European Journal of Mechanics B-fluids最新文献

The experimental and numerical investigation of the flow instabilities acting on rigid blades and vice versa was conducted for both compressor and turbine configuration. The blade cascade consisted of five rectangular NACA 0010 blades, with three middle blades capable of performing harmonic motion with one degree of freedom (pitching) using force excitation. The base case (all blades fixed) and excited regime were examined. The influence of various angles of attack, harmonic frequency values, amplitude values, inter-blade phase angles and Reynolds numbers (Re) were tested. The mean flow properties as well as the fluid - structure interaction (FSI) were studied using Particle Image Velocimetry (PIV), Reynolds-averaged Navier-Stokes (RANS) CFD methods and using force measurement. Additionally, two different approaches, namely traveling wave mode (TWM) and aerodynamic influence coefficient (AIC), were adopted to estimate the aeroelastic stability of the blade cascade, and the results were compared. The results show significant aeroelastic coupling between the blades in both compressor and turbine configuration. However, the aerodynamic coupling effect for torsional flutter is more prominent in turbine configuration.

The study delves into the dynamic behavior of fluid flows in hydrodynamic (HD) and magnetohydrodynamic (MHD) regimes, specifically focusing on the influence of varying magnetic field strengths on vortex shedding around a cylinder. Employing advanced modal decomposition techniques such as Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD), the research unveils the intricate characteristics of these flow fields. In HD scenarios, the flow exhibits complex, periodic patterns with notable vortex shedding, whereas in MHD scenarios, the introduction of magnetic fields gradually transforms the flow into a more stable and streamlined state. The study significantly demonstrates the damping effect of magnetic fields on vortex intensity and oscillations, leading to a uniform flow at higher field strengths. This study leverages DMD to predict future flow dynamics in both HD and MHD regimes around a cylinder. By using snapshots from CFD simulations at Re $=$ 120, we validate DMD’s predictive capabilities by comparing predicted snapshots with CFD results at corresponding time instants. This approach not only demonstrates DMD’s robustness in capturing complex flow behaviors but also highlights its potential for real-time monitoring and control in industrial applications. The findings provide new insights into the temporal dynamics of MHD flows and open avenues for optimizing flow control strategies in engineering systems.

In this analysis, the collective effects of rotation, viscous dissipation and vertical throughflow on the onset of convective movement in Jeffrey fluid saturated permeable layer is studied. The improved Darcy model is applied to depict the rheological performance of Jeffrey fluid flow in porous medium. The approximate analytical solution with overall error 0.4 % and numerical solution accurate to one decimal place are presented using the Galerkin process. The analysis reveals that the convective motion concentrates in the top layer if it occurred with sufficiently high value of the Darcy–Eckert number. The rotation factor and the Péclet number postponement the onset of convective drive while, the Gebhart number quicken it weakly. In the occurrence of rotation, the Jeffrey factor displays dual impact on the coming of convective movement. The magnitude of the convection cell declines with increasing the rotation factor, the Jeffrey factor and the Péclet number, while it decreases with enhancing the Gebhart number. It is also found that in the lack of rotation, the Jeffrey factor has no impression on the extent of the convective cell, whereas in the nonexistence of the Péclet number, the Gebhart number has no impact on the arrival of convective drive as well as on the magnitude of the convective cells.

This paper describes a physics-informed neural network (PINN) for determining pressure from velocity where the Navier-Stokes (NS) equations are incorporated as a physical constraint, but the boundary condition is not explicitly imposed. The exact solution of the NS equations for the oblique Hiemenz flow is utilized to evaluate the accuracy of the PINN and the effects of the relevant factors including the boundary condition, data noise, number of collocation points, Reynolds number and impingement angle. In addition, the PINN is evaluated in the two-dimensional flow over a NACA0012 airfoil based on computational fluid dynamics (CFD) simulation. Further, the PINN is applied to the velocity data of a flying hawkmoth (Manduca) obtained in high-speed schlieren visualizations, revealing some interesting pressure features associated with the vortex structures generated by the flapping wings. Overall, the PINN offers an alternative solution for the problem of pressure from velocity with the reasonable accuracy and robustness.

This study investigates the long-time vorticity dynamics of the multi-cellular configurations of the two-dimensional (2D) Taylor–Green vortex (TGV). The pseudo-spectral method is used to solve the incompressible Navier–Stokes equation to analyze the evolution of TGV arrays. The focus is on understanding vortex interactions leading to vortex filamentation and stripping (forward cascade) during primary instability; merger and reconnection (inverse cascade) among the TGV vortical cells subsequently. Here, consideration of multiple cells avoids imposing symmetries at the smallest periodic length scale, and thereby affecting disturbance growth. The initial condition is taken from the analytic solution of the TGV, and Fourier spectral method is employed to track the interactions of the initial doubly-periodic vortices. The full sequence of evolution from one equilibrium state to another for the TGV is not addressed before, as reported here to fill this gap for multiple TGV cells in both directions. By studying various vortical interactions in the ensemble, here we report the enstrophy and energy spectra for different number of TGV cells. This is crucial in understanding the very long-time evolution process, at post-critical Reynolds numbers for the 2D TGV problem in the same physical domain, ($0\le (x,y)\le 4\pi$) having $(4\times 4)$ and $(6\times 6)$ cells. Reported results show the evolution of these vortical cells from original configurations to finally a $(1\times 1)$ vortical cells — the universal state not demonstrated before.

Water-hammer waves propagation is an important phenomenon arising in numerous applications. It is also a long-standing topic in the fields of mechanics, mechanical engineering and civil engineering. This review first presents the basic mechanism associated with water-hammer waves as well as a brief historical survey of the topic. It then develops along the twentieth century progress both regarding the Fluid–Structure-Interaction (FSI) influence and wave dissipation modeling. The second part of the review presents recent developments shading new lights on some aspects of the wave propagation with a fluid mechanical viewpoint. This review covers various aspects related to the influence of visco-elastic properties of the pipe’s wall, asymptotic analysis as well as wave propagation within networks. Albeit discursive in many places, this review also tries to establish and derive many of the presented results from first principles, as well as emphasizes the theoretical understanding of the topic.

Wave breaking is a multifaceted physical phenomenon that is not fully understood and remains challenging to model. An effective method for investigating wave breaking involves utilising the two-phase Reynolds-averaged Navier–Stokes (RANS) equations to directly simulate breaking waves. In this study, we apply a RANS model with an adaptively refined mesh to simulate breaking waves in deep water using the stabilised RANS model proposed by Larsen and Fuhrman. This approach enables a more efficient simulation of the physics of breaking waves compared to Direct Numerical Simulations, as it places less stringent demands on grid resolution. Our findings demonstrate that the RANS model compares well with deep water wave breaking experiments in terms of surface elevation. We also give estimates of the breaking strength parameter of our RANS simulations and compared them with the literature.

We study theoretically the steady Newtonian flow in confined and hyperbolic long tubes (symmetric channels and axisymmetric pipes) considering slip along the walls. Using a stream function formulation, and the extended (or high-order) lubrication method in terms of the square of the aspect ratio of the tube, ε, the solution for the stream function is found analytically up to twentieth order in ε. At the classic lubrication limit, i.e. i.e. for a vanishing small aspect ratio, and for perfect slip conditions, the analysis predicts a plug-like velocity profile and a constant strain-rate on the midplane/axis of symmetry of the tube. A constant strain-rate is also predicted for the non-slip case. Furthermore, the high order asymptotic results for the stream function and fluid velocity are post-processed with an acceleration technique to investigate the convergence and accuracy of the solution. The results reveal the existence of a boundary layer at the inlet of the tube, the influence of which diminishes in a very short distance from the entrance. We discuss the effect of the contraction ratio of the tube and the dimensionless slip coefficient on the midplane/centerline and wall (slip) velocities, as well as on the average pressure-drop, required to maintain a constant flow-rate. The acceleration of converge technique on the solution for the pressure-drop revealed a remarkable convergence at a value slightly larger (∼1 %) than the value predicted by the classic lubrication theory. Finally, we comment on the common practice in the literature for approaching the velocity profile with the velocity profile at the classic lubrication limit, and we compare the high-order results for the strain rate at the midplane/centerline with the effective strain rate previously derived in the literature by Housiadas & Beris, J. Rheology, 68(3), 327–339, 2024.

This paper presents mathematical modelling and simulation of thin free-surface flows of viscoplastic fluids with a Herschel–Bulkley rheology over complex topographies with basal perturbations. Using the asymptotic expansion method, depth-averaged models (lubrication and shallow water type models) are derived for 3D (three-dimensional) multi-regime flows on non-flat inclined topographies with varying basal slipperiness. Starting from the Navier–Stokes equations, two flow regimes corresponding to different balances between shear and pressure forces are presented. Flow models corresponding to these regimes are calculated as perturbations of the zeroth-order solutions. The classical reference models in the literature are recovered by considering their respective cases on a flat-inclined surface. In the second regime case, a pressure term is non-negligible. Mathematically, it leads to a corrective term to the classical regime equations. Flow solutions of the two regimes are compared; the difference appears in particular in the vicinity of sharp changes of slopes. Nonetheless, both regime models are compared with experiments and are found to be in good agreement. Furthermore, numerical examples are shown to illustrate the robustness of the present shallow water models to simulate viscoplastic flows in 3D and over an inclined topography with local perturbations in basal elevation and basal slipperiness. The derived models are adequate for direct (engineering and geophysical) applications to real-world flow problems presenting Herschel–Bulkley rheology like lava and mud flows.