Investigation on drag performance of anti-fouling painted flat plates in a cavitation tunnel

The flat plate coated with silicone-type Tin-free self-polishing co-polymer (SPC) or the conventional metal-type Tin-free SPC is prepared to investigate the drag performance of the anti-fouling SPC. The local skin friction of anti-fouling paints is evaluated by a flat plate model test method in the cavitation tunnel. The properties of the boundary layer and the drag performance are investigated by flow and force measurement techniques. The silicone-type SPC paint shows better drag performance than the metal-type paint in the high speed regime. The silicone-type SPC paints also show decreasing roughness function (ΔU+) with the increase of displacement thickness Reynolds number (Reδ⁎) and roughness Reynolds number (ks+). Even in the same silicone-type SPC paints with similar roughness function, drag performance appears differently. The different drag performance in the silicone-type SPC painted surfaces is considered to be affected by different turbulent vortical structures caused by the surface roughness. Y-directional peak position of streamwise turbulence intensity is utilized to estimate the existence of vortical structure. To investigate the reason of the different drag performance in the silicone-type SPC painted surfaces, the POD analysis, extracting the most energetic flow fields, is adopted to find the effects of cross-flow velocity component caused by the turbulent vortical structure.