不可压平板边界层转捩机理

基于扰动形式N-S方程,从空间模式的角度,采用Fourier伪谱及MPI(massage passing interface)并行方法,模拟了不可压平板边界层从层流到湍流的转捩过程.通过对计算统计数据的分析,比较了不同幅值入口扰动引起的转捩过程的同异.研究结果表明:当层流中扰动幅值逐渐增大后,非线性作用将修正平均流剖面,表现为不稳定区域逐渐扩大,很多高次谐波被激发.当平均流剖面被修正到一定程度时,不稳定区域变得很大,这使得更多的高次谐波被快速激发并迅速增长,即转捩过程开始.由于大量谐波快速增长导致扰动能量快速增长,同时,扰动能量的快速增长又进一步加速了平均流剖面的快速修正,即平均流剖面的不稳定特性发生了改变,这样的相互作用使得层流快速变为湍流,因此,平均流剖面不稳定特性的改变在转捩过程中起到了关键作用.

Transition mechanism of an incompressible boundary layer on a flat plate

Direct numerical simulation (DNS) was carried out for laminar-turbulent transition of an incompressible boundary layer on a flat plate based on disturbance N-S equation in spatial mode with Fourier pseudo-spectral and massage passing interface(MPI) methods.The statistical data were analyzed.Transition processes caused by disturbances with different initial amplitudes were compared.Results indicate: after the amplitude of disturbances in laminar flow grows,nonlinear effect modifies the profile of mean flow,with enlargement of performed by the enlarging unsteady zones.At the same time,more and more high frequency harmonic waves are generated.When the profiles of mean flow are modified to certain degree,the unstable zones are enlarged significantly.More harmonic waves are actived and grown rapidly,triggering the breakdown of transition process.During the breakdown of transition process,the rapid growth of many harmonic waves induces the rapid growth of disturbance energy,accelerating the modification of mean flow profiles.The phenomenon means that the instability characteristic of mean flow profile is changed.The reciprocal rapid growth makes laminar flow become turbulent flow rapidly.Therefore,change of instability characteristic of mean flow profile plays a key role during process of transition.