Experiments in Fluids最新文献

Many flows have a multi-constituent nature, where understanding the transfer of mass and momentum between different parts of the flow is key. While experiments in liquid media have long used flow tagging techniques such as dye injection to isolate parts of the flow, analogous methods that do not compromise full-field velocimetry in gas flows are highly impractical. The recent introduction of a solution of Pyrromethene 567 (P567) in di-ethyl-hexyl-sebacate (DEHS) to produce a seeding fluid capable of fluorescent emission in addition to Mie scattering promises to address this need. By locally seeding a secondary flow of interest with the modified DEHS, the fluorescent signal can be used for tagging; global seeding of main flow with standard DEHS, which only produces Mie scattering of incident laser light, is used to obtain full-field velocimetry with established PIV techniques. Performing morphological image processing and intensity-based thresholding on the fluorescent particle images yields a continuum representation of the secondary flow. This can then be combined with velocimetric information from PIV to conduct quantitative zonal analyses. This technique has been applied to the flow behind an active synthetic-jet turbulence grid and a turbulent boundary layer (TBL). With the new zonal decomposition capabilities offered, data relating to the intermittency of the flows, statistical structure of the phenomena, turbulent/non-turbulent interface (TNTI) and entrainment and detrainment can be extracted.

In the present study, by adopting the advantage of ultrasonic techniques, we developed a multichannel pulsed ultrasonic Doppler velocimetry (MPUDV) to measure the two-dimension-two-component (2D-2C) velocity fields of liquid metal flow. Due to the specially designed ultrasonic host and post-processing scheme, the MPUDV system can reach a high spatiotemporal resolution of 50 Hz and 3 mm in the measurement zone of (192 times 192) (text {mm}^{2}). The experimental loop includes a cavity test section designed to establish a recirculating flow, thereby validating the accuracy of the MPUDV in measuring the velocity field with the well-developed particle image velocimetry (PIV). A comparison of the data obtained from the PIV and MPUDV methods revealed less than 3% differences exist in the 2D-2C velocity field between the two techniques during simultaneous measurements of the same flow field. This finding strongly demonstrates the reliability of the MPUDV method developed in this paper. Moreover, the ternary alloy GaInSn, which has a melting point below that of room temperature, was selected as the working liquid in the experimental loop to validate the efficacy of the MPUDV in measuring 2D-2C velocity fields. A series of tests were conducted in the cavity test section at varying Reynolds (Re) numbers, ranging from 9103 to 24,123. The measurements demonstrated that the MPUDV could accurately measure the flow structures, characterized by a central primary circulation eddy and two secondary eddies in the opaque liquid metal. Furthermore, comprehensive analyses of the velocity data obtained by the MPUDV were conducted. It was found that the vortex center of the primary circulating eddy and the size of the secondary eddies undergo significant alterations with varying Re numbers, attributed to the influence of the enhanced inertial forces on the flow within the cavity. It is therefore demonstrated that the current MPUDV methodology is applicable for measuring a 2D-2C velocity field in opaque liquid metal flows.

Accurately estimating multidimensional positions and displacements in fluid dynamic experiments is essential for understanding complex flow phenomena. This paper presents a novel optical measurement setup capable of determining velocity fields using periodic coded optical apertures in a single-camera imaging system. Our method involves placing a periodic transmission grating in front of the optical system and aligning a laser beam along the system’s optical axis. When particles intersect the laser beam, they scatter light, producing out-of-focus images on the imaging sensor resembling the periodic grating. The size and position of these images vary with the distance to the imaging system and the offset from the optical axis, allowing for accurate determination of particle positions. We derive mathematical formulas describing the imaging system and the relationship between image properties and particle positions and verify them through simulations. Additionally, we develop a calibration method enabling arbitrary optical imaging systems to be used with this technique. Finally, we demonstrate the effectiveness of our approach by measuring velocity fields near a ducted ship propeller, a practical application that underscores the real-world impact of our research in fluid dynamics.

This paper evaluates the experimental generation of internal solitary waves (ISWs) in a miscible two-layer system with a free surface using a jet-array wavemaker (JAW). Unlike traditional gate-release experiments, the JAW system generates ISWs by forcing a prescribed vertical distribution of mass flux. Experiments examine three different layer-depth ratios, with ISW amplitudes up to the maximum allowed by the extended Korteweg-de Vries (eKdV) solution. Phase speeds and wave profiles are captured via planar laser-induced fluorescence and the velocity field is measured synchronously using particle imaging velocimetry. Measured properties are directly compared with the eKdV predictions. As expected, small- and intermediate-amplitude waves match well with the corresponding eKdV solutions, with errors in amplitude and phase speed below 10%. For large waves with amplitudes approaching the maximum allowed by the eKdV solution, the phase speed and the velocity profiles resemble the eKdV solution while the wave profiles are distorted following the trough. This can potentially be attributed to Kelvin-Helmholtz instabilities forming at the pycnocline. Larger errors are generally observed when the local Richardson number at the JAW inlet exceeds the threshold for instability.

This study investigates the use of a trained neural network (NN) to enable more efficient noise reduction in processing images associated with acoustically coupled combustion phenomena, as compared with more commonly used image processing techniques. The approach is applied to experiments involving high-speed imaging of a single and coaxial methane–air jet diffusion flames exposed to various acoustically resonant environments. Proper orthogonal decomposition (POD) analysis applied to the flame imaging may be used to capture characteristic signatures in the flame dynamics and in verification of the proposed approach in this investigation. The NN trains on low-exposure input images and high-exposure response images for a steadily burning fuel jet with no coaxial flow, yet is remarkably successful when applied to a range of coaxial flow and acoustic excitation conditions. The proposed neural network approach demonstrates a significant decrease in the preprocess time required in analyzing flame images, typically by over a factor of 5, and preserves image quality. The approach replicates POD-based flame dynamics very well, for both low-amplitude and high-amplitude flame responses, the latter involving transitions in the dynamics due to the introduction of multiple timescales. The relative simplicity and success of this NN approach thus show the potential for improved image processing for complex dynamical flows and their transitional features.

The planar laser-induced fluorescence (PLIF) method has been widely applied for measuring the thickness of liquid films. To identify the liquid–gas interface, however, PLIF-based methods require an artificial threshold value of brightness or a calibration curve between the thickness and the brightness, limiting its application in measuring unknown film thickness. To overcome the drawbacks, we propose a new method, time-variant PLIF (T-PLIF), which employs an index of time variance of brightness to detect the interface. We first establish the mathematical principle of T-PLIF, wherein the time variance of a phase-dependent variable becomes the maximum exactly at the time-averaged position of the wavy interface. We then perform experiments for a well-controlled downward annular liquid film flow to test the reliability of T-PLIF. We demonstrate that T-PLIF measures liquid film thickness of (h > 0.2,textrm{mm}) with the accuracy of (varepsilon le 10%) to the theoretical reference and (h le 0.2,textrm{mm}) with (varepsilon = 20%). T-PLIF is able to quantify the film thickness with no need for any pre-/post-calibration or artificial threshold values. We further confirm the applicability of T-PLIF to the wavy film flow sheared by an airflow up to (30,text{m/s}) by measuring the phase velocity and wavelength, which well matches the theoretical results.

The Coandă effect relies on the deviation of a fluid stream, usually a jet, from a straight direction toward a lateral wall. In the past, it has been primarily investigated in the context of air flows to increase lift and optimize propulsion in aeronautical applications, while in this paper, it is considered in the context of shallow-water free-surface flows for potential applications in coastal engineering. Through an extensive experimental campaign, the influence of multiple parameters, including some previously unexplored factors related to the presence of a free surface (i.e., Froude’s number), is investigated by employing time-resolved particle image velocimetry. These parameters are considered individually and simultaneously to provide a comprehensive understanding of their effects and identify the key variables controlling the phenomenon. Results indicate that in this configuration, the Coandă effect is slightly enhanced compared to standard two and three-dimensional air jets, resulting in shorter reattaching lengths. While cross-sectional velocity profiles were measured to be symmetrical and somewhat similar to that of a free jet, it was found that the jet’s axis is not a streamline, implying a significant asymmetry in the entrainment properties on the two sides of the jet. Finally, a simple scaling system based on the predicted reattachment length was proven effective in generalizing the phenomenon’s properties across a wide range of conditions.

Graphical abstract

Leading-edge protuberances on airfoils have been shown to soften the onset of aerodynamic stall and to increase lift in the post-stall regime. The present study examines the effect of tubercles during dynamic stall. Pitching airfoils with tubercles of different amplitudes are studied by wind-tunnel experiments, where the three-dimensional time-resolved velocity field is determined using large-scale particle-tracking velocimetry. Computational fluid dynamics simulations are carried out that complement the experimental observations providing pressure distribution and aerodynamic forces. The dynamic stall is dominated by a vortex formed at the leading edge; we characterize the vorticity, circulation, and advection path of this dynamic-stall vortex (DSV). The presence of the tubercles profoundly modifies the boundary layer from the leading edge. The roll-up of the vorticity sheet is significantly delayed compared to a conventional airfoil, resulting in a weaker DSV. The vortex formation is shifted downstream, with the overall effect of a weaker and shorter lift overshoot, in turn enabling a quicker transition to deep stall. Regions of flow separation (stall cells) are visibly compartmentalized with a stable spacing of two tubercles wavelengths.

Wall reflection of laser light in particle image velocimetry (PIV) measurements is one of the limiting factors in obtaining velocity information in the vicinity of rough walls. In this wind tunnel study, wall reflections for stereoscopic PIV (SPIV) measurements over a rough wall are suppressed by coating the rough surface with fluorescent paint to shift the diffuse-reflected light to a higher wavelength. A narrow band-pass filter employed on the imaging lenses extinguishes the majority of this wavelength-shifted surface-reflected laser light, allowing particle images to be recorded in the near-wall vicinity. Three different fluorescent paints were evaluated. Rhodamine 6G was found to have the best performance for suppressing wall and background reflections. Reliable velocity measurements with this technique were obtained as close as 500 µm ((sim 5.0) wall units) from the base of the roughness valleys. The technique was successfully used with SPIV to detect the secondary motions within riblet valleys.

Acoustic streaming is a process that can be used as a flow control mechanism for mixing, sorting, and enhanced transport phenomena. In this work, we present experimental results examining the superposition of acoustic streaming and bulk flow in a microchannel that incorporates an array of sharp-edge obstacles placed uniformly inside the microchannels. In the absence of bulk flow, we perform experiments over a parameter space consisting of obstacle morphology (circle, square, triangle, cross) and input sinusoidal voltage (4–12 V) with a fixed frequency of 5.8 kHz. Microscopic particle image velocimetry (µPIV) measurements yield a velocity range from 37 to 674 µm/s. Importantly, in all shapes, an overall clockwise rotation was found at the right side of the PZT and anticlockwise rotation at the left side of PZT. Although the peak acoustic streaming velocities are different for each shape, we find that the velocity scales nearly quadratically as a function of applied voltage (({U}_{o}sim {V}_{text{app}}^{2})), which is consistent with scaling analyses of acoustic streaming in microfluidic systems. A bulk flow of ~ 185 µm/s is imposed on the microchannel at the same time as a 10 V signal. We find that the resulting flow field can be reconstructed by adding the bulk flow field without streaming to the acoustic streaming flow field without bulk flow.