About

Margaret Byron Trethewey is an Early Career Professor in the Department of Mechanical Engineering at Penn State University. Her research areas include biomechanics and mechanobiology, as well as experimental fluid dynamics. Her specific interest areas encompass multiphase flow, turbulence, animal locomotion, and intermediate Reynolds number phenomena. She holds a B.S. in Mechanical and Aerospace Engineering from Princeton University, obtained in 2010, and both a M.S. and Ph.D. in Civil and Environmental Engineering from the University of California, Berkeley, completed in 2012 and 2015 respectively. Her work involves investigating complex fluid dynamics and biological systems, contributing to the understanding of animal movement and fluid behavior at various scales.

Research topics

• Geography
• Political Science
• Computer Science
• Sociology
• Social Science
• Artificial Intelligence
• Mechanics
• Mathematics
• Physics
• Medicine
• Management science
• Geometry
• Classical mechanics
• Data science
• Nursing
• Engineering

Selected publications

• Creating Public Resources to Diversifying Content in Mechanical Engineering: Fostering Awareness and Ethical Considerations

  2025-08-21

  article
  PublisherDOI

• Hydromechanics of ammonoid conch ornamentation: trade-offs between rocking attenuation and drag reduction

  Paleobiology · 2025-08-01 · 1 citations

  articleOpen access

  Abstract Ammonoid cephalopods are excellent model systems for evolutionary biomechanics due to their volatile evolutionary dynamics and remarkable fossil record. During the Mesozoic marine revolution, natural selection increasingly favored ammonoid shells with specific ranges of ornamentation patterns (projections that influence surface roughness). While this evolutionary pattern has been attributed to enemy-driven evolution (i.e., escalation), many morphologies lack clear defensive roles. Using a combination of 3D modeling, physical experiments, and computer simulations, we investigate these patterns from a hydromechanical perspective. We model theoretical morphologies along a continuum of increasing ornamentation coarseness. Neutrally buoyant, 3D-printed models, weighted to match the mass distribution of their virtual counterparts, demonstrate that coarser patterns progressively attenuate rocking motions. Flow visualization experiments reveal these coarser patterns produce higher energy dissipation rates in the disturbed fluid. Computational fluid dynamics simulations were performed to characterize the hydrodynamic costs of ornamentation patterns over the majority of biologically relevant swimming speeds and shell sizes for planispiral ammonoids. Only the coarsest categories incur substantial increases in hydrodynamic drag. However, ornamentation patterns with intermediate coarseness effectively avoid this physical trade-off, experiencing dynamic stabilization without considerably reducing swimming efficiency. These trade-off-defying morphologies were progressively favored during the Mesozoic, becoming more abundant than others by the end of this era. Ultimately, these experiments highlight important hydromechanical selective pressures involved in ammonoid evolutionary trends and some fundamental constraints on aquatic locomotion more broadly.

  PublisherOA PDFDOI

• A Computational Analysis of Fluid-Structure Interaction in Metachronal Propulsion

  2024-07-15

  article

  Abstract Ctenophores employ flexible rows of appendages called ctenes that form the metachronal beating pattern. A complete cycle of such paddling consists of a power stroke that strokes backward to produce propulsion and a recovery stroke that allows the appendage to recover its initial position. Effective locomotion in these creatures relies on maximizing propulsion during the power stroke while minimizing drag in the recovery stroke. Unlike rigid oars, the ctenes are flexible during both the power stroke and the recovery stroke, and notably, their strokes are asymmetric, with faster movement during the power stroke. As previous research assumed uniform material properties. This assumption will eventually make the ctene deform more intensively in the power stroke than the recovery stroke due to the asymmetrical hydrodynamic forces. However, observations contradict these assumptions. One explanation posits that ctenes stiffen during the power stroke, enhancing their propulsive force, and become more flexible in the recovery stroke, reducing drag by minimizing the water-countering area. This study focusses on the influence of asymmetric stiffness on their propulsion mechanism. Inspired by nature, we conducted three-dimensional fluid-structure interaction (FSI) using an in-house immersed-boundary-method-based flow solver integrated with a nonlinear finite-element solid-mechanics solver. This integrated solver uses a two-way coupling that ensures a higher accuracy regarding the complexity due to the involvement of the multiple ctenes in a ctene row. The preliminary results show that the anisotropic stiffness of the ctene have better accuracy of deformation as compared to the deformation recorded by the high-speed camera. The asymmetric properties of the ctene material allow both the spatial and temporal asymmetry of the ctene beating pattern. Our investigation suggests that while symmetrical beating can only generate negative net thrust, a slightly asymmetrical beating can make the thrust positive. We find that power stroke period that cost 30% whole period can generates the highest thrust. As multiple ctenes involves, the interaction among ctenes can amplified the effects of the asymmetrical beating, so that the thrust generation is enhanced by 9 to 13 times because of it.

  PublisherDOI

• Fluid-Structure Interaction Analysis of Metachronal Propulsion at Intermediate Reynolds Numbers

  2024-11-17

  article

  Abstract Ctenophores swim using flexible rows of appendages called ctenes that form the metachronal paddling. To generate propulsion, each appendage operates a power stroke that strokes backward, followed by a recovery stroke that allows the appendage to readjust its position. Notably, strokes of most metachronal swimmers are asymmetric, with faster power strokes while slower recovery strokes. Previously, the material properties are assumed as isotropic. So, the faster power stoke will lead to more pronounce deformation and the slower recovery stroke will lead to less deformation. However, this contradicts with the observations that power-stroking ctenes have the least deformation and recover deforms more, indicating an anisotropic material behavior. Such anisotropic material is hard to be manufactured, but the anisotropic behavior may be achieved by making the initial structural shape curved. The pre-curved ctene, that bending towards downstream, will be straighten in power stoke while easy to bend during recovery stroke. Our study aims to demonstrate the feasibility of using pre-curved shapes to achieve anisotropic material properties during metachronal swimming. Treating it as fluid-structure interaction (FSI) problem, we integrate our in-house computational fluid dynamics (CFD) solver with a finite element method (FEM) solver, utilizing strong coupling methods for convergence. By comparing the performance of pre-curved ctenes with straight ones, which represent isotropic material properties, we found that the curved ctenes exhibited 26.05% to 65.69% higher cycle-averaged thrust compared to the straight one as stiffness is lower. However, as stiffness increased, the pre-curved ctenes produced 3.92% to 30.58% less thrust than the straight ones. Similar trends were observed in propulsive efficiency, with the pre-curved ctenes demonstrating 46.97% better efficiency at the lowest stiffness but dropping to 34.02% less efficient as stiffness rise. Thus, while the pre-curved initial shape led to better performance at lower stiffness, exceeding a certain stiffness threshold resulted in worse performance compared to straight ctenes. The thrust enhancement from pre-curve shape is due to the drag reduction during recovery stroke, where the curved shape mitigate part of force to point more downward.

  PublisherDOI

• Encoding spatiotemporal asymmetry in artificial cilia with a ctenophore-inspired soft-robotic platform

  Bioinspiration & Biomimetics · 2024-09-10 · 2 citations

  articleOpen accessSenior author

  A remarkable variety of organisms use metachronal coordination (i.e. numerous neighboring appendages beating sequentially with a fixed phase lag) to swim or pump fluid. This coordination strategy is used by microorganisms to break symmetry at small scales where viscous effects dominate and flow is time-reversible. Some larger organisms use this swimming strategy at intermediate scales, where viscosity and inertia both play important roles. However, the role of individual propulsor kinematics-especially across hydrodynamic scales-is not well-understood, though the details of propulsor motion can be crucial for the efficient generation of flow. To investigate this behavior, we developed a new soft robotic platform using magnetoactive silicone elastomers to mimic the metachronally coordinated propulsors found in swimming organisms. Furthermore, we present a method to passively encode spatially asymmetric beating patterns in our artificial propulsors. We investigated the kinematics and hydrodynamics of three propulsor types, with varying degrees of asymmetry, using Particle Image Velocimetry and high-speed videography. We find that asymmetric beating patterns can move considerably more fluid relative to symmetric beating at the same frequency and phase lag, and that asymmetry can be passively encoded into propulsors via the interplay between elastic and magnetic torques. Our results demonstrate that nuanced differences in propulsor kinematics can substantially impact fluid pumping performance. Our soft robotic platform also provides an avenue to explore metachronal coordination at the meso-scale, which in turn can inform the design of future bioinspired pumping devices and swimming robots.

  PublisherDOI

• Settling of nonuniform cylinders at intermediate Reynolds numbers

  Physical Review Fluids · 2024-07-03 · 7 citations

  articleOpen accessSenior author

  For sedimenting nonspherical particles at finite Reynolds numbers, very small offsets in the center of mass (less than 0.05% of particle length) can dramatically alter settling behavior. Nonuniformity in mass distribution enhances lateral dispersion and alters overall settling velocity; small changes in particle orientation lead to the onset of wake features which can either stabilize or destabilize the particle's trajectory, bifurcating over a relatively narrow range of Reynolds number. These results carry implications for a variety of natural and engineered processes, such as the transport and settling of microplastics and/or multimaterial aggregates in the environment.

  PublisherOA PDFDOI

• Encoding spatiotemporal asymmetry in artificial cilia with a ctenophore-inspired soft-robotic platform

  arXiv (Cornell University) · 2024-07-18

  preprintOpen accessSenior author

  A remarkable variety of organisms use metachronal coordination (i.e., numerous neighboring appendages beating sequentially with a fixed phase lag) to swim or pump fluid. This coordination strategy is used by microorganisms to break symmetry at small scales where viscous effects dominate and flow is time-reversible. Some larger organisms use this swimming strategy at intermediate scales, where viscosity and inertia both play important roles. However, the role of individual propulsor kinematics - especially across hydrodynamic scales - is not well-understood, though the details of propulsor motion can be crucial for the efficient generation of flow. To investigate this behavior, we developed a new soft robotic platform using magnetoactive silicone elastomers to mimic the metachronally coordinated propulsors found in swimming organisms. Furthermore, we present a method to passively encode spatially asymmetric beating patterns in our artificial propulsors. We investigated the kinematics and hydrodynamics of three propulsor types, with varying degrees of asymmetry, using Particle Image Velocimetry and high-speed videography. We find that asymmetric beating patterns can move considerably more fluid relative to symmetric beating at the same frequency and phase lag, and that asymmetry can be passively encoded into propulsors via the interplay between elastic and magnetic torques. Our results demonstrate that nuanced differences in propulsor kinematics can substantially impact fluid pumping performance. Our soft robotic platform also provides an avenue to explore metachronal coordination at the meso-scale, which in turn can inform the design of future bioinspired pumping devices and swimming robots.

  PublisherOA PDFDOI

• Propulsive efficiency of spatiotemporally asymmetric oscillating appendages at intermediate Reynolds numbers

  Bioinspiration & Biomimetics · 2024-09-13 · 4 citations

  articleOpen accessSenior author

  Many organisms use flexible appendages for locomotion, feeding, and other functional behaviors. The efficacy of these behaviors is determined in large part by the fluid dynamics of the appendage interacting with its environment. For oscillating appendages at low Reynolds numbers, viscosity dominates over inertia, and appendage motion must be spatially asymmetric to generate net flow. At high Reynolds numbers, viscous forces are negligible and appendage motion is often also temporally asymmetric, with a fast power stroke and a slow recovery stroke; such temporal asymmetry does not affect the produced flow at low Reynolds numbers. At intermediate Reynolds numbers, both viscous and inertial forces play non-trivial roles---correspondingly, both spatial and temporal asymmetry can strongly affect overall propulsion. Here we perform experiments on three robotic paddles with different material flexibilities and geometries, allowing us to explore the effects of motion asymmetry (both spatial and temporal) on force production. We show how a flexible paddle's time-varying shape throughout the beat cycle can reorient the direction of the produced force, generating both thrust and lift. We also evaluate the propulsive performance of the paddle by introducing a new quantity, which we term "integrated efficiency". This new definition of propulsive efficiency can be used to directly evaluate an appendage's performance independently from full-body swimming dynamics. Use of the integrated efficiency allows for accurate performance assessment, generalization, and comparison of oscillating appendages in both robotic devices and behaving organisms. Finally, we show that a curved flexible paddle generates thrust more efficiently than a straight paddle, and produces spatially asymmetric motion---thereby improving performance---without the need for complex actuation and controls, opening new avenues for bioinspired technology development.

  PublisherDOI

• Laboratory generation of zero-mean-flow homogeneous isotropic turbulence: non-grid approaches

  Flow · 2023-01-01 · 10 citations

  articleOpen access

  Over the years, many facilities have been developed to study turbulent flow in the laboratory. Homogeneous isotropic turbulence (HIT) with zero mean flow provides a unique environment for investigating fundamental aspects and specific applications of turbulent flow. We provide an extensive overview of laboratory facilities that generate incompressible zero-mean-flow HIT using different types of actuators and configurations. Reviewed facilities cover a variety of geometries and sizes, as well as forcing style (e.g. symmetric versus asymmetric and unsteady versus steady). We divide facilities into four categories, highlighting links between their geometries and the statistics of the flows they generate. We then compare published data to uncover similarities and differences among various turbulence-generation mechanisms. We also compare the decay of turbulence in zero-mean-flow facilities with that observed in wind and water tunnels, and we analyse the connections between flow characteristics and physical aspects of the facilities. Our results emphasize the importance of considering facility geometry and size together with the strength and type of actuators when studying zero-mean-flow HIT. Overall, we provide insight into how to optimally design and build laboratory facilities that generate zero-mean-flow HIT.

  PublisherOA PDFDOI

• A new propulsion enhancement mechanism in metachronal rowing at intermediate Reynolds numbers

  Journal of Fluid Mechanics · 2023-11-10 · 15 citations

  articleOpen access

  Metachronal rowing is a biological propulsion mechanism employed by many swimming invertebrates (e.g. copepods, ctenophores, krill and shrimp). Animals that swim using this mechanism feature rows of appendages that oscillate in a coordinated wave. In this study, we used observations of a swimming ctenophore (comb jelly) to examine the hydrodynamic performance and vortex dynamics associated with metachronal rowing. We first reconstructed the beating kinematics of ctenophore appendages based on a high-speed video of a metachronally coordinated row. Following the reconstruction, two numerical models were developed and simulated using an in-house immersed-boundary-method-based computational fluid dynamics solver. The two models included the original geometry (16 appendages in a row) and a sparse geometry (8 appendages, formed by removing every other appendage along the row). We found that appendage tip vortex interactions contribute to hydrodynamic performance via a vortex-weakening mechanism. Through this mechanism, appendage tip vortices are significantly weakened during the drag-producing recovery stroke. As a result, the swimming ctenophore produces less overall drag, and its thrust-to-power ratio is significantly improved (up to 55.0 % compared with the sparse model). Our parametric study indicated that such a propulsion enhancement mechanism is less effective at higher Reynolds numbers. Simulations were also used to investigate the effects of substrate curvature on the unsteady hydrodynamics. Our results illustrated that, compared with a flat substrate, arranging appendages on a curved substrate can boost the overall thrust generation by up to 29.5 %. These findings provide new insights into the fluid dynamic principles of propulsion enhancement underlying metachronal rowing.

  PublisherOA PDFDOI

Recent grants

• Collaborative Research: Scaling of ciliary flows at intermediate Reynolds number

  NSF · $319k · 2021–2025

Frequent coauthors

• Evan Variano

  University of California, Berkeley

  23 shared

• Stéphanie Condon

  19 shared

• Adrian Herrera-Amaya

  Pennsylvania State University

  10 shared

• Gabriele Bellani

  8 shared

• Yiheng Tao

  7 shared

• Colin Meyer

  Dartmouth College

  6 shared

• Keith Hoggart

  King's College London

  5 shared

• Цыпылма Дариева

  4 shared

Labs

• PSMES BoardPI

  The PSMES board of directors is made up of elected officers, six to nine at large members, the ME department head, and a mechanical engineering faculty member.

Education

• M.S., Ph.D., Civil and Environmental Engineering

  University of California Berkeley

  2015

• B.S.E., Mechanical and Aerospace Engineering

  Princeton University

  2010