FLOW OVER 2 CYLINDERS OF DIFFERENT DIAMETERS - THE LOCK-IN EFFECT

The frequencies of vortex shedding from two adjacent cylinders with th e large diameter cylinder upstream of the smaller one, were measured a t different cylinder spacings, diameter ratios and stagger angles. Ove r a range of diameter ratios, the vortex shedding frequency of the dow nstream cylinder was locked in at a frequency of twice that of the ups tream cylinder. At a diameter ratio less than two, the vortex shedding frequencies were the same for both cylinders in three of the position s tested. In general, the base pressure decreases with an increase in the diameter ratio until lock-in occurs. After lock-in, the base press ure remains approximately constant. However, the magnitude of the base pressure depends on the degree of flow interference caused by the ups tream cylinder.