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Gas–liquid two-phase flows are widely encountered in production processes of petroleum and chemical industry. Understanding the dynamic characteristics of multi-scale gas–liquid two-phase flow structures is of great significance for the optimization of production process and the measurement of flow parameters. In this paper, we propose a method of multi-scale symbolic time reverse (MSTR) analysis for gas–liquid two-phase flows. First, through extracting four time reverse asymmetry measures (TRAMs), i.e. Euclidean distance, difference entropy, percentage of constant words and percentage of reversible words, the time reverse asymmetry (TRA) behaviors of typical nonlinear systems are investigated from the perspective of multi-scale analysis, and the results show that the TRAMs are sensitive to the changing of dynamic characteristics underlying the complex nonlinear systems. Then, the MSTR analysis is used to study the conductance signals from gas–liquid two-phase flows. It is found that the multi-scale TRA analysis can effectively reveal the multi-scale structure characteristics and nonlinear evolution properties of the flow structures.

Gas–liquid two-phase flows are frequently encountered in chemical and nuclear industries. The study of gas–liquid flow structures is of great significance for understanding the mechanisms of the flow pattern transition. In this paper, a direct-image multi-electrode conductance sensor (DMCS) was used to detect the structure information of vertical gas–liquid flows. Recurrence plot (RP) and cost-based recurrence plot (CBRP) are validated using typical nonlinear systems, i.e. Lorenz system and Hénon map, and used to analyze the signals collected by the DMCS. The results indicate that the determinism (DET) derived from the CBRP is sensitive to flow pattern evolution, and can also demonstrate the internal differences in the same flow patterns.

This study aims to numerically investigate the optimal conditions for fluid flow control around a single square cylinder with the help of a pair of attached flat plates. It is comparatively a new approach for controlling fluid flows as compared to the traditional solo plate flow control devices. The plates are attached adjacent to the both rear corners of the cylinder and their length (l) is varied from $0.1$ to 4 times size of main cylinder while fixing the height (h) at $0.2$. By varying the length, the plates manage to control the flow gradually. This study discusses how a steady wake can be achieved through control plates. Results indicate that the flow regime changes from unsteady to transitional at $l=2.7$ while for $l>3.1$ the steady flow appears. The streamlines visualizations reveal different flow structures termed as the oval-eye vortex, chain necklace vortex, sphere vortex, hair pin vortex and wooden eyes vortex-like structures. Among these the oval-eye vortex structure is found to have higher flow induced forces and shedding frequencies while the wooden eyes vortex structure is found to have minimal flow induced forces and shedding frequencies. After $l=3.2$, the plates’ efficacy is proven by a 100$\%$ reduction in Strouhal number, root-mean-square values of lift coefficient and amplitudes of lift and drag coefficients. This study reveals that $l=3.2$ is the best optimal value of plates length for complete wake and fluid forces control.

The flow structures of jet impingement dominate heat and mass transfer process, even the whole thermal performance. In this study, we have inspected the flow structures and mechanism of nanofluid jet impingement onto a dimpled target surface with different design parameters. Investigations are performed for the relative depth of dimple ($\delta /D$), the jet-to-plate spacing ($H/d$), nanoparticle volume concentration ($\varphi$), and Reynolds number (Re) ranging to explore the mechanism of flow structure variations. Results indicate that these parameters have a significant effect on the flow structure of nanofluid jet impingement near the dimpled target surface. The flow begins to separate after passing the edge of the dimple along with the curvature of a dimple. $\delta /D$ will affect the form and location of flow separation and reattachment, and $\varphi$ will affect the intensity of separation flow. The length of the flow separation bubble varies in different $H/d$ cases. When $H/d$ increases, the impinging energy and the velocity near the dimple edge decreases. The different Re has little effect on the length of the flow separation bubble and the tendency of the pressure coefficient (Cp). These results can provide further mechanism inspiration for the design of the flow structure of nanofluid jet impingement.

Phase-average technique based on wavelet multi-resolution analysis and continuous wavelet transform are used to reveal the phase-averaged features of square cylinder wake measured by high-speed PIV. The one-dimensional orthogonal wavelet analysis is first applied to decompose the measured velocity fields into large-, intermediate- and small-scale structures. Then the phase information referenced with large- and intermediate-scale flow structures are clearly identified based on Morlet wavelet transform. Finally, the data ensembles are phase-sorted to give phase-averaged representations of measured flow field. The instantaneous multi-scale structures suggest that large-scale vortices are weakened and begin to transfer into intermediate-vortices at the downstream of separation region. The intermediate-scale vortex observed at the upper boundary of shear layer is considered to be associated with the secondary vortex movement. The phase-averaged intermediate-scale structures tend to convey downstream along streamwise direction, with the rotation sense varying from the first half period to the last half period. The peaks of phase-averaged large-scale Reynolds stress tend to move back and forth in the near-wake region. These findings suggest that the proposed phase-average technique is effective in revealing multi-scale fluid dynamics of wake flow structures.

This paper aims to further explore flow structure, shock waves' boundary layer interaction in supersonic inlets, and to analyze the details of the differences in supersonic flow field simulation between various turbulence models. For a typical binary mixed-compression inlet, the paper adopts the three common models of RANS SA, DES, and LES for detailed two-dimensional numerical simulation. The results show that shock waves, shock wave reflection, generation of boundary layer, boundary layer separation caused by shock wave boundary layer interaction, and flow field structure are complex in the inlet. In terms of the turbulence models, all three models can effectively predict shock waves, boundary layer thickness, shock wave boundary layer interaction, and the position and size of separation bubble, and can also visualize variation trends of various aerodynamic parameters in flow field. On this basis, contours of DES fully demonstrate hybrid RANS-LES characteristics, which is consistent with the theoretical basis of the model. In the LES model's results, the vortex structure's formation, development and shedding processes are quite clear. LES also has distinct advantages in terms of its ability to capture flow field details.

Turbulent flow over rough boundaries is a common occurrence in nature and the subject of much interest in a range of disciplines. It has long been recognized that the geometry of the boundary (or surface) dictates the flow and turbulence structure on a mean and instantaneous time scale. However, the mechanisms linking flow characteristics to roughness geometry remain poorly quantified, which has implications for our understanding of a variety of processes, particularly those occurring in the near-boundary region. It has been demonstrated that temporal and spatial variations in flow structure are sensitive to a range of geometric parameters describing the boundary geometry. We review the experimental evidence for rough boundary/flow interactions across different disciplines. A synthesis reveals that (1) different approaches have led to the adoption of a variety of parameters that are used to describe boundary roughness, and (2) that different criteria are used to evaluate the relative effects of boundary roughness. Moreover, (3) much of the experimental data relates to idealized surfaces that do not reflect the complexity of natural boundaries, or (4) is taken in low Reynolds number flows, and generally cannot be applied to aquatic flows in nature. The implications for our understanding of near-bed aquatic processes in turbulent boundary layers are discussed, and suggestions for future research approaches are presented.