Jan Trulsen - Academia.edu (original) (raw)
Papers by Jan Trulsen
Understanding Complex Systems
ABSTRACT
Journal of Fluid Mechanics, 2021
Taylor's hypothesis, or the frozen turbulence approximation, can be used to estimate also the... more Taylor's hypothesis, or the frozen turbulence approximation, can be used to estimate also the specific energy dissipation rate epsilon\epsilonepsilon by comparing experimental results with the Kolmogorov–Obukhov expression. The hypothesis assumes that a frequency detected by an instrument moving with a constant large velocity VVV can be related to a wavenumber by omega=kV\omega = k Vomega=kV . It is, however, not obvious how large the translational velocity has to be in order to make the hypothesis valid, or at least applicable with some acceptable uncertainty. Using the space–time-varying structure function for homogeneous and isotropic conditions, this question is addressed in the present study with emphasis on small velocities VVV . The structure function is obtained using results from numerical solutions of the Navier–Stokes equation. Particular attention is given to the VVV variation of the estimated specific energy dissipation, epsilonest\epsilon _{est}epsilonest , compared with the actual value, epsilon\epsilonepsilon , used i...
Plasma Physics and Controlled Fusion, 2018
A simple magnetized plasma torus is modeled by using a 'top-hat' density variation. The benefit o... more A simple magnetized plasma torus is modeled by using a 'top-hat' density variation. The benefit of this simplification is an exactly solvable model, allowing for a few additions, such as an externally maintained parabolic steady state potential variation imposed on the plasma. The combined effects of the resulting plasma rotation and the plasma polarization due to magnetic field gradient particle drifts can be described. The stabilizing and confining effects of plasma rotation are explicitly demonstrated. In a special high plasma density limit the results are confirmed by a more general model, indicating that the results of the top-hat model can be used with confidence in more general cases. A small vertical magnetic field component can be included for a plasma with neutral collisions and its influence on the electron dynamics studied. The effects of ion-neutral collisions are also included. The rotation and polarization of the plasma has different effects on the time variations of the plasma density and potential. As a reference we use data from the Blaamann device at the University of Tromsø obtained by a movable multi-probe, measuring variations in density, floating potential and an electric field component. The fluctuations in the plasma are characterized by auto-correlations and by crosscorrelations between the signal from a fixed reference probe and data. The model accounts adequately for the phase variations of the signals for varying spatial multi-probe positions.
International Astronomical Union Colloquium, 1976
The problem of the effects of mutual collisions for the dynamics of interplanetary dust particles... more The problem of the effects of mutual collisions for the dynamics of interplanetary dust particles and grains is reviewed. Collisions are shown to give a rather characteristic dynamical signature, the importance of these effects depending mainly on the mean free collision time and the degree of inelasticity. Although a few attempts to look for collisional effects in the solar system have been made, rather much work remains to be done before the problem is fully understood.
International Astronomical Union Colloquium, 1971
Jetstreams probably played an important role at an intermediate stage of the formation of the sol... more Jetstreams probably played an important role at an intermediate stage of the formation of the solar system (Alfvén and Arrhenius, 1970). A Jetstream is defined here as a collection of grains moving in neighboring elliptical orbits around a central gravitating body and with the dynamics modified by the action of complicated collision processes among the grains themselves.Three main types of collisions will take place in such a stream. Hyper-velocity impacts will lead to fragmentation of the grains involved. At lower impact velocities, the particles will retain their identities after the collision even if they might be deformed to some degree depending on impact velocity and internal structure of the grains. With still lower impact velocity, accretion can take place, the grains sticking together after collision to form larger grains. A common feature of these collision processes are that they will be partially inelastic. A certain fraction of the kinetic energy of the colliding partic...
![Research paper thumbnail of Erratum: ‘‘A statistical analysis of numerically simulated plasma turbulence’’ [Phys. Fluids B 1, 1616 (1989)]](https://attachments.academia-assets.com/113666749/thumbnails/1.jpg)
](Physics of Fluids B: Plasma Physics, 1990
AIP Conference Proceedings, 2008
ABSTRACT The parameter dependence of the ion focus [1, 2] behind perfectly conducting or alternat... more ABSTRACT The parameter dependence of the ion focus [1, 2] behind perfectly conducting or alternatively perfectly insulating spherical grains for different electron to ion temperature ratios is studied. For elongated, insulating dust grains we study the potential and plasma density wakes in drifting plasma for rods or plates of different lengths and different inclination angles. These two characteristics (i.e., the rod length and the inclination angle are important for the exact charge distribution on the surface and the wake pattern. For this case we discuss also the interaction potential between two elongated grains in a flowing plasma. Our simulations are carried out in two spatial dimensions by a Particle-in-Cell code [3], treating ions and electrons as individual particles. These studies can be relevant for finite size dust grains suspended in a plasma sheath or larger objects in space, e.g., meteoroids.
AIP Conference Proceedings, 2008
The electrostatic potential and plasma density variations around a point‐like charged object in a... more The electrostatic potential and plasma density variations around a point‐like charged object in a plasma flow are studied. These objects can represent small charged dust particles, for instance. The radiation patterns can be interpreted as the result of sound waves being ...
ERCOFTAC Series
The motion of point-particles is studied by numerical simulations. We analyze the turbulent parti... more The motion of point-particles is studied by numerical simulations. We analyze the turbulent particle fluxes to perfectly absorbing spheres, which are moving with the flow. Particular attention is given to the effect of viscosity for the case where the radius of the sphere is comparable to or smaller than the Kolmogorov length scale. By relatively simple model arguments, we arrive
Astronomy and Astrophysics
A unified derivation of the coupled mode equations for circular waveguide is presented. Also, app... more A unified derivation of the coupled mode equations for circular waveguide is presented. Also, approximate design criteria for TEOnto-TEOn' axisymmetric, TE01to-TE 1 I wriggle, and TE01to-TM11 bend converters are reviewed. Numerically solving the coupled mode equations, an optimized set of mode converters has been designed for conversion of a 2 millimeter wave TE03 mode into TE11. This set consists of axisymmetric TE 0 3to-TE 0 2 and TE 0 2to-TE 0 1 converters followed by a wriggle TE01to-TEII converter. This mode converter set was fabricated and tested using a 3 kW, 137 GHz gyrotron. A TE11 mode purity of better than 97% was achieved. The TEOIto-TE 1 I wriggle converter was experimentally optimized for a measured conversion efficiency of better than 99% not including ohmic losses.
Physics and Chemistry of the Earth, Part C: Solar, Terrestrial & Planetary Science, 2001
ABSTRACT
Physical Review E, 2002
With reference to studies of predator-prey encounters in turbulent waters, we demonstrate the fea... more With reference to studies of predator-prey encounters in turbulent waters, we demonstrate the feasibility of an experimental method for investigations of particle fluxes to an absorbing surface in turbulent flows. A laboratory experiment is carried out, where an approximately homogeneous and isotropic turbulent flow is generated by two moving grids. The simultaneous trajectories of many small neutrally buoyant polystyrene particles are followed in time. Selecting one of these to represent a predator, while the others are considered as prey, we obtain estimates for the time variation of the statistical average of the prey flux into a suitably defined ''sphere of interception.'' The variation of this flux with the radius in the sphere of interception, as well as the variation with basic flow parameters is well described by a simple model, in particular for radii smaller than a characteristic length scale for the turbulence. Also the Eulerian counterpart of the problem has been analyzed, and the particle fluxes from the two studies compared.
Physical Review E, 2003
The motion of passively convected particles in turbulent flows is studied experimentally in appro... more The motion of passively convected particles in turbulent flows is studied experimentally in approximately homogeneous and isotropic turbulent flows, generated in water by two moving grids. The simultaneous trajectories of many small passively convected, neutrally buoyant, polystyrene particles are followed in time by a particle tracking technique. We estimate the probability distribution of the occupation times of such particles in spherical volumes with a given radius. A self-consistently moving particle defines the center of the reference sphere, with the occupation time being defined as the difference between entrance and exit times of surrounding particles convected through the sphere by the turbulent motions. Simple, and seemingly universal, scaling laws are obtained for the probability density of the occupation times in terms of the basic properties for the turbulent flow and the geometry. In the present formulation, the results of the analysis are relevant for understanding details in the feeding rate of microorganisms in turbulent waters, for instance.
Understanding Complex Systems
ABSTRACT
Journal of Fluid Mechanics, 2021
Taylor's hypothesis, or the frozen turbulence approximation, can be used to estimate also the... more Taylor's hypothesis, or the frozen turbulence approximation, can be used to estimate also the specific energy dissipation rate epsilon\epsilonepsilon by comparing experimental results with the Kolmogorov–Obukhov expression. The hypothesis assumes that a frequency detected by an instrument moving with a constant large velocity VVV can be related to a wavenumber by omega=kV\omega = k Vomega=kV . It is, however, not obvious how large the translational velocity has to be in order to make the hypothesis valid, or at least applicable with some acceptable uncertainty. Using the space–time-varying structure function for homogeneous and isotropic conditions, this question is addressed in the present study with emphasis on small velocities VVV . The structure function is obtained using results from numerical solutions of the Navier–Stokes equation. Particular attention is given to the VVV variation of the estimated specific energy dissipation, epsilonest\epsilon _{est}epsilonest , compared with the actual value, epsilon\epsilonepsilon , used i...
Plasma Physics and Controlled Fusion, 2018
A simple magnetized plasma torus is modeled by using a 'top-hat' density variation. The benefit o... more A simple magnetized plasma torus is modeled by using a 'top-hat' density variation. The benefit of this simplification is an exactly solvable model, allowing for a few additions, such as an externally maintained parabolic steady state potential variation imposed on the plasma. The combined effects of the resulting plasma rotation and the plasma polarization due to magnetic field gradient particle drifts can be described. The stabilizing and confining effects of plasma rotation are explicitly demonstrated. In a special high plasma density limit the results are confirmed by a more general model, indicating that the results of the top-hat model can be used with confidence in more general cases. A small vertical magnetic field component can be included for a plasma with neutral collisions and its influence on the electron dynamics studied. The effects of ion-neutral collisions are also included. The rotation and polarization of the plasma has different effects on the time variations of the plasma density and potential. As a reference we use data from the Blaamann device at the University of Tromsø obtained by a movable multi-probe, measuring variations in density, floating potential and an electric field component. The fluctuations in the plasma are characterized by auto-correlations and by crosscorrelations between the signal from a fixed reference probe and data. The model accounts adequately for the phase variations of the signals for varying spatial multi-probe positions.
International Astronomical Union Colloquium, 1976
The problem of the effects of mutual collisions for the dynamics of interplanetary dust particles... more The problem of the effects of mutual collisions for the dynamics of interplanetary dust particles and grains is reviewed. Collisions are shown to give a rather characteristic dynamical signature, the importance of these effects depending mainly on the mean free collision time and the degree of inelasticity. Although a few attempts to look for collisional effects in the solar system have been made, rather much work remains to be done before the problem is fully understood.
International Astronomical Union Colloquium, 1971
Jetstreams probably played an important role at an intermediate stage of the formation of the sol... more Jetstreams probably played an important role at an intermediate stage of the formation of the solar system (Alfvén and Arrhenius, 1970). A Jetstream is defined here as a collection of grains moving in neighboring elliptical orbits around a central gravitating body and with the dynamics modified by the action of complicated collision processes among the grains themselves.Three main types of collisions will take place in such a stream. Hyper-velocity impacts will lead to fragmentation of the grains involved. At lower impact velocities, the particles will retain their identities after the collision even if they might be deformed to some degree depending on impact velocity and internal structure of the grains. With still lower impact velocity, accretion can take place, the grains sticking together after collision to form larger grains. A common feature of these collision processes are that they will be partially inelastic. A certain fraction of the kinetic energy of the colliding partic...
Physics of Fluids B: Plasma Physics, 1990
AIP Conference Proceedings, 2008
ABSTRACT The parameter dependence of the ion focus [1, 2] behind perfectly conducting or alternat... more ABSTRACT The parameter dependence of the ion focus [1, 2] behind perfectly conducting or alternatively perfectly insulating spherical grains for different electron to ion temperature ratios is studied. For elongated, insulating dust grains we study the potential and plasma density wakes in drifting plasma for rods or plates of different lengths and different inclination angles. These two characteristics (i.e., the rod length and the inclination angle are important for the exact charge distribution on the surface and the wake pattern. For this case we discuss also the interaction potential between two elongated grains in a flowing plasma. Our simulations are carried out in two spatial dimensions by a Particle-in-Cell code [3], treating ions and electrons as individual particles. These studies can be relevant for finite size dust grains suspended in a plasma sheath or larger objects in space, e.g., meteoroids.
AIP Conference Proceedings, 2008
The electrostatic potential and plasma density variations around a point‐like charged object in a... more The electrostatic potential and plasma density variations around a point‐like charged object in a plasma flow are studied. These objects can represent small charged dust particles, for instance. The radiation patterns can be interpreted as the result of sound waves being ...
ERCOFTAC Series
The motion of point-particles is studied by numerical simulations. We analyze the turbulent parti... more The motion of point-particles is studied by numerical simulations. We analyze the turbulent particle fluxes to perfectly absorbing spheres, which are moving with the flow. Particular attention is given to the effect of viscosity for the case where the radius of the sphere is comparable to or smaller than the Kolmogorov length scale. By relatively simple model arguments, we arrive
Astronomy and Astrophysics
A unified derivation of the coupled mode equations for circular waveguide is presented. Also, app... more A unified derivation of the coupled mode equations for circular waveguide is presented. Also, approximate design criteria for TEOnto-TEOn' axisymmetric, TE01to-TE 1 I wriggle, and TE01to-TM11 bend converters are reviewed. Numerically solving the coupled mode equations, an optimized set of mode converters has been designed for conversion of a 2 millimeter wave TE03 mode into TE11. This set consists of axisymmetric TE 0 3to-TE 0 2 and TE 0 2to-TE 0 1 converters followed by a wriggle TE01to-TEII converter. This mode converter set was fabricated and tested using a 3 kW, 137 GHz gyrotron. A TE11 mode purity of better than 97% was achieved. The TEOIto-TE 1 I wriggle converter was experimentally optimized for a measured conversion efficiency of better than 99% not including ohmic losses.
Physics and Chemistry of the Earth, Part C: Solar, Terrestrial & Planetary Science, 2001
ABSTRACT
Physical Review E, 2002
With reference to studies of predator-prey encounters in turbulent waters, we demonstrate the fea... more With reference to studies of predator-prey encounters in turbulent waters, we demonstrate the feasibility of an experimental method for investigations of particle fluxes to an absorbing surface in turbulent flows. A laboratory experiment is carried out, where an approximately homogeneous and isotropic turbulent flow is generated by two moving grids. The simultaneous trajectories of many small neutrally buoyant polystyrene particles are followed in time. Selecting one of these to represent a predator, while the others are considered as prey, we obtain estimates for the time variation of the statistical average of the prey flux into a suitably defined ''sphere of interception.'' The variation of this flux with the radius in the sphere of interception, as well as the variation with basic flow parameters is well described by a simple model, in particular for radii smaller than a characteristic length scale for the turbulence. Also the Eulerian counterpart of the problem has been analyzed, and the particle fluxes from the two studies compared.
Physical Review E, 2003
The motion of passively convected particles in turbulent flows is studied experimentally in appro... more The motion of passively convected particles in turbulent flows is studied experimentally in approximately homogeneous and isotropic turbulent flows, generated in water by two moving grids. The simultaneous trajectories of many small passively convected, neutrally buoyant, polystyrene particles are followed in time by a particle tracking technique. We estimate the probability distribution of the occupation times of such particles in spherical volumes with a given radius. A self-consistently moving particle defines the center of the reference sphere, with the occupation time being defined as the difference between entrance and exit times of surrounding particles convected through the sphere by the turbulent motions. Simple, and seemingly universal, scaling laws are obtained for the probability density of the occupation times in terms of the basic properties for the turbulent flow and the geometry. In the present formulation, the results of the analysis are relevant for understanding details in the feeding rate of microorganisms in turbulent waters, for instance.