Journal of Fluid Science and Technology最新文献

{"title":"Effects of characteristic decomposed modes of the internal flow of a circular 90-degree bent nozzle on the behavior of the oil jet interface","authors":"Mikimasa Kawaguchi, Ryoutaro Nakayama, Li-Jia Ma, K. Nishida, H. Yokohata, Masanobu Koutoku, J. Nishikawa, A. Nakashima, Y. Ogata","doi":"10.1299/jfst.2021jfst0023","DOIUrl":"https://doi.org/10.1299/jfst.2021jfst0023","url":null,"abstract":"Methods of decreasing the CO2 emissions of the internal combustion engine have been suggested. For example, an engine can be designed with a high compression ratio and/or a downsizing turbocharger. However, these methods generate high combustion temperatures that increase the heat load. The piston cooling gallery has been proposed as a system for cooling the engine piston. The piston cooling gallery is an oil flow path that is set internal to the piston. An oil jet injected from a nozzle placed under the piston flows into the piston cooling gallery through an entrance hall. It may thus be desirable to control the shape of the oil jet such that it is stable and straight. However, the interface of the ambient air and oil jet may have unstable waviness because of Kelvin– Helmholtz instability and/or Rayleigh–Taylor instability. In addition, we investigated the flow and found that the propagation of the flow speed fluctuation of the nozzle internal flow results in the waviness of the oil jet in a previous study. To further clarify the relationship between oil jet interface instability immediately after nozzle exit and flow in nozzle, this paper reports on two types of particle image velocimetry (PIV), namely twodimensional two-velocity-component PIV and two-dimensional three-velocity-component PIV, in addition to two-component and three-component snapshot proper orthogonal decompositions, and analyzes turbulence propagation adopting a cross-correlation method. We find a characteristic basis vector with large energy that propagates the fluctuation downstream under the condition that the interface between the oil jet and air has strong waviness.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66306407","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Influence of small wavy roughness on flatplate boundary layer natural transition","authors":"Hiroki Tameike, A. Yakeno, S. Obayashi","doi":"10.1299/jfst.2021jfst0008","DOIUrl":"https://doi.org/10.1299/jfst.2021jfst0008","url":null,"abstract":"One of the effective ways to reduce viscous drag around an airfoil is by delaying the boundary layer transition. In this study, we analyzed the influence of a small wavy roughness on a two-dimensional, natural boundary layer transition, using direct numerical simulation that resolved each small roughness. A parametric study was conducted on the wavy roughness wavelength. Our results show that in some cases the transition delays whose characteristics depend on the roughness wavelength. In a detailed analysis, we found that the wavy roughness firstly affects the process of primary vortex growth, Tollmien–Schlichting (TS) instability. In addition, we found that the secondary vortex pairing also depended on it. In the most transition-delayed cases, the roughness wavelength was different far from the TS instability one, and the vortex pairing occurred firstly in upstream however not much in downstream, keeping the vortex size is kept small.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66305483","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Laminarization of inhomogeneous turbulent channel flow under stable density stratification","authors":"O. Iida, Yoshito Nagase","doi":"10.1299/JFST.2021JFST0012","DOIUrl":"https://doi.org/10.1299/JFST.2021JFST0012","url":null,"abstract":"relaminarized by the viscosity due to the low-Reynolds number, the local turbulent regions in the upper and lower walls appear simultaneously, and a flow is almost homogeneous over the wall-normal direction et. al ., 2014; Abstract Effects of buoyancy force stabilizing the disturbances are investigated on a turbulent channel flow bounded by two walls of different temperatures. With a constant mean pressure gradient imposed, the buoyancy force related to the Grashof number (Gr) is systematically increased by increasing the temperature difference between upper and lower walls, which are perpendicular to the gravitational direction. As a result, the mean flow rate increases with an increase in the Gr, and turbulent structures become intermittent and inhomogeneous; turbulent and quasi-laminar regions simultaneously appear in the same computational region. Detailed visualization for instantaneous turbulent structure shows that in the upper and lower sides of a channel, the turbulent regions appear alternatively, and local one-sided turbulence is observed. Finally, with a further increase in the Gr, the stratified channel flow becomes complete one-sided turbulence where a flow becomes turbulent on one side of a channel, while it becomes laminarized on the other side.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66305919","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Bayesian optimization of traveling wave-like wall deformation for friction drag reduction in turbulent channel flow","authors":"Yusuke Nabae, K. Fukagata","doi":"10.1299/jfst.2021jfst0024","DOIUrl":"https://doi.org/10.1299/jfst.2021jfst0024","url":null,"abstract":"We attempt to optimize the control parameters of traveling wave-like wall deformation for turbulent friction drag reduction using the Bayesian optimization. The Bayesian optimization is an optimization method based on stochastic processes, and it is good at finding the parameter values to minimize (or maximize) an expensive cost function or a blackbox function. The parameter value to be tested in the next iteration step is chosen based on the acquisition function that accounts for both the exploration term searching in high uncertainty regions and the exploitation term searching in the regions of high possibility over the current best observations. First, we investigate the ef-fectiveness of the Bayesian optimization using a two-parameter test function with known optimum value. As a result, the Bayesian optimization is shown to successfully work. Next, we apply the Bayesian optimization to the control parameters of traveling wave-like wall deformation for friction drag reduction in a turbulent channel flow at the friction Reynolds number of Re (cid:28) = 180. While the wavenumber ( k + x ) is fixed, the velocity amplitude ( v + w ) and the phasespeed ( c + ) are chosen as the variable to optimize. As a result of the Bayesian optimization, although the bulk-mean velocity in the optimized case varies periodically, we achieved the maximum drag reduction rate of 60 : 5% when ( v + w ; c + ) = (10 : 0 ; 42), which is higher than that in the previous study (Nabae et al., 2020), i.e., 36 : 1%. In the optimized case, by repeating laminarization of flow field and re-transition to turbulent flow due to the inflection instability, the bulk-mean velocity increases and decreases periodically.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66306499","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Flow measurement around the edges and curved outer surface of a rotating disk","authors":"Y. Nishio, Kohei Komori, S. Izawa, Y. Fukunishi","doi":"10.1299/jfst.2021jfst0003","DOIUrl":"https://doi.org/10.1299/jfst.2021jfst0003","url":null,"abstract":"An experimental study is performed to investigate the flow structures near the edges of rotating disks with di ff erent edge shapes. By changing the rotational speed, the Reynolds number is changed from 277 to 356. It is found that the fluid motion near the sharp edge di ff ers depending on whether a rotating shaft is at the disk surface. A flow developing on the flat side of the disk surface changes its direction toward the curved outer surface on the shaft side, and the flow goes straight outward, leaving the disk on the flat side. The existence of the supporting shaft increases the radial velocity of the flow while decreasing the azimuthal velocity. The e ff ects of the edge shape of the disk on the flow fields are also investigated by changing the shape of the disk edges. Rounded and chamfered edges have no noticeable e ff ect on the azimuthal velocity on the curved outer surface, whereas changing the edge shape enhanced the velocity in the disk-thickness direction. In the FFT analyses of the azimuthal velocity measured at the edge of the curved outer surface when the disk edge is rounded, an increase in power across a range of frequencies is observed. Only in the chamfered edge disk case, a peak in the spectrum of the velocity that corresponds to a wavenumber which appears in the transitional boundary layers is observed.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66305574","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Fabrication of liquid-core fiber-optic structure for large-area CO2 sensing using ionic liquids","authors":"M. Ohkura, H. Takana, F. Ohuchi, R. Furukawa","doi":"10.1299/jfst.2021jfst0004","DOIUrl":"https://doi.org/10.1299/jfst.2021jfst0004","url":null,"abstract":"Abstract Among the ionic liquids (ILs) that are known for their CO2 absorption properties, the optical properties of 1ethyl-3-methylimidazolium acetate ([emim][Ac]) and 1-butyl-3-methylimidazolium acetate ([bmim][Ac]) have been investigated with the aim of assessing their suitability for use in a CO2 sensor with a liquid-core fiber-optic structure. Fiber-optic sensors offer multiple benefits, including a large-area sensing capability and immunity to electromagnetic interference. In these two ILs with their different cation alkyl chain extensions, similar levels of change in the refractive index were observed for both [emim][Ac] and [bmim][Ac]; this change was demonstrated to lead to a change in the numerical aperture of a waveguide equipped with an [emim][Ac] core with a maximum value of 0.017787. Waveguide samples were fabricated using both [emim][Ac] and [bmim][Ac] and the output spectra of these samples were compared in terms of their liquid absorption characteristics, which were measured before the samples were packed in a gas-permeable Teflon®AF cladding tube. The liquid-core waveguides demonstrated successful light transmission over a length of 10 cm that agreed with the absorption characteristics of each of the core liquids. The CO2 concentration level inside the core liquid was believed to cause the transparency of the waveguide to deteriorate as a result of bubble formation. The growth of the CO2 bubbles is irreversible and is assumed to be promoted by a kinetic stimulus and some other factors. The ILs comparison considered in this study will be useful for further development of the liquid-core waveguidestructured CO2 sensor. The transmission length of the sensor could be elongated by optimizing both the waveguide and the core IL.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66305620","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Fluid-structural design analysis for composite aircraft wings with various fiber properties","authors":"Shugo Date, Yoshiaki Abe, Takeki Yamamoto, T. Okabe","doi":"10.1299/jfst.2021jfst0009","DOIUrl":"https://doi.org/10.1299/jfst.2021jfst0009","url":null,"abstract":"This study performed an analysis for the fluid-structural design of aircraft wings composed of carbon fiber reinforced plastics (CFRPs). Specifically, the effects of carbon fibers on structural weight were evaluated. A multiscale computational framework was developed for designing CFRP wings so that even those CFRPs can be considered whose mechanical properties are not available as experimentally-measured data, thereby bridging two different scales by the following processes: 1) a microscale analysis for evaluating the mechanical properties (stiffness and strength) of unidirectional CFRP laminates and 2) a macroscale fluid-structural analysis that involves structural sizing of wingbox structures based on the mechanical properties given by the microscale analysis. To this end, five fibers were examined in this study, namely: T300, T700S, T800H, T800S, and T1100G. It was discovered that T1100G exhibited the lightest wingbox structures, followed by T800S, T800H, T700S, T300. This was mainly due to the difference in a thickness of the lower panels, where the thickness was minimized with T1100G among the five fibers, resulting from the tensile failure mode. Meanwhile, the upper panels under compressive load showed two different failure modes, namely: fiber microbuckling and skin buckling. In the region where the fiber microbuckling was dominant, the panel thickness was in order of the stiffness of the fiber, i.e., the panel made with T1100G having the highest stiffness was thicker than that made with T800S, T800H, T700S and T300, and vice versa in the region where the skin buckling was dominant. Based on the microscale analysis, the aforementioned failure mechanisms are consistent with the fact that a quasi-isotropic laminate with the fibers of higher stiffness is more resistant to tensile load and skin buckling but less resistant to compressive load.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66305554","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Sound radiated from low Mach number turbulent boundary layer flows（Turbulent boundary layer on a smooth plate and over a small forward facing step）","authors":"Shiko Moriyama, Yohei Inoue, H. Maekawa","doi":"10.1299/JFST.2021JFST0010","DOIUrl":"https://doi.org/10.1299/JFST.2021JFST0010","url":null,"abstract":"High-resolution numerical simulation has been performed to study aeroacoustic noise radiated from a turbulent boundary layer at freestream Mach number Ma = 0.3, which develops on a smooth flat plate and over a small forward-facing step. Sound waves radiated from the turbulent boundary layer on the flat plate are dominant in a very-low-frequency band and have characteristics of a linear sound source. The sound waves are observed in the far-field from the boundary propagate outside of the flow region where small hydrodynamic pressure fluctuations with a significant long-wavelength appear. The instantaneous hydrodynamic pressure fields that gradually develop or decay while moving downstream with the turbulent boundary layer are associated with the evolution of various vortical structures. The characteristics of the sound wave being a linear sound source and dominant in a very-low-frequency band are similar to those of the real high-speed train. The sound waves generated from the turbulent boundary layer over the forward-facing step with a height (SH) of SH/y ≅ 62 are significantly larger in a full frequency band than those radiated from the turbulent boundary layer on the flat plate. Numerical results of the turbulent boundary layer over the forward-facing step with a height of SH/y ≅ 30 show that the sound waves are dominant in the low-frequency band, and also step-specific sound waves are generated in the high-frequency band. Further, even if the step height is SH/y ≅ 7.5, the sound waves unique to the small step are generated in the high-frequency band.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66305652","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Noise generation of fluttering flag in a free stream","authors":"Reon Nishikawa, O. Terashima, Y. Konishi, Miyu Okuno","doi":"10.1299/jfst.2021jfst0005","DOIUrl":"https://doi.org/10.1299/jfst.2021jfst0005","url":null,"abstract":"Fluttering flag in flow has been investigated for a long time from both a physical and an engineering point of view. Pioneering work on fluttering flag was performed by Fairthorne (1930) and Thoma (1939). Fairthorne measured the drag coefficients of several types of flags in a wind tunnel and found the relationship between the length of the flag and its coefficient. Subsequently, significant work has been conducted to investigate the forces acting on fluttering flags (Emmanuel et al., 2013). Hoerner (1965) and Taneda (1968) investigated the relationship between the drag of a flag and its length, and obtained different relationships from those of Fairthorne. Furthermore, Taneda (1968) found that the frequency of the oscillation of the flag depended on the Reynolds number and the mass ratio of the flag. The traveling waves on a fluttering flag were found and discussed by Sparenberg (1962). These groundbreaking works led to later research on parachutes (Lokerson, 1968), vehicles (Bourrières, 1969), projectiles (Fancett and Calyden, 1972), and rockets (Auman and Wilks, 2005). The fluttering flag remains a subject of interest for researchers. The time-averaged drag coefficients of fluttering flag were investigated and discussed in previous works (e.g., Wilk and Skuta, 2009), and a simple model was presented by Moretti (2003), which was later validated by Ristroph and Zhang (2008). More recently, the unsteady fluid force acting on a fluttering flag was measured, and the drag force and the moment around its strut were discussed from the perspective of its evolution during fluttering mode switches (Emmanuel et al., 2013). In addition, for the improvement of heat transfer, the flow-induced vibration of an inverted flag (Kim et al., 2013) has been studied by numerous researchers (Yu et al., 2017, Ryu et al., 2015, and Chen et al., 2018). Abstract An experimental study on the noise from a fluttering flag was performed in a low-noise wind tunnel. In the experiment, simultaneous measurements of noise from the flag and its motion were performed using microphones and a camera, respectively, to obtain the noise characteristics and their relation. Additionally, simultaneous measurements of noise and its displacement were performed to quantitively discuss their relation using seven laser displacement sensors. The experimental results indicated that a highly periodic noise with significant directivity in the vertical direction is generated from the flag, and the dominant frequency of the noise is linearly proportional to the inlet velocity. Additionally, the constants of proportionality are inversely proportional to the length of the flag and the square root of its thickness. The results also indicated that the downstream edge of the flag rolls up and down when significant periodic sound pressure is generated by the flow near the downstream edge of the flag. Furthermore, near the center of the downstream edge, the flag flutters at the dominant frequency of the emit","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66305699","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Quantitative density measurement of wake region behind reentry capsule (Improvements in accuracy of 3D reconstruction by evaluating the view-angle of measurement system)","authors":"Masato Yamagishi, Yusuke Yahagi, M. Ota, Y. Hirose, S. Udagawa, T. Inage, Shigeya Kubota, Koji Fujita, K. Ohtani, H. Nagai","doi":"10.1299/jfst.2021jfst0021","DOIUrl":"https://doi.org/10.1299/jfst.2021jfst0021","url":null,"abstract":"","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66306018","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}