Shear Fluctuations Research Articles

Stress fluctuations and shear thickening in dense granular suspensions

We experimentally investigate the rheology and stress fluctuations of granules densely suspended in silicone oil. We find that both thickening strength and stress fluctuations significantly weaken with oil viscosity $\eta_0$. Comparison of our rheological results to the Wyart-Cates model for describing different dynamic jamming states suggests a transition from frictional contacts to lubrication interactions as $\eta_0$ increases. To clarify the contribution from viscous interactions to the rheology, we systematically measure stress fluctuations in various flow states. Reduction of stress fluctuations with $\eta_0$ indicates that a strong lubrication layer greatly inhibits force correlations among particles. Measuring stress fluctuations in the strong shear thickening regime, we observe a crossover from asymmetric Gamma to symmetric Gaussian distributions and associated with it a decrease of lateral (radial) correlation length $\xi$ with increasing shear rate.

4D-CT angiography versus 3D-rotational angiography as the imaging modality for computational fluid dynamics of cerebral aneurysms

Background and purposeComputational fluid dynamics (CFD) can provide valuable information regarding intracranial hemodynamics. Patient-specific models can be segmented from various imaging modalities, which may influence the geometric output and thus...

Combustion of H2/air supersonic mixing layers with splitter plate: Growth rates and transport characteristic

H2/air supersonic mixing layers were calculated with different splitter plates thickness in both inert and reactive cases. The decay of mixing layer growth rates with the presence of heat release indicates the inhibition of fluctuations of velocity and scalar. Analyses of mean pressure and mean scalar show that the combustion efficiency increases with thickening of splitter plate. Heat release effect strengthens local compressibility, and then the relation among local compressibility, mean shear and velocity fluctuations is built. Considering heat release effect, the peak value of turbulent kinetic energy of moderate thickness splitter plates (the thickness equals 4 mm) is smallest. Similar phenomena are observed in scalar variance and vorticity variance, indicating the turbulent development of moderate thickness splitter plates is slowest.

Long-lived predator-prey dynamics in the pedestal of near-zero torque high performance DIII-D plasmas

Quiescent high performance plasmas (often termed QH-mode) are attractive due to the replacement of potentially damaging energy and particle releases known as edge localized modes by relatively benign edge harmonic oscillations (EHOs). These EHOs are believed to be driven unstable by edge current and/or edge toroidal rotational shear and contribute to edge particle transport. Decreasing the applied neutral beam torque in standard QH-mode discharge leads to an improved quiescent phase of higher and wider pedestal, also known as the wide-pedestal QH-mode [Burrell et al., Phys. Plasmas 23, 056103 (2016)]. This work expands upon the observed limit cycle oscillation (LCO) dynamics [Barada et al., Phys. Rev. Lett. 120, 135002 (2018)] in this wide pedestal QH-mode. The onset of these LCOs after wide-pedestal transition is found to be correlated with the disappearance of coherent EHOs which happens either when the edge maximum bootstrap current decreases after the transition or when the toroidal rotation is decreased consistent with simulation predictions. Sustainment of this quasistationary oscillating regime is found to be possible due to a predator-prey type competition between E × B velocity shear and turbulence density fluctuations facilitated by an inward propagation of nonzonal flow like toroidally and poloidally symmetric E × B velocity perturbations from these LCOs. These LCO dynamics are further controlled by adding electron cyclotron heating (ECH) to a neutral beam heated wide-pedestal QH-mode discharge which led to a surprising increase in energy confinement correlated with a concomitant decrease in edge turbulence in contrast to normally observed confinement degradation in H-mode with ECH.

A review of factors affecting the efficiency of clean-in-place procedures in closed processing systems

This paper reviews the current state of researches on improvement of Clean-In-Place (CIP) procedures in closed processing system thus saving energy, with a special attention paid to the hydrodynamic effects of cleaning fluid and the numerical and experimental approaches to investigate the identified controlling factors. The paper discussed the fouling problems of processing plants and the importance of sufficient CIP procedures, the forces contributing to cleaning with a special focus on the hydrodynamic effects. In general, it is possible to enhance hydrodynamic removal forces by local introduction of, among others, high wall shear stress and fluctuation rate of wall shear stress without consuming more energy. A theoretical model of particle removal in flow was also reviewed which supports the factors identified. The paper therefore further reviewed and compared the current state of modelling and experimental techniques on CIP improvement. To simulation the CIP process, it is necessary to consider 3D time-resolved Large Eddy Simulation with a Hybrid RANS-LES WMLES as Sub-Grid-Scale model because it captures both the mean and fluctuation rate of flow variables, while affordable for industrial flows. The wall shear stress measurement techniques and cleanablity test methods were also discussed and suggested.

Impact of Wind Veer and the Coriolis Force for an Idealized Farm to Farm Interaction Case

The impact of the Coriolis force on the long distance wake behind wind farms is investigated using Large Eddy Simulations (LES) combined with a Forced Boundary Layer (FBL) technique. When using the FBL technique any mean wind shear and turbulent fluctuations can be added with body forces. The wind shear can also include the mean wind veer due to the Coriolis force. The variation of the Coriolis force due to local deviations from the mean profile, e.g., from wakes, is not taken into account in the FBL. This can be corrected for with an extra source term in the equations, hereon defined as the Coriolis correction. For a row of 4 turbines it is shown that the inclusion of the wind veer turns the wake to the right, while including the Coriolis correction turns it to the left. When including both wind veer and Coriolis correction the impact of wind veer dominates. For an idealized farm to farm interaction case, two farms of 4 ∗ 4 turbines with 6 km in between, it can be seen that when including wind veer and the Coriolis correction a approximately 3% increase in the relative production for a full wake direction can be seen and only a slightly smaller increase can be seen when including only wind veer. The results indicate that FBL can be used for studies of long distance wakes without including a Coriolis correction but efforts need to be taken to use a wind shear with a correct mean wind veer.

Fluctuation and self-diffusion research about dry granular materials under shearing

Flow and self-diffusion behaviors of granules in sheared granular systems are a typical study field but still not yet fully grasped because of the complexities and discreteness of granular matter systems. This paper presents a discrete element method-based simulation research of the fluctuation and self-diffusion characteristics of granules in a dense sheared granular system. Simulation results show that the average velocity of granules decreases with the altitude location, which is reverse to the variation of fluctuation velocity. It is noted that the model channel can be classified into “solid-like,” “oscillating” and “fluid-like” region according to the magnitude of shear rate and fluctuation velocity of granules. The self-diffusion of granules is greater in the “solid-like” region, but smaller in the “fluid-like” region. These findings emphasize the importance of research on macroscopic dynamics of granules, such as re-arrangement and self-diffusion of granular matter systems.

Buoyancy-induced effects on large-scale motions in differentially heated vertical channel flows studied in direct numerical simulations

Direct numerical simulations of turbulent convection in a differentially heated vertical channel flow with Prandtl number Pr=0.71 are conducted with a fourth-order accurate finite volume method for a bulk Reynolds number Reb=4328 and three Grashof numbers Gr ∈ {0, 6.4 · 105, 9.5 · 105}. The discussion of instantaneous flow field snapshots, first- and second-order moments, budget equations, power density spectra and quadrant analyses shows that the turbulent velocity fluctuations are attenuated in the aiding flow and enhanced in the opposing flow. In contrast, temperature fluctuations are attenuated in the opposing flow and enhanced in the aiding flow. The analyses further reveal that the low-momentum flow structures in the aiding flow are warmer than the high-momentum flow structures and vice versa in the opposing flow. Due to their different temperatures, buoyancy accelerates and decelerates the flow structures differently, which leads to reduced and increased pressure and shear fluctuations in the aiding and opposing flow. Thus, the redistribution of turbulent velocity fluctuations is lower in the aiding flow and higher in the opposing flow. The Reynolds shear stresses are consequently decreased in the former and increased in the latter, influencing the production of streamwise velocity fluctuations accordingly. In summary, the discussed physical mechanisms underline the indirect effect of buoyancy on the turbulent velocity fluctuations.

Evaluation of three mainstream numerical weather prediction models with observations from meteorological mast IJmuiden at the North Sea

AbstractNumerical weather prediction models play an important role in the field of wind energy, for example, in power forecasting, resource assessment, wind farm (wake) simulations, and load assessment. Continuous evaluation of their performance is crucial for successful operations and further understanding of meteorology for wind energy purposes. However, extensive offshore observations are rarely available. In this paper, we use unique met mast and Lidar observations up to 315 m from met mast “IJmuiden,” located in the North Sea 85 km off the Dutch coast, to evaluate the representation of wind and other relevant variables in three mainstream meteorological models: ECMWF‐IFS, HARMONIE‐AROME, and WRF‐ARW, for a wide range of weather conditions. Overall performance for hub‐height wind speed is found to be comparable between the models, with a systematic wind speed bias <0.5 m/s and random wind speed errors (centered RMSE) <2 m/s. However, the model performance differs considerably between cases, with better performance for strong wind regimes and well‐mixed wind and potential temperature profiles. Conditions characterized by moderate wind speeds combined with stable stratification, which typically produce substantial wind shear and power fluctuations, lead to the largest misrepresentations in all models.

Insights on pulsed bubble control of membrane fouling: Effect of bubble size and frequency

A three-dimensional Computational Fluid Dynamics (CFD) model was developed to study the shear stress induced by spherical cap bubbles on industrial scale hollow fibres. Simulations and experimental data were compared for water and a 0.5 g/L xanthan gum solution that mimicked the rheological behaviour of activated sludge. The model was built on a pilot-scale membrane tank configured with five 1.5 m long hollow fibre membranes. 58–290 ml spherical cap bubbles were sparged into the system at frequencies from 0.2 to 1 Hz. The Volume of Fluid (VOF) method coupled with the Realizable k–ε turbulent model was used to simulate the transient behaviour of cap bubbles rising in a Newtonian and a viscous shear-thinning liquid. CFD prediction of root mean square (RMS) shear stress induced by pulsed bubbles exhibited an inverse relationship with experimental observations of membrane fouling. At a constant flux of 25 L/m2/h (LMH), fouling decreased from 5.1 to 3.9 kPa/min as simulated RMS shear stress increased from 0.04 to 0.25 Pa. Shear fluctuations, which are considered to be beneficial for the cleaning of membranes, were less pronounced for a non-Newtonian fluid compared to in water. Increasing pulse bubble size from 115 ml to 290 ml led to a 80% increase of RMS shear stress, and therefore improved the fouling control efficiency.

Communication—Tribology of Retaining Rings in Chemical Mechanical Planarization

Stribeck and Stribeck+ curves helped determine the tribological mechanisms in the ring-slurry-pad interface. Both methods gave consistent results with the lubrication mechanism starting at “boundary lubrication” and transitioning to “mixed lubrication” as pseudo-Sommerfeld numbers increased. COF for PPS rings were higher than PEEK. They were also higher for inter-level dielectric (ILD) processes as compared to copper. Stribeck curves were used to infer wear rate information about the ring, which in conjunction with Stribeck+ curves could help choose process parameters that balanced wafer removal with ring wear. Data cluster shapes were shown to be due to shear and normal force fluctuations.

Numerical investigation on convective heat transfer to aviation kerosene flowing in vertical tubes at supercritical pressures

Numerical simulations of convective heat transfer to aviation kerosene (China RP-3) flowing in vertical circular tubes at supercritical pressures are reported in this study. Firstly, performance of a variety of Reynolds-Averaged Navier-Stokes turbulence models in predicting the fluid-thermal behaviours under both forced and mixed convection conditions are evaluated. Under forced convection conditions, all models predict a gentler growth of wall temperature along the flow direction than experimental measurements. Under mixed convection conditions, the effect of buoyancy become significant and there are large discrepancies in the predicted wall temperature by different models. Only the low-Reynolds number k-ε models are found to be able to qualitatively predict the flow laminarization and heat transfer deterioration. Profiles of thermal, flow and turbulence fields obtained using various models are studied to explain the differences in predictions. For mixed convection conditions, an examination on the turbulence production due to shear and density fluctuation indicates that the direct effect of buoyancy on the turbulence production is negligible compared with the indirect effect. Furthermore, the effect of turbulent Prandtl number on the predicted heat transfer is studied. It is found that turbulent Prandtl number has a significant influence on the simulation results. Under the conditions considered in the present study, the value of 1.0 for turbulent Prandtl number leads to a closest agreement with the experimental data.

Modeling grain fragmentation and deformation textures for titanium using a combined approach of the viscoplastic self-consistent model and a shear fluctuation model

A new grain fragmentation model is proposed for the formation of grain substructures in polycrystalline materials during plastic deformation. Micro-shear bands and twins are assumed to originate from grain boundaries due to the local stress concentrations caused by the interaction of grain boundaries and slip bands. These stress concentrations will impose shear fluctuations on the neighboring grains, which can be relaxed by the formation of corresponding substructures, i.e., micro-shear bands and twins, in these neighboring grains. Texture simulations for cold rolling and equal channel angular pressing (ECAP) of commercially pure titanium are performed using a combined approach of the viscoplastic self-consistent (VPSC) model and the shear fluctuation model. The texture update scheme in the combined approach is presented. Experimental textures at different thickness reductions of cold rolling at room temperature (300 K) and after the first pass of ECAP at 473 K are measured and compared with simulated textures obtained by the standard VPSC approach and the combined approach. In comparison with the standard VPSC approach, the combined approach gives better texture predictions for the two cases: The missing components at low to intermediate rolling reductions in the standard VPSC approach are reproduced; at high rolling reductions and after ECAP, the dominating fibers are also essentially smoothened in the combined approach, in better agreements with the experimental textures.

Radially dependent large-scale dynamos in global cylindrical shear flows and the local cartesian limit

For cylindrical differentially rotating plasmas, we study large-scale magnetic field generation from finite amplitude non-axisymmetric perturbations by comparing numerical simulations with quasi-linear analytic theory. When initiated with a vertical magnetic field of either zero or finite net flux, our global cylindrical simulations exhibit the magnetorotational instability (MRI) and large scale dynamo growth of radially alternating mean fields, averaged over height and azimuth. This dynamo growth is explained by our analytic calculations of a non-axisymmetric fluctuation-induced EMF that is sustained by azimuthal shear of the fluctuating fields. The standard "Omega effect" (shear of the mean field by differential rotation) is unimportant. For the MRI case, we express the large-scale dynamo field as a function of differential rotation. The resulting radially alternating large-scale fields may have implications for angular momentum transport in disks and corona. To connect with previous work on large scale dynamos with local linear shear and identify the minimum conditions needed for large scale field growth, we also solve our equations in local Cartesian coordinates. We find that large scale dynamo growth in a linear shear flow without rotation can be sustained by shear plus non-axisymmetric fluctuations -- even if not helical, a seemingly previously unidentified distinction. The linear shear flow dynamo emerges as a more restricted version of our more general new global cylindrical calculations.

Long-time behavior of the ω→α transition in shocked zirconium: Interplay of nucleation and plastic deformation

We study the thermally activated, slow conversion of the hysteretically retained ω phase into stable α phase in recovered samples of shocked zirconium. The ω-phase decays in time following an algebraic law, unlike the predictions of the nucleation-growth framework for first order transitions, and residual volume fractions of phases and dislocation densities are related by a power law. We propose an explanation for the annealing mechanism through coupled dynamics of dislocations and phase change. We find that the long-time behavior is controlled by the interplay of dislocations, shear fluctuations, and remnant volume fractions of phases, which lead to an algebraic decay in time. For late time, thermally activated quantities such as the dislocation mobility and nucleation rate set the timescale and control the algebraic behavior, respectively. At high enough temperatures this behavior is effectively indistinguishable from standard Avrami kinetics.

Evolution of E × B shear and coherent fluctuations prior to H-L transitions in DIII-D and control strategies for H-L transitions

While operating a magnetic fusion device in H-mode has many advantages, care must be taken to understand and control the release of energy during the H-L back transition, as the extra energy stored within the H-mode transport barrier will have the potential to cause damage to material components of a large future tokamak such as ITER. Examining a scenario where the H-L back transition sequence begins before the E × B shearing layer decays on its own, we identify a long-lived precursor mode that is tied to the events of the H-L sequence and we develop a robust control strategy for ensuring gradual release of energy during the transition sequence. Back transitions in this scenario commonly begin with a rapid relaxation of the pedestal, which was previously shown to be inconsistent with ideal peeling-ballooning instability as the trigger [Eldon et al., Phys. Plasmas 22, 052109 (2015)], despite being otherwise similar to a large type-I Edge Localized Mode (ELM). This so-called transient occurs when the E × B shearing rate ωE×B is significantly larger than the turbulence decorrelation rate ωT, indicating that this is not the result of runaway turbulence recovery. The transient is always synchronous with amplitude and propagation velocity modulations of the precursor mode, which has been dubbed the Modulating Pedestal Mode (MPM). The MPM is a coherent density fluctuation, which, in our scenario at least, reliably appears in the steep gradient region with f≈70 kHz, kθ≈0.3 cm−1, and it exists for ≳100 ms before the onset of back transitions. The transient may be reliably eliminated by reducing toroidal rotation in the co-current direction by the application of torque from counter-injecting neutral beams. The transient in these “soft” H-L transitions is then replaced by a small type-III ELM, which is also always synchronous with the MPM, and MPM shows the same behavior in both hard and soft cases.

A new formulation for rotor equivalent wind speed for wind resource assessment and wind power forecasting

AbstractThe spurt of growth in the wind energy industry has led to the development of many new technologies to study this energy resource and improve the efficiency of wind turbines. One of the key factors in wind farm characterization is the prediction of power output of the wind farm that is a strong function of the turbulence in the wind speed and direction. A new formulation for calculating the expected power from a wind turbine in the presence of wind shear, turbulence, directional shear and direction fluctuations is presented. It is observed that wind shear, directional shear and direction fluctuations reduce the power producing capability, while turbulent intensity increases it. However, there is a complicated superposition of these effects that alters the characteristics of the power estimate that indicates the need for the new formulation. Data from two field experiments is used to estimate the wind power using the new formulation, and results are compared to previous formulations. Comparison of the estimates of available power from the new formulation is not compared to actual power outputs and will be a subject of future work. © 2015 The Authors. Wind Energy published by John Wiley & Sons, Ltd.

How does a thermal binary crystal break under shear?

When exposed to strong shearing, the particles in a crystal will rearrange and ultimately, the crystal will break by forming large nonaffine defects. Even for the initial stage of this process, only little effort has been devoted to the understanding of the breaking process on the scale of the individual particle size for thermalized mixed crystals. Here, we explore the shear-induced breaking for an equimolar two-dimensional binary model crystal with a high interaction asymmetry between the two different species such that the initial crystal has an intersecting square sublattice of the two constituents. Using Brownian dynamics computer simulations, we show that the combination of shear and thermal fluctuations leads to a characteristic hierarchical breaking scenario where initially, the more strongly coupled particles are thermally distorted, paving the way for the weakly coupled particles to escape from their cage. This in turn leads to mobile defects which may finally merge, proliferating a cascade of defects, which triggers the final breakage of the crystal. This scenario is in marked contrast to the breakage of one-component crystals close to melting. Moreover, we explore the orientational dependence of the initial shear direction relative to the crystal orientation and compare this to the usual melting scenario without shear. Our results are verifiable in real-space experiments of superparamagnetic colloidal mixtures at a pending air-water interface in an external magnetic field where the shear can be induced by an external laser field.

Single-particle fluctuations and directional correlations in driven hard-sphere glasses

Via event-driven molecular dynamics simulations and experiments, we study the packing-fraction and shear-rate dependence of single-particle fluctuations and dynamic correlations in hard-sphere glasses under shear. At packing fractions above the glass transition, correlations increase as shear rate decreases: the exponential tail in the distribution of single-particle jumps broadens and dynamic four-point correlations increase. Interestingly, however, upon decreasing the packing fraction, a broadening of the exponential tail is also observed, while dynamic heterogeneity is shown to decrease. An explanation for this behavior is proposed in terms of a competition between shear and thermal fluctuations. Building upon our previous studies [Chikkadi et al., Europhys. Lett. 100, 56001 (2012)], we further address the issue of anisotropy of the dynamic correlations.

Spatial Cooperativity in Microchannel Flows of Soft Jammed Materials: A Mesoscopic Approach

The flow of amorphous solids results from a combination of elastic deformation and local structural rearrangements, which induce nonlocal elastic deformations. These elements are incorporated into a mechanically consistent mesoscopic model of interacting elastoplastic blocks. We investigate the specific case of channel flow with numerical simulations, paying particular attention to situations of strong confinement. We find that the simple picture of plastic events embedded in an elastic matrix successfully accounts for manifestations of spatial cooperativity. Shear rate fluctuations are observed in seemingly quiescent regions, and the velocity profiles in confined flows at high applied pressure deviate from those expected in the absence of nonlocal effects, in agreement with experimental data. However, we suggest a different physical origin for the large deviations observed when walls have rough surfaces, associated with "bumps" of the particles against the asperities of the walls.