DESIGN AND TEST OF A RECYCLED WATER TUNNEL FOR INVESTIGATING TURBULENCE BOUNDARY LAYER FLOW AT LOW REYNOLDS NUMBER

A low Re recycled water tunnel with a 1.5 m-long working section of 0.3 m×0.3 m is designed,manufactured and tested.The contraction section adopts the Witozinsky curve.The velocity in test section can be changed in the range of U_∞=0.2 to 1.0 m/s by a variable-frequency pump.A Laser Doppler Velocimeter(LDV) is used to determine the relation curve between the pump running frequency and the mean velocity and longitudinal turbulence intensity at inlet center of the test section.Furthermore,for a turbulent boundary layer developed from a flat plate placed in the test section,the mean velocity and longitudinal turbulence intensity distribution in normal direction are measured by the LDV,and the low-speed streaks in the buffer layer are investigated with a Particle Image Velocimeter(PIV).The results indicate that the mean velocity in the test section has an approximately linear relation with the pump running frequency.The longitudinal turbulence intensity decreases with increasing velocity,when U_∞0.7 m/s,the turbulence intensity is less than about 2.6%.A fully developed flat plate turbulent boundary layer flow is obtained in the water tunnel,and its low-speed streaks at buffer layer are studied by PIV.