Jan Visscher

PIV on Inclined Cylinder Shaped Fish Cages in a Current and the Resulting Flow Field

The present experimental investigation focuses on the flow structure around short free end cylinders with an aspect ratio of L/D = 3 at a Reynolds number of Re = 5000. Cylinders with varying degrees of inclination and porosity were tested in a towing tank, acting as models for fish farm cages. According to studies of fish cages at commercial Norwegian fish farms, the inclination angle of the structure change up to 25 degrees from the vertical when exposed to a current. Cylinders with porosities of 0% and 75% were tested, the latter representing fish cage netting with fouling. To visualize the flow around the cylinders, 2- and 3-Component (2C and 3C) Particle Image Velocimetry (PIV) was used. The effects of inclination and porosity on the three-dimensional flow field will be described and discussed.Copyright

Particle image velocimetry measurements of massively separated turbulent flows with rotation

Measurements of instantaneous velocity fields in the separated flow downstream of a backward-facing step in a rotating channel are presented for the first time. Particle image velocimetry (PIV) measurements were made for 13 different rotation numbers Ro at a bulk flow Reynolds number of about 5600. The expansion ratio 2:1 was the same as in the flow visualization study by Rothe and Johnston [ASME J. Fluids Eng. 101, 117 (1979)] which covered about the same range of Ro. The measured mean flow pattern exhibited substantial variations with the rate of system rotation. In particular, the length of the primary separation bubble decreased monotonically with increasing anti-cyclonic rotation and increased with increasing rate of cyclonic rotation, in keeping with the earlier observations. At the highest anti-cyclonic rotation rate, the flow field also separated from the planar wall where the shear layer flow was subjected to cyclonic rotation. The PIV data for the in-plane components of the Reynolds stress tenso...

An Experimental Investigation of Scale Effects on Offshore Wind Turbine Blades

In hydrodynamic model testing, the choice of the scaling factor is often a compromise between the size of the facility and close matching of the characteristic numbers such as Reynolds and Froude. When combining the two aspects into model testing of offshore floating wind turbines, limitations on the size of the model test facilities apply and it is not possible to use both Froude and Reynolds number scaling effects at the same time. To investigate Reynolds number similarity, two wing geometries, a NACA 0012 and a NREL S825 were manufactured in several sizes and tested at identical Reynolds numbers. The wakes behind the airfoils were measured using Particle Image Velocimetry. One would expect to obtain similar results, but differences in the wake profiles were observed. Understanding the mechanisms involved are important as it then enables model building techniques to be improved, and thus make model testing more effective.Copyright

Numerical and Experimental Study of the Flow Around Two Ship Sections Side-by-Side

This paper reports calculations of three dimensional (3D) unsteady cross flow over two ship sections in close proximity and compares the results with measurements. The ship sections have different breadth and draft, and represent typical situations in a ship-to-ship marine operation in a cross current. The behavior of the vortex-shedding around the two different ship hull sections is investigated numerically by CFD methods and experiments. For the two sections, simulations are done for several Reynolds numbers by using the dynamic Smagorinsky Large Eddy Simulation (LES) turbulence model. Finally the cross flow past the ship sections in side-by-side position is simulated and vortex interaction between the sections is found by using the software (Ansys) FLUENT. The numerical predictions are compared with PIV results taken in a circulating water tunnel.Copyright

Hydrodynamic Forces Acting on Objects in a Moonpool

This master thesis describes and discusses an experimental study on the hydrodynamic forces acting on objects in a moonpool, carried out at MARINTEK in 2013. The experiments were done using a simp ...