Investigation of indentations versus surface roughness for fluid induced motion of an elastically attached circular cylinder at velocities relevant to the marine environment using CFD

Vortex induced vibration aquatic clean energy (VIVACE) is a renewable energy (RE) device being developed that uses multiple oscillating circular cylinders in one degree of freedom to produce electrical energy due to fluid induced motion (FIM). VIVACE is a modular system intended for use in ubiquitous slow velocity water flows of rivers, tides and ocean currents. Velocities of 1 to 5 knots are to slow for use by turbines and represents a massive opportunity for widespread, continuous RE power. VIVACE currently uses passive turbulence control (PTC), a pair of symmetrical roughened surfaces on the circular cylinder to increase amplitudes and therefore increase power output. PTC works by effecting the fluid-structure interaction (FSI) within the boundary layer and the separation of the water from the cylinder creating changes in the characteristics of forces on the cylinder. The problem with a roughened surface in the marine environment is become fouled by marine organisms and therefore lose its effectiveness. The intent of this research is to investigate the likelihood of biofouling of the PTC and suggest an alternative using indentations partially covering a circular cylinder to act in a similar wat to PTC increasing amplitudes. Indentations are an alternative design because they have smooth surfaces which can be treated with anti-biofouling coatings while still creating the disturbance to the boundary layer mechanism that causes the increased amplitude. A review of FIM, FSI and computational fluid dynamics (CFD) is conducted to inform the research. A literature review is conducted on topics of current FIM technologies, surface roughness and indentation research on FIM and marine biofouling. The literature analysis strongly suggests that biofouling will be an issue for the current VIVACE PTC design. CFD using ANSYS is conducted to baseline the design against existing studies and then investigate four cases of indentations. Of the four indentation designs trialled they produce a reduced amplitude compared to PTC or completely dampened FIM. Further research with improved computational resources is required to complete this work