跃变分层流体中球体内波散度场特征实验研究

Experimental investigation on internal waves generated by a towed sphere in pycnocline

本文对跃变分层流体中匀速拖曳运动球体内波诱导流场PIV试验测量得到的水平速度散度场随时间的演化过程及内波时频、波谱、传播速度等特征进行了研究。球体处于跃层中,测试水平面位于球心下方一倍直径处。分析获得了速度散度场整体特征及其随内傅氏数F_(ri)=U/N_(max)D的变化规律,比较分析了体效应内波(Lee波)和尾流效应内波(WIW波)的时频、波谱、传播速度等波动特征。研究表明:(1)在低F_(ri)时Lee波占优,水平速度散度等值线呈内凹的“>”形,在最大浮力振荡频率f_(max)附近有非常明显的频率主峰,速度散度曲线的波谱主峰也非常明显,内波传播速度与球体运动速度基本一致;(2)在高F_(ri)时WIW波占优,水平速度散度等值线呈“■”形,频谱曲线上明显存在多个峰,且主频接近球体潜深位置当地浮力振荡频率,波谱曲线上也存在多个较明显的峰,对应WIW波波长在2.4D~5.6D之间,传播速度远低于球体运动速度;(3)在高F_(ri)时WIW波分界线的斜率与参数F_(ri)呈指数分布,在低F_(ri)下Lee波包络线的斜率也符合此分布规律。

This paper presents the study of the evolution of horizontal velocity divergence of the flow field produced by internal waves generated by a towed sphere in pycnocline,and measured by PIV on the horizon-tal plane.The test plane was about one diameter below the sphere center.The overall characteristics of the ve-locity divergence field and its variation with F_(ri) were obtained,and the time-frequency,spectrum and propa-gation velocity of body-generated internal waves(Lee waves)and wake-generated internal waves(WIW waves)were compared.The results show that at low F_(ri),the Lee waves dominate,and the isoline of the hori-zontal velocity divergence shows a concave“>”shape.In the frequency domain,there is a very obvious main peak near the maximum buoyant vibration frequency(fmax),and the main peak of spectral energy density of the velocity divergence curve is very obvious.The measured propagation velocity of internal waves is stable and close to that of the sphere motion.At high F_(ri),the WIW waves dominate while the isoline of horizontal ve-locity divergence is in the shape of“■”.There are obvious multiple peaks on the frequency spectrum curve,among which the vibration frequencies of several major internal waves are close to local buoyancy frequency in the vertical position of the sphere.There are several obvious peaks in the wave spectrum curve,that is,there are obvious WIW waves of multiple wavelengths,with the wavelength of 2.4D-5.6D.The propagation velocity of internal waves is much lower than that of sphere motion.At high F_(ri),the slope of WIW waves boundary and F_(ri) are of exponential distribution while the slope of Lee wave envelope at low F_(ri) also conforms to this distribution law.