Influence of coaxial cylinders on vortex breakdown in a closed flow (original) (raw)

Influence of coaxial cylinders on the vortex breakdown in a closed flow

European Journal of …, 2010

The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is studied experimentally and numerically. We find that the VB is enhanced for very small values of the rod radius d, while it is suppressed for values of d beyond a critical value. In order to characterize this effect, the critical Reynolds number for the appearance of vortex breakdown as a function of the radius of the fixed rods and the different aspect ratios was accurately determined, using digital particle image velocimetry. The numerical and experimental results are compared showing an excellent agreement. In addition, a simple model in order to show that this effect also appears in open pipe flows is presented in the appendix.

An Investigation on Vortex Breakdown Phenomena in a Vertical Cylindrical Tube

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.

Control of Vortex Breakdown Phenomenon in Confined Flows

The effect of stationary coaxial rod on the vortex breakdown (VB) in a closed flow is studied numerically. The flow takes place in a cylindrical container with a rotating top end wall. The dimensionless numbers characterizing the system dynamics are the rotational Reynolds number (Re) based on the angular velocity of the rotating end wall, the aspect ratio (H/R) of the container and rod radius ratio (R r /R).The rod dimensions and aspect ratios are varied to obtain a close understanding of the internal flow structure and the recirculation zone of the vortex breakdown. We find that the onset of VB is a function of the rotational Reynolds number and the radius of the rod.

A coaxial vortex ring model for vortex breakdown

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.

Vortex Breakdown in a Closed Cylindrical Container with a Rotating or Stationary Conical Endwall

Journal of Fluid Science and Technology, 2010

A visualization study is made of flow of a constant-density viscous fluid in a closed cylindrical container. Fluid motions are generated by the rotation of one of its endwalls. Vortex breakdown bubble(s) in the meridional plane are visualized by using a high-precision turntable rig. One endwall is a flat disk, and the other endwall is a cone. Cones of inclination angle α = 0°(flat disk), 30°, 45°and 60° are employed in the experiment. The visualization photographs, produced by employing fluorescent dye technique, reveal the vortex breakdown characteristics. The breakdown regime diagrams in the aspect ratio-Reynolds number plots are constructed. The changes incurred in the locations and sizes of the breakdown bubble(s) are elaborated. Based on the visualized flow data, plausible physical interpretations are offered.

The role of non-uniqueness in the development of vortex breakdown in tubes

Journal of Fluid Mechanics, 1992

Numerical solutions of viscous, swirling flows through circular pipes of constant radius and circular pipes with throats have been obtained. Solutions were computed for several values of vortex circulation, Reynolds number and throat/inlet area ratio, under the assumptions of steady flow, rotational symmetry and frictionless flow at the pipe wall. When the Reynolds number is sufficiently large, vortex breakdown occurs abruptly with increased circulation as a result of the existence of non-unique solutions. Solution paths for Reynolds numbers exceeding approximately 1000 are characterized by an ensemble of three inviscid flow types: columnar (for pipes of constant radius), soliton and wavetrain. Flows that are quasi-cylindrical and which do not exhibit vortex breakdown exist below a critical circulation, dependent on the Reynolds number and the throat/inlet area ratio. Wavetrain solutions are observed over a small range of circulation below the critical circulation, while above the c...

Control of vortex breakdown in a closed cylinder with a small rotating rod

Journal of Fluids and Structures, 2008

Effective control of a vortex breakdown was achieved with small rotating rods. This study was performed experimentally and numerically with high precision. After validation with accurate measurements using a novel SPIV technique, analysis of numerical simulations has been undertaken. The effect of a finite partial rod creates additional source terms in the vorticity as it rotates. These additional source terms and their spatial location influence the occurrence of the vortex breakdown.

Axisymmetric Vortex Breakdown in a Pipe

Esaim Proceedings, 1996

A new apparatus, capable of generating and controlling swirling flows in a pipe, has been designed and built in order to verify a recent theory by Wang and Rusak( Wang, S. and Rusak, Z. RPI Aero. Eng. Report) 107, 1995 on the axisymmetric vortex breakdown in a pipe. A key element in the theory is the existence of two stable solutions in the ω0 <ω < ω1 range, which suggests a hysteresis loop in the crossover between solutions while the swirl ratio ω is changed in this range. This idea has been demonstrated experimentally for the first time, using Laser Doppler Velocimetry (LDV) measurements combined with flow visualization. An additional verification of the theory is carried out experimentally by demonstrating that even a very large perturbation to the flow just below the threshold level ω0 would not permanently change the flow from being columnar, while the same disturbance to the flow just above this threshold results in vortex breakdown. The measured LDV inlet velocity profiles to a pipe have been used in flow simulations and good agreement between the theoretical predictions and the experimental results is found.

A simple mechanism for controlling vortex breakdown in a closed flow

2000

Received: date / Accepted: date Abstract This work is focused to study the development and control of the laminar vortex breakdown of a flow en- closed in a cylinder. We show that vortex breakdown can be controlled by the introduction of a small fixed rod in the axis of the cylinder. Our method to control the onset of vortex breakdown