Browsing by Author "Karola, Aaro"

Filter results by typing the first few letters
Now showing 1 - 5 of 5
  • Absorption of wave reflections in way of an inlet boundary
    (2023-08-09) Karola, Aaro; Tavakoli, Sasan; Mikkola, Tommi; Matusiak, Jerzy; Hirdaris, Spyros
     A4 Artikkeli konferenssijulkaisussa
    In recent years, Computational Fluid Dynamics (CFD) is increasingly becoming more attractive for the analysis of ship seakeeping. This is because of the increasing efficiency of computers and the accuracy of solvers (e.g. Reynold Navier Stokes Averaged – RANS) that may be more accurate in terms of idealizing nonlinear phenomena in comparison to potential flow methods. Notwithstanding this, RANS methods remain sensitive to accurate wave modelling across the fluid domain as well as the reflection of domain boundaries. This paper studies the effective absorption of the reflected waves from a structure heading back towards the inlet boundary with a relaxation zone method. This is achieved by applying the RANS CFD solver OpenFOAM-v2206 and wave generation toolbox waves2Foam. The modelled case is a 2D box in waves with different wave lengths. The distance of the relaxation zone and the distance after the relaxation zone until the box varies and their effect to excitation forces is studied. It is demonstrated that longer distances reduce the problems caused by wave reflections. The effects are reduced with coarser meshes as the mesh itself dampens the waves.
  • Asymmetry in sagging and hogging responses of a ship hull
    (2023-08-09) Matusiak, Jerzy; Karola, Aaro; Niemelä, Ari; Mikkola, Tommi; Hirdaris, Spyros
     A4 Artikkeli konferenssijulkaisussa
    In this paper asymmetry in sagging and hogging of a hull progressing in regular waves is investigated by applying a weakly nonlinear potential flow theory. At first instance ship motions in waves are evaluated using a linear strip-theory yielding transfer functions of ship motions and sectional loads. Then, by assuming time-histories of ship motions the nonlinear part of the Froude-Krylov and restoring forces and moments are evaluated using a panel representation. Linear and weakly nonlinear predictions are compared against available model tests for a roll-on/off (Ro-Pax) ship hull. The results confirm that importance of nonlinearities of restoring and Froude-Krylov part of wave loading on asymmetry of sagging and hogging.
  • Hitsattujen rakenteiden väsymislujuuden mallintaminen tehollisen lovijännityksen menetelmällä
    (2016-04-24) Karola, Aaro
     Insinööritieteiden korkeakoulu | Bachelor's thesis
  • The influence of wave modelling on the motions of floating bodies
    (2024-08-15) Karola, Aaro; Tavakoli, Sasan; Mikkola, Tommi; Matusiak, Jerzy; Hirdaris, Spyros
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In recent years OpenFOAM studies have focused on pure wave modelling and wave-structure interaction. Considering that there is a lack of transparency concerning effects of different motion modelling settings in OpenFOAM, this paper presents the influence of different numerical setups on the accurate modelling of wave-induced motions. This is achieved by applying the interFoam solver included in OpenFOAM-v2206 with the waves2Foam toolbox. The optimal numerical setup is studied with a two-dimensional box-like ship idealisation heaving in varying wave lengths while the wave steepness remains constant. Different numerical setups are considered for accurate wave modelling, the modelling of wave excitation forces for the case of a static structure, and heave motion modelling. Finally, the optimal setup that is found is applied for a 3D ship case in head waves with heave and pitch coupling. As far as practically possible, the results are validated against experimental data. It is shown that strict requirements for mesh density and time step for accurate wave modelling result in accurate excitation force and motion results. Reflections from the relaxation zones and different mesh density layers cause inaccuracies. It is concluded that simple numerical cases are suitable for studying optimal numerical setups for more complex simulation cases.
  • Nonlinear effects in wave loads analysis of a mega cruise liner
    (2019-12-16) Karola, Aaro
     Insinööritieteiden korkeakoulu | Master's thesis
    Design loads provided by Classification Societies have been traditionally applied as the basis for ship structural design. However, recent studies imply that modern ships may be subject to nonlinear wave loads that could be different to those assured by the application of empirical Rule formulae. It is believed that the influence of nonlinear wave loads and ship motions can be accurately described with well validated numerical hydrodynamic solvers. By using such direct analysis tools this thesis reviews the effects of hydrodynamic nonlinearities on global wave load predictions for a latest generation passenger vessel. Three nonlinearity levels and two wave amplitudes are investigated by using hydrodynamic solvers embedded in ANSYS AQWA and WAVETD computer algorithms. Hydrodynamic nonlinearity is idealised on the basis of the taxonomy introduced by Hirdaris et al. (Level 1 - linear and Level 3 - body-wave nonlinear modelling assumptions) and the in-between body nonlinear idealisation of Level 1.5 introduced by Kukkanen (weakly nonlinear assumptions). Accordingly, the longitudinal distributions of vertical shear force and bending moment are calculated for each Level and wave amplitude and results are compared to DNV GL Classification Society Rules. It is demonstrated that global load predictions are similar between Levels 1.5 and 3. However, results from Level 1 predictions differ greatly, because they neglect the effect of wave amplitude. Generally, AQWA and WAVETD provide similar results. Yet, the analysis with higher wave amplitude predicts larger global loads than the design loads by Classification Society Rules. It is concluded that wave amplitudes influence the numerical results computed along the lines of different nonlinear hydrodynamic idealisations and computational tools.