Research On Simulationn And Experiments Of The Hydrodynamic Drag Reduction Of The Hydrophobic Micro-groove

The drag reduction effect of hydrophobic-micro-groove, which has been applied in pipeline transport and ship’s drag control, shows more advantages, such as easy machining, low cost and no additional energy consumption, compared with other techniques. However, the mechanism of the above drag reduction is not clear. Considering the hydrophobicity is the necessary condition of the drag reduction phenomenon of hydrophobic-micro-groove, the wettability of V-groove, sawtooth-groove, dome-groove and rectangular-groove were firstly analyzed and tested with Fluent simulation. Then the drag reduction characteristics of those grooves were simulated and the mechanism of it was expounded. Lastly, the wettability of those micro-grooves were experimental verified.Firstly, the V groove, sawtooth groove, dome groove and rectangular groove with the feature size of30μm were chosen as the objects of studying. The VOF models were built and then simulated to obtain the wettability of these grooves and achieve the effect of drop diameter and initial speed on the drop’s wettability performance. According to the simulation, these grooves all displayed well hydrophobicity, especially, the V-groove and sawtooth-groove showed the super-hydrophobicity. Now, the necessary condition of the drag reduction of hydrophobic-micro-groove, that is the hydrophobicity of the micro-grooves, was proved with simulation.Secondly, with the hydrophobic properties of the micro-grooves proved, the Fluid Dynamic Model was built on the base of grid optimization, boundary setting and turbulence model application etc. The simulation was carried out in different speed regions and showed that the drag reduction efficiency was changing with speed section, which means that each groove has it’s specific speed range of optimum friction reducing efficiency. On the other hand, the structural parameters of grooves, such as gap, vertex angle and height, were simulated to illustrate their effect on the friction reducing efficiency. It was found that the structural parameters have a significantly influence on the drag reduction efficiency and that the reduction phenomenon would disappear when they beyond the corresponding limits.Then, the drag reduction mechanism of hydrophobic-micro-groove was illustrated from the aspect of velocity gradient, turbulent structure, shear stress and energy exchanging on the base of the above simulation results. The paper found that the existence of the micro grooves reduced the transverse fluctuation of near wall flow field, and weaken the turbulence burst, then forming a small velocity gradient and reducing the shear stress of the flow on the micro-groove surfaces.Lastly, a series of rectangular-grooves with different gaps were fabricated with electrical discharge machining method and it’s wettability was analyzed with the OCA20Contact Angle Meter. Then the influence of gap on the contact angle was analyzed. The experiment showed that the grooves with small gap displayed excellent hydrophobicity, besides this, it was found that the smaller gap, the bigger contact angle. The experiment verified the foregoing wettability simulation, and proved that the grooves satisfied the necessary condition of the drag reduction, which is the well hydrophobic property hydrophobic-micro-groove, meanwhile, the friction reducing efficience of hydrophobic-micro-groove was indirectly demonstrated.