Interactions for Regular Patterns of Turbulent Spots in a Laminar Boundary Layer

1980 ◽  
pp. 277-287 ◽  
Author(s):  
D. Coles ◽  
O. Savas
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Turbulent Spots ◽  
Regular Patterns

Investigation of transition to turbulence using white-noise excitation and local analysis techniques

1997 ◽  
Vol 348 ◽  
pp. 29-83 ◽  
Author(s):  
F. N. SHAIKH
Keyword(s):  
Boundary Layer ◽  
White Noise ◽  
Laminar Boundary Layer ◽  
Nonlinear Flow ◽  
Local Analysis ◽  
Hot Wire ◽  
Turbulent Spots ◽  
Analysis Techniques ◽  
White Noise Excitation ◽  
Highly Nonlinear

Weak free-stream turbulence excites modulated Tollmien–Schlichting (T–S) waves in a laminar boundary layer that grow in magnitude with downstream distance and ultimately lead to the formation of turbulent spots and then fully turbulent flow. Hot-wire experiments have indicated that the development of localized large-amplitude ‘events’ in the velocity records are the essential precursor to the eventual formation of turbulent spots in the flow field. Traditional global Fourier techniques are unable to resolve the localized nature of these events and hence provide little useful information concerning the physical processes responsible for this breakdown process.This investigation used sequences of computer-generated deterministic white noise to excite a laminar boundary layer via a loudspeaker embedded in a flat-plate model. This form of excitation generated the modulated disturbance waves of interest a short distance downstream from the source in a repeatable and deterministic manner. Further downstream the pattern of flow breakdown and subsequent generation of turbulent spots was similar to that observed in naturally excited situations. By repeatedly exciting the boundary layer with a single white-noise sequence it was possible to examine the highly nonlinear stages of ‘event’ development and breakdown with a single hot-wire probe.Two local analysis techniques, the wavelet transform (WT) and singular spectrum analysis (SSA), were used in conjunction with the white-noise excitation technique to examine the highly nonlinear flow mechanisms responsible for the localized formation of events that lead to the eventual breakdown to turbulence.

K-1404 Visualization experiment on mutual Interaction between turbulent spots in a laminar boundary layer

2001 ◽  
Vol II.01.1 (0) ◽  
pp. 211-212
Author(s):  
Ryuichi AOYAMA ◽  
Hideharu MAKITA ◽  
Nobumasa SEKISHITA ◽  
Naoyuki TUCHIYA
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Mutual Interaction ◽  
Turbulent Spots ◽  
Visualization Experiment

Transition Mechanisms in Separation Bubbles Under Low and Elevated Freestream Turbulence

2007 ◽  
Author(s):  
B. R. McAuliffe ◽  
M. I. Yaras
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Numerical Simulations ◽  
Shear Layer ◽  
Laminar Boundary Layer ◽  
Large Scale ◽  
Freestream Turbulence ◽  
Turbulent Spots ◽  
Separated Shear Layer ◽  
Separation Bubbles

Through numerical simulations, this paper examines the nature of instability mechanisms leading to transition in separation bubbles. The results of two direct numerical simulations are presented in which separation of a laminar boundary layer occurs over a flat surface in the presence of an adverse pressure gradient. The primary difference in the flow conditions between the two simulations is the level of freestream turbulence with intensities of 0.1% and 1.45% at separation. In the first part of the paper, transition under a low-disturbance environment is examined, and the development of the Kelvin-Helmholtz instability in the separated shear layer is compared to the well-established instability characteristics of free shear layers. The study examines the role of the velocity-profile shape on the instability characteristics and the nature of the large-scale vortical structures shed downstream of the bubble. The second part of the paper examines transition in a high-disturbance environment, where the above-mentioned mechanism is bypassed as a result of elevated freestream turbulence. Filtering of the freestream turbulence into the laminar boundary layer results in streamwise streaks which provide conditions under which turbulent spots are produced in the separated shear layer, grow, and then merge to form a turbulent boundary layer. The results allow identification of the structure of the instability mechanism and the characteristic structure of the resultant turbulent spots. Recovery of the reattached turbulent boundary layer is then examined for both cases. The large-scale flow structures associated with transition are noted to remain coherent far downstream of reattachment, delaying recovery of the turbulent boundary layer to an equilibrium state.

An Investigation on the Onset of Wake-Induced Transition and Turbulent Spot Production Rate Using Thermochromic Liquid Crystals

1999 ◽  
Author(s):  
C. Kittichaikarn ◽  
P. T. Ireland ◽  
S. Zhong ◽  
H. P. Hodson
Keyword(s):  
Boundary Layer ◽  
Liquid Crystals ◽  
Flat Plate ◽  
Laminar Boundary Layer ◽  
Gas Turbines ◽  
Formation Rate ◽  
Boundary Layer Transition ◽  
Published Data ◽  
Layer Transition ◽  
Turbulent Spots

Wakes shed by upstream blade rows are known to cause boundary layer transition in both the compressor and turbine stages of axial flow gas turbines. This transition process is believed to take place via discrete zones of turbulence known as turbulent spots which occur in an otherwise laminar boundary layer. However, the process of transition over the blade surface cannot, at present, be reliably predicted. This is due to a lack of information on where and when these turbulent spots form and how they grow and merge as they convect downstream to form the turbulent boundary layer. This paper presents detailed experimental information on the process of boundary layer transition induced by a bar generated wake travelling over a zero pressure gradient laminar boundary layer on a flat plate. The Reynolds number was 3×105. The peak turbulence intensity within the wakes varied from 3 to 6 % by using different bar of different diameters. An encapsulated cholesteric liquid crystals coating has been employed on a heated flat plate to reveal detailed information over the full surface. The information includes the thermal characteristics, the spot onset and formation rate. Data were obtained at high resolution on a grid of 30,000 points. The results were compared to intermittency plots and time-distance diagrams obtained by using surface-mounted thin film gauges and found to be similar. The data were also consistent with well established correlations and other published data from the literature for existing wake-induced transition models.

On the growth of turbulent regions in laminar boundary layers

1981 ◽  
Vol 110 ◽  
pp. 73-95 ◽  
Author(s):  
Mohamed Gad-El-Hak ◽  
Ron F. Blackwelderf ◽  
James J. Riley
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Roughness Element ◽  
Water Channel ◽  
Lateral Spreading ◽  
Lateral Growth ◽  
Turbulent Spots ◽  
Salinity Stratification ◽  
Laminar Boundary Layers ◽  
Probe Measurements

Turbulent spots evolving in a laminar boundary layer on a nominally zero pressure gradient flat plate are investigated. The plate is towed through an 18 m water channel, using a carriage that rides on a continuously replenished oil film giving a vibrationless tow. Turbulent spots are initiated using a solenoid valve that ejects a small amount of fluid through a minute hole on the working surface. A novel visualization technique that utilizes fluorescent dye excited by a sheet of laser light is employed. Some new aspects of the growth and entrainment of turbulent spots, especially with regard to lateral growth, are inferred from the present experiments. To supplement the information on lateral spreading, a surbulent wedge created by placing a roughness element in the laminar boundary layer is also studied both visually and with probe measurements. The present results show that, in addition to entrainment, another mechanism is needed to explain the lateral growth characteristics of a turbulent region in a laminar boundary layer. This mechanism, termed growth by destabilization, appears to be a result of the turbulence destabilizing the unstable laminar boundary layer in its vicinity. To further understand the growth mechanisms, the turbulence in the spot is modulated using drag-reducing additives and salinity stratification.

Transition Mechanisms in Separation Bubbles Under Low- and Elevated-Freestream Turbulence

2009 ◽  
Vol 132 (1) ◽  
Author(s):  
Brian R. McAuliffe ◽  
Metin I. Yaras
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Numerical Simulations ◽  
Shear Layer ◽  
Laminar Boundary Layer ◽  
Large Scale ◽  
Freestream Turbulence ◽  
Turbulent Spots ◽  
Separated Shear Layer ◽  
Separation Bubbles

Through numerical simulations, this paper examines the nature of instability mechanisms leading to transition in separation bubbles. The results of two direct numerical simulations are presented in which separation of a laminar boundary layer occurs over a flat surface in the presence of an adverse pressure gradient. The primary difference in the flow conditions between the two simulations is the level of freestream turbulence with intensities of 0.1% and 1.45% at separation. In the first part of the paper, transition under a low-disturbance environment is examined, and the development of the Kelvin–Helmholtz instability in the separated shear layer is compared to the well-established instability characteristics of free shear layers. The study examines the role of the velocity-profile shape on the instability characteristics and the nature of the large-scale vortical structures shed downstream of the bubble. The second part of the paper examines transition in a high-disturbance environment, where the above-mentioned mechanism is bypassed as a result of elevated-freestream turbulence. Filtering of the freestream turbulence into the laminar boundary layer results in streamwise streaks, which provide conditions under which turbulent spots are produced in the separated shear layer, grow, and then merge to form a turbulent boundary layer. The results allow identification of the structure of the instability mechanism and the characteristic structure of the resultant turbulent spots. Recovery of the reattached turbulent boundary layer is then examined for both cases. The large-scale flow structures associated with transition are noted to remain coherent far downstream of reattachment, delaying recovery of the turbulent boundary layer to an equilibrium state.

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