Unstable Shear Research Articles

Association between ultrasound morphology of carotid plaque and carotid wall shear rate in patients with atherosclerosis of peripheral arteries

Aim– to investigate the relationship between the values of the carotid endothelial shear rate and ultrasound morphology of atherosclerotic plaques in carotid arteries.Material and methods. The study involved 70 patients with carotid atherosclerosis, including 43 males and 27 females. The average age of patients was (61.1±8.54) years. All patients underwent ultrasound triplex scanning of carotid arteries. Ultrasonic plaque morphology was evaluated using several ultrasonographic characteristics: uniformity of echotexture and echogenicity. We used the classification of G. Geraulakos et al. (1993). Endothelial shear rate in the common carotid artery was determined in accordance with the law of Hagen – Poiseuille.Results. Most patients (30) had a plaque type I, less frequently occurred plaque II (15) and III (20 patients) types. Homogeneous echo positive plaques corresponding to «stable» phenotype were found in 5 patients. In patients with plaque type I and II, endothelial shear rate values were 373 (305; 481) s–1 and 311 (282; 419) s–1 respectively. In the groups of patients with plaque– type III and IV endothelial shear rate was significantly higher – 500 (429; 556) s–1 and 470 (440; 512) s–1 respectively. Among patients with plaque with a predominance of echolucent component, more patients with type 2 diabetes mellitus were found (p=0.006).Conclusion. Carotid endothelial shear rate was significantly lower in patients with carotid atherosclerosis and echolucent plaques compared to those having predominantly hyperechoic plaque. Prevalence of diabetes mellitus type 2 was significantly higher in the group of patients with unstable plaque and low carotid endothelial shear rate.

Effects of Axisymmetric Square-Wave Excitation on Transverse Jet Structure and Mixing

The influence of temporal square-wave excitation on structural and mixing characteristics of an equidensity, gaseous jet in crossflow (JICF) was explored in the present study. As in separate unforced and sinusoidally excited JICF experiments, acetone planar laser-induced fluorescence imaging enabled this detailed quantification for the JICF for mean jet-to-crossflow momentum flux ratios ranging from (with a convective unstable upstream shear layer, or USL, in the absence of forcing) to (with a globally unstable USL). Such square-wave excitation of the jet fluid required adaptive feedforward control, not only to create more accurate temporal square waveforms but to enable more accurate comparison among alternative forcing conditions. Square-wave excitation of the JICF demonstrated a significant influence on the naturally globally unstable JICF, where specific nondimensional stroke ratios within -dependent ranges could produce deeply penetrating, periodic vortices with improved jet penetration and spread. Enhanced jet penetration did not always correlate with better molecular mixing, however; there was a stronger correlation of improved mixing at higher values with creation of a more symmetric jet cross section via square-wave excitation, especially one with a clear counter-rotating vortex pair structure.

Effect of yaw angle on flow structure and cross-flow force around a circular cylinder

Flow around an inclined circular cylinder at yaw angles of α = 0°, 30°, 45°, and 60° has been numerically studied using the delayed detached eddy simulation at a Reynolds number of 1.4 × 104. Periodic boundary conditions are utilized to minimize the end effect. The focus is to explore the effect of yaw angle on the flow structure and the spatial distribution of the cross-flow forces. For the normal flow case, the modulation of the span-wise averaged lift force coefficient is found to be related to the unstable shear layer. For the inclined cases, contours of the sectional lift force coefficient show that the local vortex shedding staggers in time along the axial span at the early stage of the simulation, when the flow approaches the cylinder. After the flow reaches the quasi-periodic state, the axial difference disappears for α > 45° but not for α = 30°. In particular, the axial difference of the sectional lift force coefficient results in a near-zero value of the span-wise averaged lift force coefficient. The transition from a two-dimensional flow to a three-dimensional one is not captured in the current simulation. However, wake visualization indicates a mitigation of von Kármán vortex shedding when the yaw angle is greater than 30°. Although the Strouhal number is well predicted by the Independence Principle (IP), other flow properties are less agreeable with the prediction by IP.

Чисельне моделювання потоку в’язкої нестисливої рідини крізь циліндричні порожнини

The flow of viscous incompressible fluid in a cylindrical duct with two serial diaphragms is studied by the numerical solution of the unsteady Navier–Stokes equations. The discretization procedure is based on the finite volume method using the TVD scheme for the discretization of the convective terms and second order accurate in both space and time difference schemes. The resulting system of non-linear algebraic equations is solved by the PISO algorithm. It is shown that the fluid flow in the region between the diaphragms is nonstationary and is characterized by the presence of an unstable shear layer under certain parameters. A series of ring vortices is formed in the shear layer that causes quasi-periodic self-sustained oscillations of the velocity and pressure fields in the orifice of the second diaphragm. There can be four self-sustained oscillation modes depending on the length of the cavity formed by the diaphragms. With the increase in the distance between the diaphragms, the frequency of oscillations decreases within the same self-oscillation mode and rises sharply with the switch to the next mode.

Turbulent intensity effect on axial-flow-induced cylinder vibration in the presence of a neighboring cylinder

Experimentally investigation is conducted on the turbulent intensity effect on the axial-flow-induced vibration of an elastic cylinder in the presence of a neighboring rigid cylinder. The center-to-center cylinder spacing P∗ (= P/D, where D is the cylinder diameter) is 1.21, 1.36, 1.57 and ∞ (an isolated cylinder). The dimensionless velocity U¯ (=U∞L(mf∕EI)1∕2, where U∞, L, mf, EI, are the free-stream velocity, cylinder length, added mass of fluid per unit length and cylinder flexural rigidity, respectively) is 0.64 ∼ 6.92, and two ranges of incident turbulent intensity Tu are 0.71%–0.80% and 2.30%–2.91%. The cylinder vibration and the flow are simultaneously captured using two laser Doppler vibrometers and a time-resolved particle image velocimetry, respectively. It is found that the level of Tu may have a pronounced effect on the vibration of the elastic cylinder. At high Tu (2.30–2.91%), the vibration amplitude of the elastic cylinder is significantly increased compared with its counterpart at low Tu (0.71–0.80%). Two factors are found to be responsible. Firstly, the instability of the shear layer around the elastic cylinder is amplified through the interaction between the incident flow fluctuation of the high Tu and the unstable shear layer around the vibrating cylinder. As a result, eddies are prone to separation from the cylinder wall, increasing the cylinder-wall pressure fluctuations. Secondly, the eddies are observed to bounce out and then bounce back, thus enhancing unsteady fluid forces.

Evolution and characteristics of forced shear flows in polytropic atmospheres: large and small Péclet number regimes

Complex mixing and magnetic field generation occurs within stellar interiors particularly where there is a strong shear flow. To obtain a comprehensive understanding of these processes, it is necessary to study the complex dynamics of shear regions. Due to current observational limitations, it is necessary to investigate the inevitable small-scale dynamics via numerical calculations. Here, we examine direct numerical calculations of a local model of unstable shear flows in a compressible polytropic fluid primarily in a two-dimensional domain, where we focus on determining how key parameters affect the global properties and characteristics of the resulting saturated turbulent phase. We consider the effect of varying both the viscosity and the thermal diffusivity on the non-linear evolution. Moreover, our main focus is to understand the global properties of the saturated phase, in particular estimating for the first time the spread of the shear region from an initially hyperbolic tangent velocity profile. We find that the vertical extent of the mixing region in the saturated regime is generally determined by the initial Richardson number of the system. Further, the characteristic quantities of the turbulence, i.e. typical length-scale and the root-mean-square velocity are found to depend on both the Richardson number, and the thermal diffusivity. Finally, we present our findings of our investigation into saturated flows of a `secular' shear instability in the low P\'eclet number regime with large Richardson numbers.

Extension Strain and Rock Strength Limits for Deep Tunnels, Cliffs, Mountain Walls and the Highest Mountains

Brittle rock can fail in tension even when all principal stresses are compressive. The culprit is Poisson’s ratio, but marked stress anisotropy due to a neighbouring free surface, and due to a raised principal tangential stress is also necessary. Extension strain-induced failure causes fracture initiation in tension. Propagation in unstable shear may occur if the tunnels or mine openings are deep enough, and if they are located in hard, brittle, sparsely jointed rock. Both in laboratory uniaxial compression test samples with strength σc and in deep tunnels, extension fracturing and acoustic emission begin when the principal applied or induced stress reaches the magnitude of tensile strength divided by Poisson’s ratio σt/ν. The traditionally expected fracture initiation when the principal or maximum tangential stress σ1 or σθ = 0.4 ± 0.1 × σc can actually be explained with arithmetic. Using related logic, cliffs and the near-vertical mountain walls frequented by rock climbers, may have erosional or glacial origin, but extension strain limits their height, including vertical walls of sheeting joints and long continuous fractures. Shear failure seems to be reserved for occasional major rock avalanches. Equations with soil mechanics origin involving Coulomb parameters c and φ and density that may apply to vertical cuts in soil, give greatly exaggerated heights for rock cliffs and mountain walls since rock is brittle and favours failure in tension. Tensile strength, Poisson’s ratio and density are suggested for estimating the maximum heights of rock cliffs and mountain walls, not compression strength and density. However, overall mountain heights are limited by critical state maximum shear strength, or by the slightly lower brittle–ductile transition strength.

A novel approach to assess the hydrodynamic effects of a salmon farm in a Patagonian channel: Coupling between regional ocean modeling and high resolution les simulation

The interaction between local hydrodynamics conditions and salmon cages are important in disease transmission and in the transport of waste products generated by aquaculture. We propose a modeling methodology to assess the hydrodynamic effects of a salmon farm in a Patagonian channel. The method is based on the coupling between a Coastal and Regional Ocean COmmunity model (CROCO) and a high resolution Large Eddy Simulation (LES) model. A salmon farm located in the Estero Elefantes Channel (45°39′16.50″S–73°35′59.40″W) was used as a study case. The physical coherence of the results was validated by comparison with field current velocities measured with an Acoustic Doppler Current Profiler (ADCP). The model predictions were capable of reproducing the tendencies of the variation although there were some differences in magnitude. The results showed that the hydrodynamics of the Estero Elefantes Channel is dominated by two circulation modes depending on the current direction in the adjacent Moraleda channel. Both circulation modes are characterized by a highly unstable shear flow composed by turbulent structures that interact with the salmon cages. Therefore it is not possible to select a local control volume with arbitrarily defined inlets, outlets and impermeable walls to evaluate hydrodynamic processes relevant to the salmon farms. The presence of the cages modifies the natural hydrodynamics of the channel, attenuating the intensity of the local velocity magnitude and generating recirculation and retention zones near them. However, their effect is not confined locally because the perturbations introduced by the presence of cages are propagated far from them. The transport of material discharged inside the cages was also analyzed. This information should prove useful both for producers and the aquaculture management authority.

Compressibility effects on a shear flow in strongly coupled dusty plasma. I. A study using computational fluid dynamics

We study compressibility effects on the two-dimensional strongly coupled dusty plasma by means of computational fluid dynamics (CFD) with the Kolmogorov flow as an initial shear flow profile. Nonlinear compressible vortex flow dynamics and other linear and nonlinear properties of such flow in the presence of variable density, pressure, and electrostatic potential are addressed using a generalised compressible hydrodynamic model. The stabilizing effect of compressibility on the unstable shear flows in the presence of strong correlation (τm>0) is presented. Increasing the Mach number relatively reduces the growth-rate of perturbation. On the other hand, strong correlation makes the medium to be more unstable and increases the growth rate. Using an eigen value solver, various linear properties of compressible Kolmogorov flow have been investigated for a range of variable parameters, for example, Mach number, Reynolds number, and viscoelastic coefficient (τm). Compressible Kolmogorov flow becomes unstable above a critical value of the Reynolds number (Rc), and below Rc, the shear flow is found to be neutrally stable. In this study, it is found that the viscoelasticity reduces the value of Rc. For our choice of parameters, at τm=τmc, the compressible Kolmogorov flow becomes unconditionally unstable and no Rc exists for values of τm higher than τmc. To address the nonlinear properties, for example, mode-mode interaction due to the presence of nonlinearity in the fluid, vortex formation, etc., a massively parallelized Advanced Generalized SPECTral Code (AG-Spect) has been developed. AG-Spect, a newly developed code, is an efficient tool to solve any set of nonlinear fluid dynamic equations. A good agreement in linear growth rates obtained from the eigen value solver and time dependent simulation (AG-Spect) is found. In our CFD study, the suppression of instability, elongated vortex structures, pattern formation, nonlinear saturation, and visco-elastic oscillations in perturbed kinetic energy have been observed for various values of Mach number, Reynolds number and τm.

Influence of the velocity field on scalar transport in gaseous transverse jets

The present experiments explored the dynamical character of the gaseous jet injected flush into cross-flow for variable jet-to-cross-flow momentum flux ratios$J$(5, 12 and 41) and density ratios$S$(0.35 and 1.0). Contoured nozzle and straight pipe injectors were studied here, with the jet Reynolds number fixed at 1900 as other flow parameters were varied. Simultaneous acetone planar laser-induced fluorescence (PLIF) imaging and stereo particle image velocimetry (PIV) were used to study the relationships between scalar and velocity/vorticity fields, with a special focus on comparing PLIF-based extraction of scalar dissipation rates and local strain rates with PIV-based local strain rates in the upstream and downstream shear layers of the jet. There was remarkable similarity between the scalar and vorticity fields for the jet in cross-flow, spanning conditions for absolutely unstable upstream jet shear layers at low$J$or$S$values to conditions for convectively unstable shear layers for larger$J$, equidensity conditions (Megerianet al.,J. Fluid Mech., vol. 593, 2007, pp. 93–129; Getsingeret al.,Exp. Fluids, vol. 53, 2012, pp. 783–801). Proper orthogonal decomposition applied to both scalar and velocity fields revealed strengthening instabilities in both the upstream shear layer and in the jet’s wake as$J$was reduced. The simultaneous measurements allowed PLIF-extracted scalar dissipation rates and strain rates to be determined via a flamelet-like model and compared with PIV-extracted strain rates, each in the diffusion layer-normal direction. There was generally very good qualitative and quantitative agreement for these metrics in both the jet upstream and downstream shear layers for most flow conditions, with excellent correspondence to locations of shear layer vorticity roll up, although downstream shear layer strain rates in some cases showed lesser correspondence between PLIF- and PIV-based data. Such differences are shown to potentially result from diffusion and resolution effects as well as the influence of three-dimensional and transient effects which can be more significant in the lee side of the jet. Nevertheless, the present results reveal interesting dynamics and demonstrate the importance of strain fields in enhanced diffusion and transport phenomena.

Quasi-periodic and irregular motion of a solid sphere falling through a thixotropic yield-stress fluid

We report the observation of the oscillatory and irregular motion of solid spheres settling under the influence of gravity in a thixotropic yield-stress fluid, namely, a suspension of Laponite. The size of the ball and the aging time of the Laponite suspension are found to be two important parameters that determine whether oscillations occur. The irregular motion may be related to the existence of an unstable flow region and shear banding as is concluded from comparisons with rheological measurements, namely, the flow curve and creep tests, using the same Laponite suspensions.

Pulsating turbulence in a marginally unstable stratified shear flow

We describe a simple model for turbulence in a marginally unstable, forced, stratified shear flow. The model illustrates the essential physics of marginally unstable turbulence, in particular the tendency of the mean flow to fluctuate about the marginally unstable state. Fluctuations are modelled as an oscillatory interaction between the mean shear and the turbulence. The interaction is made quantitative using empirically established properties of stratified turbulence. The model also suggests a practical way to estimate both the mean kinetic energy of the turbulence and its viscous dissipation rate. Solutions compare favourably with observations of fluctuating ‘deep cycle’ turbulence in the equatorial oceans.

Coupling of damped and growing modes in unstable shear flow

Analysis of the saturation of the Kelvin-Helmholtz instability is undertaken to determine the extent to which the conjugate linearly stable mode plays a role. For a piecewise-continuous mean flow profile with constant shear in a fixed layer, it is shown that the stable mode is nonlinearly excited, providing an injection-scale sink of the fluctuation energy similar to what has been found for gyroradius-scale drift-wave turbulence. Quantitative evaluation of the contribution of the stable mode to the energy balance at the onset of saturation shows that nonlinear energy transfer to the stable mode is as significant as energy transfer to small scales in balancing energy injected into the spectrum by the instability. The effect of the stable mode on momentum transport is quantified by expressing the Reynolds stress in terms of stable and unstable mode amplitudes at saturation, from which it is found that the stable mode can produce a sizable reduction in the momentum flux.

Stop whistling! A note on fluid driven whistles in flow ducts

The generation mechanism and possible counter measures for fluid driven whistles in low Mach number flow duct networks are discussed. The vortex sound model, where unstable shear layers interact with the acoustic field and act as amplifiers under certain boundary conditions, is shown to capture the physics well. Further, for the system to actually whistle, an acoustic feedback to the amplifying shear layer is also needed. The demonstration example in this study is a generalized resonator configuration with annular volumes attached to a straight flow duct via a number of small holes, perforations, around the duct’s circumference. At each hole, a shear layer is formed and the acoustic reflections from the resonator volumes and the up and downstream sides provides a possible feedback to them. The attenuation properties as well as the whistling frequencies at varying inlet mean flow velocities for this system are studied both numerically and experimentally showing that good quality predictive simulations are possible using the vortex sound theory. Finally, a few countermeasures against whistling are tested. Both the feedback and the shear layers are manipulated. Best effect was found disturbing the shear layers by covering the holes with a coarse mesh.

Mountain Waves, Downslope Winds, and Low-Level Blocking Forced by a Midlatitude Cyclone Encountering an Isolated Ridge

Abstract The interaction of a midlatitude cyclone with an isolated north–south mountain barrier is examined using numerical simulation. A prototypical cyclone develops from an isolated disturbance in a baroclinically unstable shear flow upstream of the ridge, producing a cold front that interacts strongly with the topography. The structure and evolution of the lee waves launched by the topography are analyzed, including their temporal and their north–south variation along the ridge. Typical mountain wave patterns are generated by a 500-m-high mountain, but these waves often exhibit significant differences from the waves produced in 2D or 3D simulations with steady large-scale-flow structures corresponding to the instantaneous conditions over the mountain in the evolving flow. When the mountain height is 2 km, substantial wave breaking occurs, both at low levels in the lee and in the lower stratosphere. Despite the north–south uniformity of the terrain profile, large north–south variations are apparent in wave structure and downslope winds. In particular, for a 24-h period beginning after the cold front passes the upstream side of the ridge toward the south, strong downslope winds occur only in the northern half of the lee of the ridge. Just prior to this period, the movement of the cold front across the northern lee slopes is complex and accompanied by a burst of strong downslope winds and intense vertical velocities.

Annual and Interannual Variability of the Tropical Instability Vortices in the Equatorial Eastern Pacific Observed from Lagrangian Surface Drifters

Abstract This study documents the spatial distributions and temporal variations of anticyclonic eddies with identified radii ≥100 km in the equatorial eastern tropical Pacific Ocean [viz., tropical instability vortices (TIVs)] using Lagrangian surface drifters. The TIVs identified from Lagrangian surface drifters are distributed in a band along 5°N and are closely associated with latitudinal barotropically unstable shear between the westward South Equatorial Current (SEC) and the eastward North Equatorial Countercurrent (NECC). Fewer TIVs are identified from February to June when the shear between the SEC and NECC is weak, whereas more TIVs are found from July to January when the shear is enhanced. The number of identified TIVs also exhibits substantial interannual variability, with fewer TIVs identified during El Niño events and more TIVs found during La Niña events. This relationship is likely associated with the interannual variations of the zonal circulation in the equatorial Pacific modulated by El Niño–Southern Oscillation (ENSO).

Shear-banding in wormlike micelles: Beware of elastic instabilities

We report on the shear banding flow of semidilute and concentrated wormlike micelle systems which have been probed in the recent past using rheology, velocimetry, and small angle neutron scattering techniques. We show that the samples under consideration exhibit unstable shear banding flow due to the development of flow instabilities reminiscent of elastic or inertio-elastic instabilities and turbulence. Evidence for such unstable shear banding flows can be obtained either from direct visualizations, flow visualizations or two-dimensional velocimetry. The presence or not of instabilities on top of shear banding flows cannot be deduced from global rheology and/or 1D velocimetry alone. These flow instabilities are associated with more or less complex flow kinematics and appear to be ubiquitous in the flow of wormlike micelles. Using a pedagogical example, we show that their presence can strongly impact microstructural characterizations of the banded state.

Turbulent jet flow generated downstream of a low temperature dielectric barrier atmospheric pressure plasma device.

Flowing low temperature atmospheric pressure plasma devices have been used in many technological applications ranging from energy efficient combustion through to wound healing and cancer therapy. The generation of the plasma causes a sudden onset of turbulence in the inhomogeneous axisymmetric jet flow downstream of the plasma plume. The mean turbulent velocity fields are shown to be self-similar and independent of the applied voltage used to generate the plasma. It is proposed that the production of turbulence is related to a combination of the small-amplitude plasma induced body forces and gas heating causing perturbations in the unstable shear layers at the jet exit which grow as they move downstream, creating turbulence.

Controls on Turbulent Mixing in a Strongly Stratified and Sheared Tidal River Plume

AbstractConsiderable effort has been made to parameterize turbulent kinetic energy (TKE) dissipation rate ε and mixing in buoyant plumes and stratified shear flows. Here, a parameterization based on Kunze et al. is examined, which estimates ε as the amount of energy contained in an unstable shear layer (Ri < Ric) that must be dissipated to increase the Richardson number Ri = N2/S2 to a critical value Ric within a turbulent decay time scale. Observations from the tidal Columbia River plume are used to quantitatively assess the relevant parameters controlling ε over a range of tidal and river discharge forcings. Observed ε is found to be characterized by Kunze et al.’s form within a factor of 2, while exhibiting slightly decreased skill near Ri = Ric. Observed dissipation rates are compared to estimates from a constant interfacial drag formulation that neglects the direct effects of stratification. This is found to be appropriate in energetic regimes when the bulk-averaged Richardson number Rib is less than Ric/4. However, when Rib > Ric/4, the effects of stratification must be included. Similarly, ε scaled by the bulk velocity and density differences over the plume displays a clear dependence on Rib, decreasing as Rib approaches Ric. The Kunze et al. ε parameterization is modified to form an expression for the nondimensional dissipation rate that is solely a function of Rib, displaying good agreement with the observations. It is suggested that this formulation is broadly applicable for unstable to marginally unstable stratified shear flows.

Adaptive simulations of viscous detonations initiated by a hot jet using a high-order hybrid WENO–CD scheme

In the present work a sixth-order hybrid WENO–Centered Difference (CD) scheme with an adaptive mesh refinement method is employed to investigate gaseous detonation by injecting a hot jet into a hydrogen–oxygen combustible mixture flowing at supersonic speed. Two-dimensional reactive Navier–Stokes (NS) equations with one-step two-species chemistry model are solved numerically. The comparison between viscous and inviscid detonation structures shows that due to the absence of both the physical viscosity in Euler equations and minimization of numerical dissipation in the hybrid WENO–CD scheme, very small-scale vortices can be observed behind the detonation front. The diffusion effect in the NS equations suppresses the small-scale vortices, but it has negligible influence on the large-scale vortices generated by Richtmyer–Meshkov (RM) instability and those along the highly unstable shear layers induced by Kelvin–Helmholtz (KH) instability. When studying the same setup in an expanding channel and beyond the point of detonation initiation, it is found that because of the diffusion effect of detached shear layers, any unburned jet flow is consumed quickly and then additional energy is released periodically. Because of the formation of multiple secondary triple points and subsequent shear layers after the shutdown of the hot jet, a highly turbulent flow is produced behind the detonation front. Rather than the commonly known RM instability, the large-scale vortices involved in the highly unstable shear layers dominate the formation of the turbulent flow and the rapid turbulent mixing between the unburned jet flow and burned product. It is found that the size of unburned jets and vortices due to KH instability is growing for larger expansion angles. The further generated turbulent flow resulting from larger sized vortices, significantly enhances the mixing rate behind the Mach stem, leading to rapid consumption of the unburned reactants. Therefore, detonations propagate faster in channels with larger expansion angle and higher expansion ratio.