Lift Force Generation Research Articles

Numerical study of dynamic stall effects on VR‐12 airfoil with pitch oscillation and accelerated inflow

AbstractThis study investigates the effects of positive horizontal acceleration of the freestream velocity on a pitch‐oscillating VR‐12 airfoil using computational fluid dynamics. The shear stress transport k–ω model, coupled with a low‐Reynolds number correction, was employed for Re <105 during dynamic stall. The flow equations were solved in two‐dimensional, incompressible form using the finite volume method. The study examined various parameters, including positive acceleration values of the inflow and the angle of attack of the airfoil, to determine their impact on lift and drag coefficients, as well as the ratio. Additionally, the maximum lift coefficient was analyzed under different inflow and airfoil motion conditions. The results indicate that aerodynamic force coefficients and the ratio are influenced by both the attack angle and the acceleration of the inflow. Furthermore, inflow acceleration affects the onset of dynamic stall conditions. Generally, inflow acceleration modifies the lift coefficient of the airfoil during the upstroke, while having minimal effect on the drag coefficient, except near dynamic stall points. The findings also suggest that, for a specific airfoil, the sequence of factors with the greatest influence on lift force generation before static stall occurs is as follows: asymmetric airfoil oscillation, symmetrical airfoil oscillation, accelerated inflow, constant velocity inflow, and static airfoil.

Investigation of front hydrofoil position influence on the hydrofoil-assisted craft

The hydrofoil is utilized to generate lift force and craft center of gravity (CoG) rise-up, resulting in reduced total resistance and improved performance of hydrofoil-assisted craft. This study focuses on the performance of a hydrofoil-assisted craft equipped with a double front foil and a single rear foil. The position of the front hydrofoil is adjusted by shifting it back and ahead by one chord length in the longitudinal direction to investigate the craft's performance. Various parameters including hydrodynamic resistance, trim angle, CoG rise-up, and hydrofoil pressure coefficient are examined at different volume Froude numbers (Fn∇). The results indicate that the total resistance coefficient exhibits similar behavior across all three states within the Fn∇ range of 0.73–2.2. However, when the Fn∇ is increased to 2.93 and the front hydrofoil is shifted one chord length ahead, a significant increase of 27% in CoG rise-up and 18% in trim angle is observed compared to the base case study. Likewise, a back shift of the front hydrofoil leads to a 54% increase in CoG rise-up and a 30% increase in trim angle.

Drafting behaviors in fish induced by a local pressure drop around a hydrofoil model

Fish schooling has the improvement in hydrodynamic propulsive efficiency through the interaction of flow field induced by fish bodies and tail beat. Such energy-saving behaviors due to flow interactions also occur with changes in the flow field caused by structures. We examined the differences between a live fish swimming around a streamlined hydrofoil model prepared to represent fish body and swimming alone in a flow tank. We observed that the fish can remain in the same place without tail beating. It called “drafting” behavior. The analysis of fish drafting showed that fish obtained thrust using a local pressure drop caused by the high velocity flow even in the vicinity of the hydrofoil model at an angle of attack α of 10° to 20°without flow separation, and fish balanced forces by using an α of fish body. This tendency was confirmed in the model experiment using a two-axis load cell, and the forces acting on the fish body was the smallest value when the fish model was placed in the same conditions as a live fish experiment. We also confirmed by simulation and found that the α of fish body generated lift force and counteract the suction force. Above results indicate that a fish can balance the anterior–posterior and lateral direction forces by using a local pressure drop around a hydrofoil model as suction force, and using angle of attack on its body, thereby realizing drafting.

Self-consistent numerical model of mosquito dynamics with specified kinematic parameters of wing movement

Mosquito flight dynamics was considered using an advanced mathematical model with experimentally determined and biologically relevant parameters. The model was developed using a self-consistent algorithm. It described mosquito’s body and wing oscillations using mechanics equations and aerodynamic flows, simultaneously. Six equations were used for the mechanics of the mosquito (relative to the center of mass), and Navier–Stokes equations simulated aerodynamics. Numerical methods employed deformable computational mesh with specific mesh construction near the wing boundaries. Using size, shape and weight characteristics of the Culex quinquefasciatus (male) mosquito the flight dynamics was computed and analyzed. It was determined that the mosquito lift force was proportional to the square of the wing frequency. Specifically, we found a critical frequency of approximately 820 Hz that corresponded to mosquito freezing when the lift force compensated gravity. This number was consistent to experimentally determined mosquito flight characteristics. Mechanism of mosquito’s lift force generation was discussed.

Active Generation of Lift Force in the “Wing–Ejector” Integral Aerodynamic Configuration

Active Generation of Lift Force in the “Wing–Ejector” Integral Aerodynamic Configuration

Validation of a control-oriented point vortex model for a cyclorotor-based wave energy device

Recently conducted analytical assessment of the potential performance of cyclorotor wave energy converters (WECs) have shown that such devices offer the best wave absorption behaviour, if energy capture can be optimised through suitable control. Such claims require additional investigation. This article is dedicated to validation of the control-oriented point vortex model of cyclorotor WECs against numerical and experimental assessments conducted by various research groups. The validation is conducted in terms of the traditional metrics for cyclorotor WECs: (a) cancellation of incoming waves; (b) generation of lift and drag forces (c) mechanical power generation.It is shown that the point vortex model generally confirms the previously conducted analytical assessment of device performance. However, accounting for the influence of the hydrofoil induced wakes decreases performance estimates to some extent. It is also shown that, overall, wave cancellation metrics are more optimistic than actual shaft power generation.Analysis of the lift and drag coefficients, which were derived from experimental data, reveal a range of hydrodynamic and mechanic effects which could influence actual device performance. It has been shown that, due to the complexity of hydrodynamic effects, lift and drag coefficients for the control-oriented model should be considered not only as functions of the Reynolds number and angle of attack, but also related to submergence of the foils and direction of their rotation with respect to the free surface. This method allows us to achieve the best validation against experimental results in terms of generation of tangential and radial forces.

Some Fundamental Results for Cyclorotor Wave Energy Converters for Optimum Power Capture

Cyclorotor-based wave energy converters (WECs) present a relatively new and innovative paradigm for wave energy harvesting. Their operational principle is based on the generation of lift forces on the rotating hydrofoils due to their interaction with wave-induced circulation of water particles. As a result, relatively little is known about their optimal operation. To date, cyclorotor device performance has been measured by the power of waves radiated by the WEC, while a constant rotational velocity, consistent with the wave frequency, is employed. In this note we show (a) that variation of the cyclorotor velocity within the incoming monochromatic wave period significantly increases the generated mechanical power, while (b) optimising wave cancellation is at odds with the maximisation of shaft power. To optimise shaft power, the letter adopts a multi-harmonic solution for variable cyclorotor rotation rate, inspired by methods developed in other WEC domains.

The forewing of a black cicada Cryptotympana atrata (Hemiptera, Homoptera: Cicadidae): Microscopic structures and mechanical properties.

Insects in nature flap their wings to generate lift force and driving torque to adjust their attitude and control stability. An insect wing is a biomaterial composed of flexible membranes and tough veins. In this paper, we study the microscopic structures and mechanical properties of the forewing of the black cicada, Cryptotympana atrata. The thickness of the wing membranes and the diameter of veins varied from the wing root to the tip. The thickness of the wing membranes ranged from 6.0 to 29.9μm, and the diameter of the wing veins decreased in a gradient from the wing root to the tip, demonstrating that the forewing of the black cicada is a nonuniform biomaterial. The elastic modulus of the membrane near the wing root ranged from 4.45 to 5.03 GPa, which is comparable to that of some industrial membranes. The microstructure of the wing vein exhibited a hollow tubular structure with flocculent structure inside. The "fresh" sample stored more water than the "dry" sample, resulting in a significant difference in the elastic modulus between the fresh and dried veins. The different membrane thicknesses and elastic moduli of the wing veins near the root and tip resulted in varied degrees of deformation on both sides of the flexion line of the forewing during twisting. The measurements of the forewing of the cicada may serve as a guide for selecting airfoil materials for the bionic flapping-wing aircraft and promote the design and manufacture of more durable bionic wings in the future. RESEARCH HIGHLIGHTS: The distribution of the wing vein diameter and the wing membrane thickness indicated that the forewing of Cryptotympana atrata is composed of heterogeneous materials. The wing membrane and the outer wall of the wing vein are the layered structure with multilayer fibers, which has a great significance for improving the ability of the forewing to sustain aerodynamic loads. The elastic modulus of the wing membrane near the wing root is in the range of 4.45-5.03GPa, which is comparable to that of membranes manufactured by industries. This is a suitable reference for selecting materials for making bionic aircraft wings. We proved that the elastic moduli of the "fresh" and "dry" wing veins differ greatly compared with those of the wing membrane. Because the wing vein microstructure exhibits an internal hollow tubular structure with flocculent structure inside, the "fresh" sample stores more water than the "dry" sample. The wing membrane near the wing root is thicker and reinforced by the main wing vein with a high elastic modulus. This renders the region near the wing root difficult to deform. The membrane far from the wing root is thinner and the elastic modulus of the nearby wing veins is smaller, making them more flexible.

The effect of trip wire on transition of boundary layer on a cylinder

The effect of height of a trip and its location on the transition of boundary layer on a cylinder is studied using large eddy simulations for 2×103≤Re≤5×105. The Reynolds number, Re, is based on the free stream speed and diameter of the cylinder (D). Two modes of transition are observed: (a) natural, for a relatively small trip of height dT/D=0.25%, via formation of a laminar separation bubble (LSB) and (b) direct, for a large trip of height dT/D=1.0%, wherein the formation of LSB is bypassed and the trip disturbs the flow enough to cause separation of the boundary layer and its subsequent turbulent reattachment. Transition delays the final separation leading to a very significant reduction in drag, often referred to as drag crisis. The delay is more for natural as compared to direct transition. Consequently, the drag at the end of crisis is lower for natural transition. The 1.0% trip at 78° leads to a more delayed flow separation than one at 55° from the front stagnation point. The drag crisis takes place in two stages for a cylinder with trip. During each of the two stages, asymmetric transition on the two sides results in generation of circulation and lift force. The effect of trip is felt even by the non-trip side. The cylinder experiences a relatively large “reverse lift” during the second stage of drag crisis. While natural transition is accompanied by intermittency of LSB, direct transition is associated with intermittency in laminar vs turbulent attachment of the flow following its separation at the trip.

An Assessment of the Application of Bernoulli’s Theorem in the Generation of Lift Force

In this paper, we will be performing a detailed analysis of the application of Bernoulli’s Theorem in aviation and aerodynamics. The aim of our experiment and consequently this paper is to verify the application of Bernoulli’s Theorem in the aviation industry. In the field of aerodynamics, Bernoulli’s Theorem has been specifically used in shaping the wings of an aircraft.
 Over the years, however there has been a significant controversy in the aviation industry regarding the generation of lift force, especially the applicability of Newton’s Third Law of Motion along with Bernoulli’s Theorem. The controversy seems to be due to a combined effect of Newton’s and Bernoulli’s theorems’ (e.g. ‘Equal Transit Time Theory’), which may be incorrectly applied in the real world. Further, it seems that people are over-simplifying the problem of aerodynamic lift leading to the dismissal of either one of the theorems, when in reality both the theorems seem to be at play, as explained in this paper. For the generation of lift in air, momentum, mass and energy need to be conserved. Newton’s laws take into account the conservation of momentum, whereas Bernoulli’s Theorem considers the conservation of energy. Hence, they are both relevant for the generation of lift in air. However, no one has been able to determine accurately the working of both these theorems in the process of providing lift to an aircraft. Through this research paper, we have been able to prove the effect of Bernoulli’s Theorem in generating lift in air.

Effect of Wing Flexibility on the Lift Force Generated by a 2D Model Insect Wing Flapping in Hover Mode

In this study, we numerically examine the effect of wing flexibility on the lift force generated by a modeled flapping wing that simulates an insect flight in hover mode. The flow field around the flapping wing is assumed to be two-dimensional (2D), and the flow equations are formulated with the notion of “immersed boundary” for easy handling of the moving-boundary problem. In the 2D framework, the wing chord is treated as an elastic filament. To account for the large deformation in the wing, the motion of the filament is modeled using a geometrically nonlinear equation for bending beams. Both sets of equations are then solved according to the fluid–structure interaction concept using appropriate numerical schemes. Time variation of lift force on the wing is calculated for different values of wing flexural rigidity. Effects of elastic deformation of the wing and interactions between the bending wing and vortical flow structure are discussed. Flexural rigidity that gives the highest average lift force on the wing is sought. This study affirms that the flexibility of wing and the vortex-pair capture phenomenon are crucial to the lift force generation in the flapping-wing mechanics.

Hydrodynamics of rowing propulsion

Abstract

Aerodynamics of a half-rotating wing in hovering flight: An integrated study

This study introduces a new quasi-flapping wing driving mechanism based on a half-rotating mechanism which is capable of pure rotational flapping rather than the more traditional oscillatory flapping method. Lift models for half-rotating wing (HRW) aircraft in hovering flight are proposed based on the kinematics of a HRW prototype and the flow characteristics near the surface of its wing. Alongside further analytical expressions for lift based on kinematic extractions, computational models and a novel lift validation mechanism are used to reinforce the aerodynamic characteristics of the HRW in hovering flight. The aerodynamics of the HRW are experimentally assessed for different wing layouts and wing materials. Results indicate that the flow field generated by the motion of the wing arranged symmetrically on both sides of the body interfere with each other, causing the average lift coefficient of the paired-wing HRW to be less than that of the single-wing HRW. The average lift coefficient of the flexible wing is larger than that of the rigid wing. In addition, the average lift of the flexible wing increases with increasing flexural compliance within a particular range. Lift forces in different flight conditions are calculated using derived formulas alongside representative computational models, through which the derivation of lift variation for the HRW in hovering flight is validated. The theoretical lift curves show reasonable agreement with numerical simulation results in terms of the time course over one stroke cycle. The mechanisms of the HRW for generation and shedding of vortices in hovering flight are further revealed in computed flow field characteristics results. The velocity vectors of the flow field between the HRW and the symmetrically rotating wing indicate that the HRW with asymmetric rotation can generate lift force effectively. The velocity difference between the wing and the fluid is the key factor influencing the structure of generated vortices. In detailed three-dimensional (3D) vortex flows, our computational fluid dynamics study shows that a horseshoe-shaped vortex is first generated in the early downstroke. The horseshoe-shaped vortex subsequently grows into a doughnut-shaped vortex ring, with a jet stream appearing in its core which forms the downwash. The doughnut-shaped vortex ring eventually elongates into a long arc-shaped wake vortex ring. A large increasing lift force is generated during the upstroke, most likely due to the stable distal attached vortices; and in accordance with this, downwash becomes evident in the vortex ring during the downstroke.

Effect of apex angle on near wake vortex dynamics of triangular cylinders at intermediate Reynolds number

A study of vortex dynamics is essential in understanding the behaviour of the forces acting in a flow. This is because super positioning of vortex flow on other elementary flows results in generation of finite lift forces. The flow past a triangular cylinder is turbulent. Vortex structures dominate this type of flow. A study of vortex dynamics of this type of flow will reveal the physics involved in the flow. The vortex dynamics of near wake region is responsible for the generation of von Kármán Vortex Street. As a result, the vortex flow structures in the near wake region of 60° and 75° triangular cylinders were studied numerically using LES. The study was performed at Re = 520 in the intermediate range.A system of interdependent vortices was discovered in the near wake region. This vortex system exists behind both cylinders. The primary and secondary vortex strengths have sinusoidal variation. This variation is similar in both cases. However, the time-period and amplitude are directly proportional to apex angle. A mathematical model of the variation was presented. This model is applicable to both cylinders. The study also indicated that the time-period and amplitude of the vortex strengths are mathematically related.

Equilibrium configuration of a rectangular obstacle immersed in a channel flow

Fluid flows around an obstacle generate vortices which, in turn, generate lift forces on the obstacle. Therefore, even in a perfectly symmetric framework equilibrium positions may be asymmetric. We show that this is not the case for a Poiseuille flow in an unbounded 2D channel, at least for small Reynolds number and flow rate. We consider both the cases of vertically moving obstacles and obstacles rotating around a fixed pin.

Numerical investigation of flow field over four cylinders in square arrangement for different spacing ratios

The objective of this paper is to study the effect of spacing ratio on the wake dynamics in an in-line square array of four cylinders. Two dimensional incompressible CFD simulations are done with commercial CFD software Ansys-Fluent. Diameter of cylinder is 0.01 m, and the four cylinders have identical diameter. The spacing ratios considered are 1.5, 2.0, 2.5 and 3.0, where spacing ratio is defined as ratio of distance between center of two cylinders and diameter of cylinder. Simulations are performed at a Reynolds number (Re) of 200. Water is considered as fluid medium, and velocity of water corresponding to Re of 200 is 0.0178 m/s. SIMPLE algorithm is used to solve incompressible Navier-Stokes equations, and second order accuracy is used for spatial discretization. At the chosen Re, far wake is expected to transition to turbulence, and hence κ-ω SST turbulence model is used for turbulence closure. The flow field and associated surface properties such as drag and lift force are strong function of spacing ratio. The impingement of shear layers of upstream cylinders onto to the downstream cylinders is strongly influenced by spacing ratio. The impingement and resulting flow interaction leads to asymmetry in the surface pressure distribution. This leads to generation of lift force on the cylinders. For spacing ratio of 1.5 and 2.0, the top row cylinders have positive lift coefficient, and bottom row cylinders have negative lift coefficient. For spacing ratio of 2.5 upstream cylinders have positive lift coefficient, and downstream cylinders have negative lift coefficient. All four cylinders have positive lift coefficient for spacing ratio of 3.0. Drag coefficient found to be positive for all cylinders and for all spacing ratios.

On the formation of three-dimensional separated flows over wings under tip effects

We perform DNS of flow over finite-aspect-ratio NACA 0015 wings to characterize the tip effects on the wake dynamics. This study focuses on the development of three-dimensional separated flow over the wing, and discuss flow structures formed on the wing surface as well as in the far field wake. Vorticity is introduced into the flow in a predominantly two-dimensional manner. The vortex sheet from the wing tip rolls up around the free end to form the tip vortex. At its inception, the tip vortex is weak and its effect is spatially confined. As the flow around the tip separates, the tip effects extend farther in the spanwise direction, generating noticeable three dimensionality in the wake. For low-aspect-ratio wings, the wake remains stable due to the strong downwash over the entire span. Increasing the aspect ratio allows unsteady vortical flow to emerge away from the tip at sufficiently high angles of attack. These unsteady vortices shed and form closed vortical loops. For higher-aspect-ratio wings, the tip effects retard the near-tip shedding process, which desynchronizes from the two-dimensional shedding over the mid-span region, yielding vortex dislocation. At high angles of attack, the tip vortex exhibits noticeable undulations due to the strong interaction with the unsteady shedding vortices. The spanwise distribution of force coefficients is related to the three-dimensional wake dynamics and the tip effects. Vortical elements in the wake that are responsible for the generation of lift and drag forces are identified through the force element analysis. We note that at high angles of attack, a stationary vortical structure forms at the leading edge near the tip, giving rise to locally high lift and drag forces. The analysis performed here reveals how the vortical flow around the tip influences the separation physics, the global wake dynamics, and the spanwise force distributions.

Investigation of High Lift Force Generation of Dragonfly Wing by a Novel Advanced Mode in Hover

In the paper, a novel flapping mode is presented that can generate high lift force by a dragonfly wing in hover. The new mode, named partial advanced mode (PAM), starts pitching earlier than symmetric rotation during the downstroke cycle of the hovering motion. As a result, high lift force can be generated due to rapid pitching coupling with high flapping velocity in the stroke plane. Aerodynamic performance of the new mode is investigated thoroughly using numerical simulation. The results obtained show that the period-averaged lift coefficient, CL, increases up to 16% compared with that of the traditional symmetrical mode when an earlier pitching time is set to 8% of the flapping period. The reason for the high lift force generation mechanism is explained in detail using not only force investigation, but also by analyzing vortices produced around the wing. The proposed PAM is believed to lengthen the dynamic stall mechanism and enhance the LEV generated during the downstroke. The improvement of lift force could be considered as a result of a combination of the dynamic stall mechanism and rapid pitch mechanism. Finally, the energy expenditure of the new mode is also analyzed.

Comparative assessment of hydrodynamic performance of two-dimensional Naca0012 and Naca6612 hydrofoils under different cavitation and non-cavitation conditions

Installation of hydrofoil on a vessel results in lift force generation, especially at high speeds, and reduces its resistance. In the current paper, hydrodynamic performance of two NACA hydrofoils is numerically assessed under cavitation and non-cavitation conditions. Simulations are conducted by Ansys-CFX software and RANS equations are solved using k-e turbulence model. The obtained results are compared against available experimental data and good compliance is displayed. Results of simulations indicate that hydrodynamic forces of lift and drag of the hydrofoils vary in different operating conditions. Based on numerical findings, one may conclude that lift and drag forces of two-dimensional Naca6612 hydrofoil are larger than those of Naca0012 hydrofoil. This is true in both cavitation and non-cavitation conditions. Overall, it is also concluded that under these conditions, Naca6612 hydrofoil performs much better than Naca0012 hydrofoil, and lift-to-drag ratio of Naca6612 is larger than that of Naca0012 at all angles of attack.

Comparative assessment of hydrodynamic performance of two-dimensional Naca0012 and Naca6612 hydrofoils under different cavitation and non-cavitation conditions

Installation of hydrofoil on a vessel results in lift force generation, especially at high speeds, and reduces its resistance. In the current paper, hydrodynamic performance of two NACA hydrofoils is numerically assessed under cavitation and non-cavitation conditions. Simulations are conducted by Ansys-CFX software and RANS equations are solved using k-ε turbulence model. The obtained results are compared against available experimental data and good compliance is displayed. Results of simulations indicate that hydrodynamic forces of lift and drag of the hydrofoils vary in different operating conditions. Based on numerical findings, one may conclude that lift and drag forces of two-dimensional Naca6612 hydrofoil are larger than those of Naca0012 hydrofoil. This is true in both cavitation and non-cavitation conditions. Overall, it is also concluded that under these conditions, Naca6612 hydrofoil performs much better than Naca0012 hydrofoil, and lift-to-drag ratio of Naca6612 is larger than that of Naca0012 at all angles of attack.