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Direct numerical simulation (DNS) of a spatially evolving flat-plate boundary layer transition process at free stream Mach number 0.7 is performed. Tollmien-Schlichting (T-S) waves are added on the inlet boundary as the disturbanc...
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Direct numerical simulation (DNS) of a spatially evolving flat-plate boundary layer transition process at free stream Mach number 0.7 is performed. Tollmien-Schlichting (T-S) waves are added on the inlet boundary as the disturbances before transition. Typical coherent structures in the transition process are investigated based on the second invariant of velocity gradient tensor. The instantaneous shear stress and the mean velocity profile in the transition region are studied. In our view, the fact that the peak value of shear stress in the stress concentration area increases and exceeds a threshold value during the later stage of the transition process plays an important role in the laminar breakdown process.
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The vast majority of experiments on transition in pipe flow have been concerned with high-amplitude disturbance (of approximate to 10% of the centerline velocity) at Reynolds numbers around 2000 and above. In this experimental stu...
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The vast majority of experiments on transition in pipe flow have been concerned with high-amplitude disturbance (of approximate to 10% of the centerline velocity) at Reynolds numbers around 2000 and above. In this experimental study, we concentrate on the transition process in water pipe flow where the level of the disturbance is at least an order of magnitude smaller. We follow the sequence of transitional events while varying gradually the disturbance level from a laminar state to an almost fully turbulent one, using flow visualization, pressure-drop and hot-wire measurements. This is accomplished by injection of a very small diameter jet perpendicular to the main stream at a controllable flow rate, in a downstream distance where the flow is approximately fully developed. The injection flow rate normalized by the main stream rate is of O(0.001). With increasing injection flow rates the friction coefficient (lambda) increases along with changes in the nature of the flow structures. The transition begins with the generation of a streamwise counter-rotating vortex pair, followed by the formation of a packet of hairpins and their breakdown. As the injection level is increased, the separation distance between two consecutive bursts (wavelength) decreases and the breakdown to turbulence begins further upstream and consequently the value of the friction coefficient lambda increases. Above and below a certain threshold of the disturbance level, turbulence is either triggered or the flow is relaminarized, respectively. Finally, it is established that the evolution of the packet of hairpins is a key element during this transition scenario, and is well explained by the three-element model recently proposed by Cohen et al (2014).
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A visualization study is conducted on the excited laminar-turbulent transition within a flat plate boundary layer flow in a water tunnel. The hydrogen bubble technique is employed to investigate the complex characteristics of the ...
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A visualization study is conducted on the excited laminar-turbulent transition within a flat plate boundary layer flow in a water tunnel. The hydrogen bubble technique is employed to investigate the complex characteristics of the flow structure and its breakdown in the later stages of the transition. A new flow structure is observed, which involves two secondary hairpin vortices outboard of both legs of a primary hairpin vortex. This complex structure is argued to be a precursor of a turbulent spot in this K-type transition. Also reported in the paper is the evolution of the flow structure and its subsequent breakdown, manifested by the emergence of dark spots, low-speed fluid bumps, and near-wall hairpin vortex groups. The results indicate that the near-wall flow breakdown is the result of instability of a local three-dimensional high-shear layer between the low-speed fluid bump and the outer higher-speed region.
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Film cooling is regularly used to cool the surface of components within the turbine stage of an aero engine. This enables them to withstand the high air temperatures that are required for maximising aero engine cycle efficiency. I...
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Film cooling is regularly used to cool the surface of components within the turbine stage of an aero engine. This enables them to withstand the high air temperatures that are required for maximising aero engine cycle efficiency. It is known that the mixing of a film cooling flow with the main high temperature air flow through a turbine passage is an unsteady process, with coherent unsteady features occurring across a range of blowing ratios. Upon an aero engine the cooling holes on a turbine blade commonly have a crossflow at the hole inlet. Previous work has shown that crossflow at the hole inlet modifies the time-mean flowfield downstream of a cooling hole compared to the case without crossflow. The current paper investigates the impact of spanwise orientated crossflow on the coherent unsteadiness within film cooling flows. Both cylindrical and shaped cooling holes, located on a blade pressure surface, are studied. The range of blowing ratios considered is 0.7-1.8 and the crossflow velocity is up to 0.8 times the bulk jet velocity. High Speed Photography and Hot Wire Anemometry are used to observe the presence of coherent unsteadiness, both immediately downstream of the hole exit and within thecooling hole tube. The results show that the coherent unsteadiness downstream of the hole exit is persistent and its occurrence is not significantly affected by the magnitude of spanwise crossflow. Within the cooling hole tube the existence of coherent unsteadiness is presented for the first time, inside both cylindrical and shaped holes, with a Strouhal number of 0.6-1.2. The pattern of this in-hole coherent unsteadiness is seen to change with increasing the crossflow velocity.
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Visualization experiments are conducted to study the evolution of hairpin vortices shed from a pair of surface-mounted rectangular cylinders at a Reynolds number of approximately 300. The gap between the cylinders normalized by th...
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Visualization experiments are conducted to study the evolution of hairpin vortices shed from a pair of surface-mounted rectangular cylinders at a Reynolds number of approximately 300. The gap between the cylinders normalized by the uridth of the cylinders is fixed to the value of 0.75. Hairpin vortices are shed downstream from the combined wake of the paired cylinders due to the blocking effect of the combined wake on the flow exited from the gap. The hairpin vortices are large in scale and are lifted away from the surface toward the free stream as they move to farther downstream locations.
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The evolution of a finite-amplitude three-dimensional localized disturbance embedded in external shear flows is addressed. Using the fluid impulse integral as a characteristic of such a disturbance, the Euler vorticity equation is...
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The evolution of a finite-amplitude three-dimensional localized disturbance embedded in external shear flows is addressed. Using the fluid impulse integral as a characteristic of such a disturbance, the Euler vorticity equation is integrated analytically, and a system of linear equations describing the temporal evolution of the three components of the fluid impulse is obtained. Analysis of this system of equations shows that inviscid plane parallel flows as well as high Reynolds number two-dimensional boundary layers are always unstable to small localized disturbances, a typical dimension of which is much smaller than a dimensional length scale corresponding to an 0(1) change of the external velocity. Since the integral character of the fluid impulse is insensitive to the details of the how, universal properties are obtained. The analysis predicts that the growing vortex disturbance will be inclined at 45 degrees to the external flow direction, in a plane normal to the transverse axis. This prediction agrees with previous experimental observations concerning the growth of hairpin vortices in laminar and turbulent boundary layers. In order to demonstrate the potential of this approach, it is applied to Taylor-Couette flow, which has additional dynamical effects owing to rotation. Accordingly, a new instability criterion associated with three-dimensional localized disturbances is found. The validity of this criterion is supported by our experimental results. [References: 24]
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A recent database from direct numerical simulation (DNS) of a turbulent boundary layer up to Re_θ = 4300 (Schlatter and Orlu, 2010) is analysed to extract the dominant flow structures in the near-wall region. In particular, the q...
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A recent database from direct numerical simulation (DNS) of a turbulent boundary layer up to Re_θ = 4300 (Schlatter and Orlu, 2010) is analysed to extract the dominant flow structures in the near-wall region. In particular, the question of whether hairpin vortices are significant features of near-wall turbulence is addressed. A number of different methods based on the λ_2 criterion (Jeong and Hussain, 1995) is used to extract turbulent coherent structures: three-dimensional flow visualisation with quantitative estimates of hairpin population, conditional averaging and planar hairpin vortex signatures (HVS). First, visualisations show that during the initial phase of laminar-turbulent transition induced via tripping, hairpin vortices evolving from transitional Λ vortices are numerous and can be considered as the dominant structure of the immediate post-transition stage of the boundary layer. This is in agreement with previous experiments and low-Reynolds-number simulations such as Wu & Moin (2009). When the Reynolds number is increased, the fraction of hairpin vortices decreases to less than 2% for Re_θ > 4000. Second, conditional ensemble averages (Jeong et al., 1997) find hairpins close to the wall at low Reynolds number, while at a sufficient distance downstream from transition, the flow close to the wall is dominated by single quasi-streamwise vortices; even quantitatively, no major differences between boundary layer and channel can be detected. Moreover, three-dimensional visualisations of the neighbourhood of regions of strong swirling motion in planar cuts through the layer (the HVS) do not reveal hairpin vortices, thereby impairing statistical evidences based on HVS. The present results thus clearly confirm that transitional hairpin vortices do not persist in fully developed turbulent boundary layers, and that their dominant appearance as instantaneous flow structures in the outer boundary-layer region is very unlikely.
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Compressible subsonic turbulent starting jet with a relatively large Reynolds number of significant practical importance is investigated using large eddy simulation (LES), starting from a smooth contraction nozzle. The computation...
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Compressible subsonic turbulent starting jet with a relatively large Reynolds number of significant practical importance is investigated using large eddy simulation (LES), starting from a smooth contraction nozzle. The computational domain of truncated conical shape is determined through the comparison of the time-averaged numerical solution with the particle imaging velocimetry measurements for the steady jet. It is shown that the starting jet consists of a leading vortex ring followed by a quasi-steady jet, and the instantaneous velocity field exhibits contraction and expansion zones, corresponding to the high pressure (HP) and low pressure (LP) regions formed by the convecting vortex rings, and are related to the Kelvin-Helmholtz instability. The thin boundary layer inside the smooth contraction nozzle evolves into a shear layer at the nozzle exit and develops with the downstream penetration of the jet. Using lambda (2) criterion, the formation and evolution of the vortical structures are temporally visualized, illustrating distortion of vortex rings into lobed shapes prior to break-down. Rib-shape streamwise vortex filaments exist in the braid region between a pair of consecutive vortex rings due to secondary instabilities. Finally, formation and dynamics of hairpin vortices in the shear layer is identified.
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Direct numerical simulation (DNS) of incompressible, spatially developing square jets in the Reynolds number range of 500-2000 is reported. The three-dimensional unsteady Navier-Stokes equations are solved using high order spatial...
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Direct numerical simulation (DNS) of incompressible, spatially developing square jets in the Reynolds number range of 500-2000 is reported. The three-dimensional unsteady Navier-Stokes equations are solved using high order spatial and temporal discretization. The objective of the present work is to understand the evolution of free and forced square jets by examining the formation of large-scale structures. Coherent structures and related interactions of free jets suggest control strategies that can be used to achieve enhanced spreading and mixing of the jet with the surrounding fluid. The critical Reynolds number for the onset on unsteadiness in an unperturbed free square jet is found to be 875-900 while it reduces to the range 500-525 in the presence of small-scale perturbations. Disturbances applied at the flow inlet cause saturation of KH-instability and early transition to turbulence. Forced jet calculations have been carried out using varicose perturbation with amplitude of 15%, while frequency is independently varied. Simulations show that the initial development of the square jet is influenced by the four corners leading to the appearance hairpin structures along with the formation of vortex rings. Farther downstream, adjacent vortices strongly interact leading to their rapid breakup. Excitation frequencies in the range 0.4-0.6 cause axis-switching of the jet cross-section. Results show that square jets achieve greater spreading but are less controllable in comparison to the circular ones. (C) 2015 Elsevier Inc. All rights reserved.
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Flow visualization experiments are conducted to study the hairpin vortices which are shed from the wakes of a pair of surface-mounted hemispheres at a Reynolds number of about 500. The results show that the number of groups of the...
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Flow visualization experiments are conducted to study the hairpin vortices which are shed from the wakes of a pair of surface-mounted hemispheres at a Reynolds number of about 500. The results show that the number of groups of the hairpin vortices can be one, two or three, depending on the gap between the hemispheres. As the gap increases from zero, the number of groups varies from one to three first. Then it reduces to two when the gap is large enough for the individual wakes to prevail.
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