Rigid Channel Research Articles

Lithospheric structure and deformation in SE Tibet revealed by ambient noise and earthquake surface wave tomography: Recent advances and perspectives

High-resolution lithospheric structure is essential for understanding the tectonic evolution and deformation patterns of the southeastern Tibetan plateau. This is now possible due to recent advances in ambient noise and earthquake surface wave tomography, and great improvements in data coverage from dense portable array stations deployed in SE Tibet. In this review paper, I first give a brief overview of the tomographic methods from ambient noise and earthquake surface waves, and then summarize the major findings about the lithospheric structure and deformation in SE Tibet revealed by ambient noise and earthquake surface wave tomography as well as by other seismic and geophysical observations. These findings mainly include the 3-D distribution of mechanically weak zones in the mid-lower crust, lateral and vertical variations in radial and azimuthal anisotropy, possible interplay of some fault zones with crustal weak zones, and importance of strike-slip faulting on upper crustal deformation. These results suggest that integration of block extrusion in the more rigid upper-middle crust and channel flow in the more ductile mid-lower crust will be more compatible with the current geophysical observations. Finally I discuss some future perspective researches in SE Tibet, including array-based tomography, joint inversion using multiple seismic data, and integration of geodynamic modeling and seismic observations.

Exoribonuclease and Endoribonuclease Activities of RNase BN/RNase Z both Function in Vivo

Escherichia coli RNase BN, a member of the RNase Z family of endoribonucleases, differs from other family members in that it also can act as an exoribonuclease in vitro. Here, we examine whether this activity of RNase BN also functions in vivo. Comparison of the x-ray structure of RNase BN with that of Bacillus subtilis RNase Z, which lacks exoribonuclease activity, revealed that RNase BN has a narrower and more rigid channel downstream of the catalytic site. We hypothesized that this difference in the putative RNA exit channel might be responsible for the acquisition of exoribonuclease activity by RNase BN. Accordingly, we generated several mutant RNase BN proteins in which residues within a loop in this channel were converted to the corresponding residues present in B. subtilis RNase Z, thus widening the channel and increasing its flexibility. The resulting mutant RNase BN proteins had reduced or were essentially devoid of exoribonuclease activity in vitro. Substitution of one mutant rbn gene (P142G) for wild type rbn in the E. coli chromosome revealed that the exoribonuclease activity of RNase BN is not required for maturation of phage T4 tRNA precursors, a known specific function of this RNase. On the other hand, removal of the exoribonuclease activity of RNase BN in a cell lacking other processing RNases leads to slower growth and affects maturation of multiple tRNA precursors. These findings help explain how RNase BN can act as both an exo- and an endoribonuclease and also demonstrate that its exoribonuclease activity is capable of functioning in vivo, thus widening the potential role of this enzyme in E. coli.

Chebyshev collocation approach to stability of blood flows in a large artery

In this study, treating the large artery as a rigid channel with uniform width and the blood as an incompressible Newtonian fluid with variable viscosity due to transverse variation in hematocrit ratio, the basic flow structure and its temporal stability to small disturbances were studied. A fourth-order Eigenvalues problem which reduces to the well known Orr–Sommerfeld equation in some limiting cases was obtained and solved numerically by a spectral collocation technique with expansions in Chebyshev polynomials implemented in MATLAB. Graphical results for the basic flow axial velocity, disturbance growth rate and marginal stability curve are presented and discussed. It is worth pointing out that a transverse increase in the blood hematocrit ratio towards the central region of the artery had a stabilizing effect on the flow. Key words: Arterial blood flow, hematocrit ratio, variable viscosity, temporal stability, Chebyshev spectral collocation technique.

In situ pressure measurement within deformable rectangular polydimethylsiloxane microfluidic devices

In this paper, we present a simple procedure to incorporate commercially available external pressure transducers into existing microfluidic devices, to monitor pressure-drop in real-time, with minimal design modifications to pre-existing channel designs. We focus on the detailed fabrication steps and assembly to make the process straightforward and robust. The work presented here will benefit those interested in adding pressure drop measurements in polydimethylsiloxane (PDMS) based microchannels without having to modify existing channel designs or requiring additional fabrication steps. By using three different devices with varying aspect ratio channels ([Formula: see text], width/depth), we demonstrate that our approach can easily be adapted into existing channel designs inexpensively. Furthermore, our approach can achieve steady state measurements within a matter of minutes (depending on the fluid) and can easily be used to investigate dynamic pressure drops. In order to validate the accuracy of the measured pressure drops within the three different aspect ratio devices, we compared measured pressure drops of de-ionized water and a 50 wt. % glycerol aqueous solution to four different theoretical expressions. Due to the deformability of PDMS, measured pressure drops were smaller than those predicted by the rigid channel theories (plate and rectangular). Modification of the rigid channel theories with a deformability parameter α provided better fits to the measured data. The elastic rectangular expression developed in this paper does not have a geometric restriction and is better suited for microchannels with a wider range of aspect ratios.

Separation Zone in Flow past a Spur Dyke on Rigid Bed Meandering Channel

AbstractFlow past a spur dyke on a rigid bed meandering channel with a trapezoidal cross section has been studied experimentally. An acoustic Doppler velocimeter (ADV) was used to measure the velocities. The results show that length of the downstream separation zone changes according to the location of the spur dyke. It varies from 4.0 to 22.8 times the spur dyke length. In addition, the separation zone at higher elevations is wider compared to that near the bed for most of the locations of the spur dyke. The effect of contraction ratio and inflow Froude number on separation zone is also presented.

Enhanced control of light and sound trajectories with three-dimensional gradient index lenses

We numerically study the focusing and bending effects of light and sound waves through heterogeneous isotropic cylindrical and spherical devices. We first point out that transformation optics and acoustics show that the control of light requires spatially varying anisotropic permittivity and permeability, while the control of sound is achieved via spatially anisotropic density and isotropic compressibility. Moreover, homogenization theory applied to electromagnetic and acoustic periodic structures leads to such artificial (although not spatially varying) anisotropic permittivity, permeability and density. We stress that homogenization is thus a natural mathematical tool for the design of structured metamaterials. To illustrate the two-step geometric transform-homogenization approach, we consider the design of cylindrical and spherical electromagnetic and acoustic lenses displaying some artificial anisotropy along their optical axis (direction of periodicity of the structural elements). Applications are sought in the design of Eaton and Luneburg lenses bending light at angles ranging from 90° to 360°, or mimicking a Schwartzchild metric, i.e. a black hole. All of these spherical metamaterials are characterized by a refractive index varying inversely with the radius which is approximated by concentric layers of homogeneous material. We finally propose some structured cylindrical metamaterials consisting of infinitely conducting or rigid toroidal channels in a homogeneous bulk material focusing light or sound waves. The functionality of these metamaterials is demonstrated via full-wave three-dimensional computations using nodal elements in the context of acoustics, and finite edge-elements in electromagnetics.

Stability of two-dimensional collapsible-channel flow at high Reynolds number

AbstractWe study the linear stability of two-dimensional high-Reynolds-number flow in a rigid parallel-sided channel, of which part of one wall has been replaced by a flexible membrane under longitudinal tension ${T}^{\ensuremath{\ast} } $. Far upstream the flow is parallel Poiseuille flow at Reynolds number $\mathit{Re}$; the width of the channel is $a$ and the length of the membrane is $\lambda a$, where $1\ll {\mathit{Re}}^{1/ 7} \lesssim \lambda \ll \mathit{Re}$. Steady flow was studied using interactive boundary-layer theory by Guneratne & Pedley (J. Fluid Mech., vol. 569, 2006, pp. 151–184) for various values of the pressure difference ${P}_{e} $ across the membrane at its upstream end. Here unsteady interactive boundary-layer theory is used to investigate the stability of the trivial steady solution for ${P}_{e} = 0$. An unexpected finding is that the flow is always unstable, with a growth rate that increases with ${T}^{\ensuremath{\ast} } $. In other words, the stability problem is ill-posed. However, when the pressure difference is held fixed (${= }0$) at the downstream end of the membrane, or a little further downstream, the problem is well-posed and all solutions are stable. The physical mechanisms underlying these findings are explored using a simple inviscid model; the crucial factor in the fluid dynamics is the vorticity gradient across the incoming Poiseuille flow.

FLOW THROUGH NONUNIFORM CHANNEL WITH PERMEABLE WALL AND SLIP EFFECT

An investigation of viscous incompressible fluid flow in a nonuniform rigid channel with permeable walls is presented by taking into consideration the influence of slip velocity at the walls. The effect of fluid absorption through permeable walls is accounted for by prescribing flux as a function of axial distance. The nonlinear governing equations of motion are linearized by the perturbation method by assuming ‐ (the ratio of inlet width to the length of the channel) as a small parameter to get an approximate analytical solution. The effects of reabsorption coefficient (a), slope parameter (k), and slip parameter (fl) on the velocity profiles, mean pressure drop, and wall shear stress are studied and presented graphically. The results indicate that the slip parameter influences the flow field considerably.

Packing of Amantadine-Like Compounds in the Central Cavity of Influenza a M2

Over 291 variants of amantadine, mw 113.2 to 437.5, have been historically used as therapeutic agents for influenza A. These amantadine-like molecules have been thought to function as intrachannel blockers in the M2 channel of the influenza A virus. We explored whether these drugs fit in the central cavity of the channel using simple simulation methods with the CHARMM force field, the atomic coordinates of the solid state NMR structure for the M2 channel (PDB ID 2L0J, model 1), and a full set of amantadine-like molecules, with variations and titration states, gleaned from the literature. First, we examined the optimal water packing in the central cavity of the rigid channel using randomized, annealed water insertion, which indicates that optimally it holds 882 to 1116 Daltons of water. Drugs were inserted into water-filled channels with waters deleted based on various oxygen radius parameters. A radius of 2.2 A was found to give optimal water packing. Sterically, the amantadine-like molecules from the literature all fit readily within the central cavity, judging from drug energies inside the channel compared to those of the drugs relaxed in vacuo. Future work will focus on the relative energetics of drug binding and configuration and the solvation of amantadine-like molecules in explicit water within the central cavity. Support is from NIH AI23007 and a Mentoring Grant from BYU.

Reconfigurable virtual electrowetting channels

Lab-on-a-chip systems rely on several microfluidic paradigms. The first uses a fixed layout of continuous microfluidic channels. Such lab-on-a-chip systems are almost always application specific and far from a true "laboratory." The second involves electrowetting droplet movement (digital microfluidics), and allows two-dimensional computer control of fluidic transport and mixing. The merging of the two paradigms in the form of programmable electrowetting channels takes advantage of both the "continuous" functionality of rigid channels based on which a large number of applications have been developed to date and the "programmable" functionality of digital microfluidics that permits electrical control of on-chip functions. In this work, we demonstrate for the first time programmable formation of virtual microfluidic channels and their continuous operation with pressure driven flows using an electrowetting platform. Experimental, theoretical, and numerical analyses of virtual channel formation with biologically relevant electrolyte solutions and electrically-programmable reconfiguration are presented. We demonstrate that the "wall-less" virtual channels can be formed reliably and rapidly, with propagation rates of 3.5-3.8 mm s(-1). Pressure driven transport in these virtual channels at flow rates up to 100 μL min(-1) is achievable without distortion of the channel shape. We further demonstrate that these virtual channels can be switched on-demand between multiple inputs and outputs. Ultimately, we envision a platform that would provide rapid prototyping of microfluidic concepts and would be capable of a vast library of functions and benefitting applications from clinical diagnostics in resource-limited environments to rapid system prototyping to high throughput pharmaceutical applications.

Smoke evacuation in single-incision laparoscopic surgery

To the Editor, We read with interest the article by Dauser et al. describing the use of a peripheral venous catheter placed intraperitoneally to evacuate smoke during single-incision laparoscopic surgery (SILS) [1]. The proliferation of SILS has posed new challenges in the steps that surgeons take for granted during multiport laparoscopic surgery, and efficient evacuation of smoke to provide a clear view of the operative field is one of them. Many of the special access ports designed for SILS lack a dedicated smoke extraction channel. The problem becomes even more challenging when performing SILS using the ‘‘single-skin-incision, multi-trocar’’ approach. Most plastic, low-profile trocars used in this approach have a rubber cap with a membrane valve provided to maintain the pneumoperitoneum, but these trocars make evacuation of the smoke rather cumbersome. Even when using some of the low-profile trocars that have a built-in gas port on the side, smoke does not escape easily once a 5-mm instrument is introduced through it. Also, repeated opening of the cap covering the gas port by an assistant to release smoke may hinder the surgeon using an operative instrument through the trocar. We have used two methods for evacuation of smoke during SILS. The most common SILS procedure we undertake is cholecystectomy and it is performed without any special access ports or roticulating instruments. We use two trocars (one metal, 10 mm, and another low-profile, 5 mm) placed through separate fascial incisions along with two traction sutures on the gallbladder [2]. After using plastic low-profile trocars in several initial cases, we have now shifted to a 5-mm, low-profile, rigid metal trocar designed for thoracoscopic surgery (Karl Storz, Tuttlingen, Germany). As this trocar has a cap without a membrane valve, smoke can be evacuated easily by intermittently removing the operative instrument or during exchange of instruments. Moreover, use of this reusable, easy-to-clean, and easy-to-sterilize trocar makes SILS cost-effective. However, a drawback of using this method is the need to interrupt the operation momentarily for evacuation of smoke. Intraperitoneal placement of a Veress needle to vent the pneumoperitoneum that occurs accidentally during a totally extraperitoneal inguinal hernia repair is well described. Based on the same principle, we insert a Veress needle through the fascia alongside the trocars in all SILS procedures (Fig. 1). The camera person or nurse opens the valve on the Veress needle intermittently to release the smoke without interrupting the operation or disturbing the surgeon. We find that using a Veress needle in this manner is advantageous as it is readily available, its long length allows it to be used in patients of all body habitus, and the rigid metal channel does not collapse when gripped by a thick abdominal wall (as is sometimes likely when a peripheral venous catheter is used). As a Veress needle is guarded by a blunt obturator, there is no danger of visceral injury when using this method, even though the tip of the needle is not in constant view of the endocamera.

Synthesis of a Novel Class of Cyclodextrin-Based Nanotubes

The synthesis and characterization of a novel class of structurally well-defined nanotubes from β-cyclodextrin are described. These new hosts were formed using disulfide linkages that substitute all the primary hydroxyl groups of a β-cyclodextrin. A deep and rigid hydrophobic channel with a size of more than 1.5 nm is found in the molecules. Because of their unique geometry and the potential biodegradability of the disulfide bond, this class of molecules could find broad applications in biology and other areas of research.

Large-amplitude and narrow-band vibration phenomenon of a foursquare fix-supported flexible plate in a rigid narrow channel

An experiment was performed to analyze the flow-induced vibration behavior of a foursquare fix-supported flexible plate exposed to the axial flow within a rigid narrow channel. The large-amplitude and narrow-band vibration phenomenon was observed in the experiment when the flow velocity varied with the range of 0–5 m/s. The occurring condition and some characteristics of the large-amplitude and narrow-band vibrations were investigated.

Simplified Accurate Solution for Design of Erodible Trapezoidal Channels

The sides and bottoms of earthen irrigation channels are erodible. The main design criterion for the earthen channel is to minimize or eliminate channel erosion under design flow conditions. The Manning formula and the tractive force equation were used as governing equations to design erodible and riprap channels. The traditional design approach involves a tedious trial procedure to find the bottom width of trapezoidal channels. Some hydraulic engineering problems require the solution of the bottom width from the Manning formula for nonerodible channels. An approximate equation is available for determining channel bottom width, but it is not applicable for channels with side slopes z<1 (1 vertical and z horizontal), which are encountered in practice for nonerodible channels. This technical note presents a direct and accurate equation for determining the channel bottom width (with errors less than 0.1%) using the tractive force concept and a dimensionless form of the Manning formula. The proposed direct solution facilitates the design of erodible and riprap-lined channels. Moreover, the proposed equation can be used to accurately determine the bottom width of rigid (nonerodible) channels for any side slope.

Sloshing and slamming oscillations in a collapsible channel flow

We consider laminar high-Reynolds-number flow through a finite-length planar channel, where a portion of one wall is replaced by a thin massless elastic membrane that is held under longitudinal tension T and subject to a linear external pressure distribution. The flow is driven by a fixed pressure drop along the full length of the channel. We investigate the global stability of two-dimensional Poiseuille flow using a method of matched local eigenfunction expansions, which is compared to direct numerical simulations. We trace the neutral stability curve of the primary oscillatory instability of the system, illustrating a transition from high-frequency ‘sloshing’ oscillations at high T to vigorous ‘slamming’ motion at low T. Small-amplitude sloshing at high T can be captured using a low-order eigenmode truncation involving four surface-based modes in the compliant segment of the channel coupled to Womersley flow in the rigid segments. At lower tensions, we show that hydrodynamic modes increasingly contribute to the global instability, and we demonstrate a change in the mechanism of energy transfer from the mean flow, with viscous effects being destabilizing. Simulations of finite-amplitude oscillations at low T reveal a generic slamming motion, in which the flexible membrane is drawn close to the opposite rigid wall before recovering rapidly. A simple model is used to demonstrate how fluid inertia in the downstream rigid channel segment, coupled to membrane curvature downstream of the moving constriction, together control slamming dynamics.

Stability of pressure-driven flow in a deformable neo-Hookean channel

The stability of pressure-driven flow in a rectangular channel with deformable neo-Hookean viscoelastic solid walls is analysed for a wide range of Reynolds numbers (from Re ≪ 1 to Re ≫ 1) by considering both sinuous and varicose modes for the perturbations. Pseudospectral numerical and asymptotic methods are employed to uncover the various unstable modes, and their stability boundaries are determined in terms of the solid elasticity parameter Γ = Vη/(ER) and the Reynolds number Re = RV ρ/η; here V is the maximum velocity of the laminar flow, R is the channel half-width, η and ρ are respectively the viscosity and density of the fluid and E is the shear modulus of the solid layer. We show that for small departures from a rigid solid, wall deformability could have a destabilizing or stabilizing effect on the Tollmien–Schlichting (TS) instability (a sinuous mode) depending on the solid-layer thickness. Upon further increase in solid deformability, the TS mode coalesces with another unstable mode (absent in rigid channels) giving rise to a single unstable mode which extends to very low Reynolds number (&lt;1) for highly deformable walls. There are other types of instabilities that exist only due to wall deformability. In the absence of inertia (Re = 0), there is a short-wave instability of both sinuous and varicose modes arising due to the discontinuity of the first normal stress difference across the fluid–solid interface. For both sinuous and varicose modes, it is shown that inclusion of inertia is important even for Re ≪ 1, wherein a new class of long-wavelength unstable modes are predicted which are absent at Re = 0. These unstable modes are a type of shear waves in an elastic solid which are destabilized by the flow. These long- and short-wave instabilities are absent if a simple linear elastic model is used for the solid. At intermediate and high Re, upstream and downstream travelling waves of both sinuous and varicose modes become unstable. We show that sinuous and varicose modes become critical in different parameter regimes, thereby demonstrating the importance of capturing all the unstable modes. Inclusion of dissipative effects in the neo-Hookean model is generally shown to play a stabilizing role on the instabilities due to both sinuous and varicose modes. The predicted instabilities will be important for the flow of liquids (with viscosity ≥ 10−3 Pa s) in deformable channels of width ≤1 mm, and with shear modulus ≤ 105 Pa.

Vibration and stability of a constrained elastica with variable length

This paper studies the vibration and stability of an elastica constrained by a pair of symmetrically placed parallel plane walls. One end of the elastica is fully clamped, while the other end is allowed to slide through a rigid channel under edge thrust. In order to take into account the variation of the contact points between the elastica and the walls during vibration, an Eulerian version of the equations of motion is adopted. It is found that the lowest few natural frequencies approach and remain degenerately zero when point-contact deformations evolve to line-contact patterns. As a consequence, the stability of all line-contact deformations before secondary buckling cannot be determined from the linear vibration analysis. A load-controlled experiment was conducted to find that the elastica jumped from one-point to two-point, and then to three-point contact with the walls without going through any line-contact deformations. These experimental observations are different from the results reported previously by others with different set-ups, in which line-contact deformations did exist. Explanations based on experimental evidences and theoretical analyses are provided to confirm the validity of these previous investigations and clarify the cause of the difference.

Deformations and Stability of an Elastica Subjected to an Off-Axis Point Constraint

This paper studies, both theoretically and experimentally, the deformation, the vibration, and the stability of a buckled elastic strip (also known as an elastica) constrained by a space-fixed point in the middle. One end of the elastica is fully clamped while the other end is allowed to slide without friction and clearance inside a rigid channel. The point constraint is located at a specified height above the clamping plane. The elastic strip buckles when the pushing force reaches the conventional buckling load. At this buckling load, the elastica jumps to a symmetric configuration in contact with the point constraint. As the pushing force increases, a symmetry-breaking bifurcation occurs and the elastica evolves to one of a pair of asymmetric deformations. As the pushing force continues to increase, the asymmetric deformation experiences a second jump to a self-contact configuration. A vibration analysis based on an Eulerian description taking into account the sliding between the elastica and the point constraint is described. The natural frequencies and the stability of the calculated equilibrium configurations can then be determined. The experiment confirms the two jumps and the symmetry-breaking bifurcation predicted theoretically.

Bedload resistance in supercritical flow

Abstract The movement of bedload in subcritical flow produces additional roughness as compared to flow in a rigid bed. The magnitude of this bed load roughness is proportional to the thickness of the sediment layer moving along the bed, the particle size and the sediment concentration. In a supercritical flow, however, further resistance is expected due to the momentum absorption by the high flow velocity. In this study the effect of sediment movement on the flow resistance in supercritical flow was experimentally investigated. The experiments included flows over smooth and rough beds carrying sediment of mean diameters D 50=2.80, 5.42 and 7.06 mm in a rigid rectangular channel. The results show that the sediment transport may increase the friction factor by up to 90% and 60% in smooth and rough beds, respectively. Bedload extracts its momentum from the flow, which causes a reduction of near bed flow velocity and steeper velocity gradient near the bed resulting in an increase in shear velocity as well as in roughness height. The increase in friction factor is directly related to bedload concentration and particle size.

The effect of the structure of heterogeneous media on the evaporation of liquids under intensive thermal stimulation

Mass-transfer processes in heterogeneous media under intensive thermal stimulation are investigated. The structure of a capillary-porous material is simulated by rigid conic channels of convergent and divergent types. The viscous flow of vapor at temperatures above the boiling point of evaporating liquid is considered in a quasi-stationary approximation. Expressions for the rate of evaporation from axisymmetric capillaries of variable cross section are obtained in an analytical form. Results are given of estimation of the intensity of vaporization in channels of different types.