边界层,湍流中速度矢量场检测方案(粒子图像测速)

A flat plate boundary layer flow was generated at the open test section of the circuit type 1m- wind tunnel of the DLR, Gottingen, at a free stream velocity of U亣 = 7 m/s with zero pressure gradients. A transparent and smooth surface glass plate is utilized to provide optical access from its bottom and a good flow quality. The plate has an elliptic leading edge to prevent flow separation. Four tripping wires, one with 1.5 mm diameter and three with 1 mm diameter were positioned with 1 mm x-distance close to the surface at (Rex). . 290 along the whole span-wise elongation of the plate. This bypass transition of the flow results in the development of a turbulent boundary layer flow. In a box of about 34 x 18 x 35 mm3 in x-y and z-directions around (Rex). . 450 tracer particles has been illuminated by laser light pulses at 4 kHz with 21 mJ each in order to achieve PIV recordings without using the frame straddling method. The light source was a Nd:YAG laser from Lee Laser inc. Two highly reflective dielectric mirrors aligned in the x-y-direction are installed nearly parallel and close to the surface of the flat plate on opposite sides of the measurement region. The collimated laser beam undergoes multiple reflections between the two mirrors passing through the measurement volume approximately 15 times before exiting (see sketch left). This arrangement resulted in a light amplification with a factor ~5 with respect to a single pass system. The specific illumination arrangement has the advantage that all viewing directions collect scattered light both in forward and backward directions, which provided the level of light needed for imaging particles in the whole volume.