Chih-Lun Liu

Postdoctoral Scholar

Department of Earth System Science, University of California, Irvine

Biography

I am a postdoctoral scholar in the Department of Earth System Science at University of California, Irvine. My research interests involve stratified shear instability’s role in ocean mixing. I am also interested in flow-topography interactions, such as internal tide-driven mixing at ocean bottom, island wakes and flow over ocean ridges. My research aims to address the questions, ‘how does turbulence occur in densely-stratified flows? How efficient does mixing occur to homogenize different layers of fluids? What are the effects of turbulent mixing on larger-scale processes?’ Using numerical simulations and theories, I aim to understand the complex dynamics of ocean mixing and its impact on the Earth systems.

I have completed my PhD at Oregon State University in 2023, working with Bill Smyth on Kelvin-Helmholtz turbulence in complex environments. Feel free to explore my projects and publications to learn more about my work and contributions to the field.

Interests

Ocean Mixing
Stratified Shear Flows
Flow-topography Interactions
Internal Waves
Data Science

Education

PhD in Earth, Ocean, Atmospheric Sciences, 2019-2023
Oregon State University (Advisor: [Bill Smyth](https://blogs.oregonstate.edu/salty/))
MSc in Physical Oceanography, 2015-2017
National Taiwan University (Advisor: [Ming-Huei Chang](https://minghueichang.wixsite.com/smallocean))

Experience

Postdoctoral Scholar

Department of Earth System Science, University of California, Irvine

February 2024 – February 2026 Irvine, USA

Understanding the primary steady energy pathway from barotropic tides to the internal wave continuum and the upslope mean flow generated from tidally-driven mixing in the bottom boundary layer.
Developing a parameterization method for spatial variation of internal wave breaking efficiency and irreversible mixing efficiency.
Strengthening Julia programming skills by contributing to Oceananigans.jl through source code debugging, developing a customized internal tide simulation (including postprocessing functions), and actively engaging with software developers.
Extensively utilized high-performance computing and optimized resource allocation.
Enhancing data analysis and visualization skills.

Fluid Dynamics of Sustainability and the Environment Summer School

Department of Applied Mathematics and Theoretical Physics, University of Cambridge

September 2023 – September 2023 Cambridge, UK

Participated in an intensive two-week program focused on cutting-edge theories and applications in fluid dynamics.
Enhanced technical, communication and collaboration skills through hands-on computational and laboratory experiments.
Built valuable professional connections by engaging with talented peers and leading experts.

Graduate Research Assistant

College of Earth, Ocean, and Atmospheric Sciences, Oregon State University

September 2019 – November 2023 Corvallis, USA

Under the supervision of Bill Smyth and collaboration with Alexis Kaminski. Projects includes studying the impact on shear instability and turbulence under three conditions: (1) altering the initial random noise (butterfly effect), (2) when a stratified shear layer is near a solid boundary, and (3) when a stratified shear layer is close to another shear layer.
Enhanced analytical capabilities through the derivation of mathematical theories, improved numerical proficiency by customizing FORTRAN code, and refined coding skills through postprocessing and organizing DNS outputs using MATLAB.
Extensively utilized high-performance computing and optimized resource allocation.
Developed strong critical thinking and problem-solving abilities, alongside effective communication skills, by participating in conferences and publishing research in Journal of Fluid Mechanics.

Faculty Research Assistant

Institute of Oceanography, National Taiwan University

October 2018 – September 2019 Taipei, Taiwan

Developed a 3D nested model using MITgcm to simulate oil spill pollution, improving predictive capabilities for environmental impact assessments and response strategies for the Central Weather Bureau, Taiwan.
Leveraged MATLAB programming skills and modeling skills to develop a 2D simulation analyzing interactions between the Kuroshio current and the I-Lan Ridge, east of Taiwan. This work collabrates with observational team in optimizing field data collection strategies and interpretation during an oceanic research cruise focused on measuring turbulence.

Substitute Military Service (Obligatory Service)

October 2017 – October 2018 Chiayi, Taiwan

Delivered care to elderly residents in a nursing home.
Enhanced soft skills such as communication, adaptability, time management, and teamwork.

Graduate Research Assistant

Institute of Oceanography, National Taiwan University

September 2015 – August 2017 Taipei, Taiwan

Collaborated with Ming-Huei Chang on advanced island wake modeling projects.
Conducted in-depth analysis of the Kuroshio current using numerical simulations and field observations, providing actionable insights into oceanographic dynamics.
Participated in research cruises to collect and analyze ocean hydrology and dynamics data, enhancing data-driven decision-making processes.

Projects

Internal Tide-Driven Mixing and Energy Pathways at Rough Seafloor Topography

Turbulent mixing is a crucial process in ocean dynamics, impacting both stratification and circulation. One significant aspect of this mixing is the internal tide-driven mixing that occurs at rough topography. Our research focuses on understanding the energy pathways of the barotropic M2 tide as it interacts with a gently sloping, rough seafloor. The sloping seafloor promotes restratification, which balances with fluid homogenization due to tidal-driven mixing, allowing a non-transient flow development. A large eddy simulation is used with realistic topography based on the Brazil Basin in the South Atlantic Ocean. The energy of the internal tide is transferred to smaller-scale internal waves, which radiate into the interior of the ocean. Eventually, these internal waves break down into patches of turbulence, driven by either shear instability or convective instability. The resulting mixing near the topography generates an upslope mean flow in the bottom boundary layer and a downslope flow above, which would potentially affect the deep ocean circulation.

Turbulence and Mixing from Neighboring Stratified Turbulence

Investigated the interactions between neighboring stratified shear layers found in thermohaline density staircases and those associated with internal wave breaking. The study aimed to understand the mutual influence of closely spaced shear layers, as they are rarely isolated in natural settings. Results indicated a non-monotonic dependence on separation distance, with significant changes observed when the distance approximates half the shear layer thickness. This behavior is influenced by the dominance of two distinct modes, contingent on the separation distance. Proximity of layers led to reduced growth rates of primary and secondary instabilities, as well as diminished turbulent mixing, similar to the effects of a nearby solid boundary (left figure).

The effects of boundary proximity on Kelvin-Helmholtz instability and turbulence

Kelvin-Helmholtz (KH) turbulence is a prevalent mechanism for generating turbulence in nature. Previous research on KH instability has often overlooked boundary effects. However, in geophysical flows, significant turbulent mixing occurs near boundaries, where shear instability frequently arises. My objective is to explore how boundary proximity influences KH instability and turbulent mixing through direct numerical simulation. Using theories and simulations, our findings indicate that boundary effects suppress both vortex pairing (a key mechanism for efficient mixing) and secondary convective instability, leading to reduced quantity and efficiency of turbulent mixing.

The butterfly effect and the transition to turbulence in a stratified shear layer

In a well-known metaphor for the sensitivity of chaotic systems to initial conditions, a butterfly’s wing flap is imagined to influence the development of a storm. In a dynamically unstable stratified shear layer, small disturbances grow exponentially through various competing instabilities, indicating a similar process in a high-dimensional yet relatively simple system. The objective was to determine whether the primary instability, transition to turbulence, and overall mixing efficiency are sensitive to initial condition details. Using ensembles of nearly identical direct numerical simulations (DNS), we discovered that slight variations in initial random perturbations can lead to significantly different turbulence and mixing outcomes. The figure on the left illustrates the evolution from the initial perturbation, showing the growth and development of KH billows through a horizontal cross-section of Reynolds stress and the turbulent kinetic energy time series. Different initial perturbations result in varied timing of instability growth and kinetic energy levels. This has crucial implications, particularly for initial value problems, as a single simulation may not be representative, suggesting broader implications for the predictability of oceanic and atmospheric movements.

Instabilities and Turbulent Mixing in the Strong and Unsteady Flows Over a Sill

Motivated by a sequence of field experiments, we employed a two-dimensional ocean model implemented in MITgcm to investigate the mixing processes occurring in the vicinity of the sill. Our primary focus was to understand the fundamental mechanisms of mixing, taking into account the energetic passage of the Kuroshio current across the sill and the associated variations induced by the M2 tide. During the flood tide, lee waves are generated, subsequently undergoing disruption due to the presence of Kelvin-Helmholtz instability during various tidal phases. The figure on the left illustrates an idealized scenario where tidal currents move back and forth, generating lee waves and shear instabilities along sharp density interfaces. These findings offer valuable insights into the evolution of flow dynamics and provide predictions for different tidal phases observed in the field.

Oil Spill Modeling and Strategies

Driven by the 1994 oil spill in the Philippine Sea, visible in SAR imagery, this project focuses on understanding and predicting oceanic oil spills. We implemented a 3D nested model utilizing open boundary conditions from HYCOM velocity and temperature fields, and incorporated modulation by the barotropic tide with data from OSU TPXO. A passive tracer was released from the southern boundary, where marine traffic is most concentrated. Results indicate that the oil spill remained trapped for over 10 hours near Green Island due to island wakes creating reverse flow downstream, aligning with historical incidents where oil was found on the northern shores. The long-term goal is to develop an interface that automatically collects HYCOM and tidal data for specific times and locations, enabling high-resolution simulations to predict future or analyze past oil spill events. This project aims to provide cutting-edge solutions for industry challenges in environmental monitoring and disaster response.

Dynamics of Stratified Island Wakes in Steady Flow

Island wakes develop when flow encounters a blunt body in geophysical settings. Inspired by satellite imagery and field observations, I employed MITgcm to simulate the wakes downstream of Green Island (about 5 km in diameter) east of Taiwan, achieving patterns and periods aligned with field data. Our study revealed that (i) the wakes display sequentially detached recirculation cells with cold water upwelling, moving downstream; (ii) the island wakes resemble a von Kármán vortex street, influenced by inertial and barotropic instabilities; (iii) the hotspot of turbulent mixing is located where horizontal shear is most pronounced. Due to the island-shelf effect and vertical vorticity tilting, lateral shear partially converts to vertical shear, leading to turbulent diapycnal mixing. Compared to a circular cylinder, the conical shape of the realistic island generates more turbulence in the wake region, evident by overturned colder water (left figure).