1,721,520 research outputs found
Dynamics of finite-sized light spheres in turbulence
No full textWe report experimental results on the Lagrangian dynamics of finite-size light particles in turbulence. Using an orthogonal camera setup and 3D particle tracking, we study the velocity and acceleration statistics of rigid light spheres in a water tunnel with nearly homogeneous and isotropic turbulence. The Reynolds number (ReY) is varied from 180 to 300, and the study covers a range of size ratios (4 < D/η < 16) for marginally light spheres. We find that the normalised acceleration PDF decreases in intermittency with increasing size ratio - in qualitative agreement with the predictions of the Faxén corrected model. We also present preliminary results on the rotational dynamics of large light spheres in turbulence
Direct numerical simulation of turbulent Taylor-Couette flow with grooved walls
No full textWe present direct numerical simulations of Taylor-Couette flow with grooved walls up to inner cylinder Reynolds number of , corresponding to Taylor number of . The simulations are performed at a fixed radius ratio . The grooves are axisymmetric V-shaped obstacles attached to the wall with a tip angle of . Results are compared with the smooth wall case in order to investigate the effects of the grooved walls. In particular, we focus on the effective scaling laws for torque, boundary layers and flow structures. With increasing , the boundary layer thickness finally becomes smaller than the groove height. When this happens, the plumes are ejected from tips of the grooves and a secondary circulation between the grooves is formed. This is associated with a sharp increase of the torque and thus the effective scaling law for the torque becomes much steeper. Further increasing does not result in an additional slope increases. Instead, the effective scaling law saturates to the same ``ultimate'' regime effective exponents seen for smooth walls
Direct Numerical Simulations of two-phase Taylor-Couette turbulence
No full textTwo-phase Taylor-Couette flow is simulated using the Euler-Lagrange approach, where the dispersed phase is treated as point particles with effective forces such as drag, lift, added mass and buoyancy acting on them. Two-way coupling is implemented between the carrier and the dispersed phase allowing us to study the interaction between the point like particles and the large scale flow structure in the carrier phase. Light buoyant particles are observed to be very effective in disrupting the coherent Taylor rolls, thus reducing the overall dissipation in the system and the overall driving torque
Geostrophic convective turbulence: The effect of boundary layers
No full textWe conduct computations of rotating Rayleigh-Bénard convection in the so-called geostrophic regime, characterized by strong thermal forcing (high Rayleigh numbers) and strong rotation (small Ekman numbers). We employ the full Navier-Stokes equations in our computations and compare no-slip and stress-free boundaries for the plates. The Ekman boundary layers, that exist in the no-slip case but not for stress-free, enhance convective heat transfer and prevent the formation of large-scale flow structures
Bulk statistics of stable and decaying Taylor-Couette turbulence
No full textIn this talk we focus on the velocity fluctuations in highly turbulent Taylor-Couette flow for the case of stable flow (constant rotation) and for decaying flow. Turbulent flows are generally characterized by the range of scales of their fluctuations, and a statistical description of the flow is often done by calculating the correlations of velocity fluctuations. These correlations are found to behave like power-laws over a range of scales, and their exponents characterize a certain geometry of flow. Many systems have been investigated carefully: Pipe-flow, Von Kármán flow, Rayleigh Bénard convection, \textit{et cetera}. There are, however, few reports \cite{lew99,she01} quantifying the turbulent properties in Taylor-Couette flow. In the presented work \cite{huisman2013b} we measure the longitudinal structure functions using laser Doppler anemometry, which is a non-intrusive technique and is able to measure the components of the velocity, and thus ideal for obtaining structure functions and the local velocity. We present the statistics of the turbulent velocity fluctuations for counter rotation for varying
Phase diagram of turbulent Taylor-Couette flow
No full textWe will present the results of our recent numerical work on the nature of the phase diagram of turbulent Taylor-Couette (TC) flow, both with co- and counterrotating cylinder. The work can be seen as the extension of the famous experimental Andereck et al. phase diagram for Taylor-Couette flow just above the onset of instabilities, towards the ultimate turbulence regime, and now obtained numerically. In particular, we will understand when and why optimal transport of angular velocity from the inner to the outer cylinder is achieved and how this is connected to the angular velocity profile and the structures in the flow
Boundary layers in turbulent vertical convection at high Prandtl number
No full textMany environmental flows arise due to natural convection at a vertical surface, from flows in buildings to dissolving ice faces at marine-terminating glaciers. We use three-dimensional direct numerical simulations of a vertical channel with differentially heated walls to investigate such convective, turbulent boundary layers. Through the implementation of a multiple-resolution technique, we are able to perform simulations at a wide range of Prandtl numbers Pr. This allows us to distinguish the parameter dependences of the horizontal heat flux and the boundary layer widths in terms of the Rayleigh number Ra and Prandtl number Pr. For the considered parameter range 1 ≤ Pr ≤ 100, 106 ≤ Ra ≤ 109, we find the flow to be consistent with a ‘buoyancy-controlled’ regime where the heat flux is independent of the wall separation. For given Pr, the heat flux is found to scale linearly with the friction velocity V∗. Finally, we discuss the implications of our results for the parameterisation of heat and salt fluxes at vertical ice–ocean interfaces
Turbulence decay towards the linearly stable regime of Taylor–Couette flow
Get PDFTaylor–Couette (TC) flow is used to probe the hydrodynamical (HD) stability of astrophysical accretion disks. Experimental data on the subcritical stability of TC flow are in conflict about the existence of turbulence (cf. Ji et al. (Nature, vol. 444, 2006, pp. 343–346) and Paoletti et al. (Astron. Astroph., vol. 547, 2012, A64)), with discrepancies attributed to end-plate effects. In this paper we numerically simulate TC flow with axially periodic boundary conditions to explore the existence of subcritical transitions to turbulence when no end plates are present. We start the simulations with a fully turbulent state in the unstable regime and enter the linearly stable regime by suddenly starting a (stabilizing) outer cylinder rotation. The shear Reynolds number of the turbulent initial state is up to and the radius ratio is . The stabilization causes the system to behave as a damped oscillator and, correspondingly, the turbulence decays. The evolution of the torque and turbulent kinetic energy is analysed and the periodicity and damping of the oscillations are quantified and explained as a function of shear Reynolds number. Though the initially turbulent flow state decays, surprisingly, the system is found to absorb energy during this decay
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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