4,089 research outputs found

    Formation and evolution of vortex rings with weak to moderate swirl and their implications for enhancing vortex ring circulation

    Get PDF
    The formation of swirling vortex rings and their early time evolution, resulting from the controlled discharge of an incompressible fluid into a stationary equivalent fluid bulk, is explored both numerically and experimentally for swirl number S ∈ [0, 1]. For the numerical work, two practically realisable inlet conditions are investigated with swirl simultaneously superposed onto a linear momentum discharge; the corresponding circulation based Reynolds number is 7500. The results reveal that, for S > 1/2, the addition of swirl promotes the breakdown of the leading primary vortex ring structure, giving rise to the striking feature of significant negative vorticity, or opposite sign vorticity (OSV), generation in the region surrounding the primary vortex ring core, whose strength scales with S2. Through a non-linear interaction with the vortex breakdown, the radius of the primary toroidal vortex core is rapidly increased; consequently, the self-induced propagation velocity of the leading ring decreases with S and vortex stretching along the circular primary vortex core increases counteracting viscous diffusion effects. The latter governs the evolution of the peak vorticity intensity and the swirl velocity magnitude in the primary ring core, the circulation growth rate of this ring, as well as the vorticity intensity of the trailing jet and hence its stability. This combination of effects leads to an increased dimensionless kinetic energy for the primary ring with increasing S and results in an almost linearly decreasing circulation based formation number, F. In a rigorous complementary experimental investigation, OSV is observed by introducing swirl using a rotating pipe, varying the time period before the piston stroke to achieve the desired swirl strength at a Reynolds number of 1000. Rotating pipe is found to generate a secondary flow altering the inlet condition. Nevertheless, it is observed, using short periods of pipe rotation and higher angular speed, that it is possible to generate a swirling vortex ring with less OSV production and all the related effects discussed above. The relation between F and the radius of the vortex ring is investigated through manipulation of ring radius growth, achieved through its interaction with a preceding vortex ring. Reducing radius growth, facilitates an increase of the circulation of the vortex ring, which in turn affects its F value

    Jets and instabilities in forced magnetohydrodynamic flows

    Get PDF
    Magnetic fields are present in the solar system and astrophysical bodies (e.g. the Sun's field, the Earth's field, and the fields of giant planets, stars and galaxies). Our research examines the effect of magnetic fields on these systems, extending the work of Meshalkin and Sinai (1961) & Manfroi and Young (2002). The results will be useful for understanding the effects of the magnetic field in more turbulent regimes, although this study is concerned with the instabilities associated with classical laminar flow. We aim to investigate the role played by the magnetic field in modifying the stability properties of planar-forced fluid flows. In the absence of magnetic fields, the flow found by a body force, and nonlinear interactions with Rossby waves result in the generation of strong zonal flows. However, we find that the presence of a weak magnetic field suppresses the zonal jet generation. Here we study the instabilities of the Kolmogorov flow. We consider u_0=(0,sin x ) as a 2D incompressible flow. In the presence of a mean magnetic field, the dynamics are governed by the Navier–Stokes equations and the induction equation. We perform a classical linear analysis, in which growth rate, stability criteria, and MHD effects are derived. Instabilities are investigated associated with two magnetic field orientations, which can be x-directed (horizontal) or y-directed (vertical}) in our two-dimensional system to give an MHD version of Kolmogorov flow. In a basic equilibrium state magnetic field lines are straight for the case of vertical field and sinusoidal for horizontal field with an additional component of the external force balancing the resulting Lorentz force

    Fluidic Nozzles for Automotive Washer Systems: Computational Fluid Dynamics and Experimental Analysis

    Get PDF
    One of the main goals of this project was to cultivate an understanding of fluidic nozzle geometries and characteristic flow. Through this knowledge, three new fluidic nozzle concepts were developed to be used as components in several windscreen washer systems for an automotive part supplier, Kautex Textron CVS Ltd.Accurate and conclusive visualisation of flow through fluidic nozzles was vital in understanding how they can be best utilised for different applications. Over the past century, the specific needs of automotive cleaning systems have greatly developed with new technological discoveries, these advances allow the driver further knowledge of their surroundings. These specialised systems each require a different type of maintenance and cleaning system depending on their usage and the different size and shape of the vehicle. By completing this project, it is hoped to allow manufacturers to accurately identify what sort of fluidic nozzles are best for windscreen cleaning systems for a vehicle and how to design a nozzle to suit their specification. Fluidic nozzles have been researched experimentally and computationally to ensure an accurate comparison of results. By guaranteeing a precise comparison it will negate the need for high volume testing of nozzles in experimental situations, greatly reducing time and resources required to analyse a fluidic nozzle.The fluidic nozzles that are investigated and developed in this project were modelled and examined both experimentally and computationally, this ensured valid and accurate results were achieved by both the computational modelling and experimental testing. The development of the nozzles within this project was conducted using several experimental and computational setups to analyse the spray distribution, angle and oscillatory frequency amongst other parameters significant to the nozzle usage on a vehicle. Through this it was possible to tailor nozzle dimensions to allow for a streamlined design approach, this increased efficiency in fluidic nozzle development for any specification given by a vehicle manufacturing company customer. In addition to this the water flow emitted from the outlet was experimentally tested and modelled with both stationary and high surrounding velocities to examine how external variables affect the flow of the water from the nozzle.iiiThis project has been useful in the design manufacturing process of fluidic nozzles, by utilising computational modelling it has allowed a faster and cheaper method of analysing the effect of design alterations to fluidic nozzles. There is a greatly reduced frequency required for rapid prototyping of an array of fluidic chips with minimal dimensional differences to be used in the experimental stages of design, as once the inlet boundary conditions are established the nozzle can be redesigned completely within reason without the need for additional material wastage. This ensures a more easy and precise method of testing the manufacturing tolerances of a fluidic nozzle with a target of reaching customer specifications are always achieved.Three nozzles were aimed developed to satisfy conditions set by the customers, the vehicle manufacturers at which the new nozzle designs are aimed at are Honda, Nissan and Toyota. The nozzles to be established were designed for use on windscreen washer systems with a varying number of nozzles and with diverse windscreen sizes for different vehicles, resulting in a wide variety of specifications that must be met for each vehicle manufacturer. This meant that a single nozzle could not be utilised for all vehicles, instead a base model of fluidic chip was developed for the Nissan vehicle which was then dimensionally changed to suit the other vehicles.Throughout this project there were design specifications changes and ambiguities from the automotive company customers, leading to redesigns of the fluidic chips designed in this project. This means that although only two of the three fluidic nozzle designs are successfully in production, a much greater understanding of the mechanics of the fluid flow within the fluidic nozzle was achieved

    Linear stability and numerical analysis of dipolar vortices and topographic flows

    Get PDF
    The linear stability and numerical analysis of geophysical flow patterns is carried out on the beta-plane in a quasigeostrophic approximation. We consider initial steady state dipoles in a one-and-a-half-layer model that are capable of zonal drift in either direction. Despite previous numerical works suggesting that eastward propagating dipoles are stable, our high resolution simulations identify the spontaneous symmetry breaking of weak dipoles over time. The evolution is associated with a growing critical mode with even symmetry about the zonal axis. On carrying out a linear stability analysis, the critical modes obtained share consistency with the numerical fields. In addition, both methods of analysis show that the linear growth rate is inversely proportional to the dipole intensity. Furthermore, the partner separation becomes more pronounced after the linear growth stage, suggesting that nonlinear effects play a pivotal role in the underlying dynamics. Beyond this, the dynamics of initially tilted dipoles and dipole-rider solutions are considered, while stronger dipoles are further analysed using the method of distillation. Flows over sinusoidal bottom relief are considered in a two-layer model on the quasigeostrophic beta-plane. Fourier mode solutions are assumed for the layer-wise perturbation field in order to carry out a linear stability analysis, from which a coupled eigenproblem is derived between fluid columns for both zonal and meridional bottom irregularities. The presence of zonally oriented multiple ridges stabilises an otherwise unstable homogeneous zonal current with respect to increases in the number of ridges and ridge amplitude. Moreover, a bifurcation occurs in the unstable mode spectra and is dependent on the number of ridges. The critical eigenmodes in this case are found to be eddy chains of alternating sign, and these share remarkable resemblance with those obtained numerically. Meridionally oriented multiple ridges are also considered, but are found not to affect the maximum growth rate directly.Open Acces

    Development, Implementation, and Optimization of a Modern, Subsonic/Supersonic Panel Method

    Get PDF
    In the early stages of aircraft design, engineers consider many different design concepts, examining the trade-offs between different component arrangements and sizes, thrust and power requirements, etc. Because so many different designs are considered, it is best in the early stages of design to use simulation tools that are fast; accuracy is secondary. A common simulation tool for early design and analysis is the panel method. Panel methods were first developed in the 1950s and 1960s with the advent of modern computers. Despite being reasonably accurate and very fast, their development was abandoned in the late 1980s in favor of more complex and accurate simulation methods. The panel methods developed in the 1980s are still in use by aircraft designers today because of their accuracy and speed. However, they are cumbersome to use and limited in applicability. The purpose of this work is to reexamine panel methods in a modern context. In particular, this work focuses on the application of panel methods to supersonic aircraft (a supersonic aircraft is one that flies faster than the speed of sound). Various aspects of the panel method, including the distributions of the unknown flow variables on the surface of the aircraft and efficiently solving for these unknowns, are discussed. Trade-offs between alternative formulations are examined and recommendations given. This work also serves to bring together, clarify, and condense much of the literature previously published regarding panel methods so as to assist future developers of panel methods

    Optical Measurement of Airborne Particles on Unmanned Aircraft

    Get PDF
    Aerosols and clouds are persistent causes of uncertainty in climate and weather models, which is due to their heterogeneous suspension and occurrence within the atmosphere, and complex interactions which are chaotic and exist on small scales. Unmanned aerial vehicles (UAVs) have grown in popularity, and are becoming more commonly used for general atmospheric measurement, particularly measurement of aerosols and clouds. This thesis presents and evaluates a synergy between two UAVs, a multi-rotor: the UH-AeroSAM octocopter and a fixed-wing: the FMI-Talon, and an optical particle instrument: the Universal Cloud and Aerosol Sounding System. Computational fluid dynamics with Lagrangian particle tracking (CFD-LPT) was used as a tool for the characterisation of the velocity fields and particle trajectories around both UAVs. In both instances CFD-LPT was used to develop an operational envelope, with particular attention to angle of attack constraints and size distribution perturbation, for the UAV – instrument synergy. UCASS was the first open path instrument to be used on a UAV, and a good case has been made for its continued use, particularly on fixed-wing UAVs, which exhibit less complex aerodynamics and superior stability in the induced sampling airflow through the instrument

    A new ghost-cell/level-set method for three-dimensional flows

    Get PDF
    This study presents the implementation of a tailored ghost cell method in Hydro3D, an open-source large eddy simulation (LES) code for computational fluid dynamics based on the finite difference method. The former model for studying the interaction between an immersed object and the fluid flow is the immersed boundary method (IBM) which has been validated for a wide range of Reynolds number flows. However, it is challenging to ensure no-slip and zero gradient boundary conditions on the surface of an immersed body. In order to deal with this, a new sharp-interface ghost-cell method (GCM) is developed for Hydro3D. The code also employs a level-set method to capture the motion of the air-water interface and solves the spatially filtered Navier-Stokes equations in a Cartesian staggered grid with the fractional step method. Both the new GCM and IBM are compared in a single numerical framework. They are applied to simulate benchmark cases in order to validate the numerical results, which mainly comprise single-phase flow over infinite circular and square cylinders for low- and high-Reynolds number flows along with two-phase dam-break flows with a vertical cylinder, in which a good agreement is obtained with other numerical studies and laboratory experiments

    Dynamical description of spatio-temporally varying turbulent energy cascades

    Get PDF
    The spatio-temporally varying turbulent energy cascade dynamics in forced homogeneous/periodic turbulence is investigated with direct numerical simulations (DNS) and Helmholtz decompositions. The local in space and time cascade dynamics vastly differs from its spatio-temporal average manifestation. At scales larger than the Taylor scale, the solenoidal interscale transfer at most locations at most times increases or decreases the energy at the given scale in the frame moving with larger scales, i.e. Lagrangian transport. The solenoidal interscale transfer derives from the non-local in space vortex stretching/compression and tilting effects of its spatial vicinity. The irrotational cascade dynamics reduces to an exact balance between irrotational transport, irrotational interscale transfer and pressure-velocity. The typical fluctuations of these processes vastly exceed their spatio-temporal average values and the typical dissipation fluctuations. At scales below the Taylor scale, viscous effects increase in importance in the solenoidal dynamics. At the Kolmogorov scale solenoidal interscale transfer, Lagrangian transport and viscous effects are all important. In regions of low and moderate small-scale energy, and to a somewhat lesser extent in regions of high small-scale energy, there is rarely a local balance between interscale transfer and viscous effects. Lagrangian transport acts as a non-local in time and space link between interscale transfer and viscous effects. The spatially-averaged manifestation of the local cascade dynamics is an unsteady and approximately unidirectional energy cascade, which can be approximated with a hypothesis connecting the present interscale transfer with the future dissipation. The hypothesis can be used to develop non-equilibrium corrections to the low-pass filtered dynamics and second-order structure function scaling consistent with DNSs. We use the phenomenology of a time-lagged energy cascade to motivate a new redistributive dissipation scaling. The non-equilibrium dissipation scaling typically reduces to the redistributive dissipation scaling at low and moderate Reynolds numbers.Open Acces
    • …
    corecore