@article{ramanathan_gopalarathnam_2023, title={Prediction of leading-edge-vortex initiation using criticality of the boundary layer}, volume={5}, ISSN={["1432-2250"]}, DOI={10.1007/s00162-023-00648-z}, abstractNote={The initiation of leading-edge-vortex formation in unsteady airfoil flows is governed by flow criticality at the leading edge. While earlier works demonstrated the promise of criticality of leading-edge suction in governing LEV shedding, this criterion is airfoil and Reynolds number dependent. In this work, by examining results from Navier–Stokes computations for a large set of pitching airfoil cases at laminar flow conditions, we show that the onset of flow reversal at the leading edge always corresponds to the boundary-layer shape factor reaching the same critical value that governs laminar flow separation in steady airfoil flows. Further, we show that low-order prediction of this boundary-layer criticality is possible with an integral-boundary-layer calculation performed using potential-flow velocity distributions from an unsteady panel method. The low-order predictions agree well with the high-order computational results with a single empirical offset that is shown to work for multiple airfoils. This work shows that boundary-layer criticality governs LEV initiation, and that a low-order prediction approach is capable of predicting this boundary-layer criticality and LEV initiation.}, journal={THEORETICAL AND COMPUTATIONAL FLUID DYNAMICS}, publisher={Springer Science and Business Media LLC}, author={Ramanathan, Hariharan and Gopalarathnam, Ashok}, year={2023}, month={May} }
 @article{ramanathan_gopalarathnam_2022, title={Prediction of Vortex Initiation using an Unsteady Panel Method with an Integral-Boundary-Layer Calculation}, DOI={10.2514/6.2022-3897}, abstractNote={In our current work, we implement and evaluate the efficacy of a boundary layer parameter ( 32 ) based low-order model to signal the onset of leading-edge vortex (LEV) initiation for low Re flows (O (10 4 )) .In an earlier work(AIAA Aviation Forum 2019, p.3590) using numerical simulations, it was proven that the shape factor parameter ( 32 ) attains a critical value of ∼ 1.52 prior to the formation of dynamic stall vortex (DSV) irrespective of airfoil, motion kinematics.Numerical simulations spanned high pitch rate motions ( ≥ 0.1) and three leading edge radii (0.5 ≤ ≤ 2.0) for three different pivot locations : leading-edge (LE), quarter-chord (QC) and half-chord (HC).This finding concurred with the 32 value reported in literature for laminar separation ( 32 = 1.515) in steady incompressible flows.In the second part of the effort, we focus on low-order prediction of LEV initiation using this criticality.This was achieved using an inviscid calculation of unsteady-airfoil velocity and pressure distributions, which serve as an input to a quasi-steady integral boundary layer (IBL) method.The results provided here demonstrate the effectiveness of 32 in predicting leading-edge flow reversal.We show, by analysing different cases, that this inexpensive method has crucial ramifications in picking the appropriate for any given airfoil and motion combination.Hence, we eliminate the need for CFD, Experiments to obtain for a new parameter set.}, journal={AIAA AVIATION 2022 Forum}, publisher={American Institute of Aeronautics and Astronautics}, author={Ramanathan, Hariharan and Gopalarathnam, Ashok}, year={2022}, month={Jun} }
 @article{ramanathan_narsipur_gopalarathnam_2019, title={Boundary-Layer Characteristics at the Onset of Leading-Edge Vortex Formation on Unsteady Airfoils}, DOI={10.2514/6.2019-3590}, journal={AIAA Aviation 2019 Forum}, publisher={American Institute of Aeronautics and Astronautics}, author={Ramanathan, Hariharan and Narsipur, Shreyas and Gopalarathnam, Ashok}, year={2019}, month={Jun} }