Investigation of Turbulence Production and Counter-Gradient Heat Flux in a Strongly Stably-Stratified Flow Field

本研究旨在研究重力内波的转换过程，以阐明重力内波坍塌过程中逆梯度热流的产生与湍流产生之间的关系。采用7根冷丝、热线和多通道高精度测温系统，多点同时测量温度场和速度场。通过对速度和温度涨落的相关分析，得到了重力内波空间结构的流向变化：(1)随着重力内波在下游的发展，垂直速度涨落的能级w和温度涨落的能级θ在Brunt-Vaisala频率N_b以下的频率范围内显著增加。同时，w和θ之间的相干性增强，其相位差从-π变为-π/2。在N_B以下的频率范围内，中心冷线测得的温度涨落与其他六个探头测得的温度涨落的相关性变大，它们的相位差趋于零。(3)在波的崩塌过程的初始阶段，速度和温度涨落的相互作用形成了高次谐波的大势能温度涨落。(4)在重力内波的波前破裂时，通过将简谐分量的势能转换为随机分量的动能，在浮力作用下诱导负湍流产生，从而产生逆梯度热流。(5)两点空间相关测量结果表明，重力内波的相速度随其沿流向的发展而增大。

The present research aimed to examine the transition process of internal gravity waves to clarify relationship between the occurrence of counter-gradient heat flux and turbulence production in the collapse process of the internal gravity waves. Multi-point simultaneous measurement was conducted on temperature and velocity fields by a rake of 7 cold-and hot-wires and a multi-channel high-precision thermo-anemometer system. Streamwise change in the spatial structure of the internal gravity waves was obtained through correlation analysis on the velocity and the temperature fluctuations.(1)As the internal gravity waves developed downstream, the energy levels of the vertical velocity fluctuation, w, and the temperature fluctuation, θ, remarkably increased in the frequency range below the Brunt-Vaisala frequency, N_B. At same time, the coherence between w and θ increased and its phase difference changed from -π to -π/2.(2)With streamwise increase in the amplitude of the waves, correlations between the temperature fluctuations measured by the center cold-wire and the other six probes became larger and their phase differences approached zero in the frequency range below N_B.(3)In the initial stage of the collapse process of the waves, higher harmonic temperature fluctuations with large potential energy developed through the interaction between the velocity and the temperature fluctuations. Then, random components with higher frequencies became to intermingle with spiky wave forms intermittently.(4)At the break down of the wave fronts of the internal gravity waves, the counter-gradient heat flux was generated as negative turbulence production was induced against buoyancy force by converting the potential energy of the harmonic components to the kinetic energy of random components(5)The result of two point spatial correlation measurement showed that the phase velocity of the internal gravity waves increased as they developed in the streamwise direction.