ON ONE POSSIBLE MECHANISM OF FORMATION OF A TURBULENT SPOT (original) (raw)
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Experimental studies of a distorted turbulent spot in a three-dimensional flow
J of Fluid Mechanics, 1996
We report here on the results of a series of experiments carried out on a turbulent spot in a distorted duct to study the effects of a divergence with straight streamlines preceded by a short stretch of transverse streamline curvature, both in the absence of any pressure gradient. It is found that the distortion produces substantial asymmetry in the spot: the angles at which the spot cuts across the local streamlines are altered dramatically (in contradiction of a hypothesis commonly made in transition zone modelling), and the Tollmien-Schlichting waves that accompany the wing tips of the spot are much stronger on the outside of the bend than on the inside. However there is no strong effect on the internal structure of the spot and the eddies therein, or on such propagation characteristics as overall spread rate and the celerities of the leading and trailing edges. Both lateral streamline curvature and non-homogeneity of the laminar boundary layer into which the spot propagates are shown to be strong factors responsible for the observed asymmetry. It is concluded that these factors produce chiefly a geometric distortion of the coherent structure in the spot, but do not otherwise affect its dynamics in any significant way.
Journal of Turbomachinery, 2000
It has been suggested that a turbulent spot is formed when a transient separation occurs in the laminar boundary layer and this criterion has been successfully used by Johnson and Ercan (1996, ASME Paper No. 96-GT-44; 1997, ASME Paper No. 97-GT-475) to predict bypass transition for boundary layers subjected to a wide range of free-stream turbulence levels and streamwise pressure gradients. In the current paper experimental results are presented that support the premise that the formation of turbulent spots is associated with transient separation. Near-wall hot-wire signals in laminar and transitional boundary layers are analyzed statistically to produce probability distributions for signal level and trough frequency. In the laminar period the signal level is normally distributed, but during the inter-turbulent periods in the transitional boundary layer, the distribution is truncated at the lower end, i.e., the lowest velocity periods in the signal disappear, suggesting that these ar...
THE PERTURBATION FLOW FIELD ASSOCIATED WITH PASSAGE OF TURBULENT SPOT
2003
The flow field associated with the passage of the turbulent spot in a 3-D duct with streamline divergence under zero pressure gradients was investigated and displayed as contour plots of the velocity perturbation in plan and elevation view of the spot. It suggests that, streamline divergence has no strong effect on the internal structure of the spot and eddies and their propagation in the downstream direction is similar to 2-D flows. The orthographic views are also shown. They are rather similar to contour plots but with different kind of presentation to show velocity excess and deficit as peaks and valleys respectively in the flow field while passage of the spot. This work was a part of detailed investigation of the structure of a distorted turbulent spot in a 3-D constant pressure flow.
The role of turbulent on the initiation of
Committee Chairman: Panayiotis Diplas Civil Engineering (ABSTRACT) The present study examines the role of turbulence on the incipient motion of sediment. For this purpose, well-controlled experiments are performed at the laboratory in a tilting flume. In these tests glass beads of the same size and density are used as the testing material to isolate the role of turbulence. State of the art equipment are used during the course of this study. Specifically, a 3-D Laser Doppler Velocimetry system is employed to measure the instantaneous velocity components at different points near the vicinity of a ball while the ball motion is monitored with a video camera. An image analysis program is developed here to analyze the motion of the particles within a test area. To examine the importance of the different stress components in the entrainment of sediment, five tests of different packing configuration are performed. Specifically three different roughness regimes are examined namely, the isolated, the wake interference, and the skimming flow. The results reveal that the instantaneous normal stress in the streamwise direction is the most dominant component of the instantaneous stress tensor. The backbone of this study is the development of a methodology to link the effects of turbulence with the commencement of sediment motion. It is considered that the metastable bursting cycle (i.e. sweeps, ejections, inward and outward interactions) is responsible for the sediment entrainment. And that the sediment entrainment, if any, occurs within a bursting period.
Turbulent spots in channel flow: An experimental study
The European Physical Journal E, 2014
We present new experimental results on the development of turbulent spots in channel flow. The internal structure of a turbulent spot is measured, with Time Resolved Stereoscopic Particle Image Velocimetry. We report the observation of travelling-wave-like structures at the trailing edge of the turbulent spot. Special attention is paid to the large-scale flow surrounding the spot. We show that this large-scale flow is an asymmetric quadrupole centred on the spot. We measure the time evolution of the turbulent fluctuations and the mean flow distortions and compare these with the predictions of a nonlinear reduced order model predicting the main features of subcritical transition to turbulence.
2000
To simu late the effect of fre e st ream tu rbu lence on tu rbu lent spot format ion, Abstract experiments were conducted on the interaction of localized three-dimensional disturbances with the harmonic waves in a laminar bou ndary layer on a flat plate. E xperiments condu cted in three-dimensional diverging flow (bu t zero pressu re gradient) show, while individu ally the disturbances decay downstream, their interaction leads to amplification of three-dimensional distu rbance leading to formation of the tu rbu lent spot. Also flow divergence show the least effect in the interaction process.
Onset of Turbulence from the Receptivity Stage of Fluid Flows
Physical Review Letters, 2011
The traditional viewpoint of fluid flow considers the transition to turbulence to occur by the secondary and nonlinear instability of wave packets, which have been created experimentally by localized harmonic excitation. The boundary layer has been shown theoretically to support spatiotemporal growing wave fronts by Sengupta, Rao, and Venkatasubbaiah [Phys. Rev. Lett. 96, 224504 (2006)] by a linear mechanism, which is shown here to grow continuously, causing the transition to turbulence. Here, we track spatiotemporal wave fronts to a nonlinear turbulent state by solving the full 2D Navier-Stokes equation, without any limiting assumptions. Thus, this is the only demonstration of deterministic disturbances evolving from a receptivity stage to the full turbulent flow. This is despite the prevalent competing conjectures of the event being three-dimensional and/or stochastic in nature.
The Perturbation Flow Field Associated With Passage of
2002
The flow field associated with the passage of the turbulent spot in a 3-D duct with streamline divergence under zero pressure gradients was investigated and displayed as contour plots of the velocity perturbation in plan and elevation view of the spot. It suggests that, streamline divergence has no strong effect on the internal structure of the spot and eddies and their propagation in the downstream direction is similar to 2-D flows. The orthographic views are also shown. They are rather similar to contour plots but with different kind of presentation to show velocity excess and deficit as peaks and valleys respectively in the flow field while passage of the spot. This work was a part of detailed investigation of the structure of a distorted turbulent spot in a 3-D constant pressure flow.
A ‘turbulent spot’ in an axisymmetric free shear layer. Part 2
Journal of Fluid Mechanics, 1980
A three-dimensional ' turbulent spot ' has been induced in the axisymmetric free mixing layer of a 12.7 cm diameter air jet by a spark generated at the nozzle boundary layer upstream of the exit. The spot coherent-structure signature, buried in the largeamplitude random fluctuating signal, has been educed a t three downstream stations within the apparent self-preserving region of the mixing layer (i.e. x / D = 1.5, 3.0 and 4.5) a t the jet exit speed of 20m s-1. The eduction has been performed through digital phase averaging of the spot signature from 200 realizations. In order to reduce the effect of the turbulence-induced jitter on the phase average, individual filtered signal arrays were optimally time-aligned through an iterative process of crosscorrelation of each realization with the ensemble average. Further signal enhancement was achieved through rejection of realizations requiring excessive time shifts for alignment. The number of iterations required and the fraction of realizations rejected progressively increase with the downstream distance and the radial position.