Unsteady Nature of Wing Vortices and Their Interactions with Downstream Wings

Student thesis: Doctoral ThesisDoctor of Engineering (EngD)

Abstract

Unsteady wakes are experimentally investigated in this thesis using a water tunnel facility, with a focus on the structure of the tip vortex and wake of a plunging wing, unsteady wakes of a plunging airfoil, and unsteady aerodynamics of the stationary downstream wing in the wake of a plunging airfoil. The flow structure of a NACA0012 airfoil/wing plunging sinusoidally at a chord Reynolds number of Re= 20,000 is investigated within a wide range of reduced frequencies (k) (from 0.00 to 3.14) and Strouhal numbers based on the peak-to-peak amplitude (St) (from 0.00 to 0.24).
The amplitude of the tip vortex and the wake oscillation vary with wing oscillation frequencies and amplitudes. Both the tip vortex amplitude and wake oscillation amplitude have a weak relationship with the Strouhal number (St) and the reduced frequency (k). Wake amplitude decreases as the Strouhal number (St) and the reduced frequency (k) increase, resulting in a reverse Kármán Street in the mid-span plane and various vortical structures in the cross-flow plane due to the linked vortex loops. The ratio of the amplitude of the tip vortex to the amplitude of the wake oscillations is always smaller than unity. This means that wake oscillations are more likely to create bigger force variations than the tip vortex in interactions with the downstream wing.
Unsteady wakes of a plunging airfoil are analysed in the streamwise flow and the cross-flow in the near wake using two-point correlations and the Proper Orthogonal Decomposition (POD). It is shown that the unsteady characteristics are even better correlated with the Strouhal number (St) than the mean flow quantities and the reduced frequency (k). By increasing the Strouhal number (St), the percentage energy of the fundamental wake modes of the streamwise flow and the flapping mode of the cross-flow increases, but at different rates in the drag-producing and thrust-producing wakes. Results reveal that the Strouhal number (St) is the most important parameter in determining the degree of two-dimensionality of the wake. This implies that the spanwise coherence of the experimentally produced gusts should be considered when interpreting the amplitude of the lift fluctuations on the downstream wing.
ii
Unsteady aerodynamics of a wing in the wake of a plunging airfoil is explored by altering vortex-street configuration, frequencies and amplitudes of the plunge oscillation. When the leading-edge of the wing is placed at the wake centreline or just above it, the amplitude of the lift variations is the largest. With increasing angle of attack of the wing, the lift amplitude increases and the flow separation at the leading-edge, formation of a leading-edge vortex and formation of a vortex couple with the incident vortex become more noticeable. The lift time history has higher harmonics up to n=5 when the wing is close to the wake centreline. This is due to the cross-stream velocity profile in the undisturbed wake and can be also predicted by a point vortex model. For wing-asymmetric wake interactions, the lift amplitude of the wing is reduced as the asymmetry of the wake is increased (by increasing the angle of attack of the plunging airfoil), resulting in modification of geometry and circulation of the vortices in the reversed von Kármán Street. For an unloaded wing in the symmetric wakes, the peak lift coefficients increase with increasing frequency and amplitude of the plunging airfoil. The amplitude and form of the cross-stream velocity profiles, as well as the degree of two dimensionality in the undisturbed wake, are all determined by these kinematic parameters. The amplitude of the lift coefficient of the wing depends on a single wake parameter only, which is the Strouhal number based on the peak-to-peak amplitude of the upstream airfoil (St).
Date of Award14 Sep 2022
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorZhijin Wang (Supervisor) & Ismet Gursul (Supervisor)

Cite this

'