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In this Paper, the wake dynamics of coaxial rotors are studied using the high-wake-resolution method. The high-wake-resolution method is a novel combination of the truncated vortex tube model for i...

A single Dielectric Barrier Discharge (DBD) plasma actuator driven by Alternating Current (AC) power, capable of inducing a starting vortex and a wall jet in quiescent air, is suited for low-Reynolds-number flow control. However, the starting vortex and the wall jet are usually observed after the plasma actuator has been operated for dozens of and hundreds of cycles of the voltage, respectively. The detail of the induced flow field at the initiation stage of the plasma actuator has rarely been addressed. At the initiation stage, a thin jet that provides the impetus for the entrainment of the induced flow at the beginning of the plasma actuation is first observed by using a high-accuracy phase-lock Schlieren technique and a high-speed Particle Image Velocimetry (PIV) system. This is the initial form of the momentum transfer from the plasma to the fluid. Then, an arched type jet is created by the plasma actuator. In addition, the whole development process of the induced flow field from the starting point of the thin jet to the quasi-steady stage of wall jet is presented for providing a comprehensive understanding of the plasma actuator and proposing a relevant enhancement of the numerical simulation model.

This paper presents an experimental study on the vortex formation and frequency tuning of jet diffusion flames under periodic excitations. The state-of-art laser diagnostic techniques were applied to provide temporally-resolved measurements for flame and flow structures. The results show that the fame surface deformation is synchronized with the convection of the inner vortex rings (IVRs), demonstrating the crucial role of IVRs in affecting the flame dynamics. The quantitative study on vortex formation and evolution is intended to understand two mechanisms: how the IVR forms, and how it is tuned to the forcing frequency of the perturbed fuel. The qualitative agreement between the predicted circulation growth and experimental data verifies the relevance of the classical starting vortex jet model in addressing the current problem, indicating the vortex growth is mainly dictated by the shear layer of the upstream fuel. The vortex tuning is closely related to the detachment of the main IVR, which is attributed to a secondary “razor vortex” when the shearing between the fuel jet and the coflow switches direction.

An advanced potential flow method is used to study the aerodynamic interactions between small rotors of quadrotor configurations and the impact on the overall flight performance. The aerodynamic an...

The spectral proper orthogonal decomposition method, which extracts spatially and temporally coherent modes in stationary flowfields, is used in the current paper to analyze high-speed broadband ch...

We carry out experiments on vortex ring flows submerged underwater which are generated by a pulsatile circular jet with the stroke ratio of 10. The pulsatile Reynolds number, based on centerline exit velocity and jet diameter, is a cosinusoidal function of time with a mean part of 2000 and an oscillating part of 820. This axisymmetric flow field is measured in a meridian symmetry plane by time-resolved planar particle image velocimetry. For revealing the interaction among the starting vortex ring, the trailing jet and the ambient quiescent fluid, we apply finite-time Lyapunov exponent in the Lagrangian framework to the Eulerian-based PIV dataset. By tracking trajectories of typical virtually color-dyed fluid parcels, we obtain clear visualizations of the entrainment and detrainment processes by the vortex ring, the advection transport of the trailing jet and the advection transport of trailing shear layer, during the longitudinal propagation of the vortex ring.

The flow resulting from the rotation of a series of thin plates that initially separate two gases of different densities is analyzed using direct numerical simulations (DNSs). The 90-deg plates' rotation forms a vorticity shear layer and a density interface in between the tips of two neighboring plates. Results of this study show that the shape of these layers strongly depends on the plate's tip-based Reynolds number that can be varied thanks to a parametrization of the plates' opening law. Different regimes are identified corresponding to single- or multimode initial interfaces, with or without the occurrence of starting vortices during the formation of the shear layer. The density interfaces resulting from this procedure are particularly well-suited to serve as initial conditions for the study of the Richtmyer–Meshkov (RM) instability-induced mixing. Results of this study also provide a description of vortex formation in stratified flows.

The unsteady flow due to a sphere, immersed in a quiescent fluid, and suddenly rotated, is a paradigm for the development of unsteady boundary layers and their collision. Such a collision arises when the boundary layers on the surface of the sphere are advected towards the equator, where they collide, serving to generate a radial jet. We present the first particle image velocimetry measurements of this collision process, the resulting starting vortex and development of the radial jet. Coupled with new computations, we demonstrate that the post-collision steady flow detaches smoothly from the sphere’s surface, in qualitative agreement with the analysis of Stewartson (Grenzschichtforschung/Boundary Layer Research (ed. H. Görtler), Springer, 1958, pp. 60–70), with no evidence of a recirculation zone, contrary to the conjectured structure of Smith & Duck (Q. J. Mech. Appl. Maths, vol. 20, 1977, pp. 143–156).

Recently, it was demonstrated in an inviscid flow that burst-and-coast or intermittent swimming can save energy when compared with continuous swimming and that the energy savings are maximized for ...

This study explores the wake effects of an upstream leading insect on the flight performance of a following one. The potential-flow based aerodynamic model, which combines the unsteady panel and unsteady vortex-lattice methods, is used to compute aerodynamic loads and simulate wake structures. The accuracy of the current aerodynamic model was confirmed in this study. The paper shows that the following insect does not cause any noticeable impact on the leading insect aerodynamics, while unfavorable effects due to the presence of the leading insect were found on the following counterpart. Nonetheless, by choosing a proper wing kinematic phase, the following insect may absorb the energy of the leading insect’s trailing wake, and therefore mitigate these negative effects. The variations of the required mechanical power of the following insect against the wing kinematic phase difference were shown to be related to the travel of the leading insect’s downstroke starting vortex.

Experimental and numerical analyses of the effect of residual air bubbles in a single-hole high-pressure diesel injector nozzle are presented. Detailed information on spray structures and dynamics near nozzle exit at the Start of Injection (SOI) is described. Experimental measurements are performed using a laser-based backlit imaging technique through a long distance microscope by injecting diesel fuel into a constant volume high-pressure spray chamber. Numerical investigation of, in and near-nozzle fluid dynamics is conducted in an Eulerian framework using a Volume of Fluid (VOF) interface capturing technique integrated with Large Eddy Simulation (LES) turbulence modelling. The present flow setup includes residual air bubbles remaining from a previous injection event, in-nozzle turbulence with no-slip wall conditions. Experimental images show a toroidal starting vortex near the nozzle exit suggesting a partially filled nozzle; transparency in the emerging jet demonstrates the presence of air trapped inside the nozzle liquid from the previous injection event. The numerical model provides insight into the influence of residual air bubbles on the spray morphology and dynamics of the emerging jet at the SOI. A mathematical code is developed to replicate the backlit imaging approach with the numerical results. The virtual images demonstrate a transparent liquid jet emerging into the pressurized chamber gas showing improved agreement with experimental images. The inclusion of air bubbles in the nozzle liquid prior to injection in the numerical model also yields improved agreement in the penetration velocity profile of the jet. These results explain how inclusion of residual air bubbles inside the nozzle liquid affects the physics of the penetrating jet at the SOI. The air bubbles inclusion also provides an explanation for the transparency of the emerging jet, rough interfacial surfaces, and enhanced necking behind the jet tip captured at the SOI.

Measurements of the flow field around a free-flying model helicopter in ground effect for both quasi-steady and unsteady maneuvering flights were performed using stereoscopic particle image velocimetry (PIV). The wake features for hover and forward flight at low advance ratios were characterized and changing flow patterns like recirculation and ground vortex flow were observed to be in good agreement with existing wind tunnel data. Parameters describing both general flow patterns and single blade tip vortices were extracted and showed significant dependence on the forward flight velocity. Landing approaches were performed and large vortical structures were observed close to the rotor disk, which contained high amounts of vorticity due to entrained blade tip vortices. The vortex structures developing for unsteady landing approaches contained distinctly higher velocities and momentum fluxes than expected from quasi-steady conditions at the same advance ratios. For a vertical takeoff maneuver with a rapid increase of collective pitch, the bundling of blade tip vortices into a "starting vortex" with circulation values up to 6 times higher than for a single blade tip vortex was observed. These results show the significance of experimental data on free-flying helicopters in unsteady maneuvering flight because the resulting flow fields cannot be predicted using steady-state experiments or simulations.

The vortex force map (VFM) method is a promising low-cost technique for predicting unsteady aerodynamic load from the vorticity field. Developed under potential flow assumption and applied to high-resolution computational fluid dynamics (CFD) data, the original VFM method needs to be further explored for viscous flows, especially at low Reynolds numbers and with incomplete knowledge of the flow data. In this paper, the VFM method is extended to general viscous flows, with both forces contributed from vorticity in the flowfield and on the body surface considered. We find that viscosity contributes to not only the skin-friction force but also a pressure force term. We also show that the extended vortex pressure force maps for general flow problems without the potential flow assumption lead to vanishing contributions from the far-field. The extended VFM method is applied to CFD flowfields for the starting flow of a cylinder and a NACA0012 airfoil, and it is used to analyze the force contribution effect of a given vortex. Moreover, acceptable accuracy of the method on truncated domains and under coarse sampling of typical particle image velocimetry measurement size is demonstrated.

We consider the trailing-edge vortex produced in an inviscid fluid by the start-up motion of a two-dimensional flat plate. A general starting motion is studied that includes the initial angle-of-attack of the plate (which may be zero), individual time power laws for plate translational and rotational speeds and the pivot position for plate rotation. A vortex-sheet representation for a start-up separated flow at the trailing edge is developed whose time-wise evolution is described by a Birkhoff–Rott equation coupled to an appropriate Kutta condition. This description includes convection by the outer flow, rotation and vortex-image self-induction. It admits a power-law similarity solution for the (small-time) primitive vortex, leading to an equation set where each term carries its own time-wise power-law factor. A set of four general plate motions is defined. Dominant-balance analysis of this set leads to discovery of three distinct start-up vortex-structure types that form the basis for all vortex motion. The properties of each type are developed in detail for some special cases. Numerical and analytical solutions are described and transition between solution types is discussed. Singular and degenerate vortex behaviour is discovered which may be due to the absence of fluid viscosity. An interesting case is start-up motion with zero initial angle of attack coupled to power-law plate rotation for which time-series examples are given that can be compared to high Reynolds number viscous flows.

The development of laminar thermals and the formation of buoyant vortex rings in thermals are studied by performing direct numerical simulations. The formation number of buoyant vortex rings in thermals is also analysed. We find that the development of thermals can be classified into three modes: the starting vortex ring dominated mode; the mode with the occurrence of a secondary vortex ring with breakup; and the mode with the occurrence of a secondary vortex ring without breakup. For the latter two modes, owing to the stretching of the thermal cap, the fluid at the leading edge rolls up, and a secondary vortex ring occurs, grows and replaces the starting vortex ring. The boundary of non-occurrence and occurrence of the secondary vortex ring is determined in a space of Richardson number (Ri) and injection duration ( larger than the critical value (approximates to 1. 95 × 105) for the occurrence of the secondary vortex ring. The formation number of buoyant vortex rings in thermals is beyond the universal formation number of 4 for non-buoyant vortex rings, and increases with the increase of the Richardson number and the injection duration. The switching between the thermal modes by changing the Richardson number and the injection duration has no significant effect on the value of the formation number.

This experimental study investigates the flow field and properties of a sweeping jet emitted from a fluidic oscillator into a quiescent environment. The aspect ratio of the outlet throat is 1. Stereoscopic particle image velocimetry is employed to measure the velocity field plane-by-plane. Simultaneously acquired pressure measurements provide a reference for phase correlating the individual planes yielding three-dimensional, time-resolved velocity information. Lagrangian and Eulerian visualization techniques illustrate the phase-averaged flow field. Circular head vortices, similar to the starting vortex of a steady jet, are formed repetitively when the jet is at its maximum deflection. The quantitative jet properties are determined from instantaneous velocity data using a cylindrical coordinate system that takes into account the changing deflection angle of the jet. The jet properties vary throughout the oscillation cycle. The maximum jet velocity decays much faster than that of a comparable steady jet indicating a higher momentum transfer to the environment. The entrainment rate of the spatially oscillating jet is larger than for a steady jet by a factor of 4. Most of the mass flow is entrained from the direction normal to the oscillation plane, which is accompanied by a significant increase in jet depth compared to a steady jet. The high entrainment rate results from the enlarged contact area between jet and ambient fluid due to the spatial oscillation. The jet’s total force exceeds that of an idealized steady jet by up to 30 %. The results are independent of the investigated oscillation frequencies in the range from 5 to 20 Hz.

The characteristics of the relationship between the evolution of propeller trailing vortex wake and skew angle are numerically examined based on four different five-blade David Taylor Model Basin (DTMB) model propellers with different skew angles. Numerical simulations are based on Reynolds-averaged Navier–Stokes (RANS) equations combined with SST k−ω turbulence model. Results show that the contraction of propeller trailing vortex wake can be restrained by increasing skew angle and loading conditions, and root vortices fade away when the propeller skew angle increases. With the increase of the propeller's skew angle, the deformation of the hub vortex and destabilization of the tip vortices are weakening gradually because the blade-to-blade interaction becomes weaker. The transition trailing vortex wake from stability to instability is restrained when the skew increases. Furthermore, analyses of tip vortice trajectories show that the increasing skew can reduce the difference in trailing vortex wake contraction under different loading conditions. Keywords: Propeller, Skew angles, Tip vortices, Hub vortex, RANS

Three-dimensional numerical simulations, for confined flows and unconfined flows, have been carried out to study the Karman vortex street inversion in the wake of a square cylinder. This paper is aimed to analyze the cause of the vortex inversion and to investigate the physical mechanisms. As two independent factors for the inversion, the wall confinement and the incoming flow conditions have been proved in a more rigorous way. It is shown that the interaction between the primary vortex and the incoming flow or the wall vortex layer is the fundamental cause. In order to compare the thermal transport phenomenon between the inversion and non-inversion cases, the top wall of a channel is kept at the free stream temperature and the bottom wall is set heated, while the cylinder is maintained adiabatic. The results indicate that the inversion phenomenon is not good for the heat transfer. Additionally, the effects of streamwise magnetic field on the vortex street inversion has been considered.

Large Eddy Simulations (LES) have been performed on a pitching NACA0012 airfoil at R e c = 2 . 10 4 . The influence of the reduced frequency on the dynamic stall phenomenon is investigated. Special attention is given on the aerodynamic coefficients drop incidence delay in comparison with a static case. The analysis is based on vorticity field observations and the introduction of local lift and friction coefficients. The boundary layer events and the presence and the impact of a Leading-Edge Vortex (LEV) are investigated. The total stall delay is decomposed into two main phases: a delay in the boundary layer separation which occurs at higher incidence as the reduced frequency increases and the initiation and the growth of the LEV. For selected reduced frequencies, the influence of the LEV on the lift and drag coefficients is analysed as a function of the airfoil angle of incidence and then as a function of time. The LEV maintains a high level of lift and drag. Its lifetime on the airfoil suction side seems to be shortened by the beginning of the airfoil downstroke motion. Given the fact that the LEV appears at higher incidence as the reduced frequency grows, the LEV lifetime on the airfoil suction side decreases as the reduced frequency increases. However, the airfoil oscillation being faster, the LEV goes through larger incidence ranges during its lifetime and leads to a larger lift and drag drop incidence delay as the reduced frequency increases.

Cavitating tip-leakage vortices appear in several hydrodynamic flows such as marine propellers or Kaplan turbines. Cavitating computations are a challenging topic since several keys issues are an ongoing work such as the definition of a universal mass source term. The present study focuses on the computations of the tip-leakage vortex including the gap between the blade tip and the side wall. Two computations are performed, one without cavitation and a second one with cavitation. In both cases, the results are compared with experimental data. The cavitation influence is investigated by comparing the cavitating and the non-cavitating cases. A particular attention is focused on the vortex core trajectory, the vorticity field and the vortex core identification. It is shown that, compared to the non-cavitating case, cavitation leads to a vortex trajectory closer to the suction side and the side wall, which can be of importance regarding the cavitation erosion. Furthermore, cavitation modified the vorticity field in the vortex core region. The main feature is a misalignment between the high vorticity region and the cavitating region, which opens a discussion regarding the definition of the vortex core.

The flow structure and energy separation considering the effect of cold mass fraction in a Ranque–Hilsch vortex tube were investigated computationally based on the vortex breakdown theory. The velocity distributions and pressure fields for nine different cold mass fractions were considered. A quasi-cylindrical approximation was adopted to predict the size of the vortex core size by considering the pressure gradient. Further, a novel analysis was conducted on the energy separation mechanism, in which the large-scale vortex structure plays an important role; for example, increasing the cold mass fraction within a certain range can result in bigger vortex cores, yielding better energy separation performance. Finally, the type of vortex breakdown was also discussed for further understanding the vortex structure. This paper offers a new idea on the manner in which the external conditions (here, cold mass fraction) affect the large-scale vortex structure and on the subsequent energy separation performance.

The objective of this paper is to investigate ventilated cavitating flow structures with special emphasis on vortex shedding dynamics via combining experimental and numerical methods. In the experiments, the high-speed video and time-resolved particle image velocimetry (TR-PIV) technique are used to observe ventilated cavitating patterns, and to measure the flow velocity and vorticity fields. The numerical simulation is performed by CFX with large eddy simulation (LES) model to capture the unsteady cavity shedding process, and the corresponding velocity and vorticity dynamics. The results show that the flow patterns can be classified into two principally different categories: structures mainly with vortex shedding (namely Benard–Karman vortex street; Benard–Karman vortex street with vortex filaments and Aligned vortices) and relatively stable structures (namely Aligned vortices with Re-entrant jet; Re-entrant jet and Stable supercavity). For the structures mainly with vortex shedding, the Strouhal number St corresponding to vortex shedding frequency and ratio h/λ corresponding to vortex streets are significantly different in variable ventilated cavitating regimes: St and ratio h/λ increase with enhancement of gas entrainment coefficient Qv for the Benard-Karman vortex street, and then St declines gradually for Benard-Karman vortex street with vortex filaments and Aligned vortices, but ratio h/λ declines dramatically for the above both patterns. In addition, the influences of Qv on the velocity and vorticity distributions have also been investigated. The proper orthogonal decomposition (POD) analysis of PIV measurements is used to characterize the coherent large-scale flow unsteadiness of velocity fields. It demonstrates that the ventilated cavitation plays an important role in the first mode pairs to mainly affect the vortex shedding in the wake. Moreover, the vorticity transport equation is applied to illustrate the influence of ventilated cavitation on the vorticity distribution. It can be found that the associated vortex dilatation term and baroclinic torque term are important mechanisms for the complicated change of vortices.

Phocid seals detect and track artificial or biogenic hydrodynamic trails based on mechanical signals of their whisker arrays. In this paper, we investigated the correlations between flow structures and whisker array signals using a simplified numerical model of fluid–structure interaction (FSI). Three-dimensional (3D) wakes of moving paddles in three different shapes (rectangular plate, undulated plate, and circular cylinder) were simulated using an in-house immersed-boundary-method-based computational fluid dynamics solver. One-way FSI was then simulated to obtain the dynamic behavior and root signal of each whisker in the two whisker arrays on a seal head in each wake. The position, geometry, and material properties of each whisker were modeled based on the measurements reported in literatures. The correlations between the wake structures and whisker array signals were analyzed. It was found that the patterns of the signals on the whisker arrays can reflect the strength, timing, and moving trajectories of the jets induced by the vortices in the wakes. Specifically, the rectangular plate generates the strongest starting vortex ring as well as the strongest jets, while the undulated plate generates the weakest ones. These flow features are fully reflected by the largest whisker signal magnitude in the rectangular plate sensing and the smallest one in the undulated plate sensing. Moreover, the timing of the signal initiation and the maximum signal agree well with the timing of the jet reaching the arrays and the maximum flow speed, respectively. The correlation coefficient between the moving trajectories of the jet and the movement of the high signal level region in the array was found to be higher than 0.9 in the rectangular plate case. The results provide a physical insight into the mechanisms of seal whisker flow sensing.

Passive mass transfer enhancement from the surface of a liquefying substance using Lateral Sweep Vortex Generators (LSVG) placed in high-speed flow is presented. Numerical simulation of three-dimensional turbulent compressible flowfield involving species transport has been carried out using Advection Upstream Splitting Method (AUSM) scheme based Finite Volume solver. A temperature dependent mass efflux boundary condition has been developed and implemented in the computational procedure for updating the boundary mass flux. Computational procedure has been validated using the experimental wall pressure profile reported for a similar vortex generator placed in high speed flow and could capture the complex flow features resulted by the vortex-boundary layer interactions. Convective mass transfer of species is found to get improved in wake region of the LSVG by the horseshoe vortex and further get transported downstream by the counter-rotating vortex pair. Extensive numerical simulations have been carried out with several lateral sweep angles of LSVG to explore the role of vortices in promotion of the convective mass transfer. It has been observed that the mass efflux enhances with increase in sweep angle of the vortex generator due to the vorticity augmentation. Analysis of the vortex trajectories could figure out the influence of vortex interactions leading to the peak mass efflux. Further, an exponential-power law based correlation between relative Sherwood number and relative streamwise vorticity has been developed.

The secondary flows of different types were studied in a boundary layer of cyclonic vortex over localized heater. Near the periphery of the heating area system of horizontal rolls oriented along the basic flow is formed. Thermal plumes which are originated between counter-rotating horizontal rolls are pushed to the centre by basic flow and create spiral bands. Both types of observed secondary flows (rolls and spiral bands) are of convective nature. Cyclonic vortex becomes unstable at low values of viscosity and fast rotation of the experimental model. The instability of the vortex is tightly connected with a structure of the radial inflow. For moderate values of rotational Reynolds number Re the radial flows consist of several branches which transport angular momentum to the centre of the model. When Re exceeds critical value (about 23) radial inflow changes its structure and appears as one wide branch which does not reach the centre. As a result the vortex that slowly moves around the centre is formed instead of the vortex localized in the centre. Further increase of Re leads to chaotic state with several vortices which appears at different locations near the periphery of the heating area. The map of regimes with stable and unstable vortices is presented. The applicability of experimental results to the formation of large-scale atmospheric vortices is discussed.

The problem of asymmetric water entry of a wedge with the vortex sheet shed from its apex is considered within the framework of the ideal and incompressible fluid. The effects due to gravity and surface tension are ignored and the flow therefore can be treated as self-similar, as there is no length scale. The solution for the problem is sought through two mutually dependent parts using two different analytic approaches. The first one is due to water entry, which is obtained through the integral hodograph method for the complex velocity potential, in which the streamline on the body surface remains on the body surface after passing the apex, leading to a non-physical local singularity. The second one is due to a vortex sheet shed from the apex, and the shape of the sheet and the strength distribution of the vortex are obtained through the solution of the Birkhoff–Rott equation. The total circulation of the vortex sheet is obtained by imposing the Kutta condition at the apex, which removes the local singularity. These two solutions are nonlinearly coupled on the unknown free surface and the unknown vortex sheet. This poses a major challenge, which distinguishes the present formulation of the problem from the previous ones on water entry without a vortex sheet and ones on vortex shedding from a wedge apex without a moving free surface. Detailed results in terms of pressure distribution, vortex sheet, velocity and force coefficients are presented for wedges of different inner angles and heel angles, as well as the water-entry direction. It is shown that the vortex shedding from the tip of the wedge has a profound local effect, but only weakly affects the free-surface shape, overall pressure distribution and force coefficients.

The interaction of aircraft exhaust jet and wake vortex generated from a realistic aircraft configuration was numerically studied from the early phase until the complete roll-up. For numerical simulation we employed an unstructured mesh Reynolds-averaged Navier–Stokes (RANS) solver for the flow around an aircraft model, while a Cartesian mesh large-eddy simulation solver implemented on the framework of the building-cube method (BCM) was used for the simulation of the mid- to far-field wake. We especially investigated the feasibility of a coupling approach of the steady RANS solver and the Cartesian LES solver in the simulation of the jet-wake vortex interaction, where unsteady velocity fluctuations are stochastically reproduced based on RANS turbulence variables. The results showed that a multi-level Cartesian mesh of the BCM was effective to capture the evolution of aircraft wake including exhaust jet. The velocity profiles of a wake vortex are compared with several vortex models, which reveals that the velocity profiles fit with different vortex models depending on the vortex ages. The normalized temperature of exhaust jet is compared with the experiment, which confirms that the temperature information is appropriately transferred from the near- to the far-field simulation.

In some situations, a vortex is involved in the inflow of a propeller, e.g. a ground vortex or a vortex shed from the upstream parts of the airframe of the aircraft. In order to study the impact of the vortex on the propeller, the Batchelor vortex is utilised to model the impinging vortex, and its effect on the propeller is calculated by Computational Fluid Dynamics. The loading on the propeller increases due to the impingement of a contra-rotating vortex, and the opposite is true for a co-rotating vortex. However, the torque coefficient of the propeller changes consistently with the thrust coefficient which leads to the efficiency of the propeller being independent of the impinging vortex. With the vortex impinging on the propeller, a strong pressure fluctuation on the blade is observed, especially on the leading edge of the blade. This cyclic pressure also results in a periodic bending moment on the blade, which may have an impact on the structural fatigue of the blade.

An investigation of symmetry breaking and naturally occurring instabilities over thin slender delta wings with sharp leading edges was carried out in a water tunnel using particle image velocimetry (PIV) measurements. Time-averaged location, strength and core radius of conical vortices vary almost linearly with chordwise distance for three delta wings with $75^{\circ }$, $80^{\circ }$ and $85^{\circ }$ sweep angles over a wide range of angles of attack. Properties of the time-averaged vortex pairs depend only on the similarity parameter, which is a function of the angle of attack and the sweep angle. It is shown that time-averaged vortex pairs develop asymmetry gradually with increasing values of the similarity parameter. Vortex asymmetry can develop in the absence of vortex breakdown on the wing. Instantaneous PIV snapshots were analysed using proper orthogonal decomposition and dynamic mode decomposition, revealing the shear layer and vortex instabilities. The shear layer mode is the most periodic and more dominant for lower values of the similarity parameter. The Strouhal number based on the free stream velocity component in the cross-flow plane is a function of only the similarity parameter. The dominant frequency of the shear layer mode decreases with the increasing similarity parameter. The vortex modes reveal the fluctuations of the vorticity magnitude and helical displacement of the cores, but with little periodicity. There is little correlation between the fluctuations in the cores of the vortices.

Theoretical investigations are conducted to study the structure of stationary vortex with a free surface at the bottom of the orifice flow. A modified Lundgren-like model has come up to describe the vortex flow and the governing equations of the theoretical model are solved numerically. Based on the calculation results, the flow distribution and structure characteristic are analyzed, and several quantitative relationships between the major flow parameters at a high radial Reynolds number (including the outflow rate, tangential circulation, water level, export size, maximum tangential velocity, vortex core radius, and liquid viscosity) are obtained. Furthermore, a formula of the critical submerged depth for the stationary free surface vortices is derived and the results from the formula are consistent with the experiments.

By employing a wing-tip vortex impinging onto a propeller, the impact of the propeller on this vortex is studied with the help of Particle Image Velocimetry measurements. The effects of the propeller on the vortex are compared in the flow fields in the vicinity of the blades, i.e. one plane upstream and one plane downstream of the propeller. The meandering of the vortex downstream of the propeller increases by approximately one order of magnitude with respect to that upstream of the propeller. The circulation of the downstream vortex is at the same level as the upstream vortex, and this means the downstream vortex still has the potential to influence the airframe further downstream. As the vortex impinges at different radial positions of the propeller, the stretching effect depends on the local thrust of the propeller, which is different from the case with a vortex impinging on a steady wing. The Proper Orthogonal Decomposition analysis shows that the first and second modes of the flow upstream of the propeller are induced by the blade passing, whilst the third and the fourth modes are induced by the vortex meandering.

This paper presents a method examining static forces and moment of airfoil in steady solver. Aerodynamic stability is one subject that many scholars have begun to realize very crucial for design and daily operation. Fundamentals of aerodynamics were firstly scrutinized concerned about lift, drag and moment. Then steady simulation solver is set up instead of transient solver because we would like to examine airfoil without the effect of vortex motion, temporarily focus on static forces on airfoil. Dependent variable include cl, cd and cm of three airfoils, FLAT PLATE, NACA0012 and NACA23012 at TSR = 0, 2, 5 and 7. Stability is characterized by deviation and fluctuation at 360 degrees of azimuth position. We found that FLAT PLATE showed most deviation and fluctuation as an example of airfoil for VAWT. NACA0012 representing symmetric airfoil has more stable cm at whole time and cd just at TSR of 2 and 5. NACA23012 representing cambered airfoil has more stable cl all phases of running.

In this study vortex shedding phenomenon behind two inclined flat plates in tandem arrangement have been studied experimentally. Velocity field components were acquired by employing of Hot-wire Anemometer (HWA) for successive phases in a vortex shedding cycle at Reynolds number Re = 21,000, at three angles of attack 45°, 60° and 75°. The effects of gap ratio between the plates (g/D) together with angle of attack (α) on the asymmetric flow structure, time averaged wake flow properties and coherent structures have been investigated. Strouhal number s t decreases monotonically with increasing angle of attack, whereas, Modified Strouhal number s t ’ (based on the projected plate width as the characteristic length) is independent of angle of attack. Phase averaged contour shows significant lateral momentum transfer to a growing vortex ¼ period after a vortex was shed. The result of coherent structure shows a train of opposite vortices, with unequal coherent strength, are being shed alternating from leading edge and trailing edge of the plates.

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 Γ l . Transient lift forces measured in a wind tunnel are also compared with the potential theory results for a given Γ l 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 | Γ l | 1 ∕ 2 ) α in relation to the zero angle of attack for the same Γ l . 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 Γ l and relative separation distance. The opposite happens for a counter-clockwise vortex. This asymmetry in the maximum lift fluctuation increases slightly with | Γ l | , approaching a ratio of almost two for large | Γ l | .

We present the transient phenomena occurring during the impulsive control of flow separation over a NACA0015 airfoil at an incidence angle of 11° and a chord Reynolds number of 1 million. Actuation is performed via pneumatic vortex generators, impulsively activated in order to analyze the transient phenomena corresponding to the attachment process and, conversely, to transient re-separation occurring when the actuators are switched off. Measurements are performed using a linear array of unsteady pressure transducers and a single traversing crosswire. The pressure transducers are positioned in the separated region of the airfoil, which extends ∼ 0.3c upstream of the trailing edge at the above flow condition. To control the flow, the angled fluidic vortex generators are positioned in a single spanwise array located 0.3c downstream of the leading edge of the airfoil. We establish a statistical relationship between pressure and velocity signals during both the uncontrolled steady state and the transient processes of attachment and separation. The unsteady behavior of the attachment process is also qualitatively analyzed via a 0.3 million Reynold number visualizations. The emission of a “starting vortex” is evidenced. This corresponds to a transient increase of drag.

In aerodynamics, flow separation often results to the increasing of drag and decreasing of lift for objects traveling through the fluid. This also leads to the instability of the commercial and military vehicle. In order to circumvent this problem, a new method of setting a small plate near the leading edge of the airfoil has been proposed by our team. The principle of this method is to create a mutual interference between the trailing vortex generated by the small plate and the boundary layer of the airfoil in order to control the flow separation phenomenon. Very little study has been provided up to now about the effects of Mach number on the flow separation control of this method. Indeed, most of our previous works were dedicated to the study about the length, initial installation angle and installation position of the small plate. Accordingly, in this paper, we mostly focus our attention on the effects of Mach number on the flow separation control of this method. Meanwhile, a more detailed discussion about the effects of the length, installation angle and installation position of the small plate on the flow separation control has also been performed. Results show that: for Mach numbers inferior to 0.5, this method can lead to the airfoil (NACA4405) maintain a relatively high lift coefficient even at very large angles of attack, which is not possible for normal airfoils. A better aerodynamic performance of the airfoil can be achieved in this way. However, for Mach numbers superior to 0.5, only slight amelioration of the aerodynamic performance can be obtained.

In the present study, a steady state numerical simulation is performed for investigating the thermal and fluid flow characteristics in a turbulent boundary layer for a flat channel as an element of a compact heat exchanger. A trapezoidal winglet pair (TWP) and curved trapezoidal winglet pair (CTWP) are mounted in the computational domain in order to deliberate the effects of longitudinal vortices induced by CTWP and TWP as the vortex generators. The simulation of the turbulent flow is performed by means of the Reynolds stress model (RSM) and the flow has a Reynolds number in the range 7000–35,000. The flow structure at the presence of longitudinal vortex generator consists of corner (horseshoe) vortex, induced vortex, and the main vortex which cause significant heat transfer effects in the downstream region. The effects on the thermal boundary layer are also studied from the field synergy point of view. Numerical results showed that CTWP has a lower pressure drop and a better overall performance compared to TWP. To achieve a maximum heat transfer augmentation and a minimum pressure drop, optimal height and clearance between two vortex generators have been determined using and combining computational fluid dynamics analysis, artificial neural networks and single-objective genetic algorithm.

The interactions of two like-signed vortices in viscous fluid are investigated using two-dimensional numerical simulations performed across a range of vortex strength ratios, $\unicode[STIX]{x1D6EC}=\unicode[STIX]{x1D6E4}_{1}/\unicode[STIX]{x1D6E4}_{2}\leqslant 1$, corresponding to vortices of circulation, $\unicode[STIX]{x1D6E4}_{i}$, with differing initial size and/or peak vorticity. In all cases, the vortices evolve by viscous diffusion before undergoing a primary convective interaction, which ultimately results in a single vortex. The post-interaction vortex is quantitatively evaluated in terms of an enhancement factor, $\unicode[STIX]{x1D700}=\unicode[STIX]{x1D6E4}_{end}/\unicode[STIX]{x1D6E4}_{2,start}$, which compares its circulation, $\unicode[STIX]{x1D6E4}_{end}$, to that of the stronger starting vortex, $\unicode[STIX]{x1D6E4}_{2,start}$. Results are effectively characterized by a mutuality parameter, $MP\equiv (S/\unicode[STIX]{x1D714})_{1}/(S/\unicode[STIX]{x1D714})_{2}$, where the ratio of induced strain rate, $S$, to peak vorticity, $\unicode[STIX]{x1D714}$, for each vortex, $(S/\unicode[STIX]{x1D714})_{i}$, is found to have a critical value, $(S/\unicode[STIX]{x1D714})_{cr}\approx 0.135$, above which core detrainment occurs. If $MP$ is sufficiently close to unity, both vortices detrain and a two-way mutual entrainment process leads to $\unicode[STIX]{x1D700}>1$, i.e. merger. In asymmetric interactions and mergers, generally one vortex dominates; the weak/no/strong vortex winner regimes correspond to $MP1$, respectively. As $MP$ deviates from unity, $\unicode[STIX]{x1D700}$ decreases until a critical value, $MP_{cr}$ is reached, beyond which there is only a one-way interaction; one vortex detrains and is destroyed by the other, which dominates and survives. There is no entrainment and $\unicode[STIX]{x1D700}\sim 1$, i.e. only a straining out occurs. Although $(S/\unicode[STIX]{x1D714})_{cr}$ appears to be independent of Reynolds number, $MP_{cr}$ shows a dependence. Comparisons are made with available experimental data from Meunier (2001, PhD thesis, Université de Provence-Aix-Marseille I).

Summary In this paper, the effects of nondimensional frequency f*, pitch amplitude θ0, and airfoil thickness on the energy extraction performance of an oscillating wing wind power generator were numerically investigated. It is found that the optimum value of f* or θ0 exists to achieve the maximum energy efficiency. Additionally, the thickness of airfoil also significantly affects the efficiency and the flow patterns around the oscillating foil. For thin airfoils, a relatively large-scale vortex was normally generated at its leading edge. This vortex detached from leading edge might be able to be “caught” by the airfoil again and then reutilized to increase its work capacity. By contrast, no induced leading edge vortex is formed on the upper surface of a thick airfoil. Nevertheless, the pressure difference between the upper and lower surface of the oscillating thick airfoil is greater than that of thin airfoil. Thus, the portion of the output power contributed by the oscillatory heaving motion is greatly increased and high energy extraction efficiency can still be achieved. For airfoils with moderate thickness, both flow phenomena observed on thin and thick oscillating airfoils that have high wind energy utilization efficiency are all likely to occur, depending on the adopted motion parameters.

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.

The broadband noise generated by the scattering of turbulent flow at the trailing edge of a NACA 0018 airfoil with trailing edge serrations is investigated, varying both the airfoil angle of attack and serration flap angle. Acoustic emissions from the trailing edge are measured using a microphone array. The noise level is observed to be higher than that of the airfoil without serrations at frequencies beyond a crossover value. The latter is found to scale with a characteristic Strouhal number based upon the boundary layer thickness and the freestream velocity. A satisfactory collapse of the results under varying angles of attack and freestream velocities is observed. The modifications of the hydrodynamic behavior and the noise increase are linked by high-speed observations conducted with particle image velocimetry. An increase in the energy of turbulent fluctuations is also observed at the expected crossover frequency. The dominant cause of the increased noise is thereby identified at the pressure side edge of the serrations at a given flap angle.

The spatial relationship between the energy dissipation slabs and the vortex tubes is investigated based on the direct numerical simulation (DNS) of the channel flow. The spatial distance between these two structures is found to be slightly greater than the vortex radius. Comparison of the core areas of the vortex tubes and the dissipation slabs gives a mean ratio of 0.16 for the mean swirling strength and that of 2.89 for the mean dissipation rate. These results verify that in the channel flow the slabs of intense dissipation and the vortex tubes do not coincide in space. Rather they appear in pairs offset with a mean separation of approximately 10η.

A new vortex sheet model was proposed for simulating aircraft wake vortex evolution. Rather than beginning with a pair of counter-rotating cylindrical vortices as in the traditional models, a lift-drag method is used to initialize a vortex sheet so that the roll-up phase is taken into account. The results of this model report a better approximation to a real situation when compared to the measurement data. The roll-up induced structures are proved to influence the far-field decay. On one hand, they lead to an early decay in the diffusion phase. On the other hand, the growth of linear instability such as elliptical instability is suppressed, resulting in a slower decay in the rapid decay phase. This work provides a simple and practicable model for simulating wake vortex evolution, which combines the roll-up process and the far-field phase in simulation. It is also proved that the roll-up phase should not be ignored when simulating the far-field evolution of an aircraft wake vortex pair, which indicates the necessity of this new model.

Multi-phase flows, particularly two-phase flows, are widely used in the industries, hence in order to predict flow regime, pressure drop, heat transfer, and phase change, two-phase flows should be studied more precisely. In the petroleum industry, separation of phases such as water from petroleum is done using rotational flow and vortices; thus, the evolution of the vortex in two-phase flow should be considered. One method of separation requires the flow to enter a long tube in a free vortex. Investigating this requires sufficient knowledge of free vortex flow in a tube. The present study examined the evolution of tube-constrained two-phase free vortex using computational fluid dynamics. The discretized equations were solved using the SIMPLE method. It was determined that as the liquid flows down the length of the pipe, the free vortex evolves into combined forced and free vortices. The tangential velocity of the free and forced vortices also decreases in response to viscosity. It is shown that the concentration of the second discrete phase (oil) is greatest at the center of the pipe in the core of the vortex. This concentration is at a maximum at the outlet of the pipe.

The structure of a trailing vortex from a wing undergoing small amplitude, low frequency heaving motion is investigated using space–time representations determined from stereo particle image velocimetry. The evolution of the vortex shows large fluctuations of axial velocity deficit and circulation during the oscillation cycle. Correspondingly, large variations of swirl ratio occur and onset of pronounced azimuthal vorticity arises. At a given cross-section of the vortex, the pattern of azimuthal vorticity moves around its axis in an ordered fashion as both it and the pattern of velocity defect increase in magnitude and scale. When the swirl ratio attains its minimum value during the oscillation cycle, and this value lies below the theoretically established critical threshold for amplification of azimuthal modes, the magnitude and scale of the pattern of azimuthal vorticity is maximized. Subsequent increase of the swirl ratio yields attenuation of the azimuthal vorticity. Onset of pronounced azimuthal vorticity when the swirl ratio decreases involves rapid amplification, then disruption, of axial vorticity fluctuation.