Fa-Gung Fan

A sublayer model for turbulent deposition of particles in vertical ducts with smooth and rough surfaces

Abstract The sublayer model for deposition of spherical particles from turbulent air streams in vertical ducts with smooth and rough walls is described. The formulation is based on the coherent vortical structure of near-wall turbulence and the detailed analysis of particle trajectories in this flow field. The Stokes drag, the Staffman lift force, and the gravitational force are included in the equations of particle motion. The wall roughness is taken into account by an appropriate modification of the boundary condition for the particle capture trajectories. The predicted deposition rates under various conditions are compared with the available experimental data and a reasonable agreement is observed. It is noticed that the particle-to-fluid density ratio, the shear-induced lift force, the wall roughness, the gravity direction and the flow Reynolds number have profound effects on the particle deposition rate. Based on a perturbation method, a simplified empirical equation for evaluating the turbulent deposition rate is also proposed.

Ellipsoidal particles transport and deposition in turbulent channel flows

Abstract Ellipsoidal particle transport and deposition in dilute turbulent channel flows are studied. The instantaneous fluid velocity field is generated by the direct numerical simulation (DNS) of the Navier–Stokes equation via a pseudospectral method. The particle equations of motion used include the hydrodynamic forces and torques, the shear-induced lift and the gravitational forces. Eulers four parameters (quaternions) are used for describing the time evolution of particle orientations. Ensembles of ellipsoidal particle trajectories in turbulent channel flows are generated and statistically analyzed. The results are compared with those for spherical particles and their differences are discussed. Effects of particle size and aspect ratio, turbulence near wall eddies, and the gravitational and hydrodynamic forces are studied. The DNS predictions are compared with the available experimental data and earlier sublayer model simulation results and reasonable agreements are observed.

A sublayer model for wall deposition of ellipsoidal particles in turbulent streams

Abstract A sublayer model for deposition of ellipsoidal particles from turbulent streams on a smooth wall is developed. The analysis is based on the motion of ellipsoidal particles in the coherent vortices of near-wall turbulence. The vortical velocity field is simulated by using a sequence of viscous stagnation point flows with a superposed streamwise turbulence mean flow. The equations of particle motion include the hydrodynamic forces and torques, the shear-induced lift and the gravitational force. Eulers four parameters (quaternions) are used for describing the time evolution of particle orientations. Trajectories of ellipsoidal particles in the modeled flow field are evaluated and the differences with those of thier spherical counterparts are discussed. Based on the observed trajectories and an averaging procedure, the inertia-interception deposition velocities under various conditions are evaluated. The effects of particle size, aspect ratio, particle-to-fluid density ratio, and gravity on deposition velocity are studied. The model predictions are also compared with several available experimental results, and favorable agreements are observed.

Nonstationary Kanai-Tajimi models for El Centro 1940 and Mexico City 1985 earthquakes

Abstract An efficient and simple method for generating synthetic accelerograms is presented. The approach is based on extending the site-dependent Kanai-Tajimi model to include the nonstationary nature of amplitude and frequency content of earthquake ground accelerations. The N00W component of El Centro 1940 and the N90W component of Mexico City 1985 accelerograms are analyzed and specific stochastic models for these earthquakes are developed. It is shown that the simulated records preserve the significant properties of the original accelerograms. The statistically evaluated velocity response spectra curves for the simulated ground motions are also evaluated and compared with those for the actual records.

WALL DEPOSITION OF SMALL ELLIPSOIDS FROM TURBULENT AIR FLOWS—A BROWNIAN DYNAMICS SIMULATION

Abstract Brownian dynamics simulations of the motions of ellipsoidal particles in the turbulence near-wall coherent vortices are carried out. The kinematics and the dynamics of an ellipsoidal particle moving in shear flows are outlined. Eulers four parameters (quaternions) are used for describing the particle orientation. The particle equation of motion includes the hydrodynamic forces and torques, the shear-induced lift force, and the Brownian forces and torques. The near-wall coherent structures are simulated using vortical flow models. A turbulence mean flow is used in the streamwise direction, and the spanwise direction is assumed to be periodic. The wall is modeled as an absorbing (a sticky surface) boundary condition. Ensembles of particle trajectories are evaluated and statistically analyzed. For various particle sizes, aspect ratios, and densities, the deposition velocities of elongated particles in turbulent air streams are evaluated. An empirical equation for the turbulent deposition rate of ellipsoidal particles is also developed. The results are compared with earlier studies and the available experimental data.

On the Sublayer Model for Turbulent Deposition of Aerosol Particles in the Presence of Gravity and Electric Fields

The deposition of neutral and charged aerosol particles from turbulent channel flows is considered. The sub-layer model for the turbulent deposition process is extended to cover the effects of gravity and electrostatic forces. The model is based on the detailed analyses of particle trajectories in the turbulence coherent vortical structures near a wall. The Stokes drag, the Saffman lift, and the gravitational and electrostatic forces are included in the equations of particle motion. The Coulomb, the image, the dielectrophoretic, and the dipole-dipole forces acting on a charged particle near a conducting surface are included in the analysis. Limiting trajectories for various conditions are carefully analyzed. Electrostatic precipitation and gravitational sedimentation in turbulent channel flows are studied in details. Deposition velocities for various particle parameters, flow conditions, and electric field strengths are presented and discussed. The effects of the electrostatic force, the particle density ...

Dispersion of Ellipsoidal Particles in an Isotropic Pseudo-Turbulent Flow Field

Dispersion of ellipsoidal particles in a simulated isotropic pseudo-turbulent field is studied. A procedure using Euler`s four parameters in describing the particle orientations is used, and the governing equations for the translational and rotational motions of particles are outlined. Turbulence fluctuation velocity field is simulated by a Gaussian random model. Motions of ellipsoidal particles of different sizes and lengths are analyzed. Ensemble and time averagings are used for evaluating various statistics of particle motion. Effects of size, shape, and density ratio on the mean square particle velocities and the relative particle diffusivities are studied. By applying the orientation-averaging procedure, an analytical model for estimating the mean-square particle velocities and the relative diffusivities is developed. The predictions of the approximate model are compared with the simulation results and discussed.

Seismic responses of secondary systems in base-isolated structures

Abstract This paper presents the results of a series of numerical simulation studies on seismic responses of secondary systems in base-isolated structures including equipment-structure interactions. A three-storey building is used as the primary structure, while the equipment is modelled as a single-degree-of-freedom linear system. A number of base isolation systems such as the Laminated Rubber Bearing, the Pure Friction, the Resilient-Friction, and the Electricite de France system are considered. Several earthquake records including the N00W component of El Centro 1940, the S16E components of Pacoima Dam 1971, and the N90W component of Mexico City 1985 earthquakes are used as ground accelerations. Acceleration and deflection response spectra of the secondary system under different conditions are evaluated and the effects of equipment — structure interactions are studied. It is shown that the use of base isolation provides considerable protection for structural contents. However, peak responses of the secondary systems vary substantially depending on the base isolation system used. Among the base isolation systems considered, the Laminated Rubber Bearing system appears to be remarkably effective in reducing peak responses of secondary systems under a variety of conditions.

Performance analysis of aseismic base isolation systems for a multi-story building

Abstract A comparative study of the performances of several leading base isolation devices including the Rubber Bearing with and without lead core, the Sliding-Joint, the French System, and the Resilient-Friction isolator with and without upper sliding plate for a multi-story building under seismic ground accelerations is carried out. Several earthquake ground accelerations including El Centro 1940, Pacoima Dam 1971, and Mexico City 1985 earthquakes are considered. The acceleration, the base displacement and the structural deflection response spectra for the structure with different base isolation systems under various conditions are obtained and discussed. Several sensitivity analyses for variations in the parameters of the isolators and/or structure are also carried out. The effectiveness of various base isolators under a variety of conditions are studied and advantages and disadvantages of different systems are discussed. The results show that the acceleration transmitted and the column stresses of the structure can be effectively reduced by using properly designed base isolation systems. The results of the sensitivity analyses show the friction-type isolation systems are less sensitive to the variations in the amplitude and frequency content of the ground excitation in comparison to the Rubber Bearing base isolators. The study also shows that the peak accelerations transmitted by the friction-type isolators are not significantly affected by small variations in friction coefficient. Furthermore, variations in mass ratio and damping of the structure or the isolator do not significantly affect the peak responses.

Multi-story base-isolated buildings under a harmonic ground motion — Part II: Sensitivity analysis

Abstract A series of sensitivity analyses for multi-story base-isolated structures subject to a harmonic ground excitation is carried out. Several base isolation and energy dissipation devices are considered. The effects of variations in properties of structures and base-isolation systems on the peak responses are studied. For a range of natural frequency, friction coefficient, and damping of the isolator, the peak responses are evaluated. The sensitivity of the base-isolated structure to variations in the amplitude and the frequency of ground excitation is also studied. Particular attention is given to the effects of long-period excitations. The study shows that the peak structural responses for base isolators with a frictional element are less sensitive to variations in intensity and frequency of ground excitation when compared with non-frictional systems. Furthermore, the performances of frictional isolators are insensitive to small variations in friction coefficient and its velocity-dependence.