Seminars

Turbulence as the Stability of Three Fundamental Flow Structures

Johan Hoffman

Professor
Division of Computational Science and Technology
KTH Royal Institute of Technology.
Stockholm, Sweden

Turbulence theory builds on two main results: (i) the Navier-Stokes equations from about 200 years ago, a set of partial differential equations with the viscosity as the sole empirical parameter that under given flow conditions describe the evolution of velocity and pressure; and (ii) a statistical theory from almost 100 years ago, which under the assumptions of statistical homogeneity and isotropy states that there exists an inertial subrange of scales where energy dissipation is independent of viscosity so that the energy spectrum satisfies a certain power law. Two important developments are: (iii) the extension of the statistical theory into a theory of non-uniform, localized energy dissipation, referred to as intermittency; and (iv) the Onsager conjecture that turbulence can be modelled by weak solutions to the inviscid Euler equations, hence, without viscosity at all. In this talk we present a model of turbulence in the form of three fundamental flow structures: rigid body rotation, pure strain, and shear flow; each with its own distinct stability property. Rotation is stable and can exist for a long time; shear is linearly unstable and only exits for a short time before transforming into rotation (roll-up); and strain is exponentially unstable, therefore, only exists in short instants before transforming into shear (compression) or rotation (vortex stretching). This model connects (i)-(iv) and is consistent with classical fluid mechanics phenomena driven by flow instabilities, such as the Kelvin-Helmholtz instability, boundary layer transition to turbulence, and the turbulent flow separation (d’Alembert paradox). Further, we show how this model can be used to construct a mechanistic model of the turbulent energy cascade.

Johan Hoffman is professor and deputy head of the division of computational science and technology at KTH Royal Institute of Technology in Stockholm. He is a founder of the FEniCS open source software project, and he combines fundamental research in mathematics, fluid mechanics, and computational science with applied research in interdisciplinary collaborations with industry and society. He earned his Ph.D. in applied mathematics at Chalmers University, was a postdoc fellow at the Courant Institute at New York University, and has been a visiting researcher at Oxford University, University of Chicago, Stanford University, University of Heidelberg, UPC Barcelona, the Auckland Bioengineering Institute, and the Basque Center for Applied Mathematics.

Monday, August 25, 2025

12:00 NOON

Laufer Conference Room (OHE 406)

host: Oberai

Ice Sculpturing with Laminar Flow

Xiaojing (Ruby) Fu

Assistant Professor
Mechanical and Civil Engineering
California Institute of Technology
Pasadena, CA

This talk examines the physics of unsaturated water flow through subfreezing porous media and its role in shaping ice solidification within confined environments.

At the centimeter-to-meter scale, we use a nonequilibrium infiltration model to study refreezing structures that emerge during gravity-driven infiltration. Two freezing-induced mechanisms are shown to hinder infiltration: (i) partial freezing of infiltrating water, which reduces effective infiltration rates and can be characterized by a freezing Damköhler number, and (ii) the formation of secondary fingers—new flow paths that disrupt primary channels—leading to homogenization of the flow field and reduced infiltration efficiency. Preliminary experimental observations are presented to test these model predictions.

At the millimeter-to-centimeter scale, we investigate water imbibition in a subfreezing Hele-Shaw cell. Water at 0 °C is injected at a constant rate into a radial cell cooled by a temperature-controlled aluminum block. By varying flow rate and subcooling, we identify how solidification dynamics and flow morphology evolve across regimes.

These studies are motivated by the need to understand fundamental processes in cold-region hydrology, including snowpack, glacier, and permafrost evolution. Beyond the cryosphere, the insights extend to broader nonequilibrium solidification problems relevant to additive manufacturing, freeze casting, lava flows, and natural terrace landform development.

Xiaojing (Ruby) Fu is an assistant professor in Mechanical and Civil engineering at California Institute of Technology, where she leads the group on Mechanics and Physics of Porous Media Flow. Her group combines mathematical theory, computation and laboratory experiments to advance our predicative capability of field-scale applications in a wide range of geoscience problems, including hydrology, gas hydrate systems and soil biogeochemistry, geologic carbon sequestration and geothermal systems. A primary focus of her current work is on understanding the physics of freezing flow through porous media.

Wednesday, August 27, 2025

3:30 PM

Seaver Science Library, Room 202 (SSL 202

host: Maghsoodi

Data-Driven Nonlinear Stability Analysis in Aeroelastic Systems

Amin Ghadami

Research Assistant Professor
Sonny Astani Department of Civil and Environmental Engineering
University of Southern California
Los Angeles, CA

The demand for increased fuel efficiency in aircraft and the necessity to address structural damage caused by sudden loading are driving an interest in new aircraft designs with lighter, more slender, and flexible configurations. Increased flexibility results in diverse nonlinear response phenomena due to complex fluid-structural interactions, making the system susceptible to unexpected instabilities in their dynamics, emphasizing the necessity to incorporate nonlinear stability analysis early in the design process. However, performing such analysis in aeroelastic systems remains challenging with traditional approaches. Despite the existence of analytical and numerical methods for performing such analysis, their use is limited in systems with large dimensionality or when an accurate mathematical model is lacking, leaving time-marching simulations as the common alternative, though they are computationally expensive and difficult to interpret. In this talk, I will present recently developed data-driven techniques for nonlinear stability analysis in aeroelastic systems. In particular, I will demonstrate that a combined use of invariants in nonlinear dynamics and data-driven methods offers an alternative to demanding analytical, computational, and experimental processes for nonlinear stability analysis in aeroelastic systems. Applications of this approach to models of highly flexible wings and experimental configurations prone to flutter instabilities will be demonstrated.

Amin Ghadami is a Research Assistant Professor of Civil and Environmental Engineering and Aerospace and Mechanical Engineering at the University of Southern California. He received his Ph.D. in mechanical engineering in 2019, followed by a postdoctoral position at the University of Michigan-Ann Arbor from 2019 to 2022. His research is at the intersection of nonlinear dynamics and data-driven techniques with applications in mechanical and aerospace systems and structures. Dr. Ghadami is the recipient of the 2025 AFOSR Young Investigator Program Award for his research on nonlinear stability analysis of aeroelastic systems. He is also the recipient of several other honors and awards, including the University of Michigan Ivor K. McIvor Award, which recognized the excellence of his PhD research in applied mechanics. He is a member of the Technical Committee of Multibody Systems, Nonlinear Dynamics, and Control as well as the Technical Committee of Dynamics and Control of Systems and Structures within American Society of Mechanical Engineers (ASME).

Wednesday, September 3, 2025

3:30 PM

Seaver Science Library, Room 202 (SSL 202)

host: Ronney

Hierarchical Learning and Control for Agile and Adaptive Legged Robots

Quan Nguyen

Assistant Professor
Aerospace & Mechanical Engineering Department
University of Southern California
Los Angeles, CA

Optimization-based control methods, such as model predictive control (MPC) and trajectory optimization, are essential to advancing the locomotion capabilities of legged robots. However, for successful deployment in real-world environments, legged robots must be capable of rapid adaptation to unknown terrain, robust handling of model uncertainties, and effective interaction with unknown objects to perform practical manipulation tasks.

In this talk, I will present a hierarchical model predictive control framework that enables legged and humanoid robots to perform dynamic loco-manipulation tasks. I will also highlight our recent work on adaptive force-based control that enables legged robots to handle significant model uncertainty. These strategies allow the robots to interact robustly with unknown objects, particularly in scenarios involving the transport of heavy loads over rough terrain.

Finally, I will discuss how we extend our hierarchical control principles into the domain of reinforcement learning to acquire long-term adaptive behaviors in both control actions and trajectory planning for legged and humanoid robots.

Quan Nguyen is an Assistant Professor of Aerospace and Mechanical Engineering at the University of Southern California. He is also the Chief Scientific Officer of VinMotion, a humanoid robotics company established by Vingroup, the largest private enterprise in Vietnam. Prior to joining USC, he was a Postdoctoral Associate in the Biomimetic Robotics Lab at the Massachusetts Institute of Technology (MIT). He received his Ph.D. from Carnegie Mellon University (CMU) in 2017 with the Best Dissertation Award.

His research interests span different learning and control approaches for highly dynamic robotics, including nonlinear control, trajectory optimization, adaptive control, and reinforcement learning. His work was featured widely in many major media channels, including CNN, BBC, NBC, IEEE Spectrum, etc. Nguyen won the Best Presentation of the Session at the 2016 American Control Conference (ACC) and the Best System Paper Finalist at the 2017 Robotics: Science & Systems conference (RSS). He is also a recipient of the 2020 Charles Lee Powell Foundation Faculty Research Award.

Wednesday, September 10, 2025

3:30 PM

Seaver Science Library, Room 202 (SSL 202)

host: Ronney

Robots and Sensors for Movement Rehabilitation: What Have We Learned and What Comes Next?

David Reinkensmeyer

Professor
Departments of Mechanical and Aerospace Engineering, Anatomy and Neurobiology, Biomedical Engineering, and Physical Medicine and Rehabilitation
University of California at Irvine
Irvine, CA

A fundamental goal of movement rehabilitation following neurologic injuries such as stroke is to enhance sensorimotor plasticity through targeted movement practice. Over the past 40 years, researchers have developed robotic and sensor-based technologies to facilitate and quantify this practice. In this talk, I will describe key insights into post-stroke motor control that have emerged from these technologies. I will also discuss recent findings that reveal which individuals benefit most from robot-assisted training and why, highlighting the role of somatosensory feedback in what appears to be a reinforcement learning-driven process. Finally, I will provide examples of mechatronic rehabilitation technologies that have successfully transitioned into clinical and home settings, inviting discussion on the design factors that drive their adoption and impact.

David Reinkensmeyer is Professor in the Departments of Mechanical and Aerospace Engineering, Anatomy and Neurobiology, Biomedical Engineering, and Physical Medicine and Rehabilitation at the University of California at Irvine. He received the B.S. degree in electrical engineering from the Massachusetts Institute of Technology and the M.S. and Ph.D. degrees in electrical engineering from the University of California at Berkeley, studying robotics and the neuroscience of human movement. He carried out postdoctoral studies at the Rehabilitation Institute of Chicago developing robotic devices for rehabilitation therapy after stroke before becoming a assistant professor at U.C. Irvine in 1998. He is co-inventor of the T-WREX upper extremity training device, which was commercialized as ArmeoSpring, and the MusicGlove finger training device. He was Editor-in-Chief of the Journal of Neuroengineering and Rehabilitation for 10 years and is co-director of the NIDILRR COMET Robotic Rehabilitation Engineering Center, co-editor of the of the book Neurorehabilitation Technology, and a fellow of the National Academy of Inventors.

Wednesday, September 17, 2025

3:30 PM

Seaver Science Library, Room 202 (SSL 202)

host: Maghsoodi

—Laufer Lecture—

Probing Turbulence Physics Using High Fidelity Numerical Simulations with Application to Model-Based Engineering

Parviz Moin

Franklin P. and Caroline M. Johnson Professor of Mechanical Engineering
Center for Turbulence Research
Stanford University
Stanford, CA

High fidelity numerical simulations have provided unprecedented comprehensive datasets for the study of the mechanics of turbulent flows. A brief review of fundamental research on the structure and mechanics of wall-bounded turbulent flows will be presented with particular emphasis on the role played by large scale numerical simulations in conducting controlled experiments of discovery. Although incorporation of the results from these simulations into practical engineering models remains an important area of research, a key insight gained from these novel numerical experiments has been the demonstration of the independence of self-sustaining dynamics of near wall-turbulence from the outer layer structures. In retrospect this insight has been underpinning the success of the so-called wall modeled LES of complex wall bounded turbulent flows.

I will conclude by presenting recent progress in large eddy simulation of complex flows, specifically for prediction of aircraft performance at the edges of flight envelope. Recent work has demonstrated that leveraging large eddy simulation with appropriate wall/subgrid-scale models and low dissipation numerical methods on modern computer architectures offers a tractable path towards meeting industry’s stringent accuracy and affordability requirements. The success of these calculations in predicting important quantities of practical interest is further evidence of the robust nature of near wall turbulence.

Parviz Moin is the Franklin P. and Caroline M. Johnson Professor of Mechanical Engineering and the Founding Director of the Center for Turbulence Research (CTR) and the Institute for Computational and Mathematical Engineering (ICME) at Stanford University. Professor Moin pioneered the use of Direct Numerical Simulation for the study of turbulence physics, and has written widely on the structure of turbulent shear flows. His current interests include numerical simulation of complex multi-physics turbulent flows. Professor Moin is a Fellow of the American Physical Society, AIAA, and the American Academy of Arts and Sciences. He is a member of the United States National Academy of Engineering, and the National Academy of Sciences.

Wednesday, September 24, 2025

Reception at 2:30 PM

Seminar at 3:30 PM

Michelson Hall, Room 101 (MCB 101)

host: Ronney

Advanced Manufacturing for Extreme Environment Spacecraft Applications at NASA JPL/Caltech

Douglas Hofmann

Senior Research Scientist
Materials Development and Manufacturing Technology Group
Jet Propulsion Laboratory
Pasadena, CA

For the past 15 years, NASA’s Jet Propulsion Laboratory has had a robust research activity in advanced materials and manufacturing, with focus on the development of new metal alloys and manufacturing technologies for extreme environment spacecraft and rovers. This has involved the creation of many new applications for advanced materials, such as innovative spacecraft shielding, cryogenically capable gears for space applications, novel robotics applications, and lightweight wheels for planetary rovers, among others. The research has spanned the technology development spectrum, from early prototyping through successful infusion of hardware on the Mars Perseverance Mars rover and on the Europa Clipper flagship spacecraft. This talk will give an overview of some of the technologies and applications that have been developed as part of this materials and manufacturing effort.

Douglas Hofmann is a Senior Research Scientists, Principal Scientist, and founding member of the Materials Development and Manufacturing Technology Group at JPL. He is founder of the JPL Metallurgy Facility, co-founder of JPL’s Additive Manufacturing Center and has been a Visiting Associate and Lecturer in Applied Physics and Materials Science at Caltech for 15 years. He holds an M.S. and Ph.D. from Caltech in Materials Science and Engineering and a B.S. and M.S. in Mechanical Engineering from U.C. San Diego. He is a recipient of several awards, including the 2012 Presidential Early Career Award for Scientists and Engineers from President Obama, and the inaugural Young Innovator in the Materials Science of Additive Manufacturing Award from The Minerals, Metals and Materials Society. He is a 2024 Fellow of the National Academy of Inventors. Doug has over 35 granted patents in materials and manufacturing and has successfully founded two commercial NASA spinoff companies that are exclusively licensing JPL technology.

Wednesday, October 1, 2025

3:30 PM

Seaver Science Library, Room 202 (SSL 202)

host: Chen

USC Space Engineering Research Center Overview for AME

David Barnhart

Research Professor
Department of Astronautical Engineering
USC
Los Angeles, CA

A jointly supported center between the Department of Astronautical Engineering and the Information Sciences Institute has built up its research portfolio dedicated to Space. The Space Engineering Research Center (or SERC) was formed in 2006 with a single faculty member and now has 9 PhD’s, 3 full time staff, and supports 20-30 masters students and 10 summer intern undergrads on various space related research projects. The goal of the talk is to introduce the basics of SERC, impactful projects, and potential opportunities to help faculty/PhD students in AME in any way to secure additional resources and collaborate on new innovations in the space domain

David Barnhart is an active Research Professor in the Department of Astronautical Engineering at USC, the Director/Co- Founder of the USC Space Engineering Research Center and co-founder/CEO of Arkisys Inc.

David has specialized in developing innovative technologies and architectures for 2nd generation space morphologies, RPO technologies/techniques, and new business applications to in space activities. At USC David specializes in developing innovative technologies and architectures for 2nd generation space morphologies, rendezvous and proximity operations technologies/techniques, and hands-on projects with students, faculty and staff through an “engineering teaching hospital” construct. The SERC created and launched USC’s first three satellites in 2010, 2012 and 2021, and is working on its 4th for 2026. Over 250 students have graduated through the SERC’s hands-on training capabilities and every summer hosts US and international student interns. As part of SERC funded activities USC was part of the creation of CONFERS, and built and launched the first payload onto the ISS Astrobee free flying platform. Patented technology that came out of SERC from David and his students has provided the commercial basis for a company’s creation to develop orbital tugs and debris capture technology, and was featured by NASA on the ISS.

David has served as a senior space Project Manager at DARPA (Defense Advanced Research Projects Agency), pioneering cellular spacecraft morphologies, satbotics, space robotics and low cost high volume manufacturing on the Phoenix and SeeMe projects. He represented the first DARPA space project at the United Nations COPUOS in Vienna Austria addressing new technology pushing the need for updates to space regulations and policy issues for next generation ISAM and space servicing activities. This +$500M program is still operating and will be the first major ISAM mission from the USG in over 20 years.

Prior to USC and DARPA David helped initiate two commercial space companies; co-founding and serving as VP and CFO for Millennium Space Systems in Los Angeles CA (a Boeing Co.); and elected member of a startup in Bremen Germany, Vanguard Space, one of the first companies working commercial spacecraft servicing in early 2000.

David started his career as a civilian for the Air Force Research Labs spending over 13 years helping to birth several notable innovations in micro-miniature electronic technologies, micro-chemical/electric propulsion systems, some of the first small satellites for remote observations, and the first independent RPO missions. David has a BS from Boston University and ME from Virginia Tech both in Aerospace and Ocean engineering. He has published over 80 research papers and articles, holds 10 copyrights and patents, is a Board Member on CONFERS and Industry Oversite Board member on COSMIC (the two major space servicing sectors industry/academic consortiums), and speaks on 2nd generation space technologies nationally and internationally.

Wednesday, October 15, 2025

3:30 PM

Seaver Science Library, Room 202 (SSL 202)

host: Ronney

host: Xu

Integrating Physical Intelligence with Artificial Intelligence: Autonomous Design and Manufacturing of Material Systems