Modeling the Aerodynamic Lift Produced by Oscillating Airfoils at Low Reynolds Number

TL;DR

This paper investigates the unsteady aerodynamic forces on oscillating airfoils at low Reynolds numbers through numerical simulations, spectral analysis, and modeling with a modified van der Pol oscillator, revealing limit cycle behavior and frequency characteristics.

Contribution

It introduces a modified van der Pol oscillator model to accurately predict unsteady lift forces on oscillating airfoils, validated across different motion types and flow conditions.

Findings

Lift forces exhibit period-1 attractor at low Strouhal numbers.

Oscillating lift frequencies include fundamental and harmonic components.

The van der Pol oscillator model effectively captures unsteady lift dynamics.

Abstract

For present study, setting Strouhal Number (St) as control parameter, numerical simulations for flow past oscillating NACA-0012 airfoil at 1,000 Reynolds Numbers (Re) are performed. Temporal profiles of unsteady forces; lift and thrust, and their spectral analysis clearly indicate the solution to be a period-1 attractor for low Strouhal numbers. This study reveals that aerodynamic forces produced by plunging airfoil are independent of initial kinematic conditions of airfoil that proves the existence of limit cycle. Frequencies present in the oscillating lift force are composed of fundamental (fs), even and odd harmonics (3fs) at higher Strouhal numbers. Using numerical simulations, shedding frequencies (f_s) were observed to be nearly equal to the excitation frequencies in all the cases. Unsteady lift force generated due to the plunging airfoil is modeled by modified van der Pol oscillator. Using method of multiple scales and spectral analysis of steady-state CFD solutions, frequencies and damping terms in the van der Pol oscillator model are estimated. We prove the applicability of this model to all planar motions of airfoil; plunging, pitching and flapping. An important aspect of currently-proposed model is capturing the time-averaged value of aerodynamic lift coefficient.