Kourosh Shoele

Propulsion performance of a skeleton-strengthened fin

SUMMARY We examine numerically the performance of a thin foil reinforced by embedded rays resembling the caudal fins of many fishes. In our study, the supporting rays are depicted as nonlinear Euler–Bernoulli beams with three-dimensional deformability. This structural model is then incorporated into a boundary-element hydrodynamic model to achieve coupled fluid–structure interaction simulation. Kinematically, we incorporate both a homocercal mode with dorso-ventral symmetry and a heterocercal mode with dorso-ventral asymmetry. Using the homocercal mode, our results demonstrate that the anisotropic deformability of the ray-reinforced fin significantly increases its capacity of force generation. This performance enhancement manifests as increased propulsion efficiency, reduced transverse force and reduced sensitivity to kinematic parameters. Further reduction in transverse force is observed by using the heterocercal mode. In the heterocercal model, the fin also generates a small lifting force, which may be important in vertical maneuvers. Via three-dimensional flow visualization, a chain of vortex rings is observed in the wake. Detailed features of the wake, e.g. the orientation of the vortex rings in the heterocercal mode, agree with predictions based upon particle image velocimetry (PIV) measurements of flow around live fish.

Numerical simulation of a pectoral fin during labriform swimming

SUMMARY We numerically examine the fluid–structure interaction and force generation of a skeleton-reinforced fin that geometrically, structurally and kinematically resembles the pectoral fin of a fish during labriform swimming. This fin contains a soft membrane with negligible bending stiffness and 12 embedded rays (modeled as beams). A potential flow-based boundary element model is applied to solve the fluid flow around the fin, in which the vorticity field is modeled as thin vorticity sheets shed from prescribed locations (the sharp trailing edge). The fin motion is actuated by dorsoventral and anteroposterior rotations of the rays (the motion of each ray is controlled individually), as well as pitching motion of the baseline. Consequently, the fin undergoes a combination of flapping (lift-based) and rowing (drag-based) motions typical in labriform swimming. The fin motion contains two strokes: a recovery stroke and a power stroke. The performance of the fin depends upon kinematic parameters such as the Strouhal number, the phase lag between rays, the pitching motion of the baseline and the passive deformations of the rays. The most interesting finding is that the strengthening of the ray at the leading edge plays a pivotal role in performance enhancement by reducing the effective angle of attack and decreasing the power expenditure during the recovery stroke.

Fluid–structure interactions of skeleton-reinforced fins: performance analysis of a paired fin in lift-based propulsion

SUMMARY We investigate the thrust generation capacity of a thin foil consisting of a membrane strengthened by embedded rays that is geometrically, structurally and kinematically similar to pectoral fins of bony fishes during lift-based labriform locomotion. Our numerical model includes a fully nonlinear Euler–Bernoulli beam model of the skeleton and a boundary-element model of the surrounding flow field. The fin undergoes a dorso–ventral flapping activated by rotations of the rays. Both the trailing edge vortices (TEV) and the leading edge vortices (LEV) are accounted for and modeled as shear layers. The thrust generation and propulsion efficiency are examined and documented. Our results show that synchronization of rays is pivotal to the performance of the system. A primary factor that determines the performance of the fin is phase lags between the rays, which create variations of the effective angle of attack at the leading edge as well as shape changes throughout the fin surface. Structural flexibility of the rays leads to passive deformations of the fin, which can increase the thrust generation and the propulsion efficiency.

Computational study of flow-induced vibration of a reed in a channel and effect on convective heat transfer

The flow-induced fluttering motion of a flexible reed inside a heated channel is modeled numerically and used to investigate the relationship between the aeroelastic vibration of the reed and heat-transfer enhancement. An immersed boundary method is developed to solve the coupled flow-structure-thermal problem, and the simulations show that the vibrating reed significantly increases the mean heat flux through the channel, as well as the thermal performance, quantified in terms of the thermal enhancement factor. The effect of reed material properties on vibratory dynamics and heat transfer is studied. Changes in material properties produce a rich variety of vibratory behavior, and the thermal performance is found to depend more strongly on the reed inertia than its bending stiffness. The effects of both the Reynolds number and channel confinement are examined and it is found that the thermal performance is maximized when the reed creates large modulations in the boundary layer of the channel, while at the same time avoiding the creation of strong vortices.

Performance of a wing with nonuniform flexibility in hovering flight

The deformability of insect wings is associated with the embedded skeleton (venation). In this paper, the aerodynamic performance of wings with nonuniform flexibility is computationally investigated. By using a two-dimensional rendition, the underlying veins are modeled as springs, and the membrane is modeled as a flexible plate. The focus is on the effects of the detailed distribution of vein flexibility upon the performance of such a wing in the generation of lift force. Specifically, we are interested in finding the importance of leading edge strengthening. Towards this end, the aerodynamic performances of three wings, a rigid wing, a flexible wing with identical veins, and a flexible wing with strengthened leading edge, are studied and compared against each other. It is shown that the flexible wing with leading edge strengthening is capable of producing significantly higher lift force without consuming more energy. This is found to be related to the stabilizing and cambering effects at the leading edg...

Flow-induced vibrations of a deformable ring

To understand flow-induced vibrations of deformable objects, we numerically investigate dynamics of a pressurized elastic ring pinned at one point within a uniform flow by using an immersed-boundary algorithm. The boundary of the ring consists of a fibre with no bending stiffness, which can be modelled as a linear spring with spring constant k and zero unstretched length. The vibration of the ring is decomposed into two parts: a pitching motion that includes a rigid-body rotation and a flexible bending motion in the transverse direction, and a tapping motion in the longitudinal direction. The pitching motion is dominated by the frequency of vortex shedding, whereas the primary frequency of the tapping motion is twice the frequency of vortex shedding. At the Reynolds number of 100, resonance is observed when k ~ 0.2 (k is normalized by the diameter of the undeformed ring, the speed of the upcoming flow and the fluid density). Across the resonance region, abrupt jumps in terms of the motion amplitudes as well as the hydrodynamic loads are recorded. Within the resonance region, the lift force demonstrates a beating phenomenon reminiscent of findings through reduced models and low-degree-of-freedom systems.

Performance of synchronized fins in biomimetic propulsion

By using a two-dimensional model of ray fins, we numerically investigate the thrust generation by closely-coupled fins with an immersed boundary approach. The concentration is on the performance enhancement through fin-fin interactions and the underlying vortex control mechanisms in three representative systems, a two-fin tandem configuration, a two-fin parallel configuration, and a three-fin triangular configuration. In all these systems the thrust generation can be significantly increased in comparison with single fins. Unlike previous studies of tandem fins, in which the gap and phase lag between the two fins were considered separately, our study shows that the dynamics of the system is determined by a parameter that combines these two (the global phase difference). The optimal performance occurs as this parameter is around π (destructive mode), and the worst performance occurs when it is around 0 (constructive mode). Interestingly, contrary to the vorticity cancellation scenario implied by its name, our simulations show that in the destructive mode there is in fact a wake re-organization mechanism, during which vortices with the same rotational direction shed from the two fins are attracted towards each other and merge. Subsequently, the wake downstream becomes a strong and well-organized reverse Kármán vortex street, which explains the increased thrust. In the parallel system, the best performance occurs in cases when the two fins are in opposites phases. Both the thrust and efficiency increase as the gap between the fins decreases, until a symmetry-breaking instability occurs in the wake and the efficiency starts plunging due to the increase in lateral force generation. In the triangular formation, the highest thrust generation also occurs in the destructive mode. However, no further increase in performance is observed compared with the tandem system.

Drafting mechanisms between a dolphin mother and calf.

We numerically study the drafting mechanisms between a dolphin mother and her calf swimming near the free surface. Formation locomotion between the cetacean mother-calf pair provides a way for the mother to assist the calf in its locomotion. Depending on the age and size of the calf, it swims at neonate, echelon, and infant positions. At each position, the effects of the calfs size, swimming speed, proximity to the free surface and the formation pattern are investigated and the optimal configurations predicted by the model based on the swimming hydrodynamics are compared with previous observations. It is shown that the neonate position is the optimal formation for controlling the separation of the calf, and the echelon position is the most hydrodynamically efficient position in transferring the thrust force from the mother to the calf. The infant position, on the other hand, avoids the energy loss due to wave generation so that it improves the self-propulsion performance of an older calf.

Mechanical design, instrumentation and measurements from a hemoacoustic cardiac phantom

In this paper we discuss the design of an acoustic phantom, instrumented with acoustic sensors that is employed to validate computational hemoacoustic models (CHM) and develop/test generative (model based) statistical pattern recognition algorithms for abnormal heart conditions. The phantom of the human thorax incorporates key elements of the physical heart/thorax system.

Efficient electronic cooling via flow-induced vibrations

A novel method that exploits flow-induced vibration for enhancing heat transfer in electronic cooling applications is explored using coupled flow-structural-thermal modeling. The idea is inspired from wind-instruments where the flow-induced vibration of a “reed” generates sound. In the current approach, a reed installed in a channel with heated walls is shown to generate vortex structures that enhance thermal convection with low pressure loss. Simulations employ a multiphysics approach to model the dynamics of this coupled flow, structure and thermal problem. Through flow visualizations and analyses, the dominant heat transfer enhancement mechanism is identified. Vortical structures shed from the self-actuated fluttering reed cause jetting of cold fluid from the core of the flow towards the heated top and bottom walls of the channel, causing sharper temperature gradients and thus higher heat flux. This mechanism led to 30% higher heat transfer for a fixed flow rate, and an 11% improvement in the thermal enhancement factor.