This study explores the effects of rotational mechanisms on the characteristics of the leading edge vortex (LEV) by comparing translating and revolving flexible wings that are started from rest. Tomographic particle image velocimetry (tomographic-PIV) technique was employed to acquire three-dimensional flow fields for the revolving wings, while planar flow fields for the case of translating wings were acquired via 2D2C-PIV measurements. The comparison of flow fields between the two motion kinematics reveals similar behavior of the vortical structures yet the LEV circulation in the translating wings has higher values. The LEV centroid in the revolving cases stays above the leading-edge, while in the translating wings, it always remains at a lower position. The effect of high flexibility results in the retention of LEV closer to the wing surface for both cases. @en

This study explores the effects of rotational mechanisms on the characteristics of the leading edge vortex (LEV) by comparing translating and revolving flexible wings that are started from rest. Tomographic particle image velocimetry (tomographic-PIV) technique was employed to acquire three-dimensional flow fields for the revolving wings, while planar flow fields for the case of translating wings were acquired via 2D2C-PIV measurements. The comparison of flow fields between the two motion kinematics reveals similar behavior of the vortical structures yet the LEV circulation in the translating wings has higher values. The LEV centroid in the revolving cases stays above the leading edge, while in the translating wings, it always remains at a lower position. The effect of high flexibility results in the retention of LEV closer to the wing surface for both cases.@en

The flow field and fluid-dynamic loads of revolving low-aspect-ratio chordwise-flexible wings are studied experimentally at a Reynolds number of 10,000. The investigation involves phase-locked tomographic particle image velocimetry (PIV) complemented with force measurements. The pressure fields are reconstructed from the three-dimensional velocity fields in a complete volume around the wing. For decreasing flexural stiffness, the coherence of this vortex system and spanwise transport of vorticity along the axis of the leading edge vortex (LEV) increase, which contribute to the stability and retention of the LEV. As the LEV low-pressure region becomes smaller with increasing flexibility, the total force on the wing is reduced, while it is tilted towards the lift direction due to the wing deformation. As a result, the drag is significantly suppressed, while the lift remains relatively high. Consequently, the lift-to-drag ratio increases with increasing flexibility and correlates well with the geometric angle of attack. While the sectional lift along the full span is comparable for the different wings, the sectional drag is significantly reduced at the outboard wing for increasing flexibility. The centroids of lift and drag are located at approximately 70% of the span for all wings throughout the complete revolving motion. Finally, the process of vortex breakdown is found to be related to the formation of a positive spanwise pressure gradient.

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We present a spatially and temporally highly resolved flow measurement covering a large volume (~0.6 m³) in a pure thermal plume in air. The thermal plume develops above an extended heat source and is characterized by moderate velocities (U ~ 0.35 m/s) with a Reynolds number of Re ∼ 500 and a Rayleigh number of Ra ∼ 10 ⁶. We demonstrate the requirements and capabilities of the measurement equipment and the particle tracking approach to be able to probe measurement volumes up to and beyond one cubic meter. The use of large tracer particles (300 μm), helium-filled soap bubbles (HFSBs), is crucial and yields high particle image quality over large-volume depths when illuminated with arrays of pulsed high-power LEDs. The experimental limitations of the HFSBs—their limited lifetime and their intensity loss over time—are quantified. The HFSBs’ uniform particle images allows an accurate reconstruction of the flow using Shake-The-Box particle tracking with high particle concentrations up to 0.1 particles per pixel. This enables tracking of up to 275,000 HFSBs simultaneously. After interpolating the scattered data onto a regular grid with a Navier–Stokes regularization, the velocity field of the thermal plume reveals a multitude of vortices with a smooth temporal evolution and a remarkable coherence in time (see animation, supplementary data). Acceleration fields are also derived from interpolated particle tracks and complement the flow measurement. Additionally, the flow map, the basis of a large class of Lagrangian coherent structures, is computed directly from observed particle tracks. We show entrainment regions and coherent vortices of the thermal plume in the flow map and compute fields of the finite-time Lyapunov exponent.

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We present results from two large-volume volumetric flow experiments. The first of these, investigating a thermal plume at low velocities (up to 0.35 m/s) demonstrates the abilities and requirements to reach volume sizes up to and probably beyond one cubic meter. It is shown that the use of Helium filled soap bubbles (HFSBs) as tracers, combined with pulsed LED illumination yields high particle image quality over large volume depths. A very uniform particle imaging, both in space as well as in time enables using high particle image concentrations (up to 0.1 ppp), while still being able to accurately reconstruct the flow using Shake-The-Box particle tracking. The experiment consisted of time-resolved volumetric flow measurements of a convectional plume within a volume of approx. 0.55 m3 (550 liters). The light yield needed for such a large scale measurement is realized by using HFSBs with 300 !m diameter as tracers and illuminating the measurement region using high-power, scalable arrays of white LEDs. Applying the Shake-The-Box algorithm, up to 275,000 bubbles could be tracked simultaneously. Interpolating the results on a regular grid (using ‘FlowFit’) reveals a multitude of flow structures. The setup can be scaled to larger volumes of several cubic meters, basically only being limited by the number and power of available LEDs and high-resolution cameras with sufficient frame-rate and pixel sizes. A second experiment showcases the possibilities to reach higher flow velocities, while still measuring within a comparatively large volume, by applying high-speed imaging and advanced LED illumination. An impinging turbulent jet was investigated in volumes ranging from 13 to 47 liters, depending on the repetition rate of the camera system. The results show that even at a repetition rate of 3.9 kHz and flow speeds up to 17 m/s the tested system was able to deliver images that allowed for a reliable and accurate tracking of bubbles.@en

This study explores the flow field and fluid-dynamic loads generated by revolving low-aspect-ratio wings. The pressure field and load characteristics are successfully reconstructed from the phase-locked tomographic measurements in three independently measured volumes along the span of the wing. The vortical structures encompass a low pressure region and the spatial gradient information of the pressure field provides greater insights in their stability mechanisms. The low pressure region associated with the leading edge vortex and its close position to the wing surface are responsible for the high resultant forces acting on the wing. Simultaneous force measurements show a reasonable agreement with the reconstructed loads. The sectional lift and drag characteristics provide greater insights into the distributed load mechanisms along the span.@en

This study explores the flow field and fluid-dynamic loads generated by revolving low-aspect-ratio flat plate wings undergoing a revolving motion starting from rest. Three wings with different degree of chordwise flexural stiffness (i.e., rigid, moderate flexibility and high flexibility) have been tested in order to investigate the influence of the wing flexibility. The wings have an angle of attack of 45 deg in their undeformed condition. The measurements have been performed in a water tank at a Reynolds number of 10,000 based on the chord length and terminal velocity at the 75% span position. The experimental campaign consists of phase-locked tomographic particle image velocimetry measurements complemented with simultaneous force measurements The three-dimensional velocity fields are captured in three measurement volumes positioned side-by-side along the span of the wing for different phases of the revolving motion, generating a time-resolved volumetric velocity field data set. Subsequently, from the velocity data the pressure fields are reconstructed as well as the loads acting on the wing.@en