Whole-field density measurements by digital image correlation

A novel application of Synthetic Schlieren in a laboratory set-up yields a quantitative measurement of the density field of two-dimensional, stratified or homogeneous, transparent fluids in a laboratory set-up using a single camera. This application obtains local values of the density without the need for tomographic reconstruction algorithms that require images taken from different directions through the fluid nor does the application require regularization. This is achieved by placing the camera at a large oblique angle with respect to the experimental set-up. This step is motivated by a fallacy observed when applying ray tracing in a classical configuration, in which the camera’s optical axis is perpendicular to the flat surface of a fluid container. The application is illustrated by the optical determination of static density fields of linearly and nonlinearly stratified fluids, as well as of multi-layered fluids. The application is validated by comparing with density profiles obtained from probe measurements of conductivity and temperature. Our application yields similar density and density gradient profiles as the probe while also providing a whole-field measurement without disturbing the fluid, and allowing the determination of dynamical density fields

Frequency–wavenumber mapping in turbulent shear flows

Spatial turbulence spectra for high-Reynolds-number shear flows are usually obtained by mapping experimental frequency spectra into wavenumber space using Taylor’s hypothesis, but this is known to be less than ideal. In this paper, we propose a cross-spectral approach that allows us to determine the entire wavenumber–frequency spectrum using two-point measurements. The method uses cross-spectral phase differences between two points – equivalent to wave velocities – to reconstruct the wavenumber–frequency plane, which can then be integrated to obtain the spatial spectrum. We verify the technique on a particle image velocimetry (PIV) data set of a turbulent boundary layer. To show the potential influence of the different mappings, the transfer functions that we obtained from our PIV data are applied to hot-wire data at approximately the same Reynolds number. Comparison of the newly proposed technique with the classic approach based on Taylor’s hypothesis shows that – as expected – Taylor’s hypothesis holds for larger wavenumbers (small spatial scales), but there are significant differences for smaller wavenumbers (large spatial scales). In the range of Reynolds number examined in this study, double-peaked spectra in the outer region of a turbulent wall flow are thought to be the result of using Taylor’s hypothesis. This is consistent with previous studies that focused on examining the limitations of Taylor’s hypothesis (del Álamo &amp; Jiménez, J. Fluid Mech., vol. 640, 2009, pp. 5–26). The newly proposed mapping method provides a data-driven approach to map frequency spectra into wavenumber spectra from two-point measurements and will allow the experimental exploration of spatial spectra in high-Reynolds-number turbulent shear flows.<br/

Feather roughness reduces flow separation during low Reynolds number glides of swifts

Swifts are aerodynamically sophisticated birds with a small arm and large hand wing that provides them with exquisite control over their glide performance. However, their hand wings have a seemingly unsophisticated surface roughness that is poised to disturb flow. This roughness of about 2% chord length is formed by the valleys and ridges of overlapping primary feathers with thick protruding rachides, which make the wing stiffer. An earlier flow study of laminar–turbulent boundary layer transition over prepared swift wings suggests that swifts can attain laminar flow at low angle-of-attack. In contrast, aerodynamic design theory suggests that airfoils must be extremely smooth to attain such laminar flow. In hummingbirds, which have similarly rough wings, flow measurements on a 3D printed model suggests that the flow separates at the leading edge and becomes turbulent well above the rachis bumps in a detached shear layer. The aerodynamic function of wing roughness in small birds is, therefore, not fully understood. Here we perform particle image velocimetry and force measurements to compare smooth versus rough 3D-printed models of the swift hand wing. The high-resolution boundary layer measurements show that the flow over rough wings is indeed laminar at low angle-of-attack and Reynolds number, but becomes turbulent at higher values. In contrast, the boundary layer over the smooth wing forms open laminar separation bubbles that extend beyond the trailing edge. The boundary layer dynamics of the smooth surface varies nonlinear as a function of angle-of-attack and Reynolds number, whereas the rough surface boasts more consistent turbulent boundary layer dynamics. Comparison of the corresponding drag values, lift values, and glide ratios suggests, however, that glide performance is equivalent. The increased structural performance, boundary layer robustness, and equivalent aerodynamic performance of rough wings might have provided small (proto) birds with an evolutionary window to high glide performance

Near-wake characteristics of rigid and membrane wings in ground effect

Wind tunnel measurements are conducted at a Reynolds numbers of Re = 56,000 to examine the characteristics of near-wake vortices of rigid flat-plates and membrane wings from free-flight into ground–effect conditions. Synchronised high-speed load cell measurements, digital image correlation and particle image velocimetry are performed to resolve lift, drag and pitch oscillations simultaneously with membrane deformation and flow dynamics. Flow measurements are acquired in a crossflow-plane, one chord downstream of the trailing-edge, allowing the examination of time evolution of the wake and its relationship to the forces and membrane deformation. Membrane wings are found to delay ground–effect or high angles-of-attack induced tip-vortex break-down and result in larger tip-vortex push-out (beyond the wing span) compared to rigid flat-plate wings. The leading-edge vortex appears to shed with streamwise vorticity close to the root of the wing and this frequency of this shedding is found to match with the dominant frequency observed in membrane fluctuations. In specific resonance conditions, membrane and flow fluctuations are found to correlate well to the fluctuations in loads and moments

Aeromechanics of membrane and rigid wings in and out of ground-effect at moderate Reynolds numbers

Wind tunnel experiments are conducted using membrane wings and rigid flat-plates in ground-effect at a moderate Reynolds number of Re = 56 000 with ground clearances from 1% to 200% chord length measured from their trailing-edge. A six-axis load-cell captures time-resolved forces and moment while time-resolved stereo digital image correlation (DIC) measurements are performed to capture membrane motions. The lift and drag coefficients of the rigid wing in ground-effect follow well-established trends while the membrane wing appears to exhibit improved coefficients and efficiency (compared to the rigid wing) when in ground-effect. Proper orthogonal decomposition (POD) is applied to study the spatiotemporal structure of membrane vibrations. With increasing angles-of-attack and/or decreasing heights above ground, mode shapes of membrane deformation are dominated by large-scale fluctuations that have a smaller number of local extrema along the chord. Ground-effect induces modifications to the membrane deformation, which appear to be similar to the modifications induced by increasing angles-of-attack in free-flight. At high angles-of-attack in free-flight or at moderate angles in ground-effect, two POD modes of membrane fluctuations are found to be sufficient to capture 90% of all membrane deformations. Under these conditions, a membrane deformation with maximum camber near the trailing edge of the membrane wing is found to correlate with high lift, low drag and a nose down pitching moment. The extrema in membrane deformations and lift and drag forces occur simultaneously, while there is a time-lag between the deformation and the pitching moment

PART-1: Dataset for journal: "On the fluid-structure interaction of flexible membrane wings for MAVs in and out of ground-effect"

Complementary file for the attached files Written 24-04-2017 by Robert Bleischwitz ([email protected]) General Comments 0.) This specific upload contains PART-1 of the full dataset 1.) The attached data relates to experimental windtunnel measurements on passive membrane wings for MAVs. The data was aquired between 2012-2016 at the University of Southampton, involving Robert Bleischwitz as PhD student, who was supervised by Roeland de Kat and Bharathram Ganapathisubramani. 2.) The attached data is given time-resolved and time-synchronised at 800Hz over a imaging-period of 5000 images, involving load measurements via a 6-axis load-cell ATI Nano17 /25N, deformation measurements via Digitial Image Processing (DIC) and planar flow measurements via two side-by-side cameras. 3.) More setup and processing details can be found in the paper "On the fluid-structure interaction of flexible membrane wings for MAVs in and out of ground-effect" (2017) by the authors R. Bleischwitz, R. de Kat, B. Ganapathisubramani Published in the Journal of Fluids and Structures (http://www.sciencedirect.com/science/article/pii/S088997461630370X) 4.) All load/deformation/flow folders contain a README.txt(Use 1st) and Instructions.m (Use 2nd) file, which give further supporting details how to illustrate the data 5.) This specific upload contains PART-1 of the full dataset, including introduction file + rigid flat-plate case (Load+PIV measurements) as reference to membrane wing case (PART-2)Abstract to publication: Wind tunnel experiments are conducted at a Reynolds number of Re=56,000, measuring rigid flat-plates and flexible membrane wings from free-flight into ground-effect conditions. Load cell measurements, digital image correlation and particle image velocimetry are applied in high-speed to resolve time-synchronised lift, drag and pitch oscillations simultaneously with membrane and flow dynamics. Proper orthogonal decomposition is applied on flow oscillations to determine their spatiotemporal evolution. Loads, membrane motions and flow dynamics are correlated to each other to investigate their underlying coupling physics. A membrane wing's ability of static cambering and dynamic membrane oscillations are found to be beneficial when the wing is in ground-effect, where the descent in height forces premature leading-edge vortex-shedding and drag increase. The dynamic motions of membrane wings help to exploit vortex-shedding dynamics from the leading-edge that ensures time-averaged reattached flow over the wing upper surface, resulting in further lift enhancement. Membrane wings show lag-free fluid-membrane coupling at peak-lift conditions. In post-stall conditions, the membrane is found to lag the flow dynamics, signalling the end of direct fluid-structure coupling

Structure of high and low shear-stress events in a turbulent boundary layer

Simultaneous particle image velocimetry (PIV) and wall-shear-stress sensor measurements were performed to study structures associated with shear stress events in a flat plate turbulent boundary layer blackat a Reynolds number Re_\tau \approx 4000}. The PIV field-of-view covers 8\delta (where \delta is the boundary layer thickness) along the streamwise direction and captures the entire boundary layer in the wall-normal direction. Simultaneously, wall-shear-stress measurements were taken using a spanwise array of skin-friction sensors (spanning 2\delta). Based on this combination of measurements, the organization of the conditional wall-normal and streamwise velocity fluctuations (u and v) and of the Reynolds shear stress (-uv) can be extracted. Conditional averages of the velocity field are computed by dividing the histogram of the wall-shear-stress fluctuations into four quartiles, each containing 25\% of the occurrences. The conditional events corresponding to the extreme quartiles of the histogram (positive and negative) predominantly contribute to a change of velocity profile associated with the large structures and in the modulation of the small-scales. A detailed examination of the Reynolds shear stress contribution related to each of the four quartiles shows that the flow above a low wall-shear-stress event carries a larger amount of Reynolds shear stress than the other quartiles. The contribution of the small- and large-scales to this observation is discussed based on a scale decomposition of the velocity field.<br/

On the fluid-structure interaction of flexible membrane wings for MAVs in and out of ground-effect

Wind tunnel experiments are conducted at a Reynolds number of Re = 56,000, measuring rigid flat-plates and flexible membrane wings from free-flight into ground-effect conditions. Load cell measurements, digital image correlation and particle image velocimetry are applied in high-speed to resolve time-synchronised lift, drag and pitch oscillations simultaneously with membrane and flow dynamics. Proper orthogonal decomposition is applied on flow oscillations to determine their spatiotemporal evolution. Loads, membrane motions and flow dynamics are correlated to each other to investigate their underlying coupling physics. A membrane wing’s ability of static cambering and dynamic membrane oscillations are found to be beneficial when the wing is in ground-effect, where the descent in height forces premature leading-edge vortex-shedding and drag increase. The dynamic motions of membrane wings help to exploit vortex-shedding dynamics from the leading-edge that ensures time-averaged reattached flow over the wing upper surface, resulting in further lift enhancement. Membrane wings show lag-free fluid-membrane coupling at peak-lift conditions. In post-stall conditions, the membrane is found to lag the flow dynamics, signalling the end of direct fluid-structure coupling

Spatial-spectral characteristics of momentum transport in a turbulent boundary layer

wall, longer time and larger length scales exhibit an increasing spectral content. Wave velocities of transport events are estimated from wavenumber–frequency power spectra at different wall-normal locations. Wave velocities associated with Spectral content and spatial organization of momentum transport events are investigated in a turbulent boundary layer at the Reynolds number (Re ) = 2700, with time-resolved planar particle image velocimetry. The spectral content of the Reynolds-shear-stress fluctuations reveals that the largest range of time and length scales can be observed in proximity to the wall, while this range becomes progressively more narrow when the wall distance increases. Farther from the ejection events (Q2) are lower than the local average streamwise velocity, while sweep events (Q4) are characterized by wave velocities larger than the local average velocity. These velocity deficits are almost insensitive to the wall distance, which is also confirmed from time tracking the intense transport events. The vertical advection velocities of the intense ejection and sweep events are on average a small fraction of the friction velocity U , different from previous observations in a channel flow. In the range of wall-normal locations 60 &lt; y+ &lt; 600, sweeps are considerably larger than ejections, which could be because the ejections are preferentially located in between the legs of hairpin packets. Finally, it is observed that negative quadrant events of the same type tend to appear in groups over a large spatial streamwise extent