SMOKE-WIRE FLOW VISUALIZATION OF THE NEAR-WAKE REGION BEHIND A CIRCULAR DISC AT LOW REYNOLDS-NUMBERS

The flow structures in the near field of the unducted wake region behi nd a circular disc for annular flow at low Reynolds numbers were studi ed by smoke-wire flow visualization technique. A twisted-dual-wire was employed to perform the time evolving visualization. Three typical ch aracteristic flow modes: Q-tip, open-top toroid, and closed toroid, we re identified in the near disc region. For Reynolds number between 130 and 390, the Q-tip flow mode which subject to a periodic up-down osci llatory motion was observed. The open-top toroid mode which experience s the expelling vortex shedding was found for Reynolds number between 390 and 455 The free separation surface turns around and merges to the central axisymmetric axis to form the conventionally observed toroida l recirculation bubble for Reynolds number higher than 455 The closed toroid mode exhibits both expelling and shear-layer vortex sheddings. With the identified flow modes at low Reynolds numbers, the recirculat ion contours, recirculation length, and the shedding frequency in each mode were measured and discussed.