Date of Award

2011

Degree Type

Thesis

Department

Mechanical Engineering

First Advisor

Ibrahim, Mounir

Subject Headings

Turbines -- Blades, Boundary layer, Aerofoils, CFD, wake, low pressure high lift airfoil

Abstract

The study of a very high lift, low-pressure turbine airfoil in the presence of unsteady wakes was performed computationally and compared against experimental results. The experimental data were collected in a low speed wind tunnel under high (4.9 ) and then low (0.6 ) freestream turbulence intensity conditions on a linear cascade with wakes that were produced from moving rods upstream of the cascade. The flow coefficient was kept at 0.7 while the rod to blade spacing was changed from 1 to 1.6 to 2 blade spacings. These cases were conducted for Reynolds number equal to 25,000 and 50,000, based on the suction surface length and the nominal exit velocity from the cascade. The experimental and computational data have shown that in cases without wakes, the boundary layer separated and did not reattach. The CFD was done with LES and URANS utilizing the finite-volume code ANSYS Fluent under the same freestream turbulence and Reynolds number conditions as the experiment but only at a rod to blade spacing of 1. With wakes, separation was largely suppressed, particularly if the wake passing frequency was sufficiently high. This was validated in the CFD by comparing the experimental results for the pressure coefficients and velocity profiles, which were reasonable for all cases examined, showing very consistent results in a three dimensional domain with a larger Reynolds' number. The two dimensional domain failed to capture the three dimensionality effects of the wake and thus were less consistent with the experimental data. The effect of the freestream turbulence intensity levels also showed a little more consistency with the experimental data at higher intensities however the low intensity results were also very consistent. As a further computational study, cases were run to simulate a higher wake passing frequencies which were not run experimentally. The results of these computational cases showed that an initial 25 increase in the wake passing frequency greatly reduced the size of the separation bubble, nearly comple

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