Vortical Motion Research Articles

Single Cell Biophysics Drives Wound Healing Dynamics

The ability of cells to move in collective groups or sheets is a phenomenon observed in a number of significant biological processes, such as bone remodeling, embryonic morphogenesis, wound repair and cancer invasion and metastasis. These cells are held together through cell-cell adhesion molecules and move using heterogeneous biochemical and/or environmental cues to guide force production and morphological changes. It has been observed that these cell layers exhibit non-trivial dynamics, such as vortical motion and long-range order. This suggests that the physical interactions between neighboring cells may be an important factor guiding multicellular movements. In addition, recent findings have shown that several aspects of these collective motions can be described by the biophysics of single cells, which dictates cell speed, shape, persistence of motion, traction stress and substrate adhesion. Using live-cell imaging and traction force miscopy we experimentally measured biophysical motility parameters of isolated treated and drug-treated epithelial cells. These results were then used in a mathematical model for multicellular motion to determine the role of cellular level biophysics in the collective migration of epithelial cells. The predictions of the model were tested by comparing the results to collective cell migration experiments, where we used image processing techniques to map the velocity field and force distribution of collectively migrating cells. Deviations between the experiments and the model were used to further refine the model, thereby generating new hypotheses for the biophysical mechanisms that guide epithelial cell migration.

Blast Wave Induced Flows in Semicircular Canals

Dynamics of endolymph and cupula in a semicircular canal subject to high frequency oscillations imposed on the endolymph at the open section of the canal is investigated. Deformation of cupula inside the endolymph fluid is simulated numerically.A high frequency and high amplitude periodic oscillations may result in the formation of vortical motions inside the semicircular canals. The number of vortices increases with the oscillation frequency. As a result of asymmetry of the canal geometry, there is an asymmetry in the pressure variation across the cupula resulting in a net motion of cupula in a specific direction during each cycle. The motion of the cupula depends on the vibration amplitude and frequency.

High-resolution Euler simulation of a cylindrical blast wave in an enclosure

We solve the Euler equations to simulate a cylindrical blast wave in an enclosure. The blast is initiated close to the bottom wall of a square enclosure. An initial Mach 10 shock wave moves into ambient air, causing a mushroom structure behind the shock. We relate the development of this structure to the passage of the reflected shock from the bottom wall through the core of low-density air, which appears to trigger the Richtmyer–Meshkov instability. Use of a ninth-order derivative formula with a four-stage third-order Runge–Kutta time stepping scheme allows us to visualize the vortical motion at the core of the blast in great detail, including the spikes of ambient air penetrating the blast core and two symmetric trails of small-scale vortices behind the primary vortices.

Short- and long-term dynamic modes of turbulent swirling premixed flame in a cuboid combustor

Direct numerical simulation (DNS) of hydrogen–air turbulent swirling premixed flame in a cuboid combustor is conducted to investigate thermoacoustic instability and dynamic modes of turbulent swirling premixed flame in combustors. A detailed kinetic mechanism and temperature dependency of physical properties are considered in DNS. Two swirl number cases of 0.6 and 1.2 are investigated. Large-scale helical vortical structures are generated near the inlet of combustion chamber, and a lot of fine-scale vortices emerge downstream. Flame structure is engulfed by the vortical structures and depends largely on the swirl number. Spectral analysis of pressure oscillation on walls shows that quarter-wave mode of longitudinal acoustics has the largest energy, and that characteristic oscillations are found at higher frequencies. To investigate oscillation modes of pressure and heat release rate fields, dynamic mode decomposition (DMD) is applied to DNS results. It is shown that there are differences between dominant frequencies in spectral analysis of time-series pressure data at one point of walls and DMD of time-series pressure field data. DMD of pressure field reveals that the quarter-wave mode in longitudinal acoustics has the largest energy for the case of S=0.6. Furthermore, it is clarified that the transverse acoustic plane waves and pressure oscillations induced by large-scale vortical motions play important roles for the pressure oscillations in combustors. DMD of heat release rate field reveals that the DMD modes of pressure with high amplitude do not necessarily have coupling with fluctuations of heat release rate. Interactions between dynamic modes of pressure and heat release rate are also discussed.

Haroldo de Campos e a interpretação luciferina

The path that Haroldo de Campos takes on his reading of Dante Alighieri (Traduzir e Trovar and Pedra e Luz na Poesia de Dante) opens a new view on the work of the great Italian author. Translating parts of the Paradiso, he invokes the metaphor of light, which alludes to Robert Grosseteste vision, a great intellectual of the Middle Ages, of Dante's age. Translating, to Haroldo de Campos, is equivalent to realize a hypercritical reading, which causes a radical destabilization in the text. Tresler, tresluzir, transcriar, transparecer: these all are concepts Haroldo uses. They are the product of the coupling between reading/creating, light and the prefix trans or tres. Tresler, e.g., whose meaning may be “read in reverse”, can also be read as “losing his mind” or “talking nonsense”. There is therefore a vortical motion in diagonal or “upside down” typical of Haroldo de Campos, which highlights a tension between intellect and mystical path in Dante, revealed by the Paradise light. And this makes Harold recover Lucifer (Satan), from his etymology “which brings light”, and who, according to popular tradition, is the owner of superior wisdom, denied to the common man. A striking analogy with the Golem, of the Jewish tradition.

CFD MODELING OF A SOLITARY WAVE OVERTOPPING BREAKWATER OF VARYING SUBMERGENCE

In this paper, interaction of a nonlinear solitary wave with a submerged breakwater as well as a breakwater with zero freeboard are modeled using Computational Fluid Dynamic (CFD). Simulation results of water particle velocities as well as water surface elevations at the vicinity of breakwater are validated by comparison to experimental data. In order to find out the best turbulence modeling appropriate for wave-breakwater interaction problem, simulations are conducted using standard K-Epsilon, SST-K Omega, and inviscid models available within the CFD software. In cases involving wave breaking, simulations are also repeated using second order SST-K-Omega model. The overall pattern of the wave interaction with breakwater is adequately reproduced by the CFD software. The processes of wave reflection, wave breaking, and vortical motion at the vicinity of breakwater are correctly captured. The good results achieved are promising regarding the use of CFD for design of breakwaters and other marine structures subject to waves.

Vortex particle method in parallel computations on graphical processing units used in study of the evolution of vortex structures

Understanding the dynamics and the mutual interaction among various types of vortical motions is a key ingredient in clarifying and controlling fluid motion. In the paper several different cases related to vortex tube interactions are presented. Due to problems with very long computation times on the single processor, the vortex-in-cell (VIC) method is implemented on the multicore architecture of a graphics processing unit (GPU). Numerical results of leapfrogging of two vortex rings for inviscid and viscous fluid are presented as test cases for the new multi-GPU implementation of the VIC method. Influence of the Reynolds number on the reconnection process is shown for two examples: antiparallel vortex tubes and orthogonally offset vortex tubes. Our aim is to show the great potential of the VIC method for solutions of three-dimensional flow problems and that the VIC method is very well suited for parallel computation.

Observations of Photospheric Vortical Motions During the Early Stage of Filament Eruption

Solar filaments/prominences exhibit rotational motion during different phases of their evolution from their formation to eruption. We have observed the rotational/vortical motion in the photosphere near the ends of ten filaments during their initial phase of eruption, at the onset of the fast rise phase. All the filaments were associated with active regions. The photospheric vortical motions we observed lasted for 4--20 minutes. In the vicinity of the conjugate ends of the filament the direction of rotation was opposite, except for two cases, where rotational motion was observed at only one end point. The sudden onset of a large photospheric vortex motion could have played a role in destabilizing the filament by transporting axial flux into the activated filament thereby increasing the outward magnetic pressure in it. The outward magnetic pressure may have pushed the filament/flux rope to the height where the torus instability criterion was satisfied, and hence it could have caused the filament instability and eruption.

Flame Leading Edge and Flow Dynamics in a Swirling, Lifted Flame

Flames in high swirl flow fields with vortex breakdown often stabilize aerodynamically in front of interior flow stagnation points. In contrast to shear layer stabilized flames with a nearly fixed, well-defined flame attachment point, the leading edge of aerodynamically stabilized flames can move around substantially as a result of both the inherent dynamics of the vortex breakdown region and externally imposed oscillations. Motion of this flame stabilization point relative to the flow field may have an important dynamical role during combustion instabilities, as it creates flame front wrinkles and heat release fluctuations. For example, a prior study has shown that nonlinear dynamics of the flame response at high forcing amplitudes were related to these leading edge dynamics. This heat release mechanism exists alongside other flame wrinkling processes, arising from such processes as shear layer rollup and swirl fluctuations. This article describes an experimental investigation of flow forcing effects on the dynamics of the leading edge of a swirl stabilized flame. Flame and flow dynamics were characterized using high-speed particle image velocimetry (PIV) and CH* chemiluminescence imaging. A range of forcing conditions was achieved by varying the forcing frequency, amplitude, and acoustic field symmetry. These results show that the flame leading edge motion is dominated by the natural flow instabilities, particularly the precession of the recirculation zone around the centerline. The fluctuations in leading edge motion at the excitation frequency are much smaller than these natural motions, but are still on the order of the fluid particle displacement associated with the external excitation, indicating that they exhibit an important influence on local flame wrinkling and heat release. Flame response modeling shows that the global, spatially integrated heat release response is controlled by three factors—vortical disturbances, acoustic flow disturbances, and flame leading edge motion. The vortical flow motions dominate the flame response, and the flame leading edge motion is a minor contributor to the overall heat release response. This is an important result, as it shows that the significant motions of the flame leading edge actually have little dynamical significance for understanding the spatially integrated, forced response of the flame.

Time-distance helioseismology: A new averaging scheme for measuring flow vorticity

Time-distance helioseismology provides information about vector flows in the near-surface layers of the Sun by measuring wave travel times between points on the solar surface. Specific spatial averages of travel times have been proposed for distinguishing between flows in the east-west and north-south directions and measuring the horizontal divergence of the flows. No specific measurement technique has, however, been developed to measure flow vorticity. Here we propose a new measurement technique tailored to measuring the vertical component of vorticity. Fluid vorticity is a fundamental property of solar convection zone dynamics and of rotating turbulent convection in particular. The method consists of measuring the travel time of waves along a closed contour on the solar surface in order to approximate the circulation of the flow along this contour. Vertical vorticity is related to the difference between clockwise and counter-clockwise travel times. We applied the method to characterize the vortical motions of solar convection using helioseismic data from the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory (SDO/HMI) and from the Michelson Doppler Imager onboard the Solar and Heliospheric Observatory (SOHO/MDI). Away from the equator, a clear correlation between vertical vorticity and horizontal divergence is detected. Horizontal outflows are associated with negative vorticity in the northern hemisphere and positive vorticity in the southern hemisphere. The signal is much stronger for HMI than for MDI observations. We characterize the spatial power spectrum of the signal by comparison with a noise model. Vertical vorticity at horizontal wavenumbers below 250/R_Sun can be probed with this helioseismic technique.

Skew-symmetric couple-stress fluid mechanics

There can be no doubt as to the importance of vortical motion in fluid mechanics. Yet, very little attention is given typically to the balance law of angular momentum and to its role in defining the fundamental character of stress, which as a result is usually assumed as a symmetric tensor. Here, we allow for the possibility of couple-stresses, along with general non-symmetric force–stresses, and develop a self-consistent size-dependent theory within the context of classical continuum mechanics. This development relies upon the identification of the following key components for the dynamic response of three-dimensional fluid continua: (i) fundamental, uniquely defined kinematical measures of flow, (ii) an independent set of energy conjugate variables, (iii) the corresponding permissible natural and essential boundary conditions, and (iv) a non-redundant set of body-force and inertial contributions. Based upon this formulation, one can recognize that the previous couple-stress theory for fluids suffers from some inconsistencies, which may have restricted its applicability in the study of viscous flows. After presenting the general formulation of the new consistent theory, we specialize for incompressible viscous flow and consider the problem of generalized Poiseuille flow within this size-dependent fluid mechanics. Finally, we conclude that the theory presented here may provide a basis for a broad range of fluid mechanics applications and for fundamental studies of flows at the finest scales for which a continuum representation is valid.

Radiation induces turbulence in particle-laden fluids

When a transparent fluid laden with solid particles is subject to radiative heating, non-uniformities in particle distribution result in local fluid temperature fluctuations. Under the influence of gravity, buoyancy induces vortical fluid motion which can lead to strong preferential concentration, enhancing the local heating and more non-uniformities in particle distribution. By employing direct numerical simulations this study shows that the described feedback loop can create and sustain turbulence. The velocity and length scale of the resulting turbulence is not known a priori, and is set by balance between viscous forces and buoyancy effects. When the particle response time is comparable to a viscous time scale, introduced in our analysis, the system exhibits intense fluctuations of turbulent kinetic energy and strong preferential concentration of particles.

SIMULATIONS OF EMERGING MAGNETIC FLUX. II. THE FORMATION OF UNSTABLE CORONAL FLUX ROPES AND THE INITIATION OF CORONAL MASS EJECTIONS

We present results from 3D magnetohydrodynamic (MHD) simulations of the emergence of a twisted convection zone flux tube into a pre-existing coronal dipole field. As in previous simulations, following the partial emergence of the sub-surface flux into the corona, a combination of vortical motions and internal magnetic reconnection forms a coronal flux rope. Then, in the simulations presented here, external reconnection between the emerging field and the pre-existing dipole coronal field allows further expansion of the coronal flux rope into the corona. After sufficient expansion, internal reconnection occurs beneath the coronal flux rope axis, and the flux rope erupts up to the top boundary of the simulation domain ($\sim$ 36 Mm above the surface). We find that the presence of a pre-existing field, orientated in a direction to facilitate reconnection with the emerging field, is vital to the fast rise of the coronal flux rope. The simulations shown in this paper are able to self-consistently create many of the surface and coronal signatures used by coronal mass ejection (CME) models. These signatures include: surface shearing and rotational motions; quadrupolar geometry above the surface; central sheared arcades reconnecting with oppositely orientated overlying dipole fields; the formation of coronal flux ropes underlying potential coronal field; and internal reconnection which resembles the classical flare reconnection scenario. This suggests that proposed mechanisms for the initiation of a CME, such as "magnetic breakout", are operating during the emergence of new active regions.

Rotational bands in well deformed heavy nuclei

A method for the description of rotational states in nuclei is proposed that takes into account the dynamical stretching of the nuclear mean field, as well the dynamical reduction of pairing correlations due to rotation, and is shown to agree well with the experimental data even up to angular momenta. We also compare our new method with an approach based on an intrinsic vortical motion in rotating nuclei.

Numerical investigation of the flow behavior into a Francis runner during load rejection

The main objective of the work presented in this paper is to investigate numerically the flow behavior inside a Francis hydro-turbine during the transient event of load rejection. First, a theoretical description of the flow during the event is presented in order to predict the global flow characteristics to be anticipated since no velocity profiles are available for this transient event. The issue of choosing the proper boundary conditions to obtain the absolute pressure and the correct flow characteristics within the runner when using a typical truncated geometry is then discussed. Finally, by using a hypothesis of "quasi-stationarity" and a validated methodology, global flow characteristics within the turbine are highlighted near the no-load operating condition and the unsteady vortical motions within the runner are assessed.

Nuclear vorticity in isoscalarE1modes: Skyrme-random-phase approximation analysis

Two basic concepts of nuclear vorticity, hydrodynamical (HD) and Rawenthall-Wambach (RW), are critically inspected. As a test case, we consider the interplay of irrotational and vortical motion in isoscalar electric dipole $E1(T=0)$ modes in ${}^{208}\mathrm{Pb}$, namely the toroidal and compression modes. The modes are described in a self-consistent random-phase approximation (RPA) with the Skyrme force SLy6. They are examined in terms of strength functions, transition densities, current fields, and form factors. It is shown that the RW conception (suggesting the upper component of the nuclear current as the vorticity indicator) is not robust. The HD vorticity is not easily applicable either because the definition of a velocity field is too involved in nuclear systems. Instead, the vorticity is better characterized by the toroidal strength which closely corresponds to HD treatment and is approximately decoupled from the continuity equation.

Low‐altitude electron acceleration due to multiple flow bursts in the magnetotail

At 10:00 UT on 25 February 2008, Cluster 1 spacecraft crossed the near‐midnight auroral zone, at about 2 RE altitude, while two of the Time History of Events and Macroscale Interactions During Substorms (THEMIS) spacecraft, THD and THE, observed multiple flow bursts on the near‐conjugate plasma sheet field lines. The flow shear pattern at THEMIS was consistent with the vortical motion at duskside of a localized flow channel. Coinciding in time with the flow bursts, Cluster 1 observed bursts of counterstreaming electrons with mostly low energies (≤441 eV), accompanied by short time scale (<5 s) magnetic field disturbances embedded in flow‐associated field‐aligned current systems. This conjugate event not only confirms the idea that the plasma sheet flows are the driver of the kinetic Alfvén waves accelerating the low‐energy electrons but is a unique observation of disturbances in the high‐altitude auroral region relevant to the multiple plasma sheet flows.

Effect of Fluid Viscoelasticity on Turbulence and Large-Scale Vortices behind Wall-Mounted Plates

Direct numerical simulations of turbulent viscoelastic fluid flows in a channel with wall-mounted plates were performed to investigate the influence of viscoelasticity on turbulent structures and the mean flow around the plate. The constitutive equation follows the Giesekus model, valid for polymer or surfactant solutions, which are generally capable of reducing the turbulent frictional drag in a smooth channel. We found that turbulent eddies just behind the plates in viscoelastic fluid decreased in number and in magnitude, but their size increased. Three pairs of organized longitudinal vortices were observed downstream of the plates in both Newtonian and viscoelastic fluids: two vortex pairs were behind the plates and the other one with the longest length was in a plate-free area. In the viscoelastic fluid, the latter vortex pair in the plate-free area was maintained and reached the downstream rib, but its swirling strength was weakened and the local skin-friction drag near the vortex was much weaker than those in the Newtonian flow. The mean flow and small spanwise eddies were influenced by the additional fluid force due to the viscoelasticity and, moreover, the spanwise component of the fluid elastic force may also play a role in the suppression of fluid vortical motions behind the plates.

Evaluation of Elevated Mean Pulmonary Arterial Pressure Based on Magnetic Resonance 4D Velocity Mapping: Comparison of Visualization Techniques

PurposeThree-dimensional (3D) magnetic resonance phase contrast imaging (PC-MRI) allows non-invasive diagnosis of pulmonary hypertension (PH) and estimation of elevated mean pulmonary arterial pressure (mPAP) based on vortical motion of blood in the main pulmonary artery. The purpose of the present study was to compare the presence and duration of PH-associated vortices derived from different flow visualization techniques with special respect to their performance for non-invasive assessment of elevated mPAP and diagnosis of PH.MethodsFifty patients with suspected PH (23 patients with and 27 without PH) were investigated by right heart catheterization and time-resolved PC-MRI of the main pulmonary artery. PC-MRI data were visualized with dedicated prototype software, providing 3D vector, multi-planar reformatted (MPR) 2D vector, streamline, and particle trace representation of flow patterns. Persistence of PH-associated vortical blood flow (tvortex) was evaluated with all visualization techniques. Dependencies of tvortex on visualization techniques were analyzed by means of correlation and receiver operating characteristic (ROC) curve analysis.Resultstvortex values from 3D vector visualization correlated strongly with those from other visualization techniques (r = 0.98, 0.98 and 0.97 for MPR, streamline and particle trace visualization, respectively). Areas under ROC curves for diagnosis of PH based on tvortex did not differ significantly and were 0.998 for 3D vector, MPR vector and particle trace visualization and 0.999 for streamline visualization. Correlations between elevated mPAP and tvortex in patients with PH were r = 0.96, 0.93, 0.95 and 0.92 for 3D vector, MPR vector, streamline and particle trace visualization, respectively. Corresponding standard deviations from the linear regression lines ranged between 3 and 4 mmHg.Conclusion3D vector, MPR vector, streamline as well as particle trace visualization of time-resolved 3D PC-MRI data of the main pulmonary artery can be employed for accurate vortex-based diagnosis of PH and estimation of elevated mPAP.

Air–water mass transfer mechanism due to the impingement of a single liquid drop on the air–water interface

The mass transfer mechanism across the air–water interface due to the impingement of a single liquid drop was investigated through laboratory experiments using particle imaging velocimetry (PIV) and planar laser-induced fluorescence technique (PLIF). Velocity and CO2 concentration fields in the liquid after the impingement were visualized. The results show that the impingement of a single liquid drop on the water surface generates several vortex rings near the water surface. The vortex rings renew the water surface and also convect the CO2 gas dissolved near the water surface downward. The vortical motion clearly shows that the vortex rings work as surface-renewal eddies. The radius, center velocity and presence time of surface-renewal eddies increase with increasing momentum of the impinging drop. This suggests that surface-renewal eddies with larger radius and faster center velocity are induced by the impingement of a single drop with larger vertical momentum, and air–water mass transfer is promoted by such eddies. Based on the surface-renewal concept including the area and time fractions, a model for the air–water mass transfer due to multiple impingements of drops is also proposed.