Yiannis Andreopoulos

Energy Harvesting from Highly Unsteady Fluid Flows using Piezoelectric Materials

In the present work we explore some aspects of energy harvesting from unsteady, turbulent fluid flow using piezoelectric generators. Turbulent flows exhibit a large degree of coherence in their spatial and temporal scales, which provides a unique opportunity for energy harvesting. The voltage generated by short, flexible piezoelectric cantilever beams placed inside turbulent boundary layers and wakes of circular cylinders at high Reynolds numbers is investigated. Matching the fluid flow’s predominant frequency with the natural frequency of the piezoelectric generator appears to maximize the piezoelectric output voltage. This voltage is also dependent on the generator’s location inside the flow field. A three-way coupled interaction simulation that takes into account the aerodynamics, structural vibration, and electrical response of the piezoelectric generator has been developed. The simulation results agree reasonably well with the experimental data paving the way of using such a tool to estimate the performance of different energy harvesting devices within unsteady flow fields.

The performance of a self-excited fluidic energy harvester

The available power in a flowing fluid is proportional to the cube of its velocity, and this feature indicates the potential for generating substantial electrical energy by exploiting the direct piezoelectric effect. The present work is an experimental investigation of a self-excited piezoelectric energy harvester subjected to a uniform and steady flow. The harvester consists of a cylinder attached to the free end of a cantilevered beam, which is partially covered by piezoelectric patches. Due to fluid?structure interaction phenomena, the cylinder is subjected to oscillatory forces, and the beam is deflected accordingly, causing the piezoelectric elements to strain and thus develop electric charge. The harvester was tested in a wind tunnel and it produced approximately 0.1?mW of non-rectified electrical power at a flow speed of 1.192?m?s?1. The aeroelectromechanical efficiency at resonance was calculated to be 0.72%, while the power per device volume was 23.6?mW?m?3 and the power per piezoelectric volume was 233?W?m?3. Strain measurements were obtained during the tests and were used to predict the voltage output by employing a distributed parameter model. The effect of non-rigid bonding on strain transfer was also investigated. While the rigid bonding assumption caused a significant (>60%) overestimation of the measured power, a non-rigid bonding model gave a better agreement (<10% error).

Vorticity, strain-rate and dissipation characteristics in the near-wall region of turbulent boundary layers

Experimental results are presented that reveal the structure of a two-dimensional turbulent boundary layer which has been investigated by measuring the time-dependent vorticity flux at the wall, vorticity vector, strain-rate tensor and dissipation-rate tensor in the near-wall region with spatial resolution of the order of 7 Kolmogorov viscous length scales. Considerations of the structure function of velocity and pressure, which constitute vorticity flux and vorticity, indicated that, in the limit of vanishing distance, the maximum attainable content of these quantities which corresponds to unrestricted resolution, is determined by Taylors microscale. They also indicated that most of the contributions to vorticity or vorticity flux come from the uncorrelated part of the two signals involved. The measurements allowed the computation of all components of the vorticity stretching vector, which indicates the rate of change of vorticity on a Lagrangian reference frame if viscous effects are negligible, and several matrix invariants of the velocity gradient or strain-rate tensor and terms appearing in the transport equations of vorticity, strain rate and their squared fluctuations

Studies of interactions of a propagating shock wave with decaying grid turbulence: velocity and vorticity fields

The unsteady interaction of a moving shock wave with nearly homogeneous and isotropic decaying compressible turbulence has been studied experimentally in a large-scale shock tube facility. Rectangular grids of various mesh sizes were used to generate turbulence with Reynolds numbers based on Taylors microscale ranging from 260 to 1300. The interaction has been investigated by measuring the three-dimensional velocity and vorticity vectors, the full velocity gradient and rate-of-strain tensors with instrumentation of high temporal and spatial resolution. This allowed estimates of dilatation, compressible dissipation and dilatational stretching to be obtained. The time-dependent signals of enstrophy, vortex stretching/tilting vector and dilatational stretching vector were found to exhibit a rather strong intermittent behaviour which is characterized by high-amplitude bursts with values up to 8 times their r.m.s. within periods of less violent and longer lived events. Several of these bursts are evident in all the signals, suggesting the existence of a dynamical flow phenomenon as a common cause. Fluctuations of all velocity gradients in the longitudinal direction are amplified significantly downstream of the interaction. Fluctuations of the velocity gradients in the lateral directions show no change or a minor reduction through the interaction. Root mean square values of the lateral vorticity components indicate a 25% amplification on average, which appears to be very weakly dependent on the shock strength. The transmission of the longitudinal vorticity fluctuations through the shock appears to be less affected by the interaction than the fluctuations of the lateral components. Non-dissipative vortex tubes and irrotational dissipative motions are more intense in the region downstream of the shock. There is also a significant increase in the number of events with intense rotational and dissipative motions. Integral length scales and Taylors microscales were reduced after the interaction with the shock in all investigated flow cases. The integral length scales in the lateral direction increase at low Mach numbers and decrease during strong interactions. It appears that in the weakest of the present interactions, turbulent eddies are compressed drastically in the longitudinal direction while their extent in the normal direction remains relatively the same. As the shock strength increases the lateral integral length scales increase while the longitudinal ones decrease. At the strongest interaction of the present flow cases turbulent eddies are compressed in both directions. However, even at the highest Mach number the issue is more complicated since amplification of the lateral scales has been observed in flows with fine grids. Thus the outcome of the interaction strongly depends on the initial conditions.

Interactions of vortices with a flexible beam with applications in fluidic energy harvesting

A cantilever piezoelectric beam immersed in a flow and subjected to naturally occurring vortices such as those formed in the wake of bluff bodies can be used to generate electrical energy harvested in fluid flows. In this paper, we present the pressure distribution and deflection of a piezoelectric beam subjected to controlled vortices. A custom designed experimental facility is set up to study the interaction of individual and multiple vortices with the beam. Vortex tori are generated by an audio speaker and travel at controlled rates over the beam. Particle image velocimetry is used to measure the 2-D flow field induced by each vortex and estimate the effect of pressure force on the beam deflection.

Stress transmission in porous materials impacted by shock waves

The interaction of moving shock waves with short length elastic porous aluminum samples of various porosities was investigated in a shock tube facility in a setup where the specimens were placed in front of a long rod of a modified Hopkinson Bar. High frequency response miniature pressure transducers and semiconductor strain gages were used to measure the pore gas pressure and the transmitted stress wave to the rod respectively. It was found that the effect of pore gas flow on the total stress history was inversely proportional to the material’s porosity, permeability and length. For low porosity aluminum samples due to the very low and very confined volumetric gas flow rate within the foam, a minimal contribution of the gas pressure within the pores to the total stress was observed and the magnitude of stress wave transmitted to the rod was amplified mainly due to the lower acoustic impedance of the foams relative to the rod. However, in a high porosity aluminum specimens with a high permeability and low...

An experimental study of the dissipative and vortical motion in turbulent boundary layers

The experimental data of Honkan & Andreopoulos (1997 a ) have been further analysed and some new statistical results obtained. In the present work, particular emphasis is given to the time-dependent behaviour of the kinematic shear stress, vorticity, enstrophy, dissipation rate, vorticity stretching and several of the matrix invariants of the velocity-gradient tensor, strain-rate tensor and rotation-rate tensor. The invariants are linked with terms appearing in the transport equations of enstrophy and dissipation rate. Indicative of the existence of extremely high fluctuations is that all r.m.s. values are considerably larger than the corresponding mean values. All invariants exhibit a very strong intermittent behaviour, which is characterized by large amplitude of bursts, which may be of the order of 10 times the r.m.s. values. A substantial qualitative agreement is found between the present experimentally obtained statistical properties of the invariants and those obtained from direct numerical simulation data. Patterns with high rates of turbulent kinetic energy dissipation and high enstrophy suggest the existence of strong shear layers in the near-wall region. In many instances, locally high values of the invariants are also associated with peaks in the shear stress. Conditional analysis provides some evidence of the existence of sequences of several vortices during strong vortical activities, with an average frequency of appearance four times higher than the frequency of appearance of hairpin vortices.

Dynamic compression of highly compressible porous media with application to snow compaction

A new experimental and theoretical approach is presented to examine the dynamic lift forces that are generated in the compression of both fresh powder snow and wind-packed snow. At typical skiing velocities of 10 to 30ms

Experimental techniques for highly resolved measurements of rotation, strain and dissipation-rate tensors in turbulent flows

^{-1}

On the generation of lift forces in random soft porous media

the duration of contact of a ski or snowboard with the snow will vary from 0.05 to 0.2s depending on the length of the planing surface and its speed. No one, to our knowledge, has previously measured the dynamic behaviour of snow on such a short time scale and, thus, there are no existing measurements of the excess pore pressure that can build-up in snow on this time scale. Using a novel porous cylinder–piston apparatus, we have measured the excess pore pressure that would build-up beneath the piston surface and have also measured its subsequent decay due to the venting of the air from the snow at the porous wall of the cylinder. In further experiments, in which the air is slowly and deliberately drained to avoid a build-up in pore pressure, we have been able to separate out the force exerted by the ice crystal phase as a function of its instantaneous deformation. A theoretical model for the pore pressure relaxation in the porous cylinder is then developed using consolidation theory. Dramatically different dynamic behaviour is observed for two different snow types, one (wind-packed) giving a steady continuous relaxation of the excess pore pressure and the other (fresh powder) leading to a piston rebound with negative pore pressure. A feature of the rebound is the apparent debonding of sintered ice crystals after maximum compression. This behaviour is described well by introducing a debonding coefficient where the debonding force is proportional to the expansion velocity of the medium. The experimental and theoretical approach presented herein and the previous generalized lubrication theory for compressible porous media, have laid the foundation for understanding the detailed dynamic response of soft porous layers to rapid deformation.