Linear rapid distortion theory (RDT) is applied to unsteady, unsheared, stable, thermally stratified air (Pr=0.7), thermally stratified water (Pr=6), and salt-stratified liquid (Sc∼600) flows. The effects of diffusivity and viscosity are included in the analysis and turbulence quantities such as turbulent scalar fluxes and their cospectra are obtained. The results are compared with previous laboratory measurements and direct numerical simulations (DNS). The results show that countergradient scalar transfer (CGST), which transports the scalar counter to the mean gradient (i.e., negative eddy diffusivity), can be predicted by linear RDT, as shown in the previous studies. However, the small-scale persistent downgradient scalar transfer (P-DGST) in air flows and the small-scale persistent CGST (P-CGST) in water flows cannot be predicted by RDT. In a linear process, small-scale CGST occurs first and then it spreads on a large-scale regardless the values of Pr or Sc; then, the small-scale fluxes change their signs to become downgradient flux again (i.e., small-scale flux oscillates with time). The results suggest that the small-scale turbulent scalar transfer in a strong, stable stratified flow is dominated by nonlinear processes, and only the large-scale wave-like motions are controlled by the linear processes.
Vortex structures and Reynolds stresses around a flat paddle in a mixing vessel have been investigated experimentally by two-component LDV measurement. Phase-averaged mean velocity and Reynolds stresses were obtained to observe the relative relation between the vortical structures and momentum transport. Development of Reynolds shear stress can be recognized along with the separated shear layer released from a flat paddle. Discharge and inflow induced by the roll vortex affect the magnitude of Reynolds shear stress in the separated shear layer. A four-quadrant analysis was applied to find the turbulent motions contributing to the production of the Reynolds shear stress. Most of the Reynolds shear stresses were found to be produced by two turbulent motions, namely, inflow just behind the paddle and large-scale-motion passing from behind the paddle vertically. The two turbulent motions are closely associated with the roll vortex.
This paper analyses numerically the effect of optical rotor with N-paddle (N=3, 4, and 5) on mixing process of two fluids in Y-shaped channel flow by computational fluid dynamics (CFD). Streakline tracking simulation is employed to study dynamic mixing in the mixer. Mixing performance of the mixer is quantitatively measured by the dispersive and distributive mixing indices. The streaklines show that only a portion of one fluid can enter the rotor region where chaotic motion is observed. They also demonstrate that mixing is enhanced by high shear rate only in a portion of the perimeter area of the rotor. Consequently, mixing performance of the mixer is not dependent on number of paddle, but on the ratio of the tip paddle velocity of the rotor to the mean velocity in the mixer (kinetic parameter). Mixing is enhanced with increasing kinetic parameter. These are quantitatively confirmed by the two mixing indices.
A parametric study was conducted to improve our understanding pertaining to the fundamental physics of electrokinetic instability (EKI) and to explore the effectiveness of manipulating EKI waves to control/enhance fluid mixing inside a Y-shaped microchannel. The dependence of the critical strength of the applied static electric field to trigger the EKI waves on the conductivity ratio of the two mixing streams inside the Y-shaped microchannel was quantified at first. The effects of the applied electric field strength on the evolution of the EKI waves and the resultant fluid mixing were assessed in terms of scalar concentration distributions, shedding frequency of the EKI waves and fluid mixing efficiency. The effectiveness of manipulating the EKI waves by adding alternative perturbations to the applied static electric fields were also explored for the further enhancement of the fluid mixing inside the Y-shaped microchannel. The measurement results revealed that the relationship between the critical strength of the applied static electric field and the conductivity ratio of the two mixing streams in the Y-shaped microchannel can be represented well by a power function with the power index about -0.246. The fluid mixing efficiency was found to increase monotonically with the increasing strength of the applied electric field. The fluid mixing process was found to be further enhanced by adding alternative perturbations to the applied static electric fields with the mixing process being most enhanced when the frequency of the alternative perturbation is close to the natural shedding frequency of the EKI waves. The fluid mixing efficiency was found to increase rapidly as the amplitude of the alternative perturbation increases.
Active control of an impinging jet upon a wedge has been attempted using a sinusoidal excitation of blowing and sucking at the jet exit. The excitation sufficiently enables 'phase-lock', which is synchronization between self-oscillating flow and the excitation, so that hot-wire measurements directly provide phase averaged flow fields and they illustrate appearance of the jet swing in front of the wedge and collision of the jet on one of side of the wedge. It was demonstrated that this control set up is practical not only for illustration of the phase averaged flow field but also for reduction of the edge tone due to the flow oscillation with inverse phase excitation in half of the jet.
An unbaffled agitated vessel having an unsteadily rotating impeller was employed as an apparatus mixing liquid and solid particles with the density larger than that of liquid. For this type of vessel, the movement of solid particles on and off the vessel bottom was studied in relation to the liquid flow produced by the impeller. When a disk turbine impeller with six flat blades was rotated in the forward-reverse mode, the liquid flow and the particle movement were visualized. Concurrently, the agitation requirement for complete solid suspension where no particle remains on the vessel bottom for more than a short period and all particles are in motion was determined as a minimum rotation rate of impeller. The liquid flow and the particle movement around a tiny heap of solid particles configured on the vessel bottom were characterized through measurement of their velocities by the particle tracking velocimetry (PTV). The relative velocity of rising with off-bottom suspension of solid particles was uniform in its distribution and wholly large in its magnitude, compared with that in a baffled vessel with a unidirectionally rotating impeller of the identical design, which revealed an effectiveness of this type of vessel as an apparatus for the solid-liquid mass transfer.
For the development of an active control of a turbulent jet, measurements of an axisymmetric turbulent jet were performed with multiple “moving delta tabs.” Five computercontrolled delta tabs, which can be driven independently, were installed at the skimmer. Two modes were tested; in the first mode, all the delta tabs were stationary (i.e., conventional way), and in the second mode, the delta tabs were oscillated in phase at a frequency of 4 Hz. Simultaneous multipoint measurements of instantaneous velocity were performed using a rake of 21 I-type hot wires, specially designed and manufactured for this study. The results are compared with those in a canonical axisymmetric jet without tabs. The results show that the turbulence quantities, such as mean and rms velocities, skewness, kurtosis, probability density function, and power spectra are significantly altered by the stationary and oscillating delta tabs. The effects of the oscillation of delta tabs are found to modify the large-scale structure in the near field of the jet. The results confirm the effectiveness of the active control of an axisymmetric jet using the oscillating delta tabs.
In this study, a water solution of dye (whose Schmidt number Sc is about 3,800) was issued into the quiescent water as an axisymmetric turbulent jet and the simultaneous measurements of axial velocity and concentration have been performed using the combined probe of I-type hot-film and fiber-optic concentration sensor based on the Lambert-Beer's law. Then we calculated the PDF (Probability Density Function) for the streamwise velocity derivative ∂u/∂x and streamwise concentration derivative ∂c/∂x. It was confirmed that the PDFs for ∂u/∂x skew negatively, and the values of skewness (S∂u/∂x) and flatness factor (F∂u/∂x) are consistent with the other researcher's data (see Sreenivasan and Antonia, Annual Review of Fluid Mechanics, Vol. 29, 1997, where the extensive past data of turbulent velocity and temperature (whose Prandtl number is Pr=0.7) fields are summarized). However, with regard to the PDFs for ∂c/∂x, the skewness (S∂c/∂x) show the values very close to zero, unlikely the past other data of the temperature fields which show the magnitude of 0.5∼1.0. On the other hand, the flatness factor (F∂c/∂x) show the values of 7.0∼8.0 which are consistent with the temperature fields. This result suggests that the fine-scale structure of a high-Schmidt-number diffusion field is almost isotropic although it is intermittent.
Thermal-hydraulic design of the current boiling water reactor (BWR) is performed with the subchannel analysis codes which incorporated the correlations based on empirical results including actual-size tests. Then, for the Innovative Water Reactor for Flexible Fuel Cycle (FLWR) core, an actual size test of an embodiment of its design is required to confirm or modify such correlations. In this situation, development of a method that enables the thermal-hydraulic design of nuclear reactors without these actual size tests is desired, because these tests take a long time and entail great cost. For this reason, we developed an advanced thermal-hydraulic design method for FLWRs using innovative two-phase flow simulation technology. In this study, a detailed Two-Phase Flow simulation code using advanced Interface Tracking method: TPFIT is developed to calculate the detailed information of the two-phase flow. In this paper, firstly, we tried to verify the TPFIT code by comparing it with the existing 2-channel air-water mixing experimental results. Secondary, the TPFIT code was applied to simulation of steam-water two-phase flow in a model of two subchannels of a current BWRs and FLWRs rod bundle. The fluid mixing was observed at a gap between the subchannels. The existing two-phase flow correlation for fluid mixing is evaluated using detailed numerical simulation data. This data indicates that pressure difference between fluid channels is responsible for the fluid mixing, and thus the effects of the time average pressure difference and fluctuations must be incorporated in the two-phase flow correlation for fluid mixing. When inlet quality ratio of subchannels is relatively large, it is understood that evaluation precision of the existing two-phase flow correlations for fluid mixing are relatively low.
An agitated circular turbulent jet with a coaxial annular synthetic jet actuator is investigated experimentally. The mean and turbulent velocities are measured by a hot-wire anemometer at a jet Reynolds number of 6400. The measured iso-velocity contour maps indicate that the width of the circular jet is increased even when the intensity of the alternating flow is too weak to generate a synthetic jet. This can be attributed to the periodic disturbances by the actuator, which agitates the instability in the initial shear layer and displaces the shear layer growth upstream. With a strong agitation so as to generate a strong synthetic jet by itself, the agitated jet has the higher velocity and the larger width than those of unagitated jet. With this agitation, the momentum of the agitated jet is increased and its increment is equal to the momentum of the synthetic jet. Smoke visualization shows clearly that the actuator enhances the dispersion of primary jet fluids. These results are compared with those of a jet at a low Reynolds number of 3100 and it is found that the mixing enhancement is qualitatively same for both Reynolds numbers, but the weak agitation is more effective in the low Reynolds number jet.