Experimental investigation of the wake dynamics for a NACA0012 airfoil with a cut-in serrated trailing-edge

Abstract

An experimental investigation of the unsteady dynamics in the wake of a NACA0012 (National Advisory Committee for Aeronautics) airfoil with a narrow-angle cut-in serrated trailing-edge is presented. Time-resolved planar particle image velocimetry (PIV) has been performed in the wake region of the airfoil both with and without serrations at a Reynolds number of 78 000. The boundary layers on both sides of the airfoil surface are tripped and the angle of attack is held fixed at zero. Spectral analysis of the PIV results shows that the trailing-edge serrations are linked to increased velocity fluctuations in the wake region which are centered on a non-dimensional frequency range of fc/U∞≈3−5. Proper orthogonal decomposition (POD) is utilized to analyze the spatiotemporal characteristics of the most dominant structures in the wake. Three POD mode pairs identified in the serrated airfoil wake flow are described in detail in two different PIV measurement planes passing through the tip and root of the serrations. Overall, the serrations substantially increase the turbulent kinetic energy in the wake and concentrate this energy within the three identified mode pairs. Spectral analysis of the temporal coefficient signals pertaining to the three mode pairs shows energy concentrated within the frequency band of increased velocity fluctuations identified in the wake region. POD-based reconstruction unfolded a von Kármán-like vortex shedding from the truncated part of the airfoil with fc/U∞≈3.7, which is primarily v-fluctuation driven, convecting through the domain at close to 90% of the free stream velocity.

Wavelet analysis of the coherent structures in airfoil leading-edge separation control by bionic coverts

“…1,2 Feathers play an important role in improving the aerodynamic performance of the wings. 3,4 By learning from the bird flight, 5,6 researchers found that the covert feathers on the upper wing surfaces are lifted adaptively by airflow during taking-off and landing processes, which are referred to as the coverts and the secondary coverts. 7,8 In order to observe the covert movements in the real bird flight, Carruthers et al 9 installed a high-speed camera on the bird’s back and recorded that these soft coverts lift up at a large angle-of-attack (AoA), so that the bird can obtain higher lift during taking-off and landing.…”

“…(2) In time, frequency and space domains, evolution of multi-scale turbulent structures in shear layers could reflect the control mechanism of the artificial coverts. (3) The modulation relationship between large-and small-scale turbulent structures in the wake region could be quantified through a multidimensional genetic algorithm. Furthermore, the coherence between the trailing-and leading-edge shear layers will be easier to understand.…”