超疏水壁面湍流边界层猝发条件相位平均模态

Conditional Phase-lock Averaged Mode of Burst in Turbulent Boundary Layer over Superhydrophobic Surface

  • 摘要: 对使用TRPIV系统测量的光滑亲水壁面、各向同性和各向异性超疏水壁面湍流边界层瞬时速度场大样本时间序列进行沿流向空间多尺度小波分析,得到不同流向空间尺度的湍流脉动动能随法向坐标和尺度的分布,发现各向同性和各向异性超疏水壁面显著抑制了各尺度湍流脉动动能。用各尺度的小波系数正的极大值和负的极小值分别检测相干结构的两种猝发事件,用同尺度空间锁相条件平均的方法,得到两种猝发的流向脉动速度、法向脉动速度和展向脉动涡量的条件相位平均模态。发现两种猝发的流向脉动速度、法向脉动速度条件相位平均模态符合喷射——扫掠和扫掠——喷射的剪切层特征,脉动涡量相位平均模态为流向、法向正负交替分布的四极子涡包结构特征,其流线表现为鞍点-焦点的动力系统。各向同性和各向异性超疏水壁面显著抑制了各尺度相干结构的猝发强度。

     

    Abstract: Multi-scale wavelet analysis along the longitudinal direction was conducted on the large-sample-size time series of instantaneous velocity fields of the turbulent boundary layer over a smooth hydrophilic surface, an isotropic and an anisotropic superhydrophobic surface respectively measured by Time-Resolved Particle Image Velocimetry(TRPIV), and the distribution of turbulent fluctuating kinetic energy with different longitudinal spatial scales and different normal coordinates was obtained. It was found that the isotropic and anisotropic superhydrophobic surface significantly suppressed the kinetic energy of turbulent fluctuation at all scales. The two types of burst events of the coherent structure were detected respectively by using the positive maxima and negative minima of the wavelet coefficients at each scale. The conditional phase-locked averaged modes of the streamwise fluctuating velocity, normal-wall fluctuating velocity and spanwise fluctuating vorticity of the two burst events were obtained by the conditional phase-locking averaging method at the same scale. It was found that the phase averaged modes of the streamwise and wall-normal fluctuating velocities of the two bursts conformed to the shear layer characteristics of the eject-sweep and swept-eject modes. The phase-locked averaged mode of the fluctuating vorticity corresponded to the structural feature of a quadrupole vortex packet with alternating positive and negative distributions in the streamwise and wall-normal directions, and its streamlines were manifested as a dynamic system of saddle point-focus. Isotropic and anisotropic superhydrophobic surfaces can significantly suppress the burst intensity of coherent structures at various scales.

     

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