Separation Bubbles Under Steady and Periodic-Unsteady Main Flow Conditions

Lou, Weiliang; Hourmouziadis, J.

doi:10.1115/2000-gt-0270

Cited by 50 publications

(12 citation statements)

References 12 publications

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“…The plate has been installed between two contoured walls that produce the prescribed adverse pressure gradient, typical of Ultra-High-Lift turbine profiles. The endwall geometry is scaled from the Lou and Hourmouziadis (2000) test case.…”

Section: Test Facility and Experimental Devicesmentioning

confidence: 99%

Transition mechanisms in laminar separation bubbles with and without incoming wakes and synthetic jet effects

et al. 2012

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Laminar separation and transition processes of the boundary layer developing under a strong adverse pressure gradient, typical of Ultra-High-Lift turbine profiles, have been experimentally investigated for a low Reynolds number case. The boundary layer development has been surveyed for different conditions: with steady inflow, with incoming wakes and with the synchronized forcing effects due to both incoming wakes and synthetic jet (zero net mass flow rate jet). In this latter case, the jet Strouhal number has been set equal to half the wakereduced frequency to synchronize the unsteady forcing effects on the boundary layer. Measurements have been taken by means of a single-sensor hot-wire anemometer. For the steady inflow case, particle image velocimetry has been employed to visualize the large-scale vortical structures shed as a consequence of the Kelvin-Helmholtz instability mechanism. For the unsteady inflow cases, a phase-locked ensemble averaging technique, synchronized with the wake and the synthetic jet frequencies, has been adopted to reconstruct the boundary layer space-time evolution. Results have been represented as color plots, for several time instants of the forcing effect period, in order to provide an overall view of the time-dependent transition and separation processes in terms of ensemble-averaged velocity and unresolved unsteadiness distributions. The phase-locked distributions of the unresolved unsteadiness allowed the identification of the instability mechanisms driving transition as well as the Kelvin-Helmholtz structures that grow within the separated shear layer during the incoming wake interval and the synthetic jet operating period. Incoming wakes and synthetic jet effects in reducing and/or suppressing flow separation are investigated in depth. List of symbols ClJet momentum coefficient ¼

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Section: Test Facility and Experimental Devicesmentioning

confidence: 99%

Transition mechanisms in laminar separation bubbles with and without incoming wakes and synthetic jet effects

et al. 2012

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“…The experiments have been carried out in an Eifel-type wind tunnel with a flat plate under oscillating inlet conditions as described in detail by Lou [11][12][13]. It consists of the following modules ( Fig.…”

Section: Unsteady Profile Pressure Distribution On the Stator Bladesmentioning

confidence: 99%

“…The computational geometry, shown in Fig. 29, was chosen in accordance with experiments performed in the companion project at TU Berlin [12], see Section 3. The special shape of the upper wall generates a strongly adverse streamwise pressure At the inflow boundary of the first series of simulations an oscillating flow field (u, v, w) = U 0 1 + a sin 2π t T, 0, 0 is prescribed, where a is the amplitude and T is the period.…”

Section: Flat Plate Boundary Layer Separationmentioning

confidence: 99%

Recent German Research on Periodical Unsteady Flow in Turbomachinery

Mailach

Vogeler

2009

Flow Turbulence Combust

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The main source of flow unsteadiness in turbomachinery is the aerodynamical interaction of the rotor and stator blade rows. The blades and vanes, moving relatively to each other, interact because of the viscous wakes and the potential effects of the blades. In addition, the wakes and potential effects superimpose with other flow patterns, for instance the tip clearance vortices and other secondary flow phenomena. Furthermore in transonic compressors the interaction of wakes and shocks plays an important role. As a result, the real flow field is highly periodically unsteady and very complex, especially in multistage turbomachinery. Although this fact has received increasing attention within recent years, blade row interactions effects are not yet typically addressed in current design systems of turbomachinery. Actually, there is a requirement of the ability of modern design methods to predict unsteady flow features. With increasing aerodynamic loading of the blades and higher Mach numbers the consideration of rotor-stator-interactions gains in importance. It is therefore one of the challenges of the present and future design of compressors and turbines to include beneficial unsteady effects to improve the engine parameters. This requires a detailed physical understanding of the unsteady flow field and the resulting effects on the performance and flow stability. In 2000 the joint research program "Periodical Unsteady Flow in Turbomachinery" was initiated. Partners of this project are five research groups from four german universities:

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“…Up to date studies ( [15][16][17][18] among others) showed that a two-dimensional inviscid instability originates from the shear layer flow over the stagnation region of the separation bubbles via the Kelvin-Helmholtz mechanism. Vortical structures are generated and induce fluctuations within the flow promoting the boundary layer transition.…”

Section: Introductionmentioning

confidence: 99%

“…This plate is located between two contoured walls that produce the prescribed adverse pressure gradient, typical of Ultra-High-Lift turbine profiles. The endwall geometry is scaled from the Lou and Hourmouziadis test case [16] producing consequently the same pressure coefficient distribution along the plate. The flat plate chord is 200 mm and the span length is 300 mm.…”

Section: Introductionmentioning

confidence: 99%

Application of a Synthetic Jet to Control Boundary Layer Separation under Ultra-High-Lift Turbine Pressure Distribution

Lengani

Simoni

Ubaldi

et al. 2011

Flow Turbulence Combust

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The transition and separation processes of the boundary layer developing on a flat plate under a prescribed adverse pressure gradient typical ofUltra-High-Lift low-pressure turbine profiles have been investigated, with and without the application of a synthetic jet (zero net mass flow rate jet). A mechanical piston has been adopted to produce an intermittent flow with zero net mass flow rate. The capability of the device to suppress or reduce the large laminar separation bubble occurring under steady inflow condition at low Reynolds numbers has been experimentally investigated by means of hot-wire measurements.Wall static pressure measurements complement the hot-wire time-resolved velocity results. The paper reports the investigations performed for both steady and controlled conditions. The active device is able to control the laminar separation bubble induced at low Reynolds number conditions by the strong adverse pressure gradient. An overall view of the timedependent evolution of the controlled boundary layer is provided by the phaselocked ensemble averaging technique, triggered at the synthetic jet frequency. The separated flow transition process, which is detected for the uncontrolled condition, is modified by the synthetic jet in different ways during the blowing and suction phases. Overall, the phase-locked velocity distributions show a reduced separated flow region for the whole jet cycle as compared to the uncontrolled condition. The phase-locked distributions of the random unsteadiness allow the identification of vortical structures growing along the shear layer mainly during the blowing phase

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Separation Bubbles Under Steady and Periodic-Unsteady Main Flow Conditions

Cited by 50 publications

References 12 publications

Transition mechanisms in laminar separation bubbles with and without incoming wakes and synthetic jet effects

Transition mechanisms in laminar separation bubbles with and without incoming wakes and synthetic jet effects

Recent German Research on Periodical Unsteady Flow in Turbomachinery

Application of a Synthetic Jet to Control Boundary Layer Separation under Ultra-High-Lift Turbine Pressure Distribution

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