Bubbly drag reduction in turbulent flows is of significant interest in the naval industry. In this paper, we investigate experimentally the transition from bubbly drag increase to bubbly drag reduction at moderate to high Reynolds numbers and analyze the contributions of different mechanisms.

The experiments were performed in a water tunnel. To achieve Reynolds numbers of the boundary layer, of the same order as the ones characteristic of a flow over a$10$ meters long plate, the boundary layer at the upper wall was thickened by mounting a$2D$ obstacle. The single-phase turbulent boundary layer is strongly disturbed by the obstacle: it separates and reattaches farther in the recovery region. Air bubbles of intermediate size ($0.4-1.3mm$) were injected in the recovery region. Four upstream velocities were tested:$2,4,6$ and$8m/s$, corresponding to a Reynolds number ranging between $21000$ and $69000$ (based on the momentum thickness of the single-phase disturbed boundary layer at the measurement location of the bubbly flow). The average air volume fraction in the disturbed boundary layer $〈\alpha 〉$ was varied up to$0.08\%$. The liquid-phase velocity field in the vertical plane was investigated by particle tracking technique and the friction velocity was deduced from the logarithmic law of the wall. The gas-phase velocity field, volume fraction distribution and the bubble size were characterized by means of shadowgraphy in the vertical plane.

Two regimes are highlighted: the buoyancy dominant regime at$2m/s$ (regime I) for which bubbles are sliding along the wall and the turbulent dispersion dominant regime achieved at higher velocity (regime II) for which there is a competition between buoyancy and turbulence dispersion. It is inferred that the correlation between the gas volume fraction fluctuations and the liquid wall normal fluctuating velocity plays the major role in the friction reduction by reducing the total turbulent stress of the liquid. In regime II, gas volume fraction waves are produced through the repetitive bubble accumulation near the wall in liquid sweeping events. A maximum local reduction of the wall friction$FV=35\%$ is achieved at$6m/s$. For this regime, the gain factor $FV/〈\alpha 〉$ is hundreds of times greater than expected for a boundary layer without the obstacle at the wall. In regime I, a bubbly friction increase is observed at small air volumetric fraction, which turns into a bubbly friction reduction when increasing the air volume fraction. In this case, friction reduction is attributed to inversion of bubbles wakes: from bubbles induced jet-like flow to bubbles induced wake-like flow under bubbles deformability effect, which is responsible for bubbles wall-normal oscillating motion.

湍流中的气泡阻力减少在海军工业中具有重要意义。在本文中，我们通过实验研究了在中等至高雷诺数下从气泡阻力增加到气泡阻力减少的转变，并分析了不同机制的贡献。

实验是在水隧道中进行的。为了实现边界层的雷诺数，与流过的流的特征具有相同的阶数$10$ 米长的板，上壁的边界层通过安装一个加厚$2D$障碍。单相湍流边界层受到障碍物的强烈干扰：它在恢复区分离并重新附着得更远。中等大小的气泡（$0.4-1.3米米$) 被注入到恢复区。测试了四种上游速度：$2,4,6$ 和$8米/秒$，对应的雷诺数介于$21000$ 和 $69000$（基于气泡流测量位置单相扰动边界层的动量厚度）。受扰边界层中的平均空气体积分数$〈\alpha 〉$ 变化到$0.08\%$. 采用粒子跟踪技术研究了垂直面的液相速度场，根据壁面的对数定律推导出摩擦速度。气相速度场、体积分数分布和气泡尺寸通过垂直平面内的阴影成像来表征。

突出显示了两种机制：浮力占主导地位的机制$2米/秒$（状态 I），气泡沿壁滑动，湍流分散主导状态以更高的速度实现（状态 II），浮力和湍流分散之间存在竞争。推断气体体积分数波动与液壁法向波动速度之间的相关性通过降低液体的总湍流应力而在摩擦减小中起主要作用。在状态 II 中，气体体积分数波是通过液体吹扫事件中壁附近的重复气泡积累产生的。壁面摩擦的最大局部减少$F伏=35\%$ 达到$6米/秒$. 对于这种制度，增益因子$F伏/〈\alpha 〉$是壁上没有障碍物的边界层的预期值的数百倍。在状态 I 中，在小空气体积分数下观察到气泡摩擦增加，当增加空气体积分数时，这变成气泡摩擦减少。在这种情况下，摩擦减少归因于气泡尾流的反转：从气泡引起的喷射状流到气泡在气泡变形效应下引起的尾流状流，这是气泡壁法向振荡运动的原因。