Pertti Sarkomaa

An experimental study of vertical pneumatic conveying

Abstract This study uses a one-dimensional equation system and experimental techniques to provide a comprehensive description of vertical gas–solid two-phase flow. The results from non-accelerating flow experiments conducted with a riser tube of bore 192 mm and height 16.2 m using spherical glass beads of average diameter 64 μm are presented. The solids volume fraction, which was measured directly using quick-closing valves, was less than 0.01 in all cases. The frictional pressure drop was recognised to be an important component of the total pressure gradient in the riser. At low gas velocities, negative frictional pressure gradients occurred. The solids friction factor was found to be constant at high solids velocities and decrease to negative values as the solids velocity was reduced. The slip velocity was found to be always greater than the single-particle terminal velocity and to increase with decreasing gas velocity or increasing solids mass flux. This is different to that which has usually been reported in literature, and is thought to be due to the large diameter of riser used in this study. In addition, the slip velocity increased (independently of solids mass flux) with increasing solids concentration.

Solids friction factors in upward, lean gas-solids flows

Abstract The experimental results of measurements from a riser tube 192 mm in diameter and 16.2 m in height are presented. A new correlation for the solids friction factor was developed which predicts negative values at low solids velocities and a positive constant value at high solids velocities. Nine other correlations for the solids friction factor found in the literature are listed and compared with the results from the current experiments.

Intermittency of energy in rapid granular shear flows

Hard-disk simulations are used for two-dimensional rapid granular shear flows of circular disks between two rotating cylinders. The intermittency effects associated with the rate of the energy dissipation of collisions are studied. The statistics of intermittent signals of energy dissipation reveals that a power law governs the dynamics of rapid shear granular flows. A dynamical system approach based on the Gledzer-Ohkitani-Yamada shell model of turbulence is employed to reproduce signals for energy dissipation that are statistically consistent with those from simulations. The results suggest that rapid granular flows can be analyzed by appropriate turbulent models.

Some qualitative features of the Couette flow of monodisperse, smooth, inelastic spherical particles

Computer simulations have been performed to examine the occurrence of power-law correlations for the stresses exerted on the confining walls by the particles in the three dimensional Couette flow of hard, smooth, dissipative spherical particles of uniform size. At high particle concentrations, the wavelet analysis of the wall shear stress has revealed the existence of anomalous, long-ranged temporal correlations. Based on the results obtained, there are indications that the dense Couette flow of monodisperse, smooth, inelastic, spherical particles is a system which may be characterized by continuous distributions of the physical measures of its particles, such as size.

Shear induced diffusive mixing in simulations of dense Couette flow of rough, inelastic hard spheres

Large-scale numerical simulations of a system of inelastic, rough, hard spheres of volume fraction φs=0.565, which are initially distributed randomly in a Couette geometry, show clear evidence of the movement of the particles in directions transverse to the bulk motion. This behavior of the aforementioned system, which has been considered as a model for a granular fluid, is consistent with recent experimental observations [Hsiau and Hunt, J. Fluid Mech. 251, 299 (1993)]. Based on the results obtained, there are indications that a bounded rapid granular flow could be a diffusive system at volume fractions even higher than 0.56. This finding contradicts earlier computer experiments [Campbell, J. Fluid Mech. 348, 85 (1997)] which found a rapidly flowing granular material is a diffusive system except at large solids concentrations (i.e., φs>0.56).

Effect of the shape and configuration of smooth muscle cells on the diffusion of ATP through the arterial wall

In this study, the shape and the configuration of smooth muscle cells (SMCs) within the arterial wall are altered to investigate their influence on molecular transport across the media layer of the thoracic aorta wall. In a 2D geometry of the media layer containing SMCs, the finite-element method has been employed to simulate the diffusion of solutes through the media layer. The media is modeled as a heterogeneous system composed of SMCs having elliptic or circular cross sections embedded in a homogeneous porous medium made of proteoglycan and collagen fibers with an interstitial fluid filling the void. The arrangement of SMCs is in either ordered or disordered fashion for different volume fractions of SMCs. The interstitial fluid enters the media through fenestral pores, which are assumed to be distributed uniformly over the internal elastic lamina (IEL). Results revealed that in an ordered arrangement of SMCs, the concentration of adenosine 5′-triphosphate (ATP) over the surface of SMCs with an elliptic cross section is 5–8% more than those of circular SMCs in volume fractions of 0.4–0.7. The ATP concentration at the SMC surface decreases with volume fraction in the ordered configuration of SMCs. In a disordered configuration, the local ATP concentration at the SMC surface and in the bulk are strongly dependent on the distance between neighboring SMCs, as well as the minimum distance between SMCs and fenestral pores. Moreover, the SMCs in farther distances from the IEL are as important as those just beneath the IEL in disordered configurations. The results of this study lead us to better understanding of the role of SMCs in controlling the diffusion of important species such as ATP within the arterial wall.

Self-diffusion in unbounded rapid granular flows: a nonequilibrium approach

Using a nonequilibrium simulation scheme, the transverse diffusive motion has been investigated in unbounded shear flows of smooth, monodisperse, inelastic spherical particles. This scheme is used to obtain the concentration gradient due to the particle mass flux, which is extracted from the bulk flow using a certain labeling algorithm. The self-diffusion coefficient can then be obtained from Ficks law. Under steady conditions, the simulation results show that the particle diffusivity can be described by a linear law. This finding provides a justification for assuming a linear law relationship in the kinetic theory type derivation of an expression for self-diffusivity. Moreover, the values of self-diffusion coefficient from the computer simulations agree with those obtained using kinetic theory formulations for solid volume fractions less than 0.5.

Bond-orientational order in sheared dense flows of inelastic hard spheres

Three-dimensional bond-orientational order is studied using computer simulations with 4296 hard, monodisperse inelastic spheres flowing in a Couette geometry at a high shear rate. At an average volume fraction close to 0.6, a state with extended correlations in the orientations of particle clusters starts to develop for rough particles after sufficiently long run times. However, no clear evidence of crystallization is found in the system. Further tests of a sheared system comprised of smooth, inelastic spheres reveal crystallization consistent with the previous experimental observations.

Rough Cylindrical Object Immersed in a Granular Stream of Hard Disks

We have investigated stresses on rough cylindrical objects confronted with a granular stream of inelastic hard disks. The roughness of cylindrical objects is created via covering their surfaces with hard disks of given size and material. We have employed event-driven simulations using restitution coefficients dependent of the impact velocity in a collision. We report the effect of material property (restitution coefficient) on development of granular shock wave around the object with corresponding stresses exerted on it. The role of the roughness of the object in resulting flow is also studied. Moreover, simulations are performed in two conditions with gravity and without it.

Molecular transport in the arterial wall with variation of shape and configuration of smooth muscle cells

Molecular transport through the tunica media layer is influenced by the smooth muscle cells (SMCs). This study considers the media layer as a heterogeneous porous media composed of smooth muscle cells of elliptic and circular shapes distributed in ordered or disordered configurations. To study the role of SMCs on the transport of molecules in the media, we model the media layer as a two dimensional numerical simulation of interstitial flow through the media layer. The assumption of the elliptic shape resembles the spindled shape of SMCs. The molecular transport of ATP is considerably dependent of the shape of SMCs according to the results of our numerical model. More importantly, the ATP concentration was found to be extremely sensitive to the random configuration of SMCs, which is more close to the real arrangement of SMCs within the media layer