Numerical analysis on propulsive efficiency and pre-deformated optimization of a composite marine propeller

The objectives of this paper are to numerically investigate the performance of a composite propeller through bidirectional FSI algorithm combining CFD and FEM,and to improve its propulsive efficiency by a pre-deformated method. Numerical results are presented for the composite propeller which has been modeled by unidirectionally stacking with glass-fiber reinforced composites. The propulsive efficiency of the composite and rigid propellers with different advance coefficients J has been compared.The results show that the efficiency of the composite propeller is obviously higher than that of the rigid propeller when J≤0.8,which is attributed to the decrease of pitch angle caused by the bend-twist coupling effects. But for the design condition J=0.851 and the cases with J>0.851,the efficiency of the composite propeller is significantly lower than that of the rigid propeller,which is because the angle of attack αcomposite is deviated from the optimal angle of attack αdesign more than that for the rigid case αrigid.Based on the optimization by the proposed pre-deformated method,the efficiency improvement of the composite propeller at the conditions with J≥0.851 could be obtained,and the composite material used in this work can meet the strength requirement of the designed propellers.

基于RANSE模型CFD预测E779A型螺旋桨空化及其在通用船体后的水动力性能

Ship propulsion performance heavily depends on cavitation,increasing the recent interest in this field to lower ship emissions.Academic research on the effects of cavitation is generally based on the open-water propeller performance but the interactions of the cavitating propeller with the ship hull significantly affect the propulsion performance of the ship.In this study,we first investigate the INSEAN E779A propeller by a RANSE-based CFD in open-water conditions.The numerical implementation and the selected grid after sensitivity analysis partially succeeded in modeling the cavitating flow around the propeller.Satisfactory agreement was observed compared to experimental measurements.Then,using the open-water data as input,the propeller’s performance behind a full-scale ship was calculated under self-propulsion conditions.Despite being an undesired incident,we found a rare condition in which cavitation enhances propulsion efficiency.Atσ=1.5;the propeller rotation rate was lower,while the thrust and torque coefficients were higher.

Underlying principle of efficient propulsion in flexible plunging foils

Passive flexibility was found to enhance propulsive efficiency in swimming animals.In this study,we numerically investigate the roles of structural resonance and hydrodynamic wake resonance in optimizing efficiency of a flexible plunging foil.The results indicates that（1）optimal efficiency is not necessarily achieved when the driving frequency matches the structural eigenfrequency;（2）optimal efficiency always occurs when the driving frequency matches the wake resonant frequency of the time averaged velocity profile.Thus,the underlying principle of efficient propulsion in flexible plunging foil is the hydrodynamic wake resonance,rather than the structural resonance.In addition,we also found that whether the efficiency can be optimized at the structural resonant point depends on the strength of the leading edge vortex relative to that of the trailing edge vortex.The result of this work provides new insights into the role of passive flexibility in flapping-based propulsion.

Computational Study on a Squid-Like Underwater Robot with Two Undulating Side Fins

The undulating fin propulsion system is an instance of the bio-inspired propulsion systems. In the current study, the swimming motion of a squid-like robot with two undulating side fins, mimicking those of a Stingray or a Cuttlefish, was investigated through flow computation around the body. We used the finite analytic method for space discretization and Euler implicit scheme for time discretization along with the PISO algorithm for velocity pressure coupling. A body-fitted moving grid was generated using the Poisson equation at each time step. Based on the computed results, we discussed the features of the flow field and hydrodynamic forces acting on the body and fin. A simple relationship among the fin＇s principal dimensions was established. Numerical computation was done for various aspect ratios, fin angles and frequencies in order to validate the proposed relationship among principal dimensions. Subsequently, the relationship was examined base on the distribution of pressure difference between upper and lower surfaces and the distribution of the thrust force. In efficiency calculations, the undulating fins showed promising results. Finally, for the fin, the open characteristics from computed data showed satisfactory conformity with the experimental results.

Computational Research on Modular Undulating Fin for Biorobotic Underwater Propulsor

Biomimetic design employs the principles of nature to solve engineering problems. Such designs which are hoped to be quick, efficient, robust, and versatile, have taken advantage of optimization via natural selection. In the present research, an environment-friendly propulsion system mimicking undulating fins of stingray was built. A non-conventional method was considered to model the flexibility of the fins of stingray. A two-degree-of-freedom mechanism comprised of several linkages was designed and constructed to mimic the actual flexible fin, The driving linkages were used to form a mechanical fin consisting of several fin segments, which are able tO produce undulations, similar to those produced by the actual fins. Owing to the modularity of the design of the mechanical fin, various undulating patterns can be realized. Some qualitative observations, obtained by experiments, predicted that the thrusts produced by the mechanical fin are different among various undulating patterns. To fully understand this experimental phenomenon is very important for better performance and energy saving for our biorobotic underwater propulsion system. Here, four basic undulating patterns of the mechanical fin were performed using two-dimensional unsteady computational fluid dynamics （CFD） method. An unstructured, grid-based, unsteady Navier-Stokes solver with automatic adaptive re-meshing was used to compute the unsteady flow around the fin through twenty complete cycles. The pressure distribution on fin surface was computed and integrated to provide fin forces which were decomposed into rift and thrust. The pressure force and friction force were also computed throughout the swimming cycle. Finally, vortex contour maps of these four basic fin undulating patterns were displayed and compared.

Effect of Spanwise Flexibility on Propulsion Performance of a Flapping Hydrofoil at Low Reynolds Number

Spanwise flexibility is a key factor influencing propulsion performance of pectoral foils. Performances of bionic fish with oscillating pectoral foils can be enhanced by properly selecting the spanwise flexibility. The influence law of spanwise flexibility on thrust generation and propulsion efficiency of a rectangular hydro-foil is discussed. Series foils constructed by the two-component silicon rubber are developed. NACA0015 shape of chordwise cross-section is employed. The foils are strengthened by fin rays of different rigidity to realize variant spanwise rigidity and almost the same chordwise flexibility. Experiments on a towing platform developed are carried out at low Reynolds numbers of 10 000, 15 000, and 20 000 and Strouhal numbers from 0.1 to 1. The following experimental results are achieved: (1) The average forward thrust increases with the St number increased; (2) Certain degree of spanwise flexibility is beneficial to the forward thrust generation, but the thrust gap is not large for the fins of different spanwise rigidity; (3) The fin of the maximal spanwise flexibility owns the highest propulsion efficiency; (4) Effect of the Reynolds number on the propulsion efficiency is significant. The experimental results can be utilized as a reference in deciding the spanwise flexibility of bionic pectoral fins in designing of robotic fish prototype propelled by flapping-wing.

A CFD Study on a Biomimetic Flexible Two-body System

By studying the characteristics of the flow field around a swimming fish,useful insights can be obtained into the superior swimming capabilities developed by nature over millions of years,in comparison to what can be achieved using the standard engineering principles traditionally employed in naval and ocean engineering.In the present study,the flow field related to a single joint fish model is simulated in the framework of a commercial computational fluid dynamics software(ANSYS Fluent 18.0).The principle of the anti-Kármán vortex street is analyzed and the relationship between the direction of the tail vortex and the direction of the fin swing is determined according to the vortex structures and the pressure distribution.A parametric investigation is finally conducted to analyze in particular how the Strouhal number(St)can affect the fish propulsive performance and efficiency。

Propulsive performance of a passively flapping plate in a uniform flow

Propulsive performance of a passively flapping plate in a uniform viscous flow has been studied numerically by means of a multiblock lattice Boltzmann method. The passively flapping plate is modeled by a rigid plate with a torsion spring acting about the pivot at the leading-edge of the plate, which is called a lumped-torsional-flexibility model. When the leading-edge is forced to take a vertical oscillation, the plate pitches passively due to the fluid-plate interaction. Based on our numerical simulations, various fundamental mechanisms dictating the propulsive performance, including the forces on the plate, power consumption, propulsive efficiency and vortical structures, have been studied. It is found that the torsional flexibility of the passively pitching plate can improve the propulsive performance. The results obtained in this study provide some physical insights into the understanding of the propulsive behaviors of swimming and flying animals.

Aerodynamic performance enhancement forflapping airfoils by co-flow jet

Introducing active flow control into the design of flapping wing is an effective way to enhance its aerodynamic performance.In this paper,a novel active flow control technology called Co-Flow Jet(CFJ)is applied to flapping airfoils.The effect of CFJ on aerodynamic performance of flapping airfoils at low Reynolds number is numerically investigated using Unsteady Reynolds Averaged Navier-Stokes(URANS)simulation with Spalart-Allmaras(SA)turbulence model.Numerical methods are validated by a NACA6415-based CFJ airfoil case and a S809 pitching airfoil case.Then NACA6415 baseline airfoil and NACA6415-based CFJ airfoil with jet-off and jet-on are simulated in flapping motion,with Reynolds number 70,000 and reduced frequency 0.2.As a result,CFJ airfoils with jet-on generally have better lift and thrust characteristics than baseline airfoils and jet-off airfoil when Cμgreater than 0.04,which results from the CFJ effect of reducing flow separation by injecting high-energy fluid into boundary layer.Besides,typical kinematic and geometric parameters,including the reduced frequency and the positions of the suction and injection slot,are systematically studied to figure out their influence on aerodynamic performance of the CFJ airfoil.And a variable Cμjet control strategy is proposed to further improve effective propulsive efficiency.Compared with using constant Cμ,an increase of effective propulsive efficiency by22.6%has been achieved by using prescribed variable CμNACA6415-based CFJ airfoil at frequency 0.2.This study may provide some guidance to performance enhancement for Flapping wing Micro Air Vehicles(FMAV).

Self-propulsion of a three-dimensional flapping flexible plate

The self-propulsion of a 3-D flapping flexible plate in a stationary fluid is numerically studied by an immersed boundarylattice Boltzmann method for the fluid flow and a finite element method for the plate motion. When the leading-edge of the plate is forced to heave sinusoidally, the entire plate starts to move freely as a result of the fluid-structure interaction. Based on our simulation and analysis on the dynamical behaviors of the flapping flexible plate, we have found that the effect of plate aspect ratio on its propulsive properties can be divided into three typical regimes which are related to the plate flexibility, i.e. stiff, medium flexible, and more flexible regime. It is also identified that a suitable structure flexibility, corresponding to the medium flexible regime, can improve the propulsive speed and efficiency. The wake behind the flapping plate is investigated for several aspect ratios to demonstrate some typical vortical structures. The results obtained in this study can provide some physical insights into the understanding of the propulsive mechanisms in the flapping-based locomotion.

A Stiffness Adjustment Mechanism Based on Negative Work for High-efficient Propulsion of Robotic Fish

The applications of robotic fish require high propulsive efficiency mechanism to prolong the mission time. Though many methods were applied, robotic fish still suffers from low efficiency. To improve the efficiency of robotic fish, this paper proposes a variable stiffness mechanism which is based on the negative work. The live fish adjusts its body stiffness to save energy when the muscles do negative work. Inspired by the live fish, a control mechanism based on negative work is proposed to change the stiffness of the robotic fish for higher efficiency. Changing the stiffness of the robotic fish is to change the joint-stiffness. A fuzzy controller is introduced to mimic the variable stiffness mechanism of the fish and depicts the relationship between the stiffness and the negative work. To evaluate the performance of this controller, a two-joint robotic fish model is established based on its kinematic model and hydrodynamic model. The evaluation results show that the robotic fish reduces the energy consumption and improves the propulsion efficiency when introducing the variable stiffness mechanism. Different environments with the control mechanism impact differently on propulsive efficiency. This mechanism may provide a high efficient propulsion control method for the robotic fish.

Design and numerical investigation of swirl recovery vanes for the Fokker 29 propeller

Swirl recovery vanes（SRVs） are a set of stationary vanes located downstream from a propeller, which may recover some of the residual swirl from the propeller, hoping for an improvement in both thrust and efficiency. The SRV concept design for a scaled version representing the Fokker 29 propeller is performed in this paper, which may give rise to a promotion in propulsive performance of this traditional propeller. Firstly the numerical strategy is validated from two aspects of global quantities and the local flow field of the propeller compared with experimental data, and then the exit flow together with the development of propeller wake is analyzed in detail.Three kinds of SRV are designed with multiple circular airfoils. The numerical results show that the swirl behind the propeller is recovered significantly with Model V3, which is characterized by the highest solidity along spanwise, for various working conditions, and the combination of rotor and vane produced 5.76% extra thrust at the design point. However, a lower efficiency is observed asking for a better vane design and the choice of a working point. The vane position is studied which shows that there is an optimum range for higher thrust and efficiency.