NUMERICAL STUDY OF THE EARLY STAGES OF IMPULSIVELY STARTED UNSTEADY LAMINAR-FLOW PAST A SQUARE CYLINDER

The time development of the symmetrical standing zones of recirculatio n, which is formed in the early stages of the impulsively started lami nar flow over the square cylinder, have been studied numerically. The Reynolds number considered ranges from 25 to 1,000. Main flow characte ristics of the developing recirculation region aft of the square cylin der and its interaction with the separating shear layer from the leadi ng edges are studied through the developing streamlines. Other flow ch aracteristics are analysed in terms of pressure contours, surface pres sure coefficient, wake length and drag coefficient. Four main-flow typ es and three subflow types of regimes are identified through a detaile d analysis of the evolution of the flow characteristics. Typically, fo r a given Reynolds number, it is noted that flow starts with no separa tion (type I main-flow). As time advances, symmetrical standing zone o f recirculation develops aft of the square cylinder (type II main-flow ). The rate of growth in width, length and structure of the aft end ed dies (sub-flow (a)) depends on the Reynolds number. In time, separated flow from the leading edges of the square cylinder also develops (typ e III main-flow) and forms growing separation bubbles (sub-flow (b)) o n the upper and lower surfaces of the square cylinder. As time advance s, the separation bubbles on the upper and lower surfaces of the cylin der grow towards downstream regions and eventually merge with the swel ling symmetrical eddies aft of the cylinder. 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 junctio n. Eventually, depending on the Reynolds number, the flow develops int o a particular category of symmetrical standing recirculatory flow of specific characteristics.