Downstroke Motion Research Articles

Numerical simulations of bio-inspired approaches to enhance underwater swimming efficiency

The present study discusses the numerical simulation results of swimming similar to manta rays. The complex three-dimensional kinematics of manta rays were implemented to unravel the intricacies of its propulsion mechanisms by using the discrete vortex method (DVM). The DVM replaces the requirement for a structured grid across the computational domain with a collection of vortex elements. This method simplifies grid generation, especially for intricate geometries, resulting in time and effort savings in meshing complex shapes. By modeling the pectoral fins with discrete panels and utilizing vortex rings to represent circulation and wake, the study accurately computes the pressure distribution, circulation distribution, lift coefficient, and thrust coefficient of the manta ray. This study focuses on the modulation of aerodynamic performance by altering the span length and the length change ratio during the downstroke and upstroke motion (SV). The manta ray's three-dimensional vortex configurations comprise a combination of vortex rings, vortex contrails, and horseshoe vortices. Analysis of the three-dimensional vortex structure indicates the presence of multiple vortex rings and horseshoe vortex rings at higher SV values, while adequate formation of horseshoe vortices is not observed at lower SV values. In terms of propulsive performance, both lift and thrust increase with SV, while the propulsive efficiency demonstrates its peak at SV = 1.75. The analysis reveals that at higher SV values, the net thrust generated primarily originates from the tip of the fins. Moreover, the study illustrates a significant enhancement in propulsive efficiency, particularly in association with optimal Strouhal numbers ranging between 0.3 and 0.4. The key findings of this study may be used in efficient design of agile autonomous underwater vehicles for marine exploration and surveillance applications.

Wavy Leading Edge (WLE) Influence on a Rectangular Wing Using an Unsteady Analysis Approach

A rectangular wing with Wavy Leading Edge (WLE) effect was investigated experimentally and numerically. This research was carried out with the NACA 0018 profile. The morphology of humpback whale flippers, which are blunt and rounded in a specific pattern, inspired the design of the WLE. The rectangular wing was explored in pitching motion with a reduced frequency of k = 0.25 and varied aspect ratios. Multiple aspect ratios (AR) of the rectangular wing have been evaluated to determine the best wing aspect ratio, notably 3.9, 5.1, and 7.9. Only at AR 3.9 and 5.1 does the WLE perform efficiently in both upstroke and downstroke motion. WLE has a sinusoidal function shape. The improvement of lift force was stronger during upstroke motion than during downstroke motion. The stall is minimized during the pitching motion of the WLE wing, according to the numerical simulation. This result could be applied to fin stabilizers or wind turbines.

Plasma actuator effects on the flow physics of dynamic stall for a vertical axis wind turbine

Darrieus type vertical axis wind turbines have several advantages over other wind turbines for local electricity generation in urban environments. However, the main aerodynamic challenge is the negative impacts of the dynamic stall phenomenon on the turbine performance. This study numerically scrutinizes the effects of plasma actuators on the dynamic stall control and performance improvement of a Darrieus turbine. For this sake, unsteady Reynolds-averaged Navier–Stokes equations are solved using a pressure-based finite volume method. The Suzen–Hoang plasma actuator model is employed to calculate the body forces attributed to the plasma actuator. First, the dynamic stall characteristics of the turbine blade with no plasma actuator are explored. Then, three plasma actuator configurations (i.e., inboard, outboard, and double-sided) are considered. According to the results of the clean blade, the aerodynamic performance mainly depends on the reverse Karman vortex formation at the onset of the down-stroke motion and counterclockwise vortex formation on the blade suction side. The results of plasma configurations indicate that the inboard and double-sided actuators enhance the turbine power output by 10%, while the outboard actuator effects are negligible. Moreover, the plasma actuator is effective only in the down-stroke motion of the blade. Results also show that the inboard and double-sided plasma actuators eliminate the reverse Karman vortex and significantly reduce the counterclockwise vortex size, increasing the lift force and connection point moment. Consequently, the inboard and double-sided plasma actuators remove the negative torque generation in azimuth angles of 135° to 180°, primarily responsible for the output power enhancement.

Effects of canard oscillations on the unsteady flowfield over the wing in supersonic flow

Few studies have been devoted to effects of canard oscillations on the flowfield over the wing in supersonic flow and the associated time lags. In this article, an in-depth experimental study on a closely coupled canard-wing-body configuration in supersonic flow has been undertaken to mark the zone of influence of the oscillatory canard on the wing. Surface pressure measurements have been performed on the wing downstream of canard in both static and oscillatory cases at various wing angles of attack and canard deflection angles as well as the canard oscillation amplitudes and frequencies. The results show that the upwash and downwash flowfields due to an oscillatory canard periodically change the effective angle of attack seen by the wing and have different effects on various regions. In supersonic flow, the pressure rise due to the oblique shock waves causes the primary flow separation and the vortex break down onsets to occur at higher angles of attack, comparing to those in the lower speed regimes. According to the results, the mid-span region on the wing, which is directly in the shadow of the oscillating canard, receives more disturbances. During upstroke, the canard induces a downwash field to the front half of the wing, while in downstroke motion, the front regions of the wing experience upwash flow, giving rise to the effective angle of attack seen by the wing. Among various oscillation parameters, the pitching amplitude was observed to have a strong impact on the time lag in flowfield over the wing.

Study of wake characteristics behind an oscillating NACA 0012 airfoil in deep dynamic stall regime

PurposeThis study aims to investigate the wake behind an oscillating airfoil at a various angle of incidence and Reynolds number in a deep dynamic stall condition.Design/methodology/approachNACA 0012 airfoil is allowed to undergo harmonic pitching motion about the quarter chord axis at Reynolds numbers of 0.5 * 105, 1.17 * 105, 1.7 * 105 and 2.12 * 105, and the reduced frequency of 0.1. The experiments are conducted at a set of mean and amplitude angle of attack that covered the angle of incidence from −5° to 25°. The wake rake is placed at a distance of one chord from the trailing edge of the airfoil.FindingsThe hysteresis of the flow during the upstroke and the downstroke motion are captured. The huge growth in the velocity defect and the wake thickness beyond the angle of attack of 15° explicate the appearance of the strong unsteady effects on the wake. The results also show that at the reduced frequency of 0.1, the wake structure is of drag producing type due to the momentum deficit.Originality/valueStreamwise velocity profile and the turbulent intensity profiles are presented to show the effects of Reynolds number and angle of incidence on the wake behind the oscillating airfoil at the reduced frequency of 0.1, and in the intermediate range of Reynolds number is the novelty of the study.

An experimental study on boundary layer transition detection over a pitching supercritical airfoil using hot-film sensors

In the present work, experimental tests are conducted to study boundary layer transition over a supercritical airfoil undergoing pitch oscillations using hot-film sensors. Tests have been undertaken at an incompressible flow. Three reduced frequencies of oscillations and two mean angles of attack are studied and the influences of those parameters on transition location are discussed. Different algorithms are examined on the hot-film signals to detect the transition point. Results show the formation of a laminar separation bubble near the leading edge and at relatively higher angles of attack which leads to the transition of the boundary layer. However, at lower angles of attack, the amplification of the peaks in voltage signal indicate the emergence of the vortical structures within the boundary layer, introducing a different transition mechanism. Moreover, an increase in reduced frequency leads to a delay in transition onset, postponing it to a higher angle of attack, which widens the hysteresis between the upstroke and downstroke motions. Rising the reduced frequency yields in weakening or omission of vortical disturbances ensuing the removal of spikes in the signals. Of the other important results observed, is faster movement of the relaminarization point in the higher mean angle of attack. Finally, a time–frequency analysis of the hot-film signals is performed to investigate evolution of spectral features of the transition due to the pitching motion. An asymmetry is clearly observed in frequency pattern of the signals far from the bubble zone towards the trailing edge; this may reflect the difference between the transition and relaminarization physics. Also, various ranges of frequency were obtained for different transition mechanisms.

Effect of Viscosity and Speed on Oil Cavitation Development in a Single Piston-Ring Lubricant Assembly

A high-speed camera has been used to produce unique time-resolved images of high quality to describe the dynamics of the lubricant flow and cavitation characteristics in a sliding optical liner over a fixed single piston-ring lubricant assembly for three lubricants with different viscosities to establish their impact on cavitation formation and development. The images were obtained at two cranking speeds (or liner sliding velocity) of 300 rpm (0–0.36 m/s) and 600 rpm (0–0.72 m/s), at a lubricant temperature of 70 °C and a supply lubricant rate of 0.05 L/min. A special MATLAB programme has been developed to analyse the cavitation characteristics quantitatively. The dynamic process of cavities initiation was demonstrated by time-resolved images from fern cavity formation to fissure cavities and then their development to the sheet and strings cavities at a liner sliding velocity of around 0.17 m/s. The results for both up- and down-stroke motions showed that the cavities reach their fully developed state downstream of the contact point when the liner velocity reaches its highest velocity and that they start to collapse around TDC and BDC when the liner comes to rest. Within the measured range, viscosity had a great influence on length of cavities so that a decrease in viscosity (from Lubricant A to C) caused a reduction in length of cavities of up to 35% for Lubricant C. On the other hand, an increase in speed, from 300 rpm to 600 rpm, have increased the number of string cavities and also increased the length of cavities due to thicker oil film thickness with the higher speed. Overall, the agreement between the processed data by MATLAB and visualisation measurements were good, but further thresholds refinement is required to improve the accuracy.

Flow mechanism for the effect of pivot point on the aerodynamic characteristics of a pitching airfoil and its manipulation

The effects of pivot point location on the aerodynamic characteristics of a pitching airfoil were numerically investigated. The aerodynamic loads and evolution of flow structures for the fixed-pivot pitching airfoils with different pivot locations were studied first. It was found that the leading-edge vortex grows earlier and the lift coefficient becomes larger with the forward movement of the pivot location during the upstroke motion, which shows an opposite variation during the downstroke motion. Such trend stems from the phase lag in flow evolution of different cases. Thus, a new definition of the effective angle of attack for pitching motions was proposed, and it was found that this parameter could well unify the variations of lift coefficient and vortical evolution caused by the pivot location. The data assimilation indicates that the effective angle of attack is the dominant factor of pivot effect for most time of a period, which reveals the flow mechanism of the pivot effect. This definition was further extrapolated so that it could also be applied to the pitching-plunging motions. Furthermore, a movable pivot pitching scheme by combining respective advantages of fixed-pivot pitching airfoils with different pivot locations was proposed, and it was indicated that the lift performance of the dynamic airfoil could be improved.

Large Eddy Simulations on a pitching airfoil: Analysis of the reduced frequency influence

Large Eddy Simulations (LES) have been performed on a pitching NACA0012 airfoil at Rec=2.104. The influence of the reduced frequency on the dynamic stall phenomenon is investigated. Special attention is given on the aerodynamic coefficients drop incidence delay in comparison with a static case. The analysis is based on vorticity field observations and the introduction of local lift and friction coefficients. The boundary layer events and the presence and the impact of a Leading-Edge Vortex (LEV) are investigated. The total stall delay is decomposed into two main phases: a delay in the boundary layer separation which occurs at higher incidence as the reduced frequency increases and the initiation and the growth of the LEV. For selected reduced frequencies, the influence of the LEV on the lift and drag coefficients is analysed as a function of the airfoil angle of incidence and then as a function of time. The LEV maintains a high level of lift and drag. Its lifetime on the airfoil suction side seems to be shortened by the beginning of the airfoil downstroke motion. Given the fact that the LEV appears at higher incidence as the reduced frequency grows, the LEV lifetime on the airfoil suction side decreases as the reduced frequency increases. However, the airfoil oscillation being faster, the LEV goes through larger incidence ranges during its lifetime and leads to a larger lift and drag drop incidence delay as the reduced frequency increases.

Active Flow Control of Dynamic Stall by Means of Continuous Jet Flow at Reynolds Number of 1 × 106

We have studied the influence of a tangential blowing jet in dynamic stall of a NACA0012 airfoil at Reynolds number of 1 × 106, for active flow control (AFC) purposes. The airfoil was oscillating between angles of attack (AOA) of 5 and 25 deg about its quarter-chord with a sinusoidal motion. We have utilized computational fluid dynamics to investigate the impact of jet location and jet velocity ratio on the aerodynamic coefficients. We have placed the jet location upstream of the counter-clockwise (CCW) vortex which was formed during the upstroke motion near the leading-edge; we have also considered several other locations nearby to perform sensitivity analysis. Our results showed that placing the jet slot within a very small range upstream of the CCW vortex had tremendous effects on both lift and drag, such that maximum drag was reduced by 80%. There was another unique observation: placing the jet at separation point led to an inverse behavior of drag hysteresis curve in upstroke and downstroke motions. Drag in downstroke motion was significantly lower than upstroke motion, whereas in uncontrolled case the converse was true. Lift was significantly enhanced during both upstroke and downstroke motions. By investigating the pressure coefficients, it was found that flow control had altered the distribution of pressure over the airfoil upper surface. It caused a reduction in pressure difference between upper and lower surfaces in the rear part, while substantially increased pressure difference in the front part of the airfoil.

Perpendicular blade–vortex-interaction over an oscillating airfoil in light dynamic stall

An experimental and numerical study was performed to investigate the effects of perpendicular blade vortex interactions on the aerodynamic performance of an oscillating airfoil. The selected test cases studied the aerodynamic interaction of a stream-wise vortex impacting on a NACA 23012 airfoil oscillating in light dynamic stall regime, representing a typical condition of the retreating blade of a helicopter in forward flight. The analysis of particle image velocimetry surveys and time-accurate simulation results enabled to point out the different effects due to the blade pitching motion on the interacting flow field. Thus, numerical results enabled to achieve a detailed insight about the aerodynamic loads acting on the oscillating airfoil in the interacting cases. In particular, the comparison with the clean airfoil case shows that a severe loss of performance is produced by the interaction of the vortex during the airfoil downstroke motion, as the vortex impact triggers the local stall of the blade section.

Numerical investigation of aerodynamic flow actuation produced by surface plasma actuator on 2D oscillating airfoil

Numerical simulation of unsteady flow control over an oscillating NACA0012 airfoil is investigated. Flow actuation of a turbulent flow over the airfoil is provided by low current DC surface glow discharge plasma actuator which is analytically modeled as an ion pressure force produced in the cathode sheath region. The modeled plasma actuator has an induced pressure force of about 2kPa under a typical experiment condition and is placed on the airfoil surface at 0% chord length and/or at 10% chord length. The plasma actuator at deep-stall angles (from 5° to 25°) is able to slightly delay a dynamic stall and to weaken a pressure fluctuation in down-stroke motion. As a result, the wake region is reduced. The actuation effect varies with different plasma pulse frequencies, actuator locations and reduced frequencies. A lift coefficient can increase up to 70% by a selective operation of the plasma actuator with various plasma frequencies and locations as the angle of attack changes. Active flow control which is a key advantageous feature of the plasma actuator reveals that a dynamic stall phenomenon can be controlled by the surface plasma actuator with less power consumption if a careful control scheme of the plasma actuator is employed with the optimized plasma pulse frequency and actuator location corresponding to a dynamic change in reduced frequency.

Optimization of lift force for a bio-inspired flapping wing model in hovering flight

The lift force is an important component that affects the aerodynamic performance of bio-inspired flapping-wing micro aerial vehicles. However, there is a lack of endeavors in the optimization of the flapping wing parameters that affect the lift force of micro aerial vehicles. This research is therefore to establish a methodology that combines computational fluid dynamic simulation for the evaluation of the lift generation and wing parameters. The Taguchi’s framework for design of experiments, combined with the computational fluid dynamics simulations, is performed on a simplified robotic fly wing model used in the experiment found in Dickinson et al. (1999), under the hovering flight mode, to identify the most influential parameters on the lift generation of micro aerial vehicles. A commercial computational fluid dynamics code, ANSYS/FLUENT, along with a three-dimensional Navier–Stokes solver is used to simulate the unsteady flow field. With the optimization of the time-averaged lift force as the optimization objective, five typical parameters (flapping frequency, maximum angle of attacks during the upstroke and downstroke motions, stroke amplitude, and rotation type) in the flapping trajectory equation are selected as the input factors with each having four levels. Specific computational fluid dynamics cases are simulated in accordance with the chosen orthogonal arrays. By conducting statistical analyses with analyses of means and variance, the flapping frequency and the stroke amplitude are determined to be the two most influential parameters. The response surface of the time-averaged lift force and the power consumption contour are constructed with respect to these two parameters to determine the optimal combination for the generation of lift forces under a specific power constrain and provide a guideline for bio-inspired micro aerial vehicle designs.

Numerical simulation of the sucker-rod pumping system

The sucker rod pump is an artificial lift method frequently applied in onshore petroleum wells. This system can be described using a numerical simulation based on the behavior of a rod string. In the past, the elastic behavior of the rod string made it difficult to model the system. However, since the 1960s and with the advent of digital computers, it has been modeled numerically. The rod string behaves like a slender bar, and thus, the propagation of elastic waves along the bar can be represented by a one-dimensional equation. Gibbs (1963) presented a mathematical model based on the wave equation, which is described on the basis of the analysis of forces on the rod string and is incorporated into a boundary value problem involving partial differential equations. The use of the finite difference method allows for a numerical solution by the discretization of the wave equation developed in the mathematical formulation with appropriate boundary and initial conditions. This work presents a methodology for implementing an academic computer code that allows simulation of the upstroke and downstroke motion of the rod string described by the wave equation under ideal operating conditions, assuming a harmonic motion of the rod at one end and downhole pump at the other end. The goal of this study is to generate the downhole dynamometer card, an important and consolidated tool that controls the pump system by diagnosing operational conditions of the downhole pump.

Stereo particle image velocimetry measurements of perpendicular blade–vortex interaction over an oscillating airfoil

The aerodynamic interaction of a stream-wise vortex impacting on a NACA 23012 oscillating airfoil was investigated using stereo particle image velocimetry. The experimental rig enabled the study of the aerodynamic effects due to the blade pitching motion in the interaction with the vortex. The experimental study focused on the light dynamic stall regime, which represents a typical condition of the retreating blade of a helicopter in forward flight. Particle image velocimetry was applied to a measurement volume close to the airfoil upper surface in order to obtain the three-dimensional interacting flow field. In particular, the experimental results show that during the airfoil downstroke motion, the vortex impact triggers the stall of the local blade section, indicating that detrimental effects on the blade performance can be introduced by perpendicular vortex interactions.

Effects of adding strip flap on a plunging airfoil

Purpose – The main aim of the present work is to examine the effects of trailing edge strip (TES) on the wake region of a plunging airfoil that oscillates prior and beyond the static stall angle of attack. Design/methodology/approach – In this study, experimental investigations were carried out to explore the wake characteristics of a plunging Eppler 361airfoil equipped with TES flap. The experiments involved measurements of flapped and unflapped airfoil wake velocity for the range of initial AOA (0 and 12°). Surface pressure measurements as a supplementary data were also carried out. Data were taken at reduced frequencies of 0.03 and 0.073 and different distances downstream from trailing edge. Findings – The results showed the hysteresis between the plunging wake in the upstroke and down-stroke motion. When the airfoil oscillated beyond the static stall angle of attack, huge variations on the wake profiles were found because of the interaction between LEV and Von Karman vortices. More velocity defect in ...

Wake Measurements behind an Oscillating Airfoil in Dynamic Stall Conditions

The unsteady flow field in the wake of an NACA 23012 pitching airfoil was investigated by means of triple hot-wire probe measurements. Wind tunnel tests were carried out both in the light and deep dynamic stall regimes. The analysis of the wake velocity fields was supported by the measurements of unsteady flow fields and airloads. In particular, particle image velocimetry surveys were carried out on the airfoil upper surface, while the lift and pitching moments were evaluated integrating surface pressure measurements. In the light dynamic stall condition, the wake velocity profiles show a similar behaviour in upstroke and in downstroke motions as, in this condition, the flow on the airfoil upper surface is attached for almost the whole pitching cycle and the airloads show a small amount of hysteresis. The deep dynamic stall measurements in downstroke show a large extent of the wake and a high value of the turbulent kinetic energy due to the passage of strong vortical structures, typical of this dynamic stall regime. The comprehensive experimental database can be considered a reference for the development and validation of numerical tools for such peculiar flow conditions.

Experimental-numerical investigation of a pitching airfoil in deep dynamic stall

The results of a comprehensive experimental campaign are compared to computational fluid dynamics simulations results to assess the modelling capabilities for a NACA 23012 pitching airfoil in deep dynamic stall regime. The experimental campaign involved fast unsteady pressure measurements and particle image velocimetry. Two-dimensional simulations were carried out with EDGE, developed by FOI. The investigated test case consists in a sinusoidal pitching motion with a 10° amplitude and a reduced frequency of 0.1 around a mean angle of attack of 10°. The behaviour of the experimental lift and pitching moment coefficients is in close agreement with the two-dimensional simulations results, also during the downstroke motion where the flow field is characterised by severe unsteadiness conditions. A three-dimensional numerical model was built to evaluate the relevance of three-dimensional effects on the experiments. Three-dimensional simulations were carried out using the commercial code FLUENT. During upstroke motion, three-dimensional simulations results are in better agreement with the experiments, in particular in terms of the lift coefficient curve slope and of the pitching moment coefficient peak. The flow fields evaluated by particle image velocimetry surveys show strong vortical structures moving on the airfoil upper surface during the downstroke motion that are captured only by the three-dimensional model; then, the flow fields comparison demonstrates the importance of three-dimensional effects for a deep dynamic stall condition.

곤충비행에서 갑작스런 추력발생의 공기역학적 원인 Part 1: 와류 정지 및 와류 짝 현상

검정금파리 날개의 “8자 운동”에 의한 공기역학적 힘의 발생에 관해 수치해석을 수행하였다. 날개운동은 자유류가 있는 tethered flight 실험에서 관찰된 결과에서 인용하였다. 해석결과 양력은 downstroke 중일 때 주로 발생하였고 추력은 upstroke 끝에서 갑작스럽게 발생하였다. 본 연구에서는 양력과 추력 발생의 이러한 특성을 후류에서의 와류구조와 에어포일 주위의 압력장을 통해 물리적으로 이해하고자 하였다. 결과적으로 양력발생은 유효받음각의 증가에 따른 앞전와류와 관계있었으며, 추력발생은 유동장 형태의 측면에서 와류 짝(vortex pairing)현상과 압력장 측면에서 와류정지 현상으로 설명할 수 있었다. Numerical simulation is conducted to investigate aerodynamic force generation mechanism for the "figure-of-eight" motion of Dipteran fly, Phormia-Regina. Wing trajectory is referred to experimental result, which was observed from the tethered flight under freestream condition. Numerical simulation shows that the lift is mainly generated during downstroke motion and the large amount of thrust is generated abruptly at the end of upstroke motion. In the present work, vortical structure in the wake and the pressure field around the airfoil are examined to understand the generation of lift and thrust. Consequently, the lift generation is related with the leading edge vortex which is developed by an effective angle of attack. And the thrust generation can be explained by vortex pairing in the flow field and by vortex staying in the pressure field.

Experimental and Numerical Study on Flapping Wing Kinematics and Aerodynamics of Coleoptera

In this study we have experimentally and numerically analyzed the flapping mechanism and wing kinematics of coleoptera (Propylea japonica Thunberg). Using digital high speed camera, we captured the continuous wing kinematics and visualized the flight motion of the free-flying coleoptera. The experimental visualization shows that the elytra flapped concurrently with the main wing both in the downstroke and upstroke motions. In order to define the wing kinematics of coleoptera, the displacement of a wing cross section (50% span-wise) was measured for each sequence of the wing motion. Using these data, the flight motion of coleoptera was numerically simulated to investigate the aerodynamic performance. The computational aerodynamic simulation shows that leading edge vortex shedding plays a key role in generating lift to keep the insect aloft.