Modeling Transition in Separated and Attached Boundary
                    Layers

Roberts, S. K.; Yaras, M. I.

doi:10.1115/1.1860570

Cited by 29 publications

(21 citation statements)

References 51 publications

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“…Newer correlations were proposed by other researchers (e.g., Hatman and Wang [78]; Roberts and Yaras [79]; Praisner and Clark [71,72]), but often not in a convenient form for numerical simulations of turbomachinery flows. Suzen et al [80] constructed a correlation in the form of (17), based on a quite large data set:…”

Section: Start Of Separation-induced Transition In Steady Free-streammentioning

confidence: 99%

Transition Models for Turbomachinery Boundary Layer Flows: A Review

Dick

Kubacki

2017

IJTPP

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Current models for transition in turbomachinery boundary layer flows are reviewed. The basic physical mechanisms of transition processes and the way these processes are expressed by model ingredients are discussed. The fundamentals of models are described as far as possible, with a common structure of the equations and with emphasis on the similarities between the models. Tests of models reported in the literature are summarized and our own test is added. A conclusion on the performance of models is formulated.Keywords: turbomachinery flows; transition models; boundary layers; bypass transition; separationinduced transition; wake-induced transition; Reynolds-averaged Navier-Stokes; intermittency; laminar kinetic energy Transition MechanismsWith the objective of modelling with Reynolds-averaged Navier-Stokes (RANS) or URANS (unsteady or time-accurate RANS) description of a flow, generally, four types of transition from laminar state to turbulent state in turbomachinery boundary layer flows are distinguished [1,2]. These are: natural transition in attached boundary layer state, bypass transition in attached boundary layer state under a statistically steady mean flow, separation-induced transition in the free shear layer formed by separation of a boundary layer under a statistically steady mean flow, and wake-induced transition due to periodically unsteady impact of wakes on boundary layers in attached or separated state. Despite the vast amount of studies on these transition types during the last 4 or 5 decades, understanding of the mechanisms is still not complete, although large progress has been made in the last decade thanks to direct numerical simulation (DNS) and large-eddy simulation (LES). Hereafter, we summarise the mean mechanisms, taking into account the inherent limitations of modelling with Reynolds-averaged Navier-Stokes description of a flow. This means that secondary effects, which mostly are the least understood and sometimes still are a matter of controversy, are not discussed, because, anyhow these features cannot be represented by RANS or URANS. Natural TransitionUnder a statistically steady mean flow of low turbulence level, transition in an attached boundary layer is initiated by 2D viscosity-dependent Tollmien-Schlichting instability waves, followed by a 3D instability, leading to formation of spanwise periodic hairpin vortices, which, farther downstream, cause breakdown of the laminar layer with generation of turbulent spots. These spots finally merge resulting in the formation of a turbulent boundary layer [1,2]. This type of transition is called natural transition. It is a rather slow process and, under a very low mean-flow turbulence level, as in external Int.

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Section: Start Of Separation-induced Transition In Steady Free-streammentioning

confidence: 99%

Transition Models for Turbomachinery Boundary Layer Flows: A Review

Dick

Kubacki

2017

IJTPP

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“…Unfortunately, current transition models (e.g. Roberts and Yaras, 2005b) lack generality and thus suffer from a large uncertainty in their predictions. In order to improve such transition models, the physical mechanisms that contribute to transition in shear layers need to be further understood.…”

Section: List Of Symbolsmentioning

confidence: 99%

“…Regarding Question 4, the rate-of-production and rate-of-growth of turbulent spots following the saturation of instability modes are the basis for the majority of models that predict the transition rate (e.g. D'Ovidio et al, 2002;Roberts and Yaras, 2005b). However, the fundamental mechanisms driving the creation of coherent flow structures within the turbulent spot-the process that is primarily responsible for the spot's lateral growth (e.g.…”

Section: Instability Modesmentioning

confidence: 99%

“…These studies have resulted in increasingly-refined engineering models that allow prediction of the transition onset location and transition rate in separated and attached boundary layers (e.g., Roberts and Yaras, 2005b). A clear understanding of the physics of the instability mechanisms that contribute to transition is a prerequisite to further improvements of such transition models.…”

Section: Introductionmentioning

confidence: 99%

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Numerical Investigations of Instability and Transition in Attached and Separated Shear Layers

Brinkerhoff¹

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This integrated thesis documents a series of five complementary numerical investigations aimed at understanding the flow instability and laminar-to-turbulent transition process in attached and separated shear layers. Direct numerical simulation was used in these studies to accurately resolve the spatial and temporal scales of the simulated flows. The first two investigations simulate the flow in an idealized computational domain that approximates the conditions on the suction surface of a low-pressure turbine (LPT) blade. The first study investigates the interaction of the Tollmien-Schlichting instability in the attached-flow region with the Kelvin-Helmholtz instability in the separated-flow region. The interaction produces packets of coherent vortices that facilitate transition to a fully-turbulent boundary layer.The effect of swept-blade conditions on the transition process is investigated in the second numerical study by sweeping the simulation domain by 45 • . The resulting three-dimensional pressure field creates the potential for a crossflow-induced instability mode, but its effect on the transition process is minimal and transition follows the same mechanisms observed in the unswept configuration.The characteristics of the coherent vortices that facilitate transition to turbulence in the first study are further investigated through a series of fundamental studies of an isolated turbulent spot that is triggered by a transverse jet. The first of these fundamental studies demonstrates the sensitivity of the formation and structure of the coherent vortices to the level of free-stream acceleration. A subsequent fundamental study identifies the regeneration process by which the turbulent spot grows laterally through the formation of packets of hairpin vortices along the spanwise edges of the spot and longitudinally by the increase in the spatial scale of the hairpin vortices. The final fundamental study investigates the effects of elevated free-stream turbulence levels and LPT-realistic pressure distributions on the transition process. Under elevated turbulence conditions, bypass transition occurs as streamwise streaks i form in the laminar boundary layer and roll-up into hairpin-shaped vortices via a Kelvin-Helmholtz instability mode that is accelerated by the free-stream turbulence. The wave-packet regeneration model identified under low free-stream turbulence is promoted by elevated freestream turbulence, increasing the growth-rate of turbulent spots under elevated turbulence conditions.ii

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“…The leading edge, which consists of the heads of large hairpin vortices and reaches well into the freestream, typically has a celerity of 0.95U 0 while the trailing edge, which consists of coherent structures closer to the wall, has a typical celerity of 0.5U 0 (Brinkerhoff & Yaras, 2014). The spreading rates and celerities of the turbulent spot are affected by several flow parameters including freestream turbulence intensity and length scale, streamwise pressure gradient, streamline curvature and surface roughness (Roberts & Yaras, 2005).…”

Section: Introductionmentioning

confidence: 99%

Measurements of the Effects of Streamwise Riblets on the Turbulence Structures in Boundary Layers

Ancrum¹

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This study presents experimental results on the effects of streamwise-oriented riblets on the coherent structures of turbulence. Hotwire measurements were performed in artificially created turbulent spots. The riblet spacings of the study correspond to 0.5 and 1.5 times the natural spacing of the low-speed streaks and fall into the category identified as wider-spaced in published literature. The cross-sectional dimensions of the riblets were chosen to promote more effective control on the development and spatial distribution of wave packets consisting of streamwise-aligned hairpin vortices.The riblets proved to be highly effective in controlling the spatial positioning of wave packets. Of the two riblet spacings considered, the wider spacing increased the spanwise spacing of the low-speed streaks beyond their natural spacing and stabilized the wave packets over the riblet tips, enabling a reduction in their mutual interaction and realizing a reduction in their spanwise density compared to the conditions on a smooth surface. This effect may be optimized to achieve skin-friction drag and aerodynamic noise reduction. The closerspaced riblets were observed to have even more control on the spanwise positioning of the wave packets, and produced notably stronger sweep and ejection events by reducing the spanwise spacing between wave packets and promoting mutual interaction of hairpin vortices via spanwise-oriented vortical structures created by a Kelvin-Helmholtz instability mechanism.This effect may be used to achieve increased convection heat transfer in various applications without significant penalties in pressure loss.iii Acknowledgments This work would not have been possible without the support and mentorship of my supervisor, Professor Metin I. Yaras. I would like to thank him for his patience and his ability as an educator to bring out the best in his students while always keeping their development in mind. This manuscript represents only a small portion of the knowledge I have gained working with him.

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Modeling Transition in Separated and Attached Boundary Layers

Cited by 29 publications

References 51 publications

Transition Models for Turbomachinery Boundary Layer Flows: A Review

Transition Models for Turbomachinery Boundary Layer Flows: A Review

Numerical Investigations of Instability and Transition in Attached and Separated Shear Layers

Measurements of the Effects of Streamwise Riblets on the Turbulence Structures in Boundary Layers

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