Showing papers on "Starting vortex published in 2018"

Properties of a sweeping jet emitted from a fluidic oscillator

Abstract: 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.

RANS computations of a confined cavitating tip-leakage vortex

Abstract: 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.

Computational investigation of precessing vortex breakdown and energy separation in a Ranque–Hilsch vortex tube

Abstract: 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

A Lagrangian perspective towards studying entrainment

Abstract: This study presents a method for characterizing entrainment using Lagrangian data. Specifically, the method exploits the temporal evolution of enstrophy along pathlines to determine if pathlines undergo entrainment. The method only recently became feasible with the development of four-dimensional particle tracking velocimetry [4D-PTV; see Schanz et al. (Exp Fluids 57(5):7, 2016)], which produces pathlines of temporal length and spatial density that was previously unattainable. The proposed entrainment method was tested on experimentally acquired 4D-PTV data of a turbulent starting vortex forming behind a linearly accelerating circular plate. The method is shown to be insensitive to its control parameters. The first control parameter is an enstrophy threshold used to identify pathlines that always exhibit low enstrophy. The second control parameter is the size of an “enstrophy-source region”, which is placed within the measurement domain to identify pathlines that gain enstrophy through interactions with an enstrophy source. In the case of the starting vortex, the method reveals topological features significant to the entrainment process, such as the roll-up of irrotational fluid between the shear layer and vortex core, and pockets of entrainment that exist within undulations on the shear layer’s outboard side. Whereas the method proposed here measures the entrainment ratio directly, the use of Lagrangian coherent structures (LCSs) to quantify entrainment is shown to be infeasible for turbulent flows due to the presence of complex material manifolds for such flows. Furthermore, unlike results from Rosi and Rival (J Fluid Mech 811:3750, 2017), which analyzed the spreading of enstrophy isocontours to characterize entrainment into a starting vortex, the proposed method produces an entrainment rate that is insensitive to the enstrophy threshold, and clearly reveals mechanisms of entrainment. Using the starting vortex data, the proposed technique measured an entrainment ratio range similar to that for laminar vortex rings. The result suggests that, for this particular class of flows, turbulence does not play a significant role with respect to entrainment. The proposed method is seen as an effective tool, given its ability to quantify the entrainment, reveal its salient topological features, and potentially be easily adapted to other classes of flows.

Flow separation control by dielectric barrier discharge plasma actuation via pulsed momentum injection

Abstract: Control of a turbulent boundary layer separating on a half-cylinder mounted on a flat plate has been investigated using a Dielectric Barrier Discharge (DBD) plasma actuator placed along the apex of a cylinder. The main focus of the study has been to evaluate if the control ability of the actuator can be improved through pulsed actuation compared to its steady counterpart. Investigations of the electric wind induced by the DBD plasma actuator in still air, when mounted on the flat plate, revealed that while the steady actuation produces an electric wind similar to a wall jet, the pulsed actuation creates a train of co-rotating vortices. The vortices are the result of a starting vortex produced by the actuator at each actuation pulse. A parametric study showed a dependence of the size, shape and propagation velocity of the vortices on the pulse frequency and duty cycle. With the actuator mounted along the apex of the cylinder, Particle Image Velocimetry measurements of the uncontrolled and controlled flow with a free-stream velocity of 5 m/s showed a clear reduction of the recirculation region downstream the cylinder when using plasma actuation. An even higher control effect could be achieved with pulsed actuation compared to the steady actuation. Phase-locked measurements of the unsteady actuation showed that pulsed actuation periodically shifted the flow separation location resulting in the propagation of vortical structures in the recirculation region. The size of the vortical structures showed a dependence on the pulsed actuation timing parameters.Control of a turbulent boundary layer separating on a half-cylinder mounted on a flat plate has been investigated using a Dielectric Barrier Discharge (DBD) plasma actuator placed along the apex of a cylinder. The main focus of the study has been to evaluate if the control ability of the actuator can be improved through pulsed actuation compared to its steady counterpart. Investigations of the electric wind induced by the DBD plasma actuator in still air, when mounted on the flat plate, revealed that while the steady actuation produces an electric wind similar to a wall jet, the pulsed actuation creates a train of co-rotating vortices. The vortices are the result of a starting vortex produced by the actuator at each actuation pulse. A parametric study showed a dependence of the size, shape and propagation velocity of the vortices on the pulse frequency and duty cycle. With the actuator mounted along the apex of the cylinder, Particle Image Velocimetry measurements of the uncontrolled and controlled flow w...

Study on lift enhancement of a flapping rotary wing by a bore-hole design:

Abstract: A novel design of a micro air vehicle with flapping rotary wings was recently proposed. In this paper, the investigation is focused on aerodynamic enhancement of a bore-hole and an elastic cover added onto an intact flapping rotary wing. The aerodynamic force and the pitching motion of the perforated wing are both acquired in experiments. In comparison with an intact wing, a typical perforated wing shows a significant decrement in negative lift and thus a higher mean lift. As a supporting process, the computational fluid dynamics method is then employed to analyze the flow around the wing and the hole, and thereby make clear the underlying physical mechanism. It is found the hole opens due to the cover deflection in a passive manner and produces a secondary starting vortex on the wing lower surface of the cover during upstrokes. Together with the tip vortex around the lateral edges of the cover, the resulting vortex significantly reduces the magnitude of the negative lift, and thus explains the mean-lift ...

Water entry of a wedge with rolled-up vortex sheet

Abstract: 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.

New insights into aerodynamic characteristics of oscillating wings and performance as wind power generator

Abstract: 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.

Dynamic- and post-stall characteristics of pitching airfoils at extreme conditions:

Abstract: Post-stall flow structure and surface pressures are evaluated to determine the effects of large angles of attack, perching like manoeuvres on the flow about a NACA 0021 airfoil exposed to dynamic s...

Dynamics of nearly parallel interacting vortex filaments

Abstract: The dynamics of interacting vortex filaments in an incompressible fluid, which are nearly parallel, have been approximated in the Klein–Majda–Damodaran model. The regime considers the deflection of each filament from a central axis; that is to say, the vortex filaments are assumed to be roughly parallel and centred along parallel lines. While this model has attracted a fair amount of mathematical interest in the recent literature, particularly concerning the existence of certain specific vortex filament structures, our aim is to generalise several known interesting filament solutions, found in the self-induced motion of a single vortex filament, to the case of pairwise interactions between multiple vortex filaments under the Klein–Majda–Damodaran model by means of asymptotic and numerical methods. In particular, we obtain asymptotic solutions for counter-rotating and co-rotating vortex filament pairs that are separated by a distance, so that the vortex filaments always remain sufficiently far apart, as well as intertwined vortex filaments that are in close proximity, exhibiting overlapping orbits. For each scenario, we consider both co- and counter-rotating pairwise interactions, and the specific kinds of solutions obtained for each case consist of planar filaments, for which motion is purely rotational, as well as travelling wave and self-similar solutions, both of which change their form as they evolve in time. We choose travelling waves, planar filaments and self-similar solutions for the initial filament configurations, as these are common vortex filament structures in the literature, and we use the dynamics under the Klein–Majda–Damodaran model to see how these structures are modified in time under pairwise interaction dynamics. Numerical simulations for each case demonstrate the validity of the asymptotic solutions. Furthermore, we develop equations to study a co-rotating hierarchy of many satellite vortices orbiting around a central filament. We numerically show that such configurations are unstable for plane-wave solutions, which lead to collapse of the hierarchy. We also consider more general travelling wave and self-similar solutions for co-rotating hierarchies, and these give what appears to be chaotic dynamics.

Study of stationary vortex with a free surface at the bottom of the orifice flow

Abstract: 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.

Experiments on the drag of a flat plate moving parallel to an air-water interface

Abstract: Rowing is a competitive sport where victory is determined by fine margins. In the past, the focus has been on optimizing the shell. Due to the increasing regulations placed on the shell design, the scope for manoeuvring in this area is greatly limited. This increases the necessity to focus on the less explored option of propulsion in rowing. The propulsion is caused by the momentum transfer from the rowers to the water with the help of oar blades. The design of these oar blades has also been extensively studied. However, knowledge on the effect of the air-water interface on the drag force on the oar blades is still lacking. Therefore, visualizing the flow around the oar blades will lead to better understanding of the flow structures, which would help in optimizing the drag force acting on the oar blades. In the present study, a simplified scenario of a rectangular flat plate moving normally to its plane along a straight line and parallel to the free surface has been studied using hydrogen bubble visualization and Particle Image Velocimetry (PIV). Using a 4-axes industrial robot and a force/torque transducer, the effect of the air-water interface on the drag force on the plate has been studied at a Reynolds number, based on the longest edge of the plate, of 6×104. The analysis of the drag force profiles at different plate depths led to the identification of a high drag case, which occurred at a plate depth h of 20 mm. The hydrogen bubble visualization indicated that in this specific case, the high drag was related to the formation of a compact wake behind the plate. Hydrogen bubble visualizations were also performed at two other plate depths, h = 0 and h = 100 mm, to determine the effect of the air-water interface on the flow structures and ultimately on the drag force acting on the plate. The visualization of the deep water case (h = 100 mm) captured the formation of a vortex ring, which was found to have unique effects on the drag force. The presence of the air-water interface was found to significantly influence the drag force on the plate. Additionally, an extensive decomposition of the drag force profile showed a time-dependent added mass force and a decaying force acting on the plate. Particle Image Velocimetry (PIV) measurements were carried out in the horizontal mid-plane of the plate. The tracking of the starting vortex and its disintegration has been identified using the swirling strength analysis. Finally, combining the hydrogen bubble visualization and PIV results, the formation and the development of the starting vortex was identified.