Interaction of discrete and continuous boundary layer modes to cause transition

Durbin, Paul A.; Zaki, Tamer A.; Liu, Yang

doi:10.1016/j.ijheatfluidflow.2008.12.008

Cited by 17 publications

(6 citation statements)

References 15 publications

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“…This perturbation is far higher than previously observed for Klebanoff streaks: Jacobs & Durbin (2001) and Alfredsson & Matsubara (1996) noted perturbations of around 25% of the free-stream velocity. This result therefore suggests that these strong perturbations may correspond to the slow moving flow between the two legs of a Λ-vortex, in line with the simulations of Durbin et al (2009). These large fluctuations could also indicate varicose streak instability, as visualised by Asai, Minagawa & Nishioka (2002).…”

Section: Klebanoff Streaks In the Boundary Layer 411 Weak Klebanofsupporting

confidence: 82%

“…At the start of the APG region, Marxen et al (2009) found that the growth of the Klebanoff streaks was also driven by a modal Görtler instability, induced by the concave streamlines close to separation. Durbin, Zaki & Liu (2009) used direct numerical simulations (DNS) to examine the interaction between Klebanoff streaks and the inherent instabilities of a laminar boundary layer. Initial simulations with either streaks or Tollmien-Schlichting (TS) waves in isolation did not show transition to turbulence.…”

Section: External Disturbancesmentioning

confidence: 99%

“…The streaks in case 3 of figure 19 have a larger velocity deficit compared to the steady flow example in figure 18 and exhibit higher levels of wall-normal vorticity. Durbin et al (2009) demonstrated that Λ-vortices can emerge from the interaction of the Klebanoff streaks with the inherent instability of the boundary layer (process b in figure 3). While it is difficult to identify such three dimensional structures from the planar PIV data, there is evidence to suggest that a similar process is underway.…”

Section: Klebanoff Streaks In the Boundary Layer 411 Weak Klebanofmentioning

confidence: 99%

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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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Section: Klebanoff Streaks In the Boundary Layer 411 Weak Klebanofsupporting

confidence: 82%

Section: External Disturbancesmentioning

confidence: 99%

Section: Klebanoff Streaks In the Boundary Layer 411 Weak Klebanofmentioning

confidence: 99%

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Unsteady boundary-layer transition in low-pressure turbines

2011

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“…Transition onset is then characterized by the appearance of vortex loops or hairpin eddies at a frequency related to the dominant KH frequency of inviscid instability in the separated shear layer. This suggests that turbulent breakdown in a laminar separation bubble under these conditions occurs through interactions of TS waves with streamwise structures having a much smaller spanwise length scale 22 . Under high free-stream turbulence conditions DNS studies show turbulent breakdown originating from KH instability in the outer region of the boundary layer 23,24 .…”

Section: Boundary Layer Transitionmentioning

confidence: 95%

The Influence of Streamwise Vorticity on Transition in the Presence of Sweep

Gostelow

Garrett

Jean

2011

41st AIAA Fluid Dynamics Conference and Exhibit

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Experimental observations and theoretical predictions of streamwise vorticity on circular cylinders in crossflow and on turbine blades are considered. The observations on cylinders confirm earlier predictions and this forms a firm basis for referencing other measurements and predictions of vortical behavior. It also results directly in a correlation for predicting the spanwise wavelength of streamwise vortices on the convex surfaces of turbomachine blades. Highly resolved Large Eddy Simulation has shown that fine scale organized streamwise vorticity may exist on the convex surfaces of turbine and compressor blading and is predictable. For a turbine blade with a blunt leading edge the streamwise vorticity may persist on a time-average basis to influence the entire suction surface at suitably low Reynolds numbers typical of aircraft cruise conditions. The results emphasize the enormous computing resource required to resolve the flow on a routine basis for design purposes. It is demonstrated computationally that streamwise vorticity interacts with spanwise vorticity in leading edge bubbles to promote early transition and bubble closure. Time resolution is required to capture the flow complexity that is fundamental for an understanding of the physical behavior of the laminar boundary layer and its separation and transition. A narrow spanwise strip does not allow the streamwise vorticity to settle into the organized pattern. For streamwise vorticity to become organized, an adequate spanwise domain and run duration for time averaging are also essential. Any accurate treatment of laminar boundary layers at low Reynolds numbers needs to be performed three dimensionally and with a sufficiently fine spanwise spacing and duration of run to resolve streamwise vortical structures. Sweep of the body, wing or blade poses special problems. Not least is a serious lack of information on even the most basic cases. An attempt is made to relate the streamwise vorticity studied by Kestin and Wood to the more aggressive crossflow instability studied by Poll. More research is needed if designers are to be confident about the flow regimes they might expect to prevail for a given sweep angle. Nomenclature D Cylinder diameter n Amplification factor in inception prediction Re Reynolds number Tu Free-stream turbulence level u Velocity component in x direction x, y, z Coordinate directions    Angle between normal to the inflow and axis of the body    Spanwise spacing between vortex pairs  ___________________________________________________________________________________

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“…The transition scenario is quite similar to that of Jacobs and Durbin (2001). Durbin et al (2009) studied the interaction of a discrete mode which represents Tollmien-Schlichting (T-S) wave and a continuous mode which represents FST. A pattern of K-structures with transition was observed in their simulation.…”

Section: Introductionmentioning

confidence: 98%