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The open cavity flow contains various physical phenomena relating to vortex structures. The study on the vortex dynamics in the open cavity can provide theoretical insights into flow control and noise reduction. Direct numerical simulation is performed to obtain high order accuracy flow field of the open cavity. The vortex structures are identified using the Lagrangian coherent structures. Together with the Q-criterion and vorticity flux analysis, the formation, convection, impingement and split of the vortex in the cavity is studied and the dynamical characteristics are revealed. Results show that in a flow cycle, the rolling up and shedding of Lagrangian coherent structures at the leading edge corresponds to the formation and shedding of the vortex developed downstream; and at the trailing edge the split of Lagrangian coherent structures corresponds to the split of Q and Vortex. The main vortex in the cavity remains stable during the whole period of the flow.

A hydrodynamic and acoustic mode decomposition method for velocity and pressure based on Helmholtz decomposition and momentum equation is proposed. It is able to decompose the flow variables of velocity and pressure into their vortical, acoustic, and entropic components. The method is verified and validated by a Mach 0.9 turbulent jet flow. The spectral properties of the decomposed hydrodynamic and acoustic components verify that the method successfully separates the two parts. Spectral proper orthogonal decomposition analysis of the decomposed fields clearly identifies the spatial structures of the first three radial orders trapped waves in the potential core and validates the shallow angle noise generation mechanism of spectral leakage for subsonic wavepacket. The present work provides insights into flow–acoustic interaction and aeroacoustic source identification for turbulent jets. [ABSTRACT FROM AUTHOR]

Existing research on shale fracture conductivity is rather sparse and has not taken the effect of loading history into consideration. A splitting device was used to split the shale specimens along bedding, and a 3D scanner was used to characterize the fracture surface. Using a constant 0.3 MPa gas pressure, three shale specimens split by single fractures of different roughness were tested for gas flow during four loading-unloading cycles of 0-10 MPa. Test results show that fracture conductivity exhibited hysteresis similar to that observed in rock mechanics experiments. In addition, it is clear that fracture conductivity is smaller for fractures with rougher surfaces. Building on the cubic law and soil mechanics consolidation theory, a hydraulic aperture model for cyclic loading was developed that helps explain the influence of plastic deformation during loading and unloading on fracture conductivity. This model exhibited trends get the agreement with the data for the range of confining stresses used in the experiments. [ABSTRACT FROM AUTHOR]

Interest in shale gas as an energy source is growing worldwide. Because the rock's natural fracture system can contribute to gas production, it is important to understand the flow behavior of natural fractures in shale. Previous studies on the flow characteristics in shale fractures were limited and did not consider the effect of nonlinearity. To understand the basic mechanics of the gas flow behavior in shale fractures, laboratory investigations with consideration of the fluid pressure gradient, the confining stress, the loading history and the fracture geometry were conducted in this paper. Izbash's equation was used to analyze the nonlinearity of the flow. The results show that the behavior of the friction factors is similar to that shown in flow tests in smooth and rough pipes. The increase of the confining stress and the irreversible damage to the shale decreased the hydraulic aperture and increased the relative roughness. Thus, turbulent flow could appear at a low Reynolds number, resulting in a significant pressure loss. The limits of the cubic law and the existing correction factor for transmissivity are discussed. It is found that the previous friction models overestimate the friction factor in the laminar regime and underestimate the friction factor in the turbulent regime. For this reason, a new friction model based on a linear combination of the Reynolds number and the relative roughness was developed. [ABSTRACT FROM AUTHOR]