Interaction between the primary flow fields and the secondary flow fields under rotating condition

A series of experiments were conducted with PIV (Particle Image Velocimetry) in order to acquire the full view of primary and secondary flow fields, and investigate the interaction between them in the entry section of a rotating smooth channel with square cross section. An FPGA (Field-Programmable Gate Array) based trigger signal generator was developed to minimize the measurement errors caused by the relative movements between the rotating channel and the stationary PIV system. The measurements were conducted at three streamwise planes (X = 445 mm, 525 mm, 605 mm) and eight spanwise planes (Z = 40-75 mm) with the Reynolds number (Re=U¯D/ν) equaling to 4750 and the rotation number (Ro=ΩD/U¯) ranging from 0.25 to 0.75. Based on the experimental data, three interesting problems were discussed. Firstly, the secondary flow near the leading edge surprisingly points to the leading edge, this is attributed to the normal pressure gradient with the help of momentum integral equations. Secondly, the variations of the worst primary velocity deficit are analyzed by a method based on circulation. The results show that the trailing side secondary flow vortices can influence the variations of the worst primary velocity deficit near the leading wall. Thirdly, the critical rotation number phenomenon is analyzed. According to the force and velocity analyses, the critical rotation number phenomenon is attributed to the shift of balance between the normal pressure gradient term and Coriolis term.