An Experimental Analysis of the Laminar Separation Bubble At Low Reynolds Numbers

Author

Karla Marie Swift, University of Tennessee - Knoxville

Date of Award

12-2009

Degree Type

Thesis

Degree Name

Master of Science

Major

Aerospace Engineering

Major Professor

Ahmad Vakili

Committee Members

Basil Antar, U. Peter Solies

Abstract

This paper is an investigation into the laminar separation bubble that frequently plagues airborne vehicles operating in the low Reynolds number regime – experimentally found to be typically present in flows with Reynolds numbers below 10⁶ (Lissaman 1983). The specific application driving the present investigation is the fixed wing performance of unmanned micro air vehicles (MAVs), defined by their maximum chord length of 6 inches and current cruising speeds of 10-20 meters per second (Mueller 2001). A basic generic model was chosen for this investigation: a circular arc (section of 16 inch diameter PVC pipe) with sharp leading and trailing edges having a chord length of 9.3 inches and height of 1.5 inches. This airfoil model was tested in the UTSI water tunnel at Reynolds numbers of 27,000 and 45,000. The goal of this study was to gain some insight into the boundary layer behaviour through the use of dye injection for flow visualization and hot film anemometry for quantitative velocity measurements. Small diameter cylinders were then statically placed upstream of the model to determine their interaction with the laminar separation bubble and its effects on the boundary layer downstream over the airfoil model. The length and height of the laminar separation bubble was found to be impacted with a small cylindrical wire placed upstream at all Reynolds numbers and angles of attack with the exception of an 18 degree angle of attack at the higher Reynolds number. However, these changes did not result in a substantial or distinguishable improvement in the downstream separation point. The laminar separation bubble was found to be nearly or completely eliminated when a thermocouple wire was placed upstream of the leading edge. Although the elimination of the bubble would result in only a minor decrease in drag and increase in lift, there would be a possible improvement in the stability of the leading edge stall and possible reduction or elimination in the hysteresis associated with stall.

Recommended Citation

Swift, Karla Marie, "An Experimental Analysis of the Laminar Separation Bubble At Low Reynolds Numbers. " Master's Thesis, University of Tennessee, 2009.
https://trace.tennessee.edu/utk_gradthes/561