AD-A267 26 1 ziz 00 80 11/L%2--- Final Technical Re art 7/L/89-9430/92

The objective of the study is to explore concepts for control of turbulent boundary layers leading to skin-friction reduction using the direct numerical simulation tech-nique. This report is divided into three parts where different *control methods are investigated: an active control by sensing and perturbing structures near the wall, a feadback control procedure mitidi'd by control theory, and a passive control by long-itudinal riblets. Part 1: significant drag redtiction is achieved when the surface boundary condition is modified to suppress the dynamically significant coherent structures present in thi wall region. Part ZI: mathematical methods of control theory are applied to the problem of con-trol of fluid flow. Part III: direct numerical simulation is performed to analyze turbulent flow over longitudinal riblets, and to educe the mechanism of drag reduction by riblets