Formation Of Wake Vortices Research Articles

Wake vortex formation and its role in turbulent heat transport in jet in crossflow

Wake vortex formation and its role in turbulent heat transport in jet in crossflow

Junction flow inside and around three-row cylindrical group on rigid flat surface

Groups of bluff bodies are widespread in nature and technology. These are the supports of bridge crossings, high-rise buildings in cities, offshore drilling and wind platforms, algae and vegetation in the seas and rivers, forests and other objects. The flow of air or water around such structures has a complex vortex and jet character and requires significant efforts in the process of scientific research to improve the environmental situation and reduce material and technical costs in the process of operating such structures. The purpose of the research is study the features of the generation and evolution of vortex and jet flows near and inside the three-row group of cylinders, which are installed on the rigid flat surface. The results of experimental studies showed that the flow around the group of cylinders had a complex unsteady nature, which is due to the interaction of vortex and jet flows typical flow elements with the three-row cylindrical group, which was located installed on the rigid flat surface. The three-row cylindrical group (31 piles with a diameter of 0.027 m) is a model of a bridge support, which was streamlined at a velocity of 0.06 m/s to 0.5 m/s (Reynolds number Red=(1600–6700) and Froude number Fr=(0.04–0.18)). Visual investigations and measurements of the velocity field were carried out inside and around the three-row structure. The features of the formation and evolution of vortex and jet flows inside and near the cylindrical group were established. Integral, spectral and correlation characteristics of the velocity fluctuation field were obtained. Mean, root-mean-square values of velocity and probability density functions of velocity fluctuations integrally displayed the changes in the velocity field in the spatial and temporal domain in the junction area of grillage and plate. The power spectral densities of velocity fluctuations and mutual correlation functions made it possible to study the features of the generation of the velocity fluctuation field in the frequency domain and its interrelationships in space. It was revealed that the velocity field inside the horseshoe vortex structures was multimodal. The spectral levels of velocity fluctuations at the periphery of the quasistable horseshoe vortex structures were higher than in the cores of these structures. The highest levels of the velocity fluctuation spectra were observed in front of the second lateral cylinder where the interaction of the vortex and jet flows took place. Discrete peaks in the spectral levels of velocity fluctuations are found at the frequencies of formation of large-scale wake vortices and the frequencies of formation of small-scale vortex structures of the shear layer, which are due to the Kelvin-Helmholtz instability. It has been established that the frequency of formation of shear layer vortices is (10–40) times higher than the frequency of formation of wake vortices.

Formation number and pinch-off signals of disc vortex ring based on a Lagrangian analysis

As one fundamental flow structure, the vortex ring behind an impulsively starting disc can vividly characterize the vortex structure around the flapping flight and the drag-based paddling propulsions. In this paper, an elaborate experiment is performed to investigate the formation of the vortex rings (termed as DVR) by using digital particle image velocimetry (DPIV), as well as by investigating the ridges of finite-time Lyapunov exponent (FTLE) and the elliptic Lagrangian coherent structures (LCSs). Experimental results demonstrate that the circulation growth of DVRs satisfies the Logistic law and can be well predicted by a scaling model based on the shear-layer feeding velocity. Applying the criterion originated from the Kelvin-Benjamin variational principle, the formation number of DVRs when DVRs pinch off is found to stay within the range of F∗=3.35~4.1. Then, the pinch-off process of the DVRs is re-examined based on a Lagrangian analysis, and the dynamic signals are detected to coincide with the onset of DVRs pinch-off. Once DVRs pinch off, a trailing pressure maximum appears in the edge of disc and a closed boundary forms to define the region of DVRs based on the FTLE ridges, fluid detrainment by the dynamic lobes is observed, and a saturation phase of the circulation evolution is clearly observed in terms of the elliptic LCSs. Furthermore, the scaling laws, i.e., circulation growth and formation number, of DVR formation suggest a confined formation mode to understand the formation of wake vortices behind a moving body for reference.

壁面效应对剪切稀化流体内气泡上浮特性的影响

The wall effects on the floating motion of bubbles in shear-thinning fluids were investigated with the numerical method. The interface between the bubble and the liquid phase was tracked with the volume of fluid (VOF) method. The Carreau model and the continuous surface tension model were used to calculate the rheological properties of the shear-thinning fluid and to compute the surface tension between gas and liquid phases, respectively. For different rheological indexes, the wall effects on the bubble shape, the liquid-phase flow field and the bubble terminal velocity were studied in detail. The results show that, the strong wall effect or the weak shear-shining effect can hinder the bubble deformation and the formation of wake vortices, and can reduce the bubble terminal velocity. Among all physical parameters, the bubble terminal velocity is most liable to the wall effect. For the strong wall effect and the strong shear-shining effect, the high-shear-rate region will occur near the wall, which results in a significant decrease in the apparent viscosity of the liquid phase near the wall.

Phase field simulation of single bubble behavior under an electric field**Project supported by the National Natural Science Foundation of China (Grant Nos. 51661020, 11504149, and 11364024), the Postdoctoral Science Foundation of China (Grant No. 2014M560371), and the Funds for Distinguished Young Scientists of Lanzhou University of Technology, China (Grant No. J201304).

Based on the Cahn–Hilliard phase field model, a three-dimensional multiple-field coupling model for simulating the motion characteristics of a rising bubble in a liquid is established in a gas–liquid two-phase flow. The gas–liquid interface motion is simulated by using a phase-field method, and the effect of the electric field intensity on bubble dynamics is studied without electric field, or with vertical electric field or horizontal electric field. Through the coupling effect of electric field and flow field, the deformation of a single rising bubble and the formation of wake vortices under the action of gravity and electric field force are studied in detail. The correctness of the results is verified by mass conservation, and the influences of different electric field directions and different voltages on the movement of bubbles in liquid are considered. The results show that the ratio of the length to axis is proportional to the strength of the electric field when the air bubble is stretched into an ellipsoid along the electric field line under the action of electrostatic gravity and surface tension. In addition, the bubble rising speed is affected by the electric field, the vertical electric field accelerates the bubble rise, and the horizontal direction slows it down.

Active flow control of vortex induced vibrations of a circular cylinder subjected to non-harmonic forcing

A wide variety of waves and currents are abound in wind and ocean engineering practice. These wave forms could be harmonic as well as non-harmonic and may lead to the formation of wake vortices, behind a circular cylinder. The alternating lift forces on such structures could in turn result in damaging flow induced vibrations. In the present study, we propose a simple momentum injection based active flow control strategy to suppress such vortex induced oscillations at low Reynolds numbers. Two small control cylinders located at 120°, behind the main cylinder play the role of actuators, that enforce the desired momentum injection. Detailed Computational Fluid Dynamics (CFD) simulations are carried out, by solving mass, momentum conservation equations in conjunction with a control equation, and a dynamical evolution equation for the structural motion. Non-harmonic inlet forcing on a flexibly mounted circular cylinder generates vortex induced vibrations, which is numerically simulated. Then by controlling the wake vortices, vortex induced vibrations are completely controlled. Analysis of the leeward region behind the main cylinder reveals a different wake signature, with blobs of residual vorticity along the wake centreline. This is attributed to the phase asynchrony between the inlet forcing and the vortex induced vibrations.

Multi-scale statistical characterization of migrating pier scour depth in non-uniform sand bed channel

ABSTRACTThe present study experimentally investigates the multi-scale statistical characterization of migrating pier scour depth over non-uniform sand bed channel. Experiments were conducted for circular piers of 75 mm diameters with and without seepage cases. The results of turbulent flow parameters at the upstream of the pier such as velocity and Reynolds shear stresses (RSSs) are observed to be negative within the scour hole, while the presence of downward seepage counteracts the reversal of the flow results in decreased velocity and Reynolds stresses. RSS is higher at the downstream of the pier owing to the formation of wake vortices. The elevation of scouring is gradually decreased with an increase of time as well as with the increase of downward seepage. Time scale-averaged celerity is decreased with an increase of time and increases with increase of seepage percentage. The present study also investigates the wavelet-based semblance analysis for time series of velocity and bed elevation at different sections of channel, which will show correlations in terms of wavelength and position.

Landing Gear Influence on the Wake Vortex of a Large Transport Aircraft

An experimental investigation on the wake vortex formation and evolution of a four-engine large transport aircraft model in the near field and extended near field has been conducted by means of hot-wire anemometry in a wind tunnel. The half-model used has a scale of 1:19.25 and the tests focus on the high-lift case of a typical landing configuration at a Reynolds number of 0.52 x 10 6 based on the wing mean aerodynamic chord. The flowfield is investigated up to 4.7 spans downstream of the model, which is investigated with and without landing gear. Based on the measured time-dependent velocity components, the wake flowfield is analyzed by distributions of vorticity, turbulence intensities, and spectral densities. The landing gear consists of a stay and four wheels, which create a bluff-body wake that interferes with the shear layer caused by the wing and wing junction. The landing gear creates a low-energy vortex system and a highly turbulent wake. Due to the landing gear, the main vortex centers are located slightly further outboard and the roll-up process is retarded, especially the inboard movement of the horizontal tail plane vortex. The velocity spectra of the landing gear wake show broadband characteristics with some moderate narrowband energy concentrations in the close near field, which do not enhance inherent wake instabilities. Although the turbulence caused by the landing gear is still present in the extended near field, only a small influence on the overall vortex system of the entire aircraft can be found. The landing gear influence is therefore negligible in the context of reducing minimum aircraft separation distances.

Experimental investigation of counter-rotating four vortex aircraft wake

Abstract An experimental investigation on the wake vortex formation and evolution of a four vortex system of a generic model in the near field and extended near field as well as the behaviour and decay in the far field region has been conducted by means of hot-wire anemometry in a wind tunnel. The results were obtained during an experimental campaign as part of the EC project “FAR-Wake”. The model used consists of a wing–tail plane configuration with the wing producing positive lift and the tail plane negative lift. The circulation ratio of tail plane to wing is −0.3 and the span ratio is 0.3. Thus, a four vortex system with counter-rotating neighboured vortices exists. The model set-up was chosen on the condition to create a most promising four vortex system with respect to accelerate wake vortex decay by optimal perturbations enhancing inherent instability mechanisms. The flow field has been investigated for a half plane of the entire wake up to a distance of 48 span dimensions downstream of the model. The results obtained at 1, 12, 24 and 48 span distances are shown as non-dimensional axial vorticity and vertical turbulence intensities. A significant decay in peak vorticity, swirl velocity and circulation is observable during the downward motion of the vortices. Spectral analysis of the unsteady velocity data reveals a peak in the power spectral density distributions indicating the presence of a dominating instability. Using two hot-wire probes cross spectral density distributions have also been evaluated, which highlight the co-operative instability leading to a rapid wake vortex decay within 30 span dimensions downstream.

Large eddy simulation of flow around wavy cylinders at a subcritical Reynolds number

The cross-flow around wavy cylinders of wavelength ratios λ/ D m from 1.136 to 3.333 are investigated at Re = 3000 using large eddy simulation (LES). The mean flow field and the near wake flow structures are presented and compared with those of a circular cylinder at the same Reynolds number. The mean pressure distributions are also calculated. The results show that the mean drag coefficients of the wavy cylinders are less than those of a corresponding circular cylinder due to a longer wake vortex formation length generated by the wavy cylinders. For a subcritical Reynolds number of 3000, a maximum drag coefficient reduction of up to 18% compared with a circular cylinder is obtained corresponding to an optimal wavelength ratio of λ/ D m around 1.9 and an amplitude ratio a/ D m of 0.152. The fluctuating lift coefficients of the wavy cylinders are also greatly reduced or even suppressed. These kinds of wavy surfaces lead to the formation of three-dimensional free shear layers which are more stable than purely two-dimensional free shear layers. Such free shear layers will only roll up into mature vortices at further downstream positions. This significantly modified the near wake structures and the pressure distribution around the cylinder. It was found that the wave amplitude to wavelength ratio a/ λ plays an essential role in determining the 3D vortex structure behind the wavy cylinders which has a significant effect on the reduction of the fluctuating lift and suppression of flow-induced vibration.

On the formation of the counter-rotating vortex pair in transverse jets

Among the important physical phenomena associated with the jet in crossflow is the formation and evolution of vortical structures in the flow field, in particular the counter-rotating vortex pair (CVP) associated with the jet cross-section. The present computational study focuses on the mechanisms for the dynamical generation and evolution of these vortical structures. Transient numerical simulations of the flow field are performed using three-dimensional vortex elements. Vortex ring rollup, interactions, tilting, and folding are observed in the near field, consistent with the ideas described in the experimental work of Kelso, Lim & Perry (1996), for example. The time-averaged effect of these jet shear layer vortices, even over a single period of their evolution, is seen to result in initiation of the CVP. Further insight into the topology of the flow field, the formation of wake vortices, the entrainment of crossflow, and the effect of upstream boundary layer thickness is also provided in this study.

120 対称翼から発生する後流放出渦と離散周波数騒音について

The mechanism of the discrete frequency noise generation from the symmetric airfoils was investigated in transition Reynolds numbers. The following results are obtained. At the transition Reynolds numbers the boundary layer separate from the airfoil surface upstream of the trailing edge and the wake vortices form just at the trailing edge with inducing the shear flow fluctuation when the discrete frequency noise(DFN) generates. But the boundary layer flow attached up to the trailing edge and the wake vortices form at a downstream of the trailing edge. Then the shear flow do not fluctuate at the trailing edge and the discrete frequency noise little generate. that is, the formation of wake vortices does not always generate the DFN and the location of the wake vortices formation is closely relating to the discrete frequency noise at transition Reynolds numbers.

Simulation of the near field of a jet in a cross flow

A direct numerical simulation technique is used to predict the time-dependent behavior of a weakly compressible axisymmetric jet exiting normally into a cross flow. Time-averaged velocity fields from the unsteady simulation are in qualitative agreement with previously published experimental and numerical results. The bound vortex system that is observed experimentally is also in evidence in the results of the unsteady simulation, as is the Kelvin -Helmholtz roll-up of the jet shear layer. The horseshoe vortex system often observed experimentally is not seen in the numerical results due to insufficient resolution. The formation of wake vortices in a numerical simulation is observed for the first time. The wake vortex system is found to have a Strouhal frequency of 0.098. The results show that cross-flow boundary layer fluid is lifted up from the boundary layer into these vortices near the rear of the jet exit. This observation is consistent with experimental observations that suggest that the vorticity in the wake system originates in the cross-flow boundary layer upstream of the jet exit.

Relationship between Wake Vortex Formation and Discrete Frequency Noise in NACA Blades.

Effects of blade thickness on discrete frequency noise (DFN) have been investigated using symmetrical NACA blades of three different thicknesses. It was clarified that the mechanism of DFN generation was different depending on the thickness of the blade. In the case of a thin blade, NACA 0008, the air flow was separated upstream of the trailing edge on the suction side of the blade, but was attached to the trailing edge of the blade on the pressure side. Therefore the frequency and the sound pressure level of DFN were varied with the variation of the attack angle. On the other hand, in the case of a thick blade, NACA 0015 or NACA 0018, the flow was separated upstream of the trailing edge on the pressure side while it was attached to the trailing edge on the suction side. The frequency and sound pressure level of DFN did not vary with the variation of the attack angle in these cases. The location of the rolling up of Karman vortices was very important in the generation of DFN. When rolling up of the Karman vortex took place far from the trailing edge of the blade, the Karman vortices did not have a strong effect on the flow around the blade. However, when Karman vortices rolled up in the vicinity of the blade, the interaction of the Karman vortices with the blade was strong, causing DFN generation.

Vortical structure in the wake of a transverse jet

Structural features resulting from the interaction of a turbulent jet issuing transversely into a uniform stream are described with the help of flow visualization and hot-wire anemometry. Jet-to-crossflow velocity ratios from 2 to 10 were investigated at crossflow Reynolds numbers from 3800 to 11400. In particular, the origin and formation of the vortices in the wake are described and shown to be fundamentally different from the well-known phenomenon of vortex shedding from solid bluff bodies. The flow around a transverse jet does not separate from the jet and does not shed vorticity into the wake. Instead, the wake vortices have their origins in the laminar boundary layer of the wall from which the jet issues. It is argued that the closed flow around the jet imposes an adverse pressure gradient on the wall, on the downstream lateral sides of the jet, provoking 'separation events’ in the wall boundary layer on each side. These result in eruptions of boundary-layer fluid and formation of wake vortices that are convected downstream. The measured wake Strouhal frequencies, which depend on the jet-crossflow velocity ratio, match the measured frequencies of the separation events. The wake structure is most orderly and the corresponding wake Strouhal number (0.13) is most sharply defined for velocity ratios near the value 4. Measured wake profiles show deficits of both momentum and total pressure.

Wake Vortex Structure and Aerodynamic Origin in Severe Thunderstorms

Abstract The radar and surface structure of a severe thunderstorm's wake vortex on 25 June 1969 are examined. Two probable aerodynamic causes for wake vortex formation are Karma vortices and starting vortices to the updraft lee. Karman vortices would form, as do those observed, within the echo core at the updraft lee “edges” and move downstream with the ambient flow. However, in contrast to Karman theory, radar reflectivity distributions consistently suggest anticyclonic flow. Starting vortices should develop as observed, during transition of a thunderstorm updraft from non-rotational to rotational state, as a vortex of comparable strength but opposite circulation. Four other thunderstorms examined also produced severe weather, increased intensity rapidly, and turned right or formed hook echoes contemporaneously with vortex shedding.

Mechanisms of Flow-Induced Oscillation of Bluff Cylinders

A lightly damped elastic cylindrical bluff body, with axis normal to an incident flow, may, depending on flow and body parameters, experience one of three forms of transverse oscillation. A cylinder of noncircular section will, over certain ranges of attitude of the section to the flow, exhibit galloping oscillation arising from instability of the dynamical system to the transverse component of fluid force on the cylinder. The oscillation, which can be soft or hard, is analyzed using a quasistatic model and the theory of weakly nonlinear differential equations. Any cylindrical section has a characteristic frequency of wake vortex formation, proportional to flow velocity. At velocities for which this frequency is close to a natural frequency of the elastic system, a resonant vortex-induced oscillation will occur. No satisfactory theory is available, but measurements of surface loadings and wake phenomena during oscillation are providing data on which to build a theoretical model. Any cylinder may experience buffeting oscillations from random turbulent fluctuations in the incident flow and/or the cylinder wake. These oscillations can be analyzed by statistical methods.