Experimental Investigation of the Effect of Superhydrophobic Coating and Microbubbles Lubrication on the Resistance of a Scaled Ship

Skin friction in marine vessels constitutes one of the major issues that have negative environmental and financial impact due to the increased energy consumption. In this paper, the combination of two skin friction reduction techniques of superhydrophobic coating and microbubble lubrication are investigated experimentally. Microbubbles of up to 20 [μm] are introduced in the boundary layer through microbubble generators attached on the stem of a 2.52 [m] long ship treated with a superhydrophobic nano-ceramic coating. Resistance measurements are conducted at various towing speeds and trim angles and a skin friction coefficient reduction of up to 2.15% is noted.

Nano Bubble Lubrication for Flat Plates Skin Friction Reduction

The movement of the solution in a pipe is one of the determinants of resistance in the pipe. The resistance occurs due to the solution's movement with the pipe walls in different directions in its displacement. Then, the frictional force is generated due to these differences in movement. This results in an obstacle resulting in high-pressure drop due to a large amount of skin friction. So, in this research, we need a new method to reduce internal resistance in pipes. Before investigating the pipe's internal flow, this study wants to see its function and effect on the flat plate as an attempt to validate the investigations that will be carried out for future research efforts. The study aims to show the lubrication effect produced by using a 50 µm bubble generated by a carbon-ceramic tube. The bubbles' injector distance ratio is 0.4 to 0.85 from the end of the plate. The power needed to produce the bubble is 2.2 kW on a plate with an area of 1x104 mm2. The reduction of skin friction was analyzed by capturing the shear stress that was reviewed using a load cell at fluid velocities at intervals of 1 to 20 m.s-1 with a difference of 1 m.s-1. The results obtained in this study are that when the fluid velocity is at 7 m.s-1 to 13 m.s-1, the distribution of nanobubbles will increase. Moreover, a reduction in drag by + 60.5 percent, and the optimum skin friction (Cv) ratio is at 0.4 to 0.6.

Turbulent Skin Friction Reduction through the Application of Superhydrophobic Coatings to a Towed Submerged SUBOFF Body

In the present study, the drag-reducing effect of sprayed superhydrophobic surfaces (SHSs) is determined for two external turbulent boundary layer (TBL) flows. We infer the modification of skin friction created beneath TBLs using near-wall laser Doppler velocity measurements for a series of tailored SHSs. Measurements of the near-wall Reynolds stresses were used to infer reduction in skin friction between 8% and 36% in the channel flow. The best candidate SHS was then selected for application on a towed submersible body with a SUBOFF profile. The SHS was applied to roughly 60% of the model surface over the parallel midbody of the model. The measurements of the towed resistance showed an average decrease in the overall resistance from 2% to 12% depending on the speed and depth of the towed model, which suggests a SHS friction drag reduction of 4-24% with the application of the SHS on the model. The towed model results are consistent with the expected drag reduction inferred from the measurements of a near-zero pressure gradient TBL channel flow.

Proposal of control laws for turbulent skin friction reduction based on resolvent analysis

This paper evaluates and modifies the so-called suboptimal control technique for turbulent skin friction reduction through a combination of low-order modelling and direct numerical simulation (DNS). In a previous study, Nakashima et al. (J. Fluid Mech., vol. 828, 2017, pp. 496–526) employed resolvent analysis to show that the efficacy of suboptimal control was mixed across spectral space when the streamwise wall shear stress (case ST) was used as a sensor signal, i.e. specific regions of spectral space showed drag increment. This observation suggests that drag reduction may be attained if control is applied selectively in spectral space. DNS results presented in the present study, however, do not show a significant effect on the flow with selective control. A posteriori analyses attribute this lack of efficacy to a much lower actuation amplitude in the simulations compared to model assumptions. Building on these observations, resolvent analysis is used to design and provide a preliminary assessment of modified control laws that also rely on sensing the streamwise wall shear stress. Control performance is then assessed by means of DNS. The proposed control laws generate as much as $10\,\%$ drag reduction, and these results are broadly consistent with resolvent-based predictions. The physical mechanisms leading to drag reduction are assessed via conditional sampling. It is shown that the new control laws effectively suppress the near-wall quasi-streamwise vortices. A physically intuitive explanation is proposed based on a separate evaluation of clockwise and anticlockwise vortices.