European Journal of Mechanics B-fluids最新文献

We investigate the influence of the turbulent closure in Reynolds-Averaged Navier–Stokes (RANS) simulations for the prediction of the linear response of a turbulent boundary layer developping over a small-amplitude corrugated wall. Experimental studies by Hanratty and co-workers (Zilker et al. 1977; Abrams and Hanratty, 1985; Frederick and Hanratty, 1988) show a phase shift between the wall shear stress and the wall undulation, that depends on the wall wavenumber. Historically, this problem was studied by the means of linear forced response analyses using a mixing length model. It was shown that an ad-hoc correction is required to recover the experimental results (Thorsness et al. 1978; Charru et al. 2013). In this study, we ran Reynolds Averaged Navier–Stokes (RANS) computations using different types of turbulent closures. The results confirm the inadequacy of the Boussinesq assumption, leading to the failure of Eddy Viscosity Models (EVM) to properly recover the wall shear stress phase angle. Moreover, it is shown that a second moment closure performs better in capturing the effects of a small deformation of the wall. Additionally, a general strategy based on the modification of the balance of the closure coefficients of a $k-\omega$ model is found to be an effective approach to improve the performance of first order turbulence models. We establish corrections adapted to the $k-\omega$ model which can be seen as a pragmatic way to recover the expected behaviors.

We derive asymptotic estimates for the projection of the vorticity onto principal directions of material stretching in 3D flows. In flows with pointwise bounded vorticity, these estimates predict vorticity alignment with Lyapunov vectors along trajectories with positive Lyapunov exponents. Specifically, we find that in inviscid flows with conservative body forces, the vorticity exactly aligns with the intersection of the planes orthogonal to the dominant forward and backward Lyapunov vectors along trajectories with positive Lyapunov exponent. Furthermore, we derive asymptotic estimates for the vorticity alignment with the intermediate eigenvector of the rate-of-strain tensor for viscous flows under general forcing. We illustrate these results on explicit solutions of Euler’s equation and on direct numerical simulations of homogeneous isotropic turbulence.

With the unique structure of multi-core compound droplets, they are increasingly used in various industrial production fields, material fabrication, biological sciences, medicine, and other numerous promising large-scale applications. This study focuses on using a front tracking method to study the dynamics of a multi-core compound droplet as it moves within a ratchet microchannel. The dynamics of the multi-core droplet is assessed by deformation (determined by elongation deformation indices, and surface indentation) and the transit time of the droplet within the microchannel. The presence of the ratchet region in the microchannel increases deformation and reduces the transit time of the compound droplets. Increasing the number of ratchets leads to faster droplet motion but has no significant effect on the deformation of the compound droplet. The results indicate that the parameters such as the capillary number, microchannel geometry (i.e., number of ratchets and neck radius), droplet size and structure significantly impact the compound droplet dynamics. The compound droplet radius equal to 0.3 times the microchannel radius results in the most significant elongation deformation. The number of core droplets has minimal effect on the deformation and transit time of the compound droplet. This study provides a profound insight into the dynamics of multi-core compound droplets in a ratchet microchannel and contributes a better understanding of their behavior and potential applications in various fields.

This paper presents a comparison of experimental and numerical modelling results of the velocity field around a fume cupboard with a static and a dynamic mesh. During fume cupboard testing, components are required to move which mimic typical operating conditions, the amount of tracer gas released is then measured. This tracer gas is harmful to the environment and so an alternative is required. Advanced Computational Fluid Dynamics (CFD) techniques, such as dynamic meshing, have been utilised to replicate aspects of the current tests. The fume cupboard was tested in normal operating conditions and under the influence of a board inducing a wake close to the fume cupboard entrance. The velocity fields have been compared and show a reasonable level of accuracy with a percentage difference between experimental and simulated results of around 5% using both a static and a dynamic domain. This is an improvement on the 15–20% accuracy for detecting concentration of tracer gas using previous experimental methods. The aim of this work is to satisfy the scientific community and fume cupboard operators that CFD is sufficiently accurate to assess fume cupboard performance under real world scenarios.

This in vitro study aims at clarifying the relation between the oscillatory flow of cerebrospinal fluid (CSF) in the cerebral aqueduct, a narrow conduit connecting the third and fourth ventricles, and the corresponding interventricular pressure difference. Dimensional analysis is used in designing an anatomically correct scaled model of the aqueduct flow, with physical similarity maintained by adjusting the flow frequency and the properties of the working fluid. The time-varying pressure difference across the aqueduct corresponding to a given oscillatory flow rate is measured in parametric ranges covering the range of flow conditions commonly encountered in healthy subjects. Parametric dependences are delineated for the time-averaged pressure fluctuations and for the phase lag between the transaqueductal pressure difference and the flow rate, both having clinical relevance. The results are validated through comparisons with predictions obtained with a previously derived computational model. The parametric quantification in this study enables the derivation of a simple formula for the relation between the transaqueductal pressure and the stroke volume. This relationship can be useful in the quantification of transmantle pressure differences based on non-invasive magnetic-resonance-velocimetry measurements of aqueduct flow for investigation of CSF-related disorders.

Design load and vibration for parked conditions are gaining in importance for large-scale modern wind turbines with increasing flexibility, especially edgewise vibration when the blade is at a high angle of attack. In this work, flow-induced vibration of the wind turbine airfoil at 90 degrees of attack angle is studied with the fluid-structure interaction (FSI) simulation. The unsteady aerodynamic force due to flow separation and vortex shedding at the high angle of attack causes the chordwise vibration of the airfoil. When the vortex shedding frequency $f_v$ gets close to the chordwise natural frequency $f_n$ of the airfoil, vortex-induced vibration (VIV) of high amplitude occurs accompanied with the frequency lock-in phenomenon. In the post lock-in regime, it is found that period-3 and torus bifurcation occur successively and the vibration response becomes aperiodic. Dynamic mode decomposition(DMD) technique is used to investigate the mechanism of bifurcation from the perspective of energy balance, through analyzing the vorticity field in the wake and pressure distribution on the airfoil surface. For the certain incoming velocity in the post lock-in regime, since the frequency of the DMD mode $f=2f_v/3$ is close to the natural frequency$f_n$, both the vibration of frequency $2f_v/3$ and $f_v$ get excited, leading to the onset of bifurcation. The Lissajou curves are obtained through reconstructing the transient pressure of each DMD mode, which indicates that energy transfer mainly exists in modes $f=f_v$. In addition, the reconstructed Lissajou curves based on the leading DMD modes agree well with the original time-domain Lissajou curves.

Vortex-induced vibration (VIV) is characterized as a phenomenon of limited amplitude vibration. Understanding the basic nature and underlying mechanism of VIV is necessary for predicting the vibration amplitude. In this study, using Large Eddy Simulation (LES) of forced vibration, a detailed investigation of the flow pattern and wind load during VIV of a 4:1 rectangular cylinder is conducted. The results indicate that both vibration amplitude (y₀/D) and wind speed (U_R) significantly influence the flow pattern and wind load. Notably, an increase in vibration amplitude leads to a predominance of motion-induced force and a corresponding amplification of the fluctuating lift coefficient. Additionally, a decrease in the phase difference between lift force and displacement is observed, establishing this phase difference as a critical parameter for predicting vibration amplitude. Regarding wind speed, it is observed that as U_R increases, the predominance of motion-induced force diminishes, resulting in a concurrent decrease in the fluctuating lift coefficient. Upon further investigation into the work performed by various forces within a single vibration cycle, it has been determined that as the vibration amplitude escalates, the work of the lift force (energy input W_I) initially increases, then diminishes, whereas the work of the damping force (energy dissipation W_O) continuously rises. The intersection of these two trajectories signifies the point of energy equilibrium between input and output, thereby establishing the vibration amplitude of VIV. The predicted vibration amplitudes, grounded in this principle, have been corroborated by experimental results.

The transition from steady-state flow to periodic oscillatory flow for the natural convection by Hopf bifurcation is investigated in a three-dimensional (3D) cavity. The spectral collocation method (SCM) in combination with the artificial compressibility method (ACM), which is developed by ourselves as a numerical method SCM-ACM with high accuracy, is employed to solve the governing equations directly instead of linear stability analysis method that is commonly used for the research on flow instability. The results show that the amplitude decays exponentially with time and the decay rate is linear with the Grashof number (Gr). The critical Grashof number for steady-oscillatory transition is obtained as Gr_cr = 3.423 × 10⁶. The dimensionless angular frequency ω_cr = 0.24 is also determined by Fourier analysis. In this work, we also examine the heat-momentum interactions within the boundary layers, visualize the periodic oscillations of temperature and velocity amplitudes, and analyze the origin of instability from multiple angles. The results show that large oscillations of velocity and temperature are observed near the isothermal walls. The oscillation is enhanced by the increase of thermal boundary layer thickness and flow velocity at both ends of isothermal walls. The maximum velocity and temperature amplitudes appear at the lower left and upper right corners of the mid-plane (Z = 0.5), where are the origin of instability, and the spanwise walls are almost independent of oscillations. The oscillatory flow of natural convection in three-dimensional cavity originates from the continuously increasing buoyancy force, and its transition occurs by Hopf bifurcation. Moreover, the temperature amplitude exhibits a wavy distribution on the mid-plane (X = 0.5) and strongly depends on the depth Z. These results provide benchmark data for future numerical studies and engineering application.

Flow inside the axial test turbine VT-400 is measured by using a standard methodology of Particle Image Velocimetry (PIV). The studied area lies between the stator and rotor wheel in meridional plane. This area covers both: the regular flow near hub and the endwall secondary structures as well. Regular structure of wakes and jets past stator blades is observed in terms of mean velocities and turbulence intensity. The fluctuations are close to isotropy and the energy distribution across length-scales exhibits almost Kolmogorov scaling. Near the hub, we observe secondary vortices. Despite the positive radial ejections in that regions, these secondary vortices drift towards the hub, they dissipate energy and circulation and their core grows in size.