Corewise cross‐flow transport in hairpin vortices—The ‘‘tornado effect’’

1993 ◽  
Vol 5 (12) ◽  
pp. 3167-3174 ◽  
Author(s):  
J. P. Hagen ◽  
M. Kurosaka
Keyword(s):  
Cross Flow ◽  
Hairpin Vortices ◽  
Flow Transport

scholarly journals Flow past a square-section cylinder with a wavy stagnation face

2001 ◽  
Vol 426 ◽  
pp. 263-295 ◽  
Author(s):  
RUPAD M. DAREKAR ◽  
SPENCER J. SHERWIN
Keyword(s):  
Reynolds Number ◽  
Cross Flow ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Physical Structure ◽  
Near Wake ◽  
Hairpin Vortices ◽  
Independent State ◽  
Flow Past ◽  
Edge Surface

Numerical investigations have been performed for the flow past square-section cylinders with a spanwise geometric deformation leading to a stagnation face with a sinusoidal waviness. The computations were performed using a spectral/hp element solver over a range of Reynolds numbers from 10 to 150.Starting from fully developed shedding past a straight cylinder at a Reynolds number of 100, a sufficiently high waviness is impulsively introduced resulting in the stabilization of the near wake to a time-independent state. It is shown that the spanwise waviness sets up a cross-flow within the growing boundary layer on the leading-edge surface thereby generating streamwise and vertical components of vorticity. These additional components of vorticity appear in regions close to the inflection points of the wavy stagnation face where the spanwise vorticity is weakened. This redistribution of vorticity leads to the breakdown of the unsteady and staggered Kármán vortex wake into a steady and symmetric near-wake structure. The steady nature of the near wake is associated with a reduction in total drag of about 16% at a Reynolds number of 100 compared with the straight, non-wavy cylinder.Further increases in the amplitude of the waviness lead to the emergence of hairpin vortices from the near-wake region. This wake topology has similarities to the wake of a sphere at low Reynolds numbers. The physical structure of the wake due to the variation of the amplitude of the waviness is identified with five distinct regimes. Furthermore, the introduction of a waviness at a wavelength close to the mode A wavelength and the primary wavelength of the straight square-section cylinder leads to the suppression of the Kármán street at a minimal waviness amplitude.

scholarly journals Dynamical Chaos and Lateral Transport of a Passive Scalar in the Annular Reverse Jet Flow

2021 ◽  
Vol 17 (3) ◽  
pp. 263-274
Author(s):  
V. P. Reutov ◽  
◽  
G. V. Rybushkina ◽  
Keyword(s):  
Lyapunov Exponent ◽  
Cross Flow ◽  
Passive Scalar ◽  
Jet Flow ◽  
Wave Generation ◽  
Critical Parameters ◽  
Dynamical Chaos ◽  
Lateral Transport ◽  
Frequency Spectra ◽  
Flow Transport

The transition to dynamical chaos and the related lateral (cross-flow) transport of a passive scalar in the reverse annular jet flow generating two chains of wave-vortex structures are studied. The quasi-geostrophic equations for the barotropic (quasi-two-dimensional) flow written in polar coordinates with allowance for the beta-effect and external friction are solved numerically using a pseudospectral method. The critical parameters of the equilibrium flow with a complex “two-hump” azimuth velocity profile facilitating a faster transition to the complex dynamics are determined. Two regular multiharmonic regimes of wave generation are revealed with increasing flow supercriticality before the onset of Eulerian chaos. The occurrence of the complex flow dynamics is confirmed by a direct calculation of the largest Lyapunov exponent. The evolution of streamline images is analyzed by making video, thereby chains with single and composite structures are distinguished. The wavenumber-frequency spectra confirming the possibility of chaotic transport of the passive scalar are drawn for the basic regimes of wave generation. The power law exponents for the azimuth particle displacement and their variance, which proved the occurrence of the anomalous azimuth transport of the passive scalar, are determined. Lagrangian chaos is studied by computing the finite-time Lyapunov exponent and its distribution function. The internal chain (with respect to the annulus center) is found to be totally subject to Lagrangian chaos, while only the external chain boundary is chaotic. It is revealed that the cross-flow transport occurs only in the regime of Eulerian dynamical chaos, since there exists a barrier to it in the multiharmonic regimes. The images of fluid particles confirming the presence of lateral transport are obtained and their quantitative characteristics are determined.

Large-Eddy Simulation of Shaped Hole Film Cooling With the Influence of Cross Flow

2019 ◽  
Author(s):  
Qingsong Wang ◽  
Yifei Li ◽  
Xiutao Bian ◽  
Xinrong Su ◽  
Xin Yuan
Keyword(s):  
Large Eddy Simulation ◽  
Turbine Blade ◽  
Film Cooling ◽  
Cross Flow ◽  
Hairpin Vortices ◽  
Blade Passage ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Cooling Hole ◽  
The Asymmetry

Abstract In the modern highly-loaded gas turbine, due to the large pressure difference between the suction side and the pressure side of the turbine blade, strong cross flow is formed and it strongly affects the aerodynamic and cooling performances in the end-wall region. The film cooling behavior in the environment of strong cross flow is different from the straight channel environment widely studied in the literature. In this research, the effect of cross flow on film cooling is investigated by Large Eddy Simulation (LES) using subgrid-scale (SGS) model. Numerical simulation is carried out in a curved passage to simulate the turbine blade passage. Shaped cooling hole with blowing ratio 1 is studied. The time-averaged friction line results are compared with existing experimental ink trace results. The vortex structures, both time-averaged and instantaneous, are analyzed to study the effect of cross flow on film cooling. At the exit of the cooling hole, the hanging vortices with negative y-vorticity are more flat in shape and closer to the wall in position in contrast to hanging vortex with positive y-vorticity, which is caused by cross flow and results in the asymmetry of hairpin vortices downstream as well as the asymmetry of the distribution of coolant. It has been shown that the vortices from mainstream have a significant impact on the field near the exit of the cooling hole. Those vortices interact with the hairpin vortices from the cooling hole and directly lead to the asymmetry of the hairpin vortices. Proper Orthogonal Decomposition (POD) analysis is further conducted to extract the dominant flow structures and the physical mechanisms of primary POD modes are given to explain the distribution of film cooling effectiveness affected by cross flow. Based on the specific situation in this work, a fast incremental POD (iPOD) approach is adopted since the rank of the field matrix is far less than the rows, which is caused by the tall and thin character of the matrix, which makes the analysis less costly and more effective. This research helps to understand the cooling performance in the real turbine blade passage and to explain the coolant mixing process based on the instantaneous flow field obtained using high precision LES simulation and powerful iPOD.

Large-Eddy Simulation of Shaped Hole Film Cooling with the Influence of Cross Flow

2020 ◽  
Vol 0 (0) ◽  
Author(s):  
Wang Qingsong ◽  
Xinrong Su ◽  
Xin Yuan
Keyword(s):  
Large Eddy Simulation ◽  
Film Cooling ◽  
Large Scale ◽  
High Efficiency ◽  
Cross Flow ◽  
Vortex Pair ◽  
Hairpin Vortices ◽  
Eddy Simulation ◽  
Large Eddy ◽  
End Wall

AbstractIn the highly-loaded turbine blade passage, cross flow is driven by the lateral gradient. It strongly influences the cooling performances in the endwall region. In this research, the effect of cross flow on the shaped film cooling hole is studied by Large Eddy Simulation (LES); modal analysis is conducted with an incremental POD (iPOD) approach, which makes the analysis of the large data sets from LES feasible. It is shown that the symmetry of the counter rotating vortex pair (CRVP) is destroyed. The large-scale vortex induced by end-wall cross flow plays an important role in both shape and convection of hairpin vortices and horseshoe vortices, which influences the coolant distribution. This study suggests that the effects of cross flow should be considered for the design of end-wall film cooling. It also indicates the high efficiency of the iPOD approach, which can be used to analyze large amounts of high-dimensional data.

Dye visualisation of inclined and skewed synthetic jets in a cross flow

2005 ◽  
Vol 109 (1093) ◽  
pp. 147-155 ◽  
Author(s):  
S. Zhong ◽  
L. Garcillan ◽  
N. J Wood
Keyword(s):  
Reynolds Number ◽  
Velocity Ratio ◽  
Cross Flow ◽  
Maximum Flow ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Wall Region ◽  
Control Effect ◽  
Hairpin Vortices ◽  
Near Wall

Abstract Dye visualisation of both inclined synthetic jets and skewed synthetic jets was undertaken in a cross-flow experiment and the results were compared with those of normal synthetic jets. The process of vortex roll-up near the orifice exit and how the structure develops and interacts with the cross-flow as it propagates downstream was investigated so as to obtain an understanding of the effect of orifice orientation on the behaviour of synthetic jets. The effects of varying Reynolds number, velocity ratio and Strouhal number due to changes in diaphragm displacements and freestream velocities on the characteristics of synthetic jets were also examined. It is observed that in comparison to the normal jets vortical structures produced by both inclined and skewed jets tend to stay closer to the near wall region where maximum flow control effect is required. In both cases, at a relatively low Reynolds number and velocity ratio the active structures produced by the synthetic jet appear to be hairpin vortices which turn into vortex rings that migrate away from the wall as the Reynolds number and velocity ratio increase. These hairpin vortices are persistent in the near wall region hence are believed to be desirable structures for delaying flow separation.

scholarly journals Cross-Correlation of POD Spatial Modes for the Separation of Stochastic Turbulence and Coherent Structures

Fluids ◽  
2019 ◽  
Vol 4 (3) ◽  
pp. 134 ◽  
Author(s):  
Butcher ◽  
Spencer
Keyword(s):  
Cross Correlation ◽  
Energy Content ◽  
Flow Characteristics ◽  
Cross Flow ◽  
Turbulent Velocity ◽  
Cumulative Energy ◽  
Turbulent Length Scale ◽  
Spatial Modes ◽  
Flow Transport ◽  
Proper Orthogonal

This article describes a proper-orthogonal-decomposition (POD) based methodology proposed for the identification and separation of coherent and turbulent velocity fluctuations. Typically, POD filtering requires assumptions to be made on the cumulative energy content of coherent modes and can therefore exclude smaller, but important contributions from lower energy modes. This work introduces a suggested new metric to consider in the selection of POD modes to be included in a reconstruction of coherent and turbulent features. Cross-correlation of POD spatial modes derived from independent samples is used to identify modes descriptive of either coherent (high-correlation) or incoherent (low-correlation) features. The technique is demonstrated through application to a cylinder in cross-flow allowing appropriate analysis to be carried out on the coherent and turbulent velocity fields separately. This approach allows identification of coherent motions associated with cross-flow transport and vortex shedding, such as integral length scales. Turbulent flow characteristics may be analysed independently from the coherent motions, allowing for the extraction of properties such as turbulent length scale.

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