Title: Absolute instability in two-layer flows
Speaker: Peter Spelt (LMFA, École Centrale de Lyon, and Département Mécanique,
Speaker: Peter Spelt (LMFA, École Centrale de Lyon, and Département Mécanique,
Universite Claude Bernard Lyon1, France).
时间：8月14日下午2:30
地点：力一楼237
欢迎感兴趣的同学参加
In this talk, first the dominant mechanisms of temporal linear instability of two-layer flows are briefly recalled. The specific model analyzed semianalytically is 2D pressure-driven gas flow over a liquid layer in a channel, as a model of gas/liquid transport through pipelines; both laminar and turbulent base states are considered. Special attention will be paid to the modelling of the turbulent system, and the significance of the effects of perturbations of Reynolds stresses in turbulent gas flow. Next, the analysis is extended to linear spatio-temporal instability. In a large part of parameter space, the flow is found to be absolutely unstable. It is demonstrated that a mere modal analysis poses difficulty of interpretation of the results. We have therefore developed a ray analysis for this class of problems, and used these to confirm the results of the modal analysis. The boundaries of convective/absolute (C/A) instability in parameter space are analyzed further by making use of a newly developed approximation of the growth rate as a function of complex wave number, which yields explicit (but approximate) criteria for C/A transitions. This approximation will be demonstrated to give an accurate prediction of the exact boundaries, and to provide insight in the identification of the underlying dominant physical mechanism of the pertinent instability, as it provides a relation to the corresponding temporal problem. Results obtained with a novel extension of an energy budget to spatio-temporal instability (in conjunction with the ray analysis) will also be presented. A comparison with results obtained with DNS for the liquid/liquid system will also be presented. The talk will be concluded with a very short overview of some other work on two-phase flows.
时间：8月14日下午2:30
地点：力一楼237
欢迎感兴趣的同学参加
In this talk, first the dominant mechanisms of temporal linear instability of two-layer flows are briefly recalled. The specific model analyzed semianalytically is 2D pressure-driven gas flow over a liquid layer in a channel, as a model of gas/liquid transport through pipelines; both laminar and turbulent base states are considered. Special attention will be paid to the modelling of the turbulent system, and the significance of the effects of perturbations of Reynolds stresses in turbulent gas flow. Next, the analysis is extended to linear spatio-temporal instability. In a large part of parameter space, the flow is found to be absolutely unstable. It is demonstrated that a mere modal analysis poses difficulty of interpretation of the results. We have therefore developed a ray analysis for this class of problems, and used these to confirm the results of the modal analysis. The boundaries of convective/absolute (C/A) instability in parameter space are analyzed further by making use of a newly developed approximation of the growth rate as a function of complex wave number, which yields explicit (but approximate) criteria for C/A transitions. This approximation will be demonstrated to give an accurate prediction of the exact boundaries, and to provide insight in the identification of the underlying dominant physical mechanism of the pertinent instability, as it provides a relation to the corresponding temporal problem. Results obtained with a novel extension of an energy budget to spatio-temporal instability (in conjunction with the ray analysis) will also be presented. A comparison with results obtained with DNS for the liquid/liquid system will also be presented. The talk will be concluded with a very short overview of some other work on two-phase flows.