Diffusion Of Vorticity Research Articles

Study on the Fluctuating Load Characteristics of the Submerged Radial Gate in the High-Head Flood Discharge Outlet

The fluctuating pressure acting on the radial gate in the high-head flood discharge outlet is the main excitation source of flow-induced vibration. Therefore, this paper delves into the distribution characteristics of fluctuating pressure on the panel of the high-head submerged radial gate based on hydraulic model tests. Hydraulic tests were first conducted to obtain the distribution patterns of time-averaged pressure and the root mean square (RMS) of fluctuating pressure on the radial gate panel. Secondly, the improved complete ensemble empirical mode decomposition with adaptive noise (ICEEMDAN) and HHT method was employed to identify the causes of the fluctuating pressure on the gate panel. Finally, the ICEEMDAN-SSA (the sparrow search algorithm)–LSTM (long short-term memory) method was utilized to achieve accurate prediction of the fluctuating pressure on the gate panel. The results show that the time-averaged pressure in the middle of the gate panel is higher than that at the top and near the bottom edge, which differs significantly from the static pressure distribution. The RMS of the fluctuating pressure near the bottom edge is higher than that in the middle and at the top. The fluctuating pressure acting on the gate panel in the time domain can be regarded as a stationary process. The fluctuating pressure on the gate panel is caused by the combined diffusion and random mixing of multi-scale vortices in the turbulent eddy structure. The ICEEMDAN-SSA-LSTM combined method significantly improves the prediction accuracy of fluctuating pressure on the gate panel compared to the LSTM and ICEEMDAN-LSTM methods.

Observation of current whirlpools in graphene at room temperature.

Electron-electron interactions in high-mobility conductors can give rise to transport signatures resembling those described by classical hydrodynamics. Using a nanoscale scanning magnetometer, we imaged a distinctive hydrodynamic transport pattern-stationary current vortices-in a monolayer graphene device at room temperature. By measuring devices with increasing characteristic size, we observed the disappearance of the current vortex and thus verified a prediction of the hydrodynamic model. We further observed that vortex flow is present for both hole- and electron-dominated transport regimes but disappears in the ambipolar regime. We attribute this effect to a reduction of the vorticity diffusion length near charge neutrality. Our work showcases the power of local imaging techniques for unveiling exotic mesoscopic transport phenomena.

A numerical research on transient transport of high-temperature particles associated with air gouging process

The high-temperature particles produced by air gouging processes are common pollutants in cleaning workshops, which could endanger workers’ health. In this research, a two-way coupled Eulerian-Lagrangian method based on discrete phase modelling (DPM) was used to investigate the transport characteristics of high-temperature particles generated from air gouging processes. The particle group in the movement was divided into core zone and boundary zone, and the kinetic mechanism of particles in different zones was analysed. The results show particles in the boundary zone are more susceptible to the vortex entrainment effect than particles in the core zone, and some particles could be moved from the vortex diffusion zone to the vortex recirculation zone. The smaller the particle size, the better the followability to the hot airflow. The slower the temperature decay of particles, the more significant the thermophoresis effect, which could enhance the diffusion behaviour of the small particles. By analysing the average residence time of particles in the breathing zone, the influence of different factors on the average vertical diffusion height [Formula: see text] of particles is summarized, and a nonlinear regression prediction model of [Formula: see text] is established. The results can contribute to health risk assessment and industrial ventilation design.

Non-Gaussian anomalous diffusion of optical vortices.

Anomalous diffusion of different particlelike entities, the deviation from typical Brownian motion, is ubiquitous in complex physical and biological systems. While optical vortices move randomly in evolving speckle fields, optical vortices have only been observed to exhibit pure Brownian motion in random speckle fields. Here we present direct experimental evidence of the anomalous diffusion of optical vortices in temporally varying speckle patterns from multiple-scattering viscoelastic media. Moreover, we observe two characteristic features, i.e., the self-similarity and the antipersistent correlation of the optical vortex motion, indicating that the mechanism of the observed subdiffusion of optical vortices can only be attributed to fractional Brownian motion (FBM). We further demonstrate that the vortex displacements exhibit a non-Gaussian heavy-tailed distribution. Additionally, we modulate the extent of subdiffusion, such as diffusive scaling exponents, and the non-Gaussianity of optical vortices by altering the viscoelasticity of samples. The discovery of the complex FBM but non-Gaussian subdiffusion of optical vortices may not only offer insight into certain fundamental physics, including the anomalous diffusion of vortices in fluids and the decoupling between Brownianity and Gaussianity, but also suggest a strong potential for utilizing optical vortices as tracers in microrheology instead of the introduced exogenous probe particles in particle tracking microrheology.

New theoretical approach to dynamics of heat-mass-transfer, thermal turbulence and air ventilation in atmosphere of an industrial city II. Spectrum of thermal turbulence

In this paper we go on a development of consistent theoretical approach to modelling the turbulent regime in the atmosphere of the industrial cities and present the analytical foundations of a new model of thermal turbulence spectrum for atmosphere of an industrial city. Special attention is paid to general analytical aspects for accounting of the phenomenon of wave or vortex diffusion, which is usually ignored in most works on atmospheric ventilation modelling. Redistribution of energy over the spectrum of eddy sizes is usually called a spectral transformation, the study of which is possible only under the condition of real introduction of nonlinearity into the equation of turbulent motion. The approach presented is implemented into the general theory of heat-mass-transfer, thermal turbulence and air ventilation in atmosphere of an industrial city, including an improved theory of atmospheric circulation in combination with the hydrodynamic modelling, method of a complex geophysical plane field and the Arakawa-Schubert approach to a quantitative description of convective instability in the city’s atmosphere.

IDDES Investigation of Rotor Blade Tip Vortices in Hovering State with Tip Slots

The slots on a blade tip can effectively suppress the tip vortices by diffusing the core structure of a vortex and thus mitigate the adverse impacts of vortex interactions. In this paper, the effect of a slotted tip on vortex diffusion in the hovering state is investigated numerically by implementing the IDDES method using the fifth-order WENO scheme. The resulting wake flow field show that the number of secondary turbulent structures of so-called vortex worms near the slotted tip reduces significantly, indicating the weakening of vortex core rotation. It was found that the slotted tip reduced the peak value of velocity profiles in the vicinity of the vortex core by about 50%. The numerical result is close to the experimental conclusion. Correspondingly, the diameter of the vortex core is approximately doubled at each wake age relative to the baseline blade because the local flow passing through the slots will disintegrate the stable structure of the core region during the early stages of tip vortex generation. This promotes the turbulence mixture, resulting in the reduction in the vorticity value and rotating effect near the vortex core. Based on the overall results, the study of the slot parameters demonstrated that reducing the slot number or diameter will weaken the diffusing effect, causing a decrease in the flow rate.

Shear wave transmissivity and wave-induced vorticity diffusion through conversion scattering to slow-shear diffusion wave

In the poroelasticity theory of de la Cruz and Spanos, fluid shearing within the viscous boundary layer, i.e. fluid vorticity, manifests as an independent process, namely the diffusive slow shear wave. Quantifying its impact on elastic wave propagation remains a research challenge. We analyse the transmissivity of a horizontally polarized shear wave travelling across a stack of porous fluid-saturated layers. The fluid shearing developed at each contact is captured, in a macroscopic sense, through conversion scattering into this diffusive slow shear wave. Generalizing the reflectivity method for elastic waves to the poroelastic case, we develop semi-analytical results for an arbitrary number of layers. We find that the conversion scattering into this diffusion wave reduces the shear wave amplitude, and this reduction accumulates with the number of layers. In the limit when the layer thickness corresponds to the pore diameter, the resulting wave quality factor is close to the predictions of previously reported attempts to capture the fluid vorticity. However, our approach is distinctively different since it is purely based on a wave-theoretic analysis of interacting waves, acknowledging vorticity diffusion as an independent process, thereby avoiding the problems arising when incorporating a pore-scale phenomenon in a macroscopic wave propagation theory.

Self-adaptive turbulence eddy simulation of flow control for drag reduction around a square cylinder with an upstream rod

The use of an upstream circular rod to control the flow around a downstream square cylinder is found to efficiently reduce the drag of the square cylinder. The complex flow characteristics and mechanism of the rod-square system in tandem arrangement as a passive flow control method for various centre-to- centre gap ratios (L/D) are investigated by numerical simulation. Turbulence modelling of the rod-square cylinder system is challenging. To select a suitable turbulence modelling, the recently developed Self-Adaptive Turbulence Eddy Simulations (SATES), as well as IDDES (Improved Delayed Detached Eddy Simulation) and SBES (Stress Blended Eddy Simulation) are first assessed for single circular and single square cylinder flow test cases. The SATES method performs best among the three models for bluff body flow simulation and is thus used in the drag flow control study. For the rod-square flow control system, the Reynolds number is 34000 based on freestream velocity U0 and the edge length of the square cylinder D. The diameter of the control rod is kept constant at d/D = 0.18. The flow characteristics and mechanism are explored in detail, varying the rod and square cylinder distance of L/D = 1.5, 2.0, 2.5and 3.0. According to the absence or presence of vortex shedding from the upstream rod cylinder, two flow patterns (pattern I and pattern II) are observed. Large drag reduction is observed for all the simulation cases the maximum drag reduction is approximately 63% with L/D = 2.0, and the minimum drag reduction approximately 53% with L/D = 3.0. The secondary lock-frequency, which presents as pattern II, is caused by the excitation of upstream vortices. It is found that the side recirculation is associated with the wake recirculation length of the square cylinder, which is similar to the elongated cylinder in the flow pattern I. The flow mechanism of suppression of the vortices from the rod can be attributed to the effective diffusion of vorticity in the separating shear layer caused by the presence of the front wall of the square cylinder.

Investigation of energy dissipation of an H-type vertical axis wind turbine based on entropy production theory

In vertical axis wind turbine (VAWT) research, amount studies have been conducted about the flow field around the blades, but the effect of blade tip vortex was not revealed clearly. The present study emphasized on the effect of blade tip vortex on energy loss in wake region and employed entropy production theory to determine where and how energy loss generates quantitatively. The numerical simulation was conducted for the VAWTs versus various tip speed ratio (TSR) conditions, and the numerical results were analyzed by entropy production method. The energy loss calculated by entropy production theory in VAWTs flows increases with TSR increasing. The distribution of entropy production rate (EPR) in wake region was asymmetrical due to the influence of blade tip vortex diffusion downstream and two fluctuation peaks appeared on EPR curves. The energy loss coefficient was defined to represent the energy deficit and kinetic energy recovery in wake region quantitatively. Generally, the energy loss coefficient was reduced to 0.1 between 8D to 9D in wake region and was equal to 0.1051 at x = 8D under optimal condition TSR = 2.19. The present study can guide configuration optimization of VAWTs to improve wind energy utilization and revenue of wind farms.

Mutual friction and diffusion of two-dimensional quantum vortices

We present a microscopic open quantum systems theory of thermally-damped vortex motion in oblate atomic superfluids that includes previously neglected energy-damping interactions between superfluid and thermal atoms. This mechanism couples strongly to vortex core motion and causes dissipation of vortex energy due to mutual friction, as well as Brownian motion of vortices due to thermal fluctuations. We derive an analytic expression for the dimensionless mutual friction coefficient that gives excellent quantitative agreement with experimentally measured values, without any fitted parameters. Our work closes an existing two orders of magnitude gap between dissipation theory and experiments, previously bridged by fitted parameters, and provides a microscopic origin for the mutual friction and diffusion of quantized vortices in two-dimensional atomic superfluids.

ON THE NONLINEAR STABILITY FOR QUASI-GEOSTROPHIC FORCED ZONAL FLOWS

This paper continues a series of studies providing stability crite¬ria for quasigeostrophic forced zonal flows in in the presence of lateral diffusion and bottom dissipation of the vertical vorticity. We study the Lyapunov stability of a stationary and longitude independent ba¬sic flow, obtaining linear and nonlinear stability criteria expressed in terms of the maximum shear of the basic flow and/or its meridional derivative, extending some previous results.

Т-схемы для математического моделирования генерации завихренности на гладких профилях в вихревых методах

New numerical schemes are proposed for solving the boundary integral equation that arises in CFD vortex particle methods of when simulating a plane flow around smooth airfoils. They are based on considering the 2-nd kind integral equation with respect to vortex sheet intensity with a bounded or absolutely integrable kernel instead of traditionally solved singular integral equations of the 1-st kind with Hilbert-type singularity. To solve it, the Galerkin approach is used. It is shown that when approximating the airfoil boundary with a polygon, it is possible to develop schemes of the 1-st and 2-nd order of accuracy, considering a piecewise-constant or piecewise-linear (discontinuous or continuous) distribution of the solution along the panels. The necessary formulae are presented for calculating the components of the matrix and the right-hand side of the system of linear algebraic equations, that is a discrete analogue of the integral equation. They are suitable for modelling of the vorticity generation when simulating the flow around either single airfoil or system of airfoils, including moving and/or deformable ones. The developed schemes can be used in the framework of the viscous vortex domains method as well as other modifications of vortex particle methods, since they only concern the convective velocities of the flow near the airfoil and are not related to methods for modeling viscous diffusion of vorticity

Numerical investigation of a propeller operating under different inflow conditions

This work investigates the flow physics in propeller wakes to better understand how propeller wakes evolve under different inflow conditions from near field to far field. A rotating propeller is numerically modeled by using a dynamic overset technique that involves the improved delayed detached-eddy simulation method. To validate the numerical approach, its results are compared against experimentally determined thrust and torque coefficients and flow fields. The results show that, compared with uniform inflow, turbulent inflow significantly modifies the morphology of the vortex system behind the propeller. Under turbulent-inflow conditions, turbulent structures appear around the boundary layer of the propeller blades and interact with the boundary layer flow of the propeller blades, leading to instability and diffusion of primary tip vortices shed by the blade tips. Multiple local pairing in the circumferential direction leads to the rapid breakdown of the tip vortex system, accompanied by the generation of numerous secondary vortex structures. Tip vortices quickly lose coherence in the middle field and far field and tend to be homogeneously distributed when there is inflow turbulence. The present study gives a deeper insight into the flow physics driving the tip vortex pairing process for a propeller operating under uniform- and turbulent-inflow conditions.

A single bubble rising in the vicinity of a vertical wall: A numerical study based on volume of fluid method

Single bubble rising in the close vicinity of a vertical wall is one of the focuses in the field of gas-liquid two-phase flow. In this work, three-dimensional direct numerical simulation based on volume of fluid (VOF) and adaptive mesh refinement method is performed to investigate the bubble rising and wake structures near a vertical wall. The bubble migration trajectory, deformation, velocity and wake structures were analyzed under various Galileo numbers and initial wall distances. Bubbles with a Galileo number of 8.8 significantly migrate away from the wall, which is driven by the repulsive force as a result of the suppression of the vortex diffusion on the bubble surface. As the Galileo number increases, the asymmetry of the shedding vortex tends to play a vital role on the repulsive force, which is exacerbated by the presence of the vertical wall. A periodic shedding of the vortex appears behind the bubble when the Galileo number reaches at 95. Meanwhile, the bubble trajectory changes from rectilinear to spiral along with a periodic oscillation. Due to the interaction between the vertical wall and the bubble wake, a transition from a rectilinear rising path to a spiral rising path is more likely to occur compared to that without a vertical wall. The presence of a vertical wall contributes to bubble-rising oscillations. A decrease of the bubble terminal velocity due to viscous effect near the vertical wall is observed for bubbles at low Galileo numbers. However, the viscous effect is less pronounced as the Galileo number increases.

Performance of seal vibrissa-inspired bionic surface in suppressing aerodynamic forces and vortex shedding around a circular cylinder

In this study, circular cylinders with bionic surfaces were developed to control the flow fields around cylindrical structures such as stay cables of bridges. The bionic cylinder geometries were characterized by slanted elliptical governing sections and wave surfaces. Wind tunnel experiments were conducted to identify the surface pressure distributions, aerodynamic forces, and flow field characteristics of bionic circular cylinders at a Reynolds number (Re) of 3.8 × 104. The results show that the aerodynamic forces of the bionic cylinders are sensitive to the wind attack angle (γ) and wave surface wavelength. The control effects were excellent at γ = 0°, 90°, and 180°, whereas they vanished at γ = 45° and 135° compared with a baseline circular cylinder. When the wavelength range of the bionic cylinder is 4D–5D, the maximum reductions in the mean drag and fluctuating lift were 27% and 93%, respectively. The vortex formation region was elongated, and the alternative vortex shedding process was suppressed. Proper orthogonal decomposition was performed to determine the coherent structure of the wake field and the boundary vorticity flux analysis demonstrates the difference in the strength of the wake vortex from the viewpoint of boundary vorticity diffusion between the fluid and wall. Spatial–temporal correlation analysis of the velocity profiles behind different cross-sections shows that the wave surface troughs contribute significantly to disrupting the flow consistency in the spanwise direction.

CFD simulation and measurement of the downwash airflow of a quadrotor plant protection UAV during operation

To improve the effectiveness of unmanned aerial vehicle (UAV) spray, it is necessary to clarify the flow characteristics and penetration inside the canopy of the downwash airflow of UAV. This study used a three-dimensional computational fluid dynamic (CFD) model for the flow characteristics and velocity distribution of the downwash airflow generated by a quadrotor plant protection UAV at different flight velocities (1 ∼ 5 m/s) was established. The measurement tests for airflow velocity distribution inside the canopies were conducted. The results indicated that the spiral vortices appeared below the rotors and then evolved into inclined horseshoe vortices. The horseshoe vortices at the flight velocity of 5 m/s exceeded the flight height of the UAV and tended to cause droplet drift. When the flight velocity was larger than 4 m/s, it was difficult for the horseshoe vortex to enter the bottom of the canopy, which was not conducive to improving the droplet penetration performance. The diffusion and attenuation of the horseshoe vortex during the flow process led to an increase and then a decrease in the velocity distribution area in the Z-direction of airflow. In order to improve the droplet penetration performance, the optimal operating distance range between UAV and canopy was given when the flight velocity was less than 3 m/s based on the variation law of the velocity distribution in the Z-direction. The validation results showed that the simulated and measured values maintained the same trend.

Propeller wake instabilities under turbulent-inflow conditions

The wake instabilities of a propeller operating under turbulent-inflow conditions were studied by the improved delayed detached eddy simulation method on an unstructured mesh consisting of almost 82.5 × 106 cells, capturing propeller wakes extending to the downstream distance of 9D (where D is the propeller diameter). Two turbulent-inflow cases with the turbulence intensity of 5% and 20% were considered. The mean loads and phase-averaged flow field show good agreement with experiments. As the propeller blade interacts with the turbulent inflow, a wide peak extending approximately ±10 Hz in the power spectral density of the time histories of the thrust and torque coefficient. Simulation results reveal wake instability mechanisms of the propeller operating under different turbulent-inflow conditions. The turbulence added to the inlet boundary interacts with the tip vortices, which accelerates the destabilization processes of the tip vortex system from two aspects. First, the interaction between the inflow turbulence and the tip vortex promotes the diffusion of tip vortices. Second, the interaction between the inflow turbulence and the tip vortices magnifies the instability motion of the tip vortex. The wake vortex system of the high-turbulence inflow condition loses its stability after 2.2D downstream, while the initial instability behaviors for the low-turbulence inflow condition are observed at the location of 3.4D downstream. The present study presents a deeper insight into the flow physics driving the tip vortex pairing process for a propeller operating under turbulent-inflow conditions.

Vortex ring connection to a free surface

We perform numerical simulations of the interaction between a vortex ring and a free surface, and provide a new interpretation of the mechanism by which the vortex ring connects to the free surface. Large vorticity gradients at the free surface result in the diffusion of surface-tangential vorticity out of the fluid. This is accompanied by the diffusion of opposite-signed surface-normal vorticity away from the connection line, along the free surface, which results in the attachment of the vortex ring to the free surface. Compared to existing descriptions, this interpretation explains better how the solenoidal property that vortex lines do not end in the fluid is maintained. By including an interface vortex sheet at the free surface, the important property of vorticity conservation is maintained throughout the interaction. The upper part of the vortex ring simply diffuses out of the fluid, and into the interface vortex sheet, with the total circulation remaining constant.

Universal Anomalous Diffusion of Quantized Vortices in Ultraquantum Turbulence.

In classical viscous fluids, turbulent eddies are known to be responsible for the rapid spreading of embedded particles. However, in inviscid quantum fluids where the turbulence is induced by a chaotic tangle of quantized vortices, dispersion of the particles can be achieved via a nonclassical mechanism, i.e., their binding to the evolving vortices. However, knowledge on how the vortices diffuse and spread in quantum-fluid turbulence is very limited, especially for the so-called ultraquantum turbulence (UQT) generated by a random tangle of vortices. Here we report a systematic numerical study of the apparent diffusion of vortices in UQT in superfluid helium-4 using the full Biot-Savart simulation. We reveal that the vortices in the superfluid exhibit a universal anomalous diffusion (superdiffusion) at small times, which transits to normal diffusion at large times. This behavior is found to be the result of a generic scaling property of the vortex velocity. Our simulation at finite temperatures also nicely reproduces recent experimental observations. The knowledge obtained from this study may form the base for understanding turbulent transport and universal vortex dynamics in various quantum fluids.

Study of radiative magneto-non-Newtonian fluid flow over a nonlinearly elongating sheet with Soret and Dufour effects

Casson fluid model is extensively used for modeling of blood flow in thin arteries at low shear rates. The current article investigates the Soret and Dufour effects on free convective flow of Casson fluid over a nonlinearly elongating sheet embedded in a porous medium. Free convection: Flows arising solely due to the difference in density caused by temperature differences. The exponential space-based heat source/sink and thermal radiation are also incorporated in the heat transfer equation. Similarity solution was obtained by using suitable transformations and the transformed equations are solved by using the Runge–Kutta fourth-order method with a shooting technique by the help of the MATLAB code. The impacts of operating parameters on velocity, temperature, and concentration distributions are explained through graphs and tables. Important findings of the analysis are: Dufour and Soret effects serve as the surface coolant and maintenance of solutal deposition-free surface. Applied magnetic field and heavier diffusing species with suction lead to thinner boundary condition preventing vorticity diffusion.