Experiments in Fluids最新文献

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.

This paper presents the development of fluorescent tracer particles for use in gas flows as a countermeasure for undesired strong light reflections on surfaces of channel walls or obstacles and as a label for the discrimination of multi-constituent flows. The employment of fluorescent dye-doped tracer particles with a wavelength-specific optical filter enables the separation of the Stokes-shifted particle light emission from reflections on surfaces and Mie scattering from non-fluorescing particles. The fluorescent particles were made of Pyrromethene 567 (P567) and Di-Ethyl-Hexyl-Sebacate (DEHS), and the addition of P567 was not found to alter the characteristics of the particles generated. Investigations in a low-speed wind tunnel revealed that the intensity of fluorescent emission is proportional to the dye concentration at least up to (2.0,hbox {g l}^{-1}). The efficacy of reflection removal was investigated in a setup with a metal turbine blade placed in the flow and a laser sheet oriented to impinge the blade surface. With the installation of an appropriate optical filter, undesired light reflections were successfully removed, and reasonable vector calculations were enabled in proximity to the reflective blade surfaces. Finally, the performance of the modified DEHS was compared to conventional DEHS with the measurement of a canonical turbulent boundary layer (TBL). The flow was globally seeded with conventional DEHS and the TBL was locally seeded with fluorescing DEHS; simultaneous imaging with a notch filter confirmed that the flow is accurately tracked by the modified DEHS without additional bias. Furthermore, this indicated the possibility of using the newly developed particles to segregate portions of a flow with multiple constituents.

We study the transient dynamics of three-dimensional detonation cells when the detonation front is subjected to weak expansion due to the diffraction from a straight channel to a diverging channel. We focus on the effect of the cross-sectional shape, namely square or round, using diverging channels with the same initial cross-sectional area of 16 cm (^{2}) as the straight channels and the same expansion rate. The reactive mixture is (2,hbox {H}_{2} + hbox {O}_{2} + 2,hbox {Ar}) at the initial pressure of 20 kPa and temperature of 294 K, and we use the sooted-foil technique to record the cellular dynamics. The mean cell widths first increase from different initial values, which depend on the cross-sectional shape and then decrease to stabilize at the same value independent of the shape but larger than the initial values. We use a relation of detonation dynamics between the velocity, total curvature and acceleration of the average detonation front to interpret successfully, albeit qualitatively, all the experimental trends. This sensitivity thus makes these experimental data a reliable basis for high-resolution numerical simulations capable of handling three-dimensionality and detailed chemical kinetics mechanisms. Defining a significative mean width of detonation cells requires constant cross-sectional tubes of size and length sufficiently large. Inductively, representing three-dimensional cells requires more statistical descriptors than a single mean width.

An important step in the application of Lagrangian particle tracking (LPT) or in general for image-based single particle identification techniques is the detection of particle image locations on the measurement images and their sub-pixel accurate position estimation. In case of volumetric measurements, this constitutes the first step in the process of recovering 3D particle positions, which is usually performed by triangulation procedures. For two-component 2D measurements, the particle localization results directly serve as input to the tracking algorithm. Depending on the quality of the image, the shape and size of the particle images and the amount of particle image overlap, it can be difficult to find all, or even only the majority, of the projected particle locations in a measurement image. Advanced strategies for 3D particle position reconstruction, such as iterative particle reconstruction (IPR), are designed to work with incomplete 2D particle detection abilities but even they can greatly benefit from a more complete detection as ambiguities and position errors are reduced. We introduce a convolutional neural network (CNN) based particle image detection scheme that significantly outperforms current conventional approaches, both on synthetic and experimental data, and enables particle image localization with a vastly higher completeness even at high image densities.

The Shake-The-Box technique was applied to experimentally quantify the time-resolved volumetric flow field around a free-flying quadcopter UAV with an overall span of about 0.5 m. State-of-the-art LED illumination and high-speed camera equipment was combined with modern Lagrangian tracer particle tracking and data assimilation techniques, facilitating a measurement volume larger than ({1.5},{hbox {m}^3}). The setup allowed for both hover and limited maneuvering of the quadcopter, while resolving even small details of the complex interactional aerodynamics. In hover out of ground effect, the four individual rotor wakes merged into a single jet within a few rotor radii below the rotor planes. Evaluating the mass and momentum fluxes over suitable control volumes yields accurate estimates for the quadcopter’s total thrust, the asymmetric thrust distribution between front and back rotors, and the entrainment of external flow through turbulent mixing. Hover in ground effect decreases the power requirement and induces recirculating flow in the center of the four rotors. The outwash pattern is non-uniform with jets developing between the rotors and pointing in radially outward directions. Forward flight cases result in a skewed, rapidly merging wake flanked by the roll-up of two “supervortices” similar to the wingtip vortices of fixed-wing vehicles.

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.

Graphical abstract