Natural Vortex Research Articles

Harmonic forcing of a laminar bluff body wake with rear pitching flaps

A numerical study on the response of a two-dimensional bluff body wake subjected to harmonic forcing imposed by two rear pitching flaps is performed. The wake is generated by a rectangle at a height-based Reynolds number $Re=100$ , characterised by laminar vortex shedding. Two forcing strategies are examined corresponding to in-phase ‘snaking’ and out-of-phase ‘clapping.’ The effects of the bluff body aspect ratio ( $AR=1,2,4$ ), flapping frequency, flapping amplitude, flap length and Reynolds number are investigated. For the snaking motion, a strong fundamental resonance of the root mean square (r.m.s.) drag is observed when the wake is forced near the vortex shedding frequency. For the clapping motion, a weak subharmonic resonance is observed when the forcing is applied near twice the vortex shedding frequency resulting in an increase of the lift r.m.s. whereas the drag r.m.s. remains unaffected. Both resonances intensify the vortex shedding and a concomitant mean drag increase is observed for the snaking motion. Forcing away from the resonant regimes, both motions result in considerable drag reduction through wake symmetrisation and propulsion mechanisms. The formation of two vortex dipoles per oscillation period due to the flapping motion, which weaken the natural vortex shedding, has been identified as the main symmetrisation mechanism. A single scaling parameter is proposed to collapse the mean drag reduction of the forced flow for both motions over a wide range of flapping frequencies, amplitudes and flap lengths. Finally, the assessment of the performance of the forcing strategies has revealed that clapping is more effective than snaking.

Stochastic dynamics of vortex-acoustic lock-in: stochastic bifurcation and resonance

We perform a theoretical study to understand vortex-acoustic lock-in in the presence of an additive Gaussian white noise using a lower-order model. The acoustic field is realized as an external harmonic excitation. Frequency, harmonic excitation and noise amplitudes are varied over ranges encountered in practical and lab-scale combustors. Probability density functions (PDFs) for shedding time periods and phase instants are obtained in the Fokker–Planck framework. Unlike the general scenario, where stochastic bifurcation is identified by a qualitative change in the stationary PDFs, in this case, stochastic lock-in is identified by a qualitative change in the spectrum of the transition probability matrix. The effect of the noise is to reduce the extent of lock-in while preserving the underlying deterministic dynamical features. Although various orders of lock-in are identified, 1 : 1 stochastic lock-in with the excitation frequency close to the natural vortex shedding frequency is found to be the most favourable condition for combustion instability to occur. We show that lock-in can be accompanied by both instability and amplitude suppression. Stochastic resonance is also observed; however, its contribution to instability is marginal.

Different topologies of natural vortex dislocations in Mode A wake

The mode A wake behind a circular cylinder at Reynolds number 200 was simulated by directly solving the three-dimensional Navier–Stokes equations. A more detailed investigation of different topologies of natural vortex dislocations in the mode A wake was systematically described. In addition to the two-side natural vortex dislocation defined by Williamson [J. Fluid Mech. 243, 393–441 (1992)], three new topologies of vortex dislocations were identified. Additionally, the formation process and the mechanism of these newly identified dislocation topologies were addressed. The results in this Letter provide a more thorough understanding of the natural vortex dislocations in the mode A wake.

Effect of the Extended Rigid Flapping Trailing Edge Fringe on an S833 Airfoil

A 2D numerical simulation was conducted to investigate the effect of an extended rigid trailing edge fringe with a flapping motion on the S833 airfoil and its wake flow field, as an analogy of an owl’s wing. This study aims to characterize the influence of the extended flapping fringe on the aerodynamic performance and the wake flow characteristics downstream of the airfoil. The length (Le) and flapping frequencies (fe) of the fringe are the key parameters that dominate the impact on the airfoil and the flow field, given that the oscillation angular amplitude is fixed at 5°. The simulation results demonstrated that the airfoil with an extended fringe of 10% of the chord at a flapping frequency of fe = 110 Hz showed a substantial effect on the pressure distribution on the airfoil and the flow characteristics downstream of the airfoil. An irregular vortex street was predicted downstream, thus causing attenuations of the vorticities, and shorter streamwise gaps between each pair of vortices. The extended flapping fringe at a lower frequency than the natural shedding vortex frequency can effectively break the large vortex structure up into smaller scales, thus leading to an accelerated attenuation of vorticities in the wake.

Analysis of Separation Control Authority of DBD Plasma Actuator using Hot-Film Sensor Array

The boundary-layer control authority of a DBD plasma actuator using surface mounted hot-film sensors is evaluated. Wind tunnel experiments on a wind-turbine blade section were established at a Reynolds number of 0.27 × 106. Aerodynamic performance of the wind-turbine blade section for both plasma-ON and plasma-OFF modes are evaluated using measurements made by both surface pressure and wake survey behind the model. Two distinct boundary-layer states are recognized. A state which occurs at the onset and in proximity of the deep stall, which is affected by the low-frequency instabilities of the separated flow. In this case, the steady actuation of plasma imparts local momentum on the nearby flow, eliminating the instabilities, hence, reattaching the detached flow. The other state happens beyond the static stall angle of attack of the airfoil where the flow over the suction side of the airfoil is fully separated and coexistence of both the leading edge and the trailing edge shear-layer instabilities and natural trailing edge vortex shedding is the underlying mechanism. In this case, although the plasma actuator eliminates the instabilities, to some extent, but the corresponding momentum injection is not efficient to stabilize and reattach the flow.

Uniform suction or blowing control influence on double diffusion mechanism effectiveness in confined wall jet

Natural vortex development in double diffusive confined wall jet flow is firstly analyzed as a basic state. A consecutive vortices production and shedding processes are observed at moderated Reynolds numbers. The influence of these phenomena on combined transfers is then discussed. Strouhal number evolution was determined by spectral analysis conducted in the near-field region of the laminar wall jet. Accordingly, two critical values were found indicating the transitions between stationary, non stationary and quasi-periodic states. Uniform amounts of water are then added or extracted through a thin slot placed on the horizontal flat plate in order to control large structures creation and growth. The main purpose is to assess the effectiveness of uniform suction or blowing on each flow state and to establish the appropriate conditions under which the natural instability disappears. An Average Nusselt and Sherwood numbers as well as skin friction coefficient dependent behavior were observed which implies that vortex dynamic directly affects thermo-solutal transfer.

A Reduced-order Model for Lock-on via Vortex-combustion-acoustic Closed-loop Coupling in A Step Combustor

ABSTRACT The lock-on of the frequency of acoustic oscillations to the frequency of the vortex shedding in a reacting flow in a backward-facing step combustor is demonstrated using a reduced-order model. Individual models for the flow, flame and acoustic models are developed. A two-dimensional flow field with growth and shedding of vortices is modeled using a potential flow model derived via a conformal mapping of the step geometry. Combustion and acoustic models are derived using thermo-diffusive equations and the one-dimensional Galerkin method, respectively. A modulation of heat release rate fluctuations is thus enabled by vortices advecting past the flame, overcoming the time and space-localized assumptions in vortex kicked oscillator models. Comparisons with results from kicked oscillator models show a better prediction of the dominant frequencies during stable and unstable operations. A Reynolds number sweep is performed to reveal the transition from stable to unstable operation and thus the onset of instability. A shift in the dominant frequency of pressure fluctuations from the natural duct acoustic mode to the natural vortex shedding mode is observed, indicative of a lock-on. The results compare well with past experimental and computational data for similar geometry and flow conditions.

Evolution of Turbulent Swirling Flow in a Small-Scale Cyclone with Increasing Flow Rate: A LES Study

The flow field, vortex behaviour and pressure losses in a small-scale cyclone have been studied at a wide range of flow rate 0.23–39.7 NLPM (measured at 1 atm and $$20\,^\circ \hbox {C}$$ ) using the LES simulations that have been validated based on experimental measurements of the cyclone pressure drop. The following flow characteristics such as (1) the radial distribution of the tangential velocity; (2) the maximum tangential velocity and axial downward flow rate; (3) natural vortex length and rotation frequency of the vortex end; and (4) pressure losses in the cyclone have been analysed as a function of Reynolds number. The radial distribution of the tangential velocity inside the cyclone has been described by a proposed equation for adapted Burger’s vortex. The position of the lower end of the vortex (natural vortex length) as well as its rotational frequency have been investigated with the pressure sensing method. A unique vortex behaviour such as “vortex end jump” was revealed at some Reynolds numbers. Additionally, a deep analysis of the pressure losses in the cyclone has been performed which showed that the main pressure losses (up to 48%) occur in the vortex finder. Four flow regimes were revealed and a one-term power series model has been proposed to describe the effects of the Reynolds number on the Euler number (dimensionless pressure losses).

Acoustic analysis of a forced-oscillating cylinder in flow using a hybrid method

An acoustic analysis based on a hybrid methodology is performed for a transversely forced-oscillating circular cylinder with the aim of understanding the sound generation mechanism. The sound is predicted both in moving-observer and moving-medium systems using the unsteady near-field flow solutions as input. Different forms of integration formulation, the effect of placement of the data surface, the impact of surface closure and averaging among various closed surfaces are investigated over a range of F=0.2 to 1.4 where F is the frequency ratio of cylinder oscillation to the natural vortex shedding frequency. The results show that the sound is mainly radiated from the cylinder surface, induced by the unsteady aerodynamic loading and cylinder movement. The impact of the hydrodynamic structures of the cylinder wake on the predicted sound signal varies in different regimes of F. For 0.2≤F≤0.7, the hydrodynamic perturbation introduces spurious noise sources on the intersecting portion of the permeable data surface. For 0.8≤F≤1.05, the mean flow of the wake affects sound propagation through a convective effect, which must be taken into account in hybrid prediction. For 1.1≤F≤1.4, the effect of the wake flow on the sound generation, propagation and prediction is small. The accuracy of hybrid prediction can be significantly improved by both the open permeable surface and surface-averaging.

Investigation of the pressure drop increase in a square free-vortex cyclonic separator operating at low particle concentration

This paper reports experimental and numerical results obtained in a free-vortex gas-solid separator, in which the classic conical-section of the device is replaced by a larger square chamber, where the vortex is free to evolve. An adjustable vortex-limiting plate is introduced at the cross-section in order to allow the inversion of the coherent structure and guarantee a natural vortex length of the swirling flow according to the inlet velocity in the separator. Both single- phase and multiphase flows were analyzed. In the latter case, only very low particle concentrations, between 100 and 500 mg/m3, were considered. Compared to the classic cyclonic separators, the new geometry showed advantages in performance, such as a smaller pressure drop. Moreover, unexpected behavior was observed, since the pressure drop increased by an average of 10% when the particles were injected into the flow at the low concentrations considered.

Turner as a Daoist Sage

ABSTRACT In this paper, I provide a cross-cultural comparison between the life and work of the English land- and seascape painter, J.M.W. Turner, and the conception of aesthetic experience and artisanship found in classical Chinese Daoism. I argue that Turner exhibits certain key features of the Daoist sage. By developing some recent interpretations of his life and work, I claim that Turner, especially in his later seascapes, exhibited an approach to aesthetic experience that involves the artist’s total absorption into and identification with the natural process with which he is concerned. Next, I show how the Daoist approach to aesthetics and artisanship similarly focuses on the artist’s capacity to identify with the power and source of the world they aim to aesthetically experience. The central motif in both certain of Turner’s later work and the Daoist approach to aesthetic creativity is the figure of the vortex. By becoming the center of a natural vortex, and even the vortex that is nature itself, the Turnerian and Daoist artist experiences and embodies the wisdom of accessing and identifying with nature’s omnipotence. Such wisdom is thus expressed in the artwork by emphasizing the fundamental dynamism and vagueness of all things. I conclude by claiming that if one reads Turner as a Daoist sage, then a novel model for the aesthetic experience and appreciation of nature can be offered. I call this “the identification model” and recommend it being slotted into our taxonomy of contemporary options within environmental aesthetics.

Λ -Invariant and Topological Pathways to Influence the Strength of Submicron Crystals

In small volumes, sample dimensions are known to strongly influence mechanical behavior: especially strength and crystal plasticity. This correlation fades away at the so-called "mesoscale," loosely defined at several micrometers in both experiments and simulations. However, this picture depends on the "entanglement" of the initial defect configuration. In this Letter, we study the effect of dislocation topology through the use of a novel observable for dislocation ensembles (the Λ invariant) that depends only on mutual dislocation linking: It is built on the natural vortex character of dislocations, and it has a continuum-discrete correspondence that may assist multiscale modeling descriptions. We investigate arbitrarily complex initial dislocation microstructures in sub-micron-sized pillars using three-dimensional discrete dislocation dynamics simulations for finite volumes. We demonstrate how to engineer nanoscale dislocation ensembles that are independent from sample dimensions, either by biased-random dislocation loop deposition or by sequential mechanical loads of compression and torsion.

Upstream actuation for bluff-body wake control driven by a genetically inspired optimization

The control of bluff-body wakes for reduced drag and enhanced stability has traditionally relied on the so-called direct-wake control approach. By the use of actuators or passive devices, one can manipulate the aerodynamic loads that act on the rear of the model. An alternative approach for the manipulation of the flow is to move the position of the actuator upstream, hence interacting with an easier-to-manipulate boundary layer. The present paper comprises a bluff-body flow study via large-eddy simulations to investigate the effectiveness of an upstream actuator (positioned at the leading edge) with regard to the manipulation of the wake dynamics and its aerodynamic loads. A rectangular cylinder with rounded leading edges, equipped with actuators positioned at the front curvatures, is simulated at $Re=40\,000$ . A genetic algorithm (GA) optimization is performed to find an effective actuation that minimizes drag. It is shown that the GA selects superharmonic frequencies of the natural vortex shedding. Hence, the induced disturbances, penetrating downstream in the wake, significantly reduce drag and lateral instability. A comparison with a side-recirculation-suppression approach is also presented, the latter case being worse in terms of reduced drag (only 8 % drag reduction achieved), despite the total suppression of the side recirculation bubble. In contrast, the GA optimized case contributes to a 20 % drag reduction with respect to the unactuated case. In addition, the large drag reduction is associated with a reduced shedding motion and an improved lateral stability.

Numerical Simulation of Wake Deflection Control around NACA0012 Airfoil Using Active Morphing Flaps

This study demonstrates an active flow control for deflecting a direction of wake vortex structures behind a NACA0012 airfoil using an active morphing flap. Two-dimensional direct numerical simulations are performed for flows at the chord Reynolds number of 10,000, and the vortex pattern in the controlled and noncontrolled wakes as well as the effect of an actuation frequency on the control ability are rigorously investigated. It is found that there is an optimum actuation-frequency regime at around F + = 2.00 which is normalized by the chord length and freestream velocity. The wake vortex pattern of the well-controlled case is classified as the 2P wake pattern according to the Williamson’s categorization [1] [2], where the forced oscillation frequency corresponds to the natural vortex shedding frequency without control. The present classification of wake vortex patterns and finding of the optimum frequency regime in the wake deflection control can lead to a more robust design suitable for vortex-induced-vibration (VIV) related engineering systems.

Lock-in phenomenon of vortex shedding in oscillatory flows: an analytical investigation pertaining to combustors

An analytical investigation is performed to understand the lock-in phenomenon, observed in vortex shedding combustors. Several aeroengine afterburners and ramjets use a bluff body to stabilize the flame. The bluff body sheds vortices. During the occurrence of high-amplitude combustion instability, the frequency of vortex shedding locks in to the frequency of the chamber acoustic field. This phenomenon is termed vortex-acoustic lock-in. In general, there is a two-way coupling between the vortex shedding process and the acoustic field, making analytical investigation difficult. Since the frequency of the latter remains largely unaltered, performing an investigation to study the response of vortex shedding to external excitation not only allows one to gain insights, but also make the problem analytically tractable. We begin with a lower-order model available in the literature to describe the vortex shedding process in non-reacting flows, arising from sharp corners in the presence of upstream velocity excitation. The continuous time domain model is transformed to a discrete map, which connects the time instances of two successive vortex shedding events. The frequency and amplitude of excitation are varied to study the instantaneous vortex shedding time period, as the response of the system. In the absence of forcing, the iterates of the map form a period-1 solution with the frequency equalling the natural vortex shedding frequency. On increasing the amplitude of excitation, quasi-periodic behaviour of the iterates is observed, followed by a period-1 lock-in solution, where vortex shedding occurs at the excitation frequency. On further increasing the amplitude, de-lock-in occurs. From the map, an analytical solution is extracted, which represents the lock-in state. The condition and thereby the region in the frequency–amplitude parameter space where a general$p:1$lock-in occurs is then identified. Several important analytical expressions, such as for (1) critical threshold frequency above which lock-in occurs, (2) boundary of lock-in region in the parameter space, that are of direct importance to the design of quieter combustors are obtained. The study also identifies the transition of higher-order$p:1$to$1:1$lock-in state, through a series of lock-in and de-lock-in steps, whose occurrence could be verified from future experiments.

Mode competition in galloping of a square cylinder at low Reynolds number

Galloping is a type of fluid-elastic instability phenomenon characterized by large-amplitude low-frequency oscillations of the structure. The aim of the present study is to reveal the underlying mechanisms of galloping of a square cylinder at low Reynolds numbers ($Re$) via linear stability analysis (LSA) and direct numerical simulations. The LSA model is constructed by coupling a reduced-order fluid model with the structure motion equation. The relevant unstable modes are first yielded by LSA, and then the development and evolution of these modes are investigated using direct numerical simulations. It is found that, for certain combinations of$Re$and mass ratio ($m^{\ast }$), the structure mode (SM) becomes unstable beyond a critical reduced velocity$U_{c}^{\ast }$due to the fluid–structure coupling effect. The galloping oscillation frequency matches exactly the eigenfrequency of the SM, suggesting that the instability of the SM is the primary cause of galloping phenomenon. Nevertheless, the$U_{c}^{\ast }$predicted by LSA is significantly lower than the galloping onset$U_{g}^{\ast }$obtained from numerical simulations. Further analysis indicates that the discrepancy is caused by the nonlinear competition between the leading fluid mode (FM) and the SM. In the pre-galloping region$U_{c}^{\ast }<U^{\ast }<U_{g}^{\ast }$, the FM quickly reaches the nonlinear saturation state and then inhibits the development of the SM, thus postponing the occurrence of galloping. When$U^{\ast }>U_{g}^{\ast }$, mode competition is weakened because of the large difference in mode frequencies, and thereby no mode lock-in can happen. Consequently, galloping occurs, with the responses determined by the joint action of SM and FM. The unstable SM leads to the low-frequency large-amplitude vibration of the cylinder, while the unstable FM results in the high-frequency vortex shedding in the wake. The dynamic mode decomposition (DMD) technique is successfully applied to extract the coherent flow structures corresponding to SM and FM, which we refer to as the galloping mode and the von Kármán mode, respectively. In addition, we show that, due to the mode competition mechanism, the galloping-type oscillation completely disappears below a critical mass ratio. From these results, we conclude that transverse galloping of a square cylinder at low$Re$is essentially a kind of single-degree-of-freedom (SDOF) flutter, superimposed by a forced vibration induced by the natural vortex shedding. Mode competition between SM and FM in the nonlinear stage can put off the onset of galloping, and can completely suppress the galloping phenomenon at relatively low$Re$and low$m^{\ast }$conditions.

Transient cross flow and heat transfer over a rotationally oscillating cylinder subjected to gust impulse

Numerical analysis is carried out to investigate the effect of an upstream gust-impulse on the transient fluid flow and associated convective heat transfer from a two dimensional rotationally oscillating circular cylinder. Three distinct oscillation scenarios based on the oscillation amplitude are investigated. Forcing frequency for each rotational amplitude is varied such that the system remains within the lock-on state. Further, three different gust scenarios are chosen by relating the gust frequency, StG, to the natural vortex shedding frequency in the streamlined flow for a fixed Reynolds number of 110 at Prandtl number of 7. Balances for mass, momentum, and energy are solved by considering incompressible, viscous fluid subjected to no-slip CWT boundary at the cylinder wall. Phase diagrams, the spatiotemporal mean of the pressure coefficient and Nusselt number i.e. CP‾‾ and Nu‾‾, and vorticity and temperature contours are presented and discussed in detail. Temporal evolution of lift coefficient (CL) and Lissajous curves of lift coefficient against drag coefficient depict the perturbing effect of the gust-impulse. Time averaged drag coefficient (CD‾) and RMS values of lift coefficient (CL-RMS) show contrasting shifts in their peak values as the rotational-oscillation amplitude is increased. In addition to causing a transient phase difference between the cylinder angular velocity and the lift coefficient, introduction of the gust-impulse in the flow domain induces secondary frequencies causing transient amplification of the lift force which tends to make the system temporarily unstable by shifting it out of the lock-on state. The induced phase difference causes short term shear layer stretching and early vortex detachment around the cylinder surface thereby affecting the overall flow and the resultant convective heat transfer physics in the system. Moreover, a regression correlation is presented relating the oscillation amplitude, oscillation forcing Strouhal frequency and gust Strouhal frequency to the spatiotemporal averaged Nusselt number.

Vortex Shedding and Modal Behavior of a Circular Cylinder Equipped with Flexible Flaps

When a cylinder is subject to a flow, vortices will be shed that can lead to strong tonal noise. The modification of the cylinder with soft, flexible flaps made of silicone rubber has been shown to affect the vortex shedding cycle in a way that the Strouhal number associated with the vortex shedding suddenly jumps to a higher value at a certain Reynolds number. In the present study, the effect of the flexible flaps on the vortex shedding is further examined by subsequently reducing the number of flaps and additionally shortening their length. Acoustic measurements and camera recordings of the flap motion, performed in an aeroacoustic wind tunnel, suggest that the sudden jump of the Reynolds number is caused by the movement of the outer flaps. A comparison with the eigenfrequencies obtained from a numerical modal analysis of the different flap rings revealed that the cause of the Strouhal number jump is most likely a lock-in of the natural vortex shedding cycle with the next higher eigenfrequency of the outer flaps.

Flow-dynamics induced thermal management of crude oil wax melting: Lattice Boltzmann modeling

The flow dynamics of natural convection during paraffin wax melting is numerically investigated using double-population multi-relaxation-time lattice Boltzmann modeling. Flow dynamics manipulation is achieved using inserts of varying tilt angles θ and positions x*, while the use of insulation material isolates the effects of thermal conductivity enhancement. It is found that improvement in melting duration can reach up to 13%, particularly at low θ and high x* due to restructuring of natural convective currents. Natural convective currents show high potency to enhance melting due to a boost in thermal energy transport and solid-liquid interface progression, especially when the liquid melt phase grows significantly in size. More importantly, the mesoscopic nature of LBM permits natural convective multicellular vortex formation and phase change to naturally manifest themselves, as convective currents rotate in the clockwise direction in multiple degrees of vorticity when melting enhancement occurs. Moreover, the melting process can be expressed in four stages via time-dependent Nusselt number Nu variation. Flow dynamics analysis reveals high natural convective activity, albeit low Nu value even after the four stages of melting process complete early. Overall, the present study may provide new insights into the flow dynamic effects that are underrated knowledge in natural-convective melting, and subsequently would impact the development of available crude oil wax mitigation techniques via thermal management.

Experimental study of stall control over an airfoil with dual excitation of separated shear layers

This paper investigates the stall control over an airfoil employing dual excitation of separated shear layers with dielectric barrier discharge plasma actuators. This novel approach is studied experimentally on the NACA 0015 airfoil at the angle of attack and Reynolds number of 14° and 0.3×106, respectively. To analyze the baseline flow, Time-averaged pressure distributions on the airfoil surfaces are evaluated with 40 pressure taps. Further, the dominant natural frequencies associated with flow structures in the airfoil wake are examined utilizing a hot-wire anemometer. Two plasma actuators, one on the suction side just upstream the separation point and the other on the pressure side at the trailing edge, are exploited to control the baseline flow. Based on the active actuators, three controlled cases are considered in the present study. In the first case, the frequency of natural vortex shedding and its different harmonics are employed to excite the suction side separated shear layer. In the second case, plasma actuation is applied to the shear layer at the trailing edge. Further, in the third case, both of the actuators are utilized to excite the separated shear layers on the suction side and the trailing edge, simultaneously. The results of the pressure coefficient distributions, as well as the aerodynamic coefficients, are compared for all cases. Moreover, the actuators energy consumption is evaluated for the controlled cases and a new efficiency parameter is presented. Accordingly, the first case prevents the stall phenomenon only at the wake mode frequency and its first super-harmonic. In addition, using the plasma actuator at the trailing edge solely is an ineffective flow separation control strategy. However, dual excitation is promising for a full stall control in a wide range of excitation frequencies. Analyses of the lift-to-drag ratio and the efficiency parameter reveal that the dual excitation case is the most efficient stall control technique.