Velocity Fluctuations Research Articles

Experimental and numerical simulation research on the transport process of bubble clusters in aeration system

Improving gas-liquid mass transfer efficiency in aeration systems contributes to energy savings, cost reduction, and enhanced efficiency in wastewater treatment. However, due to the complex nonlinear interactions among bubbles in turbulence, understanding the transport mechanisms of non-uniform bubble clusters in turbulence remains unclear. This study employs a combined approach of experimental research and numerical simulations to investigate the shape, diameter distribution, trajectory, and velocity of bubbles under different aeration port sizes and flow rates. The diameter distribution of bubble clusters exhibits a bimodal distribution. Bubble trajectories during ascent mainly exhibit two types of motion patterns: “Z” shaped and linear. Increasing aeration port size and flow rate both lead to an increase in the maximum bubble diameter. Higher initial flow rates and smaller port sizes induce greater lateral velocity fluctuations in bubbles. The proposed numerical simulation method serves as a reference for simulating the transport of non-uniform bubble clusters.

Thermal-hydraulic investigation of liquid metal cross flow different arrangements of tube bundles under pulsating

The topic of this paper is to couple the passive and active approaches to enhance the heat transfer performance in the inline tube bundle heat exchanger. Firstly, the k-kl-ω turbulence model and Kays turbulent Prandtl number model are validated by liquid metal cross flow tube bundle experiments and rectangular groove pulsating flow experimental results. Then the numerical studies were carried out for pulsating velocity amplitude of 0.5, frequency of 10, and Re of 20,000 with transverse and streamwise pitch-to-diameter ratios of 1.5, 1.65, and 1.8. The introduction of pulsating flow effectively improves the turbulence characteristics between the tube bundles, especially at the first three rows, where the velocity fluctuation is enhanced by about 1.5–2 times. The tube heat transfer performances are all improved. The performance evaluation criterion (PEC) variation ranges from 1.04 to 1.32, which signifies that the introduction of pulsating flow can enhance the comprehensive heat transfer performance. The per unit flow rate thermal entropy generation decreased by about 9.7–33.3 %, in addition, a phase difference between the transient thermal entropy production and pulsating velocity was found. The correlation equations about Nu and f factor within the scope of this paper are presented.

Turbulence Modulation By Large Heavy Particles In Wall-Bounded Turbulence

"Most studies on particle laden flows have focused either on small heavy particles or large neutrally buoyant particles. We present, for the first time (to the best of our knowledge), results in the regime of large and heavy particles, investigating the particle-turbulence dynamics over a parameter space covering a wide range of particle-volume fractions, Stokes and Froude numbers. We consider a flow in a pipe aligned with the gravitational vector, thus allowing for the investigation of the two-way coupling between particles and carrier phase turbulence, while excluding the effect of gravity on the wall-normal migration of particles. Time-resolved images of both tracers (10 micrometers particles) and the dispersed particles (700 micrometers in diameter) were taken in the streamwise-wall-normal pipe plane using a planar particle image velocimetry (PIV) system consisting of a high speed camera (Phantom VEO 440), and an Nd:YLF Laser as light source. Using Voronoi tessellations to analyze the inertial particle dynamics, we show a strong deviation from Poisson behaviour for cases with and without turbophoresis. From the time-resolved PIV measurements, we also show that the carrier phase turbulence is modulated even at particle concentrations as low as 0.3% indicating that particle-induced stresses due to distortion of fluid streamlines cannot be neglected. An increase or decrease in the peak streamwise velocity fluctuation in the particle laden flow was observed, depending on the Stokes number, while the radial velocity fluctuation was found either to be the same as in the single phase flow case or higher. Furthermore, pressure drop measurements indicate a linear increase in the skin friction coefficient with volume fraction of particles. This was true for all Stokes and Froude numbers considered. The growth rate (β) of the skin friction coefficient was however observed to be only a function of the Froude number. A power-law fit to the data shows that β scales inversely with the Froude number indicating that for Froude numbers less than 1, the influence of gravity cannot be neglected. "

Turbulence Modulation By Large Heavy Particles In Wall-Bounded Turbulence

"Most studies on particle laden flows have focused either on small heavy particles or large neutrally buoyant particles. We present, for the first time (to the best of our knowledge), results in the regime of large and heavy particles, investigating the particle-turbulence dynamics over a parameter space covering a wide range of particle-volume fractions, Stokes and Froude numbers. We consider a flow in a pipe aligned with the gravitational vector, thus allowing for the investigation of the two-way coupling between particles and carrier phase turbulence, while excluding the effect of gravity on the wall-normal migration of particles. Time-resolved images of both tracers (10 micrometers particles) and the dispersed particles (700 micrometers in diameter) were taken in the streamwise-wall-normal pipe plane using a planar particle image velocimetry (PIV) system consisting of a high speed camera (Phantom VEO 440), and an Nd:YLF Laser as light source. Using Voronoi tessellations to analyze the inertial particle dynamics, we show a strong deviation from Poisson behaviour for cases with and without turbophoresis. From the time-resolved PIV measurements, we also show that the carrier phase turbulence is modulated even at particle concentrations as low as 0.3% indicating that particle-induced stresses due to distortion of fluid streamlines cannot be neglected. An increase or decrease in the peak streamwise velocity fluctuation in the particle laden flow was observed, depending on the Stokes number, while the radial velocity fluctuation was found either to be the same as in the single phase flow case or higher. Furthermore, pressure drop measurements indicate a linear increase in the skin friction coefficient with volume fraction of particles. This was true for all Stokes and Froude numbers considered. The growth rate (β) of the skin friction coefficient was however observed to be only a function of the Froude number. A power-law fit to the data shows that β scales inversely with the Froude number indicating that for Froude numbers less than 1, the influence of gravity cannot be neglected. "

Estimating Velocity Correlations Using Single Pixel PIV

In this work, we analyze the possibility of determining correlations of turbulent velocity fluctuations from PIV data with improved spatial resolution. The analysis is based on the method presented by Oldenziel et al. (2023), where a joint probability density function of the velocity at two locations is calculated from the single-pixel ensemble correlation for these locations. Synthetic PIV images are used to discuss the occurrence of random and systematic errors and to derive the number of image pairs required for a determination of the correlation coefficient with desired uncertainty, depending on the particle image density. Compared to classical window correlation, the presented method can improve the spatial resolution by about one order of magnitude if a very large number of PIV image pairs are available and/or many statistically independent points with identical flow properties can be used. As an example of application, we show the spatial distribution of a two-point correlation in a compressible turbulent boundary layer flow for a reference point near the wall.

Evaluating High-Frequency Fluctuations And Its Effect On Hemodynamics At Varying Heart Rates In Intracranial Aneurysms Via 4D PTV

Hemodynamics have been shown to affect the growth and rupture of intracranial aneurysms (IA). Studies have increasingly reported the presence of high frequency velocity fluctuations in aneurysms. However, it remains undetermined how transient physiological factors like heart rate and inflow blood waveform amplitude affect the hemodynamic parameters and high frequency fluctuations in and around an IA, as well as the risk of IA progression. In this work, we address this question using a volumetric particle tracking velocimetry (PTV) study where a patient-specific right middle cerebral artery (R MCA) aneurysm model was subjected to two heart rate conditions (60 bpm and 137 bpm). The input waveform amplitude for the 137 bpm case was increased by a factor of 1.6 to be physiologically consistent. Captured PTV images were processed using the Shake-the-Box (STB) workflow. Flow parameters including velocity, vorticity, and turbulent kinetic energy (TKE) were evaluated. To evaluate the presence of high frequency fluctuations, a direct numerical simulation (DNS) using the R MCA geometry and same two heart rate cases was conducted and a method for use with STB particle tracks was proposed. As compared to the 60 bpm case, the 137 bpm case had a more concentrated inflow jet that extended further into the aneurysmal sac and a stronger and larger vortical flow structure in the aneurysmal sac. The 60 bpm case maintained a region of slightly elevated TKE at the main junction point where the aneurysmal sac and all vessels merge. In the 137 bpm case, TKE in this region was dramatically higher. Using probe points, the DNS data revealed high frequency velocity fluctuations in an outflow vessel during the deceleration phase of the cycle. Notably, these fluctuations were only observed for the 137 bpm case. For both the DNS and STB data, high frequency components were more prevalent and stronger outside of the aneurysmal sac as opposed to inside the aneurysmal sac. Overall, these results suggest that changes in heart rate induce changes to the flow structure and can induce significant increases in TKE and high frequency fluctuations. Based on our findings, a higher heart rate would be expected to yield a flow field more akin to IA progression. The specific correlation between the high frequency components computed using our proposed STB-based method and the high frequency fluctuations in the flow field could not be well understood. Thus, additional analysis and development of this method is needed.

Wall Shear Stress Characteristics Of A Turbulent Boundary Layer Obtained With Multi-Aperture Defocusing Micro-PTV And High-Speed Stereo Profile-PIV

A multi-aperture 3d 3c micro PTV system is presented which relies on single window optical access for both high-speed tracer illumination and image recording. Similar to the “defocusing” concept described by C. Willert & Gharib (1992), the wall distance of individual particles is obtained from the size of projected particle image triplets formed by a triplet of apertures on the entrance pupil of the microscope lens. The present article extends upon previously published material Klinner & Willert (2022) by applying multi-aperture micro particle tracking velocimetry (MA-micro PTV) on a canonical turbulent boundary layer (TBL) to track the near-wall motion of tracer particles with the aim of estimating the unsteady wall-shear stress from particle tracks. The technique is validated with measurements of a TBL inside the closed test section of the 1 m wind tunnel of DLR in Göttingen (1mWK) at free- stream velocities of 5.2 to 20 m/s with corresponding shear Reynolds numbers between 560 and 1630. In the viscous sublayer, the probability density distributions of stream- and span-wise wall shear stress (WSS) components could be reliably captured down to probability densities of 10E−3. As far as we know, this has not been achieved in the past, particularly not for the span-wise component. For the stream-wise component the measured Reynolds-number dependency of the root mean squared fluctuations agrees to correlations by Örlü & Schlatter (2011). Furthermore, the skewness of the stream-wise WSS agrees well to values from DNS. On the other hand, the span-wise WSS fluctuations consistently underestimate the predicted DNS values, for which it is assumed that the deviation can be explained by the rapid decrease of the span-wise velocity fluctuations with increasing wall distance. Wall-normal profiles of 3c velocity statistics were obtained by bin averaging of particle velocities. Up into the buffer layer, these profiles agree well with profiles from stereo PIV as well as from DNS and LES, indicating an accurate determination of both near-wall velocities and inner scaling.

Evaluating High-Frequency Fluctuations And Its Effect On Hemodynamics At Varying Heart Rates In Intracranial Aneurysms Via 4D PTV

Hemodynamics have been shown to affect the growth and rupture of intracranial aneurysms (IA). Studies have increasingly reported the presence of high frequency velocity fluctuations in aneurysms. However, it remains undetermined how transient physiological factors like heart rate and inflow blood waveform amplitude affect the hemodynamic parameters and high frequency fluctuations in and around an IA, as well as the risk of IA progression. In this work, we address this question using a volumetric particle tracking velocimetry (PTV) study where a patient-specific right middle cerebral artery (R MCA) aneurysm model was subjected to two heart rate conditions (60 bpm and 137 bpm). The input waveform amplitude for the 137 bpm case was increased by a factor of 1.6 to be physiologically consistent. Captured PTV images were processed using the Shake-the-Box (STB) workflow. Flow parameters including velocity, vorticity, and turbulent kinetic energy (TKE) were evaluated. To evaluate the presence of high frequency fluctuations, a direct numerical simulation (DNS) using the R MCA geometry and same two heart rate cases was conducted and a method for use with STB particle tracks was proposed. As compared to the 60 bpm case, the 137 bpm case had a more concentrated inflow jet that extended further into the aneurysmal sac and a stronger and larger vortical flow structure in the aneurysmal sac. The 60 bpm case maintained a region of slightly elevated TKE at the main junction point where the aneurysmal sac and all vessels merge. In the 137 bpm case, TKE in this region was dramatically higher. Using probe points, the DNS data revealed high frequency velocity fluctuations in an outflow vessel during the deceleration phase of the cycle. Notably, these fluctuations were only observed for the 137 bpm case. For both the DNS and STB data, high frequency components were more prevalent and stronger outside of the aneurysmal sac as opposed to inside the aneurysmal sac. Overall, these results suggest that changes in heart rate induce changes to the flow structure and can induce significant increases in TKE and high frequency fluctuations. Based on our findings, a higher heart rate would be expected to yield a flow field more akin to IA progression. The specific correlation between the high frequency components computed using our proposed STB-based method and the high frequency fluctuations in the flow field could not be well understood. Thus, additional analysis and development of this method is needed.

Estimating Velocity Correlations Using Single Pixel PIV

In this work, we analyze the possibility of determining correlations of turbulent velocity fluctuations from PIV data with improved spatial resolution. The analysis is based on the method presented by Oldenziel et al. (2023), where a joint probability density function of the velocity at two locations is calculated from the single-pixel ensemble correlation for these locations. Synthetic PIV images are used to discuss the occurrence of random and systematic errors and to derive the number of image pairs required for a determination of the correlation coefficient with desired uncertainty, depending on the particle image density. Compared to classical window correlation, the presented method can improve the spatial resolution by about one order of magnitude if a very large number of PIV image pairs are available and/or many statistically independent points with identical flow properties can be used. As an example of application, we show the spatial distribution of a two-point correlation in a compressible turbulent boundary layer flow for a reference point near the wall.

Wall Shear Stress Characteristics Of A Turbulent Boundary Layer Obtained With Multi-Aperture Defocusing Micro-PTV And High-Speed Stereo Profile-PIV

A multi-aperture 3d 3c micro PTV system is presented which relies on single window optical access for both high-speed tracer illumination and image recording. Similar to the “defocusing” concept described by C. Willert & Gharib (1992), the wall distance of individual particles is obtained from the size of projected particle image triplets formed by a triplet of apertures on the entrance pupil of the microscope lens. The present article extends upon previously published material Klinner & Willert (2022) by applying multi-aperture micro particle tracking velocimetry (MA-micro PTV) on a canonical turbulent boundary layer (TBL) to track the near-wall motion of tracer particles with the aim of estimating the unsteady wall-shear stress from particle tracks. The technique is validated with measurements of a TBL inside the closed test section of the 1 m wind tunnel of DLR in Göttingen (1mWK) at free- stream velocities of 5.2 to 20 m/s with corresponding shear Reynolds numbers between 560 and 1630. In the viscous sublayer, the probability density distributions of stream- and span-wise wall shear stress (WSS) components could be reliably captured down to probability densities of 10E−3. As far as we know, this has not been achieved in the past, particularly not for the span-wise component. For the stream-wise component the measured Reynolds-number dependency of the root mean squared fluctuations agrees to correlations by Örlü & Schlatter (2011). Furthermore, the skewness of the stream-wise WSS agrees well to values from DNS. On the other hand, the span-wise WSS fluctuations consistently underestimate the predicted DNS values, for which it is assumed that the deviation can be explained by the rapid decrease of the span-wise velocity fluctuations with increasing wall distance. Wall-normal profiles of 3c velocity statistics were obtained by bin averaging of particle velocities. Up into the buffer layer, these profiles agree well with profiles from stereo PIV as well as from DNS and LES, indicating an accurate determination of both near-wall velocities and inner scaling.

Hot-wire spatial resolution issues in adverse pressure gradient turbulent boundary layers

The effect of a finite length of hot-wire probe sensor length on the measured streamwise velocity fluctuations is well understood in canonical wall-bounded flow, where the small-scale energy has been found to be universal and invariant with Reynolds number. A straightforward application of that assumption to non-canonical flows such as strong adverse pressure gradient (APG) flows has, however, been hampered since the effect of Re and APG could not conclusively be studied separately due to the lack of data with a clear scale separation. The present experimental investigation at Reτ≈4000 in weak, moderate and strong APGs with different wire length shows that spatial averaging effects are not only limited to the inner layer. A note of caution is hence warranted for measurements that seemingly try to take the bias effect of spatial attenuation into account by performing measurements with albeit long but fixed viscous-scaled wire length.

Fluid Dynamics Effects of a Porous Blockage on Laminar Flow in the Interior Subchannels of a 61-Pin Wire-Wrapped Rod Bundle

Flow blockages in Liquid Metal Fast Reactor (LMFR) fuel assemblies that can cause fuel pin cladding failures require further investigation for the development and optimization of these Generation IV reactor designs. The objective of this study was to experimentally evaluate the effects of a porous blockage accident scenario on laminar flow behavior in the interior subchannels of a prototypical 61-pin wire-wrapped rod bundle. The laminar flow condition is considered because of its importance to natural circulation and loss-of-coolant-accident operating scenarios as well as the limited availability of experimental data. The matched-index-of-refraction method was utilized to obtain two-dimension two-component time-resolved particle image velocimetry (TR-PIV) measurements at three planes centered on blocked and neighboring subchannel regions. Time-averaged first-order and second-order statistics, computed by Reynolds decomposition, were visualized including the mean and fluctuating streamwise and spanwise velocity components. Line profiles described the evolution of flow from upstream regions, to separated flow, and recombination downstream, while a modified Galilean decomposition distinguished differences between flow in the blocked measurement plane and flow in its neighboring counterpart region. Spatial-temporal cross-correlations were performed for the streamwise velocity fluctuations to characterize the convection velocity and decay of traveling vortices in the wake of the porous blockage. The isothermal TR-PIV measurements from this study provide high-fidelity experimental data sets for the validation of computational models and numerical studies to characterize complex fluid phenomena in wire-wrapped rod bundles during the potential accident scenario of a porous, interior flow blockage.

Effect of inlet flow turbulence on hydro–acoustic coupling and flame–vortex interactions in a premixed dump combustor

The present work examines the effect of inlet flow turbulence on the flame–vortex modulations, hydro–acoustic coupling and recirculation zone dynamics at the onset of instability. The flow turbulence intensity varied using a custom-designed turbulence generator with different slot-width blockage plates placed upstream to the nominal flame-stabilization zone. The high-speed recordings of the CH*/OH* chemiluminescence and particle image velocimetry are used to deduce the relation between the flame–acoustic, flame–vortex and hydrodynamic–acoustic features during the unsteady combustion. The bifurcation analysis map revealed the effect of high inlet flow turbulence on postponing the onset of instability to higher inlet flow parameters. The initial observations showed potential changes in the acoustic behaviour and dynamical state of premixed turbulent combustion with an increase in inlet flow turbulence. The spatial Rayleigh index map illustrates a significant change in the acoustic driving region at high inlet flow turbulence based on the flame stabilization, heat release zone and flame–acoustic modulation in the shear layer and recirculation zone. The velocity spectral analysis and dynamic mode decomposition (DMD) spectrum suggested a correlation between the acoustic modulation and hydrodynamic instabilities, resulting in higher heat release rate oscillations. The high inlet flow turbulence modulates the vertical flapping motion of the flame front and flame roll-up in the recirculation region as evident by DMD spectrum and spatial modes. The flame–vortex dynamics during the dynamic transition events showed that the high inlet flow turbulence influenced the vortex shedding along the shear layer and recirculation zone dynamics. At low turbulence intensity, the vortex, in turn, supports the bulk flame movement through the induced velocity, which interacts with the free stream to create regions of low velocity. In contrast, the vortex in the shear layer and flame resides along the shear layer at higher turbulence. The paper concludes that at higher inlet flow turbulence, the recirculating flame reduces the hydro–acoustically modulated flow velocity fluctuations, which significantly affect the upstream flame propagation propensity.

Computational modeling of the physical features that influence breast cancer invasion into adipose tissue.

Breast cancer invasion into adipose tissue strongly influences disease progression and metastasis. The degree of cancer cell invasion into adipose tissue depends on both biochemical signaling and the mechanical properties of cancer cells, adipocytes, and other key components of adipose tissue. We model breast cancer invasion into adipose tissue using discrete element method simulations of active, cohesive spherical particles (cancer cells) invading into confluent packings of deformable polyhedra (adipocytes). We quantify the degree of invasion by calculating the interfacial area At between cancer cells and adipocytes. We determine the long-time value of At vs the activity and strength of the cohesion between cancer cells, as well as the mechanical properties of the adipocytes and extracellular matrix in which adipocytes are embedded. We show that the degree of invasion collapses onto a master curve as a function of the dimensionless energy scale Ec , which grows linearly with the cancer cell velocity persistence time and fluctuations, is inversely proportional to the system pressure, and is offset by the cancer cell cohesive energy. When , cancer cells will invade the adipose tissue, whereas for , cancer cells and adipocytes remain de-mixed. We also show that At decreases when the adipocytes are constrained by the ECM by an amount that depends on the spatial heterogeneity of the adipose tissue.

Effect of impeller rotational phase on the FDA blood pump velocity fields.

The Food and Drug Administration (FDA) blood pump is an open-source benchmark cardiovascular device introduced for validating computational and experimental performance analysis tools. The time-resolved velocity field for the whole impeller has not been established, as is undertaken in this particle image velocimetry (PIV) study. The level of instantaneous velocity fluctuations is important, to assess the flow-induced rotor vibrations which may contribute to the total blood damage. To document these factors, time-resolved two-dimensional PIV experiments were performed that were precisely phase-locked with the impeller rotation angle. The velocity fields in the impeller and in the volute conformed with the previous single blade passage experiments of literature. Depending on the impeller orientation, present experiments showed that volute outlet nozzle flow can fluctuate up to 34% during impeller rotation, with a maximum standard experimental uncertainty of 2.2%. Likewise, the flow fields in each impeller passage also altered in average 33.5%. Considerably different vortex patterns were observed for different blade passages, with the largest vortical structures reaching an average core radii of 7 mm. The constant volute area employed in the FDA pump design contributes to the observed velocity imbalance, as illustrated in our velocity measurements. By introducing the impeller orientation parameter for the nozzle flow, this study considers the possible uncertainties influencing pump flow. Expanding the available literature data, analysis of inter-blade relative velocity fields is provided here for the first-time to the best of our knowledge. Consequently, our research fills a critical knowledge gap in the understanding of the flow dynamics of an important benchmark cardiovascular device. This study prompts the need for improved hydrodynamic designs and optimized devices to be used as benchmark test devices, to build more confidence and safety in future ventricular assist device performance assessment studies.

Experimental observation of the vortex flow and particle motion in a cyclonic classifier

The complex vortex structure in cyclonic classifiers lacks experimental disclosure. The vortex flow and motion of the particles in the classifier were experimentally investigated by the combination of infrared thermal imaging and particle image velocimetry. The results show that the vortex structure is complex, which is composed of the main vortex and the micro-vortices. The main vortex experiences shedding, growth coalescence. The vortex core oscillation causes the fluctuation of the instantaneous tangential velocity, and the maximum velocity varies between 9 m/s and 18 m/s. When the inlet velocity is 14 m/s, the asymmetry and stability of the vortex improve. Accelerated by the vortex flow, the fine particles (5–10 μm) form large-scale particle agglomerate and the circumferential movement rate of the fine particle reaches a maximum of 2.9 m/s. The research contributes to a profound understanding of the gas-solid flow in the cyclonic classifier.

Characteristics of the jet shear layer of a round elevated jet in crossflow

The present research aims at an experimental study to investigate the effect of velocity ratio (R) on the evolution of elevated round jet issued from a stack having an aspect ratio (AR) of 9.0 in a uniform crossflow at a Reynolds number of 2000. The experiments are conducted in a low-speed, recirculating water tunnel. Laser Doppler Velocimetry (LDV) is employed to characterize the water tunnel. Particle Image Velocimetry (PIV) is used to measure the flow field, and the dye visualization technique is utilized to capture the accompanying instantaneous flow structures. Depending on the velocity ratio (R=0.16––1.5), distinct jet shear layer (JSL) structures are observed in the symmetric plane (XY), confirmed by both the flow visualization and PIV data. These structures include clockwise vortices, anticlockwise vortices, swing-induced mushroom vortices, and jet-like vortices. For R<0.5, entrainment of a substantial amount of jet fluid into the stack-wake region (downwash) has been observed. At R=0.5, the streak image captured in the wall parallel plane reveals the ring-like vortices that appear to wrap around the jet core while upright vortices are detected downstream on the lee side of the jet. Additionally, variations in average velocity and turbulent fluctuations in the near field, contingent on different velocity ratios, are discussed.

Large-scale semi-organized rolls in a sheared convective turbulence: Mean-field simulations

Based on a mean-field theory of a non-rotating turbulent convection [T. Elperin et al., Phys. Rev. E 66, 066305, (2002)], we perform mean-field simulations (MFS) of sheared convection that takes into account an effect of modification of the turbulent heat flux by the non-uniform large-scale motions. This effect is caused by the production of additional essentially anisotropic velocity fluctuations generated by tangling of the mean-velocity gradients by small-scale turbulent motions due to the influence of the inertial forces during the lifetime of turbulent eddies. These anisotropic velocity fluctuations contribute to the turbulent heat flux. As the result of this effect, there is an excitation of large-scale convective-shear instability, which causes the formation of large-scale semi-organized structures in the form of rolls. The lifetimes and spatial scales of these structures are much larger compared to the turbulent scales. By means of MFS performed for stress-free and no-slip vertical boundary conditions, we determine the spatial and temporal characteristics of these structures. Our study demonstrates that the modification of the turbulent heat flux by non-uniform flows leads to a strong reduction of the critical effective Rayleigh number (based on the eddy viscosity and turbulent temperature diffusivity) required for the formation of the large-scale rolls. During the nonlinear stage of the convective-shear instability, there is a transition from a two-layer vertical structure with two rolls in the vertical direction before the system reaches steady-state to a one-layer vertical structure with one roll after the system reaches steady state. This effect is observed for all effective Rayleigh numbers. We find that inside the convective rolls, the spatial distribution of the mean potential temperature includes regions with a positive vertical gradient of the potential temperature caused by the mean heat flux of the convective rolls. This study might be useful for understanding the origin of large-scale rolls observed in atmospheric convective boundary layers, as well as in numerical simulations and laboratory experiments.

Interference of the direct and ground-reflected waves in the atmosphere with volumetric scatteringa).

The interference of the direct and ground-reflected sound waves is significantly affected by volumetric scattering in the atmosphere, such as scattering by turbulence and forest. In the present article, the existing theory describing this interference is generalized to three somewhat independent but equally important cases. First, the attenuation of the direct and ground-reflected waves caused by backscattering is addressed. Second, the existing theory is extended for statistically quasi-homogeneous turbulence in which the variances and length scales of the temperature and wind velocity fluctuations depend on the height above the ground. Third, the existing theory, which was previously formulated only for near-horizontal sound propagation, is generalized to slanted sound propagation as pertinent to elevated sound sources. Numerical results for slanted propagation demonstrate that atmospheric turbulence can significantly increase the sound pressure level at the interference minima. The extended theory of the interference of the direct and ground-reflected waves in the atmosphere with volumetric scattering is important for practical applications, such as auralization of flying aircraft and sound propagation in a forest, and can be adapted to radio wave propagation.

Interactions between polymer molecules and hydrodynamic responses in multi-ion induced charge electroosmosis

Induced charge electroosmosis (ICEO) can assist lab-on-a-chip devices and macromolecular separation involving interface, multi-ion species, and solution properties. To supplement the mechanistic insights into the ICEO phenomenon in complex electrochemical scenarios, we numerically address electrokinetic ion transport–fluid flow coupling regulated by multi-ion species and polymer macromolecular rheology around a polarized cylinder. The physics of the ICEO are analysed from multiple perspectives, including the azimuthal velocity, flow type, energy distribution, interfacial ion transport, and electroelastic instability. The main findings are as follows: (1) Deviations under neutral pH conditions (multi-ion species) cause flow field asymmetry and alter the input energy distribution (PE) in the tight double layer. Polymers greatly modify the interfacial force features in ion-enriched regions, as identified by the first stress difference (N1). (2) The kinetic energy vs. viscosity ratio (β) represents the nonlinear enhancement curve Ek=0.011*exp(β/0.64)-0.0138 due to the weakened effective viscosity Rvis. In addition, shear thinning notably lifts the charge density and salt concentration at the cylinder surface, which boosts the PE. (3) Quadrupolar vortex modes impact the positive correlation between β and the ion current (Jtotal) at different pH levels, resulting in less ion current dependence for small β. A low enhancement efficiency occurs at 10-4 ≤ α (molecular anisotropy) ≤ 0.1, whereas a high efficiency exists at α > 0.1. Electroelastic instabilities are confirmed by the spatiotemporal evolution of the velocity fluctuation, with a broadband structural power-law decline in the power spectral density. These results allow microdevice mixing manipulation and disclose non-Newtonian barrier of electrokinetic dynamics.