance ratios. Nevertheless, the findings in waves suggest increased turbulence, stemming from both the wave orbital velocities and the presumably increased turbulence level produced by the wave maker in the towing tank. Analysis of the CFD results obtained from the incident flow field and single-blade force and moment leads to the speculation that the observed discrepancies are associated with the inevitable asymmetric conditions and mechanical interference in the experiments which were absent in CFD. These can potentially alter the flow over the blades resulting in a different flow transition, separation, and coherent turbulent structure formation and hence forces and moments. The altered propeller performance in waves in comparison to calm water underlines the significance of waves on the propulsive factors and propeller design.
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| 8. |
- Irannezhad, Mohsen, 1990, et al.
(författare)
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Impacts of regular head waves on thrust deduction at model self-propulsion point
- 2024
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Ingår i: Ocean Engineering. - : Elsevier Ltd. - 0029-8018 .- 1873-5258. ; 309
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Tidskriftsartikel (refereegranskat)abstract
- The results obtained from the self-propulsion simulations using Computational Fluid Dynamics (CFD) in the current study, for a ship free to heave, pitch and surge with the means of a weak spring system, are combined with the formerly executed CFD results of the bare hull and propeller open water simulations to investigate the impacts of regular head waves on the propeller-hull interactions in comparison to calm water, at the self-propulsion point of the model. Despite a rather significant dependency of the nominal wake on the wave conditions, the Taylor wake fraction remains almost unchanged in different studied waves which is around 12% lower than the calm water value. The thrust deduction factor in waves is reduced (12.8%–26.1%) in comparison to the calm water value. The change of thrust deduction factor is found to be associated with the boundary layer contraction/expansion and vortical structure dynamics, originating from the wave orbital velocities as well as the significant shaft vertical motions and accelerations that resulted in a modified propeller action, and consequently diminished suction effect on the aft ship. The altered thrust deduction factor and wake fraction in waves in comparison to calm water underlines the significance of waves on the propulsive factors and propeller design.
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| 9. |
- Jalkanen, J. P., et al.
(författare)
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Modelling of ships as a source of underwater noise
- 2018
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Ingår i: Ocean Science. - : Copernicus GmbH. - 1812-0792 .- 1812-0784. ; 14:6, s. 1373-1383
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, a methodology is presented for modelling underwater noise emissions from ships based on realistic vessel activity in the Baltic Sea region. This paper combines the Wittekind noise source model with the Ship Traffic Emission Assessment Model (STEAM) in order to produce regular updates for underwater noise from ships. This approach allows the construction of noise source maps, but requires parameters which are not commonly available from commercial ship technical databases. For this reason, alternative methods were necessary to fill in the required information. Most of the parameters needed contain information that is available during the STEAM model runs, but features describing propeller cavitation are not easily recovered for the world fleet. Baltic Sea ship activity data were used to generate noise source maps for commercial shipping. Container ships were recognized as the most significant source of underwater noise, and the significant potential for an increase in their contribution to future noise emissions was identified.
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| 10. |
- Karasalo, I., et al.
(författare)
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Estimates of source spectra of ships from long term recordings in the Baltic sea
- 2017
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Ingår i: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 4:JUN
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Tidskriftsartikel (refereegranskat)abstract
- © 2017 Karasalo, östberg, Sigray, Jalkanen, Johansson, Liefvendahl and Bensow. Estimates of the noise source spectra of ships based on long term measurements in the Baltic sea are presented. The measurement data were obtained by a hydrophone deployed near a major shipping lane south of the island öland. Data from over 2,000 close-by passages were recorded during a 3 month period from October to December 2014. For each passage, ship-to-hydrophone transmission loss (TL) spectra were computed by sound propagation modeling using 1. bathymetry data from the Baltic Sea Bathymetry Database (BSBD), 2. sound speed profiles from the HIROMB oceanographic model, 3. seabed parameters obtained by acoustic inversion of data from a calibrated source, and 4. AIS data providing information on each ship's position. These TL spectra were then subtracted from the received noise spectra to estimate the free field source level (SL) spectra for each passage. The SL were compared to predictions by some existing models of noise emission from ships. Input parameters to the models, including e.g., ship length, width, speed, displacement, and engine mass, were obtained from AIS (Automatic Identification System) data and the STEAM database of the Finnish Metereological Institute (FMI).
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