To investigate the stream-wise distribution of skin-friction drag reduction on a flat plate with bubble injection, both experiments and simulations of bubble drag reduction (BDR) have been conducted in this paper. Drag reductions at various flow speeds and air injection rates have been tested in cavitation tunnel experiments. Visualization of bubble flow pattern is implemented synchronously. The computational fluid dynamics (CFD) method, in the framework of Eulerian-Eulerian two fluid modeling, coupled with population balance model (PBM) is used to simulate the bubbly flow along the flat plate. A wide range of bubble sizes considering bubble breakup and coalescence is modeled based on experimental bubble distribution images. Drag and lift forces are fully modeled based on applicable closure models. Both predicted drag reductions and bubble distributions are in reasonable concordance with experimental results. Stream-wise distribution of BDR is revealed based on CFD-PBM numerical results. In particular, four distinct regions with different BDR characteristics are first identified and discussed in this study. Thresholds between regions are extracted and discussed. And it is highly necessary to fully understand the stream-wise distribution of BDR in order to establish a universal scaling law. Moreover, mechanism of stream-wise distribution of BDR is analysed based on the near-wall flow parameters. The local drag reduction is a direct result of near-wall max void fraction. And the near-wall velocity gradient modified by the presence of bubbles is considered as another important factor for bubble drag reduction.

This study presents investigations on the internal flow behaviors and mechanisms of the ventilated partial cavity created by air ventilation behind a backward-facing step. Both planar particle image velocimetry experiments and numerical simulations have been conducted over a range of freestream velocities ($U_{\infty }$ ) for varying ventilation rates ($Q_{\mathrm{As}}$ ). The experimental and numerical internal flows are reasonably concordance, revealing three distinct internal flow regions: the ventilation region, the entrained internal boundary layer region, and the reversed region sandwiched in-between. The three-dimensional internal flow structures and the internal pressure gradients are numerically revealed. The internal recirculation vortex in the ventilation region is found to be induced by the stream-wise adverse pressure gradients, while the transverse airflow near the closure of the cavity is attributed to the presentence of the span-wise pressure gradient. Based on the internal velocity and air flux profiles, and the corresponding internal boundary layer thickness, the entrained and reversed air fluxes are revealed to initially increase, attain a maximum, and then decrease along the cavity. For the increment of $U_{\infty }$ , both entrained and reversed air fluxes increase. For the increment of $Q_{\mathrm{As}}$ , the entrained air flux rises, while the reversed air flux declines. Such feature is further verified and explained by applying the Couette flow model with adverse pressure gradients, which provides a reasonable estimation of internal flow profiles and offers a theoretical explanation on the change of internal air flux due to the variation of $U_{\infty }$ and $Q_{\mathrm{As}}$ .

The gas-liquid flow structure and interfacial behavior of submerged air jets were investigated experimentally using high speed digital video camera and image processing techniques. The jet pressure ratio varied from 1.8 to 4.8 in the experiment. And results from different jet nozzles were processed and compared. Statistical characteristics of the jet diameters along the axial distance were obtained and analyzed. Time series analysis was implemented to study the interface unsteadiness by calculating the gas-liquid interface deviation. The results showed that the jet diameters increase first linearly then nonlinearly and its growth rate decreases along the axial distance. The reason for the divergence between the result of this experiment and those done by other researchers was analyzed. Comparing the results of different pressure ratios and nozzle diameters, we found that larger jet pressure ratios have larger jet diameters and nozzle diameters nearly have no bearing on the distribution of dimensionless jet diameters. The interface unsteadiness in low and high pressure ratios exhibited totally distinct properties. And a minimum unsteady value was found along the axis of the air jets.