Wall-pressure and PIV analysis for microbubble drag reduction investigation

The effects of microbubbles injection in the boundary layer of a turbulent channel flow are investigated. Electrolysis demonstrated to be an effective method to produce microbubbles with an average diameter of 30 ??m and allowed the placement of microbubbles at desired locations within the boundary layer. Measurement of velocity fluctuations and the instantaneous wall shear stress were carried out in a channel flow facility.
The wall shear stress is an important parameter that can help with the characterization of the boundary layer. This parameter can be obtained indirectly by the measurement of the flow pressure at the wall. The wall shear stress in the channel was measured by means of three different independent methods: measurement of the pressure gradient by a differential pressure transducer, Particle Image Velocimetry (PIV), and an optical wall shear stress sensor. The three methods showed reasonable agreement of the wall shear stress values for single-phase flow. However, differences as skin friction reductions were observed when the microbubbles were injected. Several measurements of wall-pressure were taken at various Reynolds numbers that ranged from 300 up to 6154. No significant drag reduction was observed for flows in the laminar range; however, a drag reduction of about 16% was detected for turbulent Reynolds numbers. The wall-pressure measurements were shown to be a powerful tool for the measurement of drag reduction, which could help with the design of systems capable of controlling the skin friction based on feedback given by the wall-pressure signal. The proposed measurement system designed in this work has capabilities for application in such diverse fields as multiphase flows, drag reduction, stratified flows, heat transfer among others. The synchronization between independent systems and apparatus has the potential to bring insight about the complicated phenomena involved in the nature of fluid flows.