This experiment used an infrared laser technique to determine where flow separation and reattachment occurred on a backward facing step. The step size was 6.35 cm, and the overall length of the model was 60.96 cm, with 30.48 cm for the bottom step. Two types of tufts were used in this experiment to define the flow field: string tufts and thermochromic liquid crystal (TLC) tufts. In this experiment, the string tuft was attached to the end of a wand that was placed in the flow path. String tufts are intrusive to the flow and give a less accurate surface visualization because the weight of the string affects the results. Consequently, string tufts do not respond well to low flow velocities. In contrast, TLC tufts do not have these limitations. Thermochromic liquid crystals do not impede the flow and they give accurate surface visualizations. They respond to changing temperature by reflecting light at different wavelengths, resulting in a changing observable color as the temperature changes. An infrared laser heated the surface of the model in a circular spot resulting in crystal color change. Airflow over the heated surface convected energy in the direction of the flow. This resulted in a visible TLC "tail" pointing in the direction of the flow. With this technique, both forward and reverse flow can be detected. Images of these tufts were recorded with a digital video camera. Eccentricity of the tufts indicated the direction of flow. Two velocities of upstream flow were tested: 100% of the blower's voltage, a velocity of 1.13 m/s, and 80% of the blower's voltage, a velocity of 1.07 m/s. For both velocities, reattachment occurred using the TLC tuft method at approximately 17.145 cm from the step. Using the string tuft attached to a wand, these results were verified as the reattachment region was between 16.51 cm and 17.78 cm.

Volume Subject Area:
Fluids Engineering
Topics:
Flow (Dynamics), Lasers, String, Liquid crystals, Temperature, Visualization, Air flow, Crystals, Flow separation, Video cameras, Wavelength, Weight (Mass)
1.
Gregory, J. W., and Peterson, S. D., 2005, “Flow Visualization with Laser-Induced Thermal Tufts,” AIAA 2005-0699, 43rd AIAA Aerospace Sciences Meeting & Exhibit, American Institute of Aeronautics and Astronautics, Inc., Reno, NV, pp.1–10.
2.
Bakker, A. 2002, “Lecture 11: Boundary Layers and Separation,” Class Lecture.
3.
Jones, T. V., “The Use of Liquid Crystals in Aerodynamic and Heat Transfer Testing,” OX1 3PJ, Department of Engineering Science University of Oxford, UK.
4.
Byerley, A. R., Sto¨rmer, O., Baughn, J. W., Simon, T. W., Van Treuren, K. W., and List, J., 2002, “Using Gurney Flaps to Control Laminar Separation on Linear Cascade Blades,” GT-2002-30662, ASME TURBO EXPO 2002, ASME, Amsterdam, pp. 1–9.
5.
Saleh, B., and Teich, M., 1991, Fundamentals of Photonics, 1st ed., Wiley-Interscience, New York, pp. 362.
6.
Kline, S. J., and McClintock, F. A., 1953, “Describing Uncertainties in Single-Sample Experiments,” Mechanical Engineering, pp. 3–8.
7.
Holman, J. P., 2001, Experimental Methods for Engineers, 7th ed., McGraw-Hill, New York, pp. 100.
8.
Munson, B. R., Young, D. F., and Okiishi, T. H., 2002, Fundamentals of Fluid Mechanics, 4th ed., John Wiley & Sons Inc., New York, Appendix B.2
9.
Creuse´
E.
, and
Mortazavi
I.
,
2004
, “
Simulation of Low Reynolds Number Flow Control over a Backward-Facing Step Using Pulsed Inlet Velocities
,”
AMRX Applied Mathematics Research eXpress
, (
4)
, pp.
1
–
21
.
10.
Chun
K.
, and
Sung
H.
,
1998
, “
Visualization of a Locally-forced Separated Flow over a Backward-facing Step
,”
Experiment in Fluids
, Springer-Verlag, (
25)
, pp.
133
–
14
This content is only available via PDF.
Copyright © 2006
 by ASME
You do not currently have access to this content.