Large-scale Vortex Structures Research Articles

Effect of the wing trailing-edge flaps and spoilers position on the jet-vortex wake behind an aircraft during takeoff and landing run

The large-scale vortex structure of flow in the near wake behind an aircraft during its run on a runway is investigated numerically. The geometrical aircraft configuration was taken close to a mid-range commercial aircraft like Boeing 737-300. It included all essential elements: a body (fuselage), wings with winglets, horizontal and vertical stabilizers, engine nacelles, nacelle pylons, inboard flap track fairings, leading-edge and trailing-edge flaps, and spoilers. The position of flaps and spoilers corresponded to the takeoff and landing run conditions. Computational simulation was based on solving the Reynolds averaged Navier–Stokes equations closed with the Menter Shear Stress Transport turbulence model. Patterns of streamlines, fields of the axial vorticity and the turbulent intensity, vertical and horizontal velocity profiles in the wake are compared and discussed for both run regimes. The flow model was preliminary tested for validity by comparison of the calculated velocity profiles behind a reduced-scale aircraft model with those obtained in special wind tunnel experiments.

Numerical Investigation of the Influence of Special Structures On Suppression of Pressure Pulsations in The Draft Tube of a High-Head Hydraulic Turbine

The authors have performed numerical simulation of pressure pulsations in the draft tube of a hydraulic turbine that are excited by large-scale vortex structures. Numerical simulation of unsteady transfer in the draft tube in different operating regimes of the turbine has been carried out. It has been shown that the structure of flow in the outlet diffuser corresponds to vortex-core flow. Calculated data have been compared with experimental results and their agreement has been shown. To suppress pressure pulsations, a study has been made of the influence of various structural inserts into the draft tube, such as fins, a cross piece, and a hollow cylinder in the draft-tube cone. Consideration has been given to the influence of various constructions on the structure of flow behind the wheel and on pressure pulsations on the draft-tube wall. A comparison has been made of the constructions and optimum parameters have been selected to stabilize the flow in the diffuser of the hydraulic-turbine draft tube.

Applying phase averaging technique to analysis of unsteady twin vortex structure observed in tangential vortex chamber

This paper is devoted to experimental investigation of formation of precessing vortices pair in a vortex chamber with a cylindrical working area. Flow structure was registered by means of a high-speed visualization technique. Quantitative measurements characterizing the vortex properties were carried out using a 2D particle image velocimetry (PIV) technique synchronized with wall pressure measurements. The results show that two symmetrical precessing helical vortices are present downstream of the tangential nozzles.The phase- averaged analysis confirms the presence of large-scale twin vortex structure that visualized by small air bubbles dissolved in water.

Effects of Reynolds number on vortex structure behind a surface-mounted finite square cylinder with AR = 7

This paper presents the numerical solutions of flow around a surface-mounted square cylinder of aspect ratio h/d = 7 at Reynolds numbers of 652 and 13 041. The aim is to investigate the effect of the Reynolds number, between its medium-to-high range, on the flow and vortex structure around such a cylinder. The present simulations have successfully reproduced the primary flow, as well as the three-dimensional large-scale vortex structure in the wake of the finite wall-mounted body. The observation of base vortices, tip vortices, and a horseshoe vortex is consistent with previous experimental studies. A dipole wake is captured at the higher Reynolds number, while a quadrupole wake is captured for the lower, indicating that the Reynolds number strongly influences the wake structure. In the near-wake region, by plotting the isosurface of instantaneous second invariant of the velocity gradient, the full-loop structure is observed for the quadrupole wake, while the half-loop structure is observed for the dipole wake. In the far-wake region, a braided vortex structure formed by asymmetric hairpin vortices is observed at both Reynolds numbers and a new wake topology is proposed for flows with a similar geometry.

Numerical Schemes and Hybrid Approach for the Simulation of Unsteady Turbulent Flows

The paper presents a brief review of computational approaches for the simulation of turbulent flows and shows that the correct calculation of large-scale vortex structures necessarily requires the use of eddy-resolving methods, while the applied numerical schemes must be stable and accurately describe the spatial evolution of vortices. The dissipative properties and stability of most numerical schemes implemented in the OpenFOAM package are analyzed by solving problems on the decay of the homogeneous isotropic turbulence and scalar transfer. It is established that the considered schemes are not suitable for correctly calculating the propagation and dissipation of vortices in space; therefore, they are refined to eliminate oscillations and maintain an acceptable level of dissipation. An algorithm for combining the URANS and LES methods using zoning of the computational domain is described and implemented. To test the implemented calculation method of unsteady turbulent flows, we simulate the flow around a maneuvering aircraft with an installed airbrake deflection. The flow patterns of the aircraft and its aerodynamic characteristics are obtained and compared with the experimental data.

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.

Influence of Cosmic Ray Invasions and Aerosol Plasma on Powerful Atmospheric Vortices

The Earth’s atmosphere is affected by various ionizing sources. The maximum ionization of atmospheric particles by cosmic rays corresponds to the altitude of formation of tropospheric clouds. In the high-latitude troposphere for the region of the geomagnetic polar cap, in the winter period, the excitation of local cyclonic structures are observed which are accompanied with ice storms, with invasions into middle and subtropical latitudes. The time of excitation of such cyclones is about a day that is comparable with the time of excitation of tornadoes, which are generated at low latitudes. Localization of polar cyclones is not accidental. The region of the polar cap is connected with geomagnetic field lines extended into the tail of the Earth’s magnetosphere. This area is open for the penetration of cosmic rays. The ionization of aerosols in the stratosphere and the upper troposphere by precipitating particles of cosmic rays enhances the vortex activity of the atmosphere. The important role of the aerosol impurity is manifested in the generation of plasma vortices and in the accumulation of energy and mass in the atmosphere by vortices during condensation of moisture. Due to the cascade character of the ionization process, the influence of cosmic radiation turns out to be non-linear and increases with increasing pollution of the atmosphere. Aperiodic electrostatic perturbations, which play a remarkable role in the genesis of vortices, are stochastically excited in plasma inhomogeneities. During the interaction of plasma vortices and Rossby vortices, a large-scale vortex structure is formed and grows.

Characteristics of mixing enhancement achieved using a pulsed plasma synthetic jet in a supersonic flow

Supersonic mixing layers exist extensively in supersonic engineering applications. The rapid mixing of fuel and oxidant at short distances is of great importance, but makes it difficult to develop efficient propulsion systems. The plasma synthetic jet (PSJ) is regarded as a promising high-speed flow control technique. The characteristics of mixing enhancement achieved using a pulsed PSJ were investigated via experiments. Results showed that the PSJ is an effective method for mixing enhancement. Nanoparticle-based planar laser scattering (NPLS) was used to obtain flow structures in three directions. The velocity fields near the PSJ actuator orifice were measured by particle image velocimetry (PIV). Indexes of the fractal dimension and mixing layer thickness were applied to estimate the effect of the PSJ actuator on the supersonic mixing layers. The large-scale vortex structures induced by the pulsed PSJ in the supersonic mixing layers were successfully captured by NPLS. The effect of the PSJ on the supersonic mixing layers was remarkable. The mixing layer thickness under perturbation was larger than that under no perturbation in the downstream. The distribution of the fractal dimension suggests that perturbation of the PSJ cannot improve the fractal dimension values of the fully developed supersonic mixing layers.

Fine structures of self-sustaining dual jets in supersonic crossflow

The interaction between self-sustaining dual jets and supersonic laminar boundary layer is experimentally studied using the nanoparticle-based planar laser scattering in a Ma 2.95 supersonic low-turbulence wind tunnel for the first time. The typical fine flow structures, such as separation shock, “W” bow shock, barrel shock, horseshoe vortex, large-scale vortex structures and counter-rotating vortex pairs, are clearly identified by streamwise and spanwise flowfield images. The curves of jet penetration depth are fitted at the momentum ratio of 0.55 between self-sustaining dual jets and supersonic crossflow. The results play a significant role in further and comprehensive exploration of the active flow control for a supersonic flow.

Observed Kinematic and Thermodynamic Structure in the Hurricane Boundary Layer during Intensity Change

Abstract The axisymmetric structure of the inner-core hurricane boundary layer (BL) during intensification [IN; intensity tendency ≥20 kt (24 h)−1, where 1 kt ≈ 0.5144 m s−1], weakening [WE; intensity tendency <−10 kt (24 h)−1], and steady-state [SS; the remainder] periods are analyzed using composites of GPS dropwindsondes from reconnaissance missions between 1998 and 2015. A total of 3091 dropsondes were composited for analysis below 2.5-km elevation—1086 during IN, 1042 during WE, and 963 during SS. In nonintensifying hurricanes, the low-level tangential wind is greater outside the radius of maximum wind (RMW) than for intensifying hurricanes, implying higher inertial stability (I2) at those radii for nonintensifying hurricanes. Differences in tangential wind structure (and I2) between the groups also imply differences in secondary circulation. The IN radial inflow layer is of nearly equal or greater thickness than nonintensifying groups, and all groups show an inflow maximum just outside the RMW. Nonintensifying hurricanes have stronger inflow outside the eyewall region, likely associated with frictionally forced ascent out of the BL and enhanced subsidence into the BL at radii outside the RMW. Equivalent potential temperatures (θe) and conditional stability are highest inside the RMW of nonintensifying storms, which is potentially related to TC intensity. At greater radii, inflow layer θe is lowest in WE hurricanes, suggesting greater subsidence or more convective downdrafts at those radii compared to IN and SS hurricanes. Comparisons of prior observational and theoretical studies are highlighted, especially those relating BL structure to large-scale vortex structure, convection, and intensity.

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.

A review of transient flow structure and unsteady mechanism of cavitating flow

The flow structure and the unsteady mechanism of the unsteady cavitating flow are reviewed in this paper. The flow patterns and structures in different cavitation regime, for the attached cavitation and the vortical cavitation, are shown with both the visualization and the quantitative information. The attached cavitating flow around the Clark-Y hydrofoil and the vortical cavitating flow around the Tulin hydrofoil are considered. In particular, the phenomena such as the large-scale vortex structure and the re-entrant flow associated with the cloud cavitation, and the cavitating vortex street’s forming and crumbling are described. The evolution of the cavitation structure in the transient sheet/cloud cavity forming, along with the cavity collapse induced by the re-entrant flow and the shock wave propagation are discussed. The perspective future research of higher fidelity simulations, and the accurate identifications of the cavitating vortex structure is commented.

An Analytical Description of the Expansion of Plane Wall Jets

The effectiveness of forming microclimate of buildings and structures depends on the decisions of air exchange organization. One of the influencing factors of the efficiency of air exchange organization is the development of ventilation flow jets. In this case, the laying of jet streams on surfaces of the enclosing structures (wall jets) is widely used. In this work, the laws of the development of plane wall jets on the basis of geometric and kinematic analysis of large-scale vortex structure are obtained. For this purpose, a turbulent macrostructure scheme was constructed. The macrostructure is represented in the form of a sheet of tangent large-scale vortices (puffs) within the jet boundary layer. It is assumed that in the outer part between the puffs there is only the injection of the environmental liquid or gas to the jet in the direction perpendicular to the direction of its movement. In this case, when the puff moves, the jet should use the entire volume that the puff occupies on its path. Implementation of such model is most effective in computer aided design (CAD) systems, which allows automatic precise determination of the areas of complex figures. In this work, free version of DraftSight has been used To get the four exact signs of the tangent of the angle of expansion of the jet, there are only six iterations. The obtained results coincide with the known experimental data of G. Abramovych, which confirms the adequacy of the results obtained. Similar calculations have also been made for the refined hypothesis of I. Shepelev, which allows removing the wall boundary layer from consideration. Clarification of the hypothesis is that the jet boundary layer should be stretched to the wall. The difference in results is within 1 %, which allows to recommend the hypothesis for widespread practical use.

Large Eddy Simulation of Premixed Stratified Swirling Flame Using the Finite Rate Chemistry Approach

Large eddy simulations of a stratified swirling flow of a Cambridge swirl burner for both nonreacting and reacting cases are conducted using a finite rate chemistry approach represented by a partially stirred reactor model. The large eddy simulation predictions are compared with experimental measurements for velocity, temperature, and concentrations of major species. The agreement is found in overall trend of velocity prediction, but temperature and concentration of major species show slight discrepancies in the central region. Two reduced chemical mechanisms are examined in the present paper with the objective of assessing their capabilities in predicting swirling flame characteristics, and the distinct difference using two mechanisms is found in CO distribution profiles, which is considered the consequence of different kinetics of CO-CO2 equilibrium. Flow structures are qualitatively and quantitatively analyzed with numerical results. Large-scale vortex structures and precession motions are observed in both nonreacting and reacting cases. Frequency of vortex shedding is identified from the point data of instantaneous velocity in the discharging stream-induced shear layer. On this basis, the intensity and frequency of precession motion are shown to be enhanced in the presence of combustion. Large-scale wrinkling of the flame surface is resolved and characterized in the flame zone, and the effect of mixture stratification is then further discussed.

Rotationally-induced unstable flow characteristics and structural optimization of rim seals

In view of rotationally-induced unsteady rim sealing flow issues in the absence of mainflow nonuniform pressure fields, the large eddy simulation method validated by the enclosed rotating cavity flow measurement was adopted to investigate the instability of the flow in the enclosed rim seal cavity. On this basis, the unsteady Reynolds-averaged numerical calculation was carried out for three structures: typical axial and radial rim seals as well as the chute seal that is visible in actual engines. The unsteady sealing flow characteristics for typical rim seal structures are investigated. The dynamic characteristics of the large-scale vortex structures in the rim seal region are revealed by fast Fourier transform and cross-correlation analysis methods. Finally, the rotationally-induced sealing characteristics for different rim seal structures and the enlightenment of the seal structure optimization are given. The results show that the rotationally-induced rim sealing flow has inherent instability characteristics; a series of large-scale vortex structures are generated in the rim clearance region, which propagate in the circumferential direction at a speed lower than the rotor, and the vortex frequency, number of revolutions and number of vortices depend on the rim seal type; overall, the chute rim seal has the highest rotationally-induced sealing effectiveness.

Numerical investigation on flow field characteristics of Helmholtz oscillator

Self-excietd oscillation pulsed(SOP) jet, is capable of great destruction. As a typical kind of SOP nozzle, Helmhotlz oscillator has its special application. Previous researches mainly focus on optimization of the structure parameters, erosion ability of jets and so on. In this paper, the flow field of Helmhotlz oscillator was numerically simulated. 20 nozzles with different structures were manufactured according to simulation results; their performances were tested on different driving pressure and confining pressure under the condition of submerging. Conclusions can be drawn as follows: Compared with traditional conical nozzle, the flow characteristics of Helmholtz nozzle are obviously different. Change of structural parameters has obvious influence on the amplitude of pressure fluctuation. If the structure parameters are reasonably adjusted, energy dissipation will be reduced and working ability will be optimized. A large scale vortex structure is formed by the flow turbulence of SOP jet. In some cases that require suction and mixture, SOP nozzle has unique advantages. Since the theory of the SOP nozzle is still evolving, it is necessary to further study the pulse mechanism to realize artificial modulation.

The method of averaging the gas flow parameters in turbomachines to evaluate their efficiency considering the velocity-field helicity

The methodology of averaging the nonuniform swirling flow in turbomachines has been developed. It is based on preserving the mass flow rate, the fluxes of total enthalpy and momentum (or entropy) with the addition of new invariant into the algorithm, i.e., the integral of the gas flow helicity. The dependence has been obtained and validated to specify the turbine stage efficiency calculation and to take into account the presence of large-scale vortex structures in the inter-blade channel of the turbomachine (the uncertainty of the influence of secondary flows on the value of losses does not exceed 0.2%). The methods of averaging the gas flow parameters, taking into account the contribution of secondary flows to the kinetic energy loss are presented. They show a 2.4% lower efficiency compared to the calculation previously used for the turbine stage of the Leningrad Polytechnic Institute.

Experimental investigation on destruction of Reynolds stress in a plane jet

Flow structures that cause destruction of the Reynolds (shear) stress are investigated by means of simultaneous measurement of the velocity gradient and pressure. Two types of combined probes are used for the simultaneous measurement of the velocity gradient and pressure. One consists of an X-type hot-wire probe and a pressure probe; another consists of two I-type hot-wire probes arranged in parallel with a small vertical separation (double-I) and a pressure probe. The hot-wire probe and the pressure probe are aligned along the streamwise direction for both combined probes. We confirm that the simultaneous measurement of the velocity gradient and pressure in a plane jet is satisfactorily performed using the combined probes, except for the outside of the velocity half-width. In a plane jet, $$\overline{(p'/\rho )(\partial u'_{2}/\partial x_{1})}$$ does not contribute to the Reynolds stress destruction, so that the total pressure-rate-of-strain correlation is almost equal to $$\overline{(p'/\rho )(\partial u'_{1}/\partial x_{2})}$$ . The flow structures that destruct the Reynolds stress are extracted by the conditional sampling technique. We find two structures, which support the thought experiment conducted by Hinze (Turbulence, 2nd edn, McGraw-Hill College, Pennsylvania, 1975). The structures are concentrated around large-scale vortex structures, which cause strong momentum transfer. The scale of the structures is intermediate: smaller than large-scale motion contributing to the momentum transfer, and larger than small-scale motion contributing to the energy dissipation.