Research

1. Modern Numerical Method

Fully compressible multiphase model / Five-equation model/ Volume of Fluid method/ Free-surface solver/ Incompressible three-phase flow model. All papers Free download in Researchgate

2. Bubble dynamics/Cavitation bubble

Hydrodynamic cavitation and dynamic bubble processes have been studied and applied in diverse fields of science and technology such as naval structure engineering, biosciences, and biomedical technology. In a liquid, as the local pressure is reduced below its saturated pressure, the liquid breaks, and vapor bubbles appear. This phenomenon commonly occurs in the hydrodynamic cavitation process, which occurs in convergent–divergent nozzles or control valves or due to motions of bodies in a liquid such as a ship’s propellers or turbo-machineries. The cavitation bubbles often travel with the main flow. Then, the bubbles enter a region of higher pressure and collapse violently, and thereby producing water jets of liquid bubbles moving toward the wall. This in turn generates high local energy and impacts the surface with high-pressure waves that can erode the metals. Multiple events of cavitation bubble collapse that produce high pressure over time can cause detrimental effects on the mechanical components.

Conversely, this energy was observed as useful for the hydrodynamic cavitation process in cleaning technology or in industrial applications such as wastewater treatment and biofuel production. The bubble collapses lead to the reentrant jet formation, concentrated pressures, shear, and lift forces on the dirt particle or biomass, and high impulsive loads on a layer of materials. In the other approaches, cavitation bubbles can be intentionally generated by using acoustic waves or laser technologies to take advantage of local high-energy and microjets for application to biosciences, and biomedical technology such as needle-free injection devices, tissue engineering, and lithotripsy

The dynamics of bubble collapse is an interesting and challenging topic because of the difficulties involved in measuring the physical aspects of a violent event of a bubble collapse within a microsecond or even nanosecond for experimental methods and in modeling the compressibility effects and violent water jets for numerical methods. The collapses of cavitation bubbles and acoustic cavitation under various conditions have been extensively analyzed using our developed numerical methods.

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3. Partial cavitation and supercavitation

Cavitation occurs in a liquid flow owing to a pressure drop below the vapor pressure; when a cavity travels and reenters a region of high pressure, it collapses violently with very high local energy. This phenomenon that often occurs in the operation of systems in a wide range of hydraulic and industrial applications, such as ships, turbomachinery, automotive engines, and materials processing, requires careful analyses and treatments with an in-depth understanding of the flow mechanism, efficient prediction methods for optimized designationesignation.

Supercavitation refers to cavitation that envelops an underwater vehicle inside a large continuous cavity of gas while underwater. The physical phenomenon of supercavitation has been studied at length due to the fact that the drag on a moving vehicle can be reduced by supercavitation, thereby enabling it to move steadily at high speeds underwater. When the flow around the surface of the body is replaced by a layer of gas, both friction and pressure drag can be significantly reduced and the layer of gas results in a substantial reduction in near-wall density and changes in the fluid viscosity, similar to the phenomenon of supercavitating flow around underwater vehicles.

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4. Free surface flows

The computational fluid dynamic simulation of free surface flow for water impact problems is a vast topic that is still receiving increasing attention, particularly for applications in many hydraulic and hydrodynamic problems. The important examples for hydraulic problems in civil engineering are the potential risks of failures of levees, dams, reservoirs, and flood management. Although the hydraulic systems are recognized for their valuable contribution to the prosperity and wealth of societies across the world, their failures often cause tremendous economic losses, environmental damages, and significant losses of lives. Therefore, the prediction of velocities, water levels, and the trend of downstream wave front propagations with accurate times of arrival for the failures of the hydraulic structures is essential for engineers and officials involved with downstream flood prediction, early hazard warning systems, and crisis response. Free surface hydrodynamics are also very important in environmental, naval, and ocean engineering applications, and consist of a series of problems, such as water entries of objects, sloshing of liquids in tanks, in the design of ships, wave breaking in ships, ship maneuvering against waves, and green water on decks, offshore platforms, harbors, and coastal areas.

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5. Water entry

When an object falls into the water, the dynamics of the object are governed by the combined actions of the water impact and the various external forces and moments acting on the object. The complex phenomenon involves fluid kinematics and dynamics, the trajectory of the object in arbitrary translational and rotational motions, the free surface and its interaction with the rigid body surface, and air–water interfacial effects. A solid understanding of the physical characteristics of this phenomenon is essential for the effective design and usability of air-to-sea projectiles and torpedoes, as well as for numerous other applications. Numerical methods for multiphase flows in water impact problems have received increasing attention in recent years. However, a number of difficult challenges associated with computational fluid dynamics (CFD) methods remain for flows that involve strong fluid-body interactions, a large constituent density ratio, presence of discrete interfaces, and non-equilibrium interfacial dynamics.