Evolution Of Vortical Structures Research Articles

Vortical structures and passive scalar transport in starting process of annular purging jet

The evolution of vortical structures and passive scalar transport in the starting process of annular purging jets are numerically investigated by large eddy simulation. Three flow configurations with different nozzle-to-plate distances at a fixed radius ratio of 0.71 and the Reynolds number of 13 750 are simulated. The numerical results are validated against documented experimental data. Three stages during the evolution are proposed based on instantaneous flow visualizations and assessed by calculating the circulation changes of the annular jets and vortex rings. The vortical structures are identified to understand the three-dimensional characteristics. The entrainment process is analyzed focusing on the passive scalar transport in the flow fields and is correlated with the cleaning performance of annular purging jets. The flow structures dominate the process of scalar mixing, especially the inner and outer vortex rings. The large-scale motions of trailing jets cause the intermittent events of scalar transport. During the starting process, the cleaning performance is better with a smaller nozzle-to-plate distance, while the cleaning efficiency may reach the optimum at a moderate distance. The cleaning process is limited by the scalar diffusion and entrainment process. These findings highlight the significance of flow structures for effective cleanness of temperature and contaminations in the purging systems.

Flame structure and dynamics in pool fires of different geometries: Experimental and numerical investigation

Fire accidents might be caused by liquid fuel leakage, which can accumulate in a prescribed area under the action of physical barriers. Therefore, pool fires could be of an arbitrary geometry in less ideal configurations. The current objective is to uncover the underlying physics of flame structure and dynamics in pool fires of different geometries. In this work, pool fires with seven different geometries were examined. Flame base structure and instantaneous flame height in pool fires of different geometries were recorded. Temperature, velocity/vorticity fields and coherent vortical structures were further presented by Large Eddy Simulation (LES). Results show that mean flame height of circular pool fire is the highest, while that of right triangle fires is the lowest. Mean flame height of the other noncircular geometries lies in these two extremes. The presence of vertices and hypotenuses has a dramatic influence on the formation and development of vortical structures. Triangular pool fires have the highest oscillation frequency, followed by quadrangular and circular geometries. To account for the physics of air entrainment and vortex dynamics due to pool fire geometry, pool perimeter is used as a characteristic length scale to propose new scaling relations for describing flame height and oscillation frequency.

Flow behavior analysis in boundary layer transition based on the Liutex–shear decomposition

The Liutex based vortex identification method is superior to previous methods in that it overcomes the issues of threshold problem, shear contamination, etc., with a clear physical meaning that the direction of the Liutex vector represents the local axis of rotation, while the magnitude is equal to twice the angular velocity of the rigid-rotation part of the flow. The current study focuses on the interaction between the Liutex represented rotation and the residual shear part during the development of Λ vortex and hairpin vortex in boundary layer transition. The temporal–spatial evolution and development of typical vortical structures are analyzed based on the Liutex–shear decomposition with particular attention paid to the position and strength changes of Liutex and shear. Vortex core lines are extracted to investigate the mutual interactions between rotation and shear inside vortices. It is demonstrated that for the Λ vortex, spanwise rotating motions are formed at the head region and can persist for a long time under the influence of surrounding shear, while the tail region of the Λ vortex stretches down near the wall but then becomes weaker due to dissipation. High-shear regions tend to be located on top or below the vortices. When the shear layer formed between the legs gets stronger, it will be rolled up to form new vortices. On the one hand, the vortex legs together with the newly formed spanwise vortex consist of a new hairpin vortex, which, in turn, leads to the generation of the second- and third-level hairpin vortices. On the other hand, it also generates many smaller streamwise vortices in the near wall region. The results show that the interaction between Liutex and shear is very important in the development of vortical structures during transition.

Global stability analysis and direct numerical simulation of boundary layers with an isolated roughness element

Global stability analysis and direct numerical simulation (DNS) are used to study boundary layer flows with an isolated roughness element. The aspect ratio of the element ( $\eta$ ) is small, while the ratio of element height to displacement boundary layer thickness ( $h/\delta ^{*}$ ) is large. Both steady base flows and time-averaged mean flows are able to capture the frequencies of the primary vortical structures and mode shapes. Global stability results highlight that although the varicose instability is dominant for large $h/\delta ^{*}$ , sinuous instability becomes more pronounced as $Re_h$ increases for the thin geometry ( $\eta =0.5$ ), due to increased spanwise shear in the near-wake region. Wavemaker results indicate that $\eta$ affects the convective nature of the shear layer more than the type of instability. DNS results show that different instability mechanisms lead to different development and evolution of vortical structures in the transition process. For $\eta =1$ , the varicose instability is associated with the periodic shedding of hairpin vortices, and its stronger spatial transient growth indicated by wavemaker results aids the formation of hairpin vortices farther downstream. In contrast, for $\eta =0.5$ , the interplay between varicose and sinuous instabilities results in a broader-banded energy spectrum and leads to the sinuous wiggling of hairpin vortices in the near wake when $Re_h$ is sufficiently high. A sinuous mode with a lower frequency captured by dynamic mode decomposition analysis, and associated with the ‘wiggling’ of streaks, persists far downstream and promotes transition to turbulence. A new regime map is developed to classify and predict instability mechanisms based on $Re_{hh}^{1/2}$ and $d/\delta ^{*}$ using a logistic regression model. Although the mean skin friction demonstrates different evolutions for the two geometries, both of them efficiently trip the flow to turbulence at $Re_h=1100$ . An earlier location of a fully-developed turbulent state is established for $\eta =1$ at $x \approx 110h$ .

Three-dimensional direct numerical simulations of two interfering side-by-side circular cylinders at intermediate spacing ratios

Side-by-side arranged circular cylinders show fierce interferences when they are not far apart. In this paper, we present the vibration response and wake characteristics of two elastically mounted circular cylinders in a side-by-side arrangement through three-dimensional direct numerical simulations (3-D DNS). The Reynolds number is Re = 500 and two center-to-center spacings (s) of 2.0D and 2.5D are considered where D is the diameter of the two cylinders. We found that with the increasing reduced velocity, the responses of both s/D cases can be divided into three different regions, which are named desynchronization region (DS), initial branch (IB), and lower branch (LB), with the former being further divided into two subregions (DS-I and DS-II). The vibration, hydrodynamic forces, and spectral features in each region are discussed in detail. According to the characteristics of cylinder oscillation and vortex dynamics, six types of wake patterns are identified, with the deflected (DF) and flip-flopping (FF) patterns at s/D = 2.0 while in-phase (IP), anti-phase (AP), in-/anti-phase (I/AP), and hybrid (HB) patterns at both s/D = 2.0 and 2.5. Furthermore, we investigated the wake three-dimensionality through the evolution of vortical structures, and the statistics and development of the enstrophies. It was observed that the presence of the streamwise vortices modifies the strength and spanwise coherence of the spanwise vortices. Finally, flow physics for the interactions of the two cylinders at different regions is elucidated.

Analysis of shock wave-boundary layer interaction in a shock tube using higher order scheme

Over the last couple of decades, the shock-wave boundary-layer interaction has gained a lot of attention due to its practical importance in many engineering applications. It is a complex problem involving shock-wave bifurcation, boundary-layer separation, the interaction of contact discontinuity with the shock wave, the formation of shocklets, and the evolution of vortical structures having different length scales. It is difficult to capture experimentally the detailed flow field having the shocklets and vortices. Numerical solvers having negligible numerical dissipation are highly essential to predict these structures accurately. Over the years, many researchers have obtained a grid-converged solution for shock-wave boundary-layer interaction at Reynolds numbers of 200 and 1000. The shock-wave boundary-layer interaction at higher Reynolds numbers is not attempted due to the requirement of huge computational resources and challenges associated with convergence. In the investigation, the grid-converged solution is obtained for a Reynolds number of 2500 with a 13th order high-resolution hybrid scheme using 100 cores of a computational cluster equipped with 3.0 GHz Intel Xeon processors incorporating the MPI library for parallelization. The complex flow field is analysed in detail using wall density, density gradient, vorticity, pressure, and enstrophy plots after validating the solver with the benchmark wall pressure, density, and v-velocity around the primary vortex provided by Zhou et al. (2018), Phys. Fluids, 30, 016102 for a Reynolds number 1000. It is observed that the triple point height and number of vortices in both separated zone and at the shear layer increase with an increase in Reynolds number. The grid-converged data obtained from the present simulation can be used as benchmark data to validate different numerical schemes/solvers at higher Reynolds numbers.

Interplay of chordwise stiffness and shape on performance of self-propelled flexible flapping plate

The locomotion of a flapping flexible plate with different shapes and non-uniform chordwise stiffness distribution in a stationary fluid is studied numerically. The normalized effective bending stiffness K∗ for three-dimensional plates with arbitrary stiffness distribution and shape parameters is proposed, and the overall bending stiffness of non-uniform plates with different shapes is reasonably characterized. It is found that the propulsion performance in terms of cruising speed and efficiency of the self-propelled flapping plate mainly depends on the effective bending stiffness. Plates with moderate flexibility K∗ show better propulsion performance. Meanwhile, both a large area moment of the plate and a flexible anterior are favorable to significantly improve their propulsive performance. The evolution of vortical structures and the pressure distribution on the upper and lower surfaces of the plate are analyzed, and the inherent mechanism is revealed. These findings are of great significance to the optimal design of propulsion systems with different fins or wings.

Hydrodynamic study of the flow developed around a bare hull ship in static drift motion

The present study gives a Computational Fluid Dynamics (CFD) based insight into the three-dimensional incident flow developed around a very large crude carrier ship during static drift motion. The research proposes a set of virtual Planar Motion Mechanism (PMM) tests of “static drift” type conducted for a number of seven drift angles in the range of -9o to +9o. The emergence and development of vortical structures along the 1:58 KRISO Very Large Crude Carrier 2 (KVLCC2) tanker model are examined and explained, the influence of the considered drift angles being highlighted.

Numerical investigation of the effect of rotation on non-premixed hydrogen combustion in developing turbulent mixing layers

This study was aimed at examining the influence of the system rotation as an external action on the development of vortical structures and combustion. Specifically, three-dimensional direct numerical simulations of compressible mixing layers with non-premixed /air combustion were performed using a detailed chemical reaction scheme. The relationship between the developing vortical structures and chemical reactions in the flow field with the rotation was investigated. The development of combustion changed depending on the vortical structures, and the presence of roller vortices promoted the combustion phenomena. The influence of the vortical structures on the elementary reactions, which contribute to the heat release rate, was small. During the anticyclonic rotation, the roller vortices collapsed and suppressed the combustion. In contrast, the cyclonic rotation resulted in the generation of quasi-2D roller vortices, which enlarged the high-heat-release-rate regions and promoted the combustion. Overall, the vortical structures induced by the rotation can change the development of combustion even though the elementary reactions that contribute to the heat release rate remain unchanged. The presented findings can guide the prediction and control of turbulent combustion in practical situations involving fluid machinery.

Direct numerical simulation of transitional and turbulent round jets: Evolution of vortical structures and turbulence budget

Direct numerical simulation of transitional and turbulent round jets is reported in a comparative framework. Such a comparison is central toward revealing the roles that molecular viscosity and vorticity intensification play in the evolution of jets. The initial and intermediate evolution is differentiated based on the assessment of the starting jet, roll-up frequency, dynamics of vortex rings, and emergence of the secondary instability. Long-term behavior is differentiated based on the assessment of preferred mode frequency, time averaged vortical structures, half jet-width, and volume flow rate obtained from the time-averaged velocity field. The present study demonstrates that viscous damping of cross-stream vorticity plays a key role in establishing helical instability as the dominant mode in long-term evolution of the transitional jet. On the contrary, varicose mode is dominant in the turbulent jet, despite preferred mode frequency being the same in both cases. Finally, a novel attempt is made toward comparing individual terms constituting turbulence budget between both regimes. Through such a comparison, relative dominance of various transport mechanisms governing the evolution of turbulence kinetic energy (K) is revealed. It is observed that terms accounting for a forward cascade of K from inertial to smallest scales are comparatively larger for the turbulent jet, while those accounting for the backscatter of K are comparatively larger for the transitional jet. It is also established that turbulence dissipation is evidently the same for both jets. Thus, the property of turbulence dissipation being independent of Reynolds number for turbulent jets can also be extrapolated to transitional jets.

Vortical flow patterns by the cooperative effect of convective and conductive heat transfers in particle-dispersed natural convection

Cooperative effect of the conductive and convective heat fluxes on the development of vortical structures is studied in a particle-dispersed natural convection. A numerical approach is proposed that visualizes the heat transfer paths through the fluid and finite-volume particles (i.e., non-point particles) to show the spatial extension of the heat fluxes and its influence on the flow field. The approach is applied to a weakly-convective dense particulate system under the Rayleigh number 105 containing more than 4000 particles (solid volume fraction 42.8%) of various conductivities. With highly-conductive particles (in comparison to the ambient fluid), local downward convection of a large-scale vortical flow strengthens the conductive heat flux near the bottom hot wall in the counter-convective direction by the increase of the vertical temperature gradient, and the observation of the spatial extension of the heat flux lines indicates that the increase of the solid volume fraction near the hot wall further strengthens the conductive heat flux. The paired upward convection causes strengthened conductive heat flux near the top cold wall by the same increase mechanism. The above interplay of the local heat fluxes in the dense solid-dispersed media are found to influence on the formation and transition of the large-scale vortical structures through the distribution of the moment of buoyancy in a horizontal plane. An analytical model for a low-dimensional vortex system shows that buoyancy gradient and vorticity diffusion could determine the time-development of the large-scale vortical structure.

Evolution of vortical structures behind an inclined flat plate

2D3C TR-PIV technique was utilized to investigate streamwise-oriented vortical structures behind an inclined flat plate. The angle of attack was set to 7 deg, several fields of view in the wake were investigated. The instantaneous velocity vector fields were captured, dynamics of the flow was studied using POD method. The streamwise structures are determined by vorticity and low- and high-velocity streaks are defined. The acquired results are in a good agreement with the new hypothesis of a principle of flight.

DNS study on bursting and intermittency in late boundary layer transition

Experimental and numerical investigations have suggested the existence of a strong correlation between the passage of coherent structures and events of bursting and intermittency. However, a detailed cause-and-effect study on the subject is rarely found in the literature due to the complexity and the nonlinear multiscale nature of turbulent flows. The primary goal of this research is to explore the motion and evolution of coherent structures during late transition, whose structure is much more ordered than that of fully developed turbulence, and their relationship with events of bursting and intermittency based on a verified high-order direct numerical simulation (DNS). The computation was carried out on a flat plate at Reynolds number 1000 (based on the inflow displacement thickness) with an inflow Mach number 0.5. It is concluded that bursting and intermittency detected by stationary sensors in a transitional boundary layer actually result from the passage and development of vortical structures, and it would be more rational to design transitional turbulence models based on modelling the moving vortical structures rather than the statistical features and experimental experiences.

Parameter influence on the evolution of low-aspect-ratio rectangular synthetic jets

The evolution of low-aspect-ratio rectangular synthetic jets is investigated with dye and laser-induced-fluorescence (LIF) flow visualization techniques. This paper analyzes the impacts of three key parameters on the evolution of vortical structures, respectively, including orifice aspect-ratio (AR), dimensionless stroke length, and Reynolds number. Compared with circular synthetic jet, all the rectangular synthetic jets display vortex ring axis switching, and may develop into two types of stream-wise vortices I and II. The evolution of the vortex ring and the generation of stream-wise vortices are influenced by three parameters. In particular, stream-wise vortices are not detected for a low AR, stroke length or Reynolds number case.

Effects of Pivot Location and Reduced Pitch Rate on Pitching Rectangular Flat Plates

This paper presents the results of an experimental investigation of the unsteady flow about pitching flat plates. Hydrodynamic force and two-dimensional particle image velocimetry measurements are reported for three pivot locations (leading edge, midchord, and trailing edge), reduced pitch rates from 0.022 to 0.39, and in still water, which corresponds to infinite reduced pitch rate. The wing has rectangular planform with effective aspect ratio 4, and the wing pitching motion is from 0 to 45 deg angle of attack. The relation between hydrodynamic force and vortical flow development as a function of pivot location and reduced pitch rate is discussed. Reasonable agreement is found between measured hydrodynamic force and quasi-steady potential flow theoretical results. Several vortical flow features are identified and discussed, including 1) the effect of pivot location and pitch acceleration on formation and evolution of vortical structures, 2) the impact of interaction between vortical flow structures on hydrodynamic force development, and 3) three-dimensional flow development and transient vortex development.

Numerical study on the freely falling plate: Effects of density ratio and thickness-to-length ratio

A numerical study on two-dimensional (2D) rectangular plates falling freely in water is carried out in the range of 1.2 ≤ ρs/f ≤ 5.0 and 1/20 ≤ β ≤ 1/4, where ρs/f is the solid-to-water density ratio and β is the plate thickness-to-length ratio. To study this problem, the immersed boundary-lattice Boltzmann flux solver in a moving frame is applied and validated. For the numerical result, a phase diagram is constructed for fluttering, tumbling, and apparent chaotic motions of the plate parameterized using ρs/f and β. The evolution of vortical structures in both modes is decomposed into three typical stages of initial transient, deep gliding, and pitching-up. Various mean and instantaneous fluid properties are illustrated and analyzed. It is found that fluttering frequencies have a linear relationship with the Froude number for all cases considered. Lift forces on fluttering plates are linearly dependent on the angle of attack α at the cusp-like turning point when α≥π/5. Hysteresis of the lift force on fluttering plates is observed and explained whilst the drag forces are the same when α has the same value. Meanwhile, the drag force in the tumbling motion may have a positive propulsive effect when the plate begins a tumbling rotation from α = π/2.

Numerical Study of the Instability and Flow Transition in a Vortex-Ring/Wall Interaction

Instability and fl w transition of a vortex ring impinging on a wall were investig ated by means of large-eddy simulation for two vortex core thicknesses corresponding to thin and thick vortex rings. Various fundamental mechanisms dictating the fl w behaviours, such as evolution of vortical structures, instability and breakdown of vortex rings, development of modal energies, and transition from laminar to turbulent state, have been studied systematically . Analysis of the enstrophy of wrapping vortices and turbulent kinetic energy (TKE) in fl w fiel indicates that the formation and evolution of wrapping vortices are closely associated with the fl w transition to turbulent state. It is found that the temporal development of wrapping vortices and the growth rate of axial fl w generated around the circumference of the core region for the thin ring are faster than those for the thick ring. The azimuthal instabilities of primary and secondary vortex rings are analysed and the development of modal energies reveals the fl w transition to turbulent state. The law of energy decay follows a characteristic k 5=3 law, indicating that the vortical fl w has become turbulent. The results obtained in this study provide physical insight into the understanding of the instability mechanisms relevant to the vortical fl w evolution.

Evolution of vortical structures in a curved artery model with non-Newtonian blood-analog fluid under pulsatile inflow conditions

Steady flow and physiological pulsatile flow in a rigid 180° curved tube are investigated using particle image velocimetry. A non-Newtonian blood-analog fluid is used, and in-plane primary and secondary velocity fields are measured. A vortex detection scheme (d 2-method) is applied to distinguish vortical structures. In the pulsatile flow case, four different vortex types are observed in secondary flow: deformed-Dean, Dean, Wall and Lyne vortices. Investigation of secondary flow in multiple cross sections suggests the existence of vortex tubes. These structures split and merge over time during the deceleration phase and in space as flow progresses along the 180° curved tube. The primary velocity data for steady flow conditions reveal additional vortices rotating in a direction opposite to Dean vortices—similar to structures observed in pulsatile flow—if the Dean number is sufficiently high.

Prediction of thrombus formation using vortical structures presentation in Stanford type B aortic dissection: A preliminary study using CFD approach

False lumen patency has been recognized as the key determinant of aneurismal dilatation and rupture, whereas complete thrombosis has been proven clinically to improve disease outcomes. However, to date, there is no definitive method that could predict false lumen thrombosis in Stanford type B aortic dissection patient. The present study aims to use the evolution of vortical structures throughout a cardiac cycle and its interaction with the wall shear stress (WSS) to illustrate the potential mechanism behind the formation of thrombus and predict its highly possible location in an aortic dissection patient. A three-dimensional (3D) patient-specific Stanford type B aortic dissection domain was generated from computed tomography (CT) angiographic images. The effects of false lumen size on vortical structures and the subsequent risk of thrombus formation were investigated. The Carreau–Yasuda model, representing non-Newtonian fluid property was used to study the difference between Newtonian and non-Newtonian fluid settings on vortical structures. Based on our analysis, we predicted that the formation and thickening of thrombus is very much likely to occur at the posterior false lumen wall, distal to the entry tear region, in the patient. Higher λ2 intensity, associated with a higher maximum WSS and a lower minimum WSS, was achieved with an increase in the false lumen size, and we believed that this increases the risk of thrombus formation and thus aneurismal dilatation. On the other hand, percentage of flow entering the false lumen increased with an increase in the false lumen size, leading to a downward shift of the areas of thrombus formation along the false lumen wall.

Dynamics and Instability of a Vortex Ring Impinging on a Wall

AbstractDynamics and instability of a vortex ring impinging on a wall were investigated by means of large eddy simulation for two vortex core thicknesses corresponding to thin and thick vortex rings. Various fundamental mechanisms dictating the flow behaviors, such as evolution of vortical structures, formation of vortices wrapping around vortex rings, instability and breakdown of vortex rings, and transition from laminar to turbulent state, have been studied systematically. The evolution of vortical structures is elucidated and the formation of the loop-like and hair-pin vortices wrapping around the vortex rings (called briefly wrapping vortices) is clarified. Analysis of the enstrophy of wrapping vortices and turbulent kinetic energy (TKE) in flow field indicates that the formation and evolution of wrapping vortices are closely associated with the flow transition to turbulent state. It is found that the temporal development of wrapping vortices and the growth rate of axial flow generated around the circumference of the core region for the thin ring are faster than those for the thick ring. The azimuthal instabilities of primary and secondary vortex rings are analyzed and the development of modal energies is investigated to reveal the flow transition to turbulent state. The modal energy decay follows a characteristic –5/3 power law, indicating that the vortical flow has become turbulence. Moreover, it is identified that the TKE with a major contribution of the azimuthal component is mainly distributed in the core region of vortex rings. The results obtained in this study provide physical insight of the mechanisms relevant to the vortical flow evolution from laminar to turbulent state.