Characteristics of Large Vortical Structures in the Shallow Wake Flow With a Gap Near the Bed

Abstract

PIV measurements were made to investigate the turbulent wake flow generated by a vertical sharp-edged flat plate suspended in a shallow channel flow with a gap near the bed. The purpose of this study is to investigate the behaviour of large vortical structures in the wake flow. The investigation focused on the horizontal velocity field in the mid-distance between the bottom bed and the top free surface. Two different gap heights between the channel bed and the bottom edge of the bluff body was studied. These two cases were compared to the no-gap flow case which is considered as a reference case. The Reynolds number based on the water depth was 45,000. The large vortical structures were exposed by analyzing the PIV velocity fields using the proper orthogonal decomposition (POD) method. Only few modes were used for the POD reconstruction of the velocity fields to recover ∼50% of the turbulent kinetic energy. A vortex identification algorithm was then employed to quantify the number, size, circulation, and direction of rotation of the exposed vortices. A statistical analysis of the distribution of number, size, and strength of the identified vortices was carried out to explore the characteristics of the vortical structures. The results revealed that the number of vortical structures increased as a result of the gap flow with a corresponding decrease in the vortex size and strength. This behaviour is attributed to the production of new vortices and the enhancement of the tearing process.