Optimal Propulsive Efficiency Research Articles

Optimal propulsion efficiency for NACA0012 foils with asymmetries in motion: A hybrid approach using the Taguchi method and artificial neural networks

This study uses the Taguchi method and artificial neural networks to optimize the thrust and propulsion efficiency of a NACA0012 foil, examining variables like downstroke duration (S = 0.3–0.5), angle of attack during downstroke and upstroke (αmd, αmu = 20°–50°), and Strouhal number (St = 0.2–0.5). Analysis shows that the St greatly affects thrust, while the αmu is pivotal for propulsion efficiency. Thrust is optimized by using a shorter S of 0.3, a greater αmd of 50° and a larger St of 0.45–0.5 and efficiency is optimized by using symmetric S of 0.5, a smaller αmd, αmu of 20° and a lower St of 0.35–0.39. Force and flow field analysis shows that certain combinations optimize thrust by mitigating negative thrust in the early downstroke, though they increase power consumption, lowering overall efficiency. Configurations balancing stroke duration and minimizing the angle of attack enhance efficiency by decreasing power demand for steady thrust. Wake structure analysis reveals that optimal efficiency involves generating reverse Kármán vortex streets, while optimizing thrust leads to a more chaotic wake. This study advances fluid dynamics in propulsion systems and aids in designing more efficient, adaptable underwater vehicles by identifying the key balance between thrust optimization and propulsion efficiency.

Strouhal number impact on propulsion efficiency in fully-active oscillating foils: Unpacking frequency, amplitude, and velocity relationships

Oscillating foils play a crucial role in marine and aerial organisms. Propulsion efficiency is gauged by the Strouhal number, combining frequency, amplitude, and velocity. This creates three parameters: reduced frequency, dimensionless amplitude, and Reynolds number. The hydrodynamics between these parameters and their effect on propulsion efficiency is complex. Using RANS simulations, this study shows that a large amplitude significantly increases propulsion efficiency. Flow analysis shows that increasing dimensionless amplitude enhances heaving velocity without affecting pitching velocity. However, an increase in reduced frequency increases both velocities, so there is greater inflow and horizontal force. Polynomial regression shows the horizontal force coefficient correlates with the square of the Strouhal number, and the power coefficient has a cubic relationship. A rise in reduced frequency increases the gradient of the horizontal force and power coefficient curves more than an increase in amplitude, suggesting decreasing propulsion efficiency. This clarifies why larger amplitude values are more efficient and why the optimal Strouhal number is between 0.3 and 0.6. Vortex analysis shows that lower reduced frequencies produce an undulatory wake pattern, not ideal for thrust. A higher amplitude, or frequency creates a leading-edge vortex pattern, reducing propulsion efficiency. A reverse Kármán vortex pattern represents optimal propulsion efficiency.

Propulsion enhancement of flexible plunging foils: Comparing linear theory predictions with high-fidelity CFD results

The fluid–structure interaction of a flexible plunging hydrofoil immersed in a current is solved numerically to analyze its propulsion enhancement due to flexibility at Reynolds number 10000. After validating with available experimental data, the code is used to assess analytical predictions from a linear theory. We consider large stiffness ratios, with high thrust enhancement by flexibility, and small mass ratios appropriate for underwater propulsion. The maximum thrust enhancement is observed at the first natural frequency, accurately predicted by the linear theory algebraically. The magnitude of the maximum thrust is over-predicted by the theory as the flapping amplitude increases. For large Strouhal numbers the flow becomes aperiodic, which for large enough amplitudes happens at frequencies below the natural frequency. But even at these Strouhal numbers, the linear theory predicts quite well the frequency of maximum thrust enhancement and optimal propulsive efficiency. We conclude that the linear theory constitutes a reliable and useful guide for the design of underwater flexible flapping-foil thrusters, and we provide a practical chart to easily select the optimal flapping frequency as a function of the actuation point, the stiffness and the mass ratios of the hydrofoil.

A high-fidelity numerical study on the propulsive performance of pitching flexible plates

In this paper, we numerically investigate the propulsive performance of three-dimensional pitching flexible plates with varying flexibility and trailing edge shapes. We employ our recently developed body-conforming fluid-structure interaction solver for our high-fidelity numerical study. To eliminate the effect of other geometric parameters, only the trailing edge angle is varied from 45° (concave plate), 90° (rectangular plate) to 135° (convex plate) while maintaining the constant area of the flexible plate. For a wide range of flexibility, three distinctive flapping motion regimes are classified based on the variation of the flapping dynamics: (i) low bending stiffness KBlow, (ii) moderate bending stiffness KBmoderate near resonance, and (iii) high bending stiffness KBhigh. We examine the impact of the frequency ratio f* defined as the ratio of the natural frequency of the flexible plate to the actuated pitching frequency. Through our numerical simulations, we find that the global maximum mean thrust occurs near f*≈1 corresponding to the resonance condition. However, the optimal propulsive efficiency is achieved around f* = 1.54 instead of the resonance condition. While the convex plate with low and high bending stiffness values shows the best performance, the rectangular plate with moderate KBmoderate is the most efficient propulsion configuration. To examine the flow features and the correlated structural motions, we employ the sparsity-promoting dynamic mode decomposition. We find that the passive deformation induced by the flexibility effect can help in redistributing the pressure gradient, thus, improving the efficiency and the thrust production. A momentum-based thrust evaluation approach is adopted to link the temporal and spatial evolution of the vortical structures with the time-dependent thrust. When the vortices detach from the trailing edge, the instantaneous thrust shows the largest values due to the strong momentum change and convection process. Moderate flexibility and convex shape help to transfer momentum to the fluid, thereby improving the thrust generation and promoting the transition from drag to thrust. The increase in the trailing edge angle can broaden the range of flexibility that produces positive mean thrust. The role of added mass effect on the thrust generation is quantified for different pitching plates and the bending stiffness. These findings are of great significance to the optimal design of propulsion systems with flexible wings.

Optimal Efficiency and Heaving Velocity in Flapping Foil Propulsion

A novel model is proposed to estimate the optimal propulsive efficiency and the optimal dimensionless heaving velocity of a flapping foil based on a lift–drag ratio under the assumption of negligible pitching power. The optimal efficiency increases with the lift–drag ratio. The optimal Strouhal number of sinusoidal heaving motion approaches 0.318, within the range of 0.25–0.35 for optimal efficiency. In addition, the thrust generation of a foil requires that the product of the lift–drag ratio and the dimensionless heaving velocity must be greater than unity. To demonstrate how to use the new model to estimate the optimal propulsive efficiency and the optimal dimensionless heaving velocity, the lift–drag ratio is estimated using static tests of an impulsively started uniform flow over an infinitely thin flat plate for a range of angle of attack. To illustrate how to design kinematics to achieve the estimated optimal efficiency, two examples of flapping motions of a flat plate at a Reynolds number of 2000 are considered: sinusoidal and nonsinusoidal. Results show that the efficiency of nonsinusoidal flapping motion approaches the optimal efficiency using the lift–drag ratio estimated from static tests over the effective angle of attack of 6–30°.

Fluid Flow Analysis of Stern Hull MV. Kelola Mina Makmur 150 GT Based Engine Propeller and Hull Matching Using Actuator Disk Propeller Method

The results of the simulation used a propeller disk actuator method that models the propeller effect without modeling a real propeller. Direct optimization will be calculated using the CFD method of each variation of the clearance propeller configuration indicating that the amount of propulsion efficiency produced is very dependent on the clearance propeller. The most optimal propulsion efficiency occurs at a distance of 0.738 m from the steering shaft with a Dprop = 0.88, KT = 0.22, KQ = 0.028, and J = 0.40 besides having an advance velocity visualized with a Va = 10.6 knots which has a 50% propulsion efficiency, which the value will decrease according to the reduction in distance propeller. The less efficiency of the propulsion produced in the clearance propeller between -0.162 - 0.438 m from the steering shaft. This is because the slope angle between the entry of water and the longitudinal axis of the ship's hull on the stern exceeds the required conditions.

Swimming microorganisms acquire optimal efficiency with multiple cilia

Planktonic microorganisms are ubiquitous in water, and their population dynamics are essential for forecasting the behavior of global aquatic ecosystems. Their population dynamics are strongly affected by these organisms' motility, which is generated by their hair-like organelles, called cilia or flagella. However, because of the complexity of ciliary dynamics, the precise role of ciliary flow in microbial life remains unclear. Here, we have used ciliary hydrodynamics to show that ciliates acquire the optimal propulsion efficiency. We found that ciliary flow highly resists an organism's propulsion and that the swimming velocity rapidly decreases with body size, proportional to the power of minus two. Accordingly, the propulsion efficiency decreases as the cube of body length. By increasing the number of cilia, however, efficiency can be significantly improved, up to 100-fold. We found that there exists an optimal number density of cilia, which provides the maximum propulsion efficiency for all ciliates. The propulsion efficiency in this case decreases inversely proportionally to body length. Our estimated optimal density of cilia corresponds to those of actual microorganisms, including species of ciliates and microalgae, which suggests that now-existing motile ciliates and microalgae have survived by acquiring the optimal propulsion efficiency. These conclusions are helpful for better understanding the ecology of microorganisms, such as the energetic costs and benefits of multicellularity in Volvocaceae, as well as for the optimal design of artificial microswimmers.

Investigation on flapping dynamics and wake characteristics of a flexible plate in nonlinear hysteresis region

In this paper, the flapping dynamics and wake flow characteristics in the nonlinear hysteresis region are investigated experimentally by immersing a cantilevered flexible plate in uniform airflow. The experimental results show that the flapping mode of a cantilevered flexible plate in hysteresis region will be transited from periodical traveling wave mode to limited cantilever-like mode with the variation of Reynolds number. The flapping mode will greatly influence the kinetic parameters of a flexible plate and also wake flow characteristics. The comparison of Strouhal number values between flapping flexible plate and animals further indicates that the fluid dynamics between passive flapping and active swimming is similar to the obtained optimal propulsive efficiency. Flow visualization reveals that the Karman vortex street appears, vanishes and coherent structures of turbulent flow arise behind the stable flexible plate with increasing Reynolds number. Meanwhile, the measurements of pressure distribution in wake flow provide a good physical understanding of the energy-saving mechanism and the optimal arrangement in fish school. Moreover, the fast Fourier transform (FFT) spectra of fluctuating velocity indicate that the characteristics of wake flow are closely related to the flapping mode of flexible plate. The wake flow will come into strong harmonic excitation state when flapping mode transits into a limited cantilever-like mode.

Numerical Investigation of Frequency and Amplitude Influence on a Plunging NACA0012

Natural flight has always been the source of imagination for Mankind, but reproducing the propulsive systems used by animals that can improve the versatility and response at low Reynolds number is indeed quite complex. The main objective of the present work is the computational study of the influence of the Reynolds number, frequency, and amplitude of the oscillatory movement of a NACA0012 airfoil in the aerodynamic performance. The thrust and power coefficients are obtained which together are used to calculate the propulsive efficiency. The simulations were performed using ANSYS Fluent with a RANS approach for Reynolds numbers between 8500 and 34,000, reduced frequencies between 1 and 5, and Strouhal numbers from 0.1 to 0.4. The aerodynamic parameters were thoroughly explored as well as their interaction, concluding that when the Reynolds number is increased, the optimal propulsive efficiency occurs for higher nondimensional amplitudes and lower reduced frequencies, agreeing in some ways with the phenomena observed in the animal kingdom.

Propulsion efficiency in wheelchair tennis: a case study on the influence of the racket on the handrim forces

Abstract Aims: to evaluate how the act of holding a tennis racket influences the application of forces in the handrim during manual wheelchair propulsion at a self-selected comfortable speed and sprint. Methods: A case study was conducted with an experienced wheelchair tennis player who propelled the wheelchair in a straightforward trajectory at two different velocities (self-selected comfortable speed and sprint) in two different conditions (freehand and holding the racket). Kinetic and temporal data of the pushes were obtained with the SmartWheel system attached in substitution to the conventional rear wheel at the dominant side of the player. Results: holding the racket affects the propulsion pattern mainly when an accelerated movement is required (sprint). Compared to the propulsion at a self-selected speed, propelling the chair as fast as possible with the racket in hands resulted in lower total and tangential forces on the handrim, and decreased push time and increased push frequency. Conclusion: Such influence on both kinetic and temporal propulsion impact the mechanical efficiency of the manual wheelchair propulsion, which may, ultimately, affect the sport´s performance. Special attention should be directed to the propulsion training with the racket in maneuvers and motions that are characteristic of the wheelchair tennis match in an attempt to provide the athlete with proper technique for optimal propulsion efficiency and sports performance.

On the propulsive performance of a pitching foil with chord-wise flexibility at the high Strouhal number

During the predatory behaviour, the aquatic animals rapidly oscillate their fins to attack the preys or escape from the predators. In the light of this situation above, we experimentally investigate a chord-wise flexible foil pitching at the high Strouhal number, which is significantly larger than the previous studies almost performed at St<1. In the present work, the chord-wise flexibility of the foil has been considered as two components: the flexible length represented by the cf and the hardness of the flexible materials denoted by the Shore A hardness number. The experimental data indicates that the softer foil shows an overwhelming ability on generating efficient propulsion than the harder counterparts at smaller cf. Along with the increase of cf, the flexibility of foil shows the positive/negative influences on the foil’s propulsive performance, which is closely related to the temporal evolution of the foil’s shape and the effects of flexible materials on the wake structures produced by the foil. Synthetically, taking the cf and Shore A hardness number of into account, we reveal that the propulsive force of a pitching foil is relied heavily on the St, and the foil can obtain optimal propulsive efficiency when the St and the inertia of flexible section are synchronously smaller. This work is helpful for well-understanding the predatory behaviour and designing the bio-inspired underwater robotics.

Evaluation of Equine Endometrium during Maternal Recognition of Pregnancy Utilizing RNA Sequencing

During the predatory behaviour, the aquatic animals rapidly oscillate their fins to attack the preys or escape from the predators. In the light of this situation above, we experimentally investigate a chord-wise flexible foil pitching at the high Strouhal number, which is significantly larger than the previous studies almost performed at St<1. In the present work, the chord-wise flexibility of the foil has been considered as two components: the flexible length represented by the cf and the hardness of the flexible materials denoted by the Shore A hardness number. The experimental data indicates that the softer foil shows an overwhelming ability on generating efficient propulsion than the harder counterparts at smaller cf. Along with the increase of cf, the flexibility of foil shows the positive/negative influences on the foil’s propulsive performance, which is closely related to the temporal evolution of the foil’s shape and the effects of flexible materials on the wake structures produced by the foil. Synthetically, taking the cf and Shore A hardness number of into account, we reveal that the propulsive force of a pitching foil is relied heavily on the St, and the foil can obtain optimal propulsive efficiency when the St and the inertia of flexible section are synchronously smaller. This work is helpful for well-understanding the predatory behaviour and designing the bio-inspired underwater robotics.

An autonomous flexible propulsor in a quiescent flow

Flapping motions of wings and fins are common in nature. Living organisms use such motions to float in a fluid or to propel themselves forward. Some entities, such as tadpoles, use distinct flexible components to generate propulsion. Here we introduce a propulsor consisting of a rigid circular head containing an energy source and a flexible fin for propulsion. The head imparts a sinusoidal torque to the leading edge of the fin and the flexible fin flaps along the leading edge. The flexible propulsor thus moves via an oscillating relative angle between the head and the leading edge of the fin. Unlike a self-propelled heaving and pitching fin, our ‘autonomous’ flexible propulsor has no prescribed motion or constraint referenced from outside coordinates. The immersed boundary method was used to model the interaction between the flexible propulsor and the surrounding fluid. A penalty method, in which the head and fin imparted a periodic torque to each other, was used to connect the head and the fin. The cruising speed and propulsive efficiency of the propulsor were explored as a function of the ratio of the head size to the fin length (D/L), the pitching amplitude (θp) and the pitching frequency (f). The cruising speed and the equilibrium position (geq) of the flexible propulsor near the ground were also examined. The optimal propulsive efficiency was achieved at the head ratio of D/L = 0.2 at θp = 30° and f = 0.2. The cruising speed of the flexible propulsor increased when operating near the ground. The gap distance between the propulsor and the ground was dynamically determined by the pitching motion.

Numerical study of the effect of motion parameters on propulsive efficiency for an oscillating airfoil

A numerical study on an airfoil undergoing combined heaving and pitching motion in cruise condition to assess the effect of a range of kinematic parameters on the propulsive efficiency is presented. The study assesses the effect of parameters like oscillation frequency, pitch amplitude, and non-dimensional heaving amplitude on the propulsive efficiency of the airfoil and uses an in-house computational fluid dynamics code. The propulsive efficiencies of the foil in the frequency range of 1–30Hz, pitch amplitude range of 3–19º, and non-dimensional heave amplitude range of 0.1–0.9 were computed and compared in order to explore the relationship between kinematic parameters and propulsive efficiency. Results indicate that at smaller pitch amplitudes low efficiencies occur at lower frequencies and high efficiencies occur when frequencies and heave amplitudes are on higher side. The low propulsive efficiency regions disappear with increasing pitch amplitude and larger regions at higher frequencies and heave amplitudes show high efficiencies. When both heave amplitude and frequency are small, propulsive efficiency is less than 0.5; when heave amplitude range is from 0.2 to 0.7, high efficiency occurs at high frequencies, however, with increasing heave amplitudes, these high efficiencies occur at lower frequencies. When the range of frequency is from 8Hz to 30Hz, the size of high efficiency area first increases, then decreases. And we explore the reasons why the oscillation frequency, pitch amplitude, and non-dimensional heaving amplitude can affect the propulsive efficiency. Combinations of kinematic parameters for optimal propulsive efficiency have been identified.

Kinematic optimization of 2D plunging airfoil motion using the response surface methodology

The propulsive efficiency of a plunging NACA0012 airfoil is maximized by means of a simple numerical optimization method based on the response surface methodology (RSM). The control parameters are the amplitude and the reduced frequency of the harmonic sinusoidal motion. The 2D unsteady laminar flow around the plunging airfoil is computed by solving the Navier-Stokes equations for three Reynolds number values (Re = 3.3×103, 1.1×104, and 2.2×104). The Nelder-Mead algorithm is used to find the best control parameters leading to the optimal propulsive efficiency over the constructed response surfaces. It is found that, for a given efficiency level and regardless of the considered Re value, it is possible either to obtain high thrust by selecting a high oscillation frequency or to reduce the input power by adopting a low plunging amplitude.

Effects of flexibility on the aerodynamic performance of flapping wings

AbstractEffects of chordwise, spanwise, and isotropic flexibility on the force generation and propulsive efficiency of flapping wings are elucidated. For a moving body immersed in viscous fluid, different types of forces, as a function of the Reynolds number, reduced frequency (k), and Strouhal number (St), acting on the moving body are identified based on a scaling argument. In particular, at the Reynolds number regime of$O(1{0}^{3} \ensuremath{-} 1{0}^{4} )$and the reduced frequency of$O(1)$, the added mass force, related to the acceleration of the wing, is important. Based on the order of magnitude and energy balance arguments, a relationship between the propulsive force and the maximum relative wing-tip deformation parameter ($\gamma $) is established. The parameter depends on the density ratio,St,k, natural and flapping frequency ratio, and flapping amplitude. The lift generation, and the propulsive efficiency can be deduced by the same scaling procedures. It seems that the maximum propulsive force is obtained when flapping near the resonance, whereas the optimal propulsive efficiency is reached when flapping at about half of the natural frequency; both are supported by the reported studies. The established scaling relationships can offer direct guidance for micro air vehicle design and performance analysis.

Propulsive performance of a fish-like travelling wavy wall

A numerical simulation is performed to investigate the viscous flow over a smooth wavy wall undergoing transverse motion in the form of a streamwise travelling wave, which is similar to the backbone undulation of swimming fish. The objective of this study is to elucidate hydrodynamic features of the flow structure over the travelling wavy wall and to get physical insights to the understanding of fish-like swimming mechanisms in terms of drag reduction and optimal propulsive efficiency. The effect of phase speed, amplitude and Reynolds number on the flow structure over the wavy wall, the drag force acting on the wall, and the power consumption required for the propulsive motion of the wall is investigated. The phase speed and the amplitude, which are two important parameters in this problem, predicted based on the optimal propulsive efficiency agree well with the available data obtained for the wave-like swimming motion of live fish in nature.

Turbulent flow over a flexible wall undergoing a streamwise travelling wave motion

Direct numerical simulation is used to study the turbulent flow over a smooth wavy wall undergoing transverse motion in the form of a streamwise travelling wave. The Reynolds number based on the mean velocity U of the external flow and wall motion wavelength λ is 10 170; the wave steepness is 2πa/λ =0 .25 where a is the travelling wave amplitude. A key parameter for this problem is the ratio of the wall motion phase speed c to U ,a nd results are obtained for c/U in the range of −1. 0t o 2. 0a t 0. 2i ntervals. For negative c/U ,w e fi nd that f low separation is enhanced and a large drag force is produced. For positive c/U ,t he results show that as c/U increases from zero, the separation bubble moves further upstream and away from the wall, and is reduced in strength. Above a threshold value of c/U ≈ 1, separation is eliminated; and, relative to small- c/U cases, turbulence intensity and turbulent shear stress are reduced significantly. The drag force decreases monotonically as c/U increases while the power required for the transverse motion generally increases for large c/U ;t he net power input is found to reach a minimum at c/U ≈ 1. 2( for fi xedU ). The results obtained in this study provide physical insight into the study of fish-like swimming mechanisms in terms of drag reduction and optimal propulsive efficiency.