Showing papers by "Mohamed Gad-el-Hak published in 1985"

The discrete vortices from a delta wing

Abstract: Introduction: The Classical View T HE flow over delta wings at an angle of attack is dominated by two large bound vortices that result from the flow separation at the leading edge. The classical view of these vortices is sketched in Fig. la and has been discussed by Hoerner and Borst among others. With a sharp leading edge at an angle of attack a, the flow is separated along the entire leading edge forming a strong shear layer. The shear layer is wrapped up in a spiral fashion, resulting in the large bound vortex as sketched. These vortices appear on the suction surface and increase in intensity downstream. The low pressure associated with the vortices produces an additional lift on the wing, often called nonlinear or vortex lift, which is particularly important at large angles of attack. As sketched in Fig. la, small secondary vortices also appear on the wing near the points of reattachment as a result of the strong lateral flow toward the leading edge.

The pitching delta wing

Abstract: Delta wings in steady flow can provide high lift at large angles of attack and are therefore used on many highperformance aircrafts. However, the unsteady aerodynamic properties of a delta wing are practically unknown, although vital for operating and designing airplanes for poststall and other maneuvering. In this study, the flowfields around two pitching delta wings with apex angles of 90 and 60 deg were visualized in a towing tank at chord Reynolds numbers up to 3.5 xlO. The reduced frequency was varied in the range 0.05-3. The leadingedge separation vortex went through a growth-decay cycle with hysteresis during a pitching period. A distinct change of the separated flow was observed at a reduced frequency around ir.

The Dynamics of Turbulent Spots

Abstract: In this article we present a critical review of the present state of knowledge of turbulent spots. We discuss the properties of both newborn as well as fully-developed spots. Both ensemble-averaged results and also the underlying structure of the spot are presented. It is shown that the ensemble-averaged results can be misleading, and that the spot has many features similar to those of a fully-turbulent boundary layer. The mechanisms by which a turbulent spot spreads into the surrounding fluid, which were first suggested by Corrsin and Kistler (1955), will be elaborated. Finally, similarities between spots and other flows will be discussed.

The Dynwnics of Thrbulent Spots

Abstract: In this article we present a critical review of the present state of knowledge of turbulent spots. He discuss the properties of both new­ born as weIl as fully-developed spots. Both ensemble-averaged results and also the underlying structure of the spot are presented. It is shown that the ensemble-averaged results can be misleading, and that the spot has many features similar to those of a fUlly-turbulent boundary layer. The mechanisms by which a turbulent spot spreads into the surrounding fluid, which were first suggested by Corrsin and Kistler (1955), will be elaborated. Finally, similarities between spots and other flows will be discussed. 1. I ntroduction In 1951 Emmons, while studying the flow of water over a nearly horizon­ tal water table, observed that transition to turbulence occurred through the appearance of small, individual patches of turbulence in an otherwise laminar boundary layer. These patches, wh ich he termed spots, started at random instants in space and time as very small regions of turbulence, and grew in an approximately linear manner as they were swept downstream. The number and size of the spots increased in the downstream direction until the spots amalgamated into a fully­ turbulent boundary layer. Based upon these results, Emmons proposed that transition from laminar to turbulent flow occurs through the generation, growth, and amalgamation of turbulent spots. The viewpoint that turbulent spots play a central role in boundary layer transition has become generally accepted. Furthermore it has been suggested that spots are dynamically similar to turbulent boundary layers but simpler, so that their study might be profitable in shedding light on the complex dynamics of turbulent boundary layers. It has been also suggested that turbulent boundary layers might be a composite of turbulent spots, so that the spots are the basic building block of the boundary layer. Thus the study of turbulent spots has be­ come an essential part of the study of both boundary layer transition and turbulence.