on the state of these regions according to the Reynolds number, see Chapter 2.The case focuses on three-dimensional modelling of flow around a circular cylinderin steady current. The details of the flow around the body and in the near wakeare analysed.The current research for this case is mainly based on LES. Breuer (1998a,b) made adetailed study of the influence of three-dimensional effects, the subgrid scale model,discretization scheme and grid resolution on the quality of LES. Tremblay et al.(2000) carried out a series of LES and compared them to the result of direct numer-ical simulations (DNS). Wissink and Rodi (2008a) used DNS for Reynolds number3300, which is in the same flow regime. The flow is therefore considered qualita-tively equal.The details of the flow in the near wake was studied.Parnaudeauet al. (2008) compared their own LES results to experimental results obtained byparticle image velocimetry (PIV). Ong and Wallace (1996) obtained experimentalmeasurements of the flow behind a circular cylinder by hot-wire and x-ray probemeasurements. Franke and Frank (2002) performed a LES simulation for Reynoldsnumber 3900 and compared the results to direct numerical simulations of Ma et al.(2000) and the experiments of Ong and Wallace (1996). Li (2011) also applied LESfor flow over a circular cylinder at Reynolds number 3900. Lysenko et al. (2012)recently simulated to flow with Mach number 0.2.The differences between theSmagorinsky subgrid scale (SGS) model and the dynamick-equation SGS modelwere investigated.The Mach number is of adequately small magnitude and theflow is this considered to be qualitatively equal.1.2CylinderinthevicinityofarigidwallatReynolds number of 3900Flow around a circular cylinder close to a rigid wall is a topic of high interest in themarine technology research community. Challenges which can be addressed to thisproblem are for example free-spanning subsea pipelines and marine risers in thevicinity of the seabed. The risers and pipelines are subjected to a continuous straindue to exposure of wind and waves. The safety for the structures can be improvedif the flow around them and the forces acting on them are better understood.When a pipeline is placed on an erodible seabed, scour may occur below the pipedue the flow. This may lead to suspended spans of the pipeline where the pipe issuspended above the seabed with a small gap, usually in the range from O(0.1D)to O(1D). It is therefore important to know what kind of changes that take placein the flow around the pipe and forces acting on it. [Sumer and Fredsøe (1997)]The case investigated is a pipeline placed above the seabed with a gap,e, betweenthe cylinder surface and the seabed equal to0.2D. The case have been split intoto subcases, Case 2a and Case 2b.In Case 2a, the flow has a logarithmic inletvelocity profile. For Case 2a, a mesh convergence study has been performed. Case
1.3.THESIS OVERVIEW32b has a uniform velocity profile, i.e it is constant over the inlet boundary. For bothsub-cases the Reynolds number is 3900. The physics of the case are investigated
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