About

Ashok Gopalarathnam is a professor in the Department of Mechanical and Aerospace Engineering at NC State University. He directs the NCSU Applied Aerodynamics Group and is committed to developing innovative ideas, concepts, and methodologies for the design of aircraft, other types of vehicles, and alternative-energy systems. His research interests include applied aerodynamics, flight mechanics, aircraft design, adaptive aircraft, and design methodologies. At the graduate level, Dr. Gopalarathnam teaches courses such as Airfoil Theory and Wing Theory, where students explore practical requirements in aerodynamics beyond theoretical methods. For undergraduates, he teaches Aerodynamics of V/STOL vehicles and Flight Vehicle Stability and Control, involving team projects like designing, building, and flying micro-gliders. His students work on a variety of aero-hydrodynamic systems, including sailboats, race cars, aircraft, and wind turbines. Outside of his academic pursuits, he spends time with his family and enjoys recreational flying.

Research topics

• Computer Science
• Physics
• Mechanics
• Engineering
• Meteorology
• Aerospace engineering
• Simulation
• Aeronautics
• Mathematics
• Geography

Selected publications

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

  Journal of Fluid Mechanics · 2026-01-02

  articleOpen access

  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

• Investigation of thrust augmentation in multi-foil flapping propulsion systems using a low-order hydrodynamic model

  Ocean Engineering · 2026-02-17

  articleSenior author
  PublisherDOI

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

  2025-05-20

  articleSenior author

  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

• Optimal Cyclic Control of a Structurally Constrained Morphing Energy-Harvesting Kite Using an Experimentally Validated Simulation Model

  IEEE Transactions on Control Systems Technology · 2025-01-09 · 2 citations

  article

  This work presents an experimentally validated dynamic model, control trajectory optimization methodology, and representative simulation results for a morphing underwater kite. Morphing, defined as real-time modification of the kite’s geometry to either curtail structural loading or enhance power generation, is motivated by the fact that the optimal design of an energy-harvesting kite is highly sensitive to flow speed and tether length, particularly in the presence of structural limitations that render load curtailment necessary at high flow speeds and short tether lengths. To achieve morphing behavior, an inboard Fowler flap (capable of modifying the chord and camber of an inboard wing section) was employed in tandem with a symmetric aileron bias, enabling simultaneous control over both the wing’s overall lift coefficient and center of lift without requiring the mechanical complexity associated with span morphing. The effects of these morphing parameters were integrated into an existing dynamic simulation framework, and experiments were conducted using a customized scaled tow testing setup to refine and experimentally validate the simulation model. Following the refinement of this model, a morphing trajectory optimizer was designed to optimize the morphing input trajectories over a spooling cycle using flow data from the previous cycle. Finally, using the refined simulation model and multicycle controller, simulations of large-scale kites operating in a realistic flow environment were conducted. In these simulations, a kite capable of morphing was shown to generate between 8.1% and 25.3% more energy than non-morphing kite designs.

  PublisherDOI

• Effect of rounded trailing edges on unsteady airfoil loading at low reynolds numbers

  Theoretical and Computational Fluid Dynamics · 2025-07-05

  articleOpen accessSenior author

  Abstract The steady potential flow past a traditional airfoil with a round leading edge and a sharp trailing edge can usually be simulated using the assumption of Kutta condition at the trailing edge. However, for the airfoil undergoing unsteady motion, especially at high reduced frequencies, numerical and experimental studies have shown that the flow can curve around the trailing edge, resulting in the stagnation point moving away from the trailing edge. This phenomenon becomes increasingly apparent when the airfoil has a round trailing edge instead of the usual sharp one. Inspired by the success of using leading-edge suction force to represent the flow turn-around at the leading edge and the associated vortex shedding, this work introduces the trailing-edge suction force and connects it to the trailing-edge unsteady flow physics. In this work, the effect of trailing edge roundness on the unsteady airfoil flow is studied by generating airfoil shapes with various amounts of roundness. Computational fluid dynamics (CFD) studies of unsteady flow past airfoils with different round trailing edges are performed to study the effects of the trailing-edge suction force on the flowfield. A composite pressure-difference model, universally valid on the entire airfoil, is derived in this work to take into account the edge radii and the corresponding edge-suction effects. We show that, in scenarios where the stagnation point moves away from the trailing edge, a trailing-edge suction force, associated with the flow curving around the trailing edge, is necessary to better estimate the airfoil unsteady load distribution. Graphic abstract

  PublisherOA PDFDOI

• Automatic Limit Cycle Oscillation Annihilation in Aeroelastic Wings Using Prescribed Impinging Vortices

  AIAA Journal · 2025-10-28

  article
  PublisherDOI

• A Uniformly Convergent Series Expansion for the Bound Vorticity Distribution in the Unsteady Thin Airfoil Theory

  2025-01-03 · 2 citations

  article

  Classic reduced-order modeling approaches frequently rely on linear distributions of two-dimensional potential flow solutions. The solutions are not unique under specific boundary conditions and require the application of the well-known Kutta condition to determine the appropriate airfoil circulation. Traditionally, the Kutta condition is achieved by imposing zero vorticity at the trailing edge. In addition, contemporary theoretical and mathematical formulations of the unsteady Kutta condition state that the free and bound vortex sheets that meet at zero-angle edges are expected to have not only finite velocities at that point but also to be continuous there. To illustrate that, harmonically oscillating airfoils are analyzed using the Theodorsen model, and a point-wise discontinuity in the vortex sheet is observed across the trailing edge. Hence uniformly convergent series are employed to establish a valid vorticity distribution formula, and the unsteady Kutta condition is fully satisfied.

  PublisherDOI

• Utilizing Machine Learning to Predict Limit Cycle Oscillation Characteristics in Aeroelastic Wing

  2024-01-04

  articleSenior author

  Aeroelastic flutter is the oscillatory response of a structure to specific aerodynamic forces and speeds, that often results in a boundless increase in amplitude. One specific type of flutter, known as limit cycle oscillations (LCO), is instead bounded by a maximum possible amplitude. This bounded oscillation can have a multitude of effects, ranging from positives like energy harvesting, to negatives like structural damage to aircraft. As such, the ability to control this type of response is incredibly useful. To address this, an upstream vortex production system has been employed to impart specific aerodynamic forces onto a downstream wing in a wind tunnel test setting. With the wing’s structural parameters known, the frequency of oscillation while in LCO can be calculated. Using both a static bluff body tailored to produce a certain vortex shedding frequency, and a variable frequency disturbance generator that can oscillate at prescribed frequency, LCO excitation and annihilation has been observed. Specifically, using the variable frequency disturbance generator has shown that a window of frequencies surrounding the LCO frequency exists, in which LCO can be excited by upstream vortices. However, the wing response to even the variable frequency disturbances is difficult to predict, so machine learning is employed to better model the system. In particular, a recurrent neural network (RNN) is trained on the wing motion data to be able to accurately predict upcoming states of the wing, which will allow for better control of the wing response.

  PublisherDOI

• Lift tailoring on unsteady airfoils with leading-edge vortex shedding using an inverse aerodynamic approach

  Physics of Fluids · 2024-05-01 · 1 citations

  articleSenior author

  In this paper, we present a physics-informed approach to tailor the lift profile of an unsteady airfoil through the execution of an appropriate maneuver. In previous research, a low-order aerodynamic model based on the unsteady thin airfoil theory was developed for predicting the flowfield and loads on airfoils undergoing arbitrary motions. The theory was phenomenologically augmented using the concept of leading edge suction parameter (LESP) to incorporate the capability to predict intermittent leading edge vortex (LEV) shedding. The criticality of LESP was used to predict the onset and termination of LEV shedding and thus model the effect of LEVs on the flowfield and loads for a prescribed motion. In the current work, an inverse aerodynamic formulation is developed based on this framework for tackling the inverse problem: to obtain the motion kinematics required for generating a prescribed lift profile for an airfoil operating in the dynamic-stall regime. The LEV-modeling capability of the aerodynamic model enables the motion-design algorithm to take into account the effect of complex phenomena, such as dynamic stall and LEV shedding, which are not taken into account in previous research approaches. Several case studies are presented to demonstrate various scenarios such as lift tracking using pitching and heaving motions, lift cancellation during unsteady motion, and the generation of a given lift profile using two equivalent motions. The kinematic profiles generated by the inverse formulation are also simulated using a high-fidelity unsteady computational fluid dynamics solver to validate the predictions.

  PublisherDOI

• Effect of Rounded Trailing Edges on Unsteady Airfoil Loading at Low Reynolds Numbers

  Research Square · 2024-07-11

  preprintOpen accessSenior author
  PublisherOA PDFDOI

Frequent coauthors

• Jack R. Edwards

  North Carolina State University

  25 shared

• Matthew Bryant

  North Carolina State University

  22 shared

• Arun Vishnu Suresh Babu

  University of North Carolina at Charlotte

  20 shared

• Kiran Ramesh

  Arizona State University

  18 shared

• Michael Ol

  Folder Media

  16 shared

• Michael S. Selig

  University of Illinois Urbana-Champaign

  15 shared

• Kenneth Granlund

  13 shared

• Shreyas Narsipur

  Mississippi State University

  12 shared

Labs

• Applied Aerodynamics GroupPI