NDLI: Experimental Investigation of Transition to Turbulence as Affected by Passing Wakes

Content Provider	The American Society of Mechanical Engineers (ASME) Digital Collection
Author	Richard, W. Kaszeta Terrence, W. Simon David, E. Ashpis
Copyright Year	2001
Abstract	This paper presents experimental results from a study of the effects of periodically passing wakes upon laminar-to-turbulent transition and separation in a low-pressure turbine passage. The test section geometry is designed to simulate unsteady wakes in turbine engines for studying their effects on boundary layers and separated flow regions over the suction surface by using a single suction surface and a single pressure surface to simulate a single turbine blade passage. Single-wire, thermal anemometry techniques are used to measure time-resolved and phase-averaged, wall-normal profiles of velocity, turbulence intensity and intermittency at multiple streamwise locations over the turbine airfoil suction surface. These data are compared to steady-state wake-free data collected in the same geometry to identify the effects of wakes upon laminar-to-turbulent transition. Results are presented for flows with a Reynolds number based on suction surface length and stage exit velocity of 50,000 and an approach flow turbulence intensity of 2.5%. While both existing design and experimental data are primarily concerned with higher Reynolds number flows (Re > 100,000), recent advances in gas turbine engines, and the accompanying increase in laminar and transitional flow effects, have made low-Re research increasingly important. From the presented data, the effects of passing wakes on transition and separation in the boundary layer, due to both increased turbulence levels and varying streamwise pressure gradients are presented. The results show how the wakes affect transition. The wakes affect the flow by virtue of their difference in turbulence levels and scales from those of the free-stream and by virtue of their ensemble-averaged velocity deficits, relative to the free-stream velocity, and the concomitant changes in angle of attack and temporal pressure gradients. The relationships between the velocity oscillations in the freestream and the unsteady velocity profile shapes in the near-wall flow are described. In this discussion is support for the theory that bypass transition is a response of the near-wall viscous layer to pressure fluctuations imposed upon it from the free-stream flow. Recent transition models are based on that premise. The data also show a significant lag between when the wake is present over the surface and when transition begins.
Sponsorship	International Gas Turbine Institute
File Format	PDF
ISBN	9780791878521
DOI	10.1115/2001-GT-0195
Volume Number	Volume 3: Heat Transfer; Electric Power; Industrial and Cogeneration
Conference Proceedings	ASME Turbo Expo 2001: Power for Land, Sea, and Air
Language	English
Publisher Date	2001-06-04
Publisher Place	New Orleans, Louisiana, USA
Access Restriction	Subscribed
Subject Keyword	Suction Turbulence Turbine blades Reynolds number Oscillations Airfoils Pressure gradient Separation (technology) Wire Flow (dynamics) Pressure Steady state Geometry Design Gas turbines Wakes Fluctuations (physics) Shapes Boundary layers Turbines
Content Type	Text
Resource Type	Article

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