Postdoctoral scholar, Yale Univ., Mechanical Eng. & Materials Science Dept., New Haven, CT 02/2014-present
Experimental & analytical studies of thermal convection, and heat transfer/management
- Succeed in high-precision temperature measurements (with resolution < 5 milli-Kelvin), and reveal new dynamics of turbulent thermal convection
- Perform time series analysis, statistical analysis, and uncertainty analysis
- Apply low-dimensional (reduced-order) models from Navier-Stokes, developed from first principles, to model dynamics of thermal convection
- Collaborate with team members to extend the study to describe general thermal convection, cooling, and HVAC systems
Material development & testing
- Develop the world’s first conducting & magnetic liquid at room temperature (magnetic liquid-metal)
- Achieve a wide range of material properties of the liquid metal: viscosity by 3 orders of magnitude; magnetic susceptibility by a factor of 4; and electrical conductivity by a factor of 2
Research Assistant, Johns Hopkins Univ., Mechanical Engineering Dept., Baltimore, MD 09/2007-01/2014
Experimental studies of turbulent flows in complex systems
- Uncovered velocity field in complex systems (a fractal canopy), using optical setups, lasers, CCD cameras, image processing, & Particle Image Velocimetry (PIV)
- Quantified turbulent wakes, using experiments, and theoretical/time-series/spatial/spectral analysis
- Measured drag of objects, and compared results with Computational Fluid Dynamics (Large-Eddy-Simulations)
Simulation-aided design
- Designed inlet section of a water tunnel, using CFD package – Fluent
- Optimized the design by iterative simulations and experimental testing
- Performed FEA for stress distribution of meshes in the inlet section using COMSOL
- Designed/constructed a laboratory-scale water tunnel (cross-section 0.56 m * 0.23 m) and load-cell chambers
Research Assistant, Institute of Fluid Mechanics, Beihang University (BUAA), Beijing, China 09/2006-07/2007
Numerical simulation using Finite-Difference-Method/Finite-Element-Method & wind tunnel studies of aerodynamics
- Performed CFD studies of insect flight: dynamics and aerodynamics, and energy consumption
- Optimized aerodynamic features of an aircraft model using Fluent
- Tested & optimized aerodynamic characteristics of models in wind tunnels
As examples, I describe some of my projects in thermal/fluid flows along with their applications below. Please feel free to contact me about other projects and experiences.
An example of large-scale flow structures: convection rolls in the atmosphere. (Credit: figure of idealized convections from The Atmosphere, Lutgens & Tarbuck, 2001)
As a postdoctoral researcher at Yale University, I currently work on two separate projects studying turbulent convection and fluid dynamos. In the former one, I study one of the long-standing questions in turbulence, namely whether a general low-dimensional model can predict the dynamics of large-scale coherent flow structures (LSCs) in turbulence (such as convection rolls in atmosphere). A particular challenge is to obtain a general form that can predict how dynamics depend on system geometry. In this study, I extend a low-dimensional stochastic model, applicable to systems with arbitrary boundaries, to predict dynamics of LSCs. By implementing high precision temperature measurements (with resolution 2.5 mK), I tested the predictions in Rayleigh-Benard convection, and discovered a new dynamic mode of LSCs that was predicted from the model but had not been observed before. Excellent agreement is achieved between measured data and our model predictions. In principle, the methodology could be applied to predict/describe flow dynamics of large-scale coherent structures in general turbulent flows in arbitrary geometries that are typically encountered in nature.
The developed liquid metals could be used to study magnetohydrodynamics and fluid dynamos that is responsible for generating magnetic fields, e.g. in the Sun. (Credit: NASA SDO)
The large-scale flow structures also form, for example, in the Earth and Sun where large masses of conducting fluid swirl around to generate magnetic fields by dynamo effect. However, the demonstration of fluid dynamos in a laboratory experiment remains a major scientific challenge, due to the limited range of magnetic Reynolds number that is accessible in ordinary liquid metal. With my colleagues, I developed a novel material with tunable magnetic susceptibility and viscosity by suspending magnetic and non-magnetic particles in an alloy of gallium and indium, which is liquid at room temperature. Such novel suspension can produce high magnetic Reynolds number flows in a small tabletop facility. Additionally, the magnetic and hydrodynamic Reynolds number can be tuned independently. This material does not require high temperatures to operate and is easier to handle than sodium, the material used in previous dynamo studies. This new material could open up new opportunities in the field of magnetohydrodynamics and could produce the smallest dynamo systems in the universe.
(a) Trees in nature have complex shape and multiple length scales. (b) To include multi-scale property in our study, we utilized 3D fractal tree-like structures, with refractive-index matching capability.
My Ph.D. research at Johns Hopkins University focused on turbulence generated by 3D fractal tree-like structures with multiple length scales. The study was closely related to flows in vegetation canopies that had been widely studied in past few decades. However in previous studies, the inherent multi-scale properties of canopy elements were often ignored in most laboratory experiments that utilized idealized structures such as rods and tubes to represent canopy elements. In addition, the complex geometry of the canopy posed a significant challenge to capture the flow field within it. I applied large-scale optical refractive-index-matching techniques, Particle Image Velocimetry (PIV) and drag measurements to study the interactions of flows generated by the canopy at different length scales, and to bridge the gap between idealized laboratory experiments and realistic canopy flows. I was able to capture the flow field inside the complex canopy at high resolution, without optical obstructions for laser illumination and image recordings. The importance of the multi-scale property was revealed both by quantification of energy cascading across different length scales, as well as the momentum transport behind and within the canopy. Furthermore, I revealed distinct large-scale coherent structures with large inclination angles above the canopy compared to traditional wall turbulence. Such structures could have plausible impact on the long distance transport of seeds and particles in real canopies.