A theoretical approach to the passive control of spiral vortex breakdown

2012 ◽  
Author(s):  
Ubaid A. Qadri ◽  
Matthew P. Juniper
Keyword(s):  
Theoretical Approach ◽  
Passive Control ◽  
Vortex Breakdown ◽  
Spiral Vortex

Structural sensitivity of spiral vortex breakdown

2013 ◽  
Vol 720 ◽  
pp. 558-581 ◽  
Author(s):  
Ubaid Ali Qadri ◽  
Dhiren Mistry ◽  
Matthew P. Juniper
Keyword(s):  
Stability Analysis ◽  
Passive Control ◽  
Stokes Equations ◽  
Vortex Breakdown ◽  
Control Strategies ◽  
Axial Component ◽  
Azimuthal Mode ◽  
Global Mode ◽  
Structural Sensitivity ◽  
Spiral Vortex

AbstractPrevious numerical simulations have shown that vortex breakdown starts with the formation of a steady axisymmetric bubble and that an unsteady spiralling mode then develops on top of this. We investigate this spiral mode with a linear global stability analysis around the steady bubble and its wake. We obtain the linear direct and adjoint global modes of the linearized Navier–Stokes equations and overlap these to obtain the structural sensitivity of the spiral mode, which identifies the wavemaker region. We also identify regions of absolute instability with a local stability analysis. At moderate swirls, we find that the $m= - 1$ azimuthal mode is the most unstable and that the wavemaker regions of the $m= - 1$ mode lie around the bubble, which is absolutely unstable. The mode is most sensitive to feedback involving the radial and azimuthal components of momentum in the region just upstream of the bubble. To a lesser extent, the mode is also sensitive to feedback involving the axial component of momentum in regions of high shear around the bubble. At an intermediate swirl, in which the bubble and wake have similar absolute growth rates, other researchers have found that the wavemaker of the nonlinear global mode lies in the wake. We agree with their analysis but find that the regions around the bubble are more influential than the wake in determining the growth rate and frequency of the linear global mode. The results from this paper provide the first steps towards passive control strategies for spiral vortex breakdown.

A Study of Unsteady Secondary Flow in a Water Flow Axial Turbine Model

2008 ◽  
Author(s):  
Christian Kasper ◽  
Martin G. Rose ◽  
Stephan Staudacher ◽  
Jochen Gier
Keyword(s):  
Flow Visualization ◽  
Vortex Breakdown ◽  
Water Channel ◽  
Leading Edge ◽  
Secondary Flows ◽  
Stagnation Region ◽  
Spiral Vortex ◽  
Passage Vortex ◽  
High Turbulence ◽  
First Time

The influence of secondary flows on the performance of turbines has been investigated in great detail in the last decades. The interaction of vortices with following blade rows has been identified to be one of the loss mechanisms within a turbo-machine. This paper presents for the first time detailed flow visualization photographs of the interaction of the vane passage vortex with the rotor. The appearance vortex breakdown could be identified before and within the rotating passage of the turbine. The measurements were taken in a vertical water channel. Water is used instead of air because the flow visualization can be realised very easily with injected ink. For different relative positions of rotor to stator a series of photographs were taken. With an image editing process the average and the pixel RMS were calculated for each relative position. The pixel RMS is a useful indicator to identify highly turbulent regions in the flow field. The photographs of the vortex breakdown show spots of high pixel RMS which are associated with very high turbulence and therefore can be regarded as sources of loss. Insight is gained into the nature of the passage vortex breakdown mechanisms as follows: first the pressure wave of the rotor stretches the vortex causing a spiral vortex instability, then the vortex interacts with the leading edge as it attempts to cut the vortex. In the stagnation region of the blade a bubble type instability forms, expands and then convects through the rotor. The absolute trajectory of the vortex fluid reveals that it exchanges no work with the rotor.

The Effectiveness of Passive Combustion Control Methods

2004 ◽  
Author(s):  
Christian Oliver Paschereit ◽  
Ephraim Gutmark
Keyword(s):  
Passive Control ◽  
Vortex Breakdown ◽  
Flame Stabilization ◽  
Sudden Expansion ◽  
Recirculation Region ◽  
Control Methods ◽  
Diffusion Type ◽  
Recirculating Flow ◽  
Wide Range ◽  
Periodic Heat

Flame stabilization in a swirl-stabilized combustor occurs in an aerodynamically generated recirculation region which is a result of vortex breakdown. The characteristics of the recirculating flow are dependent on the swirl number and on axial pressure gradients. Coupling to downstream pressure pulsations is also possible. Control methods of unstable thermoacoustic modes and reduction of NOx and CO emissions were investigated in a low-emission swirl-stabilized industrial combustor. Several axisymmetric and helical unstable modes were identified for fully premixed and diffusion type combustion. In addition to mode variation, the instabilities spanned a wide range of frequencies. The unstable modes that were associated with flow instabilities of the wake-like region on the combustor axis due to vortex breakdown (VBD), shear layer instabilities at the sudden expansion (dump plane) and equivalence ratio fluctuations were in a range of normalized frequency St = 0.5–1.1. Other unstable modes at higher frequencies of St = 7.77, were excited by the Kelvin-Helmholtz vortices shed at the burner’s exit. The combustion structures associated with the different unstable modes were visualized using phase locked images of OH chemiluminescence and analyzed using cross-correlations between OH detecting fiberoptics. After identifying the structure of the instabilities and determining their source, different geometrical changes were applied to disrupt their formation or vary their characteristics. These modifications reduced the periodic heat release and enabled decoupling of the heat from acoustic modes that led to thermoacoustic instabilities. The passive control techniques that will be described in this paper were effective in suppressing the thermoacoustic pressure oscillations and also reduced NOx and CO emissions.

Shaping of Tip End-Plate to Control Leakage Vortex Swirl in Axial Flow Fans

2008 ◽  
Author(s):  
Alessandro Corsini ◽  
Franco Rispoli ◽  
A. G. Sheard
Keyword(s):  
Rotation Number ◽  
Passive Control ◽  
Noise Control ◽  
Vortex Breakdown ◽  
Effective Means ◽  
Plate Thickness ◽  
Axial Flow ◽  
End Plate ◽  
Blade Tip ◽  
Passive Techniques

This paper reports on quantitative tests of passive techniques for rotor-tip noise control in low-speed axial flow fans, based on blade-tip modifications involving the addition of anti-vortex appendages as end-plates. The end-plate thickness chord-wise distribution is determined to control the chord-wise evolution of the leakage vortex rotation number. The results confirm that the new end-plate configurations provide a mechanism by which leakage vortex bursting can be avoided. As such, the modified rotors represent an effective means of passive control of vortex breakdown.

Water flow model turbine flow visualization study of the unsteady interaction of secondary flow vortices with the downstream rotor

2009 ◽  
Vol 223 (6) ◽  
pp. 677-686 ◽  
Author(s):  
L Aurahs ◽  
C Kasper ◽  
M Kürner ◽  
M G Rose ◽  
S Staudacher ◽  
...  
Keyword(s):  
Flow Visualization ◽  
Vortex Breakdown ◽  
Water Channel ◽  
Axial Flow ◽  
Horseshoe Vortex ◽  
Spiral Vortex ◽  
Passage Vortex ◽  
Unsteady Interaction ◽  
Computational Fluid Dynamics Cfd ◽  
Cfd Analyses

This article presents detailed flow visualization photographs, root mean square processed photography, and computational fluid dynamics (CFD) results of the interaction of the vane passage vortex and horseshoe vortex with the rotor of an axial flow turbine model. Different modes of vortex breakdown behaviour have been experimentally observed inside the rotating passage of the turbine blade. These are spiral vortex mode and bubble mode breakdown. The breakdown mode changes as the vortices are influenced by the periodic pressure field of the rotor. The measurements were taken in a vertical water channel with ink injection for flow visualization. Unsteady CFD analyses have been made with some success in prediction of the unsteady flow structures. In particular, the pre-instability behaviour of the passage vortex in the experiments matches the results of the numerical investigations.

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