Coherent Vortical Structures Research Articles

Numerical study and POD snapshot analysis of flow characteristics for pulsating turbulent opposing jets

PurposeThe purpose of this study is to investigate the coherent structures of pulsed opposing jets by large eddy simulation (LES) model and proper orthogonal decomposition (POD) snapshot method. Flow pulsation as an active flow control method is considered for the enhancement of transport phenomena in impinging jets. The effect of flow pulsation parameters such as pulsation signal shape and frequency on the vortical coherent structures, the energy content of primary modes and their variation are studied numerically.Design/methodology/approachIn this study, flow field of turbulent pulsating opposing jets has been simulated using LES. The result of the simulation in different time steps (snapshots) are stored and POD is applied on the snapshots. In this study, the POD method and calculation of spatial modes has been done using OpenFOAM, and time coefficients have been calculated using a MATLAB code.FindingsThe results of this study show that the flow excitation has a great effect on the coherent structure formation and the energy containment of fundamental modes of the flow. When the flow was excited by a harmonic sinusoidal or step function, the turbulent kinetic energy accumulated in the set of primary modes. On the other hand, the pulsed opposing jets had more regularity compared to the steady jets. The shapes, patterns and energy values of dominant modes depended on the inlet pulsation signal. An increase in pulsation frequency leads to an augmentation in energy content of the primary modes.Research limitations/implicationsThe predictions may be extended to include various pulsation conditions such as: various amplitudes, Reynolds number and aspect ratio.Practical implicationsThe results of this study are a valuable source of information for active control of transport phenomena in opposing jet configurations which is used in different industrial applications such as cooling, combustion, reactors, heating and drying processes.Originality/valueIn this study, the coherent structures and energy content of primary modes was studied for the first time by LES model and POD snapshot method and a comprehensive discussion on numerical results is provided.

Objective magnetic vortex detection.

Magnetic coherent vortical structures are ubiquitous in space and astrophysical plasmas and their detection is key to understanding the nature of the intrinsic turbulence in those conducting fluids. A recently developed method to detect magnetic vortices is explored in problems of two- and three-dimensional magnetohydrodynamic simulations. The integrated averaged current deviation, the normed difference of the current density at a point and the mean current density in the domain, integrated along a magnetic field line, is proved to be objective, i.e., invariant under rotations and translations of the observer. The method is shown to detect accurately the boundary of magnetic vortices in two-dimensional simulations, as well as magnetic flux ropes in three dimensions.

Forced Flow Reattachment Process and the Effect of Pressure Gradient

Stereo particle image velocimetry experiments were conducted to explore the flowfield resulting from the interaction of finite-span synthetic jet actuators and a turbulent boundary layer over an S817 airfoil at various angles of attack. Three specific scenarios were investigated: a favorable pressure gradient, an adverse pressure gradient, and a separated flow in which the synthetic jet actuators reattached the flow. Multiple coherent vortical structures were shed during the reattachment process, for which the circulation associated with these structures was composed of the “trapped” vorticity within the separated shear layer. The shedding of this vorticity was shown to be an inherent part of the reattachment mechanism. In a smaller, local measurement domain, volumetric flowfields were reconstructed near the synthetic jet orifice. It was shown that the presence of an adverse pressure gradient resulted in an increase of momentum diffusion through the boundary layer, in addition to the rate at which the jet’s axis switching occurred. At the highest angle of attack tested, where the flow was reattached through actuation, the coherent structures associated with the synthetic jet dissipated significantly quicker spatially, as much of the energy associated with the actuation was used to maintain attached flow.

Analysis of the isothermal transition process in wide aspect ratio rectangular channels

This work is aiming at extending the understanding of the transition process from a laminar gas flow to a turbulent gas flow in wide aspect ratio rectangular channels under isothermal conditions. The role of the inlet geometry and its impact on the transition process have been studied. Three inlet geometries have been used to influence the transition process, each geometry having a different critical Reynolds number at which the flow is becoming turbulent. The geometries are named bellmouth, square edged and undulated geometry. Multiple measurement techniques were applied: Laser Doppler Anemometry was applied to measure the inlet boundary conditions in terms of mean and fluctuating velocity field statistics. Static pressure measurements were applied to measure the friction diagram in between 2000 < ReDh < 10,000. Double frame Particle Image Velocimetry was applied to measure velocity correlations between the stream-wise velocity component and the velocity component perpendicular to the wall and its connection to the friction diagram. Furthermore, Time Resolved Particle Image Velocimetry was applied to visualize the generation of coherent vortical structures originating at the inlet into the rectangular channel.

Friction factor for turbulent open channel flow covered by vegetation

The need for operational models describing the friction factor f in streams remains undisputed given its utility across a plethora of hydrological and hydraulic applications concerned with shallow inertial flows. For small-scale roughness elements uniformly covering the wetted parameter of a wide channel, the Darcy-Weisbach f = 8(u*/Ub)2 is widely used at very high Reynolds numbers, where u* is friction velocity related to the surface kinematic stress, Ub = Q/A is bulk velocity, Q is flow rate, and A is cross-sectional area orthogonal to the flow direction. In natural streams, the presence of vegetation introduces additional complications to quantifying f, the subject of the present work. Turbulent flow through vegetation are characterized by a number of coherent vortical structures: (i) von Karman vortex streets in the lower layers of vegetated canopies, (ii) Kelvin-Helmholtz as well as attached eddies near the vegetation top, and (iii) attached eddies well above the vegetated layer. These vortical structures govern the canonical mixing lengths for momentum transfer and their influence on f is to be derived. The main novelty is that the friction factor of vegetated flow can be expressed as fv = 4Cd(Uv/Ub)2 where Uv is the spatially averaged velocity within the canopy volume, and Cd is a local drag coefficient per unit frontal area derived to include the aforemontioned layer-wise effects of vortical structures within and above the canopy along with key vegetation properties. The proposed expression is compared with a number of empirical relations derived for vegetation under emergent and submerged conditions as well as numerous data sets covering a wide range of canopy morphology, densities, and rigidity. It is envisaged that the proposed formulation be imminently employed in eco-hydraulics where the interaction between flow and vegetation is being sought.

On the surface expression of a canopy-generated shear instability

Results are presented from a laboratory study on the free-surface signal generated over an array of submerged circular cylinders, representative of submerged aquatic vegetation. We aim to understand whether aquatic ecosystems generate a surface signature that is indicative of both what is beneath the water surface as well as how it is altering the flow. A shear layer forms over the canopy, generating coherent vortex structures which eventually manifest in the free-surface slope field. We connect the vortex properties measured at the surface with measurements of the bulk flow, and show that correlations between these quantities are adequate to create a parameterized model in which the interior velocity profile can be predicted solely from measurements taken at the free surface. Experimental surface observations yield a Strouhal number that is twice the most amplified mode predicted by linear stability theory, suggesting that vortices may evolve between generation at the canopy height and their manifestation at the water surface. Additionally, the surface signal continues evolving with distance downstream, with vortices becoming spread farther apart and the passage frequency gradually decreasing. By the trailing edge of the canopy, surface-impacting boils emerge for canopies with higher submergence ratios, suggesting a transition from coherent two-dimensional rollers to transversely varying structures.

On the inherent bias of swirling strength in defining vortical structure

The traditional practice of using rotational motion as the principal attribute of coherent vortical structures in the buffer region of near-wall turbulent flow is shown to create a biased accounting of the role of vorticity within the structures. Vorticity associated with rotation is given a favored consideration against vorticity that is equally strong but not associated with rotation. Using data from a highly resolved direct numerical simulation of channel flow, it is shown that describing the structures based on the properties of the rotational field leads to a distorted view of the actual structures that are present. As a practical matter, this means that where hairpins are typically considered to be the flow structures, a more accurate description of the coherent events is that they are elongated mushroom-shaped vortical objects ejecting over low speed streaks. In this, hairpin-shaped rotational regions are embedded in the lobes of the mushrooms.

The structure of turbulent flow through submerged flexible vegetation

The hydrodynamics of turbulent flow through submerged flexible vegetation is investigated in a flume using acoustic Doppler velocimetery (ADV) measurements. The flow characteristics such as the energetics and momentum transfer derived from conventional spectral and quadrant analyses are considered as the flow encounters a finite vegetation patch. Consistent with numerous canopy flow experiments, a shear layer and coherent vortex structures near the canopy top emerge caused by Kelvin–Helmholtz instabilities after the flow equilibrates with the vegetated layer. These instabilities are commonly attributed to velocity differences between non-vegetated and vegetated canopy layers in agreement with numerous experiments and simulations conducted on dense rigid canopies. The power-spectral density function for vertical velocity turbulent fluctuations at different downstream positions starting from the edge of the vegetation layer are also computed. For a preset water depth, the dominant dimensionless frequency is found to be surprisingly invariant around 0.027 despite large differences in vegetation densities. The ejection and sweep events significantly contribute to the Reynolds stresses near the top of the vegetation. The momentum flux carried by ejections is larger than its counterpart carried by the sweeps above the canopy top. However, the momentum flux carried by sweeps is larger below the top of the canopy.

Impingement heat transfer of quadruple synthetic jets

In this work, the heat transfer performance of impinging quadruple synthetic jets is experimentally investigated by means of infrared thermography and the heated thin foil technique. Four configurations (monopole-like, dipole-like, quadrupole-like and 90-degrees-circularly-shifted-jets), obtained by introducing different phase shifts between the single synthetic jets, are comparatively studied at constant values of the Reynolds and Strouhal numbers, equal to 4000 and 0.2, respectively. It is shown that the heat transfer behavior is strictly related to the dynamics of the coherent vortex structures that form after the interaction of the vortex rings inherent to the single jets. The signature of these vortical structures on the heat transfer distributions is evident at short impingement distances, where the most significant differences between the investigated configurations are indeed found. In the comparison, the 90-degrees-circularly-shifted-jets configuration is observed to offer the highest heat transfer rates, although with the lowest degree of uniformity, while the monopole-like jet is characterized by the lowest Nusselt number values and the highest uniformity. As the impingement distance increases, these differences are progressively smeared out and the four configurations exhibit equivalent performance from the viewpoints of both magnitude and uniformity of the heat transfer rate.

Compressible structures in incompressible hydrodynamics and their role in turbulence onset

The formation of the coherent vortical structures in the form of thin pancakes is studied for three-dimensional flows at the high Reynolds regime when, in the leading order, the development of such structures can be described within the 3D Euler equations for ideal incompressible fluids. Numerically and analytically on the base of the vortex line representation we show that compression of such structures and respectively increase of their amplitudes are possible due to the compressibility of the continuously distributed vortex lines. It is demonstrated that this growth can be considered as analog of breaking for the divergence-free vorticity field. At high amplitudes this process has a self-similar behavior connected the maximal vorticity and the pancake width by the Kolmogorov type relation ωmax ∝ l-2/3. The role of such structures in the Kolmogorov spectrum formation is also discussed.

The study of the unstably-stratified marine atmospheric boundary layer by direct numerical simulation

Prediction of the properties of the atmospheric flows over sea is important for local and regional weather forecast. This prediction typically relies on the performance of large-scale meteorological models based on bulk formulae for air velocity, temperature and humidity. The bulk formulas relate turbulent fluxes of momentum, heat and vapor to bulk air velocity, humidity and air-sea temperature difference. The most widely used parameterizations used to compute the coefficients of proportionality in the bulk formulae are formulated on the basis of the Monin-Obukhov similarity theory (MOST) for different types of air stratification (stable, neutral and unstable). In our previous studies we showed that MOST quite accurately predicts the properties of the air-flow over waved water surface under neutral and stable stratification conditions provided that stratification effects are relatively weak and flow is in a statistically stationary state. In the present work, we perform direct numerical simulation and study the air flow over a waved water surface under unstable stratification conditions where the sea surface temperature is larger than the bulk air temperature. Such situation occurs in the tropical cyclone conditions as well as in polar lows at high latitudes. Our results show that in this case, the air flow dynamics is dominated by the development of large-scale cylindrical coherent vortex structures elongated in the direction of the mean wind. These structures cause a notable deviations from the MOST predictions for the air velocity and temperature profiles.

Quantitative visualization of the three-dimensional flow structures of a sweeping jet

The flow characteristics of a sweeping jet ejecting from a typical feedback channel fluidic oscillator were investigated using time-resolved particle image velocimetry. The three-dimensional flow structure of the jet was examined through the instantaneous and averaged velocity fields measured at the streamwise center plane and cross-sectional planes of the jet for a range of Reynolds numbers (8000, 12,000, 16,000, 20,000, and 24,000). As the Reynolds number increases, the frequency of the sweeping jet linearly increases from 0.7 to 2 Hz. However, the switching time of the jet flow direction from one side to the other is very short compared to the sweeping period. As a result, the ensemble-averaged flow patterns of the sweeping jet at the streamwise center plane look like twin jets issued from a V-shaped nozzle, regardless of the Reynolds number. The rapidly switching jet contains a counter-rotating vortex at the cross-sectional plane of the jet near the nozzle, similar to a cross-flow jet. The coherent vortex structures are rapidly destroyed and become random when the jet moves downstream.

Magnetospheric Multiscale Observation of Kinetic Signatures in the Alfvén Vortex

Abstract Alfvén vortex is a multiscale nonlinear structure that contributes to the intermittency of turbulence. Despite previous explorations mostly on the spatial properties of the Alfvén vortex (i.e., scale, orientation, and motion), the plasma characteristics within the Alfvén vortex are unknown. Moreover, the connection between the plasma energization and the Alfvén vortex still remains unclear. Based on high-resolution in situ measurements from the Magnetospheric Multiscale mission, we report for the first time distinctive plasma features within an Alfvén vortex. This Alfvén vortex is identified as being a 2D ( ) quasi-monopole with a radius of 10 proton gyroscales. Its magnetic fluctuations are anti-correlated with velocity fluctuations , thus the parallel current density and flow vorticity are anti-aligned. In different part of the vortex (i.e., edge, middle, center), the ion and electron temperatures are found to be quite different and they behave in the reverse trend: the ion temperature variations are correlated with , while the electron temperature variations are correlated with . Furthermore, the temperature anisotropies, together with the non-Maxwellian kinetic effects, exhibit strong enhancement at peaks of within the vortex. Comparison between observations and numerical/theoretical results are made. In addition, the energy-conversion channels and the compressibility associated with the Alfvén vortex are discussed. These results may help to understand the link between coherent vortex structures and the kinetic processes, which determines how turbulence energy dissipates in the weakly collisional space plasmas.

GEOMETRIC CHARACTERISTICS OF WAVE-GENERATED SAND RIPPLES: A FULL-SCALE EXPERIMENTAL STUDY

In the coastal environment, wave-induced sand ripples are usually observed under moderate near-bed flow conditions. Their occurrence significantly changes the local hydrodynamics and sediment transport processes. Over the past few decades, some solid progresses have been made towards understating the ripple dimensions under wave-generated near-bed flows, e.g., O'Donoghue et al. [2006], but very few studies are targeted on the more detailed geometric characteristics, e.g., the generic shape of ripples and the sharpness of ripple crests, which are closely related to the coherent vortex structures. This study is aimed at filling this knowledge gap.

The onset of dynamic stall at a high, transitional Reynolds number

Dynamic stall due to a ramp-type pitching motion is investigated on the NACA 0012 airfoil at chord Reynolds number of $Re_{c}=1.0\times 10^{6}$ through the use of wall-resolved large-eddy simulation. Emphasis is placed on the unsteady boundary-layer interactions that develop as the airfoil approaches stall. At this Reynolds number it is shown that turbulent separation moves upstream across much of the airfoil suction surface. When turbulent separation reaches the leading-edge separation bubble, a bursting event is initiated leading to a strong coherent leading-edge vortex structure. This vortex wraps up the turbulent shear layer to form a large dynamic stall vortex. The use of large-eddy simulation elucidates the roll of the laminar separation bubble in defining the onset of the dynamic stall process. Comparisons are made to identical simulations at lower Reynolds numbers of $Re_{c}=0.2\times 10^{6}$ and $0.5\times 10^{6}$. This comparison demonstrates trends in the boundary-layer mechanics that explain the sensitivity of the dynamic stall process to Reynolds number.

HYDRODYNAMIC CHARACTERISTICS OF VORTEX MOTION INSIDE THE HEMISPHERICAL DIMPLE

Experimental researches of the features of the generation and evolution mechanism of coherent vortex structures, which are formed inside the hemispherical cavity, and space-time characteristics of the fields of velocity and pressure fluctuations that they create, both within the cavity, and in its wake, were conducted. Experiments were carried out in hydrodynamic flume on the hydraulically smooth plate with hemispherical dimple. The symmetric and asymmetric large-scale vortical systems inside a dimple are found out depending on the flow regime, and location and periodicity of their ejections are shown. The evolution of tornado-like vortices subjected to a switch mechanism that is result in appearance of low-frequency modulating transversal oscillations of vortex motion inside the hemispherical dimple. The fields of velocity, dynamic and wall pressure fluctuations by a group of hot-films, miniature piezoelectric sensors of pressure fluctuations are studied. Space-time correlations of velocity and pressure fluctuations, which generate the coherent large-scale vortex structures, the circulation flow and the vortex structure of the shear layer, which are formed inside the hemispherical dimple and ejected out into the boundary layer, are obtained. It was determined that the largest values of the coefficient of the space-time correlations of the wall pressure fluctuations observed on the aft and side walls of the hemispherical dimple, and the lowest – in its forward bottom part. It was established that the cross-correlations of the longitudinal velocity fluctuations are higher than the cross-correlations of the velocity and wall pressure fluctuations, and the cross-correlations of the dynamic and wall pressure fluctuations. On the basis of short-time spectrum and correlation analyses, the non-stationary in tame and non-uniform in space characteristics of the coherent vortical systems inside and past the open dimple have been found. Temporal periodicity of cross-correlation characteristics of the wall pressure fluctuations between two sensors, located on the fixed distance corresponds to frequencies of the shear and wake modes of resonance oscillations in the dimple. Registration of the time lag, at which the cross-correlation or anticorrelation has maximum, enables to determine the group convective velocity of the coherent vortical structures during their formation and evolution, as well as the phase of development and direction of vortex motion inside the dimple.

A versatile taxonomy of low-dimensional vortex models for unsteady aerodynamics

Inviscid vortex models have been demonstrated to capture the essential physics of massively separated flows past aerodynamic surfaces, but they become computationally expensive as coherent vortex structures are formed and the wake is developed. In this work, we present a two-dimensional vortex model in which vortex sheets represent shear layers that separate from sharp edges of the body and point vortices represent the rolled-up cores of these shear layers and the other coherent vortices in the wake. We develop a circulation transfer procedure that enables each vortex sheet to feed its circulation into a point vortex instead of rolling up. This procedure reduces the number of computational elements required to capture the dynamics of vortex formation while eliminating the spurious force that manifests when transferring circulation between vortex elements. By tuning the rate at which the vortex sheets are siphoned into the point vortices, we can adjust the balance between the model’s dimensionality and dynamical richness, enabling it to span the entire taxonomy of inviscid vortex models. This hybrid model can capture the development and subsequent shedding of the starting vortices with insignificant wall-clock time and remain sufficiently low-dimensional to simulate long-time-horizon events such as periodic bluff-body shedding. We demonstrate the viability of the method by modelling the impulsive translation of a wing at various fixed angles of attack, pitch-up manoeuvres that linearly increase the angle of attack from$0^{\circ }$to$90^{\circ }$, and oscillatory pitching and heaving. We show that the proposed model correctly predicts the dynamics of large-scale vortical structures in the flow by comparing the distributions of vorticity and force responses from results of the proposed model with a model using only vortex sheets and, in some cases, high-fidelity viscous simulation.

Influence of coherent vortex structures in subgrid scale motions on particle statistics in homogeneous isotropic turbulence

Subgrid scale (SGS) structures in large eddy simulations (LES) of turbulent particle-laden flows significantly influence particle dynamics, especially those of small inertial particles. In this study, homogenous isotropic turbulence with Taylor's Reynolds number of 102.3 is generated by a direct numerical simulation (DNS) and a wavelet-based coherent vortex extraction method is implemented to extract the coherent SGS structures and then investigate their effects on particle dynamics, including single-particle and particle-pair statistics. Compared to the classical spectral-filtered DNS (FDNS), which cuts off only the high wavenumber components regardless of the turbulence structures in the SGS motions, the wavelet-filtered DNS (WFDNS) can retain more coherent vortex structures in the SGS flow field with the help of the high compression rate characteristics of wavelet transformation. Comparing the results of WFDNS and FDNS at the identical effective grid number, it can be found that the single-particle statistics are mainly controlled by the macro energy-containing structures, and the SGS coherent vortex structures play important roles in the particle-pair dynamics, including the radial distribution function, radial relative velocity, and collision kernel. Therefore, in view of the characteristic of wavelet filtering that preserves the SGS coherent structure, the wavelet-based structural filter should be particularly suitable for LES modeling of particle-laden flow.

Letter: Evolution of coherent vortical structures in a shock wave/turbulent boundary-layer interaction flow

Evolution of coherent vortical structures (CVSs) in shock wave/turbulent boundary-layer interactions (SWBLIs) is a fundamental yet relatively unexplored physical problem. A direct insight into this issue can help understand the SWBLI phenomenon from a more microcosmic perspective and provide new experimental results to the community of high-resolution numerical simulation. Thus, ice-cluster planar laser scattering technique is applied to acquire high spatiotemporal resolution images of the turbulent boundary-layer. Two-dimensional slices (2DS) of CVSs, from which statistical information can be acquired, are extracted by a machine-learning based method. Finally, by comparing features of 2DS-CVSs in the pre-separation zone and the post-reattachment zone, the evolution of CVSs in SWBLI is analyzed.

Large Eddy Simulation of the Elliptic Jets in Film Cooling Controlled by Dielectric Barrier Discharge Plasma Actuators With an Improved Model

The dielectric barrier discharge (DBD) plasma actuator, in which electrodes are asymmetric arranged, has already demonstrated its ability in flow control. In the present work, the configuration of DBD plasma actuator defined as DBD-vortex generator (VGs), which can induce streamwise vortices, has been employed in the flow control of the inclined jet in crossflow. The coherent turbulent structures around the cooling hole are examined by the large eddy simulation (LES) method with the improved plasma model. The mechanism of coherent structure controlled by the DBD-VGs is also elucidated in the processes of parametric study with the actuation conditions. The calculation results show that the DBD-VGs provides us an effective approach to further enhance the performance of the film cooling. When it is applied into the flow, symmetrical streamwise vortices are induced to break down the coherent vortex structure, leading to more coolant gathered on the surface, especially at the lateral area of the coolant jet. What is more, an overall improvement of the film cooling performance can be obtained when the actuation strength is strong enough.