Theoretical and Computational Fluid Dynamics最新文献

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.

The problem of the detachment of a free streamline from a smoothly shaped body is considered taking into account the effect of surface tension. The fluid is assumed to be inviscid and incompressible, and the flow is assumed to be irrotational and stationary in the body-fixed frame. We employ the Brillouin-Villat criterion to determine the position of the free streamline detachment. Using the integral hodograph method, we derive expressions for the complex velocity and the derivative of the complex potential, both defined in an auxiliary parameter plane. A system of singular integral equations, formulated in terms of the velocity magnitude along the free surface and the slope of the body, is derived by applying the dynamic and kinematic boundary conditions. The numerical method is highly efficient, as it only requires the evaluation of these two nonsingular functions. Numerical results demonstrate the effect of surface tension on the free streamline detachment, the drag force and the free surface shape for a wide range of Weber numbers. It is shown that the surface tension generates capillary waves due to the distortion of the magnitude of the inflow velocity at the detachment point, which propagate along the free streamline in the freestream reference frame.

This study investigates the onset of convection in a uniformly rotating fluid layer by varying the Taylor number ((textrm{Ta})), solute Rayleigh number ((mathrm {Ra_S})), Prandtl number ((textrm{Pr})), Schmidt number ((textrm{Sc})), and boundary conditions. The control parameters are varied over the ranges (textrm{Ta} in [0, 10^5]), (textrm{Pr} in (0, 10]), (textrm{Sc} in (0, 10^3]), and (mathrm {Ra_S} in [-3 times 10^3, 3 times 10^3]). Using a staggered grid Chebyshev spectral collocation method, the analysis reveals that rotation brings stability to the system and reduces the cell size at the onset of convection. Rotation also suppresses zero critical wave number convection and concavity of stationary neutral stability curve towards the origin under the non-identical temperature and concentration boundary conditions, as discussed by D. A. Nield [“The thermohaline Rayleigh-Jeffreys problem,” J. Fluid Mech. 29(3):545-558, 1967] in the absence of rotation. It is observed that a positive solute Rayleigh number promotes stationary convection, whereas a negative solute Rayleigh number tends to favor an oscillatory onset, depending on the fluid properties. Under no-slip velocity boundaries, three transition regimes are identified: oscillatory-stationary (O-S), stationary (S), and stationary-oscillatory (S-O). In contrast, only stationary convection occurs at onset when water-ethanol is confined between two stress-free plates at fixed rotation rate and concentration gradients. Moreover, reversing the solute boundary conditions does not affect the onset threshold when both plates are perfectly thermally conducting.

This study presents a numerical investigation of the dynamic response of a Newtonian fluid interface subjected to mixed oscillatory deformations. A slender cylindrical probe, floating horizontally at a water–air interface and partially submerged by capillary forces, is driven sinusoidally in the direction perpendicular to its axis. The interface exhibits shear, dilatational, and extensional viscosities, and the system is modeled using the finite volume method within the OpenFOAM framework. The governing equations are nondimensionalized and solved for a wide range of Marangoni numbers ((Ma in [1, 10^5])) and dilatational-to-shear viscosity ratios ((Theta in [1, 10^5])). The simulations enable the decomposition of the total interfacial force into its constituent components, revealing distinct regimes dominated by Marangoni, dilatational, or extensional stresses. The results demonstrate the feasibility of isolating these contributions under specific conditions, offering insights into the design and interpretation of interfacial rheometry experiments. The influence of geometric parameters and oscillation characteristics is also explored, confirming the robustness of the observed force dynamics.

The study presents a numerical investigation of unsteady plane channel flows of an elasto-viscoplastic material using the Herschel-Bulkley variant of the Saramito elasto-viscoplastic model [1] (SRM-HB), which incorporates both viscoelastic and viscoplastic behaviour. Yield stress fluids exhibit a dual nature, behaving as solids below a critical stress threshold and flowing as non-Newtonian fluids above it. This duality is crucial for numerous industrial applications and remains an area of active research. By examining the unsteady flow dynamics driven by steady and unsteady pressure gradients in a plane channel, this work evaluates the performance of the SRM-HB model against experimental data obtained with Carbopol gels. The findings highlight the significant role of elasticity in the yielding process and the resulting flow characteristics. Various flow scenarios, including creep tests, unsteady pressure ramps, and large-amplitude oscillatory flows, are analyzed to elucidate the complex interplay between elastic and plastic responses. The results demonstrate that the SRM-HB model effectively captures the transient flow behavior and hysteresis phenomena observed experimentally, providing insight into the material’s yielding and flow mechanisms.

The wavelet-based adaptive implicit eddy-resolving simulation approach is demonstrated for linearly forced homogeneous isotropic turbulence at moderate Taylor-Reynolds number. The wavelet-filtered incompressible Navier-Stokes equations are solved using the adaptive multilevel wavelet collocation method for elliptic problems without employing any explicit modeling for the unclosed terms. Instead, the energy dissipation induced by the built-in low-pass filtering associated with the adaptive numerical scheme is effectively exploited, thus mimicking the effect of unresolved subgrid-scale coherent flow structures on the dynamics of the resolved ones. The results of various numerical simulations, performed at different spatial resolutions, and with different filtering strengths, prove both the feasibility and efficacy of the proposed no-model approach for wall-free turbulence, while addressing the corresponding limits of application.