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This paper describes a series of experiments using particle image velocimetry to investigate the dynamic stall resulting due to a rapid pitching motion of a flat plate. There exist in such unsteady separated flows multiple time-dependent coherent structures, whose interaction and evolution are complex and nonlinear. The experiments presented here are aimed at determining the behaviour of a dynamic stall vortex system in the Reynolds number range $10^{3}. Evidence is presented for the development of the three-dimensional structure associated with the dynamic stall vortex and its interaction with the no-slip boundary condition at the surface of the pitching plate. The analysis presented suggests that a centrifugal instability exists, and that the form of the three-dimensional structure is consistent with that expected of a centrifugal instability. The structure and scale dependence of the flow are explored using wavelet and Fourier methods, with the dependence of the flow on Reynolds number examined, as well as the influence of spanwise end boundary conditions.

As a novel volumetric particle image velocimetry technique, single-camera light-field PIV (LF-PIV) is able to acquire three-dimensional flow fields through a single camera. Compared with other multi-camera 3D PIV techniques, LF-PIV has distinct advantages, including concise hardware setup and low optical access requirements. Its capability has proven effective in many experimental investigations. In this study, the use of LF-PIV in measuring a self-similar adverse pressure-gradient turbulent boundary layer (APG-TBL) is demonstrated. Experiments are performed in a large water tunnel at the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC), Monash University. Sets of 250 light-field PIV image pairs are captured covering both the inner and outer regions of the boundary layer. Instantaneous 3D velocity fields are reconstructed using a GPU accelerated density ray tracing multiplicative reconstruction technique (DRT-MART) and three-dimensional cross-correlation methods. The LF-PIV results are compared with two-dimensional PIV (2D-PIV) measurements of the same flow. Comparable accuracy to 2D-PIV is achieved for first- and second-order velocity statistics above approximately $$ y/\delta_{1} = 1 $$ .

This paper applies particle image velocimetry (PIV) to a simplified, canonical, pitch-hold-return problem of a pitching plate in order to gain some understanding of how three dimensionality develops in such flows. Data from a progression of PIV studies, from stereoscopic PIV yielding three-component, two-dimensional (3C-2D) data to tomographic PIV yielding three-component, three-dimensional (3C-3D) data are presented thus providing progressively more detailed information. A comparison of results is made between the two techniques. The PIV study is performed in a water tunnel facility with cross-sectional area 500 × 500 mm, and involves a full-span (nominally two-dimensional) plate, suspended between a wall end boundary condition and a free surface, pitching at a dimensionless pitch rate of K c = 0.93 in flow at Re = 7,500. Results demonstrate the existence of spanwise flows in both the leading edge and trailing edge vortices, but with strong directionality in the leading edge vortex towards the wall end boundary condition. Observations of instantaneous flow patterns suggest also the existence of three-dimensional coherent vortex filament structures in the outer regions of the leading edge vortex.

This paper concerns the development of highly complex three-dimensional flow structures within the vortex system produced when a flat plate, pitching rapidly to high amplitude, undergoes dynamic stall at its leading edge. A single stall event is studied, produced by a single non-periodic, simplified, pitch-hold-return motion of the plate. This differs from many previous studies on pitching airfoils which measure exclusively periodic, oscillatory motions. In this way, a single event can be measured and characterised, without regard for any preceding cycles or hysteresis in the dynamical system. Two-component, two-dimensional particle image velocimetry measurements of the flow over the upper surface of the pitching plate are taken in a water tunnel. The plate is pitched at a rate relevant to rapidly perching or manoeuvring biological flyers, and at Reynolds numbers ranging from Re = 5000 to Re = 10000. Due to the non-periodic, transient, nature of the flow it becomes necessary to carefully select an appropriate analysis tool and, in this instance, a discrete wavelet approach is chosen. A wavelet based multi-resolution analysis of the coherent structures in the dynamic stall vortex system is undertaken in two dimensions, separating flow features into multiple scales. Discussion of the topology and energy content of each scale is presented, with consideration of temporal evolution and dependence on chord-based Reynolds number.

Previous works have shown that momentum transfer in the wall–normal direction within turbulent wall–bounded flows occurs primarily within coherent structures defined by regions of intense Reynolds stress [1]. Such structures may be classified into wall–attached and wall–detached structures with the latter being typically weak, small–scale, and isotropically oriented, while the former are larger and carry most of the Reynolds stresses. The mean velocity fluctuation within each structure may also be used to separate structures by their dynamic properties. This study aims to extract information regarding the scales, kinematics and dynamics of these structures within the topological framework of the invariants of the velocity gradient tensor (VGT). The local topological characteristics of these intense Reynolds stress structures are compared to the topological characteristics of vortex clusters defined by the discriminant of the velocity gradient tensor. The alignment of vorticity with the principal strain directions within these structures is also determined, and the implications of these findings are discussed.

High pitch rate manoeuvres have been studied in great depth in recent decades, in respect to rotor dynamic stall and, more recently, in understanding bird and insect flight with view to application in biomimetic micro air vehicles (MAVs). The flow topology arising from such unsteady airfoil movement is complex. This paper focuses on assessing the development of spanwise variation and three-dimensional substructure within the leading edge vortex (LEV) produced by a rapidly pitching airfoil. Two-component, two-dimensional (2C-2D) particle image velocimetry (PIV) is used to take measurements of the flow in a plane above the upper surface of an airfoil at two different Reynolds numbers. Observations conclude that flow three-dimensionality develops in the form of coherent vortical structures along the dividing stream surface (DSS) upstream of the leading edge vortex. The average spanwise spectral content of the flow immediately upstream of the leading edge vortex is calculated, allowing quantitative analysis of the dependence of leading edge vortex substructure on Reynolds number and time.

Unsteady aerodynamics in the low Reynolds number domain is of great interest in applications to Micro Air Vehicles (MAVs), especially with respect to flow over, and in the wake of, pitching aerofoils. The massively separated flow over the upper surface of the pitching aerofoil allows for the production of large transient peaks of lift at this scale. This paper extends our previous work 1 to multi-dimensional (3D), multi-component (3C) measurements of this phenomenon by using Tomographic Particle Image Velocimetry (Tomo-PIV). For this purpose the three-component, three-dimensional (3C-3D) velocity fields in the wake of a flat plate undergoing a pitch -hold-return motion is investigated. The experiments are conducted in a water tunnel at a Reynolds number of 7,500 and a dimensionless pitching rate of Kc = 0.93. The instantaneous and phase averaged evolution of the trailing edge vortices is investigated, highlighting the three -dimensional organisation of the coherent vortex structure and its spanwise variation.