Linear instability of low Reynolds number massively separated flow around three NACA airfoils

Abstract

Two- and three-dimensional modal and non-modal instability mechanisms of steady spanwise-homogeneous laminar separated flow over airfoil profiles, placed at large angles of attack against the oncoming flow, have been investigated using global linear stability theory. Three NACA profiles of distinct thickness and camber were considered in order to assess geometry effects on the laminar–turbulent transition paths discussed. At the conditions investigated, large-scale steady separation occurs, such that Tollmien–Schlichting and cross-flow mechanisms have not been considered. It has been found that the leading modal instability on all three airfoils is that associated with the Kelvin–Helmholtz mechanism, taking the form of the eigenmodes known from analysis of generic bluff bodies. The three-dimensional stationary eigenmode of the two-dimensional laminar separation bubble, associated in earlier analyses with the formation on the airfoil surface of large-scale separation patterns akin to stall cells, is shown to be more strongly damped than the Kelvin–Helmholtz mode at all conditions examined. Non-modal instability analysis reveals the potential of the flows considered to sustain transient growth which becomes stronger with increasing angle of attack and Reynolds number. Optimal initial conditions have been computed and found to be analogous to those on a cascade of low pressure turbine blades. By changing the time horizon of the analysis, these linear optimal initial conditions have been found to evolve into the Kelvin–Helmholtz mode. The time-periodic base flows ensuing linear amplification of the Kelvin–Helmholtz mode have been analysed via temporal Floquet theory. Two amplified modes have been discovered, having characteristic spanwise wavelengths of approximately 0.6 and 2 chord lengths, respectively. Unlike secondary instabilities on the circular cylinder, three-dimensional short-wavelength perturbations are the first to become linearly unstable on all airfoils. Long-wavelength perturbations are quasi-periodic, standing or travelling-wave perturbations that also become unstable as the Reynolds number is further increased. The dominant short-wavelength instability gives rise to spanwise periodic wall-shear patterns, akin to the separation cells encountered on airfoils at low angles of attack and the stall cells found in flight at conditions close to stall. Thickness and camber have quantitative but not qualitative effect on the secondary instability analysis results obtained.

Modal Analysis of Fluid Flows: An Overview

“…Three-dimensional short-wavelength instability mode determined from global stability analysis for a NACA 4415 airfoil at Re = 500 and α = 20 • . (a) Streamwise vorticity (ω ζ ) isosurface of the short-wavelength eigenmode and (b) the corresponding wall-streamlines, superposed upon the time-periodic base state[169]. Reprinted with permission from Cambridge University Press.…”

Stability of Low-Reynolds-Number Separated Flow Around an Airfoil Near a Wavy Ground

“…For example, by combining local and global linear stability analyses on the flow around airfoils at a low angle of attack, Theofilis et al [20] and Lin and Pauley [21] were able to attribute the dominant perturbation to the Kelvin-Helmholtz (KH) mode. Likewise, at a high angle of attack, a similar dominance by the KH mode was found in the massively separated flow around a series of NACA airfoils at a low Reynolds number [22,23]. For steady separated flow around an airfoil at a low Reynolds number, Kitsios et al [24] observed the existence of three-dimensional stationary modes, which were first reported by Theofilis et al [25] in a flat-plate boundary layer.…”

A robust approach for stability analysis of complex flows using high-order Navier-Stokes solvers

“…For example, earlier studies found a growing stationary mode (in a three-dimensional setup), which dominates the decaying wake mode. Detailed analysis performed later clarified the observations [44] . The problem highlights the importance of the a priori known shift value in Arnoldi-based techniques.…”

“…Results from both matrix-forming as well as time-stepper approaches [42,43,41,44] are available for comparison, albeit in an incompressible framework. While earlier studies [42,43] have used a BiGlobal code with conformally mapped curvilinear co-ordinates to obtain eigenmodes, subsequent studies [41,44] use open-source codes, including Nektar++ and Nek5000 for the time-stepper approach, and FreeFEM++ for matrix-forming. In these studies, the Arnoldi subspace iteration is used for the relevant eigenspectrum computation, with the goal of identifying the smallest modulus modes.…”