Osamu Terashima

Turbulence structure and turbulence kinetic energy transport in multiscale/fractal-generated turbulence

The turbulence structure and turbulence kinetic energy transport in multiscale/fractal-generated turbulence in a wind tunnel are investigated. A low-blockage, space-filling square-type (i.e., fractal elements with square shapes) fractal grid is placed at the inlet of the test section. On the basis of the thickness of the biggest grid bar, t0, and the inflow velocity U∞, the Reynolds numbers (Re0) are set to 5900 and 11 400; these values are the same as those considered in previous experiments [D. Hurst and J. C. Vassilicos, “Scalings and decay of fractal-generated turbulence,” Phys. Fluids 19, 035103 (2007)10.1063/1.2676448; N. Mazellier and J. C. Vassilicos, “Turbulence without Richardson-Kolmogorov cascade,” Phys. Fluids 22, 075101 (2010)10.1063/1.3453708]. The turbulence characteristics are measured using hot-wire anemometry with I- and X-type probes. Generally, good agreements are observed despite the difference in the size of the test sections used: The longitudinal integral length-scale Lu and the T...

Relevance of turbulence behind the single square grid to turbulence generated by regular- and multiscale-grids

Direct numerical simulations were carried out to study the turbulence generated by a fractal square grid at a Reynolds number of ReL0 = 20000 (based on the inlet velocity Uin and length of the largest grid bar L0). We found that in the near-field region, the fractal square grid can generate much higher turbulence levels and has a better mixing performance than the single square grid. However, the current numerical results show that a single square grid can produce a turbulence intensity and turbulent Reynolds number at the end of the simulation region (i.e., X/L0 ≃ 13) comparable to those of a higher-blockage fractal square grid because the two turbulent flows have quite different energy decay rates. We also demonstrated that for the fractal square grid, the length L0 gives a physical description of the inlet Reynolds number. An examination of the characteristic length scale for the fractal square grid reveals that the unusual high energy decay rates in previous experiments [D. Hurst and J. C. Vassilicos,...

Development of turbulence behind the single square grid

In this paper, direct numerical simulations are carried out to study single-square grid-generated turbulence at a Reynolds number ReL0 = 20 000 (based on the inlet velocity Uin and the length of grid bar L0). Different from the regular grid and the multiscale/fractal grid, here only single large square grid is placed at the center near the inlet. First, we investigate the evolutions of turbulence characteristics (e.g., mean streamwise velocity, turbulence intensity, Taylor microscale, etc.) along the centerline. The common characteristics possessed by turbulent flows generated by the single square grid and by the fractal square grid are presented. We confirm the hypothesis proposed by Mazellier and Vassilicos [“Turbulence without Richardson-Kolmogorov cascade,” Phys. Fluids 22, 075101 (2010)] that for the fractal square grid, the location of turbulence intensity peak along the centerline is mainly determined by large-scale wake interactions. Current numerical results show that in turbulence generated by t...

VISUALIZATION OF TURBULENT REACTIVE JET BY USING DIRECT NUMERICAL SIMULATION

Direct numerical simulation (DNS) of turbulent planar jet with a second-order chemical reaction (A + B → R) is performed to investigate the processes of mixing and chemical reactions in spatially developing turbulent free shear flows. Reactant A is premixed into the jet flow, and reactant B is premixed into the ambient flow. DNS is performed at three different Damkohler numbers (Da = 0.1,1, and 10). Damkohler number is a ratio of a time scale of a flow to that of chemical reactions, and in this study, the large Da means a fast chemical reaction, and the small Da means a slow chemical reaction. The visualization of velocity field shows that the jet flow is developed by entraining the ambient fluid. The visualization of concentration of reactant A shows that concentration of reactant A for Da = 1 and 10 becomes very small in the downstream region because the chemical reaction consumes the reactants and reactant A is diffused with the jet development. By comparison of the profiles of chemical reaction rate and concentration of product R, it is found that product R for Da = 10 is produced by the chemical reaction at the interface between the jet and the ambient fluids and is diffused into the jet flow, whereas product R for Da = 0.1 is produced in the jet flow after reactants A and B are well mixed.

Joint statistics between velocity and reactive scalar in a turbulent liquid jet with a chemical reaction

Joint statistics between the velocity and the concentration of reactive species are experimentally investigated in a planar liquid jet with a second-order chemical reaction A + B → R. Reactant species A and B are premixed in a jet flow and a main flow, respectively. An optical fibre probe based on light absorption spectrometry is used to measure the instantaneous concentrations of reactive species. The stream-wise velocity and the concentrations of reactive species are simultaneously measured by combining the optical fibre probe with I-type hot-film anemometry, and we investigate the influence of the chemical reaction on correlation coefficients, joint probability density functions and cospectra of u and γi, where u is the stream-wise velocity fluctuation and γi is the concentration fluctuation of species i. The results show that the absolute value of the correlation coefficient between u and γB becomes small owing to the chemical reaction, whereas that between u and γA becomes large on the jet centreline. It is also shown that the influence of the chemical reaction on the cospectrum of u and γi in the upstream region and near the jet centreline is different from that in the downstream region and the outer edge of the flow.

Simultaneous measurement of all three velocity components and pressure in a plane jet

Simultaneous measurement of all three velocity components and static pressure in a plane turbulent jet is performed by developing a new probe for the measurement. The combined probe consists of two X-type hot-wire sensors and a static pressure tube placed at the center of hot-wires. The static pressure tube is miniaturized with a micro electromechanical system fabrication technique to improve spatial resolution. The results of the calibration test show that the measurement accuracy of the pressure fluctuation with the pressure tube is 9%. Further, we are able to compensate for the cross-flow error of the pressure tube by instantaneously measuring all the velocity components with two X-type hot-wire sensors. The profile of the production term and diffusion term in the turbulent energy transport equation, directly estimated with the measured data, also shows slight improvement compared to our previous studies. This is due to the improvement in the spatial resolution of the combined probe and the improvement in the measurement accuracy of both the velocity-kinematic energy correlation and that of the velocity–pressure correlation by measuring all velocity components. Further, it is also found that the pressure diffusion of the turbulent energy in the plane jet is mainly caused due to the fluctuation whose frequency is from 20 to 40 Hz, which corresponds to the flapping frequency in the present experiment condition.

Improvement of Constant Temperature Anemometer and Measurement of Energy Spectra in a Plane Turbulent Jet

A constant temperature anemometer (CTA) is a useful instrument for measuring the velocity fluctuations in turbulent flow. However, in our calibration test, the actual frequency response of a typical CTA was no more than 5 kHz under normal laboratory conditions: for example, the diameter of the hot wire is 5 μm and the free stream velocity is 20 m/s. Therefore, in some cases, a typical CTA is not enough to measure accurately turbulent velocity fluctuations for fine scale structures. In this paper, we present a rearranged CTA circuit to obtain a faster frequency response so that in turn fine-scale structures can be more accurately investigated.A typical CTA circuit consists of a Wheatstone bridge and a feed back circuit. To improve the frequency response, the ratio of the electrical resistance of the Wheatstone bridge is set to 1 and two operational amplifiers with a gain-band width product of 100 MHz and a slew rate of 20 V/μs are used in the feedback circuit.An experiment to estimate the frequency response of the rearranged CTA circuit is performed with a free stream velocity of 20 m/s and using hot wires of diameter 5 μm and 3 μm. Experimental results show that the roll-off frequency of the rearranged CTA circuit is improved from 5 kHz to 20 kHz for the 5 μm hot wire and from 6 kHz to 40 kHz for the 3 μm hot wire.Velocity measurements are made using the rearranged CTA circuit in a plane turbulent jet where the value of the Taylor microscale λ is 3.2 mm and the Taylor-scale Reynolds number Reλ is 440. Measurements shows that the power spectrum obeys the reliable numerical profile derived by a LDIA (Lagrangian Direct-Interaction Approximation) theory until more than 0.20 of the non-dimensional wave number κ1η, which is a wider range in comparison with the results obtained when using a typical CTA circuit. Here, κ1 is the axial wave number and η is the Kolmogorov microscale.Further, velocity measurements are performed taken using the rearranged CTA circuit with a square jet where the value of λ is 6.3 mm and Reλ is 1,720. Measurements shows that the power spectrum obeys the numerical profile by the LDIA theory in the range 0.04 < κ1η < 0.20, which is a much wider range than the results obtained when using a typical CTA circuit (0.04 < κ1η < 0.08).These results indicate that the rearranged CTA circuit can be used to investigate fine-scale structures in turbulent flows more accurately.Copyright

Effect of Pressure Fluctuation at Nozzle Exit on Flapping Phenomena in a Two-Dimensional Jet

The flapping motion of the flow is one of the coherent structures in a two-dimensional turbulent jet. In past studies, the flapping phenomenon indicated that a pair of fluid lumps with the positive and negative streamwise velocity fluctuation exists on the opposite sides of the jet centerline, and the signs of the velocity fluctuation for those fluid lumps change alternately as the time advances. Additionally, it is known that the vortices at the jet exit are arranged symmetrically to the jet centerline and gradually become the alternate arrangement, and in the self-preserving region, the flapping phenomenon can be observed. However, the reason why the flapping phenomenon arises is not cleared yet. In this study, in order to clarify the influence of the velocity and pressure fluctuation on the arising of the flapping phenomenon, the characteristics of the velocity and pressure at near the jet exit are investigated. The measurements of the flapping phenomenon, the characteristics of the velocity and pressure at near the jet exit are conducted by using combined probe composed of an X-type hot-wire probe and a pressure probe, and at the same time, the measurements of streamwise velocity fluctuations at the two points in the self-preserving region are performed to determine the time when the flapping phenomenon is arising. The measured data are analyzed statistically by ensemble-averaging technique and conditional-sampling technique on the basis of the intermittency function for the flapping/non-flapping decision. The intermittency function is obtained by applying the wavelet transform analysis to the measured data by two I-type hot wire probes placed at the opposite side of the jet centerline in the self-preserving region. Measured and analyzed results show that the RMS value of the streamwise velocity fluctuation at the jet exit is clearly different according to whether flapping phenomenon arises or not. On the other hand, the RMS value of the pressure fluctuation at the jet exit is not influenced by the arising of the flapping phenomenon. In addition, the possibility that the arising of the strong negative pressure fluctuation at near the jet exit has an important role in the flapping phenomenon is shown.Copyright

603 Study on the flow characteristic in the cerebral aneurysm and effect of the medical device placement

1.緒言 クモ膜下出血をきたす原因の一つに脳動脈瘤破裂が挙げ られ,破裂を起こした場合は重篤な状態に陥る場合が多い. 脳動脈瘤の低侵襲な治療法として,コイル塞栓術が広く行わ れている.しかし,wide neck 瘤や不整形瘤に対して通常の コイル留置方法では完全閉塞が困難な場合が多く,術後の再 開通や再増大を認める場合がある.そこで,このような瘤に おいて,ステント支援コイル塞栓術が行われ,良好な治療結 果が報告されている.しかしながら,脳底動脈の分岐部に形 成される瘤に対してステント留置が血流に及ぼす影響,特に 血管分岐角度が異なるケースにおいて,ステント留置が血流 にもたらす影響については研究が行われていない. そこで本研究では,脳底動脈先端部瘤モデルを作成し,血 管分岐角度の違いによりステント留置が血流に及ぼす影響 の調査を目的として,ステント留置を施した 2つの脳底動脈 先端部瘤に対し CFD解析を行った.

DNS–PDF Simulation of Turbulent Mixing in a Reactive Planar Jet

Probability density function (PDF) method is implemented in direct numerical simulation (DNS) to simulate turbulent reactive flows (DNS–PDF method). In the DNS–PDF method, a flow field and a non reactive scalar are predicted by the DNS, whereas reactive scalars are predicted by the Lagrangian PDF method, in which a transport equation of joint PDF of reactive scalars is solved by using a large number of notional particles. A mixing time scale for a mixing model used in the PDF method is directly estimated from the DNS result. In the present model for the mixing time scale, the effect of distance between notional particles is implicitly taken into account. The DNS–PDF method is applied to a planar jet with a second-order chemical reaction. The results show that the DNS–PDF method can accurately predict the rms value of mixture fraction fluctuation, and the present model for the mixing time scale is valid. It is also found that the DNS–PDF method can accurately predict mean concentrations of reactive species.