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J. Hänninen

Bio: J. Hänninen is an academic researcher from University of Oulu. The author has contributed to research in topics: Instability & Coefficient of restitution. The author has an hindex of 2, co-authored 2 publications receiving 18 citations.

Papers
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Journal ArticleDOI
TL;DR: In this paper, the velocity dependent coefficient of restitution, α = α(Ν), was shown to lead to a many particle thick equilibrium state, independent of the initial conditions, and the essential requirement for this to take place is that dα dΝ 0, the system either disperses through growing random velocities, or flattens to a near monolayer state.
Abstract: Numerical simulations of 250 mutually colliding particles, revolving in the gravitational field of a central body, indicate that velocity dependent coefficient of restitution, α = α(Ν), can lead to a many particle thick equilibrium state, independent of the initial conditions. The essential requirement for this to take place is that dα dΝ 0, the system either disperses through growing random velocities, or flattens to a near monolayer state, depending on the initial conditions.

11 citations

Journal ArticleDOI
TL;DR: In this paper, the authors derived a criterion for gravitational instability in a gaseous medium for particulate discs, in which the equilibrium depends on the collisional energy loss, and used it to predict the formation of highly elongated clouds with a relatively long lifetime before decay.
Abstract: Collisionally-induced amplification of density fluctuations can also produce non-axisymmetric local condensations in particulate discs if the optical thickness is between definite values. Gravitational instability occurs above this interval. The theory of both phenomena is derived from collisional equations. The conventional criterion for gravitational instability in a gaseous medium cannot be used for particulate discs, in which the equilibrium depends on the collisional energy loss. These instabilities can produce an unbounded growth in density or a gravitational coagulation of particles, but the typical consequence is the formation of highly elongated clouds which are denser than the background matter and have a relatively long lifetime before decay. The third type of instability, the thermal one, appears at low values of velocity dispersion. It only affects the random motion of particles without producing condensations.

7 citations


Cited by
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Journal ArticleDOI
TL;DR: A Padé approximation is constructed for this function which may be used for a wide range of impact velocities where the concept of the viscoelastic collision is valid.
Abstract: We perform a dimension analysis for colliding viscoelastic spheres to show that the coefficient of normal restitution epsilon depends on the impact velocity g as epsilon=1-gamma(1)g(1/5)+gamma(2)g(2/5)-/+..., in accordance with recent findings. We develop a simple theory to find explicit expressions for coefficients gamma(1) and gamma(2). Using these and few next expansion coefficients for epsilon(g) we construct a Pade approximation for this function which may be used for a wide range of impact velocities where the concept of the viscoelastic collision is valid. The obtained expression reproduces quite accurately the existing experimental dependence epsilon(g) for ice particles.

276 citations

Journal ArticleDOI
Heikki Salo1
01 Apr 1991-Icarus
TL;DR: In this article, the authors examined the viscous stability properties of dense planetary rings with a local simulation method closely resembling that of Wisdom and Tremaine (1988, Astron. J. 95, 925).

92 citations

Journal ArticleDOI
Heikki Salo1
01 Mar 1992-Icarus
TL;DR: In this article, a local simulation method similar to that of Wisdom and Tremaine (1988, Astron. J. 95,925) with N up to 4000 particles is applied to the collisional dynamics of dense planetary rings with power-law distribution of particle sizes.

71 citations

Journal ArticleDOI
TL;DR: Numerical simulations of strongly vibrated granular materials designed to mimic recent experiments performed in both the presence and the absence of gravity show that this model with impact-velocity-dependent restitution coefficient reproduces results that agree with experiments.
Abstract: We report numerical simulations of strongly vibrated granular materials designed to mimic recent experiments performed in both the presence and the absence of gravity. The coefficient of restitution used here depends on the impact velocity by taking into account both the viscoelastic and plastic deformations of particles, occurring at low and high velocities, respectively. We show that this model with impact-velocity-dependent restitution coefficient reproduces results that agree with experiments. We measure the scaling exponents of the granular temperature, collision frequency, impulse, and pressure with the vibrating piston velocity as the particle number increases. As the system changes from a homogeneous gas state at low density to a clustered state at high density, these exponents are all found to decrease continuously with increasing particle number. All these results differ significantly from classical inelastic hard sphere kinetic theory and previous simulations, both based on a constant restitution coefficient.

66 citations

Journal ArticleDOI
01 Jun 1992-Icarus
TL;DR: In this paper, the influence of a perturbing satellite on a planetary ring at isolated Lindblad resonances is studied with numerical computer simulations, combining Aarseth's force polynomial method for orbit integrations with the calculation of particle-particle impacts.

35 citations