Jets issuing through small holes in a wall into a freestream has proven effective in the control of flow separation. This technique is known as the vortex generator jet (VGJs) method. If a precursor signal of separation is found, the separation control system using VGJs can be operated just before the onset of separation and the flow field with no separation is always attained. In this study, we measured the flow field and the wall static pressure in a two-dimensional diffuser to find a precursor signal of flow separation. The streamwise velocity measurements were carried out in the separated shear layer and spectral analysis was applied to the velocity fluctuations at some angles with respect to the diffuser. The pattern of peaks in the spectral analysis changes as the divergence angle increases over the angle of which the whole separation occurs. This change in the spectral pattern is related to the enhancement of the growth of shear layer vortices and appears just before the onset of separation. Therefore, the growth of shear layer vortices can be regarded as a precursor signal to flow separation.
Greenblatt, D. and Wygnanski, I. J., 2000, “The Control of Flow Separation by Periodic Excitation,” Progress in Aerospace Sciences, 36, pp. 487-545.
Shizawa, T. and Eaton, J. K., 1992, “Turbulence Measurements for a Longitudinal Vortex Interacting with a Three-Dimensional Turbulent Boundary Layer,” AIAA J. 30-1, pp. 49-55.
Compton, D. A. and Johnston, J. P., 1992, “Streamwise Vortex Production by Pitched and Skewed Jets in a Turbulent Boundary Layer,” AIAA J., 30- 3, pp. 640-647.
Hasegawa, H., Matsuuchi, K. and Yamakami, J., 1998, “The Mechanism of Active Boundary Layer Control Using Vortex Generator Jets,” 21st Congress of the International Council of the Aeronautical Sciences, ICAS-98-3.4.3.
Hasegawa, H. and Matsuuchi, K., 1998, “Effect of Jet Pitch Angle of Vortex Generator Jets on Separation Control,” Third International Conference on Fluid Mechanics (ICFM-III), pp. 526-531.
Hasegawa, H., Matsuuchi, K. and Komatsuzaki, Y., 2003, “Suppression Effect of Vortex Generator Jets with Non-Circular Orifices on Separating Flow,” 4th ASME/JSME Joint Fluids Engineering Conference.
Hasegawa, H. and Kumagai, S., 2008, “Adaptive Separation Control System Using Vortex Generator Jets for Time-Varying Flow,” Journal of Applied Fluid Mechanics, Vol.1, No.2, pp. 9-16.
Mochizuki, O., Ishikawa, H., Miura, N., Sasuga, N. and Kiya, M., 2001, “Precursor of Separation,” Transactions of JSME, Vol. B-67, No.661, pp. 2226-2233 (in Japanese).
Kumano, S., Mochizuki, O. and Kiya, M., 1999, “Optimum Timing to Start Active Control to Suppress a Dynamic Stall,” Transactions of JSME, Vol. B-65, No.638, pp. 3380-3385 (in Japanese).
Kiya, M., 1989, “Turbulence Structure of Separated-and-Reattaching Flows,” Transactions of JSME, Vol. B-55, No.511, pp. 559-564 (in Japanese).
Kiya, M., Shimizu, M., Mochizuki, O. and Ido, Y., 1992, “Response of an Axisymmetric Separation Bubble to Sinuous Disturbances,” Transactions of JSME, Vol. B-58, No.554, pp. 2953-2958 (in Japanese).
Hasegawa, H., Matsuuchi, K. and Tanaka, J., 1999, “Development of Active Separation Control System Using Vortex Generator Jets,” 3rd ASME/JSME Joint Fluids Engineering Conference, FEDSM99-6944.