Theoretical and Computational Fluid Dynamics最新文献

This study investigates the impact of elasticity and plasticity on two-dimensional flow past a circular cylinder at Reynolds number (Re = 100). Ten direct numerical simulations were performed using the Saramito-Herschel–Bulkley model to represent viscoelastic and elastoviscoplastic (EVP) fluids. The flow evolves from a periodic von Kármán vortex street to chaotic-like regimes. Proper Orthogonal Decomposition (POD) and Higher Order Dynamic Mode Decomposition (HODMD) are applied to extract dominant flow structures and their temporal dynamics. For viscoelastic fluids, increasing the Weissenberg number Wi elongates the recirculation bubble and shifts it downstream, resulting in more intricate but still periodic behavior. In EVP fluids, seven cases explore variations in Bingham number Bn, solvent viscosity ratio (beta _s), and power law index n, aiming to qualitatively assess their influence rather than determine critical thresholds. Results indicate that stronger plastic effects, especially with (n ge 1), lead to increased flow complexity. Three dynamic regimes are identified: (i) periodic; (ii) transitional, with elongated recirculation and disrupted periodicity; and (iii) fully complex, with breakdown of recirculation. Overall, the study highlights the interplay between inertia, elasticity, and yield stress in non-Newtonian flows past obstacles and identifies key parameters driving the transition from periodic to complex regimes.

The paper deals with natural and mixed convection in a vertical rectangular duct exposed to solar radiation. The physical problem under consideration describes flow and heat transfer processes in a water-filled glazing chamber irradiated from the side. Due to the absorption properties of water, the near-infrared irradiance initiates a non-uniform volumetric heat source described by the Beer-Lambert law. Based on the Navier-Stokes equations with Boussinesq approximation and the energy equation, a mathematical problem for well-developed viscous flows in a rectangular duct is formulated. New exact analytical solutions in series are obtained, which include terms accounting for the effect of the heat source. The influence of the duct aspect ratio on the temperature and velocity fields is evaluated. While natural convection flow is always reversible, in the case of combined free and forced convection, a condition for the appearance of flow reversal is derived. Hydrodynamic and thermal characteristics such as Fanning friction factor, bulk liquid temperature, and Nusselt numbers at the walls are determined. The influence of the lateral walls diminishes with the increase of the aspect ratio, and when the duct is sufficiently narrow, the thermal characteristics of the flow approach the corresponding values for a plane-parallel channel.

We use three-dimensional numerical simulations based on the lattice Boltzmann method to study how an ellipsoidal microscopic swimmer moves through a square microchannel. Three key factors are varied: the strength of inertial effects in the flow, the swimmer’s shape (from spherical to three times longer than it is wide), and the degree of confinement by the channel walls (from narrow channels about three swimmer diameters across to wider channels about eight diameters). We consider both pushers (which drive the fluid backward with their rear end like sperm cells) and pullers (which pull the fluid forward with their front end like algae). As inertia increases, pushers swim faster whereas pullers slow down, and the change in speed is much stronger for pushers. The swimming speed can be captured by a simple quadratic trend when expressed in terms of a single combined measure of inertia and swimming stroke. More elongated swimmers move faster overall and are less influenced by inertia, while nearly spherical swimmers are the most sensitive to changes in inertia. As the channel becomes wider, the walls constrain the swimmer less, and variations in inertia have a more pronounced impact on the swimming speed.

Rayleigh-Bénard convection is a canonical problem in fluid mechanics, where an adverse temperature gradient between the opposing boundaries induces instabilities that drive natural convection. For viscoplastic fluids, a minimal perturbation is required to initiate the flow. This study presents a numerical investigation of the three-dimensional Rayleigh-Bénard convection within a cubic cavity heated from below, with lateral walls subject to a linear temperature profile. The fluid behavior is modeled using the Bingham constitutive model. The moment-based Lattice Boltzmann Method was employed as the numerical method to solve the mass and momentum transport equations, with an extended formulation to incorporate the energy transport equation using a local diffusion coefficient approach. Simulations are performed for Rayleigh numbers between 10⁴ and 10⁷. Within this range, we observed a region of Yield numbers, between 0.004 and 0.007, that fluid plastifies. Increasing the Rayleigh number led to a transition from a stationary to a chaotic state, while larger Prandtl numbers damped the fluctuations in the velocity field. Notably, the imposition of a linear temperature profile on the lateral boundaries enhances flow instability, thereby amplifying plastic instabilities as the critical Yield number is approached.

This work presents a linear stability analysis of the waves involved in the screech resonance loop generated within a rectangular supersonic jet. Linear stability analysis of rectangular jets has often been performed using a planar-jet approximation, due to the significant reduction in complexity and computational cost. However, the impact of this simplification on the predictive power of the stability model has not been considered in detail thus far. In this work, the predictions of both a simplified planar model and a more representative two-dimensional model are compared for two waves of particular relevance to jet noise. These waves are the downstream-propagating Kelvin-Helmholtz (KH) instability, and the upstream-propagating guided-jet mode (GJM). Disparity in the predicted wavenumber for both waves is considered, as well as disparities in the KH growth rate, and the GJM band of existence. A parametric sweep through nozzle-pressure ratio is performed for rectangular jets with aspect ratios ranging from a square jet with (textrm{AR}=1) to an (textrm{AR}=8) rectangular jet. It is demonstrated that the KH wavenumber can be well approximated by a planar model for rectangular geometries with an aspect ratio as low as 2. The growth rate is more sensitive, but for (textrm{AR} ge 4) the differences between the two models are negligible. In contrast to the KH waves, the band of existence of the GJM, defined as the frequency range between the mode’s branch and saddle points, shows significant dependence on the modelling approach. For higher aspect ratios, the planar model can reasonably predict the saddle point of the GJM, but the predicted branch point of the GJM differs significantly from that predicted by the two-dimensional model. Performing the analysis about an experimentally derived mean-flow profile demonstrates that the predictions for the GJM are sensitive to small changes in the flow profile, with both branch and saddle points showing a strong dependence on the thickness of the shear layer. In all cases tested, the two-dimensional model predicts a much narrower band of frequencies over which the GJM is supported by the flow, as compared to the planar model. To verify the predictions made from the two models, screech frequency predictions are made using the modified weakest-link model and compared to experimental data. The planar model, though it overpredicts the band of existence of the GJM, still produces correct predictions of screech frequency. The two-dimensional model, when linearized about the experimental mean flow, also correctly predicts the screech tones, though performs no better than the planar model.

A hypothesis is proposed about the process of liquid filling the experimental closed tank (ECT) with a capillary in its wall, the appearance of which may be of manufacturing defects in the ECT walls and welded seams. The hypothesis allows for the wettability properties of the ECT wall material, the capillary effect (CE), liquid evaporation and the pressure difference between the ECT volume and the environment, as well as the duration of the process. Three options for supplying liquid to the ECT volume are considered. They correspond to the technological operations in the ECT manufacture: washing, hydraulic testing, and calibration. A physical–mathematical model (PMM) of heat and mass transfer and liquid dynamics in the ECT and a capillary is developed on the basis of the Navier–Stokes equations. The obtained PMM is used to construct a corresponding computer model (CM). On the example of the ECT washing, a numerical simulation of the process of liquid filling the capillary is carried out. The changes in the capillary volume being filled with liquid are determined with respect to time, including temperature, pressure, mass flow rate of viscous incompressible liquid at the ECT entrance, wettability of the ECT walls and the exposure time interval. The capillary volume filling value is assessed for different duration of the washing process.

We examine the flow behavior around a transversely oscillating circular cylinder using various dimensionality reduction techniques. Specifically, Fourier analysis, Proper Orthogonal Decomposition (POD), Dynamic Mode Decomposition (DMD), and multi-resolution DMD (mrDMD) are employed. Numerical simulations are performed at a cylinder-diameter-based Reynolds number of 500 for a range of oscillation displacement amplitudes. The flow field exhibits well-documented wake patterns, such as 2S, 2P, and P+S, as well as intermittent transitions between these patterns at varying amplitudes. Dimensionality reduction becomes particularly effective when the force spectrum exhibits a dominant tonal character. Under these circumstances, the selection of the modal decomposition technique has minimal impact–all approaches yield comparable mode shapes for the dominant modes. However, when the flow undergoes intermittent pattern switching (e.g., between 2P and 2S), only mrDMD is able to automatically distinguish them as distinct modes. Nonetheless, if the temporal windows over which mode switching occurs are specified a priori, POD, DMD, and DFT are also successful.

Reinforcement learning (RL)-based closed-loop flow control shows great potential for managing nonlinear and complex aerodynamic flows. In this study, we investigate RL-based flow control to enhance the lift-to-drag ratio of an NLF(1)-0115 airfoil at a chord-based Reynolds number of ( Re_c = 20{,}000 ) and an angle of attack ( alpha = 5^circ ). Key control parameters, including the reward function, agent action time, observed state, and actuator placement, are systematically examined. Our results reveal that physics-informed tuning significantly improves control performance. An optimal agent action time of ( tau = 0.07 ), corresponding to approximately 11% of the primary oscillation period, was identified. It broadens the induced forcing spectrum, enhancing interaction with a wider range of flow structures. This finding establishes a clear physical connection between the RL agent action time and the unsteady flow dynamics. However, excessively short agent action time reduce the forcing amplitude, limiting control authority. Adjusting the observed state from wake-region sensors to surface-mounted pressure sensors yields comparable improvements in lift-to-drag ratio, ranging from ( 34.1% ) to ( 35.5% ), demonstrating the practical feasibility of using surface measurements. Forcing placement based on stability analysis significantly enhances control effectiveness. To improve data efficiency, the optimized 2D RL controller is transferred to a 3D CFD environment through prescribed spanwise wavenumber superposition. This lower-cost 3D controller effectively suppresses flow separation and significantly enhances aerodynamic performance. The results present a promising and practical alternative to direct 3D RL training for airfoil flow control.

The Kelvin–Helmholtz (KH) instability in the gravitational field with various density stratifications is simulated using a two-component discrete Boltzmann method. The influence of Atwood numbers ranging from negative to positive is investigated through key aspects, including concentration gradients, mixing degree, amplitude, and vorticity dynamics. The results show that concentration fraction gradients and vorticity increase with higher Atwood numbers. Conversely, the mixing degree and amplitude initially decrease but later increase as the Atwood number rises. Furthermore, a detailed analysis of the vorticity equation terms reveals that the Atwood number significantly affects vorticity evolution. Interestingly, when the Atwood number is zero, the temporal accumulation of these terms is minimal. Physically, the KH instability enhances the growth of the interface and mixing degree, while diffusion broadens the transition layer. Additionally, the Rayleigh–Taylor (RT) instability extends the perturbed interface vertically and promotes the mixing of the two media if the upper medium is heavier than the lower one; otherwise, the RT stabilization suppresses these effects.