Natural rivers usually possess sand waves and sand bars. In this article, the rapid distortion theory was used to study the turbulent flow over sand waves. The results show that the pre-existing sheafing motion and up...Natural rivers usually possess sand waves and sand bars. In this article, the rapid distortion theory was used to study the turbulent flow over sand waves. The results show that the pre-existing sheafing motion and upstream anisotropy of the turbulence flow would have significant effect on the turbulent structures, and hence the memory effect should be taken into consideration. Furthermore, the 2-D mathematical model was employed to simulate the unsteady flow around the Taiping Sand Bar in the lower reach of the Yangtze River and the time step effect on the unsteady flow simulation with the implicit scheme was discussed at the same time. The results show that the implicit scheme keeps effective until the time step reaches a certain number, and the calculated water levels and velocities are in agreement with the observed data.展开更多
Even though a number of rapid pressure-strain models have been suggested and successfully tested for different flow situations by various authors,the model proposals still exhibit some apparent deficiencies when subje...Even though a number of rapid pressure-strain models have been suggested and successfully tested for different flow situations by various authors,the model proposals still exhibit some apparent deficiencies when subjected to the flows with rapid distortion. From Mansour's relatively straightforward rapid distortion analysis,if an initially anisotropic flow undergoes a purely rapid rotation,the anisotropy measures will exhibit the behavior of the damped oscillations. Within the current framework of modeling the rapid pressure-strain correlation,i.e.,the models based on the assumption that the M-tensor for the rapid pressure-strain term is expand-able in the Reynolds-stress anisotropy tensor alone,all the model predictions fail to give the damped oscillations in the turbulence anisotropy. In the case of initially isotropic turbulence subjected to rapid distortion,Sj?gren and Johansson showed that all the existing rapid pressure-strain models would deliver the identical path in the anisotropy-invariant map for both homogeneous plane strain and shear flows. The rapid distortion analysis shows two distinct curves reflecting different flow physics. In this work,we try to present a possible way to create a system that can overcome these deficiencies with the aid of the rapid distortion theory (RDT).展开更多
In this paper the problem of homogeneous shearing turbulence approaching and passing an obstacle is studied by vorticity method and rapid distortion theory as well as a simple Reynolds stress model. The aim is to gain...In this paper the problem of homogeneous shearing turbulence approaching and passing an obstacle is studied by vorticity method and rapid distortion theory as well as a simple Reynolds stress model. The aim is to gain some insight into the effect of streamline curvature,initial anisotropy, upwind mean shear and relative strength of uniform mean shear on the turbulent structures over the obstacle. It is found that in the outer region of the obstacle the streamline curvature is of second order importance compared with the effect of the irrotational flow field produced by the obstacle. It is also found that for shearing turbulence passing the obstacle the pre-existing shearing motion would have significant effect on the turbulent structures. Comparing the changes in the equation for Reynolds normal stresses derived by rapid distortion theory with that derived by Reynolds stress model with the pressure strain terms of Launder, Reece and Rodi form, the two results agree approximately only when the initial turbulence is isotropic and the upwind mean shear is small.展开更多
When the error probability is less than 2--"", the minimum compression rate(r-optimal rate) of arbitrarily varying source with a fidelity criterion is determined. Thisgeneralizes the result of Arutyunyan and...When the error probability is less than 2--"", the minimum compression rate(r-optimal rate) of arbitrarily varying source with a fidelity criterion is determined. Thisgeneralizes the result of Arutyunyan and Mekaushll] for discrete memoryless source witha fidelity criterion, and is called strong coding theorem of arbitrarily varying source witha fidelity criterion. We also determine the asymptotic error exponellt for arbitrarily varying source with a fidelity criterion. This generalizes Marton’s result in [2] for discretememoryless source with a fidelity criterion.展开更多
基金Project supported by the National Natural Science Foundation of China(Grant No.50539060).
文摘Natural rivers usually possess sand waves and sand bars. In this article, the rapid distortion theory was used to study the turbulent flow over sand waves. The results show that the pre-existing sheafing motion and upstream anisotropy of the turbulence flow would have significant effect on the turbulent structures, and hence the memory effect should be taken into consideration. Furthermore, the 2-D mathematical model was employed to simulate the unsteady flow around the Taiping Sand Bar in the lower reach of the Yangtze River and the time step effect on the unsteady flow simulation with the implicit scheme was discussed at the same time. The results show that the implicit scheme keeps effective until the time step reaches a certain number, and the calculated water levels and velocities are in agreement with the observed data.
基金the National Natural Science Foundation of China (Grant No 90505005)
文摘Even though a number of rapid pressure-strain models have been suggested and successfully tested for different flow situations by various authors,the model proposals still exhibit some apparent deficiencies when subjected to the flows with rapid distortion. From Mansour's relatively straightforward rapid distortion analysis,if an initially anisotropic flow undergoes a purely rapid rotation,the anisotropy measures will exhibit the behavior of the damped oscillations. Within the current framework of modeling the rapid pressure-strain correlation,i.e.,the models based on the assumption that the M-tensor for the rapid pressure-strain term is expand-able in the Reynolds-stress anisotropy tensor alone,all the model predictions fail to give the damped oscillations in the turbulence anisotropy. In the case of initially isotropic turbulence subjected to rapid distortion,Sj?gren and Johansson showed that all the existing rapid pressure-strain models would deliver the identical path in the anisotropy-invariant map for both homogeneous plane strain and shear flows. The rapid distortion analysis shows two distinct curves reflecting different flow physics. In this work,we try to present a possible way to create a system that can overcome these deficiencies with the aid of the rapid distortion theory (RDT).
文摘In this paper the problem of homogeneous shearing turbulence approaching and passing an obstacle is studied by vorticity method and rapid distortion theory as well as a simple Reynolds stress model. The aim is to gain some insight into the effect of streamline curvature,initial anisotropy, upwind mean shear and relative strength of uniform mean shear on the turbulent structures over the obstacle. It is found that in the outer region of the obstacle the streamline curvature is of second order importance compared with the effect of the irrotational flow field produced by the obstacle. It is also found that for shearing turbulence passing the obstacle the pre-existing shearing motion would have significant effect on the turbulent structures. Comparing the changes in the equation for Reynolds normal stresses derived by rapid distortion theory with that derived by Reynolds stress model with the pressure strain terms of Launder, Reece and Rodi form, the two results agree approximately only when the initial turbulence is isotropic and the upwind mean shear is small.
文摘When the error probability is less than 2--"", the minimum compression rate(r-optimal rate) of arbitrarily varying source with a fidelity criterion is determined. Thisgeneralizes the result of Arutyunyan and Mekaushll] for discrete memoryless source witha fidelity criterion, and is called strong coding theorem of arbitrarily varying source witha fidelity criterion. We also determine the asymptotic error exponellt for arbitrarily varying source with a fidelity criterion. This generalizes Marton’s result in [2] for discretememoryless source with a fidelity criterion.
