Scale dependence of cloud microphysical response to turbulent entrainment and mixing. - In: Journal of advances in modeling earth systems, ISSN 1942-2466, Bd. 10 (2018), 11, S. 2777-2785
The dynamics and lifetime of atmospheric clouds are tightly coupled to entrainment and turbulent mixing. This paper presents direct numerical simulations of turbulent mixing followed by droplet evaporation at the cloud-clear air interface in a meter-sized volume, with an ensemble of up to almost half a billion individual cloud water droplets. The dependence of the mixing process on domain size reveals that inhomogeneous mixing becomes increasingly important as the domain size is increased. The shape of the droplet size distribution varies strongly with spatial scale, with the appearance of a pronounced negative exponential tail. The increase of relative dispersion during the transient mixing process is strongly dependent on the scale of the mixing and therefore on the Damköhler number, defined as the turbulence large-eddy time scale divided by the cloud supersaturation relaxation time.
https://doi.org/10.1029/2018MS001487
Numerical studies of turbulent Rayleigh-Bénard convection in closed cells : boundary layer dynamics and large scale patterns. - Ilmenau : Universitätsbibliothek, 2018. - 1 Online-Ressource (vi, 123 Seiten)
Technische Universität Ilmenau, Dissertation 2018
Die vorliegende zweiteilige Arbeit beschäftigt sich mit verschiedenen Aspekten der Rayleigh-Benard Konvektion (RBK) in geschlossenen zylindrischen und rechteckigen Zellen. Im ersten Teil wird ein theoretisches Modell für eine laminare Grenzschichtströmung entwickelt, in dem sowohl der Auftrieb und als auch der longitudinale Druckgradient erstmals gemeinsam berücksichtigt werden. Es basiert auf der klassischen Theorie von L. Prandtl. Dieses Modell soll eine bessere Übereinstimmung mit Daten aus dreidimensionalen, direkten numerischen Simulationen (DNS) von Konvektionsströmungen in einer zylindrischen Zelle mit zwei unterschiedlichen Prandtlzahlen aufweisen als bereits bestehende Modelle. Für eine niedrige Prandtlzahl konnte eine Verbesserung erreicht werden. Mit Einbeziehung des Auftriebs für höhere Prandtlzahlen ergeben sich signifikante Unterschiede. Eine mögliche Erklärung anhand von kohärenten Strukturen wird vorgestellt. Der zweite Teil dieser Arbeit bezieht sich auf DNS von turbulenten Konvektionsströmungen in einer geschlossenen, rechteckigen Zelle mit großem Aspektverhältnis. Bisher wurden die meisten numerischen Untersuchungen der RBK in geschlossenen Zylindern oder zwischen zwei horizontalen Platten ohne Seitenwände gemacht. Die Seitenwände beschränken die Nutzung effizienter numerischer Methoden, welche bei inkompressiblen Konvektionsströmungen angewandt werden. Eine hohe Performance der numerischen Methode ist besonders für RBK mit großem Aspektverhältnis von Vorteil. In der vorliegenden Arbeit wurde deshalb eine numerische Methode für die Lösung elliptischer partieller Differentialgleichungen in einer rechteckigen Zelle entwickelt, die auf Mehrgitterlösern aus einem vorhandenen Programmpaket basiert. Diese numerische Methode ist in ein bestehendes Programm für RBK integriert worden. Der so erweiterte numerische Code ermöglicht DNS von RBK mit großem Aspektverhältnis. Eine Fragestellung zur Ergodizität von turbulenter RBK wird hierbei näher untersucht. Dazu werden Daten genutzt, welche mit dem neuen Code aus DNS mit großem Aspektverhältnis in rechteckigen Zellen erhalten wurden. Die Ergodenhypothese besagt, dass das Zeitmittel der hydrodynamischen Felder mit deren Ensemble-Mittelwert übereinstimmt. Für turbulente Strömungen wurde die Ergodenhypothese bislang nicht bewiesen. Das Ziel der Untersuchungen ist es, Gemeinsamkeiten und Unterschiede zwischen Zeitmittel und Ensemblemittel zu finden. Es stellt sich heraus, dass bei einer Zeitmittelung über ein langes Zeitfenster große, kohärente Strukturen der Strömung bestehen bleiben. Bei Ensemble-Mittelung werden diese dagegen zerstört. Dennoch stimmen viele makroskopische Parameter und Momente zweiter Ordnung einiger Größen bei den zwei unterschiedlichen Arten der Mittelung überein. Die Analyse der turbulenten Viskosität und der turbulenten thermischen Diffusivität zeigt dagegen einige Unterschiede und erfordert weitere Untersuchungen.
https://nbn-resolving.org/urn:nbn:de:gbv:ilm1-2018000457
Probing turbulent superstructures in Rayleigh-Bénard convection by Lagrangian trajectory clusters. - In: Physical review fluids, ISSN 2469-990X, Bd. 3 (2018), 11, 113501, insges. 15 S.
https://doi.org/10.1103/PhysRevFluids.3.113501
Assessment of horizontal velocity fields in square thermal convection cells with large aspect ratio. - In: Experiments in fluids, ISSN 1432-1114, Bd. 59 (2018), 11, 171, S. 1-13
Transparent heating plates, consisting of glass coated with a transparent conductive metal oxide, are applied in large aspect ratio turbulent Rayleigh-Bénard convection (RBC) to investigate the large-scale patterns of velocity fields with optical flow measurement techniques across the whole horizontal cross section. The square convection cell with an aspect ratio [Gamma] = L/h = 10 was tested inside the scaled convective airflow laboratory experiment (SCALEX) facility which enables experiments with gases as working fluids for pressures of up to 10 bar to achieve very high Rayleigh numbers Ra. For the current study, Ra = 2 x 10^4 was applied. The velocity fields are measured with 2D3C particle image velocimetry (PIV). The possibility of reliable PIV measurements with reproducible homogenous temperature boundary conditions was demonstrated in the SCALEX facility. The seeding of the tracer particles, their illumination and data evaluation are addressed in detail. The final comparison of experimental data and numerical simulations shows a good agreement for the probability density functions of the horizontal velocity components. Deviations for the vertical out-of-plane velocity component and their dependence on the thickness of the laser sheet are discussed in detail and quantified by measurements with light sheets of different thickness.
https://doi.org/10.1007/s00348-018-2626-9
Electromagnetic flow rate measurement in molten tin circulating in a closed-loop test system. - In: 9th International Symposium on Electromagnetic Processing of Materials (EPM2018)14-18 October 2018, Hyogo, Japan, (2018), S. 012084, insges. 4 S.
https://doi.org/10.1088/1757-899X/424/1/012084
Experimental and numerical investigation on particle-induced liquid metal flow using Lorentz force velocimetry. - In: 9th International Symposium on Electromagnetic Processing of Materials (EPM2018)14-18 October 2018, Hyogo, Japan, (2018), S. 012006, insges. 4 S.
https://doi.org/10.1088/1757-899X/424/1/012006
Transition to turbulence scaling in Rayleigh-Bénard convection. - In: Physical review, ISSN 2470-0053, Bd. 98 (2018), 3, 033120, insges. 8 S.
If a fluid flow is driven by a weak Gaussian random force, the nonlinearity in the Navier-Stokes equations is negligibly small and the resulting velocity field obeys Gaussian statistics. Nonlinear effects become important as the driving becomes stronger and a transition occurs to turbulence with anomalous scaling of velocity increments and derivatives. This process has been described by Yakhot and Donzis [Phys. Rev. Lett. 119, 044501 (2017)] for homogeneous and isotropic turbulence. In more realistic flows driven by complex physical phenomena, such as instabilities and nonlocal forces, the initial state itself, and the transition to turbulence from that initial state, is much more complex. In this paper, we discuss the Reynolds-number dependence of moments of the kinetic energy dissipation rate of orders 2 and 3 obtained in the bulk of thermal convection in the Rayleigh-Bénard system. The data are obtained from three-dimensional spectral element direct numerical simulations in a cell with square cross section and aspect ratio 25 by Pandey et al. [Nat. Commun. 9, 2118 (2018)]. Different Reynolds numbers 1[less-than or equivalent to]Rel[less-than or equivalent to]1000 which are based on the thickness of the bulk region l and the corresponding root-mean-square velocity are obtained by varying the Prandtl number Pr from 0.005 to 100 at a fixed Rayleigh number Ra=10^5. A few specific features of the data agree with the theory. The normalized moments of the kinetic energy dissipation rate En show a nonmonotonic dependence for small Reynolds numbers before obeying the algebraic scaling prediction for the turbulent state. Implications and reasons for this behavior are discussed.
https://doi.org/10.1103/PhysRevE.98.033120
Improvement of organic solar cell morphology and device operation due to controlled polymer aggregation in solution. - In: Joint Meeting of the DPG and EPS Condensed Matter Divisions together with the Statistical and Nonlinear Physics Division of the EPS and the Working Groups: Equal Opportunities, Industry and Business, Young DPG, Philosophy of Physics, (all DPG) EPS Young Minds, EPS History of Physics Group, (2018), CPP 46.35
Reversals in infinite-Prandtl-number Rayleigh-Bénard convection. - In: Physical review, ISSN 2470-0053, Bd. 98 (2018), 2, 023109, insges. 11 S.
Using direct numerical simulations, we study the statistical properties of reversals in two-dimensional Rayleigh-Bénard convection for infinite Prandtl number. We find that the large-scale circulation reverses irregularly, with the waiting time between two consecutive genuine reversals exhibiting a Poisson distribution on long timescales, while the interval between successive crossings on short timescales shows a power-law distribution. We observe that the vertical velocities near the sidewall and at the center show different statistical properties. The velocity near the sidewall shows a longer autocorrelation and 1/f2 power spectrum for a wide range of frequencies, compared to shorter autocorrelation and a narrower scaling range for the velocity at the center. The probability distribution of the velocity near the sidewall is bimodal, indicating a reversing velocity field. We also find that the dominant Fourier modes capture the dynamics at the sidewall and at the center very well. Moreover, we show a signature of weak intermittency in the fluctuations of velocity near the sidewall by computing temporal structure functions.
https://doi.org/10.1103/PhysRevE.98.023109
Turbulent magnetohydrodynamic flow in a square duct: comparison of zero and finite magnetic Reynolds number cases. - In: Physical review fluids, ISSN 2469-990X, Bd. 3 (2018), 8, 083701, insges. 23 S.
Three-dimensional turbulent magnetohydrodynamic flow in a duct with a square cross section and insulating walls is investigated by direct numerical simulations. The flow evolves in the presence of a uniform vertical magnetic field and is driven by an applied mean pressure gradient. A boundary element technique is applied to treat the magnetic field boundary conditions at the walls consistently. Our primary focus is on the large- and small-scale characteristics of turbulence in the regime of moderate magnetic Reynolds numbers up to Rm ˜ 10^2 and a comparison of the simulations with the quasistatic limit at Rm = 0. The present simulations demonstrate that differences to the quasistatic case arise for the accessible magnetic Prandtl number Pm ˜ 10^-2 and different Hartmann numbers up to Ha = 43.5. Hartmann and Shercliff layers at the duct walls are affected differently when a dynamical coupling to secondary magnetic fields is present. This becomes manifest by the comparison of the mean streamwise velocity profiles as well as the skin friction coefficients. While large-scale properties change only moderately, the impact on small-scale statistics is much stronger as quantified by an analysis of local anisotropy based on velocity derivatives. The small-scale anisotropy is found to increase at moderate Rm. These differences can be attributed to the additional physical phenomena which are present when secondary magnetic fields evolve, such as the expulsion of magnetic flux in the bulk of the duct or the presence of turbulent electromotive forces.
https://doi.org/10.1103/PhysRevFluids.3.083701