International Journal of Multiphase Flow最新文献

Transiently growing non-modal perturbations can play a crucial role in the transition of plane shear flows in modally stable regimes. In terms of the extent of transient amplification, three-dimensional perturbations are typically more prominent due to the lift-up effect. In contrast, two-dimensional (2D) spanwise-independent perturbations are often considered less important as they typically undergo modest levels of transient growth and are short-lived. The Orr mechanism is key to the amplification of energy for 2D perturbations. In this work, we discuss 2D non-modal perturbations of three-layer viscosity stratified flows with the mean shear rates of the outer layers being equal. Strikingly, a novel regenerative Orr mechanism is found that allows for significant amount of energy amplification despite the 2D nature of the perturbations. Moreover, these perturbations survive for considerably long times. The perturbation structure shows symmetry about the middle layer and evolves such that the Orr mechanism can repeatedly occur in a regenerative manner resulting in the perturbation energy evolving in a markedly non-monotonic fashion. When these same perturbations are introduced in a uniform plane shear flow, the corresponding non-modal transient amplifications are shown to be much smaller.

In this work, molecular dynamics (MD) simulation is applied to study the effect of heating surfaces with different hydrophobicity occupancy ratios (the ratio of the surface area of hydrophobic spots to the total area of the heating surface) on the boiling process of the liquid film explosion. At the same time, the mechanism is revealed from the trajectory of argon atoms. The simulation results showed that the onset of explosive boiling was later for purely hydrophilic surfaces than for hybrid wettability surfaces with a hydrophobicity percentage of <11%. The earliest onset of explosive boiling was observed for the heated surfaces with a hydrophobicity ratio of 6%. In addition, it was found that the superheat required for explosive boiling tended to decrease first and then increase with the gradual increase of the hydrophobicity ratio. Hydrophobic spots arranged on the surface provided bubble nucleation earlier for explosive boiling while enhancing convective heat transfer and thermal perturbation. The critical heat flux (CHF) of the heated surfaces with a hydrophobicity ratio of <11% were all greater than that of the purely hydrophilic surfaces, and all reached the CHF before the purely hydrophilic surfaces.

In the present study, we conduct numerical simulations of compressible flows around spheroid particles, for the purpose of refining empirical formulas for drag force, lift force, and pitching torque acting on them. Through an analysis of approximately a thousand numerical simulation cases spanning a wide range of Mach numbers, Reynolds numbers and particle aspect ratios, we first identify the crucial parameters that are strongly correlated with the forces and torques via Spearman correlation analysis, based on which the empirical formulas for the drag force, lift force and pitching torque coefficients are refined. The novel formulas developed for compressible flows exhibit consistency with their incompressible counterparts at low Mach number limits and, moreover, yield accurate predictions with average relative errors of less than 5%. This underscores their robustness and reliability in predicting aerodynamic loads on spheroidal particles under various flow conditions.

The discrete tone radiated from tip vortex cavitation (TVC), known as ‘vortex singing’, was recognized in 1989, but its triggering remains unclear for over thirty years. In this study, the desinent cavitation number and viscous correction are applied to describe the dynamics of cavitation bubbles and the dispersion relation of cavity interfacial waves. The wavenumber-frequency spectrum of the cavity radius from the experiment in CSSRC indicates that singing waves predominantly consist of the stationary double helical modes (k_θ = 2^- and -2⁺) and the breathing mode (k_θ = 0^-), rather than standing waves as assumed in previous literatures. Moreover, two trigger mechanisms, expressed by two triggering lines, are proposed: the twisted TVC, initially at rest, is driven into motion through the corrected natural frequency (f_n) due to the step change of the far-field pressure. Subsequently, the frequency associated with the zero-group-velocity point (ῶ_zgv) at k_θ = 0^- is excited through ῶ_i, the frequency at the intersection of dispersion curves at k_θ = 0^- and -2⁺, or ῶ_j, the frequency at the intersection of dispersion curves at k_θ = 0^- and 2^-, corresponding to two types of the vortex singing triggering. These solutions, without empirical parameters, are validated using singing conditions provided by CSSRC and G.T.H., respectively. Furthermore, the coherence and the cross-power spectral density spectrum indicates a large-scale breathing wave propagating along the singing cavity surface and travelling from downstream to hydrofoil tip, providing us a comprehensive understanding for the triggering of vortex singing.

In a recent paper, a novel coding of the Ghost Fluid Method for the variable coefficient Poisson equation with discontinuous functions (named GFMxP) was proposed. A lot of numerical tests, with all the required quantities available in a analytic form, were used to demonstrate the ability of the new procedure in modeling a sharp interface and to check the accuracy order of the solutions. In practical applications, however, the real difficulty stands in the estimation of the so-called “ghost values”, that is the values at points where the function is not only unknown, but even not defined. These values allow to compute the corrective terms enabling the use of standard finite difference formulas in presence of a singularity and/or a discontinuity, and can be only determined through some extrapolation procedure, whose truthfulness is essential to achieve a reliable result. The paper deals with such a basic issue, by testing different numerical strategies and demonstrating the strict relationship between the order of the adopted fit-model, the order of the solving scheme for the Poisson equation and the accuracy of the final solution.

Understanding the physics of phase separation between gas and liquid phases as a mixture mass has long been a challenge. In this paper, a phase separation description criterion based on heterogeneous flow model is proposed. A mathematical method similar to Lagrangian coherent structure (LCS) is used to identify the two-phase separation process, which is called relative motion Lagrangian coherent structure (rLCS). The rLCS is able to describe the dynamic evolution of the phase separation process and flow pattern transition in multiphase flows, which is very common in gas‒liquid mixture transportation and industrial processes. The most striking finding of rLCS is that phase separation and phase distribution are not in the same spatial position, that is, the process and the result of separation may not be exactly corresponding as we thought. This new flow structure reflects the underlying dynamic behavior of the multiphase flow field. In addition, the phase separation process has obvious periodicity. This paper reveals the typical phase separation process in the simulation of gas‒liquid two-phase pipe flow and gas‒liquid multiphase pump. These are very important to improve the understanding of multiphase flow processes, and can also lay a solid foundation for future flow control based on multiphase flow characteristics, highlighting the application potential of the new method.

The discrete tone radiated from a cavitating tip vortex, named as ‘vortex singing’, has remained a mystery for over thirty years. In this study, based on the dispersion relation of cavity interfacial waves, the vortex singing is proved to be generated by breathing mode waves propagating from downstream to the hydrofoil tip, solely determined by the mean cavity radius (r_c), the cavitation number (σ) and the desinent cavitation number (σ_d). Then we have identified three types of vortex singing, the universal law, such as the dimensionless singing frequency for each type (ῶ = 2πfr_c/U_∞ = 0.312, 0.037 and 0.926, f is frequency and U_∞ denotes the incoming velocity) and the wavenumber for one typical type (κ = 2πr_c /λ = 0.361, λ represents the wavelength) have been first derived and validated. Furthermore, the minimum cavitation number and desinent cavitation number required for detecting each type of vortex singing are given theoretically. Importantly, we have illustrated a long-standing perplexity: why such a whistler can appear only within a narrow range of frequency, wavelength as well as the cavitation number.