Satoru Komori

Turbulence structure and mass transfer across a sheared air-water interface in wind-driven turbulence

The mass transfer mechanism across a sheared air–water interface without bubble entrainment due to wave breaking was experimentally investigated in terms of the turbulence structure of the organized motions in the interfacial region in a wind-wave tank. The transfer velocity of the carbon dioxide (CO 2 ) on the water side was measured through reaeration experiments of CO 2 , and the fluid velocities in the air and water flows were measured using both a hot-wire anemometer and a laser-Doppler velocimeter. The results show that the mass transfer across a sheared air–water interface is more intensively promoted in wind shear, compared to an unsheared interface. However, the effect of the wind shear on the mass transfer tends to saturate in the high-shear region in the present wind-wave tank, where the increasing rate of mass transfer velocity with the wind shear decreases rapidly. The effect of the wind shear on the mass transfer can be well explained on the basis of the turbulence structure near the air–water interface. That is, surface-renewal eddies are induced on the water side through the high wind shear on the air–water interface by the strong organized motion generated in the air flow above the interface, and the renewal eddies control the mass transfer across a sheared interface. The mass transfer velocity is correlated with the frequency of the appearance of the surface-renewal eddies, as it is in open-channel flows with unsheared interfaces, and it increases approximately in proportion to the root of the surface-renewal frequency. The surface-renewal frequency increases with increasing the wind shear, but for high shear the rate of increase slows. This results in the saturated effect of the wind shear on the mass transfer in the high-shear region in the present wind-wave tank. The mass transfer velocity can be well estimated by the surface-renewal eddy-cell model based on the concept of the time fraction when the surface renewal occurs.

The relationship between surface-renewal and bursting motions in an open-channel flow

Surface-renewal motions in the interfacial region below a gas-liquid interface were experimentally investigated in relation to bursting motions in the wall region. To estimate the frequency of the appearance of surface-renewal eddies, mass-transport experiments with methylene-blue solution, together with velocity measurements, were done in an open-channel flow. The instantaneous concentration of methylene-blue tracer emitted from a point source positioned in the buffer layer was measured at the free surface downstream from the source by an optical probe. Instantaneous streamwise velocity was measured using a laser-Doppler velocimeter at a position in the buffer region. Frequencies of both surface-renewal and bursting events were computed from these concentration and velocity signals using a conditional-averaging method. In order to clarify whether the surface-renewal eddies actually dominate mass transfer across the gas-liquid interface, gas-absorption experiments were added. Carbon dioxide was absorbed into the water flow across the calm free surface and its mass-transfer coefficient on the liquid side was measured under the same flow conditions as used in the above mass-transport experiments. The results show that the surface-renewal motions originate in the bursting motions which vigorously occur in the buffer region. That is, the decelerated fluid which is strongly lifted towards the outer layer by bursting almost always arrives at the free surface and renews the free surface. The frequency of the surface renewal, as well as the bursting frequency, is uniquely determined by the wall variables or the outer-flow variables and the Reynolds number. Mass transfer across the gas-liquid interface is dominated by the large-scale surface-renewal eddies, and the mass-transfer coefficient on the liquid side is proportional to the square-root of the surface-renewal frequency.

Direct numerical simulation of three-dimensional open-channel flow with zero-shear gas-liquid interface

Turbulence structure in an open‐channel flow with a zero‐shear gas–liquid interface was numerically investigated by a three‐dimensional direct numerical simulation (DNS) based on a fifth‐order finite‐difference formulation, and the relationship between scalar transfer across a zero‐shear gas–liquid interface and organized motion near the interface was discussed. The numerical predictions of turbulence quantities were also compared with the measurements by means of a two‐color laser Doppler velocimeter. The results by the DNS show that the vertical motion is restrained in the interfacial region and there the turbulence energy is redistributed from the vertical direction to the streamwise and spanwise directions through the pressure fluctuation. The large‐scale eddies are generated by bursting phenomena in the wall region and they are lifted up toward the interfacial region. Then, the eddies renew the interface and promote the scalar transfer across the gas–liquid interface. Both the damping effect and the generation process of the surface‐renewal motions predicted by the DNS explain well the experimental results deduced in previously published studies. Furthermore, the predicted bursting frequency and mass transfer coefficient are in good agreement with the measurements.

The effects of turbulent mixing on the correlation between two species and on concentration fluctuations in non-premixed reacting flows

When two species A and B are introduced through different parts of the bounding surface into a region of turbulent flow, molecules of A and B are brought together by the combined actions of the turbulent velocity field and molecular diffusion. A random flight model is developed to simulate the relative motion of pairs of fluid elements and random motions of the molecules, based on the models of Durbin (1980) and Sawford & Hunt (1986). The model is used to estimate the cross-correlation between fluctuating concentrations of A and B, G, at a point, in non-premixed homogeneous turbulence with a moderately fast or slow second-order chemical reaction. The correlation indicates the effects of turbulent and molecular mixing on the mean chemical reaction rate, and it is commonly expressed as the ‘segregation’ or ‘unmixedness ’ parameter a( = c,C,/cA c,) when normalized by the mean concentrations CA and C,. It is found that a increases from near - 1 to zero with the time (or distance) from the moment (or location) of release of two species in highReynolds-number flow. Also, the model _- (and experiments) agrees with the exact results of Danckwerts (1952) that C,c,I(ci c;); = - 1 for mixing without reaction. The model is then extended to account for the effects on the segregation parameter a of chemical reactions between A and B. This leads to a eventually decreasing, depending on the relative timescales for turbulent mixing and for chemical reaction (i.e. the Damkohler number). The model also indicates how a number of other parameters such as the turbulent scales, the Schmidt number, the ratio of initial concentrations of two reactants and the mean shear affect the segregation parameter a. The model explains the measurements of a in previously published studies by ourselves and other authors, for mixing with and without reactions, provided that the reaction rate is not very fast. Also the model is only strictly applicable for a limited mixing time t, such that t 5 TL where TL is the Lagrangian timescale, because the model requires that the interface between A and B is effectively continuous and thin, even if highly convoluted. Flow visualization results are presented, which are consistent with the physical idea underlying the two-particle model.

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.

Simultaneous measurements of instantaneous concentrations of two reacting species in a turbulent flow with a rapid reaction

An original technique is developed to measure simultaneously the instantaneous concentrations of two reacting species being mixed and reacted in a turbulent flow with a second‐order rapid reaction between acetic acid and ammonium hydroxide. The technique combines laser fluorescence and electrode‐conductivity methods and it enables simultaneous measurements of concentrations of two reacting species with a spatial resolution much less than the concentration–dissipation scale in grid‐generated tubulence.

Simultaneous measurements of instantaneous concentrations of two species being mixed in a turbulent flow by using a combined laser‐induced fluorescence and laser‐scattering technique

A technique for the simultaneous measurements of the instantaneous concentrations of two species being mixed in a turbulent flow is developed using a combined laser‐induced fluorescence and laser‐scattering technique, and the quantitative measurements are demonstrated in a simple grid‐generated turbulent flow with two streams of nonpremixed species. It is shown that the present technique enables one to measure simultaneously the instantaneous concentrations of two species with spatial and time resolutions far less than the Kolmogorov length and time scales.

Detection of coherent structures associated with bursting events in an open-channel flow by a two-point measuring technique using two laser-Doppler velocimeters

Large‐scale structures in the wall region of an open‐channel flow were investigated experimentally using the two‐point measuring technique of Komori and Ueda [J. Fluid Mech. 152, 337 (1985)] which used two laser‐Doppler velocimeters and a conditional (pattern‐) averaging technique. Instantaneous streamwise and vertical velocities were measured simultaneously at two positions—a reference and a movable position—and the spatial and temporal distributions of the fluctuating velocity vectors, Reynolds stress, and turbulent kinetic energy associated with the bursting events were computed using a pattern‐averaging technique based on the variable‐interval time‐averaging method. The results clearly show the time evolution of the coherent structure associated with the bursting events. They also supported the flow visualization results published hitherto, and gave a physical and qualitative basis for them.

Heat and mass transfer in a stable thermally stratified flow

Abstract Heat and mass transfer mechanism in strong stable thermal-stratification is experimentally investigated in unsheared water flows downstream of turbulence-generation grids, where both active scalar (heat) with a Prandtl number of about six and passive scalar (mass) with a Schmidt number of about 600 are diffused. Instantaneous velocity, temperature and concentration are simultaneously measured using a laser-Doppler velocimeter, a resistance thermometer and a laser-induced fluorescence technique, and the turbulence quantities such as turbulent scalar fluxes, joint probability density functions and cospectra are calculated. The results show that the difference of turbulent diffusion between heat (active scalar) and mass (passive scalar) with different molecular diffusivities in thermally stratified water flows appears in the high-frequency region, and it results in a slightly larger turbulent mass flux than heat flux in strong stratification. The dissipation rate is rather different between heat and mass, and therefore the temperature fluctuation decays more rapidly than the concentration fluctuation. The counter-gradient scalar transfer occurs in strongly stably stratified conditions, and the counter-gradient transfer mechanism is explained from the relationship between buoyancy and turbulent motions. The counter-gradient scalar transfer is initiated by buoyancy-induced small-scale finger-like motions, and then the contribution of large-scale motions pushed back by buoyancy to the counter-gradient scalar transfer becomes dominant. The contributions of small- and large-scale motions in the present thermally stratified water flows are in contrast to the measurements in previously investigated thermally stratified air flows, where the counter-gradient heat transfer is generated mainly by large-scale motions.

Turbulence Structure and Heat and Mass Transfer Mechanism at a Gas-Liquid Interface in a Wind-Wave Tunnel

Turbulence structure and heat and mass transfer mechanism across a wavy sheared gas-liquid interface are fluid-mechanically investigated in a wind-wave tunnel. Heat and mass transfer velocities are reported and the relationship between the scalar transfer velocities and the turbulence structure is discussed. In addition, three-dimensional direct numerical simulation is carried out to investigate the flow structure over a rigid-wavy wall similar to that over the wave. The results show that the organized motion in the air flow intermittently appears on the front side of the wave crest, and its structure is rather similar to the flow structure over the rigid-wavy wall. The organized motion in the air flow induces the organized motion in the water flow and the organized motion renews the air-water interface. The scalar transfer across a wavy sheared gas-liquid interface is controlled by the organized surface-renewal motion in the water flow