EARLY STAGES OF AN IMPULSIVELY STARTED UNSTEADY-FLOW PAST NONRECTANGULAR PRISMS

Characteristics of the development of an impulsively started dow aroun d non-rectangular sharp-edged prisms were studied numerically. A strea m function-vorticity formulation in a body coordinate system was used to describe the developing usteady dow field. The inflow Reynolds numb er considered ranges from 25 to 1000. Main dow characteristics of the developing recirculation region aft of the non-rectangular prism and i ts interaction with the separating shear layer from the leading edges were studied through the developing streamlines. Other how characteris tics are analysed in terms of pressure contours, surface pressure coef ficient, wake length and drag coefficient. Four main-flow types and th ree sub-dow types of regimes are identified through a detailed analysi s of the evolution of the flow characteristics. Typically, for a given Reynolds number, it is noted that flow starts with no separation (Typ e I main-flow). As time advances, symmetrical standing zone of recircu lation develops aft of the non-rectangular prism (Type II main-how). T he rate of growth in width, length and structure of the aft end eddies [sub-flow (a)] depends on the Reynolds number. In time, separated flo w from the leading edges of the non-rectangular prism also develops (T ype III main-dow) and forms a growing separation bubbles [sub-flow (b) ] on the upper and lower surfaces of the non-rectangular prism. As tim e advances longer, the separation bubbles on the upper and lower surfa ces of the prism grow towards downstream regions and eventually merge with the swelling symmetrical eddies aft of the prism. This merging of the Type II and Type III flows created a complex Type IV main-flow re gime with a disturbed tertiary Row zone [sub-flow (c)] near the mergin g junction. Eventually, depending on the Reynolds number;md the prism configuration, the dow develops into a particular category of symmetri cal standing recirculatory flow of specific characteristics. (C) 1998 Elsevier Science Ltd. All rights reserved.