Yasumasa Ito

Turbulent mixing of passive scalar near turbulent and non-turbulent interface in mixing layers

A direct numerical simulation of a temporally developing mixing layer with a passive scalar transport is performed for various Schmidt numbers (Sc = 0.25, 1, 4, and 8). Turbulent mixing is investigated near the turbulent/non-turbulent interface (TNTI), which is a layer consisting of the turbulent sublayer (TSL) and viscous superlayer (VSL). The irrotational boundary, which is close to the outer edge of the TNTI layer, is detected as the isosurface of small vorticity magnitude. The movement of fluid elements relative to the irrotational boundary movement is analyzed. Once the non-turbulent fluid is entrained into the VSL across the irrotational boundary by the viscous diffusion of vorticity, the fluid moves away from the irrotational boundary in the VSL in the normal direction of the irrotational boundary. After the fluid reaches the TSL, it is transported in the tangential direction of the irrotational boundary and is mixed with the fluid coming from the turbulent core (TC) region. The boundary between th...

Enstrophy and passive scalar transport near the turbulent/non-turbulent interface in a turbulent planar jet flow

The enstrophy (ω2/2) and passive scalar (ϕ) transport near the turbulent/non-turbulent (T/NT) interface is investigated using direct numerical simulation of a planar jet with passive scalar transport. To take into account the interface movement, we derive the transport equations for the enstrophy and the scalar in a local coordinate system moving with the T/NT interface. The characteristics of the T/NT interface are analyzed for three interface orientations. The cross-streamwise edge and the leading edge face the cross-streamwise and streamwise directions, respectively, and the trailing edge is opposite to the leading edge. The propagation velocity of the T/NT interface is derived from the enstrophy transport equation in the local coordinate system. The T/NT interface propagates toward the non-turbulent region on average at the cross-streamwise and leading edges, whereas the trailing edge frequently propagates into the turbulent region. The conditional average of the enstrophy transport equation in the local coordinate system shows that viscous diffusion transports, toward the non-turbulent region, enstrophy, that is advected from the turbulent core region or is produced slightly inside the T/NT interface. Viscous diffusion contributes greatly to the enstrophy growth in the region very close to the T/NT interface. The transport equation for the scalar ϕ in the local coordinate system is used to analyze the scalar transport near the T/NT interface. The conditional average of the advection term shows that ϕ in the non-turbulent region is frequently transported into the turbulent region across the cross-streamwise and leading edges by interface propagation toward the non-turbulent region. In contrast, ϕ in the turbulent region is frequently transported into the non-turbulent region across the trailing edge. The conditional averages of the advection and molecular diffusion terms show that both the interface propagation and the molecular diffusion contribute to the scalar transport across the T/NT interface.

The effects of high-frequency ultrasound on turbulent liquid mixing with a rapid chemical reaction

The effects of high-frequency ultrasound and mean fluid shear on turbulent mixing with a rapid chemical reaction were experimentally investigated in three types of liquid mixing-layer flow downstream of a turbulence-generating grid; pure grid-generated turbulence, grid-generated turbulence with high-frequency ultrasonic irradiation, and grid-generated turbulence with mean fluid shear. Instantaneous velocity and concentration were simultaneously measured using the combination of a laser-Doppler velocimeter and a laser-induced fluorescence method. The results show that turbulent mixing and chemical reaction are promoted by ultrasonic irradiation and mean fluid shear. The amount of chemical product in grid-generated turbulence with high-frequency ultrasonic irradiation is much larger than that in grid-generated turbulence with mean fluid shear, despite turbulent mass transport being enhanced at an equivalent level in both flows. This is attributed to the difference in turbulent mass transport at small scales...

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.

Wavelet analysis of coherent vorticity near the turbulent/non-turbulent interface in a turbulent planar jet

Coherent vorticity near the turbulent/non-turbulent (T/NT) interface is investigated by using direct numerical simulation of a planar jet. The coherent vorticity extraction (CVE) method based on the orthogonal wavelet decomposition of vorticity is applied to the planar jet for extracting the coherent vorticity. We analyze the conditional statistics conditioned on the distance from the T/NT interface. The coherent vorticity is reconstructed from small number of wavelet coefficients. Nevertheless, the coherent vorticity contains most of enstrophy in the planar jet. Furthermore, the characteristics of the vorticity field are well captured even near the T/NT interface by the coherent vorticity. The coherent velocity obtained by the Biot–Savart relation shows that the large-scale motions, such as induced flow and engulfing motion in the non-turbulent region, are also well represented by the coherent field. The enstrophy transport equation is decomposed into coherent and incoherent parts by the CVE for investigating the role of the coherent vorticity in the enstrophy transport mechanism. The conditional average of the enstrophy transport equation shows that the enstrophy production and dissipation associated with the incoherent vorticity are small compared with the coherent contributions. The enstrophy diffusion near the T/NT interface, which causes the local entrainment of non-turbulent fluids, arises from the coherent vorticity. Thus, most of the enstrophy transport mechanism near the T/NT interface is well captured by the coherent vorticity extracted by the CVE method.

Spatial evolution of the helical behavior and the 2/3 power-law in single-square-grid-generated turbulence

Direct numerical simulations are performed to investigate the helical properties of single-square-grid-generated turbulence. The streamwise evolution of the probability density functions of the relative helicity density h reveals the existence of a transition from a quasi-two-dimensional state to a three-dimensional state. The correlations between the helicity and the enstrophy level as well as the dissipation level are examined. When conditioned on a high level of dissipation or enstrophy, in the energy decay region the velocity and vorticity vectors in both instantaneous and fluctuating fields become more aligned. However, this correlation does not hold in the production region. We also study the second-order structure function and reveal that a well-defined power-law can be found at a location quite close to the grid, where the turbulent flow is still in the transition state.

Mixing and chemical reaction at high Schmidt number near turbulent/nonturbulent interface in planar liquid jet

This study investigates the mixing of reactive species at a high Schmidt number (Sc ≈ 600) near the turbulent/nonturbulent (T/NT) interface in a planar liquid jet with a chemical reaction A + B → R. Reactants A and B are supplied from the jet and ambient flows, respectively. An I-type hot-film probe and optical fiber probe are used for the simultaneous measurements of the streamwise velocity, mixture fraction, and concentrations of all reactive species and for detecting the T/NT interface. Statistics conditioned on the time elapsed after interface detection are analyzed. The conditional mean mixture fraction and concentrations change sharply near the interface. The widths of these changes are independent of the chemical species. The conditional statistics reveal the dependence of the chemical reaction on the interface orientation. The segregation intensity near the interface shows that the mixing state of the two reactants also depends on the interface orientation. However, the large reaction rate near the interface is related to the large concentration of reactant A rather than the mixing state, because reactant A supplied from the jet tends to be deficient near the interface. Near the interface where the reaction rate is large, the concentration of the chemical product is also large. The difference in the product concentration between the different interface orientations is larger for the infinitely fast reaction (as investigated by using the equilibrium limit) than the finite Damkohler number case, and the dependence of the chemical reaction on the interface orientation is expected to be significant for a fast chemical reaction.

LES-Lagrangian particle method for turbulent reactive flows based on the approximate deconvolution model and mixing model

We propose a numerical method for turbulent reactive flows using a large eddy simulation (LES) based on the approximate deconvolution model (ADM). LES based on the ADM is combined with a Lagrangian notional particle (LP) method for computing reactive flows without using models for chemical source terms. In the LP method, values of scalars are assigned to each particle. The evolutions of Lagrangian particles in physical and scalar composition spaces are modeled by using the mixing model for molecular diffusion and the resolved velocity field of LES. We also propose a particles-interaction mixing model using a mixing volume concept, in which the mixing timescale is determined by relating the decay of scalar variance in the mixing volume to the scalar dissipation rate. The LES-LP method based on the ADM and the mixing model is applied to a planar jet with a second-order reaction for testing the numerical method. The statistics obtained by the LES-LP method are compared with the direct numerical simulation data. The results show that the evolutions of Lagrangian particles are well modeled in the LES-LP method by using the resolved velocity and the mixing model, and this method can accurately predict the statistical properties of reactive scalars. The mixing timescale depends on the distance among the Lagrangian particles. It is also shown that the present mixing model can implicitly take into account the effect of distance among the particles by adjusting the mixing timescale without using any model parameters.