European Journal of Mechanics B-fluids最新文献

The problem of determining a mean flow induced by longitudinal libration of a rapidly rotating fluid-filled container bounded by the surfaces of two oppositely oriented right circular cones is considered. It is shown that the problem is reduced, through introducing a cosine of the angle between the normal to the conical surfaces and the axis of rotation, to the problem of determining a mean flow induced by libration of a fluid-filled rotating cylindrical container of infinite radius. A theoretical scenario is proposed of how the solved problem can be applied to the approximate determination of the differential mean rotation in a significant part of a rotating spherical and/or oblate spheroidal cavity under the action of longitudinal libration forcing.

With the limitation of the high sensitivity of nonlinear models to initial conditions, the accurate estimation of wave spatial evolution is difficult to perform at a long distance. At this stage, a helpful approach is to improve the accuracy and robustness of the model through data assimilation technique. A robust data assimilation framework is developed by coupling ensemble Kalman filtering (EnKF) with the nonlinear wave model. The spatial evolution is obtained by numerically integrating the viscous modified Nonlinear Schrödinger (MNLS) equation. The performance of the EnKF-MNLS coupled framework is tested using synthetic data and laboratory measurements. The synthetic data is generated by the MNLS simulation superposing the Gaussian noise. In the synthetic cases, the estimated wave envelopes agree well with the clean solution. The results of laboratory experiments indicate that the EnKF-MNLS framework can improve the accuracy of wave forecasts compared to noised MNLS simulations. This study aims to enhance the noise resistance of the nonlinear wave model in spatial evolution and improve the accuracy of the model forecast.

The extensive use of lightweight materials in aerial vehicle wings involves structural flexibility phenomena that generate non-negligible deformation effects. This influence is not restricted to big aircraft but also plays a role in smaller aeroplanes and Unmanned Aerial Vehicles (UAVs). Here, we conduct wind tunnel experiments to analyze the effect of passive deformation on the wing model lift slopes. To isolate the deformation effect, we compare rigid wings with a NACA0012 airfoil imposing a prescribed spanwise deformation. We study three levels of deformation: non-deformed, around 2% and 4.5% of tip deflection. Also, we consider the effect of the wing length by using three different semi-aspect ratios (1, 2, and 4), so a total of nine rigid wing models have been analyzed for a range of Reynolds number from $80\times 10^3$ to $160\times 10^3$. Deformed wing models show an increase in lift coefficient compared to non-deformed wing cases. Both deformation levels exhibit a qualitatively similar lift increment. A correlation to predict lift coefficient slope in a flat plate is adapted for a NACA0012 airfoil and validated using our experimental results and literature data. The adjusted correlation can quantify the deformation effect on the lift slope, which is comparable to using a slightly longer wing model.

In order to improve the propulsive performance of the existing pure heaving motion hydrofoil, the effect of varying stiffness ratio K*, damping ratio C* and inertia ratio J* on the propulsive efficiency of the semi-active NACA 0012 hydrofoil is systematically investigated by using the combination of CFD, Taguchi method and neural network. The results show that the passive pitching motion can significantly affect the propulsive performance of the hydrofoil. Compared to the pure heaving hydrofoil, the propulsive efficiency of the optimized semi-active hydrofoil can be improved by up to 20.97%. Further analysis reveals that the passive pitching motion can weaken the strength of the vortex around the hydrofoil, thus reducing the thrust and lift force on the hydrofoil, which results less power consumed by the active heaving motion of the hydrofoil. Although the thrust coefficient is reduced, the energy consumed by the passive pitching hydrofoil is reduced more, which leads a higher propulsive efficiency of the semi-active hydrofoil.

A test facility was constructed to investigate the flow characteristics of lateral synthetic jets generated by an actuator composed of a cavity, an orifice and a vibrating piezoelectric disc. Subsequently, 3D unsteady numerical simulations were conducted to analyze the formation of the synthetic jets and their dependence on the height of the orifice exit from the outer surface of the cavity. The time-averaged axial velocity was measured and used to validate the numerical results, and the maximum velocity reached approximately 8 m/s. In this work, it was found that the interaction of the vortex tube generated at the edge of the orifice exit and the outer surface of the cavity has a significant impact on the axial velocity distribution and characteristics of the lateral synthetic jets. Based on the experimental data and the numerical results, an attempt has been made to understand the formation of the synthetic jets and their interaction with the outer surface.

This paper presents a numerical method for incompressible fluid flow. A difficulty in analyzing incompressible fluid flow is that the continuity equation has no time evolution term. In the marker and cell (MAC) method, Poisson’s equation is solved iteratively, which takes most of the computation time, and in the artificial compressibility method (ACM), pseudo-time iteration is necessary to solve for unsteady solutions. Here, modal analysis that uses the velocity eigenvectors corresponding to zero eigenvalues is proposed for analyzing two-dimensional incompressible fluid flow. The proposed method involves only about one third of the number of variables needed in the MAC method and the ACM, and it does not require iterative calculation of Poisson’s equation or pseudo-time iteration. Numerical results for a simple flow system and a cavity flow obtained using the proposed method are compared with those obtained using the ACM and the simplified MAC method. The results agree well, thereby validating the proposed modal analysis.

In this work, the droplet impact and rebound behaviors on a flat substrate are comprehensively investigated by using the consistent and conservative phase-field based lattice Boltzmann method, which is robust for the multiphase flow problems with the large density ratios and long-time dynamics. The dynamic behavior of the droplet considered here is governed by five key factors: the contact angle, droplet size, the Bond number, the Reynolds number and the density ratio. The effects of these parameters on the barycenter motion trajectory, contact time, the maximum spreading factor and rebound height are studied, and the results show that the Bond number and the density ratio play the critical roles in the droplet morphology when it impacts the substrate. After the impact process, there are three typical patterns: fragmentation, deposition and rebound, which are mainly controlled by the wettability, the size of droplet and the Bond number. In the rebound process, we focus on the rebound height and the number of rebound, and also give the distribution of pressure inside the droplet and the evolution of pressure before fragmentation. Finally, it is also found that under some certain conditions, the air density has a positive effect on the droplet rebound behavior and a large density ratio of 6400 can be achieved.

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.