Flapping Motion Research Articles

Influence of vortex-excited acoustic resonance on flow dynamics in channel with coaxial side-branches

The influence of vortex-excited acoustic resonance on flow dynamics in a straight channel with coaxial side branches was experimentally determined. The energetic vortex structures superimposed in the shear layer, formed at the intersection between the straight channel and the side-branches, are closely coupled with the natural acoustic modes of the side-branches, resulting in significantly intensified acoustic standing waves in the side-branches. Two configurations with symmetric (length-to-diameter ratio: L1/D = L2/D = 10) and asymmetric (L1/D = 10, L2/D = 5) arrangements of the side-branches were used to investigate the clearly different flow dynamics with and without acoustic resonance. In the experiments, wall pressure measurements and acoustic modal analyses were first conducted to determine the frequency lock-on range of the vortex-excited acoustic resonance. Subsequently, the flow fields in two configurations were measured with planar particle image velocimetry. The results indicated that the vortex-excited acoustic resonance resulted in significant flow fluctuations inside the branches together with expanded shear layers and the enlarged vortex structures. Vorticity fluctuations and Reynolds shear stresses were also increased significantly when subjected to acoustic modulation. In terms of the proper orthogonal decomposition analysis, two different motions of the energetic flow structures were identified as interacting with the acoustic resonance: the vertical synchronous oscillation mode and the streamwise vortex-shedding mode. Correspondingly, the phase-averaged low-order flow fields, determined by the mode coefficients of these two correlative modes, clearly indicated the flapping motion of the mainstream and the periodic oscillation of the shear layer.

Hydrodynamic schooling of multiple self-propelled flapping plates

While hydrodynamic interactions for aggregates of swimmers have received significant attention in the low Reynolds number realm ($Re\ll 1$), there has been far less work at higher Reynolds numbers, in which fluid and body inertia are involved. Here we study the collective behaviour of multiple self-propelled plates in tandem configurations, which are driven by harmonic flapping motions of identical frequency and amplitude. Both fast modes with compact configurations and slow modes with sparse configurations were observed. The Lighthill conjecture that orderly configurations may emerge passively from hydrodynamic interactions was verified on a larger scale with up to eight plates. The whole group may consist of subgroups and individuals with regular separations. Hydrodynamic forces experienced by the plates near their multiple equilibrium locations are all springlike restoring forces, which stabilize the orderly formation and maintain group cohesion. For the cruising speed of the whole group, the leading subgroup or individual plays the role of ‘leading goose’.

3D MHD simulation of the double-gradient instability of the magnetotail current sheet

Flapping motion of the current sheet (CS) is an important physical process in the Earths magnetotail. The magnetic double-gradient model, which includes both the instability and wave modes, offers a reasonable explanation for the exciting and propagation of the flapping wave. In this paper, we apply an advanced numerical magnetohydrodynamic (MHD) scheme (conservation element and solution element (CESE)-MHD) to simulate the magnetic double-gradient instability in an idealized current sheet that mimics the magnetotail configuration. We initialize the simulations with a numerically relaxed magnetotail equilibrium, in which the normal component of the magnetic field has a tailward gradient. It is confirmed in our simulation that the instability develops in the current layer. The growth rate of the instability yielded from the simulation is very close to the prediction of theory, with a relative deviation of only ten percent. The results demonstrate that the CESE-MHD scheme is very powerful in numerical study of the double-gradient mechanism of the CS flapping mode, and can be used for further investigations of the flapping motion in more realistic CS configurations.

Experimental and theoretical analysis of a smart transmission mechanism system

This paper focuses on a smart transmission mechanism system, designed and fabricated based on compliant mechanisms. The system was assembled using a piezo-stack actuator in conjunction with a rhombus-type amplifier, a novel transmission mechanism synthesized with flexure hinges, and an active rotatable flap. This assembly is able to be utilized in smart structures to dynamically transfer linear motion generated from some actuators to rotational motion of active rotatable flaps from a long distance at a wide range of frequencies (1 Hz ∼ 64 Hz). The Lagrangian approach was utilized for dynamic analysis of the system to evaluate the fundamental natural frequency of the smart transmission mechanism system, and the result is 385 Hz; the error is 9% compared with a finite element analysis using ANSYS. Then force and displacement transmissibility of the system was also theoretically calculated based on a newly established theoretical model, and the detailed finite element analysis was also performed to study these properties. A prototype of the smart transmission mechanism system and a special test platform were manufactured to test the force and displacement transmissibility then the transmissibility was experimentally tested. The theoretical and numerical results found a good correlation with the experimental results, and the errors between experimental and theoretical (or numerical) results are within 13% (or 11%). The newly designed transmission mechanism system and the corresponding theoretical, numerical and experimental studies are able to direct the optimal design of some smart systems which consist of compliant mechanisms and transmission mechanisms. They can also offer fundamental information on the active vibration control using some piezo-stack actuators to actuate active flaps.

Coherent structures in the flow field generated by rigid flapping wing in hovering flight mode

Galilean invariant vortex identification methods and proper orthogonal decomposition (POD) are used to capture the coherent structures (CS) in the flow field generated by rigid wing in main flapping motion. Experiments are conducted for a square rigid wing at a flapping frequency f = 1.5 Hz. Tests are performed in water as a fluid medium in hover mode. The main flapping mechanism executes asymmetric lower-upper stroke of 1:3 ratio single degree of freedom motion. Two dimensional particle image velocimetry (PIV) technique is used to generate the phase locked velocity field at each discrete flapping angle by illuminating the mid chord plane of the wing. Three different Galilean invariant methods namely λ2 criterion, Q criterion and Δ criterion are used for vortex identification. These methods were found to be consistent in detecting swirling vortices or coherent structures (CS). Proper orthogonal decomposition is used to exhibit the most energetic modes of the flow field. The captured modes were identified to be in-connect with vortex identification methods. The combination of these tools were found to be more effective in comparison with velocity field data for achieving a deeper understanding of the complex flow produced by the flapping wing.

Unsteady aerodynamic model of flexible flapping wing

Bio-inspired flapping wing has potential application to micro air vehicles (MAV). Due to the nature of lightweight and flexibility of micro flapping wing structures, elastic deformation as a result of aeroelastic coupling is inevitable in flapping motion. This effect can be significant and beneficial to the aerodynamic performance as revealed in the present investigation for a flexible flapping wing of variable camber versus a rigid one. Firstly a two dimensional (2D) unsteady aerodynamic model (UAM) based on potential flow theory has been extended from previous study. Both leading and trailing edge discrete vortices are included in the model with unsteady Kutta condition satisfied to fully characterize the unsteady flow around a flapping wing. A wall function is created to modify the induced velocity of the vortices in the UAM to solve the vortices penetration problem. The modified UAM is then validated by comparing with CFD results of a typical insect-like flapping motion from previous research. Secondly the UAM is further extended for a flexible flapping wing of camber variation. Comparing with a rigid wing in a prescribed plunging and pitching motion, the results show lift increase with positive camber in upstroke by mitigating negative lift. The results also agree well with CFD simulation. Thirdly the 2D UAM is extended to calculate the aerodynamic forces of a 3D wing with camber variation, and validated by CFD results. Finally the model is applied to aerodynamic analysis of a 3D flexible flapping wing with aeroelastic coupling effect. Significant increase of lift coefficient can be achieved for a flexible flapping wing of positive camber and twist in upstroke produced by the structure elastic deformation.

Computational Fluid Dynamics Reproduction of Nonlinear Loads on a Vertical Column During Extreme Irregular Wave Events

Recently, a method for numerical reproduction of measured irregular wave events has been developed. The measured motion of the wave maker flaps defines the wave kinematics at the boundary of the numerical simulation in order to generate the waves. When such data are not available, the control signal of the wave maker can, instead, be generated from a given free surface elevation following the same procedure as in model tests. This procedure is applied to a model test case with extreme irregular wave events and resulting nonlinear global wave loads on a vertical cylinder, focusing on higher-order ringing excitation. The purpose of the investigation is twofold: (1) to validate the wave reconstruction procedure and (2) to validate the resulting computational fluid dynamics (CFD) ringing loads with the given waves. In order to better understand the frequency content in the CFD-generated loads, wavelet analysis as well as the response of a single degree-of-freedom (SDOF) oscillator is examined and compared with the corresponding results for the third-order wave forcing based on the MacCamy–Fuchs (MF) and Faltinsen, Newman, Vinje (FNV) formulations. The results show generally good agreement between CFD and experiment both in the waves and in the loads; discrepancies found in the loads mainly originate from corresponding uncertainties in the wave reconstruction. Wave breaking may be one source of uncertainty. The MF + FNV formulation showed reasonable prediction of the maximum responses of an SDOF oscillator, but could not capture the loads well at all of the important frequencies.

Experimental Evaluation of Endovascular Fenestration Scissors in an Ovine Model of Aortic Dissection

To evaluate the experimental feasibility of endovascular fenestration using specific endovascular scissor prototypes in an ovine model of acute aortic dissection (AD). A previously described endovascular technique was used to create a model of acute type B AD in sheep. Endovascular fenestrations using either endovascular scissor prototypes or a long sheath were compared. Four prototypes of endovascular fenestration scissors were evaluated. Both validity of the experimental model of AD and technical success of endovascular fenestration were assessed by haemodynamic criteria, completion angiography, transesophageal echocardiography, and post-procedural analysis of harvested aortas. Experimental acute AD was created by endovascular means in 17 sheep, with a technical success rate of 82%. Systolic blood pressure was lower in the false lumen than in the true lumen (58±5 vs. 79±3mmHg, respectively; p<.001). Endovascular fenestration was performed in 11 models (endovascular scissors n=8; long sheath n=3). Controlled endovascular fenestration was obtained by the use of endovascular scissors (n=5/8), resulting in a significant rise in false lumen systolic blood pressure after fenestration (60±2 vs. 67±9mmHg before and after fenestration, respectively; p<.047). Long sheath fenestration resulted in an uncontrolled flap motion, leading to either pseudo-coarctation syndrome or aortic rupture (58±6 vs. 40±2mmHg before and after fenestration, respectively; p<.001). In this experimental study, a reproducible AD model has been developed in sheep using endovascular procedures exclusively to evaluate endovascular fenestration techniques. Endovascular fenestration using a long sheath appeared hazardous and risky invivo. Endovascular scissors constitute a dedicated and suitable tool to perform a safe controlled and effective endovascular fenestration in an ovine model.

Modal decomposition of fluid–structure interaction with application to flag flapping

Modal decompositions such as proper orthogonal decomposition (POD), dynamic mode decomposition (DMD) and their variants are regularly used to educe physical mechanisms of nonlinear flow phenomena that cannot be easily understood through direct inspection. In fluid–structure interaction (FSI) systems, fluid motion is coupled to vibration and/or deformation of an immersed structure. Despite this coupling, data analysis is often performed using only fluid or structure variables, rather than incorporating both. This approach does not provide information about the manner in which fluid and structure modes are correlated. We present a framework for performing POD and DMD where the fluid and structure are treated together. As part of this framework, we introduce a physically meaningful norm for FSI systems. We first use this combined fluid–structure formulation to identify correlated flow features and structural motions in limit-cycle flag flapping. We then investigate the transition from limit-cycle flapping to chaotic flapping, which can be initiated by increasing the flag mass. Our modal decomposition reveals that at the onset of chaos, the dominant flapping motion increases in amplitude and leads to a bluff-body wake instability. This new bluff-body mode interacts triadically with the dominant flapping motion to produce flapping at the non-integer harmonic frequencies previously reported by Connell and Yue (2007). While our formulation is presented for POD and DMD, there are natural extensions to other data-analysis techniques.

CFD-CSD coupled analysis of underwater propulsion using a biomimetic fin-and-joint system

The remarkable performance of various species of fish in propulsion and maneuvering has motivated the design and analysis of flexible, biomimetic underwater propulsors, which may be particularly suitable to small-scale, unmanned vehicles. In this work, we employ a novel fluid-structure coupled computational framework, referred to as FIVER (a FInite Volume method based on Exact Riemann solvers), to simulate the flapping motion of a fin-and-joint system, which mimics the caudal peduncle and caudal fin of fish, and serves as a simplified engineering model of tail-dominated fish propulsion. This problem is dominated by fluid-structure interaction, featuring a three-dimensional, unsteady fluid flow, large structural motion and deformation, and strong added-mass effect. To handle these challenges, we apply an embedded boundary method and a numerically-stable partitioned procedure to couple a hybrid finite volume – finite element computational fluid dynamics (CFD) solver and a nonlinear finite element computational structural dynamics (CSD) solver. First, we validate the CFD and CSD models using experimental data in fundamental vibration frequency, hydrodynamic forces, and structural displacement. Next, we investigate the fluid and structural dynamics, as well as the propulsive performance, focusing on the two-way fluid-structure coupling and the three-dimensional flow variation, which supplements the existing body of literature on biological and bio-inspired fluid dynamics. Further, by comparing a wide, trapezoidal fin and a narrow, forked fin, we investigate the various effects of fin geometry, and more generally, also demonstrate the use of observations and knowledge of biological diversity in the design of engineering systems.

Interaction problem between fluid and membrane by a consistent direct discretisation approach

Numerical methods for solving the flow around a moving or deforming membrane are developed by employing the ideas of the direct-discretisation of the Navier–Stokes equation even in the immediate vicinity of the object surface and consistent-discretisation of the pressure Poisson equation with the incompressible velocity field (i.e., consistent-coupling) that incorporates the boundary conditions on the surface to the pressure equation. Comparative study of three different discretisation strategies is carried out to highlight the effect of the consistent and direct discretisations. The validity of the present “consistent direct discretisation” is established through comparisons with the analytical solutions for some fundamental shear-driven 2-D flow fields with a moving membrane of flat and corrugated geometries. Also, the flows induced by a membrane of circular geometry (i.e., particle) are compared with the reference numerical results under prescribed motion in a stationary fluid as well as lateral migration in a shear flow. A three dimensional problem is presented for an interaction problem between a spherical particle and Taylor–Green vortices, and the result is verified by comparing with the independently-conducted numerical results. Finally, the method is applied to an interaction problem between the fluid and flexible filament showing strong flapping and snapping motions.

Numerical analysis of a tidal current generator with dual flapping wings

Flapping wings, inspired by the mechanism of birds and fish, can act as generators to harvest energy from tidal currents. The hydraulic system is simplified as a spring-damper system to establish the coupling equations relating to the wing motion and the hydrodynamic forces. To provide guidance for design of a fully flow-induced flapping wings energy harvesting system, the behaviors of both system response and energy extraction performance are analyzed using two-dimensional numerical approach. Depending on the rotary actuator radius R, and the volume ratio β between the cylinder and rotary actuator, three distinguishable behaviors are observed in the system response and energy extraction performance. At larger R and smaller β, the dual wings tend to undergo a damped reduction flapping motion because the pitching motion consumes a significant amount of energy. Both decreasing R and increasing β can reduce the energy consumption of the pitching motion, and thus allow the dual wings to achieve a sustainable flapping motion. Although an irregular response can achieve a self-sustained flapping motion, it is unfavorable owing to its unstable power output. The regular response essential for stable energy harvesting is realized over a range of coupling parameters. The energy extraction performance of the system is closely associated with β but also slightly depends on R.

Improvement of Energy Extraction Efficiency for Flapping Airfoils by Using Oscillating Gurney Flaps

The present paper proposes an oscillating Gurney flap to enhance the power extraction efficiency of a flapping airfoil system. Two-dimensional Navier–Stokes resolutions by the flow solver fluent are performed. To alter the flap position during the flapping cycle, a dynamic mesh technique is used. The flow regime is considered fully laminar, with a free stream Reynolds number of Re = 1100. Results show that, the synchronization of the Gurney flap motion with the central flapping airfoil motion generates a virtual camber which corrects the pressure distribution and ultimately enhances the lift force during both, the up-stroke and down-stroke stages. The lift enhancement is interpreted to an improvement in the output power as compared to a clean flapping extractor.

The Propagation of the Waves in the CTO S.A. Towing Tank

Abstract The paper presents the results of research focused on the wave propagation in the CTO S.A. deepwater towing tank. In the scope of paper, the wavemaker transfer function was determined for regular waves, based on the Biésel Transfer Function and further for irregular waves, based on Hasselman model of nonlinear energy transfer. The phenomena: wave damping, wave breakdown and wave reflection, were measured, analyzed and mathematically modeled. Developed mathematical models allow to calculate the impact of mentioned phenomena on the wave propagation and furthermore to calculate the wave characteristics along the whole measurement area in the CTO S.A. deepwater towing tank, based on wavemaker flap motion control.

The Role of the Active Site Flap in Streptavidin/Biotin Complex Formation.

Obtaining a detailed description of how active site flap motion affects substrate or ligand binding will advance structure-based drug design (SBDD) efforts on systems including the kinases, HSP90, HIV protease, ureases, etc. Through this understanding, we will be able to design better inhibitors and better proteins that have desired functions. Herein we address this issue by generating the relevant configurational states of a protein flap on the molecular energy landscape using an approach we call MTFlex-b and then following this with a procedure to estimate the free energy associated with the motion of the flap region. To illustrate our overall workflow, we explored the free energy changes in the streptavidin/biotin system upon introducing conformational flexibility in loop3-4 in the biotin unbound ( apo) and bound ( holo) state. The free energy surfaces were created using the Movable Type free energy method, and for further validation, we compared them to potential of mean force (PMF) generated free energy surfaces using MD simulations employing the FF99SBILDN and FF14SB force fields. We also estimated the free energy thermodynamic cycle using an ensemble of closed-like and open-like end states for the ligand unbound and bound states and estimated the binding free energy to be approximately -16.2 kcal/mol (experimental -18.3 kcal/mol). The good agreement between MTFlex-b in combination with the MT method with experiment and MD simulations supports the effectiveness of our strategy in obtaining unique insights into the motions in proteins that can then be used in a range of biological and biomedical applications.

Instability Mechanism of Roll/Lateral Biodynamic Rotorcraft–Pilot Couplings

The paper investigates the basic mechanism of aeroservoelastic pilot-assisted oscillation about the roll axis due to the interaction with pilot's arm biomechanics. The motivation stems from the observation that a rotor imbalance may occur as a consequence of rotor cyclic lead–lag modes excitation. The work shows that the instability mechanism is analogous to air resonance, in which the pilot's involuntary action plays the role of the automatic flight control system. Using robust stability analysis, the paper shows how the pilot's biodynamics may involuntarily lead to a roll/lateral instability. The mechanism of instability proves that the pilot biodynamics is participating in the destabilization of the system by transferring energy, i.e., by producing forces that do work for the energetically conjugated displacement, directly into the flapping mode. This destabilizes the airframe roll motion, which, in turn, causes lead–lag motion imbalance. It is found that, depending on the value of the time delay involved in the lateral cyclic control, the body couples with rotor motion in a different way. In the presence of small or no time delays, body roll couples with the rotor through the lead–lag degrees of freedom. The increase of the time delay above a certain threshold modifies this coupling: The body no longer couples with the rotor through lead–lag but directly through flap motion.

The use of dental implants, cast bars and sleeve overdentures in oral cancer patients.

Surgical resection of an oral tumour (and the associated free flap reconstruction) can significantly alter the oral anatomy. The lack of sulcus depth, alveolar ridge, presence of a mobile flap and limited tongue movement can make it impossible for patients to control a removable prosthesis. To help this cohort of patients, dental implants can be invaluable. The Oral Rehabilitation Team at Central Manchester University Dental Hospital have used dental implants to rehabilitate oral cancer patients for over thirty years. After their resective surgery, a number of patients were dentally rehabilitated with a laboratory-made, precious metal-alloy bar supported by at least four dental implants. A metal-alloy under-sleeve retained overdenture was then provided to fit over the milled bar. The majority of the 50 patients in this case series had tumours in the anterior floor of the mouth. It was noted that 76% of the patients received a rim resection and were reconstructed with a fasciocutaneous, soft tissue free flap. Six percent of patients received a segmental resection and were reconstructed with either a fibular or deep circumflex iliac artery free flap. The dental implants and sleeve overdentures had a survival rate of 100%. None of the dentures lost retention, implying that the frictional grip between the overcasting and the milled bar was sufficient to appease the retention demands of this cohort. However, 10% of patients encountered complications. This would suggest a success (or complication free) rate of 90% for this cohort of 50 oncology patients. This would still imply that milled bars/sleeve overdentures carry a relatively low maintenance burden and may be a useful treatment option for oral cancer patients.

Estimate of Rotor Controls and Blade Flapping Caused by an Orthogonal Vortex Disturbance

Based on blade element/momentum theory the aerodynamic impact on rotor controls and blade flapping caused by a straight line vortex penetrating orthogonally the plane of a rotor disk is solved using a realistic swirl velocity profile. The collective and cyclic controls needed for disturbance rejection and the coning and cyclic flapping motion developing are computed.

Aerodynamic efficiency of a bioinspired flapping wing rotor at low Reynolds number.

This study investigates the aerodynamic efficiency of a bioinspired flapping wing rotor kinematics which combines an active vertical flapping motion and a passive horizontal rotation induced by aerodynamic thrust. The aerodynamic efficiencies for producing both vertical lift and horizontal thrust of the wing are obtained using a quasi-steady aerodynamic model and two-dimensional (2D) CFD analysis at Reynolds number of 2500. The calculated efficiency data show that both efficiencies (propulsive efficiency-ηp, and efficiency for producing lift-Pf) of the wing are optimized at Strouhal number (St) between 0.1 and 0.5 for a range of wing pitch angles (upstroke angle of attack αu less than 45°); the St for high Pf (St = 0.1 ∼ 0.3) is generally lower than for high ηp (St = 0.2 ∼ 0.5), while the St for equilibrium rotation states lies between the two. Further systematic calculations show that the natural equilibrium of the passive rotating wing automatically converges to high-efficiency states: above 85% of maximum Pf can be obtained for a wide range of prescribed wing kinematics. This study provides insight into the aerodynamic efficiency of biological flyers in cruising flight, as well as practical applications for micro air vehicle design.

Egress and invasion machinery of malaria: an in-depth look into the structural and functional features of the flap dynamics of plasmepsin IX and X.

Plasmepsins, a family of aspartic proteases expressed by Plasmodium falciparum parasite, have been identified as key mediators in the onset of lethal malaria. Precedence has been placed on this family of enzymes due their essential role in the virulence of the parasite, thus highlighting their importance as novel drug targets. A previously published study by our group proposed a set of parameters used to define the flap motion of aspartic proteases. These parameters were used in the study of Plm I–V and focused on the flap flexibility as well as structural dynamics. Recent studies have highlighted the essential role played by Plm IX and X in egress and invasion of the malarial parasite. This study aims to close the gap on the latter family, investigating the flap dynamics of Plms IX and X. Molecular dynamics simulations demonstrated an “open and close” mechanism at the region of the catalytic site. Further computation of the dihedral angles at the catalytic region revealed tractability at both the flap tip and flexible loop. This structural versatility enhances the interaction of variant ligand sizes, in comparison to other Plm family members. The results obtained from this study signify the essential role of structural flap dynamics and its resultant effect on the binding landscapes of Plm IX and X. We believe that this unique structural mechanism may be integrated in the design and development of effective anti-malarial drugs.