Recent research highlights the need for comprehensive three-dimensional (3D) analysis of laryngeal flow to better understand voice production, as traditional 2D methods fail to capture the full complexity of supraglottal jet dynamics. This study employed tomographic particle image velocimetry to capture the volume velocity flow fields in a synthetic multilayered vocal fold model. The impact of increased airway resistance from different vocal tract configurations was examined. Results indicated that adding a vocal tract reduced the maximum axial velocity and jet displacement, particularly at low subglottal pressure (Psg). Higher Psg increased both the maximum axial velocity and jet displacement. For all configurations, with and without a vocal tract, the vocal folds were observed to open at the posterior and anterior edges first, indicated by a double jet formation at the beginning of the opening phase, followed by an elongated jet during peak flow and a double jet at the posterior and anterior edges during the closing phase. Contrary to previous studies, the glottal flow waveforms became more symmetric between the opening and closing phases with higher Psg and the presence of a vocal tract. Additionally, vocal efficiency (VE) decreased while cepstral peak prominence increased with higher Psg. Overall, this study provides further insights into the influence of vocal tract configurations on the supraglottal jet and supports the correlation between glottal flow skewing and VE.

Over the past decade, there has been a tremendously increased interest in understanding the neurophysiology of cerebrospinal fluid (CSF) flow, which plays a crucial role in clearing metabolic waste from the brain. This growing interest was largely initiated by two significant discoveries: the glymphatic system (a pathway for solute exchange between interstitial fluid deep within the brain and the CSF surrounding the brain) and meningeal lymphatic vessels (lymphatic vessels in the layer of tissue surrounding the brain that drains CSF). These two CSF systems work in unison, and their disruption has been implicated in several neurological disorders including Alzheimer's disease, stroke, and traumatic brain injury. Here, we present experimental techniques for quantification of CSF flow via direct imaging of fluorescent microspheres injected into the CSF. We discuss detailed image processing methods, including registration and masking of stagnant particles, to improve the quality of measurements. We provide guidance for quantifying CSF flow through particle tracking and offer tips for optimizing the process. Additionally, we describe techniques for measuring changes in arterial diameter, which is an hypothesized CSF pumping mechanism. Finally, we outline how these same techniques can be applied to cervical lymphatic vessels, which collect fluid downstream from meningeal lymphatic vessels. We anticipate that these fluid mechanical techniques will prove valuable for future quantitative studies aimed at understanding mechanisms of CSF transport and disruption, as well as for other complex biophysical systems.

Traumatic brain injury (TBI) poses a major public health challenge. No proven therapies for the condition exist so protective equipment that prevents or mitigates these injuries plays a critical role in minimizing the societal burden of this condition. Our ability to optimize protective equipment depends on our capacity to relate the mechanics of head impact events to morbidity and mortality. This capacity, in turn, depends on correctly identifying the mechanisms of injury. For several decades, a controversial theory of TBI biomechanics has attributed important classes of injury to cavitation inside the cranial vault during blunt impact. This theory explains counter-intuitive clinical observations, including the coup-contre-coup pattern of injury. However, it is also difficult to validate experimentally in living subjects. Also, blunt impact TBI is a broad term that covers a range of different head impact events, some of which may be better described by cavitation theory than others. This review surveys what has been learned about cavitation through mathematical modeling, physical modeling, and experimentation with living tissues and places it in context with competing theories of blunt injury biomechanics and recent research activity in the field in an attempt to understand what the theory has to offer the next generation of innovators in TBI biomechanics.

We present experimental results of a study on oxygen transfer rates in a water channel facility with varying turbulence inflow conditions set by an active grid. We compare the change in gas transfer rate with different turbulence characteristics of the flow set by four different water channel and grid configurations. It was found that the change in gas transfer rate correlates best with the turbulence intensity in the vertical direction. The most turbulent cases increased the gas transfer rate by 30% compared to the low turbulence reference case. Between the two most turbulent cases studied here, the streamwise turbulence and largest length scales in the flow change, while the gas transfer rate is relatively unchanged. In contrast, for the two less turbulent cases where the magnitude of the fluctuations normal to the free surface are also smaller, the gas transfer rate is significantly reduced. Since the air-water interface plays an important role in the gas transfer process, special attention is given to the free-surface deformations. Despite taking measures to minimise it, the active grid also leaves a direct imprint on the free surface, and the majority of the waves on the surface originate from the grid itself. Surface deformations were, however, ruled out as a main driver for the increase in gas transfer because the increase in surface area is < 0.25%, which is two orders of magnitude smaller than the measured change in the gas transfer rate.

In aircraft engines, thermoacoustic oscillations in the combustion chamber contribute significantly to noise emissions, which, like all other emissions, must be drastically reduced. Thermoacoustic oscillations are not only a concern, they can also be beneficial in hydrogen combustion. This work demonstrates that thermoacoustic density oscillations with amplitudes at least an order of magnitude smaller than those resulting from density gradients in a turbulent flame can be detected using laser interferometric vibrometry. This improvement was made possible by heterodyning a carrier fringe system in background-oriented schlieren (BOS) recordings, which were subsequently analyzed using techniques commonly used for holographic interferometry. In comparison with other BOS evaluation techniques, the filtering of the individual frames in the Fourier domain offers a more efficient computational approach, as it allows for phase averaging of a high number of single recordings to reduce noise from turbulence. To address fringe pattern distortions and cross talk in the Fourier domain, which both have been observed by other authors, we propose background subtraction methods and an optimized background pattern. Additionally, the procedure provides a visualization tool for marking the high turbulence regions of heat release by the variations in fringe amplitude. Finally, the line-of-sight data are reconstructed using the inverse Abel transform, with the data calibrated by laser interferometric techniques, resulting in local values for density oscillations.

The current work discusses the demonstration of an event-based (EB) camera for time-resolved imaging (10,000 frames/sec) of the primary atomization of a canonical air-assist atomizer. Experiments were performed simultaneously with conventional high-speed and event-based cameras, enabling us to quantitatively assess the performance of event-based cameras in spray imaging (particularly near-field liquid jet breakup) applications. Three atomization breakup regimes are considered: columnar, bag, and multimode. Dynamic mode decomposition (DMD) was implemented to analyze the acquired instantaneous time-dependent images from both cameras and assess their performance in extracting turbulence statistics of the primary atomization. The computed DMD frequency spectrum and spatial modes of liquid breakup characteristics from the images recorded from both cameras are comparable, highlighting the potential of event-based cameras in extracting coherent structures in the primary atomization zone and their spectral contents. However, in some instances, the EB camera underpredicts the DMD modes compared to high-speed cameras, and the reasons for these discrepancies were explained. Finally, the limitations (e.g., event saturation) of event-based cameras in the context of primary atomization imaging were also discussed.

Understanding the gust response of free-falling bodies such as plant seeds and debris is critical in predicting their dispersal. Furthermore, gusts can significantly affect the performance and survivability of low-inertia aerial vehicles. However, current methodologies for studying common gusts, particularly transverse gusts, which are characterised by the sudden appearance of a flow velocity component orthogonal to the flyer's velocity, are not applicable to untethered or free-falling bodies. This article introduces a novel approach that addresses this limitation through an accelerating reference frame generating a fictitious force that temporarily scales and redirects the gravitational force. This approach is demonstrated through a first-of-its-kind vertical wind tunnel that accelerates horizontally in the direction normal to the flow with the same acceleration as the gust. A preliminary characterisation of the facility is presented. The tunnel acceleration generates the same pressure gradient as irrotational, uniform transverse gusts, without introducing the shear layer typical of Küssner's gusts. The gust response of a free-falling dandelion diaspore to a discrete transverse gust (Wagner type) is demonstrated, but the proposed approach is suitable for arbitrary time series of transverse gusts, including Theodorsen-type periodic gusts. For the first time, this novel approach will allow investigating the dynamic response of untethered bodies to transverse gusts, including micro- and nanodrones, unpowered microrobots, plant seeds, debris and more.

Due to its importance in airborne disease transmission, especially because of the COVID-19 pandemic, much attention has recently been devoted by the scientific community to the analysis of dispersion of particle-laden air clouds ejected by humans during different respiratory activities. In spite of that, a lack of knowledge is still present particularly with regard to the velocity of the emitted particles, which could differ considerably from that of the air phase. The velocity of the particles is also expected to vary with their size. In this work, simultaneous measurements of size and velocity of particles emitted by humans while speaking have been performed by means of Interferometric Laser Imaging Droplet Sizing (ILIDS). This technique allowed us to detect emitted particles with size down to 2 µm as well as to quantify all three components of the velocity vector and the particle concentration. The outcomes of this work may be used as boundary conditions for numerical simulations of infected respiratory cloud transmission.

This paper presents a hybrid and unsupervised approach to flame front detection for low signal-to-noise planar laser-induced fluorescence (PLIF) images. The algorithm combines segmentation and edge detection techniques to achieve low-cost and accurate flame front detection in the presence of noise and variability in the flame structure. The method first uses an adaptive contrast enhancement scheme to improve the quality of the image prior to segmentation. The general shape of the flame front is then highlighted using segmentation, while the edge detection method is used to refine the results and highlight the flame front more accurately. The performance of the algorithm is tested on a dataset of high-speed PLIF images and is shown to achieve high accuracy in finely wrinkled turbulent hydrogen-enriched flames with order of magnitude improvements in computation speed. This new algorithm has potential applications in the experimental study of turbulent flames subject to intense wrinkling and low signal-to-noise ratios.

A method is proposed to obtain full-domain spatial modes based on proper orthogonal decomposition (POD) of particle image velocimetry (PIV) measurements taken at different (overlapping) spatial locations. This situation occurs when large domains are covered by multiple non-simultaneous measurements and yet the large-scale flow field organization is to be captured. The proposed methodology leverages the definition of POD spatial modes as eigenvectors of the spatial correlation matrix, where local measurements, even when not obtained simultaneously, provide each a portion of the latter, which is then analyzed to synthesize the full-domain spatial modes. The measurement domain coverage is found to require regions overlapping by to yield a smooth distribution of the modes. The procedure identifies structures twice as large as each measurement patch. The technique, referred to as , is applied to planar PIV data of a submerged jet flow where the effect of patching is simulated by splitting the original PIV data. Patch POD is then extended to robotic measurement around a wall-mounted cube. The results show that the patching technique enables global modal analysis over a domain covered with a multitude of non-simultaneous measurements.

Research on free falling particles has predominantly focused on wake dynamics and vortex shedding of individual particles in quiescent flow. However, when these particles fall collectively, the wakes of neighboring particles alter the flow fields. To investigate how the settling and wake dynamics of particles are affected by the wakes of other settling particles, we conducted volumetric experiments using the Shake-The-Box technique. Negatively buoyant 12 mm particles of four different geometries (sphere, flat cuboid, circular, and square cylinders) were first released individually into quiescent water. Subsequently, the particles were released individually into the bulk wakes of 20 monodisperse particles. Using four high-speed cameras and LEDs, we simultaneously captured both 3D particle and fluid motions in the terminal velocity regime. The imaging domain measured 90 mm × 90 mm × 40 mm. Our results show that all trailing particles settling through the bulk wakes gain additional downward momentum from the turbulent wakes, causing them to fall faster than in quiescent flow. However, when the induced velocity of the preceding wakes is subtracted, the relative settling velocity was found to be essentially the same as the particle falling in quiescent fluid. Upstream of the particle, the vortices in the bulk wake interact with the developing shear layer along the particle. The wake downstream of the trailing particle also appears more chaotic than that in quiescent flow.

Using the travel time of sound waves advected by a moving carrier medium, acoustic tomography allows to reconstruct temperature and flow fields in opaque fluids without tracers or scattering particles. Reconstruction algorithms are conventionally based on the ray approximation and pose difficulties, especially in enclosed domains: Interferences of early reflections can prevent the assignment of each arrival to the correct ray path. We develop a full-waveform inversion for acoustic tomography in laboratory-scale experiments, perform synthetic tests, and benchmark these with a straight-ray algorithm. Multiple late arrivals of reflected waves are considered in order to increase the quality of the reconstructions when restricted to a sparse transducer array. In addition, the full-waveform algorithm allows to invert simultaneously emitted signals from all sources, decreasing the acquisition time in which a flow must be assumed stationary. These findings make the new method especially interesting for researchers experimenting with enclosed, opaque fluids where no optical imaging is feasible. Furthermore, we envision a potential application of the newly developed method to map flows around objects or complex wall geometries and even multiphase flows.

We present a newly constructed jet array with a novel driving scheme for turbulence generation in a vertical water tunnel and measurements of the turbulent flow this jet array establishes. The design of the array allows us to control the mean background flow and the turbulence intensity independently of each other. The array consists of a rectangular arrangement of 112 individually computer-controlled water jets that are aligned streamwise to the measurement section of our 8-m tall vertically recirculating water tunnel. Using solenoid valves, individual jets are activated following predefined protocols that can be tailored to obtain different turbulence statistics within the measurement section. The protocols are based on four-dimensional OpenSimplex noise, a type of gradient noise that features spatial and temporal coherence. Details of the mechanical and electrical designs are presented, together with a detailed description of the protocol generation. We show that the resulting turbulence is near homogeneous and isotropic, with a turbulence intensity of Order 1, an energy dissipation rate of order and . Additionally, we present experiments that show the effects that various system and protocol parameters have on the created flow conditions and address the streamwise development, as well as the homogeneity and isotropy of the flow.

This paper investigates the characteristics and control of tip vortices generated by a finite wing, focusing on the impact of the novel grooved-tip designs. Tip vortices can lead to flow loss, noise, vibration and cavitation in hydrodynamic systems. We propose and develop a grooved-tip design, featuring multiple grooves distributed along the wing tip to alter the tip vortex structure and dynamics. Four grooved-tip designs, including tilted and shrinking grooves, were experimentally investigated. Streamwise and cross-flow particle image velocimetry (PIV) measurements were employed to visualise the flow fields near the wing tip and along the primary tip vortex trajectory. The PIV results demonstrate that the grooved-tip designs significantly reduce the velocity magnitude within the primary tip vortex. This velocity deficit is attributed to the decreased suction within the vortex core. Furthermore, cross-flow PIV measurements reveal that the tip separation vortex is substantially suppressed, and the strength of the primary tip vortex is significantly mitigated. Downstream of the wing, the grooved tips lead to a reduction in vortex swirling strength and an enlargement of the vortex dimensions, suggesting enhanced diffusion and a reduction of the pressure drop of approximately 40%, based on the estimation from a reduced-order model linking pressure to vortex swirling strength. Our findings highlight the potential of these grooved-tip designs to effectively modify tip vortex behaviour and mitigate the pressure drop within the tip vortex region, with negligible changes to the lift and drag performance. This work can inform advanced passive vortex control strategies in wing- and blade-based systems, with potential applications in hydrofoils of marine vessels and underwater vehicles, as well as in turbines and propellers.

In this work, we study the jetting dynamics of individual cavitation bubbles using x-ray holographic imaging and high-speed optical shadowgraphy. The bubbles are induced by a focused infrared laser pulse in water near the surface of a flat, circular glass plate, and later probed with ultrashort x-ray pulses produced by an x-ray free-electron laser (XFEL). The holographic imaging can reveal essential information of the bubble interior that would otherwise not be accessible in the optical regime due to obscuration or diffraction. The influence of asymmetric boundary conditions on the jet's characteristics is analysed for cases where the axial symmetry is perturbed and curved liquid filaments can form inside the cavity. The x-ray images demonstrate that when oblique jets impact the rigid boundary, they produce a non-axisymmetric splash which grows from a moving stagnation point. Additionally, the images reveal the formation of complex gas/liquid structures inside the jetting bubbles that are invisible to standard optical microscopy. The experimental results are analysed with the assistance of full three-dimensional numerical simulations of the Navier-Stokes equations in their compressible formulation, which allow a deeper understanding of the distinctive features observed in the x-ray holographic images. In particular, the effects of varying the dimensionless stand-off distances measured from the initial bubble location to the surface of the solid plate and also to its nearest edge are addressed using both experiments and simulations. A relation between the jet tilting angle and the dimensionless bubble position asymmetry is derived. The present study provides new insights into bubble jetting and demonstrates the potential of x-ray holography for future investigations in this field.

In this study we quantify the uncertainty relative to a novel Lagrangian tracking technique to measure the complete solid-body rotation rate of anisotropic micro-plastic fibers. By exploiting their geometry-specifically, their elongation and curvature for tumbling and spinning rate measurements, respectively-we address a gap in the literature regarding the tracking of fibers' unique orientation along their trajectories. The impact of fiber geometry and imaging parameters on the accuracy of the solid-body rotation rates measurements is investigated. The influence of spatial and temporal resolution on the measurement uncertainty is assessed on synthetic data. Experimental results obtained in a channel flow demonstrate the method's potential to accurately detect rotations of fibers with lengths approaching the Kolmogorov scale.

To better understand how turbulent flow structures develop within shear-thinning suspensions (STSs), we investigate the behavior of a shear layer forming within an STS downstream of a sudden expansion with an expansion ratio of 0.5. Specifically, the shear-layer reattachment behavior downstream of an axisymmetric expansion is characterized through ultrasound imaging velocimetry (UIV) and through pressure measurements, and the observed behavior is used to surmise how the shear layer is modified within the STS. Four fluids are investigated, which include pure water, as well as three 1750 ppm xanthan-gum-in-water solutions mixed with non-reactive mineral microspheres at volume fractions of 0%, 15%, and 30%. Wall-pressure measurements were collected through pressure taps located at 0 to 25.8 downstream of the expansion with subsequent UIV measurements collected from 1 to 9 downstream of the expansion, where is the step height and equals the difference between the pipe and throat radii. For single-phase cases, pressure-recovery profiles and UIV flow fields indicate a predictably large reattachment length at low Reynolds numbers, which shortens as the Reynolds number increases from to and finally stabilizes at roughly 8. In contrast, the STSs exhibit pressure-recovery and pipe-wall velocity profiles indicating a reattachment length that is consistently short (8) and independent of Reynolds number. The results indicate that the suspended phase within the STSs causes the shear layer to diffuse far more rapidly, thereby promoting momentum transfer toward the wall, which results in a consistently short reattachment length.

This investigation explores the utility of Alternating Current, Dielectric Barrier Discharge (AC-DBD) plasma actuators for producing three-dimensional disturbances of a desired spanwise wavelength via superposition. The technique utilizes two pairs of exposed and covered electrodes on a single dielectric layer arranged in streamwise succession. Two-dimensional forcing is achieved through operation of the upstream, spanwise uniform electrode pair, while three-dimensional forcing at a prescribed spanwise wavelength is attained by operating both electrode pairs simultaneously, with the downstream actuator spanwise modulating the upstream, two-dimensional output. The ability to produce disturbances of different spanwise wavelengths but with equal streamwise wavelength, frequency and total momentum is established through a combined characterization effort that considers quiescent and in-flow conditions. A demonstration of the technique in an exemplary wall-bounded shear flow, a laminar separation bubble, is provided, revealing spanwise wavelength dependent disturbance growth in the flow that could be exploited for performance gains in future flow control endeavours.

Oxygen transfer across a deforming air-water interface is studied using a synergy of particle image velocimetry and laser-induced fluorescence (LIF). Such approaches have previously been limited to flat interfaces. We develop simultaneous measurements of velocity fields, dissolved oxygen (DO) concentration fields, and interface positions for spatial and temporal tracking. The imaging process begins after the DO in the water has been chemically depleted and continues until the water is saturated with DO. The oxygen LIF intensity field is calibrated using measurements from an optical oxygen probe to ensure accurate conversion into physical unit (mg/L). A canonical air turbulent channel flow, with a centerline velocity of 6.6 m/s (Reynolds number based on channel height of 21,700), develops for more than 100 heights before the bottom boundary condition is changed from a solid wall to a water surface. This induces transient and wavy structures on the air-water interface and generates velocity fluctuations and vorticity on the water side, which drives DO transport. The spatial evolution of DO concentration reveals steep gradients near the interface that diminish with depth, while the temporal evolution shows a reduction in concentration differences between the bulk and interface from about 35% to less than 5% as the water saturates. Concentration fluctuations are lower near the interface compared to the bulk and diminish in time as the system approaches saturation. Turbulent scalar transport analysis shows high vertical flux near the interface, and this too changes as the bulk DO concentration evolves, emphasizing that the observed phenomena are transient and should be treated as such.

Experiments are carried out to study the interaction of a spray of spherical micronic oil droplets with a turbulent plane air jet impacting a wall. The context is the separation of a contaminated atmosphere with passive particles from a clean atmosphere by using a dynamical air curtain. A spinning disk is used to produce the spray of oil droplets close to the air jet. The diameter of the produced droplets varies between 0.3 and m. The jet and particulate Reynolds numbers and the jet and Kolmogorov-Stokes numbers are, respectively, equal to Re , , and . The ratio of the jet height to nozzle width is . The flow properties in the experiments are measured by particle image velocimetry and are in good agreement with large eddy simulation results. The droplet/particle passing rate (PPR) through the air jet is measured by an optical particle counter. The PPR decreases with the increase in the droplet diameter for the studied droplet size range. Whatever the droplet size is, the PPR increases with time due to the presence of two large vortices on each side of the air jet that bring the droplets back to the jet. The accuracy and repeatability of the measurements are verified. The present results can be used to validate Eulerian/Lagrangian numerical simulations on the interaction of micronic droplets with a turbulent air jet.

In urban environments, pollutant ingress from outdoor sources poses a significant challenge to indoor air quality. Cross-ventilation, while essential for passive cooling and natural airflow, can also facilitate the entry of outdoor contaminants into indoor spaces. To investigate the dynamics of outdoor-to-indoor pollutant transport, the present study employs an idealized configuration, namely, a hollow cube representing a scaled-down model building with window openings in the upstream and downstream faces, subjected to an upstream passive scalar source within an atmospheric boundary layer. The experiments are conducted in two distinct facilities: a water tunnel using Rhodamine dye as the scalar, and a wind tunnel using propane gas, all performed at a specified flow Reynolds number of for a fixed boundary layer-to-cube height ratio of about 3; here, is the streamwise velocity at cube's height () measured without the cube. The scalar, released from a ground-level upstream source, is predominantly transported by a streamwise advective flux, while relatively weaker wall-normal advective and turbulent fluxes contribute to vertical dispersion and local mixing. A fraction of the oncoming scalar enters the cube intermittently, through the upstream window. Inside, a central jet-like flow carries the scalar parcels primarily by streamwise advective flux, while also interacting with the upper and lower recirculation regions, enabling scalar exchange across these zones through wall-normal advective and turbulent fluxes. While the time-averaged concentration field inside the cube is nearly uniform, suggesting effective mixing, instantaneous concentration traces exhibit strong intermittency, with sporadic peak events, highlighting the risk of transient peak exposures. The indoor concentration exponentially decays over time once the source is turned off, with a slower decay in the upper recirculation region, implying relatively prolonged exposure near the ceiling region. Both experimental setups produce closely matching values and consistent trends in the spatio-temporal dynamics of scalar concentration, and also highlight their complementary nature, with each offering distinct advantages. The present findings will deepen our understanding of pollutant ingress and mixing in buildings in cross-ventilated flows and also offer valuable insights to future modeling of pollutant exposure in urban indoor spaces.