Kouji Nagata

Effects of molecular diffusivities on counter-gradient scalar and momentum transfer in strongly stable stratification

The effects of molecular diffusivities of heat and mass on the counter-gradient scalar and momentum transfer in strongly stable stratification are experimentally investigated in unsheared and sheared stratified water mixing-layer flows downstream of turbulence-generating grids. Experiments are carried out in two kinds of stably stratified water flows. In the case of thermal stratification, the difference between the turbulent fluxes of an active scalar (heat with the Prandtl number of Pr ≈ 6) and a passive scalar (mass with the Schmidt number of Sc ≈ 600) is investigated. In the case of salt stratification, the effects of the molecular diffusion of the active scalar (salt) with a very high Schmidt number of Sc ≈ 600 on the counter-gradient scalar transfer is studied. Comparisons of the effects of molecular diffusivities are also made between thermally stratified water and air ( Pr ≈ 0.7) flows. Further, the effects of mean shear on the counter-gradient scalar and momentum transfer are investigated for both stratified cases. Instantaneous temperature, concentration and streamwise and vertical velocities are simultaneously measured using a combined technique with a resistance thermometer, a laser-induced fluorescence method, and a laser-Doppler velocimeter with high spatial resolution. Turbulent scalar fluxes, joint probability density functions, and cospectra are estimated. The results of the first case show that both active heat and passive mass develop counter-gradient fluxes but that the counter-gradient flux of passive mass is about 10% larger than that of active heat, mostly due to molecular diffusion effects at small scales. The counter-gradient scalar transfer mechanism in stable stratification can be explained by considering the relative balance between the available potential energy and the turbulent kinetic energy as in Schumann (1987). In thermally and salt-stratified water mixing-layer flows with the active scalars of high Prandtl and Schmidt numbers, the buoyancy-induced motions with finger-like structures first contribute to the counter-gradient scalar fluxes at small scales, and then the large-scale motions, which bring fluid back to its original levels, generate the counter-gradient fluxes at large scales. The contribution of the small-scale motions to the counter-gradient fluxes in stratified water flows is quite different from that in stratified air flows. The higher Prandtl or Schmidt number of the active scalar generates both the stronger buoyancy effects and the longer time-oscillation period of the counter-gradient scalar fluxes. The time-oscillation occurs at large scales but the counter-gradient fluxes at small scales persist without oscillating. The mean shear acts to reduce the counter-gradient scalar and momentum transfer at large scales, and therefore the counter-gradient fluxes in sheared stratified flows can be seen only in very strong stratification. The behaviour of the counter-gradient momentum flux in strong stratification is quite similar to that of the counter-gradient scalar flux.

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...

The difference in turbulent diffusion between active and passive scalars in stable thermal stratification

The difference in turbulent diffusion between active scalar (heat) and passive scalar (mass) in a stable thermally stratified flow is investigated both experimentally and numerically. The experiments are conducted in an unsheared thermally stratified water flow downstream of a turbulence-generating grid. Passive mass is released into the stable thermally stratified flow from a point source located 60 mm downstream from the grid. Instantaneous streamwise and vertical velocities, the temperature of the active scalar and the concentration of the passive scalar are simultaneously measured using a combined technique with a two-component laser-Doppler velocimeter (LDV), a resistance thermometer and a laser-induced fluorescence (LIF) method. From the measurements, turbulent heat and mass fluxes and eddy diffusivities for both active heat and passive mass are estimated. To investigate the Prandtl or Schmidt number effects on the difference in turbulent diffusion between active heat and passive mass, a three-dimensional direct numerical simulation (DNS) based on a finite difference method is applied to stable thermally stratified flows of both water and air behind the turbulence grid. The Schmidt number of passive mass in the DNS is set to the same value as the Prandtl number of active heat. The results show that stable stratification causes a large difference in eddy diffusivities between active heat and passive mass. The numerical predictions by the DNS are in qualitative agreement with the measurements despite the assumption of the same molecular diffusivity for active heat and passive mass. The difference suggests that the assumption of identical eddy diffusivity for active heat and passive mass, used in conventional turbulence models, gives significant errors in estimating heat and mass transfer in a plume under stably stratified conditions.

The current profile and outcome of congenital diaphragmatic hernia: a nationwide survey in Japan.

BACKGROUND/PURPOSE Few nationwide surveys have been reported regarding the perinatal status, clinical course and postnatal outcome of cases with congenital diaphragmatic hernia (CDH). The aim of this study was to review the current profile and the outcomes of a large cohort of CDH cases in Japan. METHODS A nationwide retrospective cohort study was conducted on neonates diagnosed to have CDH between January 2006 and December 2010. The questionnaires were sent to 159 representative regional institutions and 109 (68.6%) institutions responded to the preliminary survey which had 674 cases. Eleven institutions which had 60 CDH neonates did not respond to the second questionnaire, and 26 institutions had no cases. Finally, 614 CDH neonates from 72 institutions had been collected and were used in the detailed survey. The perinatal status, clinical course and the postnatal outcome were reviewed. Survival was defined as infants alive at hospital discharge, at the time of transfer or still in the hospital at the time of questionnaire, which was confirmed during the period from July 2011 to November 2011 by the investigators. RESULTS Four hundred sixty-three (75.4%) of 614 CDH neonates survived in this study. The overall survival rate of neonates with isolated CDH was 84.0%. A total of 444 (72.0%) patients were prenatally diagnosed, and had a survival rate of 70.8%. Four hundred thirty-three (70.9%) patients were treated with high-frequency oscillatory ventilation (HFOV) as the initial ventilation, 344 (56.0%) patients received inhaled nitric oxide (iNO) and 43 (7.0%) required extracorporeal membrane oxygenation (ECMO). The overall survival rates of the CDH neonates who had been treated using HFOV, iNO and ECMO were 74.3%, 68.3% and 37.2%, respectively. CONCLUSIONS This study demonstrated that the current status for CDH treatment in Japan and the overall survival rate were comparable to those of recent reports from other countries.

High-Schmidt-number scalar transfer in regular and fractal grid turbulence

Turbulent mixing of high-Schmidt-number passive scalars in regular and fractal grid turbulence is experimentally investigated using a water channel. A turbulence-generating grid is installed at the entrance of the test section, which is 1.5 m in length and 0.1 m×0.1 m in cross section. Two types of grids are used: one is a regular grid consisting of square-mesh and biplane constructions, and the other is a square-type fractal grid, which was first investigated by Hurst and Vassilicos (2007 Phys. Fluids 19 035103) and Seoud and Vassilicos (2007 Phys. Fluids 19 105108). The two grids have the same solidity of 0.36. The Reynolds number based on the mesh size, ReM=U0Meff/ν, is 2500 in both flows, where U0 is the cross-sectionally averaged mean velocity, Meff is the effective mesh size and ν is the kinematic viscosity. A fluorescent dye (rhodamine B) is homogeneously premixed only in the lower stream and therefore the scalar mixing layers with an initial step profile develop downstream of the grids. The Schmidt number of the dye is O(103). The time-resolved particle image velocimetry and the planar laser-induced fluorescence techniques are used to measure the velocity and concentration fields. The results show that the turbulent mixing in fractal grid turbulence is more strongly enhanced than that in regular grid turbulence for the same mesh Reynolds number ReM. The profile of instantaneous scalar dissipation shows that scalar dissipation takes place locally even in the far downstream region at x/Meff=120 in fractal grid turbulence.

Direct numerical simulation of turbulent mixing in grid-generated turbulence

Turbulent mixing of passive scalar (heat) in grid-generated turbulence (GGT) is simulated by means of direct numerical simulation (DNS). A turbulence-generating grid, on which the velocity components are set to zero, is located downstream of the channel entrance, and it is numerically constructed on the staggered mesh arrangement using the immersed boundary method. The grid types constructed are: (a) square-mesh biplane grid, (b) square-mesh single-plane grid, (c) composite grid consisting of parallel square-bars and (d) fractal grid. Two fluids with different temperatures are provided separately in the upper and lower streams upstream of the turbulence-generating grids, generating the thermal mixing layer behind the grids. For the grid (a), simulations for two different Prandtl numbers of 0.71 and 7.1, corresponding to air and water flows, are conducted to investigate the effect of the Prandtl number. The results show that the typical grid turbulence and shearless mixing layer are generated downstream of the grids. The results of the scalar field show that a typical thermal mixing layer is generated as well, and the effects of the Prandtl numbers on turbulent heat transfer are observed.

The effects of unstable stratification and mean shear on the chemical reaction in grid turbulence

The effects of unstable thermal stratification and mean shear on chemical reaction and turbulent mixing were experimentally investigated in reacting and non-reacting liquid mixing-layer flows downstream of a turbulence-generating grid. Experiments were carried out under three conditions: unsheared neutrally stratified, unsheared unstably stratified and sheared neutrally stratified. Instantaneous velocity and concentration were simultaneously measured using the combination of a laser-Doppler velocimeter and a laser-induced fluorescence technique. The results show that the turbulent mixing is enhanced at both large and small scales by buoyancy under unstably stratified conditions and therefore the chemical reaction is strongly promoted. The mean shear acts to enhance the turbulent mixing mainly at large scales. However, the chemical reaction rate in the sheared flow is not as large as in the unstably stratified case with the same turbulence level, since the mixing at small scales in the sheared neutrally stratified flow is weaker than that in the unsheared unstably stratified flow. The unstable stratification is regarded as a better tool to attain unsheared mixing since the shearing stress acting on the fluid is much weaker in the unstably stratified flow than in the sheared flow.

Risk Factors for the Recurrence of the Congenital Diaphragmatic Hernia—Report from the Long-Term Follow-Up Study of Japanese CDH Study Group

AIM OF THE STUDY Few follow-up studies focused on the recurrence regarding the postoperative course of congenital diaphragmatic hernia (CDH) survivors. The aim of this study was to report on risk factor for CDH patients who had the recurrence during the follow-up. MATERIALS AND METHODS A multicenter retrospective survey was conducted on neonates diagnosed to have CDH between January 2006 and December 2010. Follow-up survey was conducted between September 2013 and October 2013 (ethical approval: No. 25-222). Nine institutions agreed to participate in this survey. Out of 228, 182 (79.8%) patients were alive and 180 patients were included in this study. Two patients were excluded because the defect had not repaired at the primary operation. The patients were divided into the recurrence group (n=21) and the nonrecurrence group (n=159). Postnatal and postoperative variables were compared between these two groups. Baseline variables which showed significance in univariate analysis were entered into multiple logistic regression analysis for analyzing the recurrence. A value of p<0.05 was considered to be statistically significant by using the JMP software program (version 9; SAS Institute, Inc, Cary, North Carolina, United States). MAIN RESULTS Out of 180, 21 (11.7%) CDH neonates had the recurrence during the course of the follow-up. Five (2.8%) patients had the recurrence before primary discharge and 16 (8.9%) patients had the recurrence after discharge. Univariate analysis showed that liver herniation (crude odds ratio [OR], 7.4; 95% confidence interval [CI], 2.73-23.68), defect size C and D, proposed by the CDH Study Group (crude OR, 7.09; 95% CI, 2.73-19.99) and patch repair (crude OR, 5.00; 95% CI, 1.91-14.70) were risk factors. Multivariate logistic regression analysis showed liver herniation (adjusted OR, 3.96; 95% CI, 1.01-16.92) was the risk factor for the recurrence. CONCLUSION A wide spectrum of the disease severity and the rarity of the disease mask the risk of the recurrence for CDH patients. This study showed the only factor to predict the recurrence was the liver herniation. These data will be helpful for providing information for the long-term follow-up of the CDH patients.

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...