Abstract

The liquid–gas interface (LGI) on submerged microstructured surfaces has the potential to achieve a large slip effect, which is significant to the underwater applications such as drag reduction. The mechanism of drag reduction in the laminar flow over the LGI has been well recognized, while it is yet not clear for the turbulent boundary layer (TBL) flow over the LGI. In the present work, an experimental system is designed to investigate the mechanism of drag reduction in TBL flow over the LGI. In particular, the flow velocity profile near the LGI is directly measured by high-resolution particle image velocimetry by which the shear stress and the drag reduction are calculated. It is experimentally found that the drag reduction increases as the friction Reynolds number (Reτ0) increases. An analytical expression is derived to analyze the effect of the LGI on drag reduction, which consists of two parts, i.e., the slip property and the modifications to the turbulence structure and dynamics near the LGI. Importantly, the measured slip property also increases as Reτ0 increases, which is demonstrated to be the key effect on drag reduction. This has revealed the mechanism of drag reduction in TBL flow over the LGI. The present work provides physical insights for the drag reduction in TBL flow over the LGI, which is significant to the underwater applications.

Surface walkers are widely studied as carriers for biomedical applications, such as targeted drug delivery and cell manipulation. For in vivo/in vitro therapeutic applications, high movement velocity is necessary for efficient operation on targets with high throughput. Herein, a fast underwater microstructured spherical surface walker (i.e., microsphere) driven by a rotating magnetic field is reported, and the effect of the surface microstructures on the mobility of microspheres is explored. Compared with the motion of smooth sphere walking nearby a smooth plane, a twofold and a fourfold increases in the velocity are found for the microstructured sphere (MS) walking nearby a smooth plane and a patterned plane, respectively. A hydrodynamic model of MS moving nearby a plane is used to reveal the underlying mechanism of the enhanced motion, which demonstrates that the slippage of fluid on the microstructured surface and the interaction between fluid and microstructures are crucial to the motion enhancement. The result of motion enhancement induced by microstructures can be used for the design of fast biomimetic microrobots, anti‐adhesion, cell manipulation, and targeted drug delivery in complex aqueous environments.

Taylor-Couette flow with micro-grooves on the rotating inner cylinder is investigated to reveal the effect of surface structures on drag reduction. The Reynolds number ( Re) ranges from 160 to 18700. On the one hand, in the regimes of wave vortex flow (WVF, 160＜ Re＜1010) and modulated wavy vortices (MWV, 1010＜ Re＜1380) flow, the micro-grooves always reduce the torque, indicating drag reduction. Increasing either the size of micro-groove or Re, drag reduction will be enhanced. On the other hand, when the flow regime enters into turbulent Taylor vortices (TTV, Re＞1380), drag reduction will be suppressed as Re increases, and eventually turns to drag increase. The bigger the groove size, the smaller the critical Re where it turns from drag reduction to drag increase. To reveal the underlying mechanisam of the effect of micro-grooves on drag reduction, particle image velocimetry (PIV) measurements are conducted to observe the vortex flow structures, which demonstrates two aspects affecting the drag of Taylor-Couette flow over micro-grooved wall. First, the weakening of the large-scale Taylor vortex will lead to drag reduction. Second, the roughness effect will result in drag increase. In WVF/MWV, the former plays a dominant role, while in TTV, the latter dominates. In addition, a relationship between the micro-groove size and the predictive critical Reynolds number ( Rec) is developed, providing a method for controlling the wall drag.

Unmanned equipment, such as unmanned underwater vehicles (UUVs) and unmanned surface vehicles (USVs), are widely used in marine science for underwater observation, rescue, military purposes, etc. However, current vehicles are not applicable in complex cross-domain scenarios, because they can only perform well in either surface navigation or underwater diving. This paper deals with the design and fabrication of a cross-domain vehicle (CDV) with four hydrofoils that can both navigate at high speed on the surface, like a USV and dive silently underwater, like a UUV. The CDV’s propulsion is provided by a water jet propeller and its dive is achieved by a vertical propeller. The effect of hydrofoils and the performance of the CDV were tested and characterized in experiments, which showed that the hydrofoils improved the stability and surface sailing speed of the CDV. The maximum speed of the CDV was up to 14 kn, which is the highest of its kind according to current knowledge. This work confirmed the feasibility of high-performance CDVs and provided useful information for further improvements to the design.