Integral Length Research Articles

The influence of thinking dispositions on integration and recall of multiple texts.

We investigated the association between thinking dispositions and two outcomes of multiple‐texts comprehension: integration of conflicting information in argumentative essays; and recall of inferential information as an index of deep comprehension. We focused on two thinking dispositions, need for cognition (NFC) and actively open‐minded thinking (AOT), as relevant individual differences in the processes involved in multiple‐texts comprehension. NFC is the tendency to engage in and enjoy cognitively demanding activities, whereas AOT is the tendency to rationally evaluate arguments and supporting evidence, without being influenced by biases from one's own prior beliefs and prior knowledge. 73 university students completed perceived topic knowledge, perceived exposure to argumentative writing, and perceived competence in argumentative writing, NFC and AOT questionnaires, read two contradictory texts, wrote an argumentative essay, and recalled the information read 1 month later. Argumentative essays were assessed by length and level of integration of conflicting perspective. Text recalls were assessed by number of valid inferences included. Research questions were investigated through a path analysis model. The path analysis model had a good fit. NFC was indirectly associated with argumentation quality of the essay via the essay length. AOT was directly associated with the inferences included in the recall task. The present study contributes to the literature on multiple‐texts comprehension by emphasizing the role of thinking dispositions.

Computational efficiency of CFD modeling for building engineering: An empty domain study

In computational wind engineering, Large Eddy Simulation (LES) operates with superior accuracy. However, this comes with a higher computational cost compared to Reynolds Averaged Navier-Stokes (RANS) models. We propose multiple strategies to reduce the computational cost without compromising accuracy. The turbulence synthesis approach is adopted for efficient inflow generation. To alleviate the issues with tedious and resource-consuming computations, the concept of ‘interpolation’ is endorsed. This approach allows the usage of identical inflow information for different arrangements of meshing. The effect of employing larger time-step and coarser mesh, on the performance of LES with wall functions, is also investigated. Several LES cases are studied, initially with special near-wall treatment, and later, with three different Subgrid Scale (SGS) models. The profiles of mean velocities, turbulence intensities, and integral length scales are examined, in addition to the velocity spectra. The horizontal homogeneity is assessed for all cases. The results show that the wall-adapting eddy viscosity (WALE) SGS model with LES can reduce the computational time at a lower error range. Besides, among three investigated hybrid models, the Detached Eddy Simulation (DES) and the Delayed Detached Eddy Simulation (DDES) provide a significant reduction in the computational cost, compared to LES with the dynamic one-equation eddy-viscosity SGS model, when the objective is to achieve a similar level of accuracy.

Numerical Study of the Influence of the Inlet Turbulence Length Scale on the Turbulent Boundary Layer

In the past half century, large eddy simulations (LESs) have played an important role in turbulent flow simulation and improving the performance of computing technology. To generate a fully developed turbulent boundary layer in the channel domain using LES, suitable inflow conditions along with turbulent characteristics are required. This study aimed to clarify the effect of the integral length scale on the generation of turbulent boundary layers. To accomplish this, an artificially created boundary layer was imposed on the inlet section, which gradually evolved into a fully developed turbulent boundary layer flow inside the numerical domain. In this study, the synthetic inflow method, which is a commonly employed technique, was used by imposing the spatial and temporal correlation between two different points on the inlet section. In addition, we conducted parametric length scale studies on the inlet section and compared our results with existing data. The results showed that the larger length scales in the spanwise direction were not only effective in achieving the target shape of a fully developed turbulent boundary layer, but also developed it faster than the smaller length scales.

Analysis of energy transfer through direct numerical simulations of magnetohydrodynamic Orszag–Tang vortex

Direct numerical simulations of magnetohydrodynamic turbulence with Orszag–Tang vortex initial conditions are presented here for two different Reynolds numbers. Simulations of this transition-to-turbulence problem are carried out using a pseudo-spectral code with a grid resolution fine enough to resolve the Kolmogorov scale. The power-law behavior of bulk quantities like kinetic energy, magnetic energy, integral length scales, and enstrophy is analyzed. For a low Reynolds number, weak turbulence spectra are observed with a slope of −2.0 in the inertial subrange, while for a higher Reynolds number, the spectra exhibit a slope very close to the Kolmogorov spectra of −5/3. The Q-criteria isosurfaces at the time instant of maximum enstrophy are used for the visualization of turbulence developed within the flow. Furthermore, a new approach is presented to understand the dynamics of the transfer between kinetic and magnetic energies in the early period of development using the energy flux spectra, rate of change of kinetic and magnetic energies, and current density structures, which employ a three-dimensional representation of the alignment between velocity and magnetic field across the structures.

Nonequilibrium dissipation scaling in large Reynolds number turbulence generated by rectangular fractal grids

We present experimental observations of the turbulence field generated by a rectangular fractal grid. For engineering tests in wind tunnels, the grid can be used to produce turbulent flows with high turbulence intensities and large integral length scales. Our results also add to previous knowledge of nonequilibrium dissipation scaling, which is observed in decaying turbulence, even though homogeneity and isotropy are not strictly satisfied.

Analysis of Different Mould Section Sizes to Optimize the Submerged Entry Nozzle to Measure the Meniscus Fluctuation in a Continuous Casting Mould

An experimental investigation has been carried out to analyse different mould section sizes to measure the meniscus fluctuation by varying different liquid flow rates and different submerged entry nozzle port angles, i.e., 0° port angles, 15° downward and 15° upward port angles. The terms of maximum surface wave fluctuation and standard deviation have been analysed for the above mentioned parameters. It was observed that a submerged entry nozzle with 0° port was found to be superior when it was compared with a 15° downward and 15° upward port nozzle. By conducting an experiment, it was observed that as the water flow rate increased, the maximum wave amplitude was found to be increasing, which results in more turbulence. Different mould section sizes were analysed to provide background information to the steelmaker to analyse the behaviour of fluid flow pattern. The operating parameters of the result obtained from the present setup were compared with the published literature, and a scale down of slab moulds can be justifiedregarding the rough flow pattern in the mould but can lack accuracy. The reason behind this statement is that the integral length scales of the turbulent flow between scaled down and full scale models can be different. Therefore, details of the flow pattern can become great differences between both types of models.

Turbulent flow structure over rough side-wall surface under wave-against-current motion

An experimental study of periodic surface waves acting against the turbulent currents under the influence of smooth and rough side wall is presented. Flow structure for current alone, and combined wave against current flows over smooth and rough side walls were measured with Acoustic Doppler Velocimeter to resolve the changes in the mean flow and turbulence characteristics resulting from the interaction of the flow and the wall roughness. Turbulent velocity fluctuations were analysed to characterise the turbulence and coherent structures and integral length and time scales were evaluated from the autocorrelation function estimates. Further the Kolmogorov length scales were determined using energy dissipation rate. Results show that the dissipation rate is greater for wave against current flow compared to the current alone flow for rough side walls. The streamwise integral length scale at the measured locations reduced with the increase in wave frequency acting against the current flow. Close to the wall surface, the effect of roughness attenuated the integral length scale and increased the turbulent dissipation rate. Continuous wavelet transform (CWT) was used as a tool to detect and establish the average time and length scales of largest horizontal coherent structures that existed in the wave against current combined flow.

Turbulence-Driven Blowout Instabilities of Premixed Bluff-Body Flames

Bluff-body flame instabilities are experimentally investigated under varying turbulence conditions during lean blowout. For all turbulence conditions, the blowout process is induced through a temporal reduction of the fuel flow rate to capture the flame-flow dynamics approaching blowout. Simultaneous high-speed particle image velocimetry (PIV), stereoscopic PIV, and C2*/CH* chemiluminescence imaging are employed, along with an independent CH* imaging system, to capture flame-flow instabilities. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) techniques are used to identify prominent flame oscillations and evaluate recurring spatiotemporal modes during blowout. The results reveal that the dominant flame oscillations and wrinkling characteristics are directly dependent on the turbulence conditions in the combustor. Specifically, the flame-flow oscillations are strongly coupled with the integral length scales, which were able to collapse the oscillation frequencies to a unified value. The turbulence-driven flame-flow oscillations are shown to largely impact the magnitude, temporal evolution, and oscillatory behavior of the flame strain rate. As the turbulence intensity is increased, the oscillation of the flame strain rate increases in frequency, making it more likely for localized extinctions to occur. Additionally, the magnitude of the flame strain rate increases at high turbulence intensities and accelerates the lean blowout process.

Tidal turbine performance and near-wake characteristics in a sheared turbulent inflow

Turbulent non-homogenous flow environments at high-energy tidal sites lead to variation in power performance of tidal stream turbines. In addition, non-uniform loading due to combined effects of shear and elevated inflow turbulence affects device survivability. In the current study, an active-grid turbulence generator is used to mimic sheared-turbulent inflow representative of the flood (low shear) and ebb tides (high shear). The performance and near-wake characteristics of a turbine model measured in the sheared-turbulent inflow conditions are compared to conditions where the flow velocity is uniform with low and elevated turbulence intensities (Ti). The sheared-turbulent inflows were observed to result in a 5–15% drop in maximum power coefficient when compared to the low Ti case. An increase of ∼30% in torque- and ∼50% in thrust- fluctuations were observed for sheared inflows. Unlike the near axisymmetric wake profiles observed for homogeneous cases, inflow shear was observed to incite asymmetry in the near-wake with an immediate impact in wake turbulence and a more gradual effect on the velocity deficit and integral length scales. The data sets are deemed valuable in calibrating computational models to better predict turbine response and optimize turbine efficiency and the complementing power take-off system.

Investigations of plastic contamination of seawater, marine and coastal sediments in the Russian seas: a review.

Twelve seas with an integral coastline length of about 38,000 km wash upon the Russian coasts. They belong to the basins of the Atlantic, the Arctic, and the Pacific Oceans and stretch over temperate, subpolar, and polar climate zones. This review of 32 studies published between 2015 and August 2020 analyses the available peer-reviewed scientific publications related to the topic of plastic contamination. At present, plastic contamination of the marine environments is confirmed by field investigations in 7 out of 12 Russian seas. Pollution levels vary widely: from 0.6 to 336,000 items/m3 for microplastics in water and from 1.3 to 10,179 items/kg (DW)-in sediments, while median macroplastics abundance is around 1.0 item/m2 at the coast. One monitoring survey of the Barents Sea reported mean macroplastics concentration in the upper 60 m as 0.011 mg/m3 and 2.9 kg/km2 at the sea floor. The identification of the polymer types with spectroscopy techniques is performed only in 9 studies (28%); most researchers use visual identification which makes the results difficult to compare. Most projects aimed at the plastic contamination research use their own collection and extraction methods that poorly agree with other studies. Since the pollution levels in most of the areas are relatively low, sampling is inhomogeneous in space and time. The most extensively studied areas are the beaches of the Baltic Sea, while the least examined is the Arctic region. Our study highlights the need for a discussion on harmonizing sampling methodology and identification techniques among different studies.

Free-Stream Turbulence-Induced Boundary-Layer Transition in Low-Pressure Turbines

Abstract The aerodynamic efficiency of turbomachinery blades is profoundly affected by the occurrence of laminar-turbulent transition in the boundary layer since skin friction and losses rise for the turbulent state. Depending on the free-stream turbulence level, we can identify different paths toward a turbulent state. The present study uses direct numerical simulation as the primary tool to investigate the flow behavior of the low-pressure turbine blade. In the simulations, the flow past only one blade is computed, with periodic boundary conditions in the cross-flow directions to account for the cascade. Isotropic homogeneous free-stream turbulence is prescribed at the inlet. The free-stream turbulence is prescribed as a superposition of Fourier modes with a random phase shift. Two levels of the free-stream turbulence intensity were simulated (Tu=0.19% and 5.2%), with the integral length scale being 0.167c, at the leading edge. We observed that in the case of low free-stream turbulence on the suction side, the Kelvin–Helmholz instability dominated the transition process and full-span vortices were shed from the separation bubble. Transition on the suction side proceeded more rapidly in the high-turbulence case, where streaks broke down into turbulent spots and caused bypass transition. On the pressure side, we have identified the appearance of longitudinal vortical structures, where increasing the turbulence level gives rise to more longitudinal structures. We note that these vortical structures are not produced by Görtler instability.

Large-Eddy Simulations of Stability-Varying Atmospheric Boundary Layer Flow over Isolated Buildings

AbstractThis study explores the response of flow around isolated cuboid buildings to variations in the incoming turbulence arising from changes in atmospheric boundary layer (ABL) stability using a building-resolving large-eddy simulation (LES) technique with explicit representation of building effects through an immersed body force method. An extensive suite of LES for a neutral ABL with different model resolution and advection scheme configurations reveals that at least 6, 12, and 24 grid points per building side are required in order to resolve building-induced vortex shedding, mean-flow features, and turbulence statistics, respectively, with an advection scheme of a minimum of third order. Using model resolutions that meet this requirement, 21 building-resolving simulations are performed under varying atmospheric stability conditions, from weakly stable to convective ABLs, and for different building sizes (H), resulting inLABL/H≈ 0.1–10, whereLABLis the integral length scale of the incoming ABL turbulence. The building-induced flow features observed in the canonical neutral ABL simulation, e.g., the upstream horseshoe vortex and the downstream arch vortex, gradually weaken with increasing surface-driven convective instability due to the enhancement of background turbulent mixing. As a result, two local turbulence kinetic energy peaks on the lateral side of the building in nonconvective cases are merged into a single peak in strong convective cases. By considering the ABL turbulence scale and building size altogether, it is shown that the building impact decreases with increasingLABL/H, as coherent turbulent structures in the ABL become more dominant over a building-induced flow response forLABL/H> 1.

Numerical and experimental study on turbulence statistics in a large fan-stirred combustion vessel

Particle image velocimetry (PIV) has become a popular non-intrusive tool for measuring various types of flows. However, when measuring three-dimensional flows with two-dimensional (2D) PIV, there are some uncertainties in the measured velocity field due to out-of-plane motion, which might alter turbulence statistics and distort the overall flow characteristics. In the present study, three different turbulence models are employed and compared. Mean and fluctuating fields obtained by three-dimensional computational fluid dynamics modeling are compared to experimental data. Turbulence statistics such as integral length scale, Taylor microscale, Kolmogorov scale, turbulence kinetic energy, dissipation rate, and velocity correlations are calculated at different experimental conditions (i.e., pressure, temperature, fan speed, etc.). A reasonably isotropic and homogeneous turbulence with large turbulence intensities is achieved in the central region extending to almost 45 mm radius. This radius decreases with increasing the initial pressure. The influence of the third dimension velocity component on the measured characteristics is negligible. This is a result of the axisymmetric features of the flow pattern in the current vessel. The results prove that the present vessel can be conveniently adopted for several turbulent combustion studies including mainly the determination of turbulent burning velocity for gaseous premixed flames in nearly homogeneous isotropic turbulence.Graphic abstract

Large Eddy Simulation of the By-pass Transition Process under Different Inlet Turbulence Conditions

The transition process of the boundary layer developing over a flat plate with elevated inlet Free Stream Turbulence Intensity (FSTI) has been studied by means of Large Eddy Simulation (LES). To this purpose, four cases with different inflow disturbances have been tested varying the magnitude and the length scale of turbulence. LES has been performed by using the finite-volume ANSYS Fluent code. The computational domain, which was constituted by a rectangular domain with a zero thickness plate, was based on an ERCOFTAC test case in order to provide a validation with a well-known set of data by comparing the boundary layer integral parameters and mean and fluctuating streamwise velocity profiles.The four cases were discussed within the paper by looking at classical statistical properties as well as advanced post-processing tools. It was shown that the decrease in the free stream turbulence level postpones the transition location, whereas the variation of the integral length scale has a very low influence on the distribution of the time-mean flow properties. Proper Orthogonal Decomposition (POD) has been applied to the instantaneous LES flow fields in order to provide a statistical representation of the structures responsible for transition and their response to free-stream turbulence intensity and length scale. The presence of vortical filaments parallel to the wall, typically referred as boundary layer streaks, is clearly identified; their characteristic dimensions and how they change as a function of FSTI properties were analyzed within the paper.

Spinning and tumbling of long fibers in isotropic turbulence

We simultaneously measure both spinning and tumbling components of rotation for long near-neutrally buoyant fibers in homogeneous and isotropic turbulence. The lengths and diameters of the measured fibers extend to several orders of the Kolmogorov length of the surrounding turbulent flow. Our measurements show that the variance of the spinning rate follows a -4/3 power law scaling with the fiber diameter ($d$) and is always larger than the variance of the tumbling rate. This behavior surprisingly resembles that observed previously for sub-Kolmogorov fibers. The general picture that emerges from this study is that long fibers preferentially align with vortex filaments that can be as long as the integral length of turbulence. We compute the Lagrangian time scale and the distribution of both tumbling and spinning that supports this outlook. Our measurements also allow us to quantify the importance of the Coriolis term on the rotational dynamics of fibers in turbulent flows.

The scattering mechanism of squall lines with C-Band dual polarization radar. Part II: the mechanism of an abnormal ZDR echo in clear air based on the parameterization of turbulence deformation

In part I, the clear air echo in front of the squall line is caused by turbulence diffraction, which makes the ZDR echo characteristics different from particle scattering. To study the turbulence deformation phenomenon that is affected by environmental wind, the turbulence-related method is used to analyze the characteristics of three-dimensional turbulence energy spectrum density, and the parametric model of turbulence integral length scale and environmental wind speed is established. The results show that the horizontal scale of turbulence is generally larger than the vertical scale. The turbulence is nearly isotropic in the horizontal direction, presenting a flat ellipsoid with the vertical orientation of the rotation axis when there is no horizontal wind or the horizontal velocity is small. When horizontal wind exists, the turbulence scale increases along the dominant wind direction. The turbulence scale is positively correlated with the wind speed. The power function is used to fit the relationships of turbulence integral length scale and horizontal wind speed, which obtains the best fitting effect, and the goodness of fit (GF) is above 0.99 in each direction. The deforming turbulence can cause 8–9 dB ZDR anomalies in the echo of dual polarization radar, which the ratio of scales in the dominant wind and the vertical direction of deforming turbulence (Lu/Lw) is around 4.3. The variation in ZDR depends on the turbulence shape, orientation and the relative position between turbulence and radar. The shape of turbulence derived from radar detection results is consistent with that of the parametric model, which can provide a parametric scheme for turbulence research. The results reveal the mechanism of abnormal ZDR echo caused by deforming turbulence.

Transport of large particles through the transition to turbulence of a swirling flow

In inhomogeneous turbulent flows particles whose size are close to the flow integral length scale do not sample the flow homogeneously. Instead they explore preferentially regions of low velocity fluctuations. We study how this preferential sampling becomes more and more accentuated when the flow fluctuations decrease as the Reynolds number is lowered. In the absence of fluctuations, the large particles are trapped in the vicinity of the islands of the laminar flow, due to a shear-induced lift force, for times several order of magnitude longer than those associated with the flow forcing.

Empirical numerical model of tornadic flow fields and load effects

Tornadoes are the most devastating meteorological natural hazards. Many empirical and theoretical numerical models of tornado vortex have been proposed, because it is difficult to carry out direct measurements of tornado velocity components. However, most of existing numerical models fail to explain the physical structure of tornado vortices. The present paper proposes a new empirical numerical model for a tornado vortex, and its load effects on a low-rise and a tall building are calculated and compared with those for existing numerical models. The velocity components of the proposed model show clear variations with radius and height, showing good agreement with the results of field measurements, wind tunnel experiments and computational fluid dynamics. Normal stresses in the columns of a low-rise building obtained from the proposed model show intermediate values when compared with those obtained from existing numerical models. Local forces on a tall building show clear variation with height and the largest local forces show similar values to most existing numerical models. Local forces increase with increasing turbulence intensity and are found to depend mainly on reference velocity Uref and moving velocity Umov. However, they collapse to one curve for the same normalized velocity Uref / Umov. The effects of reference radius and reference height are found to be small. Resultant fluctuating force of generalized forces obtained from the modified Rankine model is considered to be larger than those obtained from the proposed model. Fluctuating force increases as the integral length scale increases for the modified Rankine model, while they remain almost constant regardless of the integral length scale for the proposed model.

A new linear forcing method for isotropic turbulence with controlled integral length scale

Turbulence is a common feature to all flows that surround us. Despite its ubiquity, particularly in industrial flows, it is very difficult to provide a mathematical framework for the generation of turbulent eddies. Several methods have been proposed which are able to reproduce realistic features for velocity fluctuations, exhibiting proper space- and time-correlations. Focusing on physical space forcing, these methods are usually first evaluated upon sustained homogeneous isotropic turbulence by introducing a body force to the Navier–Stokes equations. Since the pioneering work of Lundgren, these techniques usually experience difficulties in predicting the integral length scale. The present study provides a forcing through a reconstruction approach which consists in building velocity fluctuations with a prescribed energy spectrum model. The proposed approach is assessed by performing large-eddy simulations of a sustained homogeneous isotropic turbulence in a triply periodic box. Properties of this forcing technique are discussed, drawing on both spatial and time correlations and also on the shape of energy spectrum together with the level of resolved turbulent kinetic energy. A special attention is put on the control of resolved turbulent energy. In this framework, an efficient selective forcing technique is derived, making use of spectral space features. The results show that the proposed approach allows to drive efficiently the resolved kinetic energy toward its target value while preserving the integral length scale independent of the domain size. It is observed that the resulting longitudinal length scale is overestimated by 13%, while the two-time correlations are recovered when using stochastic frequencies.

Experimental investigation of turbulent characteristics in pore-scale regions of porous media

The study of flow dynamics in porous media or randomly packed beds is essential because these systems are commonly applied in a wide array of engineering applications, such as thermal energy storage, catalytic reactors for processing and distillation, oil recovery, fuel cells, and pebble bed nuclear reactor cores. The flow mixing and turbulence characteristics in pore-scale regions of an experimental facility composed of randomly packed spheres with an aspect ratio of 4.4 were experimentally investigated in this study. Velocity measurements at several pores in the test facility were conducted by combining the matched-index-of-refraction and time-resolved particle image velocimetry techniques, and these measurements were performed at Reynolds numbers of 490, 1023, and 1555. The results obtained from the velocity measurements in the pore regions revealed different and complex flow patterns depending on the pore geometries. In pore region 1, it was found that a strong axial flow entered the pore and impinged into the sphere’s surface when it encountered another flow entering laterally. These dynamic interactions created a high level of turbulent mixing, a recirculation flow region, and a shear layer, as observed in the computed statistical results for $$u^{\prime }_\mathrm{{rms}}$$ , $$v^{\prime }_\mathrm{{rms}}$$ , and $$u^{\prime }v^{\prime }$$ . In other pore regions, the flow field results exhibited complex interactions among jet flows discharging into the pores from various flow gaps created by neighboring spheres. These mutual interactions between the entering jet flows were found to create highly turbulent mixing regions at the pore’ centers and low-velocity or stagnation-flow regions in the vicinity of the sphere surfaces. Flapping shear layers were also observed when two jet flows with unequal strengths entered the pores. The characteristics of turbulent flow mixing in different pore regions were investigated via the spatiotemporal two-point correlation of fluctuating velocities along the velocity streamlines. This approach mitigated the constraints of the random geometries of pore regions on the extent of spatial separation lengths in the two-point velocity correlations. Using this approach, spatial distributions of integral length scales and convection velocities were estimated along the streamlines within the random geometries of pore regions. It was found that the estimated values of the integral length scales and convection velocities were spatially dependent on the localized flow characteristics within the pores. The integral length scales were found to be less than $$0.4D_\mathrm{{sp}}$$ , experimentally confirming the pore-scale prevalence hypothesis that turbulent structures in porous media are restricted by the pore sizes. In addition, the ratio of the convection velocity to the interstitial velocity (or local velocity magnitude) decreased as the Reynolds number increased.