Browsing by Author "Viitanen, Ville"
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- Cavitation on Model- and Full-Scale Marine Propellers : Steady And Transient Viscous Flow Simulations At Different Reynolds Numbers
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-02-01) Viitanen, Ville; Siikonen, Timo; Sanchez-Caja, AntonioIn this paper, we conducted numerical simulations to investigate single and two-phase flows around marine propellers in open-water conditions at different Reynolds number regimes. The simulations were carried out using a homogeneous compressible two-phase flow model with RANS and hybrid RANS/LES turbulence modeling approaches. Transition was accounted for in the model-scale simulations by employing an LCTM transition model. In model scale, also an anisotropic RANS model was utilized. We investigated two types of marine propellers: a conventional and a tip-loaded one. We compared the results of the simulations to experimental results in terms of global propeller performance and cavitation observations. The propeller cavitation, near-blade flow phenomena, and propeller wake flow characteristics were investigated in model- and full-scale conditions. A grid and time step sensitivity studies were carried out with respect to the propeller performance and cavitation characteristics. The model-scale propeller performance and the cavitation patterns were captured well with the numerical simulations, with little difference between the utilized turbulence models. The global propeller performance and the cavitation patterns were similar between the model- and full-scale simulations. A tendency of increased cavitation extent was observed as the Reynolds number increases. At the same time, greater dissipation of the cavitating tip vortex was noted in the full-scale conditions. - CFD predictions of unsteady cavitation for a marine propeller in oblique inflow
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-12-15) Viitanen, Ville; Sipilä, Tuomas; Sánchez-Caja, Antonio; Siikonen, TimoIn this paper the Potsdam Propeller Test Case is numerically investigated in oblique inflow conditions. We consider three different topics: open water performance curves, cavitation observations, and pressure pulses induced by the propeller to the ceiling of the cavitation tunnel. In the oblique flow case, the inflow is not uniform from the perspective of the propeller, which results in the dependency of the propeller blade loading and cavitation on the blade rate frequency. The numerical simulations were compared to experimental results for each investigated case. Additionally, we analyzed the unsteady features of the cavitation on the blades as well as the pressure peaks in the propeller wake due to collapsing cavities. We found that the global performance and cavitation patterns close to the blades agree well with the tests in the numerical simulations. The agreement with the tests for the pressure pulses on the tunnel ceiling was better in the non-cavitating case. The unsteady cavitation shed behind the propeller and the subsequent collapse events induced a vast increase in recorded maximum pressure values. Root cavitation collapse produced pressure pulses an order of magnitude greater than the collapse of tip vortex cavitation. Also the collapse of cavities on the blades contributed to a significant increase in the pressure fluctuations on the blades. - Geotekniset olosuhteet, pohjatutkimus ja pohjarakentaminen Marsissa
Insinööritieteiden korkeakoulu | Bachelor's thesis(2016-12-07) Viitanen, Ville - An investigation on using the volume of fluid method to study the bubble sweep-down phenomenon in marine context
Insinööritieteiden korkeakoulu | Master's thesis(2021-03-15) Aarnikoivu, RoniThe bubble sweep-down phenomenon is relevant to oceanographic research vessels because bubbles travelling under a ship's hull can degrade the performance of sonar mapping systems. The problem, however, remains experimentally unexplored due to cost issues at full-scale and scalability issues at model-scale. As a result, there is growing interest in the use of computational fluid dynamics (CFD) as a tool to bypass these issues. Although various CFD methods can be used, in this thesis only the Volume of Fluid (VOF) method was investigated. Specifically, the suitability of the VOF method was evaluated in terms of how well it captured the events leading up to the bubble sweep-down phenomenon. All investigations were carried out using OpenFOAM-7. Various techniques including algebraic and geometric phase-advection methods, adaptive mesh refinement (AMR), different time stepping methods and algorithmic manipulations were explored both theoretically and in application. It was recognised that ship-bow generated plunging type breaking waves are the primary mechanism driving the bubble sweep-down phenomenon. Therefore, two reference cases involving such waves and with direct relevance to ship applications were used to carry out numerical validations. The bulk of these were done in two-dimensions, which allowed relatively efficient testing of the different numerical techniques.~The findings were then leveraged in subsequent three-dimensional studies. The two-dimensional results showed that the VOF method was capable of producing a plunging wave with air entrainment. It was found that the algebraic phase-advection technique performed better than its geometric counterpart and that both adaptive time stepping and mesh refinement could be used to reduce the computational effort without compromising accuracy. These findings, however, did not translate well to three dimensions, where it was found that the VOF method failed to accurately capture the breaking wave and, therefore, its associated events as well. Hence, further work is required to establish the propriety of the VOF method as a tool to replicate the bubble sweep-down phenomenon. - Kitkavastuksen arviointi laivan vastusennusteissa
Insinööritieteiden korkeakoulu | Bachelor's thesis(2011) Viitanen, Ville - Parhaat käytännöt katutyömaan hukan ja aiheutuneen haitan minimoimiseksi
Insinööritieteiden korkeakoulu | Master's thesis(2020-08-17) Viitanen, VilleKatuhankkeet aiheuttavat merkittävää haittaa kadunvarren asukkaille, hankkeiden vaikutusalueella oleville yrityksille, sen läpi liikkujille sekä muille katualueen käyttäjille. Katu-alueella sijaitsevat kunnallistekniset järjestelmät ovat useilta osin tulleet elinkaarensa päähän ja tarve katujen korjausrakentamiselle on tällä hetkellä suuri. Katujen korjausrakentamisen haasteet eivät vastaa tyypillisen talonrakentamisen tai infrarakentamisen haasteita, jolloin ratkaisut on kehitettävä katurakentamista tukeviksi. Tämä diplomityö on toteutettu osana Helsingin kaupungin Aalto yliopistolta tilaamaa Toimivat katuhankkeet projektia, jonka tavoitteena oli tunnistaa katuhankkeiden ongelmakohdat ja löytää rakentamista nopeuttavia ratkaisuja. Tässä työssä koottiin Helsingin katuhankkeiden haasteet, sekä analysoitiin katuhankkeissa syntyvää hukkaa ja jaoteltiin se kirjallisuudesta löytyviin hukkatyyppeihin. Lisäksi kartoitettiin olemassa olevia hyviä käytäntöjä, joilla voidaan vaikuttaa tunnistettuihin haasteisiin. Lopuksi työssä arvioitiin kunkin toimintamallin soveltuvuutta Helsingin kontekstiin, sekä niiden vaikutusta tunnistettuun hukkaan. Työn aineisto kerättiin haastattelemalla eri toimijoita, sekä Toimivat katuhankkeet projektin yhteydessä pidetyistä työpajoista Katujen saneeraushankkeiden haasteiksi tunnistettiin käytössä olevat urakoitsijoiden valintakriteerit ja sopimusmallit, jotka ovat liian suppeita ja jäykkiä katuhankkeiden suju-vaan ja joustavaan toteuttamiseen. Katuhankkeet toteutetaan poikkeuksellisen haastavassa toimintaympäristössä. Lisäksi monen eri toimijan ja tilaajan työmaat ovat haastavia kokonaisuuksia hallita suunnittelun, aikataulujen ja työn yhteensovittamisen osalta. Hankkeiden lähtötiedot ovat hyvin puutteelliset, joka yhdessä valmiiksi laadittujen suunnitelmien kanssa aiheuttaa jatkuvia muutostarpeita hankkeiden aikana. Nykyinen muutostenhallintaprosessi aiheuttaa keskeytyksiä rakentamiseen hitautensa takia. Nykyisessä toteutusmallissa hukkaa esiintyy lähes jokaisessa hankkeen eri vaiheessa. Tässä työssä tunnistettiin jo olemassa olevia käytäntöjä, joilla katuhankkeiden haasteita voitaisiin ratkoa. Yhteistyötä ja joustavuutta edistävät sopimusmallit helpottavat rakentamisen aikaisten ongelmien ratkaisua, erilaisilla työtavoilla voidaan vaikuttaa esimerkiksi lähtötietojen laatuun ja nykyaikaisten teknologioiden hyödyntämisellä voidaan helpottaa muun muassa osapuolten välistä kommunikaatiota. Katurakentamisen haasteisiin on siis olemassa malleja, joilla suurimman osan haasteista voi ratkaista. Uusien käytäntöjen toimeenpano vaatii yhteistyötä ja ponnisteluja. - Three-dimensional computational fluid dynamics analysis of cyclorotor propulsion system
Insinööritieteiden korkeakoulu | Master's thesis(2023-01-23) Salminen, JuhoThis thesis is an investigation into a cyclorotor propulsion system. The principles and kinematics of cyclorotor propeller are described, along with the reasons why cyclorotor propulsion systems have grown more popular in recent years. Also the advantages it can bring over conventional propulsion systems are discussed. Then relevant theory of fluid dynamics and Computational Fluid Dynamics (CFD)are presented to bring more in-depth approach and justify some of the CFD methods used in the research case. Simulations are done using ANSYS Fluent with High Performance Clusters (HPC) provided by VTT (VTT Technical Research Centre of Finland Ltd) and CSC (IT Center for Science, Finland) to compute thrust coefficient, torque coefficient and efficiency of the propulsion system in a model scale open-water conditions. To approximate accuracy of the simulations and minimize uncertainties due to discretization methods, a verification study has been done. To choose most suitable turbulence model for the case, comparison of selected turbulence models were conducted. The simulation results are then compared to measured results obtained from open-water experiments to validate the used CFD method and to obtain more understanding how the CFD models predict the performance of the propeller compared to open-water experimental results. The reference case of the propeller had open-water efficiency around 0.71 and simulations corresponded well compared to the experimental results. Error estimation indicated that discretization error magnitude would be very low. The most accurate turbulence model was k − ω SST model with a low Reynolds number correction. Last the research work and results are summarized and next research cases regarding CFD simulations of cyclorotor propulsion system are proposed. - Verification of a homogeneous mixture model for the free surface problem
Insinööritieteiden korkeakoulu | Master's thesis(2015-01-26) Viitanen, VilleIn this thesis, the applicability of the homogeneous mixture model of Finflo for the free surface problem is studied. The free surface problem is fundamental in marine hydrodynamics, and a special case in two phase flows. The work explores the basis of this type of modelling from mathematical and numerical viewpoint, and verifies the mixture model for the problem. The mathematical background of the problem is presented, together with the nature of it from the perspective of marine hydrodynamics. The bulk flow equations are usually averaged conditionally such that the governing equations of the multiphase model are formally the same as in the case of single phase flow. It can be shown that one additional equation suffices for the description of the segregated phases. Here, the convection equation of the void fraction is utilized. The void fraction equation is derived in conservative form based on the incompressibility constraint of the individual phases. The convection of the void fraction corresponds to the so-called Riemann problem. This is studied thoroughly by developing a two-dimensional solver for the comparison of some well-known schemes for the spatial discretisation of the convective quantity. This solver is applied to the convection of a discontinuous distribution of the void fraction. In addition, the so-called SUPERBEE limiter is implemented to the Finflo code for the extrapolation of the convective void fraction. The numerical solution of the Navier-Stokes equations for simulations of two phase flows is covered comprehensively. The code Yaffa, developed at the Aalto University, has a modern VOF model implemented, and for this reason, it is here used as a reference code. The solution algorithms, the computation of the convective quantities, the pressure correction stages as well as the treatment of the segregated phases in both of the codes are discussed in detail. The two phase flow over a submerged ground elevation is computed using the codes Finflo and Yaffa, and the forming free surface wave is compared to those found from the literature. The aim of this thesis is to get acquainted with the nature of the problem in conjunction with the specific methodology used to solve such flows. This is done in order to understand the requirements and possible modifications needed for the model when we wish to accurately predict ship flow phenomena that are not solvable using the traditional free surface tracking strategies. This way, the verification of the mixture model of Finflo is achieved.