Early stages of an impulsively started unsteady flow past non-rectangular prisms

Abstract

Characteristics of the development of an impulsively started flow around non-rectangular sharp-edged prisms were studied numerically. A stream function-vorticity formulation in a body coordinate system was used to describe the developing unsteady flow field. The inflow Reynolds number considered ranges from 25 to 1000. Main flow characteristics of the developing recirculation region aft of the non-rectangular prism and its interaction with the separating shear layer from the leading edges were studied through the developing streamlines. Other flow characteristics are analysed in terms of pressure contours, surface pressure coefficient, wake length and drag coefficient. Four main-flow types and three sub-flow types of regimes are identified through a detailed analysis of the evolution of the flow characteristics. Typically, for a given Reynolds number, it is noted that flow starts with no separation (Type I main-flow). As time advances, symmetrical standing zone of recirculation develops aft of the non-rectangular prism (Type II main-flow). The rate of growth in width, length and structure of the aft end eddies [sub-flow (a)] depends on the Reynolds number. In time, separated flow from the leading edges of the non-rectangular prism also develops (Type III main-flow) and forms a growing separation bubbles [sub-flow (b)] on the upper and lower surfaces of the non-rectangular prism. As time advances longer, the separation bubbles on the upper and lower surfaces 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 regime with a disturbed tertiary flow zone [sub-flow (c)] near the merging junction. Eventually, depending on the Reynolds number and the prism configuration, the flow develops into a particular category of symmetrical standing recirculatory flow of specific characteristics.