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Research Papers: Flows in Complex Systems

Identification of Flow Structures on a LP Turbine Blade Due to Periodic Passing Wakes

[+] Author and Article Information
S. Sarkar

Department of Mechanical Engineering, Indian Institute of Technology, Kanpur 208016, Indiasubra@iitk.ac.in

J. Fluids Eng 130(6), 061103 (Jun 09, 2008) (10 pages) doi:10.1115/1.2911682 History: Received August 27, 2007; Revised December 29, 2007; Published June 09, 2008

Abstract

The paper describes the flow structures on the suction surface of a highly cambered low-pressure turbine (LPT) blade (T106 profile) subjected to periodic convective wakes. A separation bubble on the rear half of the suction side of the blade was found to form under the operating condition due to the highly diffusive boundary layer. Interactions of migrating wakes with this separated boundary layer trigger rollup of the shear layer leading to transition and the appearance of coherent vortices. To characterize the dynamics of these large-scale structures, a proper orthogonal decomposition is pursued on both the fluctuating velocity and the vorticity fields generated by large-eddy simulations (LESs) of wake passing over the LPT blade for a Reynolds number $Re=1.6×105$. The first two modes clearly depict the rollup of the unstable shear layer and formation of large-scale vortex loops that contain a major fraction of the fluctuation energy. The present LES, at least in a qualitative sense, illustrates the large-scale motions in the outer layer and dynamics of vortical structures in a separated boundary layer excited by external perturbations.

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Figures

Figure 1

Geometry of T106 low-pressure cascade and a schematic of a row of wake-generating cylinders sweeping at a speed Ub ahead of cascade

Figure 2

Instantaneous isosurface of vorticity ∣ω∣ at three equal time intervals during the wake passing cycle

Figure 3

Snapshots of instantaneous spanwise vorticity when the wake is ahead of the separation region over the rear half of the suction surface (the location of separation is marked by an arrow)

Figure 4

Snapshots of (a) instantaneous spanwise vorticity and (b) streamlines as the wake passes over the separation region of the suction surface

Figure 5

Power spectrum of pressure histories from different points, fr=fC∕V2is

Figure 6

Time-averaged velocity profiles and their derivatives at different sections along the rear half of the suction surface

Figure 7

Velocity fluctuations (rms and phase averaged) at three sections of the boundary layer along the rear half of the suction surface

Figure 8

Sum of eigenvalues divided by their total sum, indicating energy content

Figure 9

Sum of eigenvalues divided by their total sum, indicating energy content for the midspan and span-averaged data

Figure 10

First four eigenvectors of velocity disturbances for the midspan data during the wake-passing cycle: (a) indicates the shear layer instability via the KH mechanism and (b) illustrates the formation of large-scale vortex loops and the leading-edge contamination

Figure 11

First four eigenvectors of disturbances of vorticity for the midspan data during the wake-passing cycle

Figure 12

First three eigenvectors of velocity disturbances (streamlines) during the wake-passing cycle: (a) for the midspan data and (b) for the span-averaged data

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