Journal of Fluids and Structures最新文献

This study is inspired by the extraordinary suppression performance of the intrinsic vortex-induced vibration (VIV) of a harbor seal vibrissa, which is renowned for its excellent detection characteristics in sensing upstream flow information. A physical understanding of the suppression mechanism of vibrissal cylinder oscillation is mandatory for the design of a sensor detection system. In this study, the suppression performances are studied for a vibrissal cylinder at a reduced velocity U_r = U/(f_nw D) of 5, with the corresponding Reynolds number of Re = 3,750, according to the swimming velocity and real geometrical scale for a harbor seal vibrissa. In addition, a verified fluid and structure interaction (FSI) solver implemented with an improved dynamic mesh strategy is adopted. Different transverse spring-mounted oscillation responses are obtained by alternating the angle of attack (AOA). It is found that the attachment and secondary separation of vortices in the near wake significantly influence the shedding pattern and can lead to the distinct suppression of oscillating responses. A pair of attached vortices are observed for the vibrissal cylinder at AOA = 0°, where the oscillating response is almost fully suppressed. Forced vibration is applied to address the role of attached vortices in the suppression mechanism. In contrast, when the suppression mechanism diminishes at AOA = 90°, the vortex-shedding pattern is characterized by unstable discrete secondary vortices. These secondary vortices re-separate from the primary vortices, indicating high stability of the primary vortices in the near wake, which represents the primary mechanism deteriorating the suppression of vortex-shedding. Furthermore, the interaction of attaching and secondary discrete vortices results in a partially suppressed oscillating response for the harbor seal vibrissa at AOA = 45°. Of note, these two mechanisms cause the shedding mode to switch between the “2S” and “P + S” modes while partially suppressing the oscillation response.

This study delves into the aerodynamic behaviour of a highly flexible NACA 0012 aerofoil, drawing inspiration from avian feathers to handle a gust. We first examined unsteady flow on a rigid wing both experimentally and numerically and then explored the implications of introducing wing flexibility purely numerically. Our findings underscore the potential of composite materials in alleviating the oscillating aerodynamic forces on a wing under a streamwise gust. This behaviour is attributed to its capacity to destabilize the laminar separation bubble, fostering a more stable turbulent boundary layer. While direct avian evidence remains limited, it is postulated that in nature, such mechanisms could mitigate undesired wing flapping, optimizing energy consumption in perturbed environments.

This work assesses the validity of transfer learning the hydrodynamic coefficient database, consisting of the added mass and lift coefficients, applicable to flexible bodies undergoing vortex-induced vibrations. Specifically, the hydrodynamic coefficient database learned on data collected by Braaten and Lie (2005) are used to predict the motions observed during in house bare riser model experiments at the MIT Towing Tank. A fully immersed vertical flexible riser model with a length-to-diameter ratio of 145 is towed at different flow speeds and top tensions. Motion is tracked using underwater cameras and the motions are reconstructed using a machine-vision framework eliminating the need for expensive sensing hardware. The vibration amplitude, frequency, and mode shape are determined and the results are compared with those in the literature. Finally, blind predictions of the in-house observed experiments are made using the software VIVA informed with transfer learned hydrodynamic coefficients learned on the experiments by Braaten and Lie (2005).

The main goal of this work is to develop a data-driven Reduced Order Model (ROM) strategy from high-fidelity simulation result data of a Full Order Model (FOM). The goal is to predict at lower computational cost the time evolution of solutions of Fluid–Structure Interaction (FSI) problems. For some FSI applications, the elastic solid FOM (often chosen as quasi-static) can take far more computational time than the fluid one. In this context, for the sake of performance one could only derive a ROM for the structure and try to achieve a partitioned FOM fluid solver coupled with a ROM solid one. In this paper, we present a data-driven partitioned ROM on two study cases: (i) a simplified 1D-1D FSI problem representing an axisymmetric elastic model of an arterial vessel, coupled with an incompressible fluid flow; (ii) an incompressible $2D$ wake flow over a cylinder facing an elastic solid with two flaps. We evaluate the accuracy and performance of the proposed ROM-FOM strategy on these cases while investigating the effects of the model’s hyperparameters. We demonstrate a high prediction accuracy and significant speedup achievements using this strategy.

Flexible wingtip extensions matched to the Cauchy and Reynolds numbers of a peregrine falcon’s primary feather in flight have been tested in differing configurations and compared to rigid ones for reference. The wingtip configurations were attached to the end of a symmetric (NACA 0012) aerofoil and were tested at 5° and 10° angles of attack and Reynolds numbers of $70k$ and $90k$. Time resolved particle image velocimetry (TR-PIV) was used to study the dynamics of the individual vortices. The results show that, at increased angle of attack the configuration with C-type variation of the free length of the winglets is spreading the vorticity into spanwise and vertical directions, generating a circular multi-core vortex arrangement. In contrast, for the case of winglets of the same length (I-type configuration) a continuous vortex sheet is formed which rolls up into a single core, dislocated outboards and upwards from the original tip-vortex location. This remains the case even for larger angle of attack. It is concluded that – besides the known reduction of induced drag – the former configuration is also beneficial for a more rapid disintegration of the tip-vortex in the wake, while the latter shows less instability. This let us speculate that the latter could be relevant for reducing the tip-noise at higher angle of attack such as for Owls, who hunt during night and have adapted to fly silently.

The static and dynamic interaction of a normal shock wave (upstream Mach number 1.35) with a compliant wall is characterized experimentally by schlieren visualizations and an optical displacement sensor. Depending on the location of the shock wave along the compliant wall, three different regimes of interaction are found: large-amplitude synchronized regime, small-amplitude synchronized regime and unsynchronized regime. The regime of large-amplitude synchronized oscillations is found for shock locations close to the mid-point of the compliant wall along the streamwise direction; at this location, the coupled system locks to the second vibration frequency of the structure. Three regimes of small-amplitude synchronized oscillations are found depending on the shock position. When the shock is located upstream the center of the compliant wall, the shock may oscillate either periodically at the frequency of the first vibration mode or quasi-periodically with highest amplitudes at the three frequencies of the vibration modes. When the shock is located downstream the center of the compliant wall, the shock oscillates periodically at the frequency of the third vibration mode. Finally, close to the trailing edge of the compliant wall, the shock oscillation is not synchronized with the compliant wall which oscillates with a very small amplitude. An empirical model is proposed to investigate the energy exchange between the flow and the compliant wall during the limit cycle oscillations. A negative aerodynamic damping – and, hence, the possibility of a limit cycle – is observed when a sufficiently extended separation is considered in the model for the pressure distribution at the wall.

The effect of angle of attack on the flow-induced vibration (FIV) response of an elastically mounted thin elliptical cylinder has been investigated by measuring the structural displacement and fluid forces acting on the body in water-channel experiments. Specifically, an elliptical cylinder with a cross-sectional elliptical ratio of $\varepsilon =b/a=5$ was chosen due to the presence of a region of vibration response associated with the combined effect of vortex-induced vibration (VIV) and galloping, where large vibration amplitudes nearly eight times the cross-flow dimensions can be sustained. Here, $a$ and $b$ are the semi-minor axis (aligned with the streamwise direction) and the semi-major axis, respectively. The present experimental results demonstrated that the large vibration amplitudes (i.e. where the maximum observed value was approximately 6$b$) generally decrease with the angle of attack, resulting in substantial reductions for $\alpha \gtrsim 2^{\circ }$ (with $\alpha =3.50^{\circ }$ corresponding to a $\sim$ 60% decrease in the maximum vibration amplitude). Particle image velocimetry (PIV) measurements revealed that the dominant vortex shedding mode consists of two single opposite-signed vortices shed per body vibration cycle. The presence of additional vorticity regions that were absent in the zero angle of attack case was also observed, including crescent-shaped wake structures and secondary inline vortices. This study shows the importance of maintaining axial symmetry in such an FIV system, and that the flow incidence angle is an essential consideration for efficient energy harvesting using this elliptical geometry.

Machine learning was used to optimize the geometric arrangement of a pair of unsteady actuators on flow separation over an efficient low Reynolds number airfoil in post-tall conditions. Large eddy simulation was used to validate the results. Two actuators: one with blowing and the other with suction openings were installed on the top surface of an airfoil at low Reynolds number of 60,000. An SD7003 airfoil at a post stall angle of attack of 13° was utilized. The boundary layer flow of the top surface was manipulated by the actuators to control flow separation. The influence of several actuator parameters: frequency, energy input, opening area, location and orientation angle were considered in an optimization of the dual actuator configuration. A genetic algorithm-based optimization was implemented to find the most effective configuration of this coupling. Since the optimization process is time-consuming, machine learning was used to train artificial neural networks to be coupled with genetic algorithm to reduce the computational cost. The artificial neural networks and their training was constantly upgraded during the optimization cycle. Results for the optimal case indicated an increase in lift coefficient and the objective function in comparison to uncontrolled case by factors of 1.88 and 3.33 respectively. We also found a reduction in drag coefficient. It was also found that using a pair of actuators was more efficient than using a single actuator.