稀疏气固两相湍流边界层拟序结构变动特性

应用PIV两相同时测量方法,对壁面Reynolds数为430的水平槽道稀疏气固两相湍流边界层拟序结构变动特性进行了研究。选取质量载荷为10^-4~10^-3的110 μm聚乙烯颗粒作为离散相。结果表明,低载荷颗粒仍能显著改变湍流拟序结构,进而影响宏观湍流属性。颗粒重力沉降形成的粗糙壁面增强了壁面附近湍流猝发行为,导致黏性底层中的气相法向脉动速度和雷诺剪切应力显著增大。颗粒与壁面的碰撞加强了低速流体上抛、削弱了高速流体下扫,同时增强了轨道交叉效应,从而抑制了湍流拟序结构发展,显著减小了黏性底层以上区域的法向脉动速度和雷诺剪切应力。此外,颗粒惯性还减小了黏性底层厚度、增大了流向速度梯度,导致气相流向脉动速度峰值增大,且其对应位置也更加靠近壁面。

Modifications of coherent structures in turbulent boundary layer of dilute gas-particle horizontal channel flow

Modifications of coherent structures in a turbulent boundary layer of dilute gas-particle horizontal channel flow were experimentally investigated by means of simultaneous two-phase PIV(particle image velocimetry)measurement.The measurements were conducted in the near-wall region at Reynolds number of 430 based on the wall friction velocity and half channel height.Polyethylene beads with the diameter of 110 μm were used as the dispersed phase,and three low mass loading ratios ranging from 10^-4 to 10^-3 were tested.Spatial two-point correlations of fluctuation velocities and quadrant analysis were adopted to examine the change of characteristics of turbulent coherent structures in the lower boundary layer.It was found that the addition of particles noticeably modified the turbulence structures in meso-scale near the wall event at mass loading ratio 0.025%,thereby affected the gas-phase turbulence characteristics in macro-scale,e.g.fluctuation velocities and Reynolds shear stress etc.Particle sedimentation increased the roughness of the bottom wall,which significantly enhanced turbulent bursting in the inner region of the boundary layer.And then the wall-normal fluctuation velocity and Reynolds shear stress were both increased in the viscous sublayer.Under the effect of particle-wall collision,the Q2 events(ejections)of gas-phase were slightly increased by particles,while the Q4 events(sweeps)were obviously decreased.In addition,the spatial scale of the wall structures also remarkably shrink under the intensified crossing-trajectory effects due to particle saltation near the bottom wall.Consequently,the presence of the particles suppressed the turbulent coherent structures in the outer region of the boundary layer,which resulted in significantly decreasing wall-normal fluctuation velocity and Reynolds shear stress of the gas-phase above the viscous sublayer.Particle inertia changed the viscous sublayer of gas turbulence with a smaller thickness and a larger streamwise velocity gradient,which increased the peak value of streamwise fluctuation velocity of the gas-phase with its location shifting to the wall.