Experiments in Fluids最新文献

{"title":"Time-variant planar laser-induced fluorescence for thickness measurement of wavy liquid films: a calibration-free and threshold-free method","authors":"Yoshiaki Kamada,&nbsp;Zhenying Wang,&nbsp;Chihiro Inoue,&nbsp;Shigeki Senoo","doi":"10.1007/s00348-025-03989-z","DOIUrl":"10.1007/s00348-025-03989-z","url":null,"abstract":"<div><p>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 <span>(h &gt; 0.2,textrm{mm})</span> with the accuracy of <span>(varepsilon le 10%)</span> to the theoretical reference and <span>(h le 0.2,textrm{mm})</span> with <span>(varepsilon = 20%)</span>. 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 <span>(30,text{m/s})</span> by measuring the phase velocity and wavelength, which well matches the theoretical results.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 3","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-02-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143423175","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Coandă effect in free-surface shallow-water offset jets","authors":"M. Sollevanti,&nbsp;M. Miozzi,&nbsp;G. P. Romano","doi":"10.1007/s00348-024-03892-z","DOIUrl":"10.1007/s00348-024-03892-z","url":null,"abstract":"<div><p>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.</p><h3>Graphical abstract</h3>\u0000<div><figure><div><div><picture><source><img></source></picture></div></div></figure></div></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 3","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-02-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-024-03892-z.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143423312","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Dynamic stall on airfoils with leading-edge tubercles","authors":"Pere Valls Badia,&nbsp;Stefan Hickel,&nbsp;Fulvio Scarano,&nbsp;Mogeng Li","doi":"10.1007/s00348-025-03981-7","DOIUrl":"10.1007/s00348-025-03981-7","url":null,"abstract":"<div><p>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.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 3","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-02-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-025-03981-7.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143423311","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"PIV measurements within roughness grooves with enhanced wall reflection suppression","authors":"Wagih Abu Rowin,&nbsp;Peter Manovski,&nbsp;Daniel Chung,&nbsp;Nicholas Hutchins","doi":"10.1007/s00348-025-03986-2","DOIUrl":"10.1007/s00348-025-03986-2","url":null,"abstract":"<div><p>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 (<span>(sim 5.0)</span> wall units) from the base of the roughness valleys. The technique was successfully used with SPIV to detect the secondary motions within riblet valleys.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 3","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-02-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-025-03986-2.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143423308","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Acoustic streaming flow driven about and array of sharp-edged obstacles","authors":"Md. Abdul Karim Miah,&nbsp;Michael G. Olsen,&nbsp;Jaime J. Juárez","doi":"10.1007/s00348-025-03987-1","DOIUrl":"10.1007/s00348-025-03987-1","url":null,"abstract":"<div><p>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 (<span>({U}_{o}sim {V}_{text{app}}^{2})</span>), 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.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 3","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-02-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143423307","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Material and surface texture-dependent fluorescence behavior of liquid coolant wall films","authors":"Matthias Koegl,&nbsp;Leon Breitenbach","doi":"10.1007/s00348-025-03990-6","DOIUrl":"10.1007/s00348-025-03990-6","url":null,"abstract":"<div><p>The study investigates the liquid film thickness-dependent laser-induced fluorescence of a dye in a heat transfer oil affected by wall reflections at different solid surface materials (aluminum, copper, steel) and surface textures (polished and sandblasted). A specially designed fluorescence cell allows a precise adjustment of the film thickness at a fixed temperature and allows the investigation of various substrate materials and textures. Photo-dissociation free measurements are ensured due to a closed-loop circuit, driven by a pump. The LIF signal was generated by admixture of the fluorescent dye Nile red to the heat transfer oil Marlotherm LH. A CW laser at 532 nm was applied for excitation, and emissions were recorded by using a spectrometer. The use of a relatively low dye concentration (0.59 mg/l) ensures negligible reabsorption of the fluorescence and thus minimal spectral changes due to a variation in film thickness, which is indispensable for precise temperature measurements. A comparison of the dye fluorescence affected by reflections at different solid materials and surface treatments for a 1-mm film thickness reveals a similar trend for all investigated materials, except for copper. Copper leads to a surface texture-dependent spectral shift of the peak emission (polished: 3.8 nm, sandblasted: 4.3 nm) toward larger wavelengths in comparison with the remaining materials (peak always at 586.4 nm). This is attributed to the more distinct wavelength-dependent reflection behavior of copper evaluated in a theoretical study. Since the fluorescence signal experiences a stronger reflection in comparison with the incident laser beam, this leads to a spectral shift of the emission spectra toward larger wavelengths. A model approach is developed describing effects of direct and non-direct reflection of fluorescence for different materials and textures. A diffusive reflection leads to an overall decrease of reabsorption. This is caused by the reduced direct reflection of laser light, which passes through the liquid film a second time (or multiple times) and consequently less emission signal. Temperature-dependent measurements in combination with a two-color measurement approach showed the significant influence of wavelength-dependent reflection behavior on the temperature determination on liquid films.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 3","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-02-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-025-03990-6.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143423227","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Flow sensing through unsteady pressure measurements during transverse wing–gust encounters","authors":"Antonios Gementzopoulos,&nbsp;Oliver Wild,&nbsp;Anya Jones","doi":"10.1007/s00348-025-03992-4","DOIUrl":"10.1007/s00348-025-03992-4","url":null,"abstract":"<div><p>Flight vehicles can optimize their performance by sensing unsteady flow phenomena and leveraging this information to improve decision-making and actuation. This study experimentally investigates the use of surface pressure measurements as unsteady flow sensors during large-amplitude transverse wing–gust encounters. An instrumented wing model is developed to overcome difficulties associated with unsteady pressure measurements in water towing tank facilities. The measurement system is validated through steady and unsteady experiments and is used to study the unsteady pressure distributions associated with transverse wing–gust encounters. Concurrent analysis of the pressure distributions and flowfields yields the following flow event sequence for high gust ratio (GR) experiments: As the wing enters the gust, a large suction peak forms on the leading edge. The suction peak widens and eventually splits into two distinct peaks. The secondary suction peak is associated with the leading-edge vortex (LEV) and its suction strength is found to be proportional to the gusting flow dynamic pressure, <span>(textrm{GR}^2 + 1)</span>. Integration of the sectional pressure distributions resulted in accurate estimates of the overall wing loads during the vortex formation stage of the dynamic stall process but not during the vortex separation stage. Dynamic stall initiation is shown to be associated with an inflection point on the leading-edge suction transient and an abrupt drop in the leading-edge pressure gradient. The timing of LEV formation is found to be associated with a maximum in leading-edge suction and an increase in leading-edge pressure gradient.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 3","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-02-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143404179","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Applying A-PTV to RBC suspension flows","authors":"Ang Sun,&nbsp;Till Werner,&nbsp;Finn Knüppel,&nbsp;Frank-Hendrik Wurm,&nbsp;Benjamin Torner,&nbsp;Jeanette Hussong","doi":"10.1007/s00348-025-03975-5","DOIUrl":"10.1007/s00348-025-03975-5","url":null,"abstract":"<p>In strongly confined flow geometries, red blood cells migrate normal to the flow direction, thereby altering the flow rheology of blood. Direct optical measurements can help to gain an improved understanding of these migration processes. In the present study, we demonstrate that astigmatism particle tracking velocimetry is a suitable 3D-particle tracking method that allows to directly measure both 3D concentration and velocity distributions of red blood cells in a flow. Red blood cells assume a non-spherical shape; therefore, the influence of their orientation on the reconstruction of the out-of-plane particle position is evaluated through a ray tracing approach of synthetic, astigmatic images. While for noise-free images, the resulting absolute out-of-plane reconstruction error <span>(sigma _z)</span> is small for different red blood cell orientations (<span>(sigma _z)</span> = 0.98 <span>(upmu text {m})</span>), it triples for experimentally relevant signal-to-noise ratios (SNR = 1.2). Reconstruction errors are compared to those of spherical particles. Overall, both the red blood cell orientation and the increase in signal-to-noise ratio induce similar out-of-plane reconstruction error values. Experimental analyses are also performed using both a red blood cell suspension system and a refractive index-matched suspension system of identical volume fraction (<span>({1.5,mathrm{%}})</span>). Comparing results from the red blood cell suspension flow with those of the particulate suspension under identical parameters for volume fraction, particle Reynolds number, and bulk Reynolds number, a similarity in lateral migration behavior is observed under the given conditions. The results indicate that the absolute out-of-plane reconstruction error in the red blood cell suspension system (<span>(sigma _z = {4.50,mathrm{{upmu text {m}}}})</span>) is approximately 1.5 times larger compared to the refractive index-matched system.</p>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 3","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-02-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-025-03975-5.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143404249","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"An intensity-based LIF measurement technique to quantify film thicknesses in the air gap of an electric motor with direct liquid cooling","authors":"A. Auernhammer,&nbsp;M. Bonarens,&nbsp;G. Goet,&nbsp;S. Wagner,&nbsp;J. Schorr,&nbsp;A. Dreizler,&nbsp;B. Böhm","doi":"10.1007/s00348-025-03976-4","DOIUrl":"10.1007/s00348-025-03976-4","url":null,"abstract":"<div><p>The air gap of an optically accessible model of a directly cooled radial flux electric motor is investigated using laser-induced fluorescence (LIF). The cooling oil enters the air gap and a film forms on the stator surface, which resembles a thin film in a shear flow. The flow phenomena at different rotational speeds (2000 rpm to 10,000 rpm) are described. An intensity-based LIF measurement technique is developed and used to measure the film thickness on the stator in a realistic air gap environment. The rotational speed influences the flow phenomena and the film thickness of the stator film. With increasing rotational speed, i.e., increasing gas Reynolds number, the film thickness probability density functions (PDFs) shift to lower film thicknesses and become narrower, which is in agreement with the characteristic behavior of films in shear flows as reported in the literature. Additionally, the velocities of the wave crests which move across the film surface are evaluated and used to calculate the film Reynolds number Re_F, which characterizes the investigated operating points.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 3","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-02-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-025-03976-4.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143388868","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Characterization of 3D oxygen concentrations in hydrogels combining astigmatic particle tracking with phosphorescence decay measurements","authors":"Till Werner,&nbsp;Mareike Schmidt,&nbsp;Bastian J. M. Etzold,&nbsp;Anna Fritschen,&nbsp;Andreas Blaeser,&nbsp;Jeanette Hussong","doi":"10.1007/s00348-025-03977-3","DOIUrl":"10.1007/s00348-025-03977-3","url":null,"abstract":"<p>In this work, we combine astigmatic particle tracking with phosphorescence decay measurements to determine 3D oxygen concentration distributions in microfluidic systems. Out-of-plane positions are reconstructed to an accuracy of 1.5 <span>(upmu)</span>m. The calibrated measurement range covers oxygen concentrations between 0.6 to 27.6 ppm. A method is presented to systematically correct for measurement errors caused by photobleaching taking into account the excitation energy, the cumulative laser irradiation time and the spatially varying intensity profile of the laser. With this method, low measurement errors of less than 2 ppm at ambient oxygen levels can be achieved even after thousands of excitation cycles. To demonstrate the capability of the measurement technique, 3D oxygen concentrations are measured in an agarose hydrogel filled microfluidic chamber across which different pressure and oxygen gradients can be set independently. The results show that oxygen diffusion is superposed by a convective transport of interstitial flow. By matching numerical simulations to the experimental data, further insights into the ratio of convective and diffusive transport are given and a methodology for estimating relevant material parameters is presented. This in situ measurement technique can be applied to improve the design process of supply networks for tissue models.</p>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 3","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-02-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-025-03977-3.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143388870","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}