Yongsheng Lian

Laminar-Turbulent Transition of a Low Reynolds Number Rigid or Flexible Airfoil

4-10 5 . In order to gain better understanding of the fluid physics and associated aerodynamics characteristics, we have coupled (i) a NavierStokes solver, (ii) the e N method transition model, and (iii) a Reynolds-averaged two-equation closure to study the low Reynolds number flow characterized with laminar separation and transition. A new intermittency distribution function suitable for low Reynolds number transitional flow is proposed and tested. To support the MAV applications, we investigate both rigid and flexible airfoils, which has a portion of the upper surface mounted with a flexible membrane, using SD7003 as the configuration. Good agreement is obtained between the prediction and experimental measurements regarding the transition location as well as overall flow structures. In the current transitional flow regime, though the Reynolds number affects the size of the laminar separation bubble, it does not place consistent impact on lift or drag. The gust exerts a major influence on the transition position, resulting in the lift and drag coefficients hysterisis. It is also observed that thrust instead of drag can be generated under certain gust condition. At α=4 o , for a flexible wing, self-excited vibration affects the separation and transition positions; however, the time-averaged lift and drag coefficients are close to those of the rigid airfoil.

Membrane wing aerodynamics for micro air vehicles

Abstract The aerodynamic performance of a wing deteriorates considerably as the Reynolds number decreases from 106 to 104. In particular, flow separation can result in substantial change in effective airfoil shape and cause reduced aerodynamic performance. Lately, there has been growing interest in developing suitable techniques for sustained and robust flight of micro air vehicles (MAVs) with a wingspan of 15 cm or smaller, flight speed around 10 m / s , and a corresponding Reynolds number of 104–105. This paper reviews the aerodynamics of membrane and corresponding rigid wings under the MAV flight conditions. The membrane wing is observed to yield desirable characteristics in delaying stall as well as adapting to the unsteady flight environment, which is intrinsic to the designated flight speed. Flow structures associated with the low Reynolds number and low aspect ratio wing, such as pressure distribution, separation bubble and tip vortex are reviewed. Structural dynamics in response to the surrounding flow field is presented to highlight the multiple time-scale phenomena. Based on the computational capabilities for treating moving boundary problems, wing shape optimization can be conducted in automated manners. To enhance the lift, the effect of endplates is evaluated. The proper orthogonal decomposition method is also discussed as an economic tool to describe the flow structure around a wing and to facilitate flow and vehicle control.

Aerodynamics of Low Reynolds Number Plunging Airfoil under Gusty Environment

It is known that plunging airfoil can produce both lift and thrust with certain combination of plunging amplitude and frequency. Motivated by our interest in micro air vehicles (MAVs), we utilize a NavierStokes equation solver to investigate the aerodynamics of a flapping airfoil. The roles of the plunging and pitching amplitude and frequency, and Strouhal number are studied. For a symmetric plunging airfoil NACA0012 at zero geometric angle of attack and chord Reynolds number of 2×10 4 , at the same plunging frequency, it can produce either drag or thrust depending on the plunging amplitude. At the considered plunging amplitude (from 0.0125c to 0.075c), the flow history has more influence than the kinematic angle of attack to determine the lift. When drag is produced, the viscous force dominates the total drag with decreasing influence as the plunging amplitude increases. For an airfoil experiencing combined plunge and pitch motion, both thrust and input power increase with the Strouhal number (within the range of 0.03 to 0.5). For the case studied, the thrust is induced by the lift, which approximately follows the curve of the kinematic angle of attack. Leading edge vortex moves downstream and interacts with the trailing edge vortex. We also study the impact of gust on stationary airfoil and flapping airfoil. Within the range of the parameters tested, for stationary airfoil the lift is in phase with the velocity but the drag is slightly out of phase. For flapping airfoil, neither lift nor drag is in phase with the velocity. Nomenclature CD =Drag coefficient per unit span CL =Lift coefficient per unit span CP =input power coefficient CP,mean =time-averaged input power coefficient CT =thrust coefficient CT,mean =time-averaged thrust coefficient c =Chord length

Numerical simulations of membrane wing aerodynamics for micro air vehicle applications

To gain insight into the aerodynamics of flexible wing-based micro air vehicles (MAVs), we study the threedimensional interaction between a membrane wing and its surrounding fluid flow. A nonlinear membrane structural solver and a Navier‐Stokes flow solver are coupled through the moving boundary technique and time synchronization. Under the chord Reynolds number of 9 × × 10 4 , the membrane exhibits self-initiated vibrations in accordance with its material properties and the surrounding fluid flow. The vortical flow structure, its effect on the aerodynamic parameters, and the implications of the membrane deformation on the effective angle of attack and flow structure are discussed. Nomenclature C D = drag coefficient CL = lift coefficient c = chord length c p = pressure coefficient D = drag Fpx = form drag Fpy = lift caused by pressure force Fτ x = drag caused by friction L = lift U = freestream speed u = chordwise velocity v =v ertical velocity x = chordwise distance from the leading edge Z = half-wing span z = spanwise distance from the root α = angle of attack

Flapping and flexible wing aerodynamics of low reynolds number flight vehicles

For flight vehicles operated at the low Reynolds number regime, such as birds, bats, insects, as well as small man-made vehicles, flapping and fixed wings are employed in various ways to generate aerodynamic forces. For flapping wings, the unsteady fluid physics, interacting with wing kinematics and shapes determine the lift generation. For fixed wings, laminar-turbulent transition, three dimensional flows around low aspect ratio vehicles, and coupling between flexible wing structures and surrounding fluid flows are of major interest. In the present paper we discuss recent progress in understanding the low Reynolds number unsteady fluid dynamics associated with flapping wings, including leading-edge vortices, pitching-up rotation and wake-capturing mechanisms. For fixed wings, recent efforts in fluid-structure interaction and laminar-turbulent transition are highlighted.

Résumé of the AIAA FDTC Low Reynolds Number Discussion Group's Canonical Cases

The AIAA Fluid Dynamics Technical Committee’s Low Reynolds Number Discussion Group has introduced several “canonical” pitch motions, with objectives of (1) experimental-numerical comparison, (2) assessment of closed-form models for aerodynamic force coefficient time history, and (3) exploration of the vast and rather amorphous parameter space of the possible kinematics. The baseline geometry is a flat plate of nominally 2.5% thickness and round edges, wall-to-wall in ground test facilities and spanwise-periodic or 2D in computations. Motions are various smoothings of a linear pitch ramp, hold and return, of 40 and 45 amplitude. In an attempt to discern acceleration effects, sinusoidal and linear-ramp motions are compared, where the latter have short runs of high acceleration and thus high noncirculatory lift and pitch. Parameter variations include comparison of the flat plate with an airfoil and ellipse, variation of reduced frequency, pitch pivot point location and comparison of pitch to quasi-steady equivalent plunge. All motions involve strong leading edge vortices, whose growth history depends on pitch pivot point location and reduced frequency, and which can persist over the model suction-side for well after motion completion. Noncirculatory loads were indeed found to be localized to phases of motion where acceleration was large. To the extent discernable so far, closed-form models of lift coefficient on the pitch upstroke are relatively straightforward, but not so on the downstroke, where motion history effects complicate the return from stall. Broad Reynolds number independency, in flowfield evolution and lift coefficient, was found in the 10 to 10 range.

Reliability-Based Design Optimization of a Transonic Compressor

A multiobjective, reliability-based design optimization method for computationally intensive problems is proposed. In this method we use a genetic algorithm to facilitate the multiobjective optimization. To further improve the convergence of the genetic algorithm, we augment it with a local search. Reliability analysis is performed using Monte Carlo simulation. Quadratic design response surfaces are utilized to filter the noise from the Monte Carlo simulation and facilitate the multidisciplinary design optimization. In addition, response surface approximations greatly reduce the computational cost. To improve the accuracy of probability computation in the regions of low probability of failure and to provide useful information for the optimization, oprobabilistic sufficiency factor is used as an alternative measure of safety. To demonstrate the capabilities of this approach, we employ it to optimize the NASA rotor67 transonic blade. Numerical results show that with this proposed approach we can obtain a reliable design with better aerodynamic performance and less weight. Error analysis is also reported so that readers can understand not only the advantages but also the disadvantages of this approach.

Numerical Investigation of Energy Extraction in a Tandem Flapping Wing Configuration

A number of flying insects make use of tandem-wing configurations, suggesting that such a setup may have potential advantages over a single wing at low Reynolds numbers. Dragonflies, which are fast and highly maneuverable, demonstrate well the potential performance of such a design. In this paper, a tandem-wing flapping configuration is simulated at a Reynolds number of 10,000 using an incompressible Navier–Stokes solver and an overlapping gridmethod. The flappingmotion consists of a simple sinusoidal pitch and plungemotionwith a spacing of one chord length between both wings. The arrangement was tested at a Strouhal number of 0.3 for three different phase angles: 0, 90, and 180 deg. The aerodynamics of the hindwing was compared in detail to a single wing, with the same geometry and undergoing the same flapping kinematics, to determine the effect of vortex shedding from the forewing on the hindwing, as well as how the phase angle affects the interaction. The average lift, thrust, and power coefficients and the average efficiency of the foreand hindwings were comparedwith a single wing to determine how the tandem-wing interaction affects performance. The results show that adjusting the phase angle allows the tandem wing to change the flight mode. At 0 deg phase lag, the tandem wing produces high thrust at high propulsive efficiency, but low lift efficiency. Switching to 90=180 deg phase lag decreases the thrust production and propulsive efficiency but greatly increases the lift efficiency. At 90=180 deg, the power coefficient is much lower than at 0 deg, due to the hindwing extracting energy from the wake of the forewing.

Low Reynolds Number Turbulent Flows around a Dynamically Shaped Airfoil

Abstract A computational investigation for flows surrounding a dynamically shaped airfoil, at a chord Reynolds number of 78,800, is conducted along with a parallel experimental effort. A piezo-actuated flap on the upper surface of a fixed airfoil is adopted for active control. The actuation frequency focused on is 500 Hz. The computational framework consists of a multi-block, moving grid technique, the en-based laminar–turbulent transition model, the two-equation turbulence closure, and a pressure-based flow solver. The moving grid technique, which handles the geometric variations in time, employs the transfinite interpolation scheme with a spring network approach. Comparing the experimental and computational results for pressure and velocity fields, implications of the detailed flap geometry, the flapping amplitude, turbulence modeling, and grid distributions on the flow structure are assessed. The effect of the flap movement on the separation location and vortex dynamics is also investigated.

Numerical investigation of vortex dynamics in an H-rotor vertical axis wind turbine

We study the vortex dynamics of a two-dimensional H-rotor wind turbine using a Navier-Stokes solver. The turbulence model with the wall function is used as the turbulence closure. A sliding mesh technique is employed to handle the blade rotation. The vortex-blade interaction is systematically investigated and its influence on the force generation is discussed. Our simulations show that the vortex-blade interaction largely depends on the solidity and tip speed ratio. We further study the impact of solidity on the turbine performance. Our simulations show that the peak torque per blade decreases with the solidity while the peak torque azimuthal angle increases with the solidity. Our simulations also show that the increase in the azimuthal angle is more significant at low tip speed ratios than at high tip speed ratios. The impact of blade thickness is studied. Our simulations show that a thicker airfoil has a higher torque coefficient than a thinner airfoil. However, because for the thinner airfoil its peak torque occurs at a high tip speed ratio, the thinner airfoil has an overall higher power coefficient than the thicker airfoil.