层结流体中湍流锋面特征的实验研究

AN EXPERIMENTAL STUDY ON SOME FEATURES OF TURBULENT FRONTS IN STRATIFIED FLUID

在线性分层的水体中；由拖动坚条形栅格产生湍流锋面，通过阴影可视化技术考察了锋面的流态和运动行为，将锋面迹线经过图像处理，获得了锋面运动特征的有关信息：运动速度随时间的增长关系；锋面坍塌的平均无因次特征时间为NTC＝2．9；它随湍流Froude数Fri的增大而减小；锋面的特征尺度Hf与栅格运动的参数有关；当湍流演化成内波时，对锋面与内波运动间的关系进行了初步探讨和分析．

Some features on the turbulent fronts were studied experimentally. The turbulence was produced by towing a grid with vertical square bars through a tank filled with linear stratified fluid. The shapes and behaviors of front motions were observed and recorded by shadowgraph and photograph. The data and paxameters of fronts were collected and computed by using an im age-digitized-microcomputer processing system from the traces of fronts.The variation of front pattern with time was examined. The front rapid collapsed from a nearly vertical surface to a sawtooth-like wave surface. In the volume of fluid behind the front the turbulence was no longer active by the effect of stratification. Coverted two-dimensional turbulence or internal wave and fossilized microstructure could also be seen. The speed of front motion is different before and after the collapse, and is obviously faster than front collapse. Averaged nondimensional characteristic time NTc was 2.9 at front collapse (from reference [5]), where N is Brunt-Vaisala frequency, and To is the transition time of front collapse. The NTc was related to the level of turbulent disturbance and stratification. Collecting previous results obtained by several authors, it was found that with disturbance Froude number Fri increasing the NTc tended to decrease, where Fri = U'/NLc, U' and Lc are the fluctuation velocity and integral scale of turbulence at the start of collapse, respectively.The mean vertical distance between peaks on the front Hf was measured. The value of Hf varied with the parameters of grid motion. A rough relation .was found at front collapse, which is Hf = (1.1～1.2)(U/NM)1/2, where U is velocity of towing grid and M is grid mesh size. In fact, the value of Hf was associated with turbulence in the fluid involved by fronts, so that Hf could be regarded to a characteristic length of fronts. The group trelocity of internal wave was explored and examined. The group velocity of internal wave Cg was estimated from wavelength λ converted by the number of peaks on the front, Cg=Nλ/2π. The rough relation was given by Cg=(0.14～0.15)(U/NM)1/2. The range of Cg was about 0.2～0.6cm/s obtained in present experiment. l) The project supported by the Foundation of the National Turbulent Laboratory of Peking University.