Browsing by Author "Schweighofer, Juha"
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Item Evaluation of the Fully Turbulent Flow over a Flat Plate for a Large Range of Reynolds Numbers(1997) Schweighofer, Juha; Hellsten, Antti; Konetekniikan osasto; Teknillinen korkeakoulu; Helsinki University of Technology; Matusiak, JerzyTämän työn ensimmäisenä tavoitteena tutkitaan ITTC-57 malli-laivakorrelaatiokäyrän suhdetta sileän tasolevyn eri kitkavastuskäyriin. Virtaus oletetaan tasolevyn alusta lähtien täysin turbulenttiseksi. Toisena tavoitteena on antaa kuva tässä työssä käytettyjen turbulenssimallien käyttäytymisestä laivamittakaavaa vastaavilla Reynoldsin luvuilla. Teoreettisessa osassa kuvataan rajakerrosteorian pääpiirteet. Joitain kitkavastuskäyrä, samankaltaisuuslait ja kokeellisesti saatuja tuloksia esitetään. Turbulenssin teoria ja sen mallitus kuvataan. Virtausongelmaa kuvaava osittaisdifferentiaaliyhtälöryhmä annetaan ja numeerinen ratkaisumenetelmä selitetään. ITTC-57 malli-laivakorrelaatiokäyrä verrataan sileän tasolevyn kitkavastuskäyriin, jotka pohjautuvat kokeisiin, teoriaan ja suoritettuihin numeerisiin laskuihin. Täysin turbulenttinen virtaus sileän tasolevyn ylitse lasketaan numeerisesti eri Reynoldsin luvuilla käyttäen algebraalisia ja kahdenyhtälön turbulenssimalleja. Reynoldsin luvut vastaavat mallimittakaavan ja laivamittakaavan arvoja. Numeerinen laskenta suoritetaan käyttämällä FINFLO- virtausratkaisijaa. FINFLO on kehitetty Helsingin teknillisen korkeakoulun Aerodynamiikan laboratoriossa. Laivamittakaavaa vastaavilla Reynoldsin luvuilla ITTC-57 käyrä antaa kitkavastuskertoimelle lähes samat arvot kuin sileän tasolevyn kitkavastuskäyrä. Mallimittakaavassa sen antamat arvot ovat suurempia. Algebraaliset turbulenssimallit ja Menterin k-omega/epsilon-turbulenssimalli (SST) antavat hyviä tuloksia. Chienin k-epsilon-turbulenssimallin tulokset ovat suurimmalta osalta huonompia.Item Evaluation of the Scale Effect on the Flow around a Ship Hull Using CFD(2005) Hänninen, Satu; Schweighofer, Juha; Konetekniikan osasto; Teknillinen korkeakoulu; Helsinki University of Technology; Matusiak, JerzyThe purpose of this work is to study the scale effect on ship hull flows using the Reynolds-averaged Navier-Stokes solver FINFLO-SHIP. The understanding of the scale effect is important, as the hydrodynamic characteristics of a new ship are usually predicted by performing simulations at model scale. In this study, the combination of the chosen hull form and the features used in the modelling of the flow situation enables the presentation of new information in comparison to the other published papers of the same subject. This thesis begins with a theoretical discussion on the scale effect. The dimensionless equations describing the viscous flow in general are presented. The approximations in the numerical method, that possibly affect the evaluation of the scale affect, are discussed. The assumptions in the ITIC-57 method, related to the scale effect, are also paid attention to, as the lTTC-57 method is needed for the validation of the computation at full scale. Thereafter, the structure and contents of the flow solver FINFLO-SHIP are presented. Finally, the computational cases are described and the results analysed. To study the scale effect, the flow field around a typical container vessel is computed at model and full scale. The Froude number is 0.238. The Reynolds numbers are 1.0 x 107 and 1.2 x 109 at model and ship scale, respectively. The flow in the boundary layer is resolved using y+-values of about one. The turbulence is treated with Menter's k - w SST model. At ship scale, the free stream turbulence quantities are validated by computing the flow past a flat plate at the respective Reynolds number. A moving-grid technique is applied to predict the shape of the free surface. The influence of the grid on the results is observed by repeating the computations with three different grid densities. The results are validated by comparing them to model test results. The results demonstrate that the Reynolds number has an important influence on ship-hull flows. The frictional resistance coefficient of a smooth hull seems to follow the ITTC 1957 model-ship correlation line. The pressure resistance coefficient is significantly smaller at ship scale than at model scale. As a consequence of the different pressure resistance, the wave heights are higher at the stern at ship scale than at model scale. In the wake, the boundary layer becomes thinner at the higher Reynolds number. The study of the streamlines shows that the separation of the flow from the hull is delayed at ship scale.Item Investigation of two-dimensional transom waves using inviscid and viscous free-surface boundary conditions at model- and full-scale ship Reynolds numbers(Helsinki University of Technology, 2003-08-15) Schweighofer, Juha; Department of Mechanical Engineering; Konetekniikan osasto; Ship Laboratory; Laivalaboratorio; Matusiak, JerzyTwo-dimensional transom waves are computed using inviscid and viscous free-surface boundary conditions at model- and full-scale ship Reynolds numbers. The computations are carried out solving the steady Euler or RaNS equations with the Navier-Stokes solver, FINFLO. The viscous free-surface boundary conditions are obtained from a flat-surface approximation. Different numerical schemes used when evaluating the free-surface deformation are presented. Their effect on the evaluated transom waves and the flow field is discussed at model and full scale. Further, computations of turbulent free-surface flows carried out at full-scale ship Reynolds numbers using the moving-grid technique and no wall functions are presented and discussed. An improved extrapolation method combining model testing and CFD is proposed. The simulations in this work demonstrate the significant effect of the numerical realization of the free-surface boundary conditions and the decreasing Froude number on the computed transom waves, the flow field and the total resistance. At full-scale ship Reynolds numbers, multigridding will speed up the convergence. The free-stream dissipation of the turbulent kinetic energy has to be treated like a material property when using Chien's low-Reynolds number k-ε turbulence model. The scaling of the computed results is in excellent agreement with the modified ITTC-78 method. The convected turbulent kinetic energy is amplified by the transom waves. At the vicinity of the transom, a significant increase of the nondimensional vorticity is obtained at full scale.