Research in Engineering and Aviation

Comparison of Intermittency Detection Algorithms in a Transitional Boundary Layer

January 2012

Author(s): Null, J., and McQuilling, M.

AIAA-2012-0749, presented at the 50th AIAA Aerospace Sciences Meeting, Nashville, TN, January 9-12, 2012.

Abstract

Intermittency algorithms provide a mathematical means to determine how turbulent a flow undergoing laminar to turbulent transition may be.  Jordan Null (BSAE 2012) is comparing four different intermittency detection algorithms: Clark, Hedley-Keffer, MTERA, and the Volino-Hultgren. The algorithms are each used to process thermal anemometry data sets from a single normal hot-wire taken over the suction surface of a transitional low-pressure turbine airfoil (L2F) at Reynolds numbers of 2.5x10^5, 5.0x10^5, and 7.5x10^5. Through comparing these algorithms further insight can be gained about which algorithms may be more appropriate than others for transitional low pressure turbine boundary layers. Along with this, the comparisons will show which algorithms pick up the onset of transition at the anticipated location and if the resulting intermittency intensities accurately reflect the actual flow over the airfoil. Intermittency contours of the entire flowfield along with intermittency profiles will be compared. Current results show that three algorithms indicate the same transition onset location, but the intermittency contours differ in magnitude and in surface normal extent before dropping back down to a zero or near zero level. To also confirm if the algorithms’ results are accurate, the velocity data is compared to Coles’ Law of the Wake, an accepted turbulent velocity profile. This will help indicate when the flow actually has begun transitioning and when it fully transitions to turbulent flow, and if the algorithms are correctly reflecting these facts. In addition, points of interest throughout the flowfields will be examined in more scrutiny using a combination of Fourier transforms, spectrograms (short time Fourier transforms), and wavelets. This work was presented at the 50th AIAA Aerospace Sciences Meeting in Nashville, TN, from January 9 - 12, 2012 as paper AIAA-2012-0749, and after wavelet analyses have been completed the paper will be submitted to the Physics of Fluids journal by the end of the spring 2012 semester.