Experiments in Fluids最新文献

We describe an oscillating boundary layer apparatus (OBLA) to investigate mass and momentum transfer in the wave bottom boundary layer. The facility is designed such that near-bed shallow water orbital velocities are physically modeled in full scale. A PIV/PLIF system allows for simultaneously resolving the intra-ripple velocity and dye concentration fields. We examine two cases by injecting dye at the trough and crest of the rippled boundary. The extent of the plume is the largest near the zero-crossing of the free-stream velocity and 40(^circ) later for the trough and crest case, respectively. Both cases showed periodic turbulent vortical structures influencing the phase-averaged concentration plumes. For normalized concentrations greater than 0.01, the plumes remained within the boundary layer and traveled half a ripple length for both cases. Dye spread vertically upward about 2 and 1.5 ripple heights from the crest and trough sources, respectively. Stronger advection was observed over the crests, along with a clear dependence on bedform asymmetry.

Many problems in fluid mechanics require single-shot 3D measurements of fluid flows, but are limited by available techniques. Here, we design and build a novel flexible high-speed two-color scanning volumetric laser-induced fluorescence (H2C-SVLIF) technique. The technique is readily adaptable to a range of temporal and spatial resolutions, rendering it easily applicable to a wide spectrum of experiments. The core equipment consists of a single monochrome high-speed camera and a pair of ND: YAG lasers pulsing at different wavelengths. The use of a single camera for direct 3D imaging eliminates the need for complex volume reconstruction algorithms and easily allows for the correction of distortion defects. Motivated by the large data loads that result from high-speed imaging techniques, we develop a custom, open-source, software package, which allows for real time playback with correction of perspective defects while simultaneously overlaying arbitrary 3D data. The technique is capable of simultaneous measurement of 3D velocity fields and a secondary tracer in the flow. To showcase the flexibility and adaptability of our technique, we present a set of experiments: (1) the flow past a sphere, and (2) vortices embedded in laminar pipe flow. In the first experiment, two channel measurements are taken at a resolution of 512 × 512 × 512 with volume rates of 65.1 Hz. In the second experiment, a single-color SVLIF system is integrated on a moving stage, providing imaging at 1280 × 304 × 256 with volume rates of 34.8 Hz. Although this second experiment is only single channel, it uses identical software and much of the same hardware to demonstrate the extraction of multiple information channels from single channel volumetric images.

This investigation characterized the relation between the breathing motion and wall-pressure fluctuations for a turbulent separation upstream of the trailing edge of a two-dimensional wing with NACA 4418 profile. The experiments were conducted at a freestream velocity U_∞ = 10.2 m/s with a turbulent intensity of 0.4%. The wing had an aspect ratio of 1.2 and an angle of attack of 9.7°. The corresponding chord-based Reynolds number was 620,000. The measurements consisted of simultaneously acquired wall-pressure measurements at various streamwise locations and time-resolved particle image velocimetry (PIV) in a streamwise-wall-normal plane. Both measurements showed unsteadiness related to the breathing motion at low Strouhal number St_l ≈ 0.05. Here, St_l is defined based on the characteristic length, l, of the mean turbulent separation bubble (TSB). Cross-correlation between the measured wall pressures at different streamwise locations revealed that the breathing motion propagated at approximately 0.8U_∞ downstream of the mean detachment (MD) point. The breathing motion was observed to establish a stronger correlation with the low-frequency wall-pressure fluctuations in the low-intermittency regions as opposed to the high-intermittency regions. Spectral proper orthogonal decomposition was performed using the combined PIV velocity fields and wall-pressure measurements. The results revealed that the expansion (or contraction) of TSB preceded a reduction (or increase) in wall pressure measured upstream of MD and an increase (or reduction) in wall pressure measured downstream of MD. The findings align with the fact that TSB expansion occurs when local adverse pressure gradient (APG) increases, whereas contraction corresponds to a decrease in APG.

The present work proposes a fast and optimized experimental approach for pressure reconstruction and far-field noise prediction for flow past tandem cylinders based on time-resolved particle image velocimetry (PIV). The low-order reconstruction of the velocity fields based on proper orthogonal decomposition (POD) is applied, which effectively mitigates the incoherent measurement noise by selecting the low-order modes representing the dominant coherent structures. The preprocessing of velocity fields significantly improves the accuracy of both field and surface pressure fluctuations estimated by solving the Poisson equation. The time-marching enhancement algorithm uses the pressure field from the preceding snapshot as the initial guess in the iterative process, which accelerates convergence and reduces the computational cost for solving the Poisson equation of the PIV database with a large ensemble size. The estimated surface pressure fluctuations are used to predict the far-field noise through Curle’s analogy with the correction based on the spanwise correlation length. Comparisons are performed with reference signals, yielding good agreement on both pressure and noise spectra.

This study experimentally investigates turbulent flow separation around a slanted-back Ahmed body with different leading edge configurations [rounded (RL) and square (SL)] using the particle image velocimetry technique. The Reynolds number (based on freestream velocity and body height) is ({text{Re}}_{text{H}}) = 0.17 × 10⁵. Spatiotemporal flow characteristics, including mean flow, vorticity flux, two-point correlation, reverse flow area, frequency spectra, and proper orthogonal decomposition, are analyzed to elucidate the effects of leading edge shape on separated shear layer interactions over the roof, slanted surface, and the wake region. The results show that rounding the leading edge of the Ahmed body leads to the formation of smaller coherent structures with lower shedding frequency over the roof. While, the structures are still smaller over the slanted surface and the wake region of the RL body, the shedding frequency surpasses that of the SL ones. The results also show evidence of the identical shedding frequency over the roof, slanted surface, and wake region of the RL body. Detailed analyses, including convective velocities of vortices over the roof, slanted surface, and in the wake region, vary significantly between SL and RL cases, indicating distinct shedding mechanisms influenced by leading edge geometry.

In this article, structured laser illumination planar imaging and polarization ratio techniques are successfully combined to size droplets in various optically dense sprays. The polarization ratio approach is based on the acquisition of the perpendicular and parallel polarized components of Lorenz–Mie scattered light, for which the ratio is proportional to the surface mean diameter, D₂₁. One of the main advantages of this technique, compared to some other laser imaging techniques for particle sizing, is that no fluorescent dye is required. This makes the technique suitable for characterizing sprays under evaporation conditions, such as combustion or spray drying applications. In addition, the SLIPI technique aims at suppressing the detection of multiple light scattering and at extracting the desirable single-light scattering signal. To test the reliability of this novel approach, an industrial hollow-cone nozzle is used, injecting at 50 bar water mixed with Glycerol (in the range of 0–60%). The first aim of this work is to study the experimental parameters that influence the reliability of the technique, such as the polarization orientation of the incident light, the refractive index of the injected liquid and the variation of the droplet size distribution. Using Phase Doppler Anemometry, the results show that a linear calibration is obtained for droplets ranging between 10 and 70 μm, when the incident illumination has a polarization set to 10° and 20°. In addition, this article demonstrates the feasibility of the technique for the measurement of liquids having a refractive index reaching 1.41. In the last stage of this work, after rotating the nozzle every 5°, a 3D tomographic reconstruction of D₂₁ is performed. This demonstrates the robustness and efficiency of the technique for droplet sizing in 3D, under challenging conditions.

Preventing fluid penetration poses a challenging reliability concern in the context of power electronics, which is usually caused by unforeseen microfractures along the sealing joints. A better and more reliable product design heavily depends on the understanding of the dynamic wetting processes happening inside these complex microfractures, i.e. microchannels. A novel automated image processing procedure is proposed in this work for analyzing the moving interface and the dynamic contact angle in microchannels. In particular, the developed method is advantageous for experiments involving non-transparent samples, where extracting the fluid interface geometry poses a significant challenge. The developed method is validated with theoretical values and manual measurements and exhibits high accuracy. The implementation is made publicly available. The developed method is validated and applied to experimental investigations of forced wetting with two working fluids (water and 50 wt% glycerin/water mixture) in four distinct microchannels characterized by different dimensions and curvature. The comparison between the experimental results and molecular kinetic theory (MKT) reveals that the dynamic wetting behavior can be described well by MKT, even in highly curved microchannels. The dynamic wetting behavior shows a strong dependency on the channel geometry and curvature.

This paper delves into the methodology employed in examining lean premixed turbulent flame fronts extracted from Planar Laser Induced Fluorescence (PLIF) images at elevated pressures. In such flow regimes, the PLIF signal suffers from significant collisional quenching, typically resulting in image data with low signal-to-noise ratio (SNR). This poses severe difficulties for conventional flame front extraction algorithms based on intensity gradients and requires intense user intervention to yield acceptable results. In this work, we propose Convolutional Neural Network (CNN)-based Deep Learning (DL) models as an alternative to problem specific conventional methods. The pretrained DL models were fine-tuned, employing data augmentation, on a small annotated dataset including a variety of conditions between SNR (approx) 1.6 to 2.6 and subsequently evaluated. All DL models significantly outperformed the best-scoring conventional implementation both quantitatively and visually, while having similar inference times. IoU-scores and Recall values were found to be up to a factor (approx) 1.2 and (approx) 2.5 higher, respectively, with (approx) 1.15 times improved Precision. Small-scale structures were captured much better with fewer erroneous predictions, becoming particularly pronounced for the lower SNR data investigated. Moreover, by applying artificially modeled noise, it was shown that the range of image conditions in terms of SNR that can be reliably processed extends well beyond the images included in the training data, and satisfactory segmentation performances were found for SNR as low as (approx) 1.1. The presented DL-based flame front detection algorithm marks a methodology with significantly increased detection performance, while a similar computational effort for inference is achieved and the need for user-based parameter tuning is eliminated. It enables a very accurate extraction of instantaneous flame fronts in large image datasets where supervised processing is infeasible, unlocking unprecedented possibilities for the study of flame dynamics and instability mechanisms at industry-relevant conditions.

This paper discusses the extension of an optical liquid film thickness measurement technique to characterize liquid film flow rate in wavy thin liquid film flow. The technique, based on laser refractometry, is used to measure wave height, shape, frequency, and velocity. A two-zone model to process the measured wave characteristics is used to estimate the liquid film flow rate. The method is validated in a falling film facility where easy optical access allows comparisons of the wave velocity measurements with high-speed videos and where the calculated liquid film mass flow rate can be compared with actual measurements. The paper provides a framework for analyzing time-resolved film thickness data using multizone models in more complex liquid film flows, such as in two-phase annular flow.

Fuel injection systems significantly impact the combustion process and play a key role in reducing harmful exhaust emissions in internal combustion engines. For dual-fuel (DF) engines operating in gas mode, ignition of the main fuel is typically controlled by directly injected liquid pilot fuel. Liquid pilot fuel’s initial penetration and total mass considerably impact exhaust emissions and combustion stability. We investigated the spray morphology of a multi-hole diesel fuel injector within a constant-volume spray chamber using high-speed shadowgraphy and Mie-scattering measurements. Two methodologies were employed. The first one utilized a nozzle equipped with a thimble structure to isolate a single plume. The second methodology known as plume-blocking, involved sealing the orifices of the multi-hole nozzle to generate a single-spray plume. Our findings revealed that the plume-blocking approach demonstrated greater penetration than the thimble-equipped nozzle. The rapid penetration of this method may restrict its applicability to single-spray studies. Sprays generated from this partially sealed nozzle exhibited noticeable disparities compared to an unblocked nozzle, whereas a nozzle equipped with a thimble produced similar outcomes to the standard nozzle. The orifices when sealed, modify the flow distribution within the sac volume, which consequently affects the spray characteristics. In summary, this research provides insights into the impacts of various plume isolation methods on spray morphology, thereby enhancing the understanding of spray behaviour in transient conditions by comparing plume variations and disturbances under various fuel pressure and ambient conditions.