To investigate the mechanism of vortex motion, a pitched synthetic jet is arranged on the bottom of a cross flow and simulated by incompressible Naiver–Stokes equations with large eddy simulation. The Liutex vector identification method is utilized to quantitatively study the rotation strength and the core size of the vortex structures according to various driving frequencies ( St = 0.25, 0.5, 0.75, 1.0) and amplitudes ( A0 = 1.0, 1.5, 2.0, 2.5) of the synthetic jet during 21 periods. The results show that both the frequency and the amplitude play a significant role in the vortex generation mechanism of the synthetic jet. However, the amplitude makes a positive contribution to the rotation strength and the vortex core size, while the frequency makes a negative contribution. The vortex motion mechanism of a synthetic jet in a cross flow can be summarized as follows: low frequency and small amplitude favor the primary clockwise vortex, while high frequency and large amplitude motivate the anticlockwise vortex generation.

This paper seeks to increase our understanding on the fluid mechanics and heat transfer in a transitional mixed convection flow between two vertical plates. Direct numerical simulation by the spectral method, with a weak formulation, is used to solve the transient 3–D Navier‐Stokes equations and energy equation. Initial disturbances consist of the finite‐amplitude 2–D Tollmien‐Schlichting wave and two 3–D oblique waves. The transition phenomena in a mixed‐convection flow can be significantly different from the isothermal flow. Disturbance competitions among different modes are also found to be different from those known for an isothermal flow. In a mixed‐convection flow, there exist thresholds for the low‐mode Fourier waves. The intensified vortices are concentrated left of the central surface between the two plates. Hairpin vortices are formed with high Ri. Based on the flow visualization, the λ vortices are found to be staggered on the surfaces parallel to the plates. The Ri number seems to be the main parameter governing the transition mechanism. The Nu number is found to increase during transition.

Although the coal pore structure is attracting the attention of researchers, studies on the multifractal and gas adsorption/desorption capacity are relatively few. In this study, multifractal characteristics were analyzed, and their effects on gas adsorption/desorption were discussed. The results showed that metamorphism and tectonism affected gas adsorption/desorption by changing the coal pore structure. The limit gas adsorption volume and initial desorption rate of tectonic coals increased by 3.9%–23.9% and 1.96–2.7 times, respectively. The information dimension (D 1), correlation dimension (D 2), and classical Hurst index gradually decreased with metamorphism and tectonism, while the Hausdorff dimension and Δα gradually increased, indicating that pore size distribution (PSD) was more decentralized or inhomogeneous. The evaluation results of the correlation between multifractal parameters and complexity confirmed its accuracy in assessing pore complexity. Multifractal parameters have a significant impact on gas adsorption/desorption capacity overall, mainly by reflecting the concentration degree, local density, and homogeneity of pores.

No abstract

This research seek lo increase our understnnding of fluid mechanics and heaf transfer phenomena in transitional mired-convection Jows between two heated vertical plates. A direct numerikal simulation by the spectral method with weak formulation is used lo sobe the transient three-dimensional (3-0) Nauier-Stokes equations and energy equation. Initial disturbances consist oflhefinite amplitude two-dimensional Tollmien-Schlichting wave and two 3-D oblique waves. The NusseU number (Nu) is mainly a function of the Rayleigh number (Ra) and alro depends on the Reynolds number (Re) and the RichardFon number (Ri). Nu dependence on Re increases with increase of Ra. Disturbance competitions among different modes during transition are shown for different sets of dimensionless parameters.