Mixing in oscillating columns: Experimental and numerical studies

In this paper, mixing in an oscillating column was experimentally and numerically studied as a function of power applied through vibrations. The mixing experiments were performed using phenolphthalein and NaOH solution, and the mixing time was computed using a simple image processing algorithm to track intensity changes implemented in MATLAB. Numerically, the air-solution interface was tracked using the VOF model and the solution was vertically disturbed by oscillating the base of the column. The bottom boundary was treated as a rigid moving boundary and a compiled user-defined function (UDF) was applied to the boundary to impose a sinusoidal displacement of the lower boundary. The interior of the column was assumed to be a deforming body and a dynamic mesh was employed to improve the mesh quality. It was found that the mixing time is highly nonlinear with respect to the applied power. The stability chart mapped in Benjamin and Ursell (1954) by solving a series of Mathieu equations was applied to our system and the behavior of mixing in the vibrating column was interpreted. Pseudo steady states were observed, however they lasted only for few minutes and then switched back to 'real' steady states. These findings were supported by the images captured during experiments and numerically-produced iso-surface and contour plots.