Journal of Fluid Science and Technology
Online ISSN : 1880-5558
ISSN-L : 1880-5558
Volume 16, Issue 1
Recent Advances in Flow Dynamics 2019
Displaying 1-9 of 9 articles from this issue
Recent Advances in Flow Dynamics 2019
Papers
  • Takashi NAKAZAWA, Takashi MISAKA, Clair POIGNARD
    2021 Volume 16 Issue 1 Pages JFST0002
    Published: 2021
    Released on J-STAGE: January 01, 2021
    JOURNAL FREE ACCESS

    This paper presents an optimal design obtained as a shape optimization problem in a domain with a singular point. For shape optimization, the eigenvalue in Snapshot Proper Orthogonal Decomposition (Snapshot POD) is defined as a cost function. The main problems are a Non-stationary Navier–Stokes problem and eigenvalue problem of Snapshot POD. An objective functional is described using Lagrange multipliers and finite element method. Two-dimensional open cavity flow is adopted for an initial domain, where the domain includes a singular point. In this paper, two kinds of sensitivities assuming velocity vector in H1 and H2 are used. Using H1 gradient method for domain deformation, all triangles over a mesh are deformed as the cost function decreases. Finally, eigenvalues of Snapshot POD are compared in the initial and optimal domains.

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  • Yu NISHIO, Kohei KOMORI, Seiichiro IZAWA, Yu FUKUNISHI
    2021 Volume 16 Issue 1 Pages JFST0003
    Published: 2021
    Released on J-STAGE: January 01, 2021
    JOURNAL FREE ACCESS

    An experimental study is performed to investigate the flow structures near the edges of rotating disks with different 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 differs 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 effects 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 effect 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.

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  • Miku OHKURA, Hidemasa TAKANA, Fumio S. OHUCHI, Rei FURUKAWA
    2021 Volume 16 Issue 1 Pages JFST0004
    Published: 2021
    Released on J-STAGE: January 01, 2021
    JOURNAL FREE ACCESS

    Among the ionic liquids (ILs) that are known for their CO2 absorption properties, the optical properties of 1-ethyl-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 waveguide-structured CO2 sensor. The transmission length of the sensor could be elongated by optimizing both the waveguide and the core IL.

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  • Reon NISHIKAWA, Osamu TERASHIMA, Yasufumi KONISHI, Miyu OKUNO
    2021 Volume 16 Issue 1 Pages JFST0005
    Published: 2021
    Released on J-STAGE: January 01, 2021
    JOURNAL FREE ACCESS

    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 emitted noise with high two-dimensionality. Therefore, the fluttering region was observed as the source of significant periodic noise from the flag. It is also found that the vibration of the downstream edge which brought the noise generation was caused by the strong upward or downward flow which occurs periodically.

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  • Koji SHIMOYAMA, Yoshio SATO, Jun ONODERA, Jun LIU
    2021 Volume 16 Issue 1 Pages JFST0006
    Published: 2021
    Released on J-STAGE: January 01, 2021
    JOURNAL FREE ACCESS

    A heating ventilation and air conditioning (HVAC) unit is an essential unit to adjust temperature for passenger’s comfortability in an automotive cabin. For efficient and reliable design and development of the HVAC unit, the interior thermal flow needs to be simulated and the performance needs to be evaluated with low cost and high fidelity. Hence, this paper develops measurement-based strategies for high-fidelity thermo-fluid dynamics simulation of an HVAC heat exchanger. These strategies tune up the parameters of a porous media model in the governing equations, which model the interaction between the heat exchanger and the surrounding thermal flow field and are conventionally fixed to certain constants, by functionalization or data assimilation with actual measurement data. The present results show that both strategies are able to reduce discrepancies between the simulation and the actual measurements, and improve fidelity to simulate the temperature field without sacrificing the simulation cost very much. Especially, the data assimilation strategy is more effective to yield more accurate simulation results only with the measurement data while the functionalization strategy needs to derive theoretical correlations. It demonstrates that data assimilation is helpful to assist reliable and efficient design and development of an HVAC unit regardless of designer’s professional skills or knowledge.

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  • Hiroki YAO, Taisuke NAMBU, Yasuhiro MIZOBUCHI
    2021 Volume 16 Issue 1 Pages JFST0007
    Published: 2021
    Released on J-STAGE: January 01, 2021
    JOURNAL FREE ACCESS

    A combustion simulation software tool, “HINOCA”, has been developed for automotive engine analysis. HINOCA is based on fully compressible Navier-Stokes equations, which are Reynolds-averaged (RANS) or spatially-filtered (LES), and employs the Cartesian grid and immersed boundary (IB) methods to reduce the mesh generation cost. In the present paper, focusing on flow simulations using k-ε models, a robust and reliable IB method coupled with wall functions is proposed. One major aspect of the method is that different IB cell information is employed for inviscid and viscous flux evaluations at fluid-IB cell interfaces. To improve the evaluation of wall shear stress, the shear stresses on the boundaries of an IB cell are transformed into a body force acting on the adjacent fluid cell. The computational method for ε-equation and the source terms of the k-equation near IB cells are modified so that the development of the turbulent boundary layer on a flat plate is well reproduced. The effects of these modifications are validated by the 2D Zero Pressure Gradient Flat Plate problem. To improve the mass conservation property of the IB method, multiple geometric parameters are defined for IB cells; that is, different image point information is immersed on IB cell centers for evaluating the inviscid flux on each cell interface. Evaluation with the Steady State Flow Bench problem shows that the proposed method drastically improves the mass conservation property of simulations and is able with a coarse mesh to reproduce flow structures obtained by LES with a much finer mesh.

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  • Hiroki TAMEIKE, Aiko YAKENO, Shigeru OBAYASHI
    2021 Volume 16 Issue 1 Pages JFST0008
    Published: 2021
    Released on J-STAGE: January 01, 2021
    JOURNAL FREE ACCESS

    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.

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  • Shugo DATE, Yoshiaki ABE, Takeki YAMAMOTO, Tomonaga OKABE
    2021 Volume 16 Issue 1 Pages JFST0009
    Published: 2021
    Released on J-STAGE: January 01, 2021
    JOURNAL FREE ACCESS

    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 quasiisotropic laminate with the fibers of higher stiffness is more resistant to tensile load and skin buckling but less resistant to compressive load.

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