Large-scale Coherent Vortex Structure Research Articles

Large eddy simulation of shock wave/boundary layer interactions in a transonic compressor cascade

In this paper, large eddy simulation (LES) was performed to investigate the shock wave/boundary layer interaction (SBLI) phenomenon in transonic compressor cascades with a chord Reynolds number of 2.12 × 106. A comprehensive analysis was conducted on both the SBLI structures inherent to the transonic compressor cascade and the coherent vortex structures within the boundary layer. The underlying mechanisms of the shock-induced boundary layer transition and the shock low-frequency unsteadiness in the transonic compressor cascade were elucidated through spectral and dynamic mode decomposition (DMD) analysis. The results revealed that boundary layer separation induced by the SBLI cannot reattach, leading to the formation of large-scale coherent vortex structures. Spectral analysis revealed that the shock-induced boundary layer transition in the transonic compressor cascade was dominated by inviscid Kelvin–Helmholtz (K–H) and secondary instability mechanisms, characterized by a dimensionless Strouhal number of 0.06. Additionally, pressure signals showed the variations in sub-frequency from the separated shear layer to the main flow. The oscillation amplitude of the shock foot was significantly greater than that of the shock main body, and the oscillation frequency of the shock foot was consistent with the sub-frequency. The oscillation frequency of the shock main body coincided with that of the compression ramp and flat plate configurations. Finally, DMD modal analysis indicated that high-frequency modes were correlated with turbulent fluctuations in the boundary layer, while medium- and low-frequency modes corresponded to shedding motion in the separated shear layer and low-frequency motion of the shock. This work promotes the understanding of the complex flow mechanisms of SBLI in the transonic compressor cascades.

Predicting rocker shoe parameters for reducing plantar pressure using machine learning methods

The formation and evolution of quadruple synthetic jets issuing into quiescent air are experimentally investigated. Four different configurations (monopole-like, dipole-like, quadrupole-like and 90-degrees-circularly-shifted-jets) have been studied by varying the phase lags between the ejection strokes of the four jets, at constant values of the source Reynolds and Strouhal numbers, equal to 4000 and 0.2, respectively. For each configuration instantaneous three-component in-plane velocity fields in the most relevant streamwise planes are measured with stereoscopic particle image velocimetry. The evolution of the turbulent jet flow is analyzed by the velocity field triple decomposition, thus separating the time-averaged component from the forced periodic oscillation and the turbulent fluctuation. It is observed that the near region of the flow domain is dominated by the formation and advection of large-scale coherent vortex structures, which are originated by the interaction of the vortex rings inherent to the single synthetic jets. The vortex dynamics is captured by phase-averaged measurements and its role in determining the flow field structure of each of the investigated configurations is widely discussed. The vortex structures are observed to lose their coherence and break down into turbulence while propagating downstream. The decay of the large-scale coherent motions is accompanied by the establishment of a self-preserving behavior of both the mean flow and turbulent quantities. Nevertheless, it is shown that the far-field topology depends critically on the source conditions and near-field evolution.

Electrical and Flow Characteristics of a Double-Side Sliding Pulsed Discharge Plasma Actuator

The development of next-generation high-maneuverability aircraft requires new technologies for controlling asymmetric separated vortices at high angles of attack. Plasma flow control is an emerging type of active flow control technology that can be used to control asymmetric vortices over slender bodies by means of pulsed dielectric barrier discharge (DBD) plasma actuators. However, some problems worthy of further study remain unsolved. For example, the stimulation strength is inadequate, the region of discharge is small, and the control mechanism is unclear. This paper proposes a double-side sliding pulsed discharge (DSPD) plasma actuator that combines a sinusoidal alternating current (AC) pulsed plasma actuator with two high-voltage direct current (DC) components. This actuator can generate two groups of unsymmetrical large-scale coherent vortex structures to balance the vorticity of the asymmetric vortices over the slender forebody of an aircraft and increase the distance between the cores of the asymmetric vortices, weakening the interaction between the vortices and hindering the emergence of the asymmetric phenomenon. Focusing on the key scientific issue of DSPD plasma actuation, the discharge morphologies and electrical properties under different DC conditions are studied, and the unsteady vortex structures induced by the DSPD plasma actuator under a static atmosphere are revealed through high-speed schlieren system and particle image velocimetry (PIV) measurements. The flow control ability of the DSPD plasma actuator is verified through wind tunnel experiments, and the main rules governing its influence on the external flowfield are determined. It is speculated that this actuator will further improve the aerodynamic flow control of slender aircraft.

Three-dimensional visualization of destruction events of turbulent momentum transfer in a plane jet

Destruction events of momentum transfer as represented by the Reynolds (shear) stress via the pressure–rate-of-strain interaction in a plane jet are investigated using direct numerical simulation data. To obtain three-dimensional pictures of the turbulent structures, pseudo-three-dimensional data are constructed on the basis of Taylor’s frozen hypothesis from time series of the relevant quantities. The coherent motion in the plane jet is extracted by means of conditional sampling analogous to variable-interval time averaging. We obtain a large-scale coherent structure consisting of horseshoelike vortices and a Kármán-vortex-like very large spanwise vortex street. Furthermore, we find that the active Reynolds stress destruction is localized around the large-scale coherent vortex structure. However, the coherent structure itself does not contribute to the Reynolds stress destruction, but the remaining incoherent motions do. For the significant pressure–rate-of-strain events in the incoherent motion, we display invariant maps of the velocity gradient tensor and the principal axes of strain to facilitate a discussion of the consistency of the numerical results with an illustration by Hinze [Turbulence, 2nd ed. (McGraw-Hill, 1975), pp. 325–328] and with our own experimental results [Takahashi et al., “Experimental investigation on destruction of Reynolds stress in a plane jet,” Exp. Fluids 60, 46 (2019)]. Consequently, the present results suggest that the illustration of the Reynolds stress destruction by Hinze is not completely applicable to a plane jet.

Vortex evolution in the near wake behind polygonal cylinders

The near wake of the polygonal cylinder with the side number N = 3~∞ is systematically studied using particle image velocimetry (PIV) at Re = 1.6 × 104. The proper orthogonal decomposition (POD) analysis is carried out to extract the large-scale coherent vortex structures and their evolution. It has been found that the vortex circulation grows to the maximum at the vortex formation length by entraining the vorticity from the separated shear layer and then undergoes a two-stage decay. The maximum circulation scales with the wake width, defined as the vertical distance between the two peaks of streamwise velocity fluctuation at vortex formation length. The vortex center trajectory indicates that the vortices move towards the centerline first and then away, with the vortex size monotonically increasing over the examined streamwise range. The vortex size at the maximum circulation also scales with the wake width. The vortex convection velocity increases gradually in the streamwise direction, and the ratio of the lateral and streamwise components of the vortex convection velocity, when scaled by wake width and vortex formation length respectively, approaches asymptotically 0.18 in the downstream, irrespective of the cylinder orientation or N.

Formation of Turbulence at Astronomical Observatories in Southern Siberia and North Caucasus

The main characteristics of the astroclimate (the structure of turbulent fields and conditions for their formation) are studied for several Russian astronomical observatories in southern Siberia and North Caucasus, including Sayan Solar, Baikal Astrophysical, Special Astrophysical, and Kolyvansky Ridge Observatories and the Center for Laser Atmosphere Sensing of Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences. The experimental results have been received from long-term astroclimate observations (expeditions in 2006–2017) with the use of small-size ultrasonic weather stations; and theoretical results, from the numerical solution of different boundary problems for the Navier–Stokes equations. The presence of regions with coherent (non-Kolmogorov) turbulence over the observatories has been ascertained, the formation of which is caused by the topography and the nonuniform heating of the underlying surface. Large-scale coherent vortex structures and coherent turbulence have been detected inside dome rooms. Numerical simulation allows us to analyze the influence of design features and temperature regimes of the telescope components and to test the telescope optical characteristics, including the minimization of the effects of external and under-dome turbulence. The paper includes the review of the history and evolution of the “coherent structure” and “coherent turbulence” concepts based on the world scientific literature and our own researches.

The evolution of quadruple synthetic jets

The formation and evolution of quadruple synthetic jets issuing into quiescent air are experimentally investigated. Four different configurations (monopole-like, dipole-like, quadrupole-like and 90-degrees-circularly-shifted-jets) have been studied by varying the phase lags between the ejection strokes of the four jets, at constant values of the source Reynolds and Strouhal numbers, equal to 4000 and 0.2, respectively. For each configuration instantaneous three-component in-plane velocity fields in the most relevant streamwise planes are measured with stereoscopic particle image velocimetry. The evolution of the turbulent jet flow is analyzed by the velocity field triple decomposition, thus separating the time-averaged component from the forced periodic oscillation and the turbulent fluctuation. It is observed that the near region of the flow domain is dominated by the formation and advection of large-scale coherent vortex structures, which are originated by the interaction of the vortex rings inherent to the single synthetic jets. The vortex dynamics is captured by phase-averaged measurements and its role in determining the flow field structure of each of the investigated configurations is widely discussed. The vortex structures are observed to lose their coherence and break down into turbulence while propagating downstream. The decay of the large-scale coherent motions is accompanied by the establishment of a self-preserving behavior of both the mean flow and turbulent quantities. Nevertheless, it is shown that the far-field topology depends critically on the source conditions and near-field evolution.

Novel Quadruple Synthetic Jet Device: Flowfield and Acoustic Behavior

This paper reports on an experimental investigation of the flowfield and acoustic behavior of an innovative quadruple synthetic jet device operated in different configurations. The device consists of four resonant cavities, each driven separately by a loudspeaker in such a way that it is possible to implement any configuration in terms of the frequency, amplitude, and initial phase angle of the exit velocities of the four embedded jets. Here, the main goal is to investigate the effect of varying the phase delay between the ejection strokes of the single synthetic jets. At first, a calibration procedure of the driving signals is designed in order to ensure that the four jets have the same exit conditions, except for a desired phase shift. Second, four different configurations are investigated at fixed values of the Reynolds and Strouhal numbers of each synthetic jet, equal to 4000 and 0.2, respectively. Stereoscopic particle image velocimetry measurements highlight the central role of the near-field large-scale coherent vortex structures in determining the flow behavior. Far-field noise measurements show that, by introducing a phase shift between the jet exit velocities, a significant noise reduction can be obtained (up to 23.9% for the examined configurations).

On the velocity and pressure statistics during the flapping motion in a planar turbulent jet

The role of a large-scale coherent vortex structure in a planar jet, which is intermittently observed and called “flapping motion”, on the turbulent energy transport process is investigated experimentally. The experiment is performed by simultaneously measuring the two components of velocity and pressure in the self-preserving region of the jet. The probe for the simultaneous measurement of the velocity and pressure consists of an X-type hot-wire sensor and a static pressure probe. The measurement data are analyzed using a conditional sampling technique and the phase-averaging technique, on the basis of an intermittency function which shows whether the jet is now flapping or not. This intermittency function is obtained by the measurement results of the streamwise velocity fluctuation by means of the two I-type hot-wire sensors set in the self-preserving region of the jet with applying a continuous wavelet transform analysis to the data. The experimental results show that the phase-averaged velocity field during the flapping motion shows a good agreement with those obtained through the 23 points simultaneous measurement of the streamwise velocity in the previous studies. Further, the phase-averaged pressure field during the flapping motion indicates the existence of a large-scale coherent vortex structure, interpreted as a combination of the flapping and puffing motions in the self-preserving region of the jet. In addition, it is found that the production of the turbulent energy and its diffusion from the inner region to the outer region of the jet are enhanced by the flapping motion. In particular, this enhancement of the turbulent energy diffusion is caused by the combination of an increase of the turbulent diffusion to the outer region of the jet and a decrease of the pressure diffusion to the inner region of the jet.

0603 KL 展開による加熱二次元噴流の温度場の固有モード解析(OS6-1 噴流,後流およびはく離流れ現象の探求と先端的応用,OS6 噴流,後流およびはく離流れ現象の探求と先端的応用)

In this study, simultaneous measurement of the temperature at two points with cold-wire sensors is performed at three different downstream locations of a heated plane jet. The measurement apparatus consists of a wind tunnel, a skimmer, a side-wall, and a heater set at the blower intake of the wind tunnel. The temperature of the jet is controlled to become 5 K higher than the ambient fluid. A thin tungsten wire whose diameter is 3.0 μm is used as a sensor and is connected to the self-made constant current circuit. The output of the cold-wire sensor is compensated theoretically to increase the temporal resolution in the measurement. By applying the Karhunen-Loeve (KL) expansion in space and time, the development of the structure in the heated plane jet is investigated from the viewpoint of both space and frequency. Measurement results show that the dominant mode of the fluctuating temperature field in a plane jet expresses and relates to a flapping motion, which is the large-scale coherent vortex structure in the plane jet.

Investigation of gas-solid two-phase flow across circular cylinders with discrete vortex method

In present paper, a Lagrangian–Lagrangian model is proposed to study gas-solid two-phase flow across single cylinder and two tandem cylinders at high Reynolds number. In this model, the single-phase flow is simulated by discrete vortex method and the particle trajectories are tracked by particle motion equation. A sub-cycle is introduced to adjust time-step in the particle collision model, through which the simulation of two phases is coupled. Validated by the comparison of the particle trajectory in Rankine vortex with literature, this model is used to study single-phase flow across single cylinder and cylinders with different arrangements firstly to get transient flow field and drag coefficients. Then, gas-solid two-phase flow across cylinders with different arrangement is studied and particle distribution is obtained under different Stokes number for horizontal and vertical particle transport cases. The settlement, entrainment and aggregation of solid particles moving with the large-scale coherent vortex structure in the wake of single cylinder and between two cylinders are numerically investigated, and the effects of St number on the distribution of solid particles are obtained.

A new electromagnetic wave and associated vortex motions in nonuniform dusty magnetoplasmas

It is known that the presence of stationary charged dust grains produces rotational ion motion due to the J × B force in a uniform dusty magnetoplasma, where J is the plasma current density and B is the magnetic field. Our objective here is to show the possibility of a new low-frequency electromagnetic mode in a nonuniform dusty magnetoplasma. The nonlinear dynamics of newly found electromagnetic mode is governed by a Charney–Hasegawa–Mima (C-HM) equation. Possible stationary solutions of the latter can be represented in the form of a vortex street and a dipolar vortex. The present results should help to understand the origin of large scale coherent vortex structures that are frequently observed in astrophysical objects (e.g., in molecular clouds) and in galaxies. Besides, new laboratory experiments should be conducted for verifying the linear and nonlinear properties of the low-frequency electromagnetic wave that is reported herein.

Combustion dynamics of turbulent swirling flames

In the present paper the influence of a periodic excitation of the mass flow rate, varied in frequency and in amplitude, on the isothermal flow field and on the flame characteristics of swirl burners with different swirl intensities S 0,th is investigated. At first the fluid dynamical conditions for the formation of ring-vortex structures in the burner near flow field are determined. The results indicate that the minimum level of excitation of the mass flow rate for vortex formation decreases hyperbolically with increasing frequency of pulsation and characteristic Strouhal number, respectively. In further studies, the dynamical behavior of lean-premixed swirl flames is investigated, whereas the fuel gas/air mixture mass flow rate at the burner exit was modulated sinusoidally. The dynamical behavior of the investigated flames is found to be dominated by two different effects in certain frequency ranges: For moderate pulsation frequencies (f puls ≲ 50 Hz) the detected periodic heat release rate of pulsated, premixed swirl flames is dominated by an effect that inhibits the strong entrainment of ambient medium in comparison with the corresponding quasi steady-state swirl flames. With increasing frequency (f puls ≳ 50 Hz) this effect will be overlaid by the periodical formation of ring-vortices entraining additional ambient medium and interfering with the combustion process. The physical understanding of the frequency-dependent flame dynamics (flame transfer function) on periodic disturbances is indispensable for the prediction of the formation of combustion-driven oscillations that occur in technical combustion systems (e.g., gas turbines, industrial combustors). It is also evident for the development of methods to prevent or suppress periodic combustion instabilities. The results of the flame transfer measurements presented in this paper will lead to a basic understanding of the formation and reaction of large-scale coherent vortex structures in turbulent flames, that are well known as drivers of combustion instabilities.

On the stability of periodic unidirectional flows of micropolar fluid

The paper treats the stability of a periodic unidirectional two-dimensional flow of micropolar fluid. An exact value of the critical Reynolds number R C is found for a wide class of such flows. As well as for the Newtonian fluid, the secondary flow proves to be a large-scale coherent vortex structure near the stability threshold, R - R C⪡1 .