Fine-scale Vortices Research Articles

On the propeller wake evolution using large eddy simulations and physics-informed space-time decomposition

A novel modal analysis methodology, denoted as the physics informed sparsity-promoting dynamic mode decomposition (pi-SPDMD) model, was introduced for the reduction and reconstruction analysis of intricate propeller wake flows, aiming to provide insight into the inherent flow structures spanning diverse temporal and spatial scales. Large-Eddy Simulation (LES) was employed to numerically model the wake dynamics of a four-bladed propeller, providing a comprehensive resolution from the proximate to the distant wake regions. The findings indicate that the pi-SPDMD model enhances the efficiency of the sparse-promoting algorithm, producing modes that gravitate towards stability, and the resulting decomposition maintains commendable physical fidelity. Integrating the results from the LES solution and the modal decomposition of pi-SPDMD, the tip vortex exhibits a uniform topological configuration with notable coherence in the proximate domain. In this region, the large-scale vortex is the dominant feature of the propeller wake, and there is a marked intermittency in the turbulence. In the mid-field, the tip vortex system transitions into fine-scale vortices, rapidly diminishing in coherence due to the onset of elliptic instability and subsequent secondary vortex generation. As the tip vortex structures related to physical quantities become fully discretized, the small-scale turbulent patterns quickly intermingle, leading to a more homogeneous distribution in the distant wake.

The flow characteristics for gas jet in liquid crossflow with special emphasis on the vortex-cavity interaction

The objective of this paper is to investigate the complex three-dimensional vortex structures created by the interaction of gas jet with liquid crossflow. A high-speed camera technique is used to record the evolution patterns of the jet cavity. High-precision numerical methods with the Chorin projection method and volume of fluid method (VOF) are employed to understand the complex flow features associated with jet–freestream interaction. The results present that three distinct regions of the jet cavity could be observed, referred to as the transparent cavity region (TCR), the transition region (TR), and the foam cavity region (FCR). The interaction between the flow of gas jet and liquid crossflow creates multiscale vortex structures, including the counter-rotating vortex pair (CVP), the upper-deck counter-rotating vortex pair (up-CVP), the horseshoe vortices, the shear layer vortices, and the fine-scale vortices, respectively. The relationship between multiscale vortex structures and the pulsation of the gas-liquid interface is analyzed in detail by analyzing the spatial distribution of different vortex structures and the fluctuations of the gas-liquid interface. In addition, the effect of the gas entrainment coefficient on cavity flow patterns and vortex structures is compared and analyzed.

Simulations of isotropic turbulent flows using lattice Boltzmann method with different forcing functions

Three different forcing functions are used with the lattice Boltzmann method (LBM) to simulate the forced isotropic turbulence in periodic boxes at different resolutions ranging from [Formula: see text] to [Formula: see text] grid points using the D3Q19 model. The aims of this study are to examine the effect of using different forcing functions on the LBM stability; to track the development of the turbulent fields at several resolutions, to investigate the effect of the weak compressibility due to change of fluid density on the flow simulations, and to identify the effective force type. The injection is performed through adding the force randomly to the collision term. The three forcing methods depend on sine and cosine as functions of the wave numbers and space. The forcing amplitude values of [Formula: see text] and the relaxation time [Formula: see text] are fixed in all cases. The single relaxation time model is found stable at such values of the forcing amplitude and the relaxation time. However, the development of the turbulent data at the different resolutions needs about 10000 time-steps to reach the required statistical state including clear visualizations of fine scale vortices. Many simulations have been tested using different values of the relaxation time [Formula: see text] and the development of the turbulent fields is found faster with fewer time-steps but the stability of the LBM is broken at some resolutions (not necessary the higher resolution). The statistical features of all fields, such as the Taylor and the Kolmogorov micro-scales, the Taylor Reynolds number, the flatness and the skewness, are calculated and compared with the previous efforts. The worm-like vortices are visualized at all cases and it is found that more fine vortices can be extracted as the resolution increases. The energy spectrum has a reasonable Kolmogorov power law at the resolutions of [Formula: see text] and [Formula: see text], respectively. Results show that the third forcing method that uses a cosine disturbance function has the best statistical features and the finest visualized vortical structures especially at higher resolutions. Extensive discussions about the density field and its evolution with time at different forcing functions, comparison to Navier–Stokes solutions and the time development of the energy spectra for all cases are also carried out.

Numerical investigation of the round jet in crossflow at high velocity ratios with special emphasis on the evolution of vortex structures

We investigate the evolution and interaction mechanism of different vortex structures for the jet in crossflow by a high precision numerical method. To verify the accuracy of the numerical method, the numerical and experimental results are compared. Numerical results show a reasonable agreement with the experimental data. The typical vortex structures can be clearly identified in the flow field, including shear layer vortices, horseshoe vortices, counter rotating vortices pairs, and wake vortices. Through the analysis of spatial distribution of different vortex structures, the formation and interaction mechanisms of different vortices are discussed in detail. The results show that the shear layer rolling up appears due to the strong rotation, inducing the formation of the shear layer vortices. The influences of velocity ratios on the vortex structures are further investigated. At low velocity ratios, the rotation is weak along the windward of the jet. With the increase in the velocity ratios, the stronger rotation is formed near the jet exit hole, inducing the instability of interface and formation of the shear layer vortices to occur earlier. In the far flow field, as the shear layer vortices gradually break up into the fine-scale vortices, both the rotation and shear tend to become weaker at different velocity ratios.

On the Impact of Left Upper Lobectomy on the Left Atrial Hemodynamics.

The left atrium (LA) functions to transport oxygenated blood from the pulmonary veins (PVs) to the left ventricle (LV). LA hemodynamics has received much attention because thrombosis in the LA in pathological states, such as atrial fibrillation, is a major factor leading to thromboembolic stroke. In the last 5 years, multiple cohort studies have revealed that left upper lobectomy (LUL) with PV resection risks thrombus formation in the PV stump even in the normal LA without a history of cardiac disease; the causal mechanism is, however, an open question. The present study investigated the potential effect of an LUL on LA hemodynamics associated with thrombus formation through computational simulation using four-dimensional computed tomography (4D-CT) images. Time series of patient-specific LA geometries before and after LUL were extracted from the 4D-CT images and these motions were estimated through non-rigid registration. Adopting the LA geometries and prescribed moving wall boundary conditions, the LA blood flow was determined using a Cartesian-grid computational fluid dynamics solver. The obtained results show that the LUL resulted in blood flow impingement from the left and right PV inflows into the LA upper region throughout most of the cardiac cycle. This characteristic alteration of the LA hemodynamics generated fine-scale vortices with viscous energy dissipations, enhancing the flow stasis associated with thrombus formation in the PV stump. These findings show that an LUL affects the hemodynamics not only in the PV stump but also throughout the LA region. They also highlight the importance of computational analysis of LA hemodynamics in understanding the underlying mechanism of LUL-induced thrombus formation.

Wake behind circular cylinder excited by spanwise non-uniform disturbances

We studied a modification of wake behind a circular cylinder using a plasma actuator. The plasma actuators were arranged in the spanwise direction of the cylinder to give temporal periodic disturbances having Strouhal number St = 0.18-2.3 with a burst ratio BR = 20 and 40%. The Reynolds number was set in a rage of Re = 4200 to 8400. Two types of plasma actuator were prepared; one is a single strip of the actuator placed at each side of the cylinder to give a spanwise uniform disturbance, and another is an array of small piece of actuators placed at the same location to create a spanwise non-uniform disturbance with temporal phase difference, φ = 0 or π, between adjacent electrodes. A conventional two-component PIV and stereo PIV was used to measure the flow field. Figure 1 shows the instantaneous spanwise component of vorticity at Re = 4200 evaluated by two-component PIV. Under no disturbance condition, the laminar shear layer extends straight to around x / d = 1.5 and then forms a wake vortex, as shown in Fig.1(a). In the case of spanwise non-uniform forcing with St = 1.09 and φ =π, rapid roll up of the initial shear layer leads to arrangement of wake vortices closer to the cylinder., as shown in Fig.1(b). With higher Strouhal number case with St = 1.09 and φ = 0, shown in Fig.1(c), a series of fine scale vortices are generated behind both side of the cylinder without forming regular Karman vortices. The spanwise non-uniform forcing was effective to suppress the formation of large scale vortices just behind the cylinder. Figure 2 shows surface of constant vorticity magnitude and vortex lines under St =1.09 and φ = π case. These were computed from a phase-averaged threecomponents velocity field evaluated by stereo PIV. The value of the surface was selected to display the boundary layer formed on the cylinder, and the vortex lines are selected to visualize the vortex structure formed in the following shear layer. A bundle of vortex lines are shaped in a wavy pattern along spanwise direction with 180 degrees out of phase to the adjacent bundle upstream of downstream. This structure, so called ‘chain-line fence structure’ was already found in planar free shear layer [Nygaard, K.J. and Glezer, A., 1990, Phys. Fluids A, 2, 461] and planar jet [Sakakibara, J., Anzai, T., 2001, Phys. Fluids, 13, 1541], but it became evident to create it in the wake of circular cylinder in this study.

Voronoi analysis of vortex clustering in homogeneous isotropic turbulence

Clustering of fine-scale dissipative vortices is quantified using a new technique based on the Voronoi diagram. We consider datasets of numerical simulations of decaying and forced homogeneous turbulence up to Reλ≈400. Velocity gradient events are more intense and intermittent if fine-scale vortices have a higher number density. However, clustering is observed only if the vortices are thresholded according to their intensity. Therefore, a fine-scale vortex cluster is the accumulation among stronger vortices rather than the accumulation of all of the vortices in the flow. Turbulence statistics in intense vortex clusters share characteristics with those of the outside when they are renormalized by their local magnitude. This suggests that the clusters are the product of passive amplification of the swirling intensity by underlying larger-scale motions. Comparing the decaying and forced flows reveals that the large-scale forcing affects the vortex distribution although other turbulence statistics are unaffected. The rotation axes of the vortices in the cluster are not preferentially aligned for either flow, which implies that large-scale shear layers are not a vital feature of the clusters.

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.

Flows around a cascade of flat plates with acoustic resonance

In order to better understand the effects of acoustic resonance on flows around a cascade of flat plates, fluid structures and acoustic fields were examined by aeroacoustic direct simulations and wind tunnel experiments. Computations and experiments were performed for the flows around five parallel plates with and without the acoustic resonance changing the freestream velocity. The aspect ratio of the plates, C/b, is 15.0, and the separation-to-thickness ratio, s/b, is 6.0. For the resonant condition of a freestream velocity of 44 m/s, the Reynolds number based on the plate thickness, b, and the freestream velocity is 5.8 × 103. The computational results revealed that large-scale vortices composed of fine-scale vortices are shed in the wake of the plates. Both experimental and computational results indicated that the shedding of the large-scale vortices is more synchronized in the spanwise direction for the resonant condition. Moreover, the shedding of the large-scale vortices from one plate and those from the neighboring plates become more synchronized in the resonant condition. The mode of this synchronization was found to be an anti-phase mode, in which the vortex is shed from the upper or lower face of one plate when the vortex is shed from the lower or upper faces of the neighboring plates. Computation of the flow around a single plate was also performed, and the radiation of the acoustic waves from the downstream edge due to vortex shedding from the plate was indicated. When acoustic resonance occurs in the flows around the cascade of flat plates, vortex shedding in the above-mentioned mode contributes to the intensification of the standing waves between the plates. Moreover, standing waves were demonstrated to induce new vortices around the upstream edges of the plates in synchronization.

Modeling of the Subgrid-Scale Pressure Distribution in Turbulent Mixing Layer

Attention was focused on the statistical properties of pressure fluctuation, which is caused by fine-scale vortices in turbulent flow. The aim is improvement in the prediction of cavitation inception due to fine-scale turbulence vortices, which are usually in subgrid-scale (SGS) in Large-eddy simulation (LES). We conducted a finely-resolved direct numerical simulation (DNS) of a spatially-developing turbulent mixing layer in cavitating and non-cavitating (single-phase) conditions. The result under cavitating condition suggested that low-pressure region corresponding to the core of fine-scale vortices could become an origin of cavitation. We applied filtering technique to DNS database under non-cavitating condition to model the low-pressure region of subfilter vortices observed in result under cavitating condition. Filtering volume is corresponding to that in the probable LES. In fully developed turbulence, proportional relation was found out between intensity of SGS pressure fluctuation and turbulent kinetic energy. In addition, Gaussian profile reasonably approximated instantaneous pressure distribution within a filtering volume. We therefore think the possibility of cavitation inception can be accurately estimated by using the intensity of SGS kinetic energy in couple with filtered pressure field.

J056045 マイクロ燃焼器における乱流及び火炎構造の動的特性([J05604]乱流における運動量,熱,物質の輸送現象(4))

Direct numerical simulation (DNS) of hydrogen-air premixed flame with swirling flow is conducted to investigate dynamic characteristics of turbulent and flame structures in a micro-scale combustor. Size of the combustor is assumed to be 15.0 mm in streamwise direction and 10.0 mm X 10.0 mm cross-section. DNS is carried out under a swirl number: S = 0.6. Large-scale helical vortical structures are generated near the inlet of combustion chamber, and a lot of fine-scale vortices appear downstream. Interactions between dynamics of large-scale vertical structures and pressure fluctuations in the combustor are focused on. The large-scale coherent structures generated near the inlet grow, pass through regions of high heat release rate and disappear. The period of this transition corresponds to the period of the pressure oscillation on side wall of the combustor. Fluctuations of heat release rate induced by progress of the large-scale vortical structures influence pressure fluctuations. These results suggest that controlling the occurrence of the large-scale eddies is important for suppression of pressure fluctuations.

Evolution of isolated turbulent trailing vortices

In this work, the temporal evolution of a low swirl-number turbulent Batchelor vortex is studied using pseudospectral direct numerical simulations. The solution of the governing equations in the vorticity-velocity form allows for accurate application of boundary conditions. The physics of the evolution is investigated with an emphasis on the mechanisms that influence the transport of axial and angular momentum. Excitation of normal mode instabilities gives rise to coherent large scale helical structures inside the vortical core. The radial growth of these helical structures and the action of axial shear and differential rotation results in the creation of a polarized vortex layer. This vortex layer evolves into a series of hairpin-shaped structures that subsequently breakdown into elongated fine scale vortices. Ultimately, the radially outward propagation of these structures results in the relaxation of the flow towards a stable high-swirl configuration. Two conserved quantities, based on the deviation from the laminar solution, are derived and these prove to be useful in characterizing the polarized vortex layer and enhancing the understanding of the transport process. The generation and evolution of the Reynolds stresses is also addressed.

The influence of turbulence on a columnar vortex with axial flow

AbstractThe interaction between a columnar vortex and external turbulence is investigated numerically. A q ‐vortex is immersed in an initially isotropic homogeneous turbulence field, which itself is produced numerically by a direct numerical simulation of decaying turbulence. The formation of turbulent eddies around the columnar vortex and the vortex‐core deformations are studied in detail by visualizing the flow field. In the less‐stable case with q = –1.5, small thin spiral structures are formed inside the vortex core. In the unstable case with q = –0.45, the linear unstable modes grow until the columnar vortex make one turn. Its growth rate agrees with that of the linear analysis of Mayer and Powell[1]. After two turns of the vortex, the secondary instability is excited, which causes collapse of the columnar q ‐vortex and the sudden appearance of many fine scale vortices. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Turbulence Production and Energy Transfer Mechanism in the Transition Process of a Two-Dimensional Jet

The effects of coherent vortices on turbulent energy transfer were examined in an acoustically excited two-dimensional parabolic jet. Production terms in the energy transport equation were evaluated based on the results of hotwire conditional measurements. Difference in vortex arrangement gave significant influence on the energy transport process among mean flow, coherent structure and incoherent turbulence. When the symmetrical vortex arrangement was excited, most of the mean flow energy directly dissipated into random incoherent turbulence. For the anti-symmetrical mode, large amounts of positive and negative turbulent energy fluxes were produced around strong large-scale coherent vortices. Fine-scale vortices between the streamwisely adjacent coherent vortices played important roles in the turbulent energy transport process for the anti-symmetrical vortex arrangement.

Current convection in solar active regions

Current carrying magnetic fields which penetrate sunspots can be unstable to current convective modes caused by the large gradient of electrical conductivity. The linear growth rates and wavelengths of the unstable modes are found. The unstable modes produce fine-scale vortices perpendicular to the magnetic field, which overshoot well into the solar corona. The modes provide a turbulent vorticity source at the photospheric footpoints of the field. This can cause braiding and reconnection of the coronal magnetic field. The modes twist the coronal magnetic field into loops with a typical radius of 200 km, consistent with recent X-ray observations.

Numerical modeling of viscous separated flow in the near wake

The present paper studies laminar flow in the near wake behind a short blunt body. We have obtained the dependence on the Mach number and Reynolds number of the incident stream of a number of geometrical characteristics of the wake, the relative base pressure, and its contribution to the total body drag. Examples are given of the influence of body shape and its surface thermal regime on the base pressure. It has been observed that for some incident stream parameters fine-scale vortices form near the separation point (an analogous phenomenon has been mentioned earlier in individual experimental studies), and a local supersonic zone can occur on the axis of symmetry in the reverse flow. i. Formulation of the Problem and Method of Solution. We consider flow about an axisymmetric body with a vertical base truncation. A uniform supersonic stream of viscous compressible heat-conducting gas flows over the body at zero angle of attack. In accordance with theoretical ideas and on the basis of the available test data (see, e.g., [i, 3]) we can give the following scheme for the separated flow in the near wake. Under the influence of inertia forces, which predominate over the viscous forces, the boundary layer leaving the body separates from the body corner at the point G (Fig. i), located somewhat below the corner point F. The separated viscous layer entrains part of the gas occupying the wake region, thereby lowering the pressure there, and together with the external stream it is deviated toward the axis, where at some distance from the base rim it merges with the symmetric viscous layer, forming the wake throat. In the merge zone the base pressure increases, the flow is turned again (almost parallel to the axis), and a tail shock forms. The internal part of the viscous layer, close to the axis, losing speed due to interaction with the gas from the separated region, cannot overcome the increased pressure in the wake throat, and turns back toward the base of the body, and in the established near wake there forms a closed region of recirculating flow, separated from the rest of the flow by the dividing streamline GH. In the external inviscid flow there is flow expansion in the vicinity of the corner point F; immediately beyond the fan of expansion waves there is a stern shock wave, due to re-expansion of the wall viscous layer at the corner point. (Figure 1 also shows stream lines and pressure profiles obtained in one of the computations, and in these one can see the location of the tail shock.)