The Role of Temperature in Shear Instability and Bifurcation of Internally Pressurized Deep Boreholes

2017 ◽  
Vol 50 (11) ◽  
pp. 3003-3017 ◽  
Author(s):  
Manman Hu ◽  
Manolis Veveakis ◽  
Thomas Poulet ◽  
Klaus Regenauer-Lieb
Keyword(s):  
Shear Instability ◽  
Deep Boreholes

scholarly journals Gravity Wave Emission by Shear Instability

2019 ◽  
Vol 49 (9) ◽  
pp. 2393-2406 ◽  
Author(s):  
Carsten Eden ◽  
Manita Chouksey ◽  
Dirk Olbers
Keyword(s):  
Gravity Wave ◽  
Single Layer ◽  
Internal Gravity Waves ◽  
Shear Instability ◽  
Vertical Shear ◽  
Wave Signal ◽  
Wave Emission ◽  
Balanced Flow ◽  
Single Layer Model

AbstractGravity wave emission by geostrophically balanced flow is diagnosed in numerical simulations of lateral and vertical shear instabilities. The diagnostic method in use allows for a separation of balanced flow and residual wave signal up to fourth order in the Rossby number (Ro). While evidence is found for a small but finite gravity wave emission from balanced flow in a single-layer model with large lateral shear and large Ro, a vertically resolved model with moderate velocity amplitudes appropriate to the interior ocean hardly shows any wave emission. Only when static instabilities generated by the shear instability of the balanced flow are allowed can a gravity wave signal similar to the ones reported in earlier studies be detected in the vertically resolved case. This result suggests a relatively small role of spontaneous wave emission in the classical sense of Lighthill radiation, and emphasizes the role of convective or symmetric instabilities during frontogenesis for the generation of internal gravity waves in the ocean and atmosphere.

The role of shear instability in amorphization of cold-rolled NiTi

1990 ◽  
Vol 62 (4) ◽  
pp. 257-264 ◽  
Author(s):  
Jun-ichi Koike ◽  
D. M. Parkin ◽  
M. Nastasi
Keyword(s):  
Shear Instability ◽  
Cold Rolled

Modeling the Role of Microstructure on Shear Instability with Reference to the Formability of Automotive Aluminium Alloys

2006 ◽  
Vol 519-521 ◽  
pp. 183-190 ◽  
Author(s):  
David S. Wilkinson ◽  
Xin Jian Duan ◽  
Ji Dong Kang ◽  
Mukesh K. Jain ◽  
J. David Embury
Keyword(s):  
Aluminium Alloys ◽  
Particle Density ◽  
Shear Bands ◽  
Rolling Direction ◽  
Shear Instability ◽  
Fe Model ◽  
Intermetallic Particles ◽  
Automotive Sheet ◽  
Cast Alloys

This paper addresses the effect of microstructure on the formability of aluminium alloys of interest for automotive sheet applications. The bulk of this work has been on the alloy AA5754 – both conventional DC cast alloys and continuous cast alloys made by twin belt casting. It is known that alloys such as these contain Fe as a tramp impurity which results in Fe-based intermetallic particles distributed through microstructure as isolated particles and in stringers aligned along the rolling direction. It is thought that these particles are the cause, both of the reduced ductility that is observed as the Fe level rises, and the relatively poor formability of strip cast alloys, as compared with those made by DC cast. Conventional wisdom suggests that the reduction of ductility is due to the effect of particles as nucleating sites for damage. However, most studies show that these materials are resistant to damage until just before fracture. We now believe that effect is actually related to the development of shear bands in these materials. We present experimental data which supports this conclusion. We then show how the FE models we have developed demonstrate the role of shear instability on fracture and the role played by hard particles. We show how a unit cell approach can be used to incorporate the effect of particle density and morphology on shear localization in a way that includes statistical variability due to microstructural heterogeneity. This leads to a set of constitutive equations in which the parameters are distributed from one region to another. These are then fed into a macroscopic FE model at the level of the specimen or the component in order to determine the effect of microstructural variability on shear instability and ductility.

A numerical investigation of fine particle laden flow in an oscillatory channel: the role of particle-induced density stratification

2010 ◽  
Vol 665 ◽  
pp. 1-45 ◽  
Author(s):  
CELALETTIN E. OZDEMIR ◽  
TIAN-JIAN HSU ◽  
S. BALACHANDAR
Keyword(s):  
Boundary Layer ◽  
Particle Transport ◽  
Fine Particle ◽  
Richardson Number ◽  
Density Stratification ◽  
Shear Instability ◽  
Bulk Richardson Number ◽  
Turbulence Modulation ◽  
Oscillatory Boundary Layer

Studying particle-laden oscillatory channel flow constitutes an important step towards understanding practical application. This study aims to take a step forward in our understanding of the role of turbulence on fine-particle transport in an oscillatory channel and the back effect of fine particles on turbulence modulation using an Eulerian–Eulerian framework. In particular, simulations presented in this study are selected to investigate wave-induced fine sediment transport processes in a typical coastal setting. Our modelling framework is based on a simplified two-way coupled formulation that is accurate for particles of small Stokes number (St). As a first step, the instantaneous particle velocity is calculated as the superposition of the local fluid velocity and the particle settling velocity while the higher-order particle inertia effect neglected. Correspondingly, only the modulation of carrier flow is due to particle-induced density stratification quantified by the bulk Richardson number, Ri. In this paper, we fixed the Reynolds number to be ReΔ = 1000 and varied the bulk Richardson number over a range (Ri = 0, 1 × 10−4, 3 × 10−4 and 6 × 10−4). The simulation results reveal critical processes due to different degrees of the particle–turbulence interaction. Essentially, four different regimes of particle transport for the given ReΔ are observed: (i) the regime where virtually no turbulence modulation in the case of very dilute condition, i.e. Ri ~ 0; (ii) slightly modified regime where slight turbulence attenuation is observed near the top of the oscillatory boundary layer. However, in this regime a significant change can be observed in the concentration profile with the formation of a lutocline; (iii) regime where flow laminarization occurs during the peak flow, followed by shear instability during the flow reversal. A significant reduction in the oscillatory boundary layer thickness is also observed; (iv) complete laminarization due to strong particle-induced stable density stratification.

scholarly journals The coexistence of the streaming instability and the vertical shear instability in protoplanetary disks

2020 ◽  
Vol 635 ◽  
pp. A190 ◽  
Author(s):  
Urs Schäfer ◽  
Anders Johansen ◽  
Robi Banerjee
Keyword(s):  
Mach Number ◽  
Protoplanetary Disks ◽  
Scale Height ◽  
Shear Instability ◽  
Vertical Shear ◽  
Two Dimensional ◽  
Dust Layer ◽  
Fundamental Nature ◽  
Streaming Instability

The streaming instability is a leading candidate mechanism to explain the formation of planetesimals. However, the role of this instability in the driving of turbulence in protoplanetary disks, given its fundamental nature as a linear hydrodynamical instability, has so far not been investigated in detail. We study the turbulence that is induced by the streaming instability as well as its interaction with the vertical shear instability. For this purpose, we employ the FLASH Code to conduct two-dimensional axisymmetric global disk simulations spanning radii from 1  to 100 au, including the mutual drag between gas and dust as well as the radial and vertical stellar gravity. If the streaming instability and the vertical shear instability start their growth at the same time, we find the turbulence in the dust midplane layer to be primarily driven by the streaming instability. The streaming instability gives rise to vertical gas motions with a Mach number of up to ~10−2. The dust scale height is set in a self-regulatory manner to about 1% of the gas scale height. In contrast, if the vertical shear instability is allowed to saturate before the dust is introduced into our simulations, then it continues to be the main source of the turbulence in the dust layer. The vertical shear instability induces turbulence with a Mach number of ~10−1 and thus impedes dust sedimentation. Nonetheless, we find the vertical shear instability and the streaming instability in combination to lead to radial dust concentration in long-lived accumulations that are significantly denser than those formed by the streaming instability alone. Therefore, the vertical shear instability may promote planetesimal formation by creating weak overdensities that act as seeds for the streaming instability.

scholarly journals Role of overturns in optimal mixing in stratified mixing layers

2017 ◽  
Vol 826 ◽  
pp. 522-552 ◽  
Author(s):  
A. Mashayek ◽  
C. P. Caulfield ◽  
W. R. Peltier
Keyword(s):  
Numerical Simulations ◽  
Ocean Circulation ◽  
Turbulent Mixing ◽  
Isotropic Turbulence ◽  
Life Cycles ◽  
Shear Instability ◽  
Mixing Efficiency ◽  
Small Scale ◽  
Mean Flow

Turbulent mixing plays a major role in enabling the large-scale ocean circulation. The accuracy of mixing rates estimated from observations depends on our understanding of basic fluid mechanical processes underlying the nature of turbulence in a stratified fluid. Several of the key assumptions made in conventional mixing parameterizations have been increasingly scrutinized in recent years, primarily on the basis of adequately high resolution numerical simulations. We add to this evidence by compiling results from a suite of numerical simulations of the turbulence generated through stratified shear instability processes. We study the inherently intermittent and time-dependent nature of wave-induced turbulent life cycles and more specifically the tight coupling between inherently anisotropic scales upon which small-scale isotropic turbulence grows. The anisotropic scales stir and stretch fluid filaments enhancing irreversible diffusive mixing at smaller scales. We show that the characteristics of turbulent mixing depend on the relative time evolution of the Ozmidov length scale $L_{O}$ compared to the so-called Thorpe overturning scale $L_{T}$ which represents the scale containing available potential energy upon which turbulence feeds and grows. We find that when $L_{T}\sim L_{O}$, the mixing is most active and efficient since stirring by the largest overturns becomes ‘optimal’ in the sense that it is not suppressed by ambient stratification. We argue that the high mixing efficiency associated with this phase, along with observations of $L_{O}/L_{T}\sim 1$ in oceanic turbulent patches, together point to the potential for systematically underestimating mixing in the ocean if the role of overturns is neglected. This neglect, arising through the assumption of a clear separation of scales between the background mean flow and small-scale quasi-isotropic turbulence, leads to the exclusion of an highly efficient mixing phase from conventional parameterizations of the vertical transport of density. Such an exclusion may well be significant if the mechanism of shear-induced turbulence is assumed to be representative of at least some turbulent events in the ocean. While our results are based upon simulations of shear instability, we show that they are potentially more generic by making direct comparisons with $L_{T}-L_{O}$ data from ocean and lake observations which represent a much wider range of turbulence-inducing physical processes.

scholarly journals The role of hydrochlorofluorocarbon densifiers in the formation of clathrate hydrates in deep boreholes and subglacial environments

2007 ◽  
Vol 47 ◽  
pp. 109-114 ◽  
Author(s):  
M. Mangir Murshed ◽  
Sérgio H. Faria ◽  
Werner F. Kuhs ◽  
Sepp Kipfstuhl ◽  
Frank Wilhelms
Keyword(s):  
Ice Core ◽  
Clathrate Hydrates ◽  
Deep Borehole ◽  
X Ray Diffraction ◽  
X Ray ◽  
Dronning Maud Land ◽  
Ice Conditions ◽  
Deep Boreholes ◽  
Dissolved Air

AbstractClear evidence for the formation of mixed clathrate hydrates of air and hydrochlorofluorocarbon densifier (known as HCFC-141b, sometimes also called R-141b) is found by means of synchrotron X-ray diffraction and Raman spectroscopy on a sample recovered from the bottom of the EPICA Dronning Maud Land deep borehole in Antarctica. Subglacial water (SGW) appears to have reacted with the drilling liquid to build a large lump of clathrate hydrate. The hydrate growth may well have been accelerated by the stirring of the SGW–densifier mixture during drilling. Moreover, dissolved air in the SGW appears to have participated in the formation of mixed hydrates of air and HCFC-141b as evidenced by the concomitant appearance of Raman signals from both constituents. Our findings elucidate to some extent the meaning of earlier accounts of the formation of ‘heavy chips’ that may sink to the bottom of the borehole, possibly affecting or even impeding the drilling advance. These observations raise concerns with respect to the use of HCFC-141b densifiers in ice-core drilling liquids under warm ice conditions.

Shear and Convective Turbulence in a Model of Thermohaline Intrusions

2007 ◽  
Vol 37 (10) ◽  
pp. 2534-2549 ◽  
Author(s):  
Rachael D. Mueller ◽  
William D. Smyth ◽  
Barry Ruddick
Keyword(s):  
Shear Instability ◽  
Convective Turbulence ◽  
Convective Mixing ◽  
Independent Laboratory ◽  
Numerical Studies ◽  
Thermohaline Intrusions ◽  
In Situ Observations ◽  
Parameter Values

Abstract Thermohaline interleaving is an important mechanism for laterally fluxing salt, heat, and nutrients between water masses. Interleaving is driven by a release of potential energy resulting from the differing diffusivities of heat and salt in seawater. The flows are composed of stacked intrusions that flux more and less buoyant water in opposite directions. In this paper, the role of shear instability caused by this juxtaposed motion is investigated. The model described in Walsh and Ruddick is modified to include both the effects of shear-induced turbulence and an improved convective mixing parameterization. Shear and convective mixing play a similar and significant role in interleaving dynamics. In the absence of either instability, cross-front fluxes are increased by approximately 30%. While in situ observations of horizontal diffusivity resulting from interleaving are not yet precise enough to calibrate the parameterizations independently, parameter values based on independent laboratory and numerical studies lead to diffusivity predictions that are within the error of the observations.

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