Dynamical Interactions Between The Coherent Motion And Small Scales Of Passive Scalar Field In A Cylinder Wake

Most turbulence contains coherent motion,whose characteristics can reflect the initial conditions and boundary conditions,while the random motion is difficult to reflect these informations.The main problem we explore here is the interaction between coherent motion and random motion at a specific scale.Therefore,coherent motion is required to have a highly periodic flow field,which can easily distinguish coherent motion from random motion.This problem is related to the flow field of any Reynolds number,but it is more suitable under the condition of finite Reynolds number,because there is no obvious separation between large scale and small scale.In this dissertation,the interaction between coherent motion and random motion in a cylindrical wake scalar field is studied by theoretical derivation and experimental analysis.Based on the heat conduction equation,the scalar variance equilibrium equations of coherent motion and random motion in scalar fields are derived.The additional terms due to coherent motion are revealed in the equation of random motion,and these terms may be the key to the effect of coherent motion on random motion in scalar fields.The processing method of second order phase-averaged structure function is mainly adopted.This method can relate the distributions of all scales and phases to the effect of coherent motion,and estimate the scale range of the effect of coherent motion.From the analysis of the velocity signal and temperature signal of the center line and half width of the velocity in the three downstream positions of cylinder wake,the following conclusions can be drawn: In the velocity field,the stress caused by large-scale motion can strengthen the energy at all scales.For passive scalar,the scalar variance distribution of temperature fluctuations is also affected by coherent motion.Close to the cylinder,coherent motion in scalar field can produce obvious effect even at very small scales.In the near wake,the second order phase average structure function of q is similar to that of u.Put forward a model about the distribution of(?) at half-width,which can be qualitatively able to reproduce the shape of the phase-averaged functions.At last,the isotropic formulation of the random motion scalar variance budget equation compares well with experimental results at relative small scales.