Non-equilibrium Molecular Dynamics Simulation Of Cavitating Flow Phenomena And Mechanism Study

This work is financially supported by National Natural Science Foundation of China Project 20473073. This project was aimed to study high-velocity flow mechanism in molecular scale, which was very important in both industrial and military application. One of the most important phenomena was supercavitation, which directly affect the research progress on high-velocity torpedo. The traditional experiment and numerical simulation were unable to provide insight for molecular scale flow, so we involve Non-equilibrium Molecular Dynamics simulation to study these phenomena.In this work, we studied multiple flows in micro scale system and their molecular distribution, physical and chemical properties and interfacial behaviors. Particularly the simulations for open system supercavitation studied molecular scale mechanism and behavior of this phenomena, provided valuable data for supercavitation study in real system.In order to study flow in different systems and conditions, our simulation configuration varied from simple slitpore to open system, from simple LJ particle to real molecule like methane and water, from simple system to dual systems coupling. NEMD methods were applied to calculate fluid properties like pressure, density, velocity, interfacial tensor and friction coefficient. By studying detail data in flow field, several mechanism about high-velocity flow were found and associated with macro scale fluid dynamics.In order to verify the simulation model and NEMD program, methane fluid was studied in macro volume system using Equilibrium Molecular Dynamics conditions for its PVT relation. The result agreed with previous literature. Also force field driven flows in slitpore were studied using NEMD, the results were agreeable with hydrodynamics theory, showing the program was able to reproduce and predict flow phenomena in molecular scale.With verified model, we simulated different obstructed methane flows in slitpore, under 160K. Different driving force fields and shape of obstacles were applied to investigate their effect on flow field detail. Simulation results showed that under high-velocity, obstructed flow would form high-density and low-density area around obstacle, causing corresponding reaction on pressure. Flow velocity was the critical factor on the formation of low-density area, which can be treated as a diluted phase. Front shape of obstacle would affect the formation of diluted phase and frontal pressure gradient but has less effect on interfacial tensor. Thus flat-head obstacle was receiving more frontal drag force from press gradient. Increasing the length of obstacle or driving force field intensity would greatly increasing the system press, preventing gas phase to form. Oscillation wave was predicted in the simulation and agreed with Rayleigh-Taylor criterion, provided a molecule scale insight for macro scale flow phenomena.Since supercavitation drag reduction was very important for both industrial and military application, we introduced open system model to expand our simulation from slitpore. By coupling EMD simulation with NEMD simulation, confine effect of wall boundary was removed, and constant flow velocity could be used instead of driving force field, making the whole simulation system better reflect flow in real circumstance. By introducing local cavity numberσ_l, simulation cell was divided into multiple bins in order to study the formation of cavitation. Simulation results showed that the conversional cavity criterionσ<0.1 was applicable in molecule scale. Lowσ_l area was spatially separated with actually cavity, and generating micro bubble that was quickly filled with surrounding fluids. These bubbles were not able to nucleate for gas phase but still causing periodical density drop. When lowσ_l area developed beyond 2 fluid molecular diameter, stable supercavitation would like to form. Velocity of underwater object was the critical factor for supercavitation.With valid model for supercavitation simulation, we carried out cavity flow for water. By applying SPC/E water molecular model, different cavitators and object velocities were simulated. Local density profile, local cavity number profile, gas volume percentage of cavity and skin friction coefficient was recorded and compared with numerical simulation result. Results showed that flat-head cavitator was easer to generate cavity with high gas percentage, hence further lowing friction under similar σthan other streamline cavitators. Drag reduction of supercavitation can range from 50% to 90% depending on different shapes of cavitator.In this work, we have reached our aim to create a theory tool for studying flow mechanism in molecular scale. This work will benefit consequent works, which can focus on more complicated cavitation flow setup.