About

Chi-An Yeh is an Assistant Professor in the Department of Mechanical and Aerospace Engineering at NC State University. His research focuses on the intersection of unsteady fluid mechanics, data science, and network science, with particular emphasis on innovating active flow control techniques for unsteady aerodynamic applications. He is also interested in computational fluid dynamics, dynamical systems, and optimization. Prior to joining NC State, Dr. Yeh was a Postdoctoral Scholar at the University of California, Los Angeles. He holds a Ph.D. from Florida State University, an M.S. from National Taiwan University, and a B.S. from National Chiao Tung University, all in Mechanical Engineering.

Research topics

• Mechanics
• Computer Science
• Physics
• Mathematics
• Algorithm
• Mathematical analysis
• Database

Selected publications

• Discrete vortex-based broadcast mode analysis for mitigation of dynamic stall

  Journal of Fluid Mechanics · 2026-01-02

  articleOpen accessSenior author

  We integrate a discrete vortex method (DVM) with complex network analysis to strategise dynamic stall mitigation over aerofoils with active flow control. The objective is to inform the actuator placement and the timing to introduce control inputs during the highly transient process of dynamic stall. To this end, we treat a massively separated flow as a network of discrete vortical elements and quantify the interactions among the vortical nodes by tracking the spread of displacement perturbations between each pair of vortical elements using a DVM. This allows us to perform network broadcast mode analysis to identify an optimal set of discrete vortices, the critical timing and the direction to seed perturbations as control inputs. Motivated by the objective of dynamic stall mitigation, the optimality is defined as maximising the reduction of total circulation of the free vortices generated from the leading edge over a prescribed time horizon. We demonstrate the use of the analysis on a two-dimensional flow over a flat plate aerofoil and a three-dimensional turbulent flow over an SD $7003$ aerofoil. The results from the network analysis reveal that the optimal timing for introducing disturbances occurs slightly after the onset of flow separation, before the shear layer rolls up and forms the core of the dynamic stall vortex. The broadcast modes also show that the vortical nodes along the shear layer are optimal for introducing disturbances, hence providing guidance to actuator placement. Leveraging these insights, we perform nonlinear simulations of controlled flows by introducing flow actuation that targets the shear layer slightly after the separation onset. We observe that the network-guided control results in a $21 \,\%$ and $14\,\%$ reduction in peak lift for flows over the flat plate and SD $7003$ aerofoil, respectively. A corresponding decrease in vorticity injection from the aerofoil surface under the influence of control is observed from simulations, which aligns with the objective of the network broadcast analysis. The study highlights the potential of integrating the DVMs with the network analysis to design an effective active flow control strategy for unsteady aerodynamics.

  PublisherOA PDFDOI

• Using optimal transport aligned latent embeddings for separated flow analysis

  Journal of Fluid Mechanics · 2026-01-16 · 2 citations

  articleOpen access

  Quantifying differences between flow fields is a key challenge in fluid mechanics, particularly when evaluating the effectiveness of flow control or other problem parameters. Traditional vector metrics, such as the Euclidean distance, provide straightforward pointwise comparisons but can fail to distinguish distributional changes in flow fields. To address this limitation, we employ optimal transport (OT) theory, which is a mathematical framework built on probability and measure theory. By aligning Euclidean distances between flow fields in a latent space learned by an autoencoder with the corresponding OT geodesics, we seek to learn low-dimensional representations of flow fields that are interpretable from the perspective of unbalanced OT. As a demonstration, we utilise this OT-based analysis on separated flows past a NACA 0012 airfoil with periodic heat flux actuation near the leading edge. The cases considered are at a chord-based Reynolds number of 23 000 and a free-stream Mach number of 0.3 for two angles of attack (AoA) of $6^\circ$ and $9^\circ$ . For each angle of attack, we identify a two-dimensional embedding that succinctly captures the different effective regimes of flow responses and control performance, characterised by the degree of suppression of the separation bubble and secondary effects from laminarisation and trailing-edge separation. The interpretation of the latent representation was found to be consistent across the two AoA, suggesting that the OT-based latent encoding was capable of extracting physical relationships that are common across the different suites of cases. This study demonstrates the potential utility of optimal transport in the analysis and interpretation of complex flow fields.

  PublisherOA PDFDOI

• Duty-cycle actuation for drag reduction of deep dynamic stall: Insights from linear stability analysis

  ArXiv.org · 2025-01-14 · 1 citations

  preprintOpen accessSenior author

  A flow control framework based on linear stability analysis is proposed focusing on reducing the aerodynamic drag due to dynamic stall through a finite-window temporal actuation. The methodology is applied on a periodically plunging SD7003 airfoil.Finite-time Lyapunov exponent (FTLE) fields reveal a saddle point near the airfoil leading edge, where a shear layer forms and feeds a dynamic stall vortex (DSV). A local stability analysis conducted at this saddle point identifies a Kelvin-Helmholtz instability, and the most unstable eigenvalue frequencies remain constant when the variation in the effective angle of attack is minimal. The findings from the FTLE fields and the stability analysis are used to inform the position and finite duty cycle of a periodic blowing and suction actuation applied in a wall-resolved large eddy simulation (LES). The present framework reduces the actuation duty cycle by 77.5% during the airfoil plunging motion, while maintaining the same performance as a continuous actuation throughout the entire cycle. The LES results demonstrate that disturbances from the stability-analysis-informed actuation modify the leading-edge dynamics, preventing the formation of the coherent DSV and significantly reducing the drag.

  PublisherOA PDFDOI

• On the Interactions Between Wake and Tip-Vortex Instabilities in Asymmetric Flows Over Finite Wings

  2025-01-03

  articleSenior author

  The influence of flow asymmetries on the coupling between wake and wing-tip vortex instabilities in flows over a finite wing is investigated using incompressible resolvent and spatio-temporal harmonic resolvent analyses. Here, we focus on a symmetry-enforced and a asymmetrically perturbed wake flows downstream of a finite-span NACA 0012 wing with an aspect-ratio of 2.5 at a chord-based Reynolds number of 1000 and angle of attack of 10 degrees. In a parallel-flow setting, the resolvent analysis is performed about the time and streamwise-averaged velocity field in a box downstream of the wing; meanwhile, the spatio-temporal harmonic resolvent analysis adopts the dominant harmonics extracted from the space-time Fourier transform of the velocity in the box as the base flow, where periodicity in streamwise direction is assumed. To study the effect of flow asymmetries on the vortex dynamics, the results of these analyses are compared to those obtained from a pair of Batchelor vortices as the base flow. We observe that flow asymmetries result in higher amplification of the perturbations, compared to symmetric vortex pairs. Also, the presence of the wake significantly changes the dynamics of the perturbations, as the most energetic disturbances are the result of the interaction between wake and the wing-tip vortices. Spatio-temporal harmonic resolvent analysis shows that the cross-frequency interaction results in instability coupling between wake and tip-vortices at the first harmonic, which is not observed in regular resolvent analysis. It also shows that instabilities at higher harmonics are dominated by the wake, whereas regular resolvent analysis indicates that instabilities develop over the stronger wing-tip vortex. The cross-frequency map of the harmonic resolvent operator highlights a strong interaction between the first and second harmonics, leading to the amplification of perturbations primarily in the wake region with minor contribution over the wing-tip vorticies. This interaction leads to the wake-dominated modes at the second harmonics of the time and space, which deviates from resolvent analysis modes. It also reveals the presence of absolute instability waves that propagate upstream in both the wake and wing-tip vortex regions due to perturbations introduced in these regions.

  PublisherDOI

• A CFD-informed barn-level swine disease dissemination model and its use for ventilation optimization

  Epidemics · 2025-05-24 · 2 citations

  articleOpen accessSenior author

  The airborne spread of infectious livestock diseases plays a crucial role in the propagation of epidemics, particularly in populations confined to densely populated facilities, such as commercial swine barns. In this study, we present a framework to study airborne disease dissemination within commercial swine barns and facilitate the strategic design of control actions, including optimization of ventilation and placement of sick animals (sick pen). This framework is based on a susceptible-infected-recovered (SIR) model that accounts for the between-pen disease spread within swine barns. A pen-to-pen contact network is used to construct a transmission matrix according to the transport of airborne respiratory pathogens across pens in the barns, via our Reynolds-averaged Navier–Stokes computational fluid dynamics (CFD) solver. By employing this CFD-augmented SIR model, we demonstrated that the location of the sick pen and the barn ventilation configuration played crucial roles in modifying disease dissemination dynamics at the barn level. In addition, we examined the effect of natural ventilation through different curtain adjustments. We observed that curtain adjustments either suppress the disease spread by an average of 64.8% or exacerbate the outbreak potential by an average of 5.8%, compared to the scenario where side curtains are not raised. Furthermore, we optimize the ventilation configuration via the selection and placement of ventilation fans through the integration of the CFD-augmented framework with the genetic algorithm to minimize the dissemination of swine disease within barns. Compared to the original barn ventilation settings, our optimized ventilation system significantly reduced disease spread by an average of 20%. Our study demonstrates that the use of the proposed framework provides a detailed understanding of the flow physics and the transport of airborne pathogens, which facilitate the optimization of ventilation systems and strategic management of sick pens within the swine barns. • We presented a CFD-augmented disease dissemination model to predict the airborne disease spread dynamics in swine barns. • Ventilation configuration and the sick pen location in mitigating the spread of disease within the barn was investigated. • Natural ventilation for curtain-sided facility can either suppress the disease spread or exacerbate the outbreak situation. • Optimized ventilation configuration, including number of fans, types of the fans, and their locations on the barn boundaries, can significantly mitigate disease transmission.

  PublisherDOI

• Control of Deep Dynamic Stall by Duty-Cycle Actuation Informed by Stability Analysis

  AIAA Journal · 2025-07-02 · 4 citations

  articleSenior author

  A flow control framework based on linear stability analysis is proposed, focusing on reducing the aerodynamic drag due to dynamic stall through a finite-window temporal actuation. The methodology is applied to a periodically plunging SD7003 airfoil. Finite-time Lyapunov exponent (FTLE) fields reveal a saddle point near the airfoil leading edge, where a shear layer forms and feeds a dynamic stall vortex (DSV). A local stability analysis conducted at this saddle point identifies a Kelvin–Helmholtz instability, and the most unstable eigenvalue frequencies remain constant when the variation in the effective angle of attack is minimal. The findings from the FTLE fields and the stability analysis are used to inform the position and finite duty cycle of a periodic blowing and suction actuation applied in a wall-resolved large-eddy simulation (LES). The present framework reduces the actuation duty cycle by 77.5% during the airfoil plunging motion while maintaining the same performance as a continuous actuation throughout the entire cycle. The LES results demonstrate that disturbances from the stability-analysis-informed actuation modify the leading-edge dynamics, preventing the formation of the coherent DSV and significantly reducing the drag.

  PublisherDOI

• Using Optimal Transport Aligned Latent Embeddings for Separated Flow Analysis

  ArXiv.org · 2025-09-09

  preprintOpen access

  Quantifying differences between flow fields is a key challenge in fluid mechanics, particularly when evaluating the effectiveness of flow control. Traditional vector metrics, such as the Euclidean distance, provide straightforward pointwise comparisons but can fail to distinguish distributional changes in flow fields. To address this limitation, we employ optimal transport (OT) theory, which is a mathematical framework built on probability and measure theory. By aligning Euclidean distances between flow fields in a latent space learned by an autoencoder with the corresponding OT geodesics, we seek to learn low-dimensional representations of flow fields that are interpretable from the perspective of unbalanced OT. As a demonstration, we utilize this OT-based analysis on controlled, separated flows past a NACA 0012 airfoil with a chord-based Reynolds number of 23,000 and a freestream Mach number of 0.3 for two angles of attack of $6^\circ$ and $9^\circ$. For each angle of attack, we identify a two-dimensional embedding that succinctly captures the different effective regimes of flow responses and control performance, characterized by the degree of suppression of the separation bubble and secondary effects from laminarization and trailing-edge separation. The interpretation of the latent representation was found to be consistent across the two angles of attack, suggesting that the OT-based latent encoding was capable of extracting physical relationships that are common across the different suites of cases. This study demonstrates the potential utility of optimal transport in the analysis and interpretation of complex flow fields.

  PublisherOA PDFDOI

• Modeling the transmission dynamics of African swine fever virus within commercial swine barns: Quantifying the contribution of multiple transmission pathways

  Epidemics · 2025-04-24 · 4 citations

  articleOpen access

  The transmission of African swine fever virus (ASFV) within swine barns occurs through direct and indirect pathways. Identifying and quantifying the roles of ASFV dissemination within barns is crucial for developing disease control strategies. We created a stochastic transmission model to examine the ASFV dissemination dynamics through transmission routes within commercial swine barns. We consider seven transmission routes at three disease dynamics levels: within-pens, between-pens, and within-room transmission, along with the transfer of pigs between pens within rooms. We simulated ASFV spread within barns of various sizes and layouts from rooms with a median of 32 pens (IQR: 28-40), where each pen housed a median of 34 pigs (IQR: 29-36). Our model enables tracking the viral load in each pen and monitoring the disease status at the pen level. Results show that between-pen transmission pathways exhibited the highest contribution in spread, accounting for 66.76%, whereas within-pen and within-room pathways account for 26.12% and 7.12%, respectively. Nose-to-nose contact between pens was the primary dissemination route, comprising an average of 46.04%. On the other hand, aerosol transmission within pens had the lowest contribution, accounting for less than 1%. Furthermore, we show that the daily transfer of pigs between pens did not impact the spread of ASFV. On average, at the room level, the combined approach of passive daily surveillance and mortality-focused surveillance enabled ASFV detection within 18 (IQR: 16-19) days. The model allows us to monitor the viral load variation across the room over time, revealing that most of the viral load accumulates in pens closer to the exhaust fans after a month. This work significantly deepens our understanding of ASFV spread within commercial swine production farms in the U.S. and highlights the main transmission pathways that should be prioritized when implementing ASFV countermeasure actions at the room level.

  PublisherDOI

• Effectiveness of Critical Leading-Edge Suction from 2D CFD in Predicting Dynamic-Stall Onset on a Rotor

  2025-05-20

  article

  Dynamic stall is an undesirable flow phenomenon that could occur on rotor blades of helicopters in forward flight due to azimuthal changes in local angle of attack resulting from blade motion, blade deformation and blade-vortex interactions. It is characterized by leading-edge vortex (LEV), or dynamic-stall vortex (DSV) shedding and significantly affects rotor performance and longevity. Therefore, the capability to predict dynamic stall, especially using rapid low-order approaches, is beneficial for vehicle design and flight-dynamics simulation. Recent work has resulted in the development of a theoretical parameter called leading-edge section parameter (LESP), which provides a measure of the suction force acting on the leading edge. It has been shown that the occurrence of dynamic stall on airfoils and finite wings corresponds to the time in an unsteady motion when the instantaneous LESP crosses a predetermined critical value. The current work shows that the critical LESP value, determined from relatively inexpensive 2D computational fluid dynamics (CFD) on an airfoil undergoing pitch and surge motions, can be used to predict the onset of dynamic stall on the section of a rotor blade in forward flight.

  PublisherDOI

• An Invitation to Resolvent Analysis

  arXiv (Cornell University) · 2024-04-17

  preprintOpen access

  Resolvent analysis is a powerful tool that can reveal the linear amplification mechanisms between the forcing inputs and the response outputs about a base flow. These mechanisms can be revealed in terms of a pair of forcing and response modes and the associated gains (amplification magnitude) in the order of energy contents at a given frequency. The linear relationship that ties the forcing and the response is represented through the resolvent operator (transfer function), which is constructed through spatially discretizing the linearized Navier-Stokes operator. One of the unique strengths of resolvent analysis is its ability to analyze statistically stationary turbulent flows. In light of the increasing interest in using resolvent analysis to study a variety of flows, we offer this guide in hopes of removing the hurdle for students and researchers to initiate the development of a resolvent analysis code and its applications to their problems of interest. To achieve this goal, we discuss various aspects of resolvent analysis and its role in identifying dominant flow structures about the base flow. The discussion in this paper revolves around the compressible Navier-Stokes equations in the most general manner. We cover essential considerations ranging from selecting the base flow and appropriate energy norms to the intricacies of constructing the linear operator and performing eigenvalue and singular value decompositions. Throughout the paper, we offer details and know-how that may not be available to readers in a collective manner elsewhere. Towards the end of this paper, examples are offered to demonstrate the practical applicability of resolvent analysis, aiming to guide readers through its implementation and inspire further extensions. We invite readers to consider resolvent analysis as a companion for their research endeavors.

  PublisherOA PDFDOI

Frequent coauthors

• Kunihiko Taira

  University of California, Los Angeles

  50 shared

• C. Elachi

  16 shared

• Jean Hélder Marques Ribeiro

  Universidade Estadual de Campinas (UNICAMP)

  14 shared

• Bernd R. Noack

  14 shared

• L. Bergman

  12 shared

• K. F. Casey

  11 shared

• F. I. Shimabukuro

  9 shared

• John Michael Morookian

  Jet Propulsion Laboratory

  9 shared