Tero Tynjälä

Magnetodissipation in ferrofluids

In a ferrofluid, the interaction between the hydrodynamics and an externally applied magnetic field leads to extraordinary phenomena, such as magnetodissipation. The origin of the magnetodissipative field, however, has been a subject of controversy. The purpose of this study is to identify an approach to scrutinize the mesoscopic and hydrodynamic theories, to determine which theory could describe the existence of effects such as magnetodissipation. To this end, the natural convection of a ferrofluid is investigated via simulations in a two-dimensional isosceles triangular enclosure.

Particle interactions in a dense monosized granular flow

Abstract Simulations of bounded dense sheared granular flows were performed to investigate multi-phase flow phenomena, in particular the behavior of the ordered phase. In the absence of gravity, collapse of the granular structures was found to occur for a wide range of shear rate, as evidenced by the disappearance of the signature of the ordered structure in the wall normal stress signal. However, normal stress signals matching those detected in recent experiments were obtained for a system whose dynamics were collision-dominated rather than friction-dominated. Moreover, the system was found to exhibit a similar flow behavior in the presence of gravity. The stress signals were analyzed using wavelet transforms, which indicated the existence of stick–slip dynamics, characterized by harmonic frequencies. It is shown that these observations might elucidate the origin of stick–slip dynamics in the system, which experienced instability due to gravitational compactification at shear rates below a certain critical value.

Magneto-Hydrodynamic Interaction in an Inclined Layer of Ferrocolloid Heated from Below

The instability of convection patterns representing a combination of vertical Rayleigh rolls and horizontal rolls resulting from longitudinal horizontal magnetic field is investigated in an inclined layer of magnetic colloid by experiments and numerical simulations. Visualization of convection patterns is provided by a temperature-sensitive liquid crystal film. The rich spectrum of convection structures is observed against different values of inclination angles and uniform magnetic fields. If the horizontal longitudinal magnetic field is strong enough it extinguishes the convection perturbations along the field direction and stabilizes Rayleigh flows. Observed patterns at about two supercriticalities and with small inclination angles show mostly spatially and temporally chaotic structures.

On pattern formation in ferrocolloid convection

Experimental studies and numerical simulations of stability of buoyancy-driven flows in a ferrocolloid for the cases of horizontal and inclined vertical orientation of a thin cylindrical cavity are performed. The influence of a homogeneous longitudinal magnetic field on convective instability and spatio-temporal patterns were also investigated. In the case of ferrocolloids the gradients of magnetic permeability may arise due to both temperature and particle concentration gradients. The particle mass flux in a classical form is summarized from the translation diffusion coefficient and the thermal diffusion ratio. However, the explanation for the observed self-oscillation regimes in magnetic fluid for the cavities of sufficiently large thickness is conditioned by the competition of density variations originating from the fluid thermal expansion and barometric sedimentation. The results prove that a uniform longitudinal magnetic field allows to control the stability and the shape of secondary convection motions at inclined orientation of layer. In a ferrocolloid the repeated transients involving localized roll convection and pure shear flow took place. Under action of uniform longitudinal magnetic field orientated perpendicular to flux velocity of shear motion on such long-wave transients can lead to complicated types of chaotic localized states or solitary vortices.

A spatiotemporal tree model for turbulence in dispersed phase multiphase flows: Energy dissipation rate behavior in single particle and binary particles arrays

In this article, a spatiotemporal dynamical system model (tree model) is utilized for investigating the features of forced and unforced turbulence in a dispersed phase two-phase system. The tree model includes a variable for spatial dimension in addition to variables of wavenumber and time, which display both spatial and temporal intermittencies. The focus of this paper is to study the turbulence modulation due to the presence of rigid particles. The study considers particles with the sizes of 32, 64, and 128 times the Kolmogorov length scale. Specifically, the study of the energy dissipation rate (EDR) at the particle-fluid interface is considered. Two models, namely, A and B with different types of interaction connections between nearby shells, are used first to compare the results of the particle-laden case with decaying turbulence. The number of tree connections in the model is found to affect the amount of augmentation of EDR near the particle surface. Model B is studied further with different sizes of particles in forced turbulence cases and compared to the unladen case with the same parameters. Also, the model expression is studied in the forced turbulence case of dual particles separated by given distances. The results of spatiotemporal shell models provide new approach of handling high Reynolds turbulence in dispersed phase multiphase systems.

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

Stability of flow and kinetic energy dissipation in 2D annular shear flows of inelastic hard disk assemblies

We have used simulations of inelastic hard disks in two-dimensional shear flows to investigate the stability conditions of kinetic energy upon shearing through different circumstances. We study the characteristics of instability via the signals of kinetic energy dissipation and the statistics associated with this quantity. Our results reveal how the flow characteristics of hard-disk assembly can be modelled by a dynamical model for turbulence named as the GOY shell model. Our results clearly show that the behaviour of rapidly sheared hard-disk assemblies is similar to a fluid in highly chaotic state, so called turbulent state. The current results assist us in finding proper modelling ways in simulation of suspension flows, colloids and magnetic fluids.