Periodic Transition on an Axial Compressor Stator — Incidence and Clocking Effects: Part II — Transition Onset Predictions

Solomon, W. J.; Walker, G. J.; Hughes, J. D.

doi:10.1115/98-gt-364

Cited by 5 publications

(3 citation statements)

References 4 publications

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“…The 0.7 U f s celerity rate is also strongly evident in experimental studies of wake-induced transition in compressors (e.g Solomon, Walker & Hughes 1999;Wheeler, Sofia & Miller 2007)…”

mentioning

confidence: 91%

Unsteady boundary-layer transition in low-pressure turbines

2011

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This paper examines the transition process in a boundary layer similar to that present over the suction surfaces of aero-engine low-pressure (LP) turbine blades. This transition process is of significant practical interest since the behaviour of this boundary layer largely determines the overall efficiency of the LP turbine. Modern 'high-lift' blade designs typically feature a closed laminar separation bubble on the aft portion of the suction surface. The size of this bubble and hence the inefficiency it generates is controlled by the transition between laminar and turbulent flow in the boundary layer and separated shear layer. The transition process is complicated by the inherent unsteadiness of the multi-stage machine: the wakes shed by one blade row convect through the downstream blade passages, periodically disturbing the boundary layers. As a consequence, the transition to turbulence is multi-modal by nature, being promoted by periodic and turbulent fluctuations in the free stream and the inherent instabilities of the boundary layer. Despite many studies examining the flow behaviour, the detailed physics of the unsteady transition phenomena are not yet fully understood. The boundary-layer transition process has been studied experimentally on a flat plate. The opposing test-section wall was curved to impose a streamwise pressure distribution typical of modern high-lift LP turbines over the flat plate. The presence of an upstream blade row has been simulated by a set of moving bars, which shed wakes across the test section inlet. Further upstream, a grid has been installed to elevate the free-stream turbulence to a level believed to be representative of multi-stage LP turbines. Extensive particle imaging velocimetry (PIV) measurements have been performed on the flat-plate boundary layer to examine the flow behaviour. In the absence of the incoming bar wakes, the grid-generated free-stream turbulence induces relatively weak Klebanoff streaks in the boundary layer which are evident as streamwise streaks of low-velocity fluid. Transition is promoted by the streaks and by the inherent inflectional (Kelvin-Helmholtz (KH)) instability of the separation bubble. In unsteady flow, the incoming bar wakes generate stronger Klebanoff streaks as they pass over the leading edge, which convect downstream at a fraction of the free-stream velocity and spread in the streamwise direction. The region of amplified streaks convects in a similar manner to a classical turbulent spot: the leading and trailing edges travel at around 88% and 50% of the free-stream velocity, respectively. The strongest disturbances travel at around 70% of the free-stream velocity. The wakes induce a second type of disturbance as they pass over the separation bubble, in the form of short-span KH structures. Both the streaks and the KH structures contribute to the early wake-induced transition. The KH structures are similar to those observed in the simulation of separated flow transition with high free-stream turbulence by McAuliffe & Yaras (ASME Unsteady ...

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“…The 0.7 U f s celerity rate is also strongly evident in experimental studies of wake-induced transition in compressors (e.g Solomon, Walker & Hughes 1999;Wheeler, Sofia & Miller 2007)…”

mentioning

confidence: 91%

Unsteady boundary-layer transition in low-pressure turbines

2011

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“…It was also found by Gostelow and Thomas [63] that for wake-perturbed boundary layer flow, the law (1) describes well transition in separated state under low free-stream turbulence in between wake impacts. Furthermore, growth and spreading of turbulent spots induced by wake impact and those induced by a statistically steady free stream, are similar (Walker et al [64,65]; Schobeiri et al [31]). Thus, the evolution law may also be used for describing transition induced by wakes in wake-perturbed flow.…”

Section: Intermittencymentioning

confidence: 64%

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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“…In the calculations performed with the aid of those codes the stator wakes were obtained as part of the solution and their characteristics did not have to be assumed a priori. The RANS codes enabled studying numerous aspects of W/R interaction, including wake deformation [9], profile losses [10], unsteady heat transfer [11], boundary layer transition [12], pressure fields and blade load fluctuations [13], etc. Experimental verification of the obtained results was, generally, good -at the level determined by the accuracy and resolution of the measuring devices used in those times.…”

Section: Wake Modellingmentioning

confidence: 99%