Bubble collection by a point sink in a uniform flow

A point sink sampling device immersed in a flowing bubbly liquid collects more bubbles than would be expected on the basis of the bubble number density in the bulk fluid because the bubbles accelerate towards the sampling device faster than does the surrounding liquid. Bubble trajectories towards the sink are determined by integrating the equation of motion for the bubbles, which are assumed to be subject to pressure forces, acceleration reaction (added mass) and (possibly) drag. The number density of bubbles in the sample is predicted to be higher than that in the bulk fluid by a factor 2.37 in the absence of drag. This highlights the importance of isokinetic sampling, in which the disturbance to the streamlines is minimal. As drag increases, so bubbles tend to follow streamlines more closely and the bubble number density in the sample approaches that in the bulk fluid.