Local bubble slip velocity in a downward laminar tube flow

The paper presents a study of the slip velocity for almost spherical bubbles in a downward laminar tube flow. The local slip velocity is defined as a difference between the mean liquid velocity (measured with electrodiffusional method) and the mean bubble velocity (measured with a modified LDA tool with a small size of measurement volume). The law for slip velocity for downward flow is significantly different from a similar law for upward flow. This reveals that the bubble flow motion is rather random than structured in the upward flow.