LDV Measurements of Orifice Jet Flows at Low Reynolds Numbers

Measurements of orifice jet flows in oil hydraulic tubes with LDV were carried out in order to clarify the flow structure which causes the peculiar behavior in the pressure recovery previously discovered by some of the present authors. The findings are as follows: with Re around 200, the orifice jet flow starts to become turbulent, exhibiting observable pressure recovery. In a typical turbulent jet flow of a low Reynolds number the axial velocity rapidly decreases downstream, and then gradually increases until it reaches the velocity of the corresponding Poiseuille laminar flow. In this rapid deceleration field, a small percentage of the kinetic energy of the main flow is restored to the static pressure, while most of it is consumed to develop turbulence which becomes most intense at the end of the field. The deceleration field travels noticeably downstream as Re decreases below 1000, which coincides with the movement of the pressure recovering region.