Session M6: Aerodynamics: Flutter, Vibration and Morphing Membranes

Aerodynamic Coupling between Two Side-by-Side Piezoelectric Harvesters in Grid Turbulence

Experimental and analytical results relating to the extraction of fluidic energy from decaying homogeneous and isotropic turbulence using two side-by-side piezoelectric beams are reported. Turbulence carries mechanical energy distributed over a range of temporal and spatial scales and the resulting interaction of these scales with the immersed piezoelectric beams creates a strain field in the beam which generates electric charge. Experiments are carried out in a large scale wind tunnel in which a passive turbulence-generating grid is used to excite various piezoelectric cantilever beam configurations positioned parallel to the flow with different gap widths between the beams at various distances from the grids and for different flow velocities. We observe that the aerodynamic coupling is stronger at higher velocities and when longer beams are paired together and decays exponentially with increasing gap width between the beams. More importantly, however, it is observed that the aerodynamic coupling due to the presence of a second beam greatly improves the energy harvesting process, so much so that the average power generated per beam increases by up to 20 times, potentially allowing for significant power extraction from a random, non-resonant phenomenon such as turbulence.   

Heat Transfer Enhancement in High Performance Heat Sink Channels by Autonomous, Aero-Elastic Reed Fluttering

Heat transport within high aspect ratio, rectangular mm-scale channels that model segments of a high-performance, air-cooled heat sink is enhanced by the formation of unsteady small-scale vortical motions induced by autonomous, aeroelastic fluttering of cantilevered planar thin-film reeds. The flow mechanisms and scaling of the interactions between the reed and the channel flow are explored to overcome the limits of forced convection heat transport from air-side heat exchangers. High-resolution PIV measurements in a testbed model show that undulations of the reed's surface lead to formation and advection of vorticity concentrations, and to alternate shedding of spanwise CW and CCW vortices. These vortices scale with the reed motion amplitude, and ultimately result in motions of decreasing scales and enhanced dissipation that are reminiscent of a turbulent flow. The vorticity shedding lead to strong enhancement in heat transfer that increases with the Reynolds number of the base flow (e.g., the channel's thermal coefficient of performance is enhanced by 2.4-fold and 9-fold for base flow Re $=$ 4,000 and 17,400, respectively, with corresponding decreases of 50 and 77{\%} in the required channel flow rates). This is demonstrated in heat sinks for improving the thermal performance of low-Re thermoelectric power plant air-cooled condensers, where the global air-side pressure losses can be significantly reduced by lowering the required air volume flow rate at a given heat flux and surface temperature.   

Interaction of Gap Flow With Flapping Dynamics of Two Side-by-Side Elastic Foils

We present a numerical analysis on the interaction between two side-by-side elastic foils with their leading edges clamped and the gap flow between them. We perform systematic parametric direct numerical simulations as a function of nondimensional bending rigidity, $K_B \in [1\times 10^{-4}, 3\times 10^{-3}]$ and mass-ratio, $m^*\in [0.05,0.2]$, for a fixed gap, $d_p=0.2/L$, at the leading edge and Reynolds number, $Re=1000$ to explain the underlying physical mechanism behind the in-phase and out-of-phase coupled flapping modes. The parametric simulations show that the parallel foil system exhibits predominant out-of-phase coupling for low mass-ratio $m^*\le 0.1$ and in-phase coupling for higher mass-ratios $m^*>0.1$. We also show that the two side-by-side elastic foils always exhibit out-of-phase coupling initially irrespective of whether the fully developed flapping show out-of-phase or in-phase coupled mode. Finally, we show that the transition from the initial out-of-phase to stable in-phase is characterized by loss of gap flow symmetric stability to undergo oscillations at the gap exit.   

Comparison of driven and simulated ``free'' stall flutter in a wind tunnel

Stall flutter and dynamic stall have received a significant amount of attention over the years. To experimentally study this problem, the body undergoing stall flutter is typically driven at a characteristic, single frequency sinusoid with a prescribed pitching amplitude and mean angle of attack offset. This approach allows for testing with repeatable kinematics, however it effectively decouples the structural motion from the aerodynamic forcing. Recent results suggest that this driven approach could misrepresent the forcing observed in a ``free'' stall flutter scenario. Specifically, a dynamically pitched rigid NACA 0018 wing section was tested in the wind tunnel under two modes of operation: (1) Cyber-Physical where ``free'' stall flutter was physically simulated through a custom motor-control system modeling a torsional spring and (2) Direct Motor-Driven Dynamic Pitch at a single frequency sinusoid representative of the cyber-physical motion. The time-resolved pitch angle and moment were directly measured and compared for each case. It was found that small deviations in the pitch angle trajectory between these two operational cases generate significantly different aerodynamic pitching moments on the wing section, with the pitching moments nearly 180$^{\mathrm{o}}$ out of phase in some cases.   

Water channel experiments of a novel fully-passive flapping-foil turbine

Experiments have been conducted to assess the performances of a fully-passive flapping-foil hydrokinetic turbine for which the blade's motions are stemming from the interaction between the blade's elastic supports (springs and dampers) and the flow field. Previous numerical studies conducted by Peng \& Zhu (2009) and Zhu (2012) have proved that a simplified version of such a turbine can extract a substantial amount of energy from the flow while offering the potential to greatly simplify the complex mechanical apparatus needed to constrain and link the blade's pitching and heaving motions in the case of the more classical flapping-foil turbine (e.g., Kinsey et al., 2011). Based on the promising numerical investigations of Veilleux (2014) and Veilleux \& Dumas (2016), who proposed a more general version of this novel concept, a prototype has been built and tested in a water channel at a chord Reynolds number of 17,000. Periodic motions of large amplitudes have been observed leading to interesting energy harvesting efficiencies reaching 25\% for some specific sets of structural parameters. The sensitivity of the turbine's dynamics to each of the seven structural parameters appearing in the equations of motion has been experimentally evaluated around a case close to the optimal one.   

Coupled-Mode Flutter of Bending-Bending Type in Highly-Flexible Uniform Airfoils

We study the behavior of a highly flexible uniform airfoil placed in wind both numerically and experimentally. It is shown that for a non-rotating highly-flexible cantilevered airfoil, placed at very small angles of attack (less than 1 degree), the airfoil loses its stability by buckling. For slightly higher angles of attack (more than 1 degree) a coupled-mode flutter in which the first and the second flapwise modes coalesce toward a flutter mode is observed, and thus the observed flutter has a bending-bending nature. The flutter onset and frequency found experimentally matched the numerical predictions. If the same airfoil is forced to rotate about its fixed end, the static deflection decreases and the observed couple-mode flutter becomes of flapwise-torsional type, same as what has already been observed for flutter of rotating wind turbine blades.   

Experimental investigation of the dynamics of a hybrid morphing wing: time resolved particle image velocimetry and force measures

A quantitative characterization of the effects obtained by high frequency-low amplitude trailing edge actuation is performed. Particle image velocimetry, as well as pressure and aerodynamic force measurements, are carried out on an airfoil model. This hybrid morphing wing model is equipped with both trailing edge piezoelectric-actuators and camber control shape memory alloy actuators. It will be shown that this actuation allows for an effective manipulation of the wake turbulent structures. Frequency domain analysis and proper orthogonal decomposition show that proper actuating reduces the energy dissipation by favoring more coherent vortical structures. This modification in the airflow dynamics eventually allows for a tapering of the wake thickness compared to the baseline configuration. Hence, drag reductions relative to the non-actuated trailing edge configuration are observed.   

Unsteady aerodynamics of membrane wings with adaptive compliance

Membrane wings are known to provide superior aerodynamic performance at low Reynolds numbers ($Re = 10^4-10^5$), primarily due to passive shape adaptation to flow conditions. In addition to this passive deformation, active control of the fluid-structure interaction and resultant aerodynamic properties can be achieved through the use of dielectric elastomer actuators as the wing membrane material. When actuated, membrane pretension is decreased and wing camber increases. Additionally, actuation at resonance frequencies allows additional control over wing camber. We present results using synchronized (i) time-resolved particle image velocimetry (PIV) to resolve the flow field, (ii) 3D direct linear transformation (DLT) to recover membrane shape, (iii) lift/drag/torque measurements and (iv) near-wake hot wire anemometry measurements to characterize the fluid-structure interactions. Particular attention is paid to cases in which the vortex shedding frequency, the membrane resonance, and the actuation frequency coincide. In quantitatively examining both flow field and membrane shape at a range of actuation frequencies and vortex shedding frequencies, this work seeks to find actuation parameters that allow for active control of boundary layer separation over a range of flow conditions.   

Unsteady fluid-structure interactions with a heaving compliant membrane wing

Membrane wings have been shown to provide some benefits over rigid wings at the low Reynolds number regime ($Re \sim 10^3$ to $10^5$), specifically improved thrust in flapping flight. Here we present results from a theoretical framework used to characterize the unsteady aeroelastic behavior of compliant membrane wings executing a heaving motion. An analytical model is developed using 2D unsteady thin airfoil theory, coupled with an unsteady membrane equation. Chebyshev collocation methods are used to solve the coupled system efficiently. The model is used to explore the effects of wing compliance, inertia (including added mass effect) and flapping kinematics on the aerodynamic performance, identifying optimal conditions for maximum thrust and propulsive efficiency. A resonant frequency of the coupled system is identified and characterized for different fluid-structure interaction regimes. Extensions to pitching kinematics are also discussed.   

Combining spanwise morphing, inline motion and model based optimization for force magnitude and direction control

Bats and other animals rapidly change their wingspan in order to control the aerodynamic forces. A NACA0013 type airfoil with dynamically changing span is proposed as a simple model to experimentally study these biomimetic morphing wings. Combining this large-scale morphing with inline motion allows to control both force magnitude and direction. Force measurements are conducted in order to analyze the impact of the 4 degree of freedom flapping motion on the flow. A blade-element theory augmented unsteady aerodynamic model is then used to derive optimal flapping trajectories.