A LASER-DOPPLER VELOCIMETRY STUDY OF ENSEMBLE-AVERAGED CHARACTERISTICS OF THE TURBULENT NEAR WAKE OF A SQUARE CYLINDER

Ensemble-averaged statistics at constant phase of the turbulent near-w ake flow (Reynolds number approximate to 21400) around a square cylind er have been obtained from two-component laser-Doppler measurements. P hase was de-tined with reference to a signal taken from a pressure sen sor located at the midpoint of a cylinder sidewall. The distinction is drawn between the near wake where the shed vortices are 'mature' and distinct and a base region where the vortices grow to maturity and are then shed. Differences in length and velocity scales and vortex celer ities between the flow around a square cylinder and the more frequentl y studied flow around a circular cylinder are discussed. Scaling argum ents based on the circulation discharged into the near wake are propos ed to explain the differences. The relationship between flow topology and turbulence is also considered with vorticity saddles and streamlin e saddles being distinguished. While general agreement with previous s tudies of flow around a circular cylinder is found with regard to esse ntial flow features in the near wake, some previously overlooked detai ls are highlighted, e.g. the possibility of high Reynolds shear stress es in regions of peak vorticity, or asymmetries near the streamline sa ddle. The base region is examined in more detail than in previous stud ies, and vorticity saddles, zero-vorticity points, and streamline sadd les are observed to differ in importance at different stages of the sh edding process.