Transition in a Separation Bubble

Malkiel, Ed; Mayle, R. E.

doi:10.1115/1.2840931

Cited by 91 publications

(41 citation statements)

References 15 publications

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“…9 Comparisons of measured separation bubble development over airfoils and flat plates with predictions of stability theory suggests that the initial stage of this process is dominated by a linear inviscid mechanism. [10][11][12][13][14][15] As disturbances grow, they begin to interact, leading to secondary disturbance amplification centered around subharmonics of the primary frequency and eventually a saturation of disturbance growth. In the late stages of transition, shear layer roll-up at the fundamental shear layer instability frequency has been observed in separation bubbles over flat plates and airfoils in direct numerical simulations [16][17][18][19][20][21][22] and experiments.…”

Section: Introductionmentioning

confidence: 99%

Separated shear layer transition over an airfoil at a low Reynolds number

2012

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Shear layer development over a NACA 0018 airfoil at a chord Reynolds number of 100 000 was investigated using a combination of flow visualization, velocity field mapping, surface pressure fluctuation measurements, and stability analysis. The results provide a detailed description of shear layer transition on an airfoil at low Reynolds numbers. An extensive comparison of measured surface pressure and velocity fluctuations demonstrated that time-resolved surface pressure sensor arrays can be used to identify the presence of flow separation, estimate the extent of the separated flow region, and measure disturbance growth rate spectra in significantly less time than is required by conventional techniques. Surface pressure sensor measurements of disturbance growth rate, wave number, and convection speed are found to compare well with predictions of linear stability theory, supporting the claim that convection speeds measured in separation bubbles over low Reynolds number airfoils are associated with wave packets of growing disturbances propagating through the shear layer. Through a comparison of measured convection speeds in this investigation and prior low Reynolds number airfoil experiments, it is shown that disturbance convection speeds of between 30% and 50% of the edge velocity are typical for this type of flow, consistent with phase speed estimates from previous analytical studies on transitional separation bubbles. Modal RMS velocity profiles were measured and found to be reasonably predicted by stability theory. The results suggest that, even for the relatively thick NACA 0018 airfoil profile, disturbance development over the majority of the laminar separated shear layer is primarily governed by a linear inviscid mechanism. C 2012 American Institute of Physics. [http://dx.

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Section: Introductionmentioning

confidence: 99%

Separated shear layer transition over an airfoil at a low Reynolds number

2012

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“…Unfortunately, the available experimental information on the onset and length of the transition re gion in typical separation bubbles encountered on turbomachiner y blades is quite sparse. The state of knowledge on the role of transition in gas turbine engines has been well summarized by Mayle (1991) and valuable additional experiments have recentl y been performed b y Malkiel and Mayle (1996) and Walraevens and Cumpst y (1995). Recognizing the lack of information on the effect of pressure gradient and free-stream turbulence on transition Gostelow et al (1994Gostelow et al ( , 1996 have conducted careful attached flow experiments and Solomon et al (19%) have formulated a new prediction model for the transition len gth.…”

Section: Introductionmentioning

confidence: 99%

On the Calculation of Laminar Separation Bubbles Using Different Transition Models

Sanz

Platzer

1997

Volume 1: Aircraft Engine; Marine; Turbomachinery; Microturbines and Small Turbomachinery

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Laminar separation bubbles are commonly observed on turbomachinery blades and therefore require effective methods for their prediction. The location and size of the bubbles is critically dependent on the laminar-to-turbulent transition process. Therefore, in this paper the transition models of Solomon et al., Abu-Ghannam & Shaw, Mayle, Calvert, and Choi & Kang are incorporated into an upwind-biased Navier-Stokes solver and the computed results are compared with the measurements of Elazar & Shreeve in a cascade with controlled-diffusion blades. It is found that none of the models predicts the measured bubbles very well, although most of them give reasonable results as long as transition is predicted to occur within the bubble.

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“…The spanwise non-uniformity has been tied to the development of elongated, counter-rotating streamwise vortices (Pierrehumbert and Widnall, 1982;Huang and Ho, 1990) which interact with the primary spanwise vortices produced by the K-H mechanism of instability. Malkiel and Mayle (1996) suggest that the interaction of the streamwise and spanwise vortices is responsible for generating small-scale turbulence near the core of the streamwise vortices, likely through the creation of coherent vortical structures in the separated shear layer. Watmuff (1999) and McAuliffe and Yaras (2007a) have observed hairpin-like structures in the separated shear layer-referred to by Watmuff (1999) as "vortex loops"-which provide a mechanism for the wall-normal exchange of streamwise momentum through the induced velocities of the vortex loops.…”

Section: Secondary Instabilitiesmentioning

confidence: 99%

“…As the K-H waves convect downstream, their induced velocity field amplifies the perturbation of the separated shear layer until the shear layer rolls-up into discrete spanwise vortices that shed from the shear layer and convect downstream (Malkiel and Mayle, 1996;Spalart and Strelets, 2000).…”

Section: Primary Shear-layer Instabilitymentioning

confidence: 99%

“…Transitional separation bubbles are frequently observed on the suction surface of highly-loaded airfoils, deteriorating the performance of the airfoil through reduced lift and increased drag (Mayle, 1991;Hodson and Howell, 2005). Numerous studies have been conducted to understand the effects of flow Reynolds number, streamwise pressure gradient, surface roughness, periodic free-stream unsteadiness, and vortical and acoustical disturbances on separation-bubble transition (Roberts and Yaras, 2005a;Malkiel and Mayle, 1996;Yang and Voke, 2001;Volino and Bohl, 2004).…”

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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Transition in a Separation Bubble

Cited by 91 publications

References 15 publications

Separated shear layer transition over an airfoil at a low Reynolds number

Separated shear layer transition over an airfoil at a low Reynolds number

On the Calculation of Laminar Separation Bubbles Using Different Transition Models

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

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