Vertical Turbulent Channel Flow Research Articles

Simultaneous measurement of fluid and dispersed phases in a particle-laden turbulent channel flow with the aid of 3-D PTV

Three-dimensional high-definition particle tracking velocimetry is successfully applied to simultaneous measurement of fluid and dispersed phases in a vertical turbulent water channel flow. Turbulence statistics are substantially modified by the solid particles having a diameter of twice the Kolmogorov length scale. The streamwise turbulence intensity of the fluid phase is significantly increased in the channel, whilst it is unchanged at y + < 20. In contrast, the wall-normal and spanwise components are increased in the whole cross-section. In the center of the channel, the wake behind a solid particle has almost the same streamwise length as that around a sphere in the uniform flow, and particles tend to align in the streamwise direction as they flow downstream. The quadrant analysis shows that the solid particles are densely distributed along the low-speed streaks, especially beneath the ejection event. The difference in traceability of particles to the ejection and sweep events should contribute to the particle agglomeration

Particle-Laden Turbulent Flow Measurement with the Aid of 3-D High-Definition PTV.

Three-dimensional high-definition particle tracking velocimetry (3-D HDPTV) is successfully applied to simultaneous measurement of fluid and dispersed phase in a vertical turbulent water channel flow. On average, 400 and 200 velocity vectors are simultaneously obtained for tracer and solid particles, respectively. Turbulent statistics are substantially modified by the solid particles having a diameter of 2-3 times the Kolmogorov length scale. When the turbulent intensities are nondimensionalized with the wall friction velocity, the streamwise component of the fluid phase is significantly increased at the center of the channel, whilst it is unchanged at y+<20. Since the turbulent production due to shear is unchanged by the presence of the dispersed phase, the turbulence energy should be directly generated by the wake of solid particles. The quadrant analysis shows that the solid particles are densely distributed along the low-speed streaks, especially beneath the ejection event. The difference in traceability of particles to the ejection and sweep events should contribute to the particle agglomeration.

On the role of the lift force in turbulence simulations of particle deposition

Most calculations of particle deposition in turbulent boundary layers have been performed using an equation of motion in which the form for the lift force is that in a linear shear flow for a particle far from any boundaries, the so-called Saffman formula. Both direct and large eddy simulations of particle deposition in turbulent channel flow have shown that the dependence of the deposition velocity on particle relaxation time is over-predicted using the Saffman force. Since the derivation of the Saffman force there have been more general derivations of the lift on a particle in a shear flow. In this paper an ‘optimum’ lift force is formulated which represents the most accurate available description of the force acting on a particle in a wall-bounded shear flow. The effect of the force was examined through large eddy simulation (LES) of particle deposition in vertical turbulent channel flow. The optimum force for depositing particles is approximately three times smaller than the lift obtained using the Saffman formula. LES results also show that use of the optimum force yields a dependence of the deposition velocity on particle relaxation time less than that obtained using the Saffman form and in better agreement with experimental measurements. Neglecting the lift force altogether leads to an even smaller dependence of the deposition velocity on particle relaxation time and is in better agreement with empirical relations, although the deposition rates are smaller than experimental measurements for particles with intermediate relaxation times.

Large eddy simulation of particle deposition in a vertical turbulent channel flow

The deposition of particles in fully-developed turbulent channel flow has been calculated using large eddy simulation of the incompressible Navier-Stokes equations. Calculations were performed for Reynolds numbers of 11,160 and 79,400 (based on bulk velocity and hydraulic diameter); subgrid-scale stresses were parameterized using the dynamic eddy viscosity model. Particle motion was governed by both drag and lift. The effect of particle-particle interactions as well as modification of the turbulent carrier flow by the particles was neglected. In agreement with previous studies, LES results show that particles accumulate in the near-wall region. Statistical measures from the LES calculations such as the particle deposition rate are in reasonable agreement with the direct numerical simulation results of McLaughlin (1989). For particles with identical relaxation times in wall units, deposition rates at the two Reynolds numbers were nearly identical. Issues relevant to application of LES for predicting particle deposition at high Reynolds numbers are also discussed.

Aerosol Particle Deposition with Electrostatic Attraction in a Turbulent Channel Flow

A digital simulation procedure for studying deposition rates of aerosol particles in a vertical turbulent channel flow including electrostatic effect is developed. It is assumed that the aerosol particles have a Boltzmann charge distribution. The effect of Coulomb and image forces on deposition rate is studied. The deposition velocities of particles carrying one unit of charge and average charges in accordance with the Boltzmann distribution are also analyzed. Several electric field intensities are used in the simulation studies and the resulting deposition velocities are compared with those obtained from empirical equations. The results show that the use of the mean number of charge is not sufficient and the actual charge distribution must be used in the simulation.