Stability boundaries of axisymmetric and two-dimensional perturbations in MHD Dean flow. - In: 9th International Conference on Fundamental and Applied MHD, Thermo Acoustic and Space Technologies, (2014), S. 230-234
We study the linear stability of annular MHD channel flow with a uniform axial magnetic field in order to determine when two-dimensional instabilities of Orr type can appear. They are a prerequisite for intermittent turbulent behavior known from plane MHD channel flow with a spanwise field. The annular flow is driven by Lorentz forces caused by a radial electric current and the imposed axial field. Stability of this MHD Dean flow is investigated for axially uniform Orr modes and axisymmetric Dean modes. Orr mode instability dominates only for small gap width and in strong magnetic fields.
Lagrangian mixing dynamics at the cloudy-clear air interface. - In: Journal of the atmospheric sciences, ISSN 1520-0469, Bd. 71 (2014), 7, S. 2564-2580
Corrigendum. - Bd. 72.2015, 3, S. 1276-1278
The entrainment of clear air and its subsequent mixing with a filament of cloudy air, as occurs at the edge of a cloud, is studied in three-dimensional direct numerical simulations that combine the Eulerian description of the turbulent velocity, temperature, and vapor fields with a Lagrangian cloud droplet ensemble. Forced and decaying turbulence is considered, such as when the dynamics around the filament is driven by larger-scale eddies or during the final period of the life cycle of a cloud. The microphysical response depicted in nd - (r3) space (where nd and r are droplet number density and radius, respectively) shows characteristics of both homogeneous and inhomogeneous mixing, depending on the Damköhler number. The transition from inhomogeneous to homogeneous mixing leads to an offset of the homogeneousmixing curve to larger dilution fractions. The response of the system is governed by the smaller of the single droplet evaporation time scale and the bulk phase relaxation time scale. Variability within the nd - (r3) space increases with decreasing sample volume, especially during the mixing transients. All of these factors have implications for the interpretation of measurements in clouds. The qualitative mixing behavior changes for forced versus decaying turbulence, with the latter yielding remnant patches of unmixed cloud and stronger fluctuations. Buoyancy due to droplet evaporation is observed to play a minor role in the mixing for the present configuration. Finally, themixing process leads to the transient formation of a pronounced nearly exponential tail of the probability density function of the Lagrangian supersaturation, and a similar tail emerges in the droplet size distribution under inhomogeneous conditions.
http://dx.doi.org/10.1175/JAS-D-13-0294.1
Laminar-turbulent transition in magnetohydrodynamic duct, pipe, and channel flows. - In: Applied mechanics reviews, ISSN 1088-8535, Bd. 66 (2014), 3, S. 030802, insges. 17 S.
A magnetic field imposed on a flow of an electrically conducting fluid can profoundly change flow behavior. We consider this effect for the situation of laminar-turbulent transition in magnetohydrodynamic duct, pipe, and channel flows with homogeneous magnetic field and electrically insulating walls. Experimental and recent computational results obtained for flows in pipes, ducts and channels are reviewed.
http://dx.doi.org/10.1115/1.4027198
Transitional and turbulent magnetohydrodynamic flows in uniform and non-uniform magnetic fields. - In: NIC Symposium 2014, (2014), S. 375-382
We investigate the effects of static magnetic fields on wall-bounded flows of liquid metals using direct numerical simulations. Such flows are modified by the Lorentz forces arising from the electromagnetic induction of eddy currents. In a homogeneous field we find that electromagnetic boundary layers at the walls parallel to the magnetic field are essential for transition to turbulence. A strongly inhomogeneous dipole field can act as a magnetic obstacle and cause transition to turbulence in its wake.
Pattern formation and mass transfer under stationary solutal Marangoni instability. - In: Advances in colloid and interface science, ISSN 1873-3727, Bd. 206 (2014), S. 344-371
According to the seminal theory by Sternling and Scriven [1], solutal Marangoni convection during mass transfer of surface-active solutes may occur as either oscillatory or stationary instability. With strong support of Manuel G. Velarde, a combined initiative of experimental works, in particular to mention those of Linde, Wierschem and coworkers, and theory has enabled a classification of dominant wave types of the oscillatory mode and their interactions. In this way a rather comprehensive understanding of the nonlinear evolution of the oscillatory instability could be achieved. A comparably advanced state-of-the-art with respect to the stationary counterpart seemed to be out of reach a short time ago. Recent developments on both the numerical and experimental side, in combination with assessing an extensive number of older experiments, now allow one to draw a more unified picture. By reviewing these works, we show that three main building blocks exist during the nonlinear evolution: roll cells, relaxation oscillations and relaxation oscillations waves. What is frequently called interfacial turbulence results from the interaction between these partly coexisting basic patterns which may additionally occur in different hierarchy levels. The second focus of this review lies on the practical importance of such convection patterns concerning their influence on mass transfer characteristics. Particular attention is paid here to the interaction between Marangoni and buoyancy effects which frequently complicates the pattern formation even more. To shed more light on these dependencies, new simulations regarding the limiting case of stabilizing density stratification and vanishing buoyancy are incorporated.
http://dx.doi.org/10.1016/j.cis.2013.10.003
Cloud formation studies in moist Rayleigh-Benard convection. - In: DPG-Frühjahrstagung (DPG Spring Meeting) of the Condensed Matter Section [SKM], with its Divisions: Biological Physics, Chemical and Polymer Physics, Crystallography, Dielectric Solids, Dynamics and Statistical Physics, Low Temperature Physics, Magnetism, Metal and Material Physics, Physics of Socio-Economic Systems, Semiconductor Physics, Surface Science, Thin Fils, Vacuum Science and Technology, together with the Divisions: Microprobes, and Radiation and Medical Physics as well as the Working Groups: Industry and Business, and "Young DPG" ; March 10 - 15, 2013, University of Regensburg, 2013, DY 29.4
On the volume of fluid method for multiphase fluid flow simulation. - In: International journal of modeling, simulation, and scientific computing, ISSN 1793-9623, Bd. 4 (2013), 2, S. 1350002, insges. 27 S.
Numerische Untersuchung wandnaher Transport- und Strukturbildungsprozesse in turbulenter Rayleigh-Bénard-Konvektion, 2013. - Online-Ressource (PDF-Datei: VI, 93 S., 36,81 MB) : Ilmenau, Techn. Univ., Diss., 2013
Parallel als Druckausg. erschienen
In der vorliegenden Dissertation wurde die Struktur der Grenzschichten in turbulenter Rayleigh-Bénard-Konvektion durch dreidimensionale direkte numerische Simulationen untersucht. Zuerst wurde die Konvektion in einer zylindrischen Zelle mit einem Seitenverhältnis von Eins bei den Rayleighzahlen Ra = 3*10^9 und Ra = 3*10^10 und bei fester Prandtlzahl Pr = 0.7 betrachtet. Ähnlich wie bei den experimentellen Ergebnissen in der gleichen Konfiguration und für die gleiche Prandtlzahl wichen die Grenzschichten der Geschwindigkeits- und Temperaturfelder von den Vorhersagen der Prandtl-Blasius-Pohlhausen Theorie ab. Die Abweichungen werden kleiner, wenn ein dynamisches Reskalieren der Daten mit einer instantan definierten Grenzschichtdicke durchgeführt und die Analyseebene mit der momentanen Richtung der großskaligen Zirkulation in der geschlossenen Zelle ausgerichtet wurde. Die physikalischen Gründe für die Abweichungen der Grenzschichtprofile von der klassischen Prandtl-Blasius-Pohlhausen und Stewartson-Theorien für erzwungene und natürliche Konvektion wurden im Detail untersucht. Die numerischen Ergebnisse zeigen, dass wichtige Annahmen für die klassischen laminaren Grenzschichttheorien für erzwungene und natürliche Konvektion verletzt werden, wie die strikte Zweidimensionalität der Dynamik oder der Stationarität. Schließlich wurde die Grenzschichtdynamik bei drei unterschiedlichen Prandtlzahlen Pr = 7, 0.7, 0.1 bei Ra = 3*10^9 und [Gamma] = 1 verglichen. Hier zeigte sich, dass mit zunehmender Prandtlzahl dynamisches Skalieren die Vereinbarung mit der Prandtl-Blasius-Pohlhausen Theorie verbessert. Die thermische Plumeablösung scheint eine der Hauptursachen für die Abweichungen von der klassischen Grenzschichttheorie. Mit abnehmender Prandtlzahl wird die thermische Grenzschichtdicke dicker. Der Winkel der momentanen großskaligen Zirkulation hat wenigere Fluktuation für die größere Prandtlzahl. Mit abnehmender Prandtlzahl nimmt die mittlere Amplitude der momentanen großskaligen Zirkulation zu.
http://www.db-thueringen.de/servlets/DocumentServlet?id=23592
Toward a mode reduction strategy in shallow moist convection. - In: New journal of physics, ISSN 1367-2630, Bd. 15 (2013), 125025, insges. 24 S.
https://doi.org/10.1088/1367-2630/15/12/125025
Resolving the fine-scale structure in turbulent Rayleigh-Bénard convection. - In: New journal of physics, ISSN 1367-2630, Bd. 15 (2013), 113063, insges. 32 S.
We present high-resolution direct numerical simulation studies of turbulent Rayleigh-Bènard convection in a closed cylindrical cell with an aspect ratio of one. The focus of our analysis is on the finest scales of convective turbulence, in particular the statistics of the kinetic energy and thermal dissipation rates in the bulk and the whole cell. The fluctuations of the energy dissipation field can directly be translated into a fluctuating local dissipation scale which is found to develop ever finer fluctuations with increasing Rayleigh number. The range of these scales as well as the probability of high-amplitude dissipation events decreases with increasing Prandtl number. In addition, we examine the joint statistics of the two dissipation fields and the consequences of high-amplitude events. We have also investigated the convergence properties of our spectral element method and have found that both dissipation fields are very sensitive to insufficient resolution. We demonstrate that global transport properties, such as the Nusselt number, and the energy balances are partly insensitive to insufficient resolution and yield correct results even when the dissipation fields are under-resolved. Our present numerical framework is also compared with high-resolution simulations which use a finite difference method. For most of the compared quantities the agreement is found to be satisfactory.
https://doi.org/10.1088/1367-2630/15/11/113063