Experimental Study on Turbulent Entrainment in an Enclosed Density Stratified Flow owing to Wind Action

研究了两层密度分层流中由于施加表面风应力而产生的界面湍流夹带。观测到的夹带速度：(1)当浮力作用强，风应力较弱时，夹带速度较小。(2)夹带现象的机理分为扩散型、内波破碎型和穿透型三种。湍流结构：(1)上层湍流层存在整体尺度和有组织的大尺度涡。(2)强的间歇湍流影响夹带速度，但其寿命短。(3)界面处的湍动能方程可以有效地引入夹带速度定律。携风速度规律判定：通过湍流动能方程的参数化，得到了风致携风速度方程u_e/u_*=0.039(7.694+R_<i*>^<1/2>)/R_<i*>-2.5)。式中Ri*为具有水摩擦速度u_*的总体Richardson数，R_<i*>=<DELTA><rho>gh/<rho>u_*^2，u_e为夹带速度，<DELTA><rho>为上下层密度差，g为重力加速度，h为上层深度。

A study is made of the interface turbulent entrainment in a two-layered density stratified flow owing to an imposed surface wind stress.Observed entrainment velocity : (1) When the buoyancy effeet is strong and the wind stress is weak, the entrainment velocity is small. (2) Mechanism of entrainment phenomenon are classified three types, which are diffusion,internal wave breaking and penetrating type.Turbulent structure : (1) lntegral scale and organized large scale vortex are existed in the upper turbulent layer. (2) A strong intermittent turbulence,which is short life time,influences entrainment velocity. (3) The kinetic turbulent energy equation at the interface is effectively to introduce the entrainment velocity law.Decission of entrainment velocity law : By using the parameterization of kinetic turbulent energy equation, We obtained an equation for wind-induced entrainment velocity,u_e/u_*=0.039(7.694+R_<i*>^<1/2>)/R_<i*>-2.5).Where Ri* is an over all Richardson number with a water friction velocity u_*, R_<i*>=<DELTA><rho>gh/<rho>u_*^2,u_e is the entrainment velocity, <DELTA><rho> is the density difference between upper and lower layer, g is the gravitational acceleration , and h is the upper layer depth.