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Vertical axis wind turbine blades are subject to rapid, cyclical variations in angle of attack and relative airspeed which can induce dynamic stall. This phenomenon poses an obstacle to the greater implementation of vertical axis wind turbines because dynamic stall can reduce turbine efficiency and induce structural vibrations and noise. This study seeks to provide a more comprehensive description of dynamic stall in vertical axis wind turbines, with an emphasis on understanding its parametric dependence and scaling behaviour. This problem is of practical relevance to vertical axis wind turbine design but the inherent coupling of the pitching and velocity scales in the blade kinematics makes this problem of more broad fundamental interest as well. Experiments are performed using particle image velocimetry in the vicinity of the blades of a straight-bladed gyromill-type vertical axis wind turbine at blade Reynolds numbers of between 50 000 and 140 000, tip speed ratios between $\unicode[STIX]{x1D706}=1$ to $\unicode[STIX]{x1D706}=5$, and dimensionless pitch rates of $0.10\leqslant K_{c}\leqslant 0.20$. The effect of these factors on the evolution, strength and timing of vortex shedding from the turbine blades is determined. It is found that tip speed ratio alone is insufficient to describe the circulation production and vortex shedding behaviour from vertical axis wind turbine blades, and a scaling incorporating the dimensionless pitch rate is proposed.

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.

We detail the analysis of centrifugal homogenization process by a hydrodynamic model and the model-guided design of a low-cost centrifugal homogenizer. During operation, centrifugal force pushes a multiphase solution to be homogenized through a thin nozzle, consequently homogenizing its contents. We demonstrate and assess the homogenization of coarse emulsions into relatively monodisperse emulsions, as well as the application of centrifugal homogenization in the mechanical lysis of mpkCCD mouse kidney cells. To gain insight into the homogenization mechanism, we investigate the dependence of emulsion droplet size on geometrical parameters, centrifugal acceleration, and dispersed phase viscosity. Our experimental results are in qualitative agreement with models predicting the droplet size. Furthermore, they indicate that high shear rates kept constant throughout operation produce more monodisperse droplets. We show this ideal homogenization condition can be realized through hydrodynamic model-guided design minimizing transient effects inherent to centrifugal homogenization. Moreover, we achieved power densities comparable to commercial homogenizers by model guided optimization of homogenizer design and experimental conditions. Centrifugal homogenization using the proposed homogenizer design thus offers a low-cost alternative to existing technologies as it is constructed from off-the-shelf parts (Falcon tubes, syringe, needles) and used with a centrifuge, readily available in standard laboratory environment.

Dynamic stall is often found in unsteady aerodynamic flows where the angle of attack can vary over a large range. It is of particular interest in the context of vertical axis wind turbines, where dynamic stall is the principal impediment to achieving improved aerodynamic efficiency. Here, we report computations using the unsteady Reynolds-averaged Navier–Stokes (URANS) equations with the Menter-SST turbulence model on a two-dimensional domain, over a range of tip speed ratios typical of the operation of vertical axis wind turbines. Comparisons are made against high resolution experimental data from particle image velocimetry (PIV), with special attention to the ability of the turbulence model to emulate the turbulence properties of the flow. It is shown that the computations approximate the experimental results well in most respects.

The flow topology arising from unsteady airfoil motion is complex, but the benefits of understanding such flows extend to a wide variety of applications. Natural flows such as those induced by birds or insects during flapping flight, or by the rapidly moving fins of swimming fish, as well as flows over the manoeuvring wings of micro air vehicles (MAVs) or the rotors of helicopters or wind turbines exhibit behaviours typical of rapidly manoeuvring airfoils. This paper presents measurements and discussion on a set of recent experiments performed on a rapidly pitching plate in low Reynolds number flow. Time resolved particle image velocimetry (PIV) data with a high spatio-temporal resolution are presented, and the main features of the flow are identified. Measurements are taken at six Reynolds numbers ranging between 1500 and 10 000 and an analysis is given of the temporal evolution of the massively separated flow at the leading edge as well as at the trailing edge and in the wake. Individual coherent structures are located and tracked, and the variation of their strength and fluctuation are quantified as a function of time. Several key observations are made regarding the effect of Reynolds number on the topology of the wake structure of a pitching flat plate.

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.

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.