Control of Vortex Breakdown Phenomenon in Confined Flows

Journal of Fluid Mechanics, 2014

Experiments were conducted to determine the effectiveness of controlling vortex breakdown in a confined cylindrical vessel using a small rotating disk, which was flush-mounted into the opposite endwall to the rotating endwall driving the primary recirculating flow. The results show that the control disk, with relatively little power input, can modify the azimuthal and axial flow significantly, changing the entire flow structure in the cylinder. Co-rotation was found to precipitate vortex breakdown onset whereas counter-rotation delays it. Furthermore, for the Reynolds-number range over which breakdown normally exists, co-rotation increases the bubble radial and axial dimensions, while shifting the bubble in the upstream direction. By contrast, counter-rotation tends to reduce the size of the bubble, or completely suppress it, while shifting the bubble in the downstream direction. These effects are amplified substantially by the use of larger control disks and higher rotation ratios. A series of numerical simulations close to the onset Reynolds number reveals that the control disk acts to generate a rotation-rate-invariant local positive or negative azimuthal vorticity source away from the immediate vicinity of the control disk but upstream of breakdown. Advection of this source along streamlines modifies the strength of the azimuthal vorticity ring, which effectively controls whether the flow reverses on the axis, and thus, in turn, whether vortex breakdown occurs. The vorticity source generated by the control disk scales approximately linearly with rotation ratio and cubically with disk diameter; this allows the observed variation of the critical Reynolds number to be approximately predicted.

Journal of King Abdulaziz University-Engineering Sciences, 2010

Vortex flows are subject to a number of major structural changes involving very large disturbances when a characteristic ratio of azimuthal to axial velocity components is varied. Vortex breakdowns are among the structural forms that may occur. This phenomenon is one of the hydrodynamic instability problems and it is encountered in many practical application, such as, aerodynamics (in aeronautics), combustion chamber, diffusers and nozzle. The objectives of the present research are to investigate the effect of Reynolds number (Re) and circulation number (Ω) on the observed modes of the vortex breakdown. The position of the vortex breakdown with two vertical cylinder tube length as well as for clockwise and anticlockwise flow direction is also investigated. The results revealed that there are a total of six distinct modes of the disruption of the vortex core as Re and Ω of the flow were varied. The breakdown position was found to be dependent on both Re and Ω of the flow. Whereas, for all Re values an increase in Ω always results in moving the breakdown position upstream for all Re values. The breakdown position is smaller for anticlockwise flow direction than that, when vanes were set at clockwise flow direction for long and short tube.

Journal of Fluid Science and Technology, 2007

Numerical simulation is performed for the vortex breakdown flows induced in a cylindrical container with rotating top disk. Solutions are obtained for the parameter ranges where steady axisymmetric flows are stable with respect to asymmetric disturbances. By varying the number of grid points and applying the Richardson extrapolation, resolution independent solutions are obtained and flow state diagram is numerically constructed on (h, Re)-plane (Re is the Reynolds number and h is the cylinder aspect ratio). The value of critical Reynolds number for the creation of breakdown bubbles agrees on the average within 1% discrepancy with the previous experiments. Solutions obtained in the present study are expected to be useful as a benchmark solution visa -vis newly developed numerical methods are compared.

Physics of Fluids, 2004

As part of a program to investigate the effectiveness of vortex breakdown in bioreactors for cell and tissue growth, a nonintrusive method of flow control is presented. A small rotating disk flush-mounted opposite to the rotating lid in a confined cylindrical vessel is found to precipitate or delay the onset of vortex breakdown depending on whether it is corotating or counterrotating, respectively. Furthermore, corotation increases the bubble radial and axial dimensions while shifting the bubble in the upstream direction. By contrast, counterrotation tends to reduce the size of the bubble, or completely suppress it, while shifting the bubble in the downstream direction. It has also been shown that corotating swirl addition using the small disk is orders of magnitude more energy efficient in manipulating the vortex breakdown bubble than using end wall rotation.

Physics of Fluids, 2012

This numerical study of the steady axisymmetric motion of a viscous incompressible fluid in a sealed cylindrical container with one end wall rotating reveals that swirl decay, induced by friction at the sidewall, plays an important role in the development of vortex breakdown (VB). When the flow is slow, it is multi-cellular. As the flow strength increases (i) meridional circulation becomes global, (ii) flow convergence toward the axis focuses near the still end wall, (iii) a few local minima of pressure appear, (iv) a few flow reversals occur near the axis, and (v) circulation regions merge and an elongated double counterflow develops. Stages (i)-(v) are common for a number of vortex devices. If the swirl decay is diminished by additional rotation of the sidewall, VB disappears.

Physica D-nonlinear Phenomena, 2008

A simple -yet plausible -model for B-type vortex breakdown flows is postulated; one that is based on the immersion of a pair of slender coaxial vortex rings in a swirling flow of an ideal fluid rotating around the axis of symmetry of the rings. It is shown that this model exhibits in the advection of passive fluid particles (kinematics) just about all of the characteristics that have been observed in what is now a substantial body of published research on the phenomenon of vortex breakdown. Moreover, it is demonstrated how the very nature of the fluid dynamics in axisymmetric breakdown flows can be predicted and controlled by the choice of the initial ring configurations and their vortex strengths. The dynamic intricacies produced by the two ring + swirl model are illustrated with several numerical experiments.

Confined steady swirling flows, driven by the end disks of a cylindrical/truncated conical enclosure have been numerically studied. Particular attention is focused on combined kinematics and geometric conditions of generation and control of the vortex breakdown phenomenon. First, the basic steady flow topology in a truncated conical cavity is described, which is shown to depend strongly on the direction as well as the rate of rotation of the end disks. For a set of governing flow parameters, the computations revealed the occurrence of bubble-like reverse flows, characterised by on axis stagnation oints. The present work, explores means of controlling the evolution of this physical phenomenon, by modifying the boundary conditions upstream the vortex breakdown. These means are found to either suppress or enhance the occurrence and size of the bubbles.

Applied Mathematical Modelling, 1992

The axisymmetric vortex breakdown of a laminar, incompressible, viscous, and swirling flow in a cylindrical tube is simulated. The numerical solutions of the time-dependent axisymmetric angular momentum and the vorticity transport equations are obtained by using the jmite element method. Linear and quadratic isoparametric elements are used for the space discretization, while the leapfrog time integration scheme is employed for the time discretization. The iteration process is started with the numerical solution of the linearized equations as the Reynolds number approaches zero. As the Reynolds number is changed, the numerical solution proceeds from one steady state to another. The numerical solutions are obtainedfor different initial velocity projZes and a range of Reynolds numbers and swirl ratios. The numerical solutions are compared with experimental data and prior finite difference studies.

International Journal of Thermal Sciences, 2020

Vortex breakdown plays a central role in the performance of countless rotating machinery applications, many of which contain thermal gradients either inadvertently or by design. The effect of thermal gradients on vortex breakdown and further flow development in a cylindrical domain with a rotating bottom plate is examined using the Generalized Integral Transformation Technique (GITT) with a streamfunction-only formulation. A thermal gradient is imposed in the axial direction, such that the buoyancy forces oppose the base flow driven by the rotation of the lower plate, i.e. the temperature difference acts to stabilize the flow. The hybrid numericalanalytical approach is shown to accurately capture vortex breakdown phenomena for a variety of conditions involving single, double and triple recirculation bubbles. The buoyancy forcesexpressed in terms of the Richardson number (Ri)-act to suppress vortex breakdown in all cases examined and led to a series of flow transitions with increasing Ri, characterized by the appearance of a stratified structure with multiple fluid layers. These flow transitions have a significant impact on the overall performance of the system. The torque coefficient decreases with Ri, compared to the base (isothermal) case following an empirical power law relationship, which is independent of Reynolds number, aspect ratio or number of fluid layers present. Flow stratification suppresses the transport of angular momentum; azimuthal velocity is shown to decline exponentially in the regions where layering occurs, accompanied by a sharp reduction in the Nusselt number, as fluid layers act to insulate the upper plate.