Experiments in Fluids最新文献

{"title":"Experimental investigation of non-linear standing internal waves using combined density and velocity measurements","authors":"Mohammad Mohaghar,&nbsp;Donald R. Webster","doi":"10.1007/s00348-023-03621-y","DOIUrl":"10.1007/s00348-023-03621-y","url":null,"abstract":"<div><p>To provide insight to the dynamics of weakly non-linear standing internal waves, the density and velocity fields are measured using combined planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) techniques. A laboratory-scale apparatus was created to generate standing internal waves in a two-layer stratified system. Experimental results are presented for two configurations with a density jump of 1.1 kg/m<span>(^3)</span> and 1.5 kg/m<span>(^3)</span> (separately). The interface location, wave amplitude and period, interface thickness, convection transport terms, fluid velocity, shear strain rate, and vorticity are quantified and analyzed at fixed phases in the wave cycle. The comparison between the internal wave frequency computed from the experimental results and the dispersion relationship resulting from the theoretical third-order Stokes internal-wave solution confirms that the laboratory-generated waves demonstrate non-linear behavior. The interface detected from experimental PLIF images indicated that due to the non-linear effects, a steeper wave with a sharper-looking interface at anti-node locations was formed in comparison with the theoretical linear sinusoidal shape. Further, the magnitude of shear strain rate and vorticity computed from experimental PIV measurements had a sharp, non-linear increase along the interface compared to the one computed from the linear theory. This non-linear trend in shear strain rate and vorticity can lead to the generation of sharper interface and short-period (i.e., higher frequency) non-linear internal waves.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"64 4","pages":""},"PeriodicalIF":2.4,"publicationDate":"2023-03-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-023-03621-y.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"5090993","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":"Measurements on the effect of steps on the transition of laminar boundary layers","authors":"Alexander Heintz,&nbsp;Peter Scholz","doi":"10.1007/s00348-023-03614-x","DOIUrl":"10.1007/s00348-023-03614-x","url":null,"abstract":"<div><p>The effects of steps on the transition of laminar boundary layers were measured on a flat plate for low Reynolds numbers with critical and subcritical step heights. The transition position was measured by determining the intermittency distribution in streamwise direction, including the characteristic length of the transitional region. The results are compared with formulations of a critical step Reynolds number <span>({textrm{Re}_textrm{h}})</span>, i.e., the step height that will instantly trigger transition at the step position, and—for subcritical step heights—with <span>({{Delta N}})</span>-formulations from the literature. For backward-facing steps, the concept of a step Reynolds number can be used to distinguish between subcritical and critical step heights, whereas for forward-facing steps there seems not to be one unique <span>({textrm{Re}_textrm{h}})</span>. Furthermore, for subcritical backward-facing steps the concept of a <span>({{Delta N}})</span>-approximation gives a reasonable description of the experimental observations. Again in contrast, for forward-facing steps a <span>({{Delta N}})</span>-approach scattered a lot and no clear dependency was found between the reduction in the critical N-factor of transition and the relative step height.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"64 4","pages":""},"PeriodicalIF":2.4,"publicationDate":"2023-03-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-023-03614-x.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"5049370","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":"Investigation of fluidic thrust vectoring for scramjets","authors":"Chris Hambidge,&nbsp;William Ivison,&nbsp;David Steuer,&nbsp;Andrew Neely,&nbsp;Matthew McGilvray","doi":"10.1007/s00348-023-03607-w","DOIUrl":"10.1007/s00348-023-03607-w","url":null,"abstract":"<p>Fluidic thrust vectoring (FTV) offers a novel approach to aerodynamic control, circumventing some of the issues associated with mechanical systems. One method is shock vector control which involves injecting a fluid into the exhaust nozzle of an engine to redirect the gases and thus, produce a control force. An experimental model which incorporated FTV was designed and tested at Mach 6 in the Oxford high density tunnel (HDT). The model was a simplified two-dimensional scramjet geometry with two different configurations to compare an internal and external exhaust nozzle. The FTV injection system consisted of a slot at the rear edge of the exhaust nozzle fed from an internal plenum. In the experimental campaign, a range of gas injection pressures and free stream stagnation pressures were tested to assess the effectiveness of both configurations. Two new measurement methods were successfully implemented in the HDT: pressure sensitive paint and a 6-axis load cell. The FTV system has been shown to be effective with observable increases in lift and pitching moment. A linear relation between the injection pressure ratio and the control forces could be observed for both configurations.</p>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"64 4","pages":""},"PeriodicalIF":2.4,"publicationDate":"2023-03-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-023-03607-w.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4979941","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":"Reconstructing temperature fields for thermally-driven flows under quasi-steady state","authors":"Daisuke Noto,&nbsp;Hugo N. Ulloa,&nbsp;Juvenal A. Letelier","doi":"10.1007/s00348-023-03620-z","DOIUrl":"10.1007/s00348-023-03620-z","url":null,"abstract":"<div><p>This letter introduces a method to reconstruct the temperature field of thermally-driven Boussinesq fluid flows. The technique relies on prescribed boundary conditions and a velocity field measured from particle image velocimetry. This information is then plugged into the heat equation to integrate and map the temperature field in space. We demonstrate the method’s skill by applying it to the problem of thermal convection in Hele-Shaw cells under a quasi-steady state. The limitations and strengths of this approach are discussed in the context of thermally-driven flows in laboratory experiments.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"64 4","pages":""},"PeriodicalIF":2.4,"publicationDate":"2023-03-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-023-03620-z.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4871655","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":"A lightweight convolutional neural network to reconstruct deformation in BOS recordings","authors":"Claudio Mucignat,&nbsp;Lento Manickathan,&nbsp;Jiggar Shah,&nbsp;Thomas Rösgen,&nbsp;Ivan Lunati","doi":"10.1007/s00348-023-03618-7","DOIUrl":"10.1007/s00348-023-03618-7","url":null,"abstract":"<div><p>We introduce a Convolutional Neural Network (CNN) that is specifically designed and trained to post-process recordings obtained by Background Oriented Schlieren (BOS), a popular technique to visualize compressible and convective flows. To reconstruct BOS image deformation, we devised a lightweight network (<span>LIMA</span>) that has comparatively fewer parameters to train than the CNNs that have been previously proposed for optical flow. To train <span>LIMA</span>, we introduce a novel strategy based on the generation of synthetic images from random-irrotational deformation fields, which are intended to mimic those provided by real BOS recordings. This allows us to generate a large number of training examples at minimal computational cost. To assess the accuracy of the reconstructed displacements, we consider test cases consisting of synthetic images with sinusoidal displacement as well as images obtained in the experimental studies of a hot plume in air and a flow past and inside a heated hollow hemisphere. By comparing the reconstructed deformation fields using the <span>LIMA</span> or conventional post-processing techniques used in Direct Image Correlation (<span>DIC</span>) or conventional image cross-correlation, we show that <span>LIMA</span> is more accurate and robust in the synthetic test case. When applied to experimental BOS recordings, all methods provide similar and consistent deformation fields. As <span>LIMA</span> is capable of achieving a comparable or better accuracy at a fraction of the computational costs, it represents a valuable alternative to conventional post-processing techniques for BOS experiments.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"64 4","pages":""},"PeriodicalIF":2.4,"publicationDate":"2023-03-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-023-03618-7.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4829843","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":"Time-resolved determination of velocity fields on the example of a simple water channel flow, using a real-time capable FPGA-based camera platform","authors":"Tino Steinmetz,&nbsp;Martin Schaeper,&nbsp;André Kleinwächter,&nbsp;Jonas Otto,&nbsp;Andreas Richter,&nbsp;Ralf Poosch,&nbsp;Nils Damaschke","doi":"10.1007/s00348-023-03617-8","DOIUrl":"10.1007/s00348-023-03617-8","url":null,"abstract":"<div><p>Field Programmable Gate Arrays (FPGAs) have become a widely available computational resource for real-time image processing and can be used in time-critical optical process measurements. In this study, we investigate the applicability of the spatial filtering technique for time-resolved velocity field determination, on the example of a simple water tunnel flow. For this, the spatial filtering technique was implemented for a custom FPGA platform and analyzed regarding the effect of spatial and temporal resolution on the accuracy of the results. It was possible to prove the algorithm as suitable for velocity field estimation and identify algorithm parameters, which allow for increasing the accuracy of the results.</p><h3>Graphical abstract</h3>\u0000 <figure><div><div><div><picture><source><img></source></picture></div></div></div></figure>\u0000 </div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"64 4","pages":""},"PeriodicalIF":2.4,"publicationDate":"2023-03-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-023-03617-8.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4829858","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":"Practical aspects of designing background-oriented schlieren (BOS) experiments for vortex measurements","authors":"Clemens Schwarz,&nbsp;Johannes N. Braukmann","doi":"10.1007/s00348-023-03602-1","DOIUrl":"10.1007/s00348-023-03602-1","url":null,"abstract":"<div><p>Setup-related aspects of background-oriented schlieren (BOS) experiments are discussed focusing on a sensitivity parameter <i>S</i>, which represents the relation between light deflection and resulting BOS signal, and the geometric blur. An analytic expression for the geometric blur by means of the circle of confusion (CoC) was derived which shows a proportional relation to the sensitivity factor <i>S</i>. The theoretical findings were validated in a reference experiment using generic distortions in glass plates. It was found that the filtering effect of the blur decreases the maximum background shift and its influence can be expressed with a blur loss factor <i>B</i>, which depends on the size of the CoC in relation to the investigated object. Multiplying the setup sensitivity <i>S</i> with the blur loss <i>B</i> results in the effective sensitivity <span>(S _{{rm eff}})</span> that determines the maximum achievable BOS signal of a schlieren object. For the investigated reference objects, the maximum effective sensitivity <span>(S _{{rm eff}})</span> was found to occur at CoC sizes in the object domain from 2.5 to 3.8 times the extent of the investigated objects. A step-by-step method is proposed for designing BOS experiments to obtain a maximum signal strength. The design parameters are further discussed specifically in regard to rotor tip vortex visualization, for which a variety of previously reported experiments are compared. A simple prediction method for the BOS signal of blade tip vortices is proposed and validated with experimental data from a rotor test stand. The application of the method to rotor systems of different size shows the requirement for increasingly higher sensitivity values for visualizing vortices of small-scale rotors.</p><h3>Graphical abstract</h3>\u0000 <figure><div><div><div><picture><source><img></source></picture></div></div></div></figure>\u0000 </div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"64 4","pages":""},"PeriodicalIF":2.4,"publicationDate":"2023-03-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-023-03602-1.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"5022796","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":"Effect of Gurney flap on the vortex-dominated flow over low-AR wings","authors":"D. Arivoli,&nbsp;Ishan Singh","doi":"10.1007/s00348-023-03605-y","DOIUrl":"10.1007/s00348-023-03605-y","url":null,"abstract":"<div><p>A Gurney flap is a simple passive flow control device that, when employed on wings, can significantly augment its lift with minor changes in drag and stall angle. In this work, a Gurney flap of height 0.02c was employed on low-AR flat plate wings (AR ≤ 1.5) of different planforms (rectangular, Zimmerman and inverse Zimmerman). Measurements were carried out at a low Reynolds number (1 × 10⁵) to understand the flap's influence on the flow field over the wings. Time-averaged 2D particle image velocimetry (PIV) measurements on the wings' mid-span plane revealed a leading-edge separation bubble. The early reattachment of the leading edge separated flow is consistent with the results obtained from surface oil flow visualization. Stereoscopic-PIV measurements carried out at different cross-flow planes showed an increase in the strength of wingtip vortices and higher downwash for the Gurney flapped configuration. The collective information from these measurements suggests that the Gurney flap increases the strength of wingtip vortices via enhanced pressure difference between the upper and lower surface of the wing. The strong wing tip vortices promote higher downwash between them that reattaches the separated shear layer leaving a leading-edge separation bubble in between. The increased downwash delays complete flow separation to higher angles of attack. The strong tip vortices retain the lift-generating vortical flow closer to the wing. Hence, the maximum lift coefficient and stall angle were significantly increased.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"64 4","pages":""},"PeriodicalIF":2.4,"publicationDate":"2023-03-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-023-03605-y.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4727927","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":"A sparse optical flow inspired method for 3D velocimetry","authors":"George Lu,&nbsp;Adam Steinberg,&nbsp;Masayuki Yano","doi":"10.1007/s00348-023-03593-z","DOIUrl":"10.1007/s00348-023-03593-z","url":null,"abstract":"<div><p>We introduce a three-dimensional three-component particle-based velocimetry method that expands the methodology of optical flow to three dimensions. The proposed scheme, sparse particle flow velocimetry (SPFV), uses a sparse representation of intensity fields with kernel functions to facilitate efficient computation in 3D. In addition, to provide robust performance for the large particle displacements seen in images, the sparse representation is combined with a multi-resolution optimization scheme based on an energy functional derived from the displaced frame difference equation; however, this formulation is not reliant on linearized coarse-to-fine warping schemes to enable estimations of large displacements at the cost of potentially freezing large scale velocity features. Performance of SPFV is evaluated in terms of accuracy and spatial resolution, using synthetic particle images from a direct numerical simulation of isotropic turbulence. SPFV yields lower errors than tomographic PIV (T-PIV) and is capable of resolving finer scale features, even for large particle displacements and in the presence of artificial tomographic reconstruction artifacts. The method is also validated on experimental images of reacting flows and shows good agreement with T-PIV results.</p></div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"64 4","pages":""},"PeriodicalIF":2.4,"publicationDate":"2023-03-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-023-03593-z.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4730510","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 a high-pressure flame facility using high-speed chemiluminescence and OH LIF imaging","authors":"Will Swain,&nbsp;Yejun Wang,&nbsp;Pradeep Parajuli,&nbsp;Matthew Hay,&nbsp;Ahmad Saylam,&nbsp;Thomas Dreier,&nbsp;Christof Schulz,&nbsp;Waruna Kulatilaka","doi":"10.1007/s00348-023-03611-0","DOIUrl":"10.1007/s00348-023-03611-0","url":null,"abstract":"<div><p>The combustor and turbine inlet pressures of modern aviation and power-generation gas turbine engines can vary between 30 and 50 bar. Innovative diagnostic methods are needed to understand the complex thermo-physical processes taking place under these conditions. Non-intrusive, spatially, and temporally resolved optical and laser diagnostic methods such as chemiluminescence and laser-induced fluorescence imaging (LIF) can be used to gain insights into flame stability, heat release, and pollutant formation processes. In this work, a laboratory-scale, optically accessible, high-pressure burner facility operating on premixed CH₄/air flames is developed and characterized using kHz-rate hydroxyl and methylidyne chemiluminescence imaging, OH-LIF imaging, and two-color OH-LIF thermometry. For the latter two measurements, the flames were stabilized up to 10 bar using a stainless-steel disk mounted above the burner surface. Approximately 10-ns duration Nd:YAG laser pulses at 283.305 nm were used to excite the Q₁(7) rotational line of the A²Ʃ⁺←X²∏ (1,0) band of the OH radical, followed by fluorescence detection from the A←X (1,1) &amp; (0,0) bands. A linear dependence of the OH-LIF signal on the laser energy was observed. An increase in pressure from 1 to 10 bar showed a nonlinear decay of the OH-LIF signal. While quenching corrections accounted for a fraction of the signal loss, additional mechanisms, such as laser beam absorption and signal trapping, need to be considered for complete signal quantification. The measured excitation spectrum compared well with the LIFBASE model predictions. The flame equivalence ratio scans at different pressures agreed with the Cantera equilibrium flame code calculations. 2D OH-concentration distributions and two-color OH-LIF temperature maps agreed qualitatively with flame simulations performed using the ANSYS Fluent software package. This well-characterized burner facility provides a testbed for combustion and soot formation studies and investigates the role of minor species at elevated pressures at gas-turbine-relevant flame conditions.</p><h3>Graphical Abstract</h3>\u0000 <figure><div><div><div><picture><source><img></source></picture></div></div></div></figure>\u0000 </div>","PeriodicalId":554,"journal":{"name":"Experiments in Fluids","volume":"64 4","pages":""},"PeriodicalIF":2.4,"publicationDate":"2023-03-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00348-023-03611-0.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4727047","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}