Experiments in Fluids最新文献

{"title":"Automatic extraction of wall streamlines from oil-flow visualizations using a convolutional neural network","authors":"Jonas Schulte-Sasse,&nbsp;Ben Steinfurth,&nbsp;Julien Weiss","doi":"10.1007/s00348-025-04016-x","DOIUrl":"10.1007/s00348-025-04016-x","url":null,"abstract":"<div><p>Oil-flow visualizations represent a simple means to reveal wall streamline patterns. Yet, the evaluation of such images can be a time-consuming process and is subjective to human perception. In this article, we present a fast and robust method to obtain quantitative insight based on qualitative oil-flow visualizations. Specifically, the local wall streamline direction is predicted by a convolutional neural network. The supervised training of this network was based on an extensive dataset involving approximately one million image patches that cover variations of the flow direction, the wall shear-stress magnitude and the oil-flow mixture. For a test dataset that is distinct from the training data, the mean prediction error of the flow direction is as low as three degrees. A reliable performance is also noted when the model is applied to oil-flow visualizations obtained from the literature, demonstrating the generalizability required for an application in diverse flow configurations. The trained model is available at https://github.com/AeroTUBerlin/OilFlowCNN.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 5","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-04-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-025-04016-x.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143793132","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":"On the unsteady aerodynamics of a surging airfoil at (90{^circ }) incidence","authors":"Guanqun Xu,&nbsp;Andrea Sciacchitano,&nbsp;Carlos Ferreira,&nbsp;Wei Yu","doi":"10.1007/s00348-025-04011-2","DOIUrl":"10.1007/s00348-025-04011-2","url":null,"abstract":"<div><p>Wind turbine blades in standstill or parked conditions often experience large angles of attack (AoA), where vortex-induced vibrations (VIV) may occur that increase the risk of structural damage. To better understand the VIV of airfoils at high AoA from an aerodynamic perspective, we conducted experimental investigations into the vortex dynamics of a surging airfoil at a <span>(90^circ)</span> incidence undergoing forced vibrations. Experiments were conducted at two reduced frequencies (<i>k</i>) to demonstrate the lock-in effect, where the vortex shedding frequency aligns with the motion frequency. Results indicate distinct vortex shedding behaviors: at higher <i>k</i> value of 0.38, downstream wake vortices form when the airfoil is moving upwind, while upstream vortices emerge during the downwind motion, interacting with the downstream vortices and leading to an outward flow. At lower <i>k</i> value of 0.19, the wake remains directed to the downwind side, regardless of the airfoil’s motion direction. Lock-in is evident in both cases, with one vortex pair shed per cycle at lower <i>k</i> and two pairs at higher <i>k</i>. Furthermore, the study examines the influence of vortex dynamics on unsteady aerodynamic loads. The results show that drag peaks when the airfoil moves upwind near the center position of its trajectory; at higher <i>k</i>, negative drag occurs as the airfoil moves downwind near the center, driven by the interactions among convection, turbulent momentum, pressure, and viscous forces. A reduced-order load estimation model for a flat plate is applied to the experimental data, showing good agreement during the upwind motion of the airfoil, which is the design condition for the original flat plate model. However, during the downwind motion, as the flow condition does not match the original flat plate design condition, the circulatory part of the model is modified to account for the presence of two pairs of vortices in the flow field, yielding improved agreement with the drag values determined from the measured flow field. The findings highlight distinct flow patterns and vortex interactions for the two motion cases, offering insights into their impact on aerodynamic loads.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 5","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-04-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-025-04011-2.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143793264","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":"Evolution of flow and turbulence over the entire tip region of an axial compressor rotor","authors":"Ayush Saraswat,&nbsp;Subhra Shankha Koley,&nbsp;Michael Joly,&nbsp;Joseph Katz","doi":"10.1007/s00348-025-04013-0","DOIUrl":"10.1007/s00348-025-04013-0","url":null,"abstract":"<div><p>Stereo-PIV measurements performed in a refractive index-matched facility examine the mean flow and normal Reynolds stresses over the entire axial, radial, and circumferential extents of an axial compressor rotor at two operating conditions, including pre-stall. In the tip region, we follow the backward leakage jet and double leakage across the tip gap as well as the rollup, evolution, and breakdown of tip leakage vortex (TLV). With decreasing flowrate, these phenomena shift closer to the blade leading edge (LE). The TLV is surrounded by a region with elevated circumferential velocity, which expands once vortex breakdown occurs, especially at pre-stall. Conditional averaging highlights the effects of transient pre-stall features, which are ‘smeared’ by averaging, such as LE spillage, circumferential velocity exceeding the blade speed, backflow vortices (BFVs), and blade boundary layer separation. Cavitation-based flow visualization under pre-stall and stall shows the prevalence and evolution of BFVs and their role in the formation of high circumferential velocity regions. The operating conditions and transients affect the spatial distributions of normal Reynolds stresses and turbulent kinetic energy (TKE). Far from stall, the TKE peaks near the TLV center and is dominated by the radial stress. At pre-stall, the TKE increases rapidly following the TLV breakup along the periphery of high circumferential velocity regions, where the BFVs form. It is dominated by the circumferential stress within the rotor and by the axial stress downstream of the trailing edge. Many, but not all, of the observed trends can be elucidated based on the turbulence production, advection, and diffusion terms.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 5","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-04-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143793131","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":"Evaluation of an event-based camera for time-resolved imaging of primary atomization in an air-assist atomizer","authors":"Kuppuraj Rajamanickam,&nbsp;Yannis Hardalupas","doi":"10.1007/s00348-025-04009-w","DOIUrl":"10.1007/s00348-025-04009-w","url":null,"abstract":"<div><p>The current work discusses the demonstration of an event-based (EB) camera for time-resolved imaging (10,000 frames/sec) of the primary atomization of a canonical air-assist atomizer. Experiments were performed simultaneously with conventional high-speed and event-based cameras, enabling us to quantitatively assess the performance of event-based cameras in spray imaging (particularly near-field liquid jet breakup) applications. Three atomization breakup regimes are considered: columnar, bag, and multimode. Dynamic mode decomposition (DMD) was implemented to analyze the acquired instantaneous time-dependent images from both cameras and assess their performance in extracting turbulence statistics of the primary atomization. The computed DMD frequency spectrum and spatial modes of liquid breakup characteristics from the images recorded from both cameras are comparable, highlighting the potential of event-based cameras in extracting coherent structures in the primary atomization zone and their spectral contents. However, in some instances, the EB camera underpredicts the DMD modes compared to high-speed cameras, and the reasons for these discrepancies were explained. Finally, the limitations (e.g., event saturation) of event-based cameras in the context of primary atomization imaging were also discussed.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 4","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-025-04009-w.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143769679","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":"Elliptic synthetic jet vortex rings impinging onto a solid wall: effect of Reynolds number","authors":"Guoan Wen,&nbsp;Lei Wang,&nbsp;Yang Xu","doi":"10.1007/s00348-025-04004-1","DOIUrl":"10.1007/s00348-025-04004-1","url":null,"abstract":"<div><p>Time-resolved stereoscopic particle image velocimetry is employed to analyze the behavior of elliptic synthetic jet vortex rings impinging onto a solid wall. Reconstruction of three-dimensional flow field is achieved using a phase-locked method. Three jet Reynolds numbers (<i>Re</i>_<i>sj</i> = 318, 477, and 636) are investigated while maintaining a constant orifice-to-wall distance (<i>H</i>₀/<i>D</i>₀ = 5) and orifice aspect ratio (<i>AR</i> = 3). The results show that the elliptic vortex ring with non-uniform distribution of the circulation induces asymmetric secondary vortex, which is different from circular ring-wall interaction. The process of impingement is divided into three stages: strong interaction, weak interaction, and stable expansion. During the stable expansion stage, the elliptic vortex ring exhibits two scenarios: into a circle and into an ellipse. The difference can be explained as follows: in the strong interaction stage, the expansion of the primary vortex ring after the impingement is mainly influenced by both the self-induction of the noncircular vortex ring and the vortex strength. However, in the weak interaction stage, it is primarily affected by the latter effect owing to the reduced three-dimensionality of the vortex ring. Under different Reynolds numbers, the vortex rings undergo different phases of the axis switching process before the vortex-wall interaction, resulting in their different final shapes. In addition, the time-averaged flow characteristics are investigated by considering azimuthally averaged velocity fields. With increasing Reynolds number, the maximum radial velocity, turbulent kinetic energy, radial mass flow rate, and momentum flux increase. In particular, the maximum radial velocity distribution can match well with the final shapes of the vortex rings.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 4","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143769812","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":"An experimental investigation on the effect of laser energy deposition in an over-expanded jet","authors":"S. Syam,&nbsp;Gauresh Raj Jassal,&nbsp;Bryan E. Schmidt","doi":"10.1007/s00348-025-04008-x","DOIUrl":"10.1007/s00348-025-04008-x","url":null,"abstract":"<div><p>This experimental investigation focuses on understanding the influence of perturbations due to short-duration energy deposition on the shock train structure and flow dynamics in an axisymmetric over-expanded Mach 2.52 jet. The flow is perturbed by localized laser-induced breakdown at various locations within the jet, creating a shock wave and a high-temperature plasma zone in the shock train. A high-speed self-aligning focusing schlieren system is used to visualize the flow and characterize the shock train dynamics and the flow structure recovery process by measuring the distance to the first shock reflection point from the nozzle exit. The response of the jet flow is similar for cases with the perturbation at the nozzle exit and the pre-reflection point across a range of jet total pressures, but the response is qualitatively different when the perturbation occurs downstream of the first shock reflection in the jet, with the flow structures being forced upstream toward the nozzle. The frequency of the oscillations of the shock height is found to be the same for all cases, approximately 10 kHz, independent of the jet total pressure, laser energy, and deposition location. The oscillations reduce in magnitude over time, and the damping ratio for cases with the energy deposition at the pre-reflection point and nozzle exit is found to be nearly constant with respect to jet total pressure and deposition energy, varying within the range of 0.06–0.12, whereas it is dependent on the jet chamber pressure for the post-reflection case, varying from 0.07 to 0.14.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 4","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-04-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-025-04008-x.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143761682","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":"Three-dimensional tracking of microswimmer suspensions","authors":"Junaid Mehmood,&nbsp;Koen Muller,&nbsp;Sowmya Kumar,&nbsp;Abel-John Buchner,&nbsp;Daniel Tam","doi":"10.1007/s00348-025-04002-3","DOIUrl":"10.1007/s00348-025-04002-3","url":null,"abstract":"<div><p>Understanding the locomotion of microorganisms is essential for insights into microbial ecology, infection, and colonization processes. Although two-dimensional microscopy has been widely used to study microswimmer motility, it does not capture the full extent of their three-dimensional (3D) movement. Recent advances in defocused particle tracking, holographic tracking velocimetry, and stereo-microscopy face challenges in achieving high resolution at larger particle densities and tracking multiple microswimmers in suspension. In this work, we introduce a novel multi-camera microscopy system that significantly improves the accuracy of 3D microswimmer tracking. Our system uses four sCMOS cameras to image microorganisms within a 2.5 <span>(times)</span> 2.5 <span>(times)</span> 2 mm<span>(^3)</span>. We assess the performance of our microscopy system by tracking a population of the unicellular motile algae <i>Chlamydomonas reinhardtii</i>. An in-house tracking algorithm based on the projective geometry framework enables tracking with reprojection errors below 0.3 body lengths. This system supports imaging and tracking particle source densities of 0.32, higher than other existing single camera 3D microscopy techniques. Analysis of <i>C. reinhardtii</i> trajectories in 3D reveals a predominance of left-handed chirality and helical swimming patterns. Moreover, our 3D tracking data provide translational and rotational diffusion coefficients that differ from those obtained using traditional two-dimensional methods.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 4","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-03-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-025-04002-3.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143726700","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":"Non-intrusive estimation of the buffet loads on a supercritical airfoil with SCBs","authors":"A. D’Aguanno,&nbsp;A. Corduas,&nbsp;F. F. J. Schrijer,&nbsp;B. W. van Oudheusden","doi":"10.1007/s00348-025-04000-5","DOIUrl":"10.1007/s00348-025-04000-5","url":null,"abstract":"<div><p>This study experimentally investigates the effect of three-dimensional shock control bumps (SCBs) on the aerodynamic loads of a supercritical airfoil under transonic buffet conditions. The experiments consisted in planar particle image velocimetry (PIV) measurements and have been carried out in the transonic-supersonic wind tunnel of TU Delft under fully developed buffet conditions (<span>(text {Ma}=0.7)</span> and <span>(alpha =3.5^{circ })</span>). The bumps are wedge-shaped and have been placed in the center of the shockwave oscillation range. Shock detection and phase-averaged velocity fields confirm that properly designed and spaced (<span>(Delta y_text {SCB}/c = 25%)</span>) SCBs reduce the shockwave oscillation range (compared to the clean case). The velocity data have been further used to evaluate the pressure field around the entire airfoil, and afterward, lift and drag coefficients have been retrieved, respectively, from momentum contour and wake integral approaches. Results demonstrate that SCBs have a beneficial effect on the aerodynamic loads with an increase in lift and a decrease in drag under fully developed buffet conditions. More importantly, a strong reduction of the amplitude of oscillations of both lift and drag coefficient, within the different buffet phases, was noted. Tests at multiple spanwise locations revealed relevant differences, with lower drag and higher lift values being achieved in the symmetry plane of a SCB, while a worse performance (with values comparable to the clean case) was achieved in the symmetry plane in between two adjacent bumps.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 4","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-03-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-025-04000-5.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143706932","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":"High-speed fs/ps-CARS thermometry for supersonic H2/air combustion studies","authors":"Clément Pivard,&nbsp;Michael Scherman,&nbsp;Rosa Santagata,&nbsp;Guillaume Pilla,&nbsp;Guillaume Pelletier,&nbsp;Thomas Le Pichon","doi":"10.1007/s00348-025-04007-y","DOIUrl":"10.1007/s00348-025-04007-y","url":null,"abstract":"<div><p>We present the results of a hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps-CARS) thermometry campaign performed in a H₂/air research supersonic scramjet combustor. In situ vertical and horizontal temperature profiles were retrieved upstream and downstream the combustion zone. The repetition rate of the measurement was adjusted in order to optimize the signal-to-noise ratio of the CARS signal depending on the local turbulence of the flow. Near the flame front, single-shot measurements were demonstrated at kHz rate to catch the high-speed temperature fluctuations. In steady flow zones downstream of the combustion, up to 100 shots integration was performed in order to increase the precision of the measurement. This measurement campaign allowed to build a valuable experimental database for comparison with a 3D numerical unsteady computational fluid dynamic (CFD) simulation developed at ONERA. This work demonstrates the efficiency of hybrid fs/ps-CARS to perform single-shot kHz thermometry inside large-scale supersonic combustor.</p><h3>Graphical abstract</h3><div><figure><div><div><picture><source><img></source></picture></div></div></figure></div></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 4","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-03-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143688493","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":"Density measurements via background-oriented schlieren and parallel-ray omnidirectional integration","authors":"Jonathan Davami,&nbsp;Thomas J. Juliano,&nbsp;Jose R. Moreto,&nbsp;Xiaofeng Liu","doi":"10.1007/s00348-025-04012-1","DOIUrl":"10.1007/s00348-025-04012-1","url":null,"abstract":"<div><p>Most measurements of density via background-oriented schlieren (BOS) numerically integrate the Poisson equation to calculate the density field from density gradients, which is susceptible to errors in measurements and uncertainties in boundary values. An alternative method, parallel-ray omnidirectional integration (ODI), was implemented and found to be significantly more accurate and precise. To compare the performance of the integration algorithms, a BOS displacement field was synthesized for the Taylor–Maccoll solution for inviscid, supersonic, conical flow. The impact of measurement error was simulated by adding noise to the synthetic displacement field. Density was reconstructed from 200 statistically independent displacement fields for two noise levels. The ODI algorithm resulted in higher accuracy and precision for all cases analyzed. In fact, the mean error for ODI at the highest noise level was found to be lower than that of Poisson integration, even when Poisson is evaluated without any input noise. These algorithms were also used to reconstruct density from experimental BOS measurements on a cone-flare model in hypersonic flow and zero angle of attack. This geometry exhibits a shock-wave/boundary-layer interaction which consists of bow, separation, and reattachment shocks and a recirculation bubble. The reconstructed density agreed excellently with the inviscid solution outside the boundary layer and recirculation bubble. The ODI-derived density field provided a closer match to the anticipated result than the Poisson-derived density, and is thus the recommended method. This work emphasizes the exceptional capability of ODI in reconstructing accurate and precise density fields from BOS measurements, thereby advancing the high spatial resolution, non-intrusive, and quantitative measurement technique used to deepen the understanding of complex fluid flows.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"66 4","pages":""},"PeriodicalIF":2.3,"publicationDate":"2025-03-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143676444","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}