Innovative Research and Applications in Hydrodynamics and Flow Control

This work aims to collect cutting-edge developments in the area of hydrodynamics and flow control, including both fundamental and engineering research. Hydrodynamics is of great importance in many fields including ships, marine engineering, energy, etc. It influences the kinetics and kinematics of ships, underwater vehicles, and marine structures. Flow control techniques can manipulate hydrodynamics and lead to benefits such as energy-saving capabilities, high performance, etc. The following topics are covered: the design of ships, underwater vehicles, and marine structuresß; shape optimization of marine vehicles; experimental fluid mechanics of marine structures; computational methods for fluids; applications of CFD (Computational Fluid Dynamics) methods; flow control techniques; fluid mechanics; and machine learning methods for fluids. Thirteen works have been collected, comprising one review article and twelve research articles.

The review article focuses on chemical reactions (contribution 1), which comprise an important area in flow chemistry. This work aims to solve the problem of mixer design in order to realize the purpose of reaction controls, especially for those experiments that are difficult to explain. These data were identified and rational explanations were proposed, based on a supramer approach.

The jet phenomenon is commonly seen when a stream of fluid is projected into a surrounding medium. A computer vision algorithm for the parameter determination of a two-phase jet flow was proposed (contribution 2). The mass exchange can thus be evaluated in real time. This approach is useful for the real-time control of a multi-phase jet. Central jet injection was utilized as a flow control strategy in a model swirl-stabilized combustor (contribution 3). It was found that the asymmetric hydrodynamics mode is suppressed, while the symmetrical one becomes dominant.

A pump can convert electrical energy into hydraulic energy by moving fluids using mechanical action. A mathematical model to predict the working processes of the piston pump cap was established (contribution 4), taking into account the phase transition process and gas solubility. It was found that increasing the pipeline length and the revolution of the crankshaft were necessary to decrease the feed irregularity. A study of a multistorey hydro-pump was presented (contribution 5), which promoted further understanding of its physical properties in order to strengthen its performance. It was found that a small chamber diameter and a high supply head resulted in high pressure.

Several fundamental fluid mechanics problems were also investigated in several of the collected works. A modified phase-transition model was proposed (contribution 6) for the study of the multi-oscillation of bubbles. The transition of the phase exhibited a strong influence on bubbles during this process. An approximation of the thermal boundary layer was implemented into the Rayleigh–Plesset equation and rigorous validation was conducted, using experimental data. Finally, the condensation rate and its dependence on phase transition and the air were investigated. The adverse effect of backward step on airfoil was studied to evaluate the performance of the airfoil (contribution 7). A three-dimensional low-profile serrated pattern was used to weaken the adverse effect.

The machine learning approach is currently a popular approach in the field of fluid mechanics, and an emerging approach in intelligent fluid mechanics. A transformer neural network was used for two-phase flow pattern identification in vertical pipes (contribution 8). Various configurations were proposed and tested to discern the one with the most accurate predictions. Order reduction optimization was used for smart structures’ vibration suppression (contribution 9). A directive of the controller was set, with the allowance of a lower order.

Several other works with various engineering backgrounds were also included. Echo-PV (particle velocimetry) methods and computer simulation were conducted to obtain information about vortex formation in blood flow (contribution 10). Microbubble injection was utilized to investigate the effect of gas on the drag reduction effect (contribution 11). The influence of support stiffness nonlinearity on vibro-acoustic properties was studied (contribution 12). Finally, a part-nested four-nozzle ejector was presented (contribution 13), which aimed to strengthen hydrogen recycling efficiency by limiting the flow in the nozzle.

In conclusion, this Special Issue collects 13 works, comprising innovative research and applications in hydrodynamics and flow control. These works are promising for relevant fundamental research and engineering purposes.