FRONTAL INSTABILITY OF LOCK-EXCHANGE GRAVITY CURRENTS

2010 ◽  
Vol 24 (13) ◽  
pp. 1369-1372 ◽  
Author(s):  
MING PENG ◽  
C. B. LEE
Keyword(s):  
Convective Instability ◽  
Precise Measurement ◽  
Vortex Breakdown ◽  
Gravity Currents ◽  
Flow Structures ◽  
Complex Structures ◽  
Three Dimension ◽  
Turbulent Structures ◽  
Planar Laser ◽  
Frontal Instability

In this work we address the frontal instability of gravity currents. The planar laser-induced fluorescence (PLiF) flow visualization is utilized to analyze the detailed dynamics of the current, which are generated in a lock-exchange Perspex tank. We believe that two dominant modes of instability determine the complex structures at the head of the flow. The first one resembles Kelvin–Helmholtz instability, which results in Kelvin–Helmholtz billows rolling up in the shear zone above the head. The other, categorized as convective instability known as "lobes and clefts", which stems from ground friction as well as unstable inverse density stratification, and is considered to be the cause for the disruption of the span-wise symmetry of Kelvin–Helmholtz billows. Moreover, our observations indicate that the convective instability also contributes to a secondary instability associated with Kelvin–Helmholtz vortex breakdown. These instabilities not only play a central role in shaping the three-dimension characteristics of the currents, but also govern the mixing and entrainment mechanisms. Therefore, more precise measurement of the positions of the frontal instability and the flow structures, especially the turbulent structures is indeed necessary.

Topological Flow Structures of Annular Swirling Jets

2001 ◽  
Vol 17 (3) ◽  
pp. 131-138
Author(s):  
Feng Chin Tsai ◽  
Rong Fung Huang
Keyword(s):  
Vortex Breakdown ◽  
Swirling Flow ◽  
Single Bubble ◽  
Flow Structures ◽  
Complex Flow ◽  
Swirl Number ◽  
Recirculation Bubble ◽  
Annular Jet ◽  
Ring Mode ◽  
Dual Ring

AbstractThe effects of blockage and swirl on the macro flow structures of the annular jet past a circular disc are experimentally studied through the time-averaged streamline patterns. In the blockage-effect regime, the flows present multiple modes, single bubble, dual rings, vortex breakdown, and triple rings, in different regimes of blockage ratio and swirl number. The topological models of the flow structures are proposed and discussed according to the measured flow fields to manifest the complex flow structures. The single bubble is a closed recirculation bubble with a stagnation point on the central axis. The dual-ring flow is an open-top recirculsation zone, in which a pair of counter-rotating vortex rings exists in the near wake. The fluids in the dual rings are expelled downstream through a central jet-like swirling flow. A vortex breakdown may occur in the central jet-like swirling flow if the exit swirl number exceeds critical values. When the vortex breakdown interacts with the dual rings, a complex triple-ring flow structure forms. Axial distributions of the local swirl number are presented and discussed. The local swirl number increases with the increase of the exit swirl number and attains the maximum in the dual-ring mode. At large exit swirl numbers where the vortex breakdown occurs, the local swirl number decreases drastically to a low value.

Visualization of intricate flow structures for vortex breakdown analysis

2005 ◽  
Author(s):  
X. Tricoche ◽  
C. Garth ◽  
G. Kindlmann ◽  
E. Deines ◽  
G. Scheuermann ◽  
...  
Keyword(s):  
Vortex Breakdown ◽  
Flow Structures

Visualization of Mach 3.0/3.8 Flow around Blunt Cone with Supersonic Film Cooling

2013 ◽  
Vol 419 ◽  
pp. 432-437
Author(s):  
Yang Zhu Zhu ◽  
Shi He Yi ◽  
Li Feng Tian ◽  
Lin He ◽  
Zhi Chen
Keyword(s):  
Film Cooling ◽  
Mixing Layer ◽  
Flow Structures ◽  
Spatiotemporal Resolution ◽  
Blunt Cone ◽  
Laser Scattering ◽  
Thin Film Layer ◽  
Planar Laser ◽  
Expansion Fan ◽  
Film Layer

Fine instantaneous flow structures of different scales around a blunt cone with or without supersonic film cooling were visualized via nanotracer planar laser scattering (NPLS), which has a high spatiotemporal resolution. The Mach number of the freestream is 3.0 and 3.8 respectively and the air injection is at Mach 2.5. Lots of typical flow structures were visible clearly, such as shock wave, expansion fan, shear layer, mixing layer, K-H vortices and turbulent boundary layer. With injection, the model wall surface can be covered by a thin film layer. While no injection, the flow is similar to the supersonic flow over a backward-facing step and the structures are simpler relatively and there is a longer laminar region. Flow structures with or without film cooling at Mach 3.0 and 3.8 were compared.

Effects of Swirler Configurations on Flow Structures and Combustion Characteristics

2004 ◽  
Author(s):  
Guoqiang Li ◽  
Ephraim J. Gutmark
Keyword(s):  
Fuel Injection ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Combustion Characteristics ◽  
Pollutant Emissions ◽  
Flow Structures ◽  
Inlet Temperature ◽  
Nox Emissions ◽  
Atmospheric Conditions ◽  
Fuel Injector

Modern gas turbine combustion technologies are driven by stringent regulations on pollutant emissions such as CO and NOx. A combustion system of multiple swirlers coupled with distributed fuel injection was studied as a new concept for reducing NOx emissions by application of Lean Direct Injection (LDI) combustion. The present paper investigates the effects of swirler configurations on the flow structures in isothermal flow and combustion cases using a multiple-swirlers fuel injector at atmospheric conditions. The swirling flow field within the combustor was characterized by a central recirculation zone formed after vortex breakdown. The differences between the tangential and axial velocity profiles, the shape of the recirculation zones and the turbulence intensity distribution for the different fuel injector configurations impacted the flame structure, the temperature distribution and the emission characteristics both for gaseous and liquid fuels. Co-swirling configuration was shown to have the lowest NOx emission level compared with the counter-swirling ones for both types of fuels with lower inlet temperature. In contrast to this, the swirl configuration had less effect on the combustion characteristics in the case of gaseous fuel with high air inlet temperature. The differences in NOx emissions were shown to be closely related to the Damkohler number or the degree to which the flame resembled well-mixed combustion, which is the foundation for LDI combustion.

scholarly journals Turbulent structures in planar gravity currents and their influence on the flow dynamics

2008 ◽  
Vol 113 (C8) ◽  
Author(s):  
Mariano I. Cantero ◽  
S. Balachandar ◽  
Marcelo H. García ◽  
David Bock
Keyword(s):  
Gravity Currents ◽  
Flow Dynamics ◽  
Turbulent Structures

Hydrodynamic Forces and Flow Structures in Flow Past a Cylinder Forced to Vibrate Transversely and Inline to a Steady Flow

2015 ◽  
Vol 138 (1) ◽  
Author(s):  
S. Peppa ◽  
L. Kaiktsis ◽  
G. S. Triantafyllou
Keyword(s):  
Strong Dependence ◽  
The Body ◽  
Flow Structures ◽  
Complex Structures ◽  
Power Transfer ◽  
Positive Power ◽  
Flow Past ◽  
Flow Past A Cylinder ◽  
Two Dimensional Flow ◽  
Oscillation Frequencies

This paper reports computational results of forces and wake structure in two-dimensional flow past a circular cylinder forced to vibrate both transversely and inline to a uniform stream, following a figure-eight trajectory. For a flow stream from left to right, we distinguish between a counterclockwise mode and a clockwise mode, if the upper part of the trajectory is traversed counterclockwise or clockwise, respectively. The present computations correspond to a range of transverse oscillation frequencies close to the natural frequency of the Kármán vortex street and several oscillation amplitudes, both for counterclockwise motion and clockwise motion. The nondimensional forces and nondimensional power transfer from the fluid to the body are calculated. The results demonstrate a strong dependence of the forces and power transfer on the direction in which the figure-eight is traversed. In general, counterclockwise motion maintains positive power transfer at higher oscillation amplitudes. Flow visualizations show that the wakes are characterized by the presence of two single (2S) vortex shedding mode at low oscillation amplitudes and can attain more complex structures at higher amplitudes.

scholarly journals Flow structures in a swirl flow - vortex breakdown condition

2018 ◽  
Vol 1045 ◽  
pp. 012031
Author(s):  
P Novotny ◽  
B Weigand ◽  
F Marsik ◽  
C Biegger ◽  
M Tomas
Keyword(s):  
Vortex Breakdown ◽  
Swirl Flow ◽  
Flow Structures ◽  
Flow Vortex

Mechanisms of the Nonlinear Response of a Swirl Flame to Harmonic Excitation

2007 ◽  
Author(s):  
Sai Kumar Thumuluru ◽  
Mohan K. Bobba ◽  
Tim Lieuwen
Keyword(s):  
Vortex Breakdown ◽  
Separation Bubble ◽  
Turbulent Flame ◽  
Harmonic Excitation ◽  
Forced Oscillations ◽  
Turbulent Flame Speed ◽  
Annular Jet ◽  
Planar Laser ◽  
Flame Response ◽  
Swirling Flame

This paper describes an experimental investigation of the amplitude dependent processes controlling the response of a swirling flame to harmonic excitation. Simultaneous measurements of the acoustic pressure, velocity, and CH* and OH* radical chemiluminescence were obtained over a range of forcing frequencies, amplitudes, and nozzle exit velocities. These were supplemented with OH planar laser induced fluorescence (PLIF) measurements at several representative conditions. The results presented show that there exist substantial nonlinearities in the flame response to forced oscillations. Furthermore, while the flame response monotonically and linearly increases with perturbation amplitude for low forcing levels, its behavior becomes much more complex at higher levels — this includes saturation as well as more complex, non-monotonic behaviors. Analysis of the OH PLIF images show that the observed dynamics result from a superposition of at least five flame/flow processes — (1) the oscillating velocity of the annular jet, oscillations in (2) position and (3) strength of the vortex breakdown bubble and separation bubble, (4) unsteady liftoff of the flame, and (5) an oscillating turbulent flame speed. These processes generally occur simultaneously, with non-monotonic dependencies upon forcing amplitude. This sheds some light on the complex overall flame heat release response measurements and suggests a fruitful area for detailed computational studies that can better elucidate the underlying physics controlling the phenomenon.

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