The impact of buoyancy on the development of starting flows in the near field was experimentally investigated using the Digital Particle Image Velocimetry and Planar Laser Induced Flourescence techniques. The experiments were conducted by releasing cylindri-cal columns of fluid into a glass water tank. Two diameters (0.95 and 1.9 cm) and four aspect ratios, ranging from 2 to 8, were examined. The fluid was released by bursting the thin latex membrane that held it in the tube. The buoyant fluid had a density difference of 4.7%. The flow was imaged at 60 Hz up to 7 diameters downstream. For the aspect ratio of 2, the flow developed into a single buoyant vortex ring (BVR), and was compared to a purely momentum driven vortex ring (MVR) generated with the same setup. For the aspect ratios of 4, 6, and 8, the flow was similar to a starting plume, with a vortical cap, followed by a columnar tail. The BVR's diameter grew linearly in space, with a full spreading angle of 18 degrees, while the MVR's diameter remained constant. The BVR started out as an axis touching ring, and transitioned to non-axis touching, opposite of the behavior of the MVR. The total circulation for the BVR was more than twice the amount predicted by the slug flow model, and the impulse grew linearly in time. The impulse of the MVR decayed slightly after the intial growth. The flows began to transition to thermal behavior at down-stream distance proportional to the cube root of the initial fluid volume. For all aspect ratios the impulse grew linearly in time. The growth rate was roportional to the initial buoyant force. The circulation generated by the addition of buoyancy was proportional to the square root of the initial buoyant force. Also the addition of buoyancy suppressed the separation of a starting vortex.
Worcester Polytechnic Institute
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Bond, Derek P., "Near Field Development of Buoyancy Driven Flows" (2002). Masters Theses (All Theses, All Years). 46.
near field, starting flow, buoyant, unsteady, Buoyant flow, Vortex motion