14:00
Rotating Flows 2
14:00
15 mins
#94
|
HEAT TRANSFER AND TEMPERATURE MEASUREMENTS IN EXTREME RAPIDLY ROTATING CONVECTION
Matteo Madonia, Jonathan Cheng, Andrés Aguirre Guzmán, Herman Clercx, Rudie Kunnen
Abstract: Rayleigh-Bénard convection and rapidly rotating flows are two topics that have been frequently explored by fluid dynamicists; the former is driven by buoyancy, proportional to the Rayleigh number Ra, and the latter is dominated by the Coriolis force, inversely proportional to the Ekman number Ek. The interiors of many celestial bodies (e.g. Earth’s dynamo and Jupiter’s interior) consist of fluids that experience these two effects at extreme values of the governing parameters. TROCONVEX (see figure 1 in the PDF version) is an experimental setup built to explore the geostrophic regime of rotating convection, a regime characterized by both strong thermal forcing and rapid rotation. Literature, for example reference [see reference 1 in the PDF version], suggests that it is not possible to extrapolate data of the geostrophic regime of rotating convection from data with Ek > 1e-7. Previous experiments and present-day numerical simulations have difficulties reaching these extreme parameters. TROCONVEX is able to cover more than 3 decades of Ek, reaching Ek ≈ 5e-9, by using a variety of tank heights (up to 4 m tall), allowing us to explore the geostrophic regime in unprecedented detail and check prior theoretical predictions made for this regime [see reference 2 in the PDF version].
Here we present results from TROCONVEX, showing heat transfer measurements (expressed in dimensionless form as the Nusselt number Nu) from different states of the geostrophic regime (see figure 2 in the PDF version). Various power-law scaling ranges can be inferred, indicating that many different flow morphologies are anticipated. With additional temperature measurements in the sidewall we can more clearly distinguish these transitions based on the mid-plane vertical temperature gradient. For Ra ≤ 2e10, a good agreement with accompanying direct numerical simulations (DNS) is observed. In the presentation these data will be extended with the first measurements in the 4 m tank.
|
14:15
15 mins
#310
|
Influence of internal heating on convection in the rotating spherical gap.
Florian Zaussinger, Christoph Egbers, Vadim Travnikov, Peter Haun
Abstract: Internal heating processes in the rotating spherical gap geometry are of great interest in geophysical- and astrophysical applications. Important sources for internal heating are thermonuclear reactions eg. in stars, tidal- and gravitational heating eg. moons and gas giants or the radioactive decay in terrestrial planets. Dielectric heating is a volumetric heating process which occurs in situations where an alternating electric field is applied on an insulating dielectric material (cf. in domestical micro-wave stoves). This effect can produce thermal convection through the thermo-electric coupling by the dielectrophoretic (DEP) force. The onset of convection and flow properties of the thermal convection under the influence of internal heating are investigated in a spherical gap geometry by means of an experiment under microgravity conditions on the ISS, linear stability analysis and numerical simulations. Two scenarios are investigated, namely the non-rotating case and the rotating case with Ekman numbers Ek=0.001, cf. Fig. 1 (top ;eft).
Numerical results are compared with interferograms from the GeoFlow II experiment to validate the model, Fig. 1 (top right). For the comparison, a numerical interferometry is applied to temperature fields obtained in the simulation, Fig. 1 (bottom). The onset of convection as well as global spatial properties of the resulting internally heated convective zone in the experiment are in good agreement with numerical simulations and analytical calculations.
|
14:30
15 mins
#508
|
Effects of thermal stratification on the axisymmetric state in spherical Couette flow
Tomoaki Itano, Taishi Inagaki, Fumitoshi Gotoh, Masako Sugihara-Seki
Abstract: The shear flow between double concentric spherical boundaries rotating differentially, Spherical Couette Flow (SCF), under the unstable thermal stratification, is numerically investigated. We restrict our attention in the first transition to the non-axisymmetric state in a wide gap case. It is elucidated that the axisymmetric state loses its stability against the non-axisymmetric disturbance due to the thermal perturbation, in the different ways depending on the rotation rate.
|
14:45
15 mins
#614
|
Localized structures and solitary states in a vertical Taylor-Couette system with a radial temperature gradient
Changwoo Kang, Arnaud Prigent, Innocent Mutabazi
Abstract: We present results from direct numerical simulations of a flow in a vertical large-aspect ratio Taylor-Couette system with a rotating inner cylinder and a fixed radial temperature gradient corresponding to a Grashof number Gr=4000. Different flow structures are computed for increasing the Taylor number Ta from 0 to 500. The resulting structures are (localized structures, solitary states, modulated vortices,...) are parametrized by the Richardson number Ri ~ Gr/Ta^2.
|
15:00
15 mins
#211
|
Transition in rotating plane Couette flow, revisited
Masato Nagata, Baofang Song, Darren P Wall
Abstract: The problem of transition from laminar flow to turbulence in plane Couette flow subject to a span-wise system rotation is considered, where the fluid motion is governed by two non-dimensional parameters, R (shear-induced Reynolds number) and Ω (system rotation rate). This system has long been serving as a testing ground for comparing experimental and theoretical results towards understanding mechanisms of transition in fluid dynamics. It is believed that the basic laminar state first loses stability to stream-wise independent disturbances with increasing R and/or Ω, resulting in the onset of a secondary flow with a stream-wise independent roll-cell structure (TV1), followed by successive supercritical bifurcations originating from three-dimensional instabilities of TV1. In the present paper, we re-examine this seemingly well-known instability property of TV1, especially for small R, and find some unexpected result.
We shall demonstrate that the solution branch for a three-dimensional tertiary flow (WVF) with small stream-wise wavenumbers resulting from the afore-mentioned secondary instability connects with the so-called Ribbon flow, which is composed of two tilted vortex flows, elongated in the stream-wise direction with the same but opposite tilted angles. Similar to TV1 this Ribbon flow bifurcates from the laminar basic state, and we find the bifurcation point coincides with that of TV1 in the limit of vanishing stream-wise wavenumber. Thus, the stream-wise independent secondary flow, TV1, becomes unstable to disturbances with small stream-wise wavenumbers as soon as it bifurcates from the basic state, and so no stable stream-wise independent roll-cell structures can exist.
It should be noted that one of the sides is bounded by a free surface and the other by a solid surface in experiments, whereas a periodic boundary condition in the span-wise direction has been applied in our theoretical analysis. Nevertheless, we conclude that the roll-cells, which are observed in experiments as the secondary flow, might actually be a three-dimensional tilted vortex flow with long stream-wise wave lengths, which exists intrinsically, being independent of the different types of boundary condition at the sides.
|
15:15
15 mins
#272
|
Effect of eccentricity in a counter-rotating Taylor-Couette flow
Kameswararao Anupindi, Dhaval Paghdar
Abstract: In the present work, we study the effect of eccentricity in a counter-rotating Taylor-Couette (CRTC) flow for two different Reynolds numbers 1500 and 4000 and for two different eccentricity ratios 0.2 and 0.5. The numerical methodology used is first validated with the reference data available in the literature for the concentric CRTC flow. In the eccentric CRTC system, the smaller-gap region seems to contain dense small-scale structures whereas the larger-gap region is devoid of these structures. The turbulent flow in the eccentric CRTC system is analyzed using mean and second-order statistics, instantaneous and mean velocity vectors and with the contours of turbulent kinetic energy. The physics of the eccentric CRTC system is studied by making comparisons to the concentric case.
|
15:30
15 mins
#129
|
Heat transfer in rotating wall-bounded flows
Geert Brethouwer
Abstract: The effect of system rotation on passive scalar transport in plane Couette flow is studied by DNS. It is found that at certain rotation rates the passive scalar transport is much faster than momentum transfer, which clearly violates the Reynolds analogy.
|
|