Fluid Dynamics最新文献

The lock-exchange flow of two miscible fluids in horizontal and inclined rectangular channels is investigated to establish a unified description of interface evolution and front dynamics across flow regimes. In horizontal and weakly inclined channels, the flow is shown to be viscous-dominated and adequately described in a quasi-parallel formulation. In this regime, the temporal evolution of the interface is derived directly from the Navier–Stokes equations, providing a simple analytical framework for predicting the interface shape and the associated front velocity. As the channel inclination or the density contrast increase, the flow transitions to an inertia-dominated regime, in which front propagation is governed by the local pressure and momentum balances rather than the viscous stresses. An analytical criterion is proposed to identify the onset of inertia-dominated behavior. At higher inclinations or density contrasts, interfacial instabilities and mixing lead to deviations from simplified predictions that neglect mixing; in these cases, experimental observations are used to interpret variations in the front velocity. The analysis also clarifies how inertia-dominated flows adjust the bulk flow rate to accommodate front motion. Overall, this study provides a consistent framework linking viscous-dominated interface evolution and inertia-dominated front dynamics in rectangular channels, thereby extending previous two-dimensional descriptions and offering a physically grounded interpretation of experimental observations.

Dual droplets are used as the research object to investigate the capture characteristics of droplet swarms on fine particulate matter, and a simulation method is employed to explore the influence rules of the dimensionless spacing and the Reynolds number on the capture efficiency. The results show that the capture effect of dual droplets is better than that of a single droplet (up to twice that of a single droplet). At the same Reynolds number, the capture efficiency first increases and then decreases with increase in dimensionless spacing. At the same spacing, the capture efficiency increases with the rise in the Reynolds number, and the capture efficiency growth rate slows down when the Reynolds number reaches 500. The optimal operating conditions for capturing fine particulate matter are the dimensionless spacing 3.5 and the Reynolds number 500. The mechanism through which the front-end velocity of the droplet affects the capture efficiency is clarified, and it is revealed that excessively large dimensionless spacing will expand the “escape space” of particulate matter, while overly small spacing will reduce the “combined capture range.” The findings of this study can provide a theoretical support for droplet swarm capture systems and have practical guiding significance for improving the dust removal efficiency of fine particulate matter.

The compressible turbulent boundary layer in supersonic flow on a cooled wall is numerically studied using the three-parameter differential RANS turbulence model. The study is carried out for a number of the Mach numbers (from 2 to 8) and a number of the temperature factors (from 0.2 to 0.8). The dependences of the Reynolds analogy coefficient on the Mach number and the temperature factor are found using the results of calculations of the flow and heat transfer characteristics.

The displacement of oil phase in pores near the wellbore during the pre-flush stage prior to acidizing operations significantly affects the acidizing effect, particularly, in fractured carbonate reservoirs. In this study, a numerical simulation program for oil–water two-phase flow in fractured carbonate reservoirs covering the entire pre-flush-acidizing process was developed to investigate this problem. The results indicate that the pre-flush stage can effectively enhance the transport capacity of hydrogen ion H⁺ within pores and improve the acidizing effect; specifically, the higher the volume of injected fluid, the more significant the improvement in the acidizing effect. However, the extent to which excessive fluid injection further enhances the acidizing effect gradually diminishes, and there exists an optimal injection volume, which is determined to be 1.0 pore volume (PV) in this study. The fractures disturb the development trend of wormholes, and this disturbance becomes more pronounced as the fracture density increases. The shielding effect induced by the fractures weakens the efficacy of pre-flushing and elevates the optimal injection volume, which increases from 1.0 to 1.5 pore volume in this study. Overall, the pre-flush stage can enhance the acidizing effect under high acid injection rates by reducing the acid-rock reaction rate.

This paper presents a new approach to modeling an intravascular stent in a two-dimensional problem formulation. This approach uses a variable-slip Navier boundary condition, simulating the periodic structure of the stent (wire thickness and number of turns). Unlike explicit modeling of stent geometry or a porous-medium approach, the proposed method reduces computational costs and takes into account major geometric parameters. The steady-state flow of a viscous incompressible fluid in a flat channel is supplemented by an analytical description of planar Hill vortex analogs in sinusoidal valleys. Numerical experiments in ANSYS are used to study the influence of wire thickness (120–400 μm) and the number of peaks (10, 30, 50) on the change in flow velocity, wall shear stress (WSS), and the specific dissipation function. It was found that increasing the wire thickness leads to an increase in velocity (up to 24%) and a decrease in WSS (up to 13%), with a weak dependence on the number of peaks in steady-state mode. In quasi-steady (pulsating) flow, the influence of stent periodicity becomes more pronounced. The value of the dissipative function decreases with increasing wire thickness (up to 25%), indicating a reduction in viscous losses due to sliding. The proposed method may be useful for rapid screening assessment of the hemodynamic effects of stents during the preclinical design stage.

Fully developed turbulent channel flow at Re = 4000 controlled by the composite approach based on the Lorentz force and sinusoidal grooves is numerically studied. The expanded Fourier–Chebyshev spectral method with coordinate transformation is applied together with the direct numerical simulation (DNS) method to solve the turbulent flow. In-depth relations among the characteristic structures in the flow field, the mean Reynolds shear stress, and the effect of drag reduction are discussed and compared with those in the flow field controlled by the Lorentz force only. The results show that the flow field controlled by the composite approach affords a drag reduction rate of 47%, while that controlled by the only Lorentz force approach affords a rate of 34.6%. For the composite approach, an additional flow with two velocity components along the spanwise and normal directions, respectively, is induced. The distributions of the two velocity components exhibit periodicity with alternating positive and negative values, thereby generating array-based quasi-streamwise vortices. Such vortices are capable of altering streak structures, suppressing bursting events, reducing the magnitude of the mean Reynolds shear stress, and modulating the mean velocity profiles in the near-wall region—ultimately yielding the drag reduction effect. Furthermore, the organized quasi-streamwise vortices induced via the composite approach demonstrate greater stability compared to those generate by the only Lorentz force method. Therefore, the composite approach is advantageous in terms of the drag reduction effect and efficiency.

The high-speed water entry of trans-media vehicles involves complex fluid-structure interactions; the strong nonlinearity of impact loads and structural dynamic responses poses core challenges for accurately characterizing impact environments and revealing shock response spectrum characteristics in the flight vehicle safety design. This paper utilizes the arbitrary Lagrangian–Eulerian (ALE) method to develop a high-fidelity predictive numerical model for simulating high-speed water entry impacts. The impact pressure, the cavitation evolution, and variations in the axial/normal forces are systematically analyzed by simulating the impact process. The acceleration time histories at critical measurement point are also extracted, and its shock response spectrum (SRS) is developed to characterize the frequency-domain attributes of the shock environment. Then, the effects of various water-entry velocities and angles are studied. The results show that the vehicle undergoes extreme transient loads during initial water entry, with the wetting pressure playing a critical role in multi-directional loading. Further analysis of the shock response spectrum reveals that the axial impact load (peak: 1.46 × 10⁵ N) demonstrates a peak amplification factor of 1.409 at approximately 400 Hz, while a local monitoring point’s axial acceleration (peak: 762 g) exhibits a significantly higher amplification factor of 5.47 at around 30 kHz. Additionally, the increased speed and angle markedly alter the time-domain response characteristics, yet negligibly affect the peak amplification factor in the shock response spectrum. Instead, the primary effect is a shift in the peak frequency toward higher frequencies. These findings provide important insights for the structural design and protection of water-entry vehicles, highlighting the need to consider not only extreme load-induced damage but also resonance failure caused by specific frequency components.

The inherent high conductivity of liquid metals ensures efficient heat transfer, motivates the study of forced magnetohydrodynamic (MHD) flow past a sphere essential for advanced thermal management applications. This article presents a numerical investigation of thermo-physical characteristics, and entropy generation in liquid-metal MHD flow past a heated sphere under uniform heat flux (UHF) condition. A fourth-order accurate finite difference method is employed to discretize the Navier–Stokes equation coupled with the energy equation, implementing the pseudo-time iterative method. Among the highlighted results, the separation length and angle become non-monotonic against the interaction parameter (M) for each Reynolds number (({text{Re}} > 22)). The mean Nusselt number (overline {{text{Nu}}} ) shows a non-monotonic dependence on (M) with the transitions at a critical interaction parameter M_cr. Alongside, a correlation for calculating (overline {{text{Nu}}} ) has also been developed. Notably, the drop in heat transfer from (M = 0) to (M = {M_{{text{cr}}}}) is more significant under UHF compared to constant wall temperature condition (CWT), highlighting around 173% of drop under liquid lithium at Re = 200. The total entropy production is non-monotonic on sphere surface for every Re and (M). The Bejan number ({text{Be}}) decreases with increase in Re and (M), falling below 0.5 at Re = 200 for liquid lithium within the critical magnetic field regime ((M = 6{-} 8)).

High-flow nasal cannula (HFNC) therapy, often combined with mask nebulization, enables uninterrupted oxygen delivery in acute respiratory care. However, the effects of the flow rate and the respiratory frequency on aerosol deposition remain elusive. A patient-specific upper-airway model is reconstructed based on high-resolution computed tomography. Computational fluid dynamics (CFD) simulations are conducted for the HFNC flow rates ranging from 0 to 50 L min^–1 and respiratory rates of 16, 22, and 28 breaths min^–1. The airflow patterns, the pressure distribution, and the regional aerosol deposition are analyzed. The aerosol deposition followed a unimodal distribution, peaking at 20 L min^–1. At this flow rate, a coherent inspiratory jet enhance bronchial penetration, whereas the flow rates ≥ 40 L min^–1 produce nasal vortices that divert ~12% of particles outward and suppress distal delivery. Aerosol deposition is mainly concentrated in the proximal airways (oropharynx 61%, trachea 18%, and nasal cavity 15%), with bronchial deposition remaining ≤3%. The lower respiratory rates markedly increase distal delivery by prolonging particle residence time; deposition at 16 breaths min^–1 more than doubled compared with 28 breaths min^–1. Although the airway resistance declines at the higher flow rates, the deposition efficiency remarkably decreases. CFD simulations revealed that aerosol deposition during HFNC-assisted mask nebulization could be influenced by the interaction between external flow and inspiratory dynamics. An HFNC flow rate of ~20 L min^–1 maximizes pulmonary deposition at all breathing frequencies, while the lower rates further enhance bronchial delivery. These results challenge the assumption that a higher flow rate universally improves therapeutic outcomes and highlight the need for individualized HFNC flow adjustment to optimize aerosol delivery in clinical practice.