# Research

Bio-inspired Flexible Propulsors for Fast, Efficient Swimming

Fish locomotion, through the benefit of evolution, is considered a highly efficient form of propulsion under the water. In collaboration with a $7.4 million-funded project to investigate the hydrodynamic principles of fish swimming, I am developing two- and three-dimensional CFD models to simulate the flow field as well as the force and power acting on different fish models. In particular, I am investigating the physical mechanisms behind the common kinematics behaviors among fish at cruise including the observed tight range of Strouhal number among swimming fish. The outcome of this work will allow us to formalize the design of bio-inspired fish robots and lay out a platform to pinpoint the major elements of efficient fish swimming. Phase Change for Extracting Extremely High Heat Fluxes The future of technological advancement in many industries is tied to the ability to extract and dissipate extremely high heat fluxes. Examples include computer chip industry and hypersonic flight. Owing to the unique thermal characteristics of phase-change heat transfer, two-phase flow cooling systems are shown to provide highly effective means of heat dissipation. As part of the University of Virginia-Max Planck Society partnership targeting energy research, I developed two- and three-dimensional multiphase flow analytical/CFD models to improve on the efficiency of a novel thermal management system. In particular, both Volume of Fraction and evaporative moving front methods were used to calculate the evaporative mass and heat flux at the liquid-vapor interface. It is shown that the micro-grooved structure can dissipate heat fluxes as high as 10MW/m^2 for superheats as low as 5 degrees Kelvin. The findings of this work has profound implications in the design of micro-grooved heat pipes. Towards a Mission-Configurable Stealth Underwater Batoid While most Autonomous Underwater Vehicles (AUVs) are capable of performing complex missions, their performance remains limited when compared with solutions from nature. For instance, screw propellers have a narrow window of efficiency, are noisy, and also lead to vibrations. AUVs also have low maneuverability (e.g. large turning radius), and limited supply of power. In collaboration with a$6.5 million-funded project (University of Virginia, Princeton University, UCLA, and West Chester University), I studied the swimming hydrodynamics of manta rays. In particular, I developed CFD models that provided insight into the role of spanwise and chordwise flexibility in the efficient thrust production of batoid-shaped flapping fins. Additionally, I performed multi objective CFD/FSI optimization to design a flexible flapping wing. The project attracted broad media coverage including a TV program in Discovery Channel, Science Magazine, ABC News, Science Daily, and UVa Today.

Modeling of Turbo Air Classifier

Classification devices are important elements in various industries to isolate the particles of certain sizes. Examples include medicine, chemical production, metallurgy, mining, energy production, pulp and paper science and cosmetics production. Turbo Air Classifiers, in particular, are among the most popular dry classification devices due to their high classification performance, adjustable cut size and controllable product granularity. As an important indicator for evaluating the classification performance of a turbo air classifier, cut size is often predicted in advance to evaluate classification effect so that the operation parameters can be adjusted suitably according to the production requirement. Owing to the limitation of theoretical/empirical predictions, CFD is often required to predict the accurate cut size. In collaboration with University of Virginia Rotating Machinery and Controls (ROMAC) Laboratory, I developed three-dimensional CFD models for analysis of two-phase flow (gas-solid) in turbo air classifiers and predicting the cut size for various operating conditions. The simulation results were further validated against the experimental data.

Supersonic Turbulent Mixing of Air-Ethylene System

Today’s air travel between long distances is costly, time consuming and produce high amount of carbon pollution. Novel forms of propulsion are needed to overcome today’s challenges facing the air transport industry. Hypersonic propulsion is one promising candidate that offers dramatically higher speed and fuel efficiency. Hypersonic propulsion will revolutionize the intercontinental air transportation and consequently affect many other industries. This technology, however, still faces fierce challenges yet to be solved before the technology becomes fully available. One of the main issues in developing engines based on hypersonic technology is the low air-fuel mixing efficiency that takes place inside the combustor. In a conventional aircraft jet engine, fuel is injected into the compressed air and combustion will take place. The resulting hot and high-speed gas will then be discharged out of the nozzle and produce forward thrust force. In hypersonic propulsion, the mixing of fuel with the incoming air is inefficient due to the very short time scale of the mixing process (the speed of the incoming air is extremely high, thus the injected fuel does not have time to mix efficiently with the air). In collaboration with the National Center for Hypersonic Combined Cycle Propulsion, I developed three-dimensional CFD models to simulate the turbulent ethylene-air mixing process at very high speeds. I further calculated the optimum configuration of the injectors around the channel (wind tunnel) as well as the required dynamic pressure ratio to maximize the mixing properties of ethylene with air including dispersion, penetration and efficiency of the mixing. Simulations results were further validated against experiment.

A computational study was conducted to explain the physical mechanisms responsible for the aerodynamic effects of leading edge tubercles. A model exploiting the spanwise periodicity and symmetry of the geometry was proposed for studying the stall mechanism of airfoils with leading edge tubercles. Based on the model, the tubercled airfoil was broken into a sequence of (symmetry) wall-bounded swept wings. The expedited (retarded) stall near outboard (inboard) regions of the wall-bounded swept wing were explained using interaction of S-shaped streamlines over the suction surface of the airfoil with the symmetry walls. There were three mechanisms that disallow the maximum lift of airfoils with leading edge tubercles be higher than that of their smooth leading-edge counterpart: i) a lower effective nose curvature seen by each streamline, ii) expedited stall behind the trough of the tubercles, and iii) relief of the pressure difference because of sweep. The improved post-stall behavior of tubercled airfoils was attributed to the appearance of sacrificial separation bubbles which stir high-momentum fluid from the free stream towards the airfoil. The physical reason underlying the presence of non-tubercled inboard portions of biological flippers was also explained. The importance of rotation about the root of the wing in experiments involving full-scale flipper models was suggested.

Effects of formation time and flexibility on the starting and stopping vortices in piston-cylinder devices

A computational study has been conducted to address the formation of vortex rings ejected from a weakly flexible nozzle at the end of a piston-cylinder device. The first observation is that, regardless of flexibility, for small enough duration of a push on the piston, the negatively-signed induced vorticity on the outside of the cylinder merges with the stopping vortex and spills into the core and pairs up with the primary (starting) vortex. The second observation is that the flexibility of the exit nozzle affects the behavior of the vortex by changing the effective diameter of the exit. Also, the snapping back of the nozzle strengthens this interaction. To model the flexibility, two approaches have been used in the study. First, the nozzle is modeled using a torsional-spring-mass-rigid system which is allowed to rotate about a hinge and its rotation is coupled with the flow solver. Second, the nozzle is considered as an elastic material and its deflection is solved using a FEM solver. It is hypothesized that the presence of flexibility in the model increases the time scale as well as the thrust as compared with results from rigid nozzle models. A study is conducted to find the highest thrust generated versus the flexural parameters.

Occupant-Oriented Heating and Cooling Technology

Heating, Ventilation, and Cooling (HVAC) accounts for 38% of building energy usage, and over 15% of all U.S. energy usage, making it one of the nation’s largest energy consumers. This research (\$2 million-funded by NSF) was focused on producing intelligent (occupant-oriented) heating-cooling technology for the buildings with the aim of reducing the national energy consumption by 28 percent, thus making it a highly effective energy saving technology to help United States achieving its target for energy reduction. As a part of the fluid and heat transfer group, I developed multi-phase flow CFD models to design and analyze an efficient buoyancy-driven heat pump. In particular, an Eulerian-Eulerian method was used to account for condensation of the water vapor in the humid air. The geometry of the heat pipe was further optimized for the maximum outflux energy given air input conditions (velocity and humidity ratio).