Experiments in Fluids最新文献

Boundary layer transition triggered by a discrete roughness element generates a turbulent wedge that spreads laterally as it proceeds downstream. The historical literature reports the spreading half angle is approximately 6(^{circ }) in zero-pressure gradient flows regardless of Reynolds number and roughness shape. Recent simulations and experiments have sought to explain the lateral spreading mechanism and have observed high- and low-speed streaks along the flanks of the wedge that appear central to the spreading process. To better elucidate the roles of Reynolds number and of streaks, a naphthalene flow visualization survey and hotwire measurements are conducted over a wider range of Reynolds numbers and longer streamwise domain than previous experiments. The naphthalene results show that while the mean spreading angle is consistent with the historical literature, there may be a weak dependency on x-based Reynolds number, which emerges as a result of the large sample size of the survey. The distance between the roughness element and the wedge origin exhibits a clear trend with the roughness–height-based Reynolds number. The hotwire measurements explain that this difference originates from whether breakdown occurs first in the central lobe or flanking streaks of the turbulent wedge. This observation highlights different transition dynamics at play within the supercritical regime. In agreement with past experiments, the hotwire measurements reveal that breakdown occurs in the wall normal shear layer above low-speed streaks. Due to the elongated streamwise extent of this experiment, secondary streak dynamics are also uncovered. A high-speed streak is produced directly downstream of the initiating low-speed streak. Subsequently, a new low-speed streak is observed outboard of the previous high-speed streak. This self-sustaining process is the driving mechanism of turbulent wedge spreading.

The generation process and behavior of non-Newtonian viscoelastic vortex rings generated with a piston cylinder apparatus are studied through fluorescent dye visualizations. The generalized Reynolds numbers targeted by this study are ({varvec{Re}}in mathbf{[10,600]}) and allow the identification of different regimes leading gradually from the generation of a blob that remains attached to the cylinder to that of a starting jet and then to a self-propagating vortex ring. Experiments are performed in eight different viscoelastic solutions and allow to evaluate the influence of the rheological properties of the fluid on the dynamics of the coherent structure. Regardless of the viscoelastic fluid used, the kinematics of the structure exhibits two steps: an initial propagation until reaching a maximum penetration position which depends on the parameters of the experiment (Reynolds number and elasticity number) and then a backward movement over a distance which also depends on these same parameters. The visualizations highlight important deformations of the structure envelope, in particular a spanwise flattening just before reaching the maximum position and a refocusing around the propagation axis during the backward movement. Results are interpreted in terms of Reynolds number and elasticity number.

Extensive experimental research on high-pressure spray has been conducted for decades to deepen our understanding and optimize its use in transportation, aviation, and propulsion applications; however, the near-field and in-nozzle flow characteristics are not fully understood. Dense near-field spray is among the most challenging diagnostic tasks since light is severely scattered and diffused by the liquid droplets and columns. In this work, the near-field spray and in-nozzle flow characteristics of an aeration nozzle at elevated pressures were characterized by neutron radiography imaging at the Oak Ridge National Laboratory High Flux Isotope Reactor. Neutron imaging benefits via strong penetration depths for some metals (i.e., aluminum, lead, and steel) and is sufficiently sensitive to detection of light elements, especially for hydrogen-based molecules, due to the large incoherent scattering cross section of neutrons. Both two-dimensional snapshots of the near-field spray and a three-dimensional tomographic scan of the nozzle geometry and in-nozzle water were obtained. This work provides new quantitative characterization of practical metal nozzle geometry for accurate boundary conditions, internal flow patterns inside the nozzle, and high-pressure spray flows. The findings may be used to improve performance and operating conditions of transportation vehicles and propulsion systems.

We present a novel bifocal imaging method enabling three-dimensional particle tracking and size determination employing a single camera only. The method is based on double refraction causing a particle to be imaged twice, each particle image of different blur. From these double images, a linear calibration function can be derived allowing to determine the three-dimensional particle position unambiguously over the entire depth of measurement volume. As this calibration function is independent of the particle size used, the particle size can be determined simultaneously by relating size of the double images and depth position of the particle. To prove the applicability, a co-laminar flow of two particle suspensions with particles of 1.14 (upmu)m and 2.47 (upmu)m in diameter was measured in a Y-shaped microchannel. While the laminar flow field was measured with very low uncertainty and independent of the particle size, the particle size distributions determined reproduced reliably the size distributions expected for the co-laminar flow applied, with a precision of about 98.6 (%) regarding the particle size discrimination. The progress for research is a new method readily to implement in common optical setups, promising, for example, valuable insights in polydisperse suspension flows—the vast majority of flows in fundamental research and applications.

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Experimental investigations on the motion of rigid particles in microcirculation environments are still scarce owing to the three-dimensional (3D) motion of the particles and to the particle image masking due to the presence of the red blood cells (RBCs). Despite the recent progress on the 3D tracking of rigid particles in RBC flows with defocus particle tracking (DPT) methods, the problem of particle image masking remains to be solved. Here, we propose, test, and evaluate the use hemoglobin-free RBCs, also known as ghost RBCs, as a replacement for normal RBCs in experiments with rigid particles in microcirculation environments. We performed DPT measurements of a pressure-driven flow of normal and ghost RBC suspensions seeded with rigid particles at three different flow rates. We show that the quasi-transparent appearance of ghost RBCs, as a result of the lack of hemoglobin, eliminates the RBC-induced masking of the defocused particle images and allows to achieve the particle matching standards found in cell-free experiments. In fact, ghost RBC suspensions enable the tracking of the rigid particles across the entire height of the microchannel, which was not possible in normal RBC flows. On a fluid dynamic level, we show that ghost RBC suspensions provide similar conditions to normal RBCs in terms of the velocity of the rigid particles and the rigid particles exhibit similar lateral dynamics in both types of cell suspensions. In essence, the findings from this work demonstrate that ghost RBCs are a well-suited replacement for normal RBCs in experiments aiming at deciphering the motion of rigid particles in microcirculation environments.

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A conductance-based sensor to measure liquid film thickness during annular two-phase flows in microchannels has been developed in the present study. The liquid film plays an important role on the characterization of two-phase annular flows. The mean thickness and the presence of interfacial waves influence the heat transfer rate, critical heat flux and pressure drop. The proposed sensor has a ring-shaped design and targets the measurement of films thinner than 50 µm in order to provide detailed information on the liquid film behavior during wall dryout events. It is fabricated on a TEMPAX wafer with micro-electro-mechanical systems (MEMS) technologies. The performance of the prototype device is assessed by using aqueous solutions of known conductivity and imposing liquid films with prescribed thicknesses above the sensor. The effects of the geometrical parameters on the sensor behavior are discussed with the aid of numerical simulation and experimental results. It is found that increasing the size of the electrodes increases the measured electrical signals, while increasing the spacing between the electrodes decreases the measured signal.

Ultrasonic velocity profiler (UVP) can be used for opaque and multiphase flows where particle image velocimetry (PIV) cannot be applied. The time resolution of PIV has greatly improved over the last few decades with the development of high-speed cameras and has been further improved using data-driven approaches. On the other hand, there have been very few works to improve the time resolution of UVP, which is already much lower than that of PIV. This study presents a proof of concept for time resolution improvement of UVP measurement, using an extended proper orthogonal decomposition (EPOD) method with optimized sensors. In this study, the EPOD method is improved by combining it with the sensor selection method, which eliminates the three-sigma ((sigma)) rule-based filtering introduced by Discetti et al. 2018 in the original work of Hosseini et al. 2015. In this study, sensor locations are optimized using sensor selection methods, and time-resolved flow fields are reconstructed using the EPOD method. The sensors’ locations along the line are optimized using non-time-resolved UVP velocity data by two sensor selection methods: determinant greedy (DG) and Bayesian determinant-based greedy (BDG). The performance of DG and BDG-optimized sensors is compared in reconstructing time-resolved flow fields. The technique is demonstrated with two sets of experimental data of flow over a cylinder: first, PIV data, which are down-sampled in the time domain and sampled along a line to mimic the UVP data, and second, actual UVP experimental data conducted in the wake of cylinder. The EPOD method’s time-resolved reconstruction capability was found to depend on the sensors’ location, and both sensor selection methods yielded similar results.

Ultrasound imaging velocimetry (UIV) is a maturing technique for measuring the dispersed phase in two-phase flows. It enables measurements of dense suspensions when optical methods fail. This study explores UIV’s applicability to measure the flow field in a swirling flow reactor (SFR) for solid–liquid mixing of dense suspensions. Despite UIV’s historical focus on unidirectional flows like arteries and axisymmetric pipes, this research demonstrates its adaptation to an inherently complex 3D flow field, i.e., a swirling sudden expansion flow in an SFR. Using high-speed plane-wave imaging and correlation averaging techniques, satisfactory velocity profiles are achieved while preserving sufficient temporal information. Firstly, the applicability of UIV in this specific setup is demonstrated by comparing UIV with stereoscopic particle image velocimetry measurements of a single-phase flow in the SFR, both indicating a Coandă jet flow (CoJF). Secondly, several bulk velocities and volume concentrations (up to 50 vol%) are measured with UIV for a suspension of water and 2.3-mm glass beads. A transducer is installed in two orientations and captures all three velocity components when combining the two datasets. A timestep optimization process is implemented to avoid the need for manual finetuning of the acquisition frequency. A time-domain spectral analysis on the dispersed phase velocity fields in the SFR reveals dominant frequencies between 1.21 and 2.42 Hz, similar to those found in single-phase flow. The general flow structure of the dispersed phase in suspension is very similar to the latter; however, the addition of particles confines the central recirculation zone (CRZ) to the center. Finally, the implementation of swirl to keep solid–liquid mixtures in suspension in the SFR is experimentally confirmed by this study. Quantitative UIV measurements confirm favorable flow structures for mixing, specifically a CoJF that avoids sedimentation. The concentration of solids in an SFR can even be increased up to 50 vol% while still maintaining a uniform suspension.

This paper introduces a new stereo-video-based free-surface reconstruction system developed for wave-tank experiments. The originality of the proposed approach relies on the use of short water waves and an adapted lighting system to create a fine texture suitable for the cross-correlation of the stereo image pairs. The feasibility of the approach is demonstrated experimentally in a wave flume. The accuracy of the stereo-video free-surface reconstruction method is assessed through comparisons with measurements performed with a servo-controlled wave gauge. The reconstruction of the free surface at rest and during different regular (periodic) long-crested wave experiments are considered for this purpose. The results demonstrate that, with a suitable free-surface roughness, the accuracy of the stereo-system can be similar to the accuracy of the wave gauge. The accuracy, the simplicity and the flexibility of the approach, which does not necessitate any seeding or dying of the water, nor the use of a laser light source, make it a promising measurement technique for water-wave experiments.

This paper reports an algorithm for measuring the time-averaged skin friction vector field (overline{pmb {tau }}(pmb {X})) starting from time-resolved temperature maps, acquired by a functional coating of temperature-sensitive paint. The algorithm is applied to a large area around a wall-mounted cube, immersed in the turbulent boundary layer over a flat plate. The method adopts a relaxed version of the Taylor Hypothesis operating on time-resolved maps of temperature fluctuations (T') measured on the slightly warmer bounding surface. The procedure extracts ({overline{U}}_T(pmb {X})), the celerity of displacement of (T'), as the best approximation of the forecasting provided by the frozen turbulence assumption near the wall, where its rigorous application is inappropriate. The (overline{pmb {tau }}(pmb {X})) estimation is based on the hypothesis of a linear relationship between ({overline{U}}_T(pmb {X})) and ({overline{U}}_U(pmb {X})), chained to the one between ({overline{U}}_U(pmb {X})) and ({overline{U}}_tau (pmb {X})). We assess the outcomes of the proposed algorithm against those derived by the 2D and 3D Lagrangian particle tracking (LPT) methodology ’Shake-The-Box’, whose advent has made available high-quality near-wall flow field information. Furthermore, data from high-density 2D time-resolved LPT allows exploring the suitability of the linear relationships chain between ({overline{U}}_T(pmb {X})) and ({overline{U}}_tau (pmb {X})) in the proposed context.