Pseudo Turbulence Research Articles

Bubble-induced pseudo turbulence in laminar pipe flows

Turbulence intensities and Reynolds stresses of bubble-induced pseudo turbulence in air–water laminar bubbly flows in a vertical pipe of 20mm diameter are measured at a low liquid Reynolds number, Re=900. A pipe made of fluorinated ethylene propylene resin, which has the same refractive index as water, is utilized to enable the LDV measurement of Reynolds stresses. Experimental data show that (1) turbulence kinetic energy of bubble-induced turbulence is proportional to local void fraction, (2) the ratio among axial, radial and azimuthal components of turbulence intensity is 2:1:1, (3) Sato’s eddy viscosity model underestimates the Reynolds shear stress of bubble-induced pseudo turbulence, whereas it qualitatively well represents the Reynolds stress distribution, (4) the Reynolds shear stress of bubble-induced turbulence depend not only on velocity gradient but also on gradient of void fraction and (5) the dissipation process of bubble-induced turbulence is analogous to that of shear-induced turbulence.

Evaluation of Turbulence Kinetic Energy Budget in Turbulent Flows by Using a Photobleaching Molecular Tagging Velocimetry

Understanding turbulence kinetic energy (TKE) budget in gas-liquid two-phase bubbly flows is indispensable to develop and improve turbulence models for the bubbly flows. Simultaneous measurement of velocity and velocity gradients with a spatial resolution smaller than the Kolmogorov scale is required to evaluate the TKE budget experimentally. We therefore proposed a molecular tagging velocimetry based on photobleaching reaction (PB-MTV) and applied it to turbulent flows in a square duct to demonstrate the possibility of evaluation of TKE budget. In this study, we improved PB-MTV in its processing speed by utilizing GPGPU(General Purpose Graphic Processing Unit) to increase sample number in measurements. We measured TKE budget in a turbulent water flow in a square duct by using the PB-MTV at the same turbulent Reynolds number as DNS data provided by Horiuchi, and compared the measured data with the DNS data to validate PB-MTV for evaluation of TKE budget. We also measured TKE budget in a bubbly flow in the square duct to examine effects of bubbles on TKE budget. As a result, we found that (1) PB-MTV can accurately evaluate TKE budget in turbulent flows, (2) bubbles affect the production and diffusion rates of TKE and do not affect the dissipation rate so much, and (3) the model proposed by Troshko and Hassan can reasonably estimate the production rate of the bubble-induced pseudo turbulence.

1106 気泡誘起擬似乱れに関する研究(GS-5 混相流-気泡)

1106 気泡誘起擬似乱れに関する研究(GS-5 混相流-気泡)

Influence of local void fraction distribution on turbulent structure of upward bubbly flow in vertical channel

Turbulent structure of upward dilute bubbly flow with 1 mm bubbles in a vertical channel is investigated experimentally. Small amount of surfactant is added to water to avoid bubble coalescence and to control local void fraction distribution. Liquid phase velocity is measured using two-dimensional laser Doppler Velocimetry. In 1-Pentanol solution of 20 ppm, bubbles have half-slip surface and migrate strongly toward the channel wall due to the shear-induced lift force which leads to wall-peaked distribution of local void fraction. On the other hand, in Triton X-100 solution of 2 ppm, bubbles become fully-contaminated and do not migrate toward the wall or the channel centre due to near-zero lift force, causing uniform distribution of local void fraction in the wall-normal direction. Once bubbles accumulate near the wall, transport of turbulent energy produced by the wall shear towards the channel centre is blocked. Then turbulence induced by the bubble motion becomes dominant in a wide core region (so-called pseudo turbulence). By contrast, in the case of the uniform distribution of bubbles, a mechanism of a turbulent energy transport which is the same as that of a single-phase turbulence still exists and furthermore the bubble-induced turbulence is added on it.

Pseudo turbulence in PIV breaking-wave measurements

The particle image velocimetry (PIV) technique was employed to measure the instantaneous velocity distribution under nonbreaking and breaking water waves. The corresponding turbulence intensity was calculated by the ensemble average of repeated measurements. The pseudo turbulence found was large enough to affect the accuracy of the turbulence measurements. We follow Prasad et al.'s (1992) approach to demonstrate that the pseudo turbulence is related to the bias error, which is the discrepancy between the true position of the particle image and the position calculated from the pixel array data with inadequate pixel resolution. To reduce the bias error (or the pseudo turbulence), we first calculate it from a turbulence-free flow with the same experimental set-up as that used for the targeted experiments (i.e., we use the same size of field of view, seeding particles, seeding density, lens aperture, and laser wavelength in both experiments). Then we minimize the bias error from the turbulence measurements in the actual experiments. To demonstrate the procedure, the evolution of a breaking wave is investigated.

Turbulent Dispersion of Heavy Particles With Nonlinear Drag

The analysis of Reeks (1977) for particle dispersion in isotropic turbulence is extended so as to include a nonlinear drag law. The principal issue is the evaluation of the inertial time constants, βα−1, and the mean slip. Unlike what is found for the Stokesian drag, the time constants are functions of the slip velocity and are anisotropic. For settling velocity, VT, much larger than root-mean-square of the fluid velocity fluctuations, u0, the mean slip is given by VT. For VT→0, the mean slip is related to turbulent velocity fluctuation by assuming that fluctuations in βα are small compared to the mean value. An interpolation formula is used to evaluate βα and VT in regions intermediate between conditions of VT→0 and VT≫ u0. The limitations of the analysis are explored by carrying out a Monte-Carlo simulation for particle motion in a pseudo turbulence described by a Gaussian distribution and Kraichnan’s (1970) energy spectrum.

気泡プルームの微細流動構造の数値解析

A numerical analysis using the Lagrangian tracking method for bubbly flow was carried out to elucidate the three-dimensional detailed flow structure and the turbulence mechanism of a bubble plume, which has various interaction between gas and liquid phases. The results revealed that the bubble plume exhibits several kinds of three-dimensional structures with spiral and sinuous behav-iors, depending on bubble size and void fraction. This macroscopic structure shows good agreement with experimental observation. Also, the bubble plume has a turbulence component, called pseudo turbulence with high frequency which is caused by the migration of large bubbles with large slip velocity inside the bubble plume.

Effect of turbulence on the particle settling velocity in the nonlinear drag range

Particle settling velocity in the nonlinear drag range is investigated using a Monte Carlo simulation for particles in a low Reynolds number, isotropic, Gaussian, pseudo turbulence. The settling velocity is affected by both the trajectory bias, which enhances the settling velocity, and the nonlinearity of the drag associated with the turbulence, which reduces the settling velocity. The effect of the trajectory bias is important in an almost frozen turbulence when the settling velocity is comparable to the turbulence and particle motion in the creeping-flow regime. For a nonfrozen turbulence, the effect of the trajectory bias on the settling velocity may be overwhelmed by the effect of the nonlinear drag associated with the turbulence. The ensemble average of the second invariant of the turbulence deformation tensor, 〈IId〉, along the particle trajectories is obtained to characterize statistically the trajectory bias and the correlation between the particle concentration and turbulence structure. 〈IId〉 attains its maximum value at zero settling for a given particle inertia. The effect of increasing the settling velocity leads to an exponentially decreasing 〈IId〉 for a large settling rate, and hence a significant reduction in the trajectory bias and the concentration-structure correlation. For very small or large particle inertia, 〈IId〉 vanishes.

Real and pseudo-turbulence in the discharge stream from a rushton turbine

The trailing vortex system leaving the blade tips dominates the velocity field in the vicinity of a turbine agitator. This paper relates velocity measurements made with a stationary conical hot film probe to the known flow field defined relative to the impeller. It is shown that the ordered pseudo turbulence deviates so much from isotropic conditions that the usual interpretation in terms of the turbulence parameters of scale intensity is of doubtful value in this region.