Incoming Vortex Research Articles

Numerical Study on the Influence of Inlet Guide Vanes on the Internal Flow Characteristics of Centrifugal Pump

In order to make the centrifugal pump run efficiently and stably under various working conditions, the influences of the incoming vortex flow in the inlet pipe on the main flow in the impeller is studied numerically, based on the k − ω SST turbulence model. Some guide vanes with different offset angle were added to change the statistical characteristic of the internal flow in the inlet pipe of the centrifugal pump. Both contour distributions of internal flow and statistical results of external performance are obtained and analyzed. The results show that the existence of vanes can divide the large vortex because of the reversed flow from the rotating impeller at low flow rate conditions into small vortices, which are easier to dissipate, make the velocity and pressure distribution more uniform, improve the stability of the flow in the impeller, reduce the hydraulic loss, and improve the hydraulic performance of the pump. The pump with vanes of offset angle 25° has a small pressure pulsation amplitude at each monitoring point. Comparing with the performance of the original pump, the head increased by around 2% and efficiency increased by around 2.5% of the pump with vanes of offset angle 25°.

Transmission of optical vortices through Bragg optical multihelicoidal fibers of heterogeneous type

In this paper we have theoretically shown that two-part and three-part multihelicoidal fibers of heterogeneous type in the presence of twist defects are able to invert the topological charge of incoming optical vortices. We have shown that three-part multihelicoidal fibers of that type can be used as compact comb filters for optical vortices. Also we have studied the emergence of topologically charged fields localized near defects in such fibers. We have established that strongly localized fields can emerge only in three-part multihelicoidal fibers.

Streamwise vortex transportation and loss generation in an intermediate turbine duct

Annular S-shaped intermediate turbine ducts are used in modern turbofan engines with large by-pass ratios. To reduce the weight of an engine, the intermediate turbine ducts should be as short as possible, while keeping the loss at an acceptable level. Understanding the flow physics within the intermediate turbine ducts is the key to improve the intermediate turbine duct design. This paper aims to understand the transportation of the inlet streamwise vortices and loss generation in intermediate turbine ducts. First, cases with isolate incoming streamwise vortices at different spanwise locations and different axial velocities are investigated. The transportation of isolated vortex and loss generation are highly related to the interaction between vortex and boundary layer, which are mainly determined by the streamwise pressure gradient. When the axial velocity of the streamwise vortex is different to the main flow, the radial pressure gradient also has an effect. Then, the inlet condition of the intermediate turbine ducts is setup based on the flow field at the exit of a cascade, which contains the flow structures such as the tip leakage vortex, hub secondary vortex and the wake. The flow physics and the loss mechanism are analysed in detail. The formation mechanism of counter-rotating vortices pair and the influence of inlet vortex on loss generation within the intermediate turbine ducts are also presented.

Modified Mach-Zehnder interferometer for determining the high-order topological charge of Laguerre-Gaussian vortex beams.

This study demonstrates a self-referenced interferometric method to estimate the magnitude and sign of a high-order topological charge (TC) carried by incoming optical vortex beams. The proposed method uses a right-angle prism in a Mach-Zehnder interferometer setup with controlled lateral shift and tilt between the wavefronts of interfering beams. The in-line interference with its conjugate results in a petal-shaped fringe pattern, where the number of petals reveals the magnitude of the TC. When the direction of one of the interfering beams is laterally displaced and rotated to emerge at a small angle for off-axis interference such that the vortices overlap at the output plane, then a fork-like interference pattern with better visibility is obtained, which can be used for estimating the magnitude as well as the sign of the TC. Through numerical simulation and optical experiment, it is shown that the technique is capable of estimating the TC of Laguerre-Gaussian beams even with a nonzero radial index.

Aerodynamic Interaction Between an Incoming Vortex and Tip Leakage Flow in a Turbine Cascade

In turbines, secondary vortices and tip leakage vortices form in the blade passage and interact with each other. In order to understand the flow physics of this vortices interaction, the effects of incoming vortex on the downstream tip leakage flow are investigated by experimental, numerical, and analytical methods. In the experiment, a swirl generator was used upstream of a linear turbine cascade to generate the incoming vortex, which could interact with the downstream tip leakage vortex (TLV). The swirl generator was located at ten different pitchwise locations to simulate the quasi-steady effects. In the numerical study, a Rankine-like vortex was defined at the inlet of the computational domain to simulate the incoming swirling vortex (SV). The effects of the directions of the incoming vortices were investigated. In the case of a positive incoming SV, which has a large vorticity vector in the same direction as that of the TLV, the vortex mixes with the TLV to form one major vortex near the casing as it transports downstream. This vortices interaction reduces the loss by increasing the streamwise momentum within the TLV core. However, the negative incoming SV has little effects on the TLV and the loss. As the negative incoming SV transports downstream, it travels away from the TLV and two vortices can be identified near the casing. A triple-vortices-interaction kinetic model is used to explain the flow physics of vortex interaction, and a one-dimensional mixing analytical model are proposed to explain the loss mechanism.

Experimental investigation of wake-induced aeroelastic limit cycle oscillations in tandem wings

This paper aims to experimentally investigate multi-body aeroelastic wing–wake interactions to quantify how structural properties dictate vortex energy transfer. The paper examines three different components needed to study this phenomenon; first, single wing aeroelastic response and wake characteristics; second, flutter boundary characteristics of tandem aeroelastic wings with wing–wake interactions; third, limit cycle behavior and wake frequency content in tandem oscillating wings. Single wing and tandem wing–wake studies conducted through hot-wire anemometer sweeps downstream of the trailing edge find dominant frequency content and wake velocity profiles. Pitch stiffness on the downstream wing is varied to be less than or equal to that of the upstream wing. It is hypothesized that pitch stiffness modulates the sensitivity of the wing to incoming vortex disturbances thus changing vortex energy transfer to the wing. The experimental results showed that the limit cycle and transient response of the downstream wing in tandem configuration are dependent on its pitch stiffness, while the aeroelastic stability and flutter point of the downstream wing is dictated by the upstream wing. In addition, the wake frequency content in tandem configuration demonstrates strong dependence on downstream wing pitch stiffness. The results show that highest wake energy transfer occurs when the downstream wing pitch stiffness is less than pitch stiffness of the upstream wing.

Effect of the angle of attack on the transient lift during the interaction of a vortex with a flat plate. Potential theory and experimental results

The dynamics of a two-dimensional vortex interacting with a flat plate at different angles of attack α is analysed using potential flow theory based on conformal mapping varying the nondimensional separation distance h∕c of the upstream incoming vortex to the plate (c is the chord length of the plate) and the vortex intensity Γl. Transient lift forces measured in a wind tunnel are also compared with the potential theory results for a given Γl and several values of h∕c and α. For the Reynolds number considered in the experiments (about 25 000) it is found that the potential theory predicts reasonably well the transient fluctuation in the lift force provided that the separation distance is not too close to the critical one h∗∕c at which the vortex trajectory given by the potential theory bifurcates. We find that the separation distance generating the highest induced lift is around this critical value h∗∕c, which, according to the potential theory, is displaced about −2.3(1−0.07|Γl|1∕2)α in relation to the zero angle of attack for the same Γl. Potential theory also predicts that the maximum peak of the lift fluctuation depends on α only through the relative separation |h−h∗|∕c, and that the maximum lift is substantially larger when a clockwise vortex passes below the plate than when it passes above the plate, for the same vortex intensity Γl and relative separation distance. The opposite happens for a counter-clockwise vortex. This asymmetry in the maximum lift fluctuation increases slightly with |Γl|, approaching a ratio of almost two for large |Γl|.

Energy-Harvesting Mechanism of a Heaving Airfoil in a Vortical Wake

Energy conversion from vortical flows using flapping wings is widely observed in nature flyers and has great potential for improving energy efficiency of micro aerial vehicles. Therefore, it has recently gained a significant research attention. The interactions between the incoming vortices and wings have previously been studied, but the mechanism inducing such interactions and its impact on energy-conversion efficiency has only marginally been discussed. To bridge this gap, this paper carries out a numerical study on the response of a two-dimensional heaving airfoil in a vortical wake generated by an upstream oscillating D-shaped cylinder. The study first emphasizes the influence of the leading- and trailing-edge vortices on energy conversion. The formation of these vortices is then linked to two major interaction modes observed between the airfoil and the incoming vortices: the suppressing mode and the reinforcing mode. Furthermore, a model of vortex–airfoil interactions is developed from potential theory of fluid dynamics to further understand the mechanism of inducing different interaction modes. The potential theory demonstrates that the topology of vortices surrounding the airfoil is crucial for triggering different interaction modes. The effects of the interaction modes on the energy-conversion efficiency are also demonstrated.

Forced convection of flow past two tandem rectangular cylinders in a channel

ABSTRACTThis work performed the first numerical investigation on the forced convection of flow past two tandem rectangular cylinders in a channel at Re = 100. The aspect ratio (AR) and gap ratio (GR) of the two cylinders are chosen at AR = 1(1)4 and GR = 1(1)8, respectively. The objective of the present work is to explore the effects of AR and GR on the characteristic flow and heat transfer quantities for the rectangular geometry that has not been studied before. The effects of the two parameters are presented by the instantaneous flow pattern, characteristic aerodynamic and heat transfer quantities, local heat transfer rate, flow patterns in the gap and near wake, and temperature distribution on the channel walls. Both time-averaged and fluctuating quantities are analyzed and presented. Numerical results reveal that for cylinders of all ARs, there are two flow regimes categorized based on the GR: the steady flow regime at GR ≤ 3, where the gap flow is steady, and the unsteady flow regime at GR ≥ 4. The characteristic aerodynamic and heat transfer quantities abruptly change as the flow transits from steady to unsteady regime especially for the downstream cylinder. The time-averaged and maximum fluctuating local heat transfer rate for the upstream cylinder almost does not vary with the GR, whereas they substantially vary for the downstream cylinder. The AR affects the magnitude of the quantities but not their variation trends. For flows in the unsteady regime, the recovery of the wake flow after the downstream cylinder is much more rapidly than those of steady flows due to the acceleration arising from the instability brought by the incoming shedding vortices. The violent shedding also effectively enhances heat transfer and increases the temperature of the channel walls.

Comparison of aerodynamic characteristics between a novel highly loaded injected blade with curvature induced pressure-recovery concept and one with conventional design

This paper introduces a novel design method of highly loaded compressor blades with air injection. CFD methods were firstly validated with existing data and then used to develop and investigate the new method based on a compressor cascade. A compressor blade is designed with a curvature induced pressure-recovery concept. A rapid drop of the local curvature on the blade suction surface results in a sudden increase in the local pressure, which is referred to as a curvature induced ‘Shock’. An injection slot downstream from the ‘Shock’ is used to prevent ‘Shock’ induced separation, thus reducing the loss. As a result, the compressor blade achieves high loading with acceptable loss. First, the design concept based on a 2D compressor blade profile is introduced. Then, a 3D cascade model is investigated with uniform air injection along the span. The effects of the incidence are also investigated on emphasis in the current study. The mid-span flow field of the 3D injected cascade shows excellent agreement with the 2D designed flow field. For the highly loaded cascade without injection, the flow separates immediately downstream from the ‘Shock’; the initial location of separation shows little change in a large incidence range. Thus air injection with the same injection configuration effectively removes the flow separation downstream from the curvature induced ‘Shock’ and reduces the size of the separation zone at different incidences. Near the endwall, the flow within the incoming passage vortex mixes with the injected flow. As a result, the size of the passage vortex reduces significantly downstream from the injection slot. After air injection, the loss coefficient along spanwise reduces significantly and the flow turning angle increases.

Inversion of the topological charge of optical vortices in a coil fibre resonator

We have analytically studied the evolution of optical vortices in a fibre resonator with two coils in the presence of an inter-coil coupling. We have shown that such a system can invert the orbital number of the incoming optical vortices if the resonance condition is fulfilled.

Propeller and inflow vortex interaction: vortex response and impact on the propeller performance

The aerodynamic operating conditions of a propeller can include complex situations where vorticity from sources upstream can enter the propeller plane. In general, when the vorticity enters in a concentrated form of a vortex, the interaction between the vortex and blade is referred to as blade–vortex interaction or BVI. The interaction may affect the propeller performance as well as its noise production. In the present paper, investigations of the interaction of a wing tip vortex generated by a lifting surface upstream of the rotor plane and an eight-bladed propeller are reported. Utilizing two ends of an upstream wing with non-symmetrical airfoil, the rotation of the incoming vortex could be made to co-rotate or to contra-rotate with the propeller. The ensuing velocity fields were quantified with the help of particle image velocimetry (PIV), and the propeller performance was evaluated with the help of a rotating shaft balance (RSB) mounted on the propeller shaft. The results describe the displacement of the vortex core, as it moves through the rotor plane as well as the positive effect on the thrust and torque of the contra-rotating vortex and the opposite of it in the case of the co-rotating vortex. The current research could be applied to analyse the influence of the incoming vortex on the propeller, e.g., ground vortex, tip vortex shed from a control surface, etc.

Local orbital angular momentum revealed by spiral-phase-plate imaging in transmission-electron microscopy

The orbital angular momentum (OAM) of light and matter waves is a parameter that has been getting increasingly more attention over the past couple of years. Beams with a well-defined OAM, the so-called vortex beams, are applied already in, e.g., telecommunication, astrophysics, nanomanipulation, and chiral measurements in optics and electron microscopy. Also, the OAM of a wave induced by the interaction with a sample has attracted a lot of interest. In all these experiments it is crucial to measure the exact (local) OAM content of the wave, whether it is an incoming vortex beam or an exit wave after interacting with a sample. In this work we investigate the use of spiral phase plates (SPPs) as an alternative to the programmable phase plates used in optics to measure OAM. We derive analytically how these can be used to study the local OAM components of any wave function. By means of numerical simulations we illustrate how the OAM of a pure vortex beam can be measured. We also look at a sum of misaligned vortex beams and show how, by using SPPs, the position and the OAM of each individual beam can be detected. Finally, we look at the OAM induced by a magnetic dipole on a free-electron wave and show how the SPP can be used to localize the magnetic poles and measure their ``magnetic charge.'' Although our findings can be applied to study the OAM of any wave function, our findings are of particular interest for electron microscopy where versatile programmable phase plates do not yet exist.

Propagation of light in a circular array of elliptical fibres

We have studied transformation of discrete light beams in circular arrays of elliptical fibres, in which the orientation of ellipses’ axes linearly depends on the angular position of the fibre in the array and makes an half-integer number p of full rotations while tracing along its contour. We have derived analytical expressions for the spectra and supermodes that allow for evanescent coupling between the fibres in the next-neighbour approximation. We have studied the transformative properties of such an array and shown that it can generate cylindrical vector beams (CVBs) of TE and TM types. We have shown that the type of generated beam depends on the orientation of linear polarization of the incident beam. In this way, the circular array of strongly elliptical fibres enables polarization control over the type of the generated CVB. We have also shown that such arrays can change the topological charge of an incoming discrete optical vortex by the doubled array’s index p.

Acoustics of finite Bessel tractor beams

Surprises and counterintuitive effects occurring in physical phenomena rank high on the list of things that make science discoveries a source of perpetual excitement. Some examples illustrated here include, (i) the effects of attracting (i.e., pulling) a spherical particle placed in the field of a Bessel acoustical beam of progressive waves back towards the radiator’s surface [Mitri, “Single Bessel tractor-beam tweezers,” Wave Motion 51, no. 6, pp. 986–993, 2014], (ii) suppressing some of the sphere’s resonances [Mitri, “Near-field acoustic resonance scattering of a finite Bessel beam by an elastic sphere,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 61, no. 4, pp. 696–704, 2014], (iii) and the emergence of a “negative radiation torque,” meaning that an incoming high-order Bessel vortex beam with a positive topological charge (known also as the order of the beam) would rotate a viscoelastic sphere in the opposite sense of the beam handedness [Silva et al., “Radiation torque produced by an arbitrary acoustic wave,” Europhys. Lett., 97, art. no. 54003, 2012]. Results and numerical predictions illustrate the analysis and extensions to other geometries, such as rigid oblate and prolate spheroids [Mitri, “Axisymmetric scattering of an acoustical Bessel beam by a rigid fixed spheroid,” http://arxiv.org/abs/1505.06754v3, ibid. "Acoustic radiation force on oblate and prolate spheroids in Bessel beams,” Wave Motion, 57, pp. 231–238, 2015] are also mentioned.

Interactions of a streamwise-oriented vortex with a finite wing

AbstractA canonical study is developed to investigate the unsteady interactions of a streamwise-oriented vortex impinging upon a finite surface using high-fidelity simulation. As a model problem, an analytically defined vortex superimposed on a free stream is convected towards an aspect-ratio-six ($\mathit{AR}=6$) plate oriented at an angle of ${\it\alpha}=4^{\circ }$ and Reynolds number of $\mathit{Re}=20\,000$ in order to characterize the unsteady modes of interaction resulting from different spanwise positions of the incoming vortex. Outboard, tip-aligned and inboard positioning are shown to produce three distinct flow regimes: when the vortex is positioned outboard of, but in close proximity to, the wingtip, it pairs with the tip vortex to form a dipole that propels itself away from the plate through mutual induction, and also leads to an enhancement of the tip vortex. When the incoming vortex is aligned with the wingtip, the tip vortex is initially strengthened by the proximity of the incident vortex, but both structures attenuate into the wake as instabilities arise in the pair’s feeding sheets from the entrainment of opposite-signed vorticity into either structure. Finally, when the incident vortex is positioned inboard of the wingtip, the vortex bifurcates in the time-mean sense with portions convecting above and below the wing, and the tip vortex is mostly suppressed. The time-mean bifurcation is actually a result of an unsteady spiralling instability in the vortex core that reorients the vortex as it impacts the leading edge, pinches off, and alternately attaches to either side of the wing. The increased effective angle of attack inboard of impingement enhances the three-dimensional recirculation region created by the separated boundary layer off the leading edge which draws fluid from the incident vortex inboard and diminishes its impact on the outboard section of the wing. The slight but remaining downwash present outboard of impingement reduces the effective angle of attack in that region, resulting in a small separation bubble on either side of the wing in the time-mean solution, effectively unloading the tip outboard of impingement and suppressing the tip vortex. All incident vortex positions provide substantial increases in the wing’s lift-to-drag ratio; however, significant sustained rolling moments also result. As the vortex is brought inboard, the rolling moment diminishes and eventually switches sign as the reduced outboard loading balances the augmented sectional lift inboard of impingement.

Numerical Study of Microscale Shock-Vortex Interaction

Numerical studies of microscale shock-vortex interaction were conducted by particle-based direct simulation of Monte Carlo (DSMC). The enstrophy is found to be increased in the strong microscale shock-vortex interaction, which is not observed in the previous DSMC studies within the limited cases. Investigations also show that the increase of the enstrophy results in an increase in dissipation rate during the strong interaction. The incoming Mach number, vortex size, and vortex Mach number turn out to play a critical role in the strength of interaction, which in turn govern the change in the dissipation rate and the increase or decrease in enstrophy during the microscale shock-vortex interaction. It is also observed that the incoming Mach number is the most dominant parameter, followed by vortex size and vortex Mach number, during the microscale shock-vortex interaction.

Simulations of passive oscillation of a flexible plate in the wake of a cylinder by immersed boundary method

The behavior of a passive plate placed behind a D-cylinder is numerically studied by using the modified immersed boundary methods. The linear Euler–Bernoulli Beam theory is employed as the structure model for the flexible plate. The effects of the Reynolds number, the mass ratio and rigidity of the material and the distance between the D-cylinder and the plate are investigated. Results show that, the initial perturbation is inhibited when the Reynolds number is small. By increasing the Reynolds number, the larger the Reynolds number the larger amplitude of the plate’s oscillation. When the plate is placed close to the D-cylinder, its surface is surrounded by the vortical layer and there is no vortex shed from the D-cylinder, the ‘attached vortex mode’. The ‘Kármán vortex street’ is formed at the front of the plate when it is placed further behind the D-cylinder, the ‘vortex street mode’. Compared with the effects of Reynolds number, the material parameters do not play a crucial role on the plate’s oscillation behavior. The drag forces which act on the plate are related to the flow structures. When the distance is smaller with S/L=1.5, the plate is located in the suction domain and negative drag acts on the plate initially. For the large distance case, when the incoming shed vortex contacts the plate’s head, a low pressure domain is generated and this results in lower drag. The ‘vortex street mode’ can get more kinetic and strain energy by the plate, since the shed vortices make the plate’s deformation mode more complex and the oscillation frequency is also larger than the one of the ‘attached vortex mode’.

Unsteady Aerodynamics of a Low Aspect Ratio Turbine Stage: Modeling Issues and Flow Physics

In this paper the three-dimensional unsteady aerodynamics of a low aspect ratio, high pressure turbine stage are studied. In particular, the results of fully unsteady three-dimensional numerical simulations, performed with ANSYS-CFX, are critically evaluated against experimental data. Measurements were carried out with a novel three-dimensional fast-response pressure probe in the closed-loop test rig of the Laboratorio di Fluidodinamica delle Macchine of the Politecnico di Milano. An analysis is first reported about the strategy to limit the CPU and memory requirements while performing three-dimensional simulations of blade row interaction when the rotor and stator blade numbers are prime to each other. What emerges as the best choice is to simulate the unsteady behavior of the rotor alone by applying the stator outlet flow field as a rotating inlet boundary condition (scaled on the rotor blade pitch). Thanks to the reliability of the numerical model, a detailed analysis of the physical mechanisms acting inside the rotor channel is performed. Two operating conditions at different vane incidence are considered, in a configuration where the effects of the vortex-blade interaction are highlighted. Different vane incidence angles lead to different size, position, and strength of secondary vortices coming out from the stator, thus promoting different interaction processes in the subsequent rotor channel. However some general trends can be recognized in the vortex-blade interaction: the sense of rotation and the spanwise position of the incoming vortices play a crucial role on the dynamics of the rotor vortices, determining both the time-mean and the time-resolved characteristics of the secondary field at the exit of the stage.

Optical vortices routing in coupled elliptical spun fibers

We have studied the tunneling of a circularly polarized optical vortex (OV) in parallel strongly spun elliptical fibers. In this case it is possible to route the OV in a pure state from one of the fibers to another. We have determined the power efficiency of this process and have shown that such a directional coupler can serve for inversion of the topological charge of the incoming vortex.