INVESTIGATION OF TURBULENT BOUNDARY LAYER OVER FORWARD-FACING STEP BY MEANS OF DNS

This paper presents observations and investigations of the detailed structure of a turbulent boundary layer over a forward-facing step. A turbulent boundary layer with reattachment and separation which causes an increase in friction of flow and disturbance in transportation of scalar often occurs in fluid machinery, the atmosphere, etc. Thus, it is important for friction reduction and environmental protection to know the transport phenomena of a boundary layer with reattachment and separation. However, little data on a boundary layer with reattachment and separation are available in comparison with an internal turbulent flow such as a channel flow. Data of turbulent heat and mass transfer in the boundary layer over a block are especially difficult to obtain. Therefore, in order to investigate the detailed turbulent quantities of a boundary layer over an obstacle which causes reattachment and separation, a DNS of the turbulent boundary layer over a forward-facing step must first be carried out. The present DNSs are conducted under conditions with various Reynolds numbers based on step height, or based on momentum thickness so as to investigate the effects of step height and inlet boundary layer thickness. DNS results show the quantitative turbulent statistics and structures of boundary layers over a forward-facing step, in which the characteristic transport phenomena in the recirculation regions which occur in front of and on the step are demonstrated.