Mehdi Soltani

On particle adhesion and removal mechanisms in turbulent flows

Particle removal mechanisms in a turbulent flow are examined and two models which are based on the structure of turbulence near wall flow are described. The theory of critical moment, along with the sliding detachment models, is used, and the effects of the near-wall coherent vortices, as well as turbulence burst/inrush phenomena, are included. The down sweep flow patterns are modeled as viscous stagnation point flows. The hydrodynamic force and torque acting on a spherical particle attached to a wall are used in the model developments. For different adhesion models, the minimum critical shear velocities for removing particles of different sizes are evaluated. The model predictions are compared with the available data and discussed.

Direct numerical simulation of particle entrainment in turbulent channel flow

Particle entrainment process in a turbulent channel flow is studied. The time history of the instantaneous turbulent velocity vector field is generated by the direct numerical simulation of the Navier–Stokes equation with the aid of a pseudospectral code. The equation of motion of submicrometer particles including Stokes drag and Brownian diffusion is used, and typical entrained particle trajectories are evaluated and statistically analyzed. It is shown that the wall coherent structure plays a dominant role on the particle entrainment process. Particles are removed from the wall region by being captured in the high speed streams moving away from the wall, which are formed by the flow structure. Furthermore, single streamwise vortices are shown to be more frequent than pairs of counter‐rotating ones at every instance of time. Temporal average of the vorticity field, however, shows roughly periodic sequence of counter‐rotating vortices in the wall region.

Particle Detachment from Rough Surfaces in Turbulent Flows

Two flow structure-based models for particle resuspension from rough surfaces in turbulent streams are developed. It is assumed that the real area of contact is determined by elastic deformation of asperities and the effect of topographic properties of surfaces are included. The JKR adhesion model is used to analyze the behaviour of individual asperities. The theories of rolling and sliding detachment are used and the flow-induced resuspension is studied. The effects of the near-wall coherent eddies, and turbulence urst/inrush motion are included in the model development. The critical shear velocities needed to detach different sized particles from rough surfaces under various conditions are evaluated and discussed. The model predictions are compared with the available experimental data and good agreement is obtained.

Particle Removal Mechanisms Under Substrate Acceleration

Abstract Particle detachment due to substrate acceleration is studied. The magnitude of the critical acceleration required to remove a particle from a surface based on the theory of critical moment and sliding detachment is determined. The special cases of spherical and cylindrical particles are examined, and the role of particle geometry on adhesion and detachment is studied. For different adhesion models, the critical substrate acceleration for particle removal is evaluated. The theoretical predictions are compared with the available experimental data and discussed.

Particle detachment mechanisms from rough surfaces under substrate acceleration

Particle removal mechanisms due to an accelerating substrate from rough surfaces are studied. The rough surface is modeled by asperities all of the same radius of curvature and with heights following a Gaussian distribution. The Johnson-Kendall-Roberts (JKR) adhesion model is used and the procedure for analyzing the particle pull-off force is described. The theory of critical moment, in addition to the sliding and lifting detachment models, is used, and the critical substrate accelerations for particle removal are evaluated and discussed. The model predictions for aluminum and glass particles are also compared with the experimental data and reasonable agreement is observed. The application of the results to surface-cleaning equipment is discussed.

Detachment of rough particles with electrostatic attraction from surfaces in turbulent flows

The detachment of particles with coarse and fine roughnesses from surfaces in a turbulent boundary layer flow including electrostatic effects is studied. It is assumed that the real area of contact is determined by elastic deformation of asperities, and the effect of topographic properties of surfaces is included. The Johnson-Kendall-Roberts (JKR) adhesion model is used for analyzing the behavior of individual asperities. For an average Boltzmann charge distribution, the saturation charge condition as well as a fixed charge per unit mass, the Coulomb, the image, the dielectrophoretic, and the polarization forces acting on the particle in the presence of an imposed electric field are evaluated. The theories of rolling and sliding detachment are used to study the onset of removal of bumpy particles and those with fine roughness from plane surfaces. The hydrodynamic forces and torques acting on the particle attached to a wall, along with the adhesion force for the particle, are used in the model development....

Charged Particle Trajectory Statistics and Deposition in a Turbulent Channel Flow

The statistical properties of charged particles and their wall deposition in a turbulent channel flow in the presence of an electrostatic field is studied in this paper. For a dilute concentration, the influence of small particles on the fluid motion is neglected. The instantaneous velocity field is generated by a direct numerical simulation of the Navier-Stokes equation via a pseudospectral method. The case in which each particle carries a single unit of charge and the case in which the particles have a saturation charge distribution are analyzed. Ensembles of 8192 particle trajectories are used for evaluating various statistics. Effects of size and electric field intensity on particle trajectory statistics and wall deposition rate are studied. RMS particle velocities and particle concentrations at different distances from the wall are evaluated and discussed. The results for deposition rates are compared with those obtained from empirical equations.

Direct Numerical Simulation of Curly Fibers in Turbulent Channel Flow

Wall deposition of rigid-link fibrous aerosols in a turbulent channel flow is studied. The instantaneous turbulent velocity vector field is generated by the direct numerical simulation of the Navier-Stokes equation with the aid of a pseudospectral code. It is assumed that the fiber is composed of five rigidly attached ellipsoidal links. The dynamic behavior of these elongated and irregular shaped particles is markedly different from the spherical ones. The hydrodynamic forces and torques acting on the fiber are evaluated and the equations governing the translational and rotational motions of the fiber are analyzed. Eulers four parameters are used, and motions of fibrous particles in the turbulent channel flow field are studied. Ensembles of 8000 fiber trajectories are generated and are used for evaluating various statistics. Root mean-square fiber velocities and fiber concentrations at different distances from the wall are evaluated and discussed. Empirical models for the deposition rate of curly fibers are also developed. The model predictions are compared with the simulation data and good agreement is observed.

Flow-Induced Resuspension of Rigid-Link Fibers from Surfaces

ABSTRACT Flow-induced removal of curly fibers from surfaces is studied. A generic rigid-link fiber model consisting of a chain of five ellipsoids with different orientations is developed, and used to study fiber resuspension in laminar and turbulent flows. The contact surfaces of the fiber with the wall are modeled as spherical joints. The hydrodynamic forces and torques acting on the ellipsoidal segments as well as the adhesion forces of nodes in contact with the surface are evaluated. Various possible modes of fiber detachment from the surface are identified, and the state of limiting equilibrium are analyzed. The critical velocities for detaching fibers of various sizes, orientations, and filament thicknesses are evaluated and discussed.

Detachment of rigid-link fibers with linkage contact in a turbulent boundary layer flow

Fiber removal with linkage contact in a turbulent boundary layer flow is studied. A rigid-link fiber model which is composed of two ellipsoids and one cylindrical link is considered. It is assumed that the fiber is in contact with the wall by the cylindrical linkage and a spherical end joint. The onset of rolling and sliding detachments is analyzed to determine the condition for removal of fibers from the surface. The hydrodynamic forces and torques acting on the fiber attached to a wall, along with the adhesion forces of contact node and linkage, are used in the model development. For different adhesion models, the minimum critical shear velocities for removing fibers of different sizes, orientations, and thicknesses are evaluated and discussed.