Showing papers on "Pitching moment published in 2013"

Dynamic Stall Control by Passive Disturbance Generators

Abstract: Passive cylindrical disturbance generators mounted near the leading edge of an airfoil significantly improved its performance under dynamic stall conditions. Time-resolved particle image velocimetry and simultaneous pressure measurements were conducted at the midchord of a pitching airfoil equipped with passive disturbance generators. The disturbance generators were effective in reducing the strength of the dynamic stall vortex and therefore the negative pitching moment peak and hysteresis effects. When the disturbance generators were applied, the flow separation type was altered from leading- to trailing-edge stall. In contrast to the clean case, reattachment was initiated immediately after the separation reached the leading-edge region. In addition to the circular shape, also backward- and forward-wedge-shaped disturbance generators were investigated. Although the backward wedge also showed favorable results, the forward wedge was less successful. The shape of the disturbance generators appears to have a strong influence on the effectiveness of reducing the negative impact of dynamic stall, depending on the sense of rotation of a pair of weak trailing vortices.

On the Role of Leading-Edge Bumps in the Control of Stall Onset in Axial Fan Blades

Abstract: Taking a lead from the humpback whale flukes, characterized by a series of bumps that result in a sinusoidal-like leading edge, this paper reports on a three-dimensional numerical study of sinusoidal leading edges on cambered airfoil profiles. The turbulent flow around the cambered airfoil with the sinusoidal leading edge was computed at different angles of attack with the open source solver OpenFOAM, using two different eddy viscosity models integrated to the wall. The reported research focused on the effects of the modified leading edge in terms of lift-to-drag performance and the influence of camber on such parameters. For these reasons a comparison with a symmetric airfoil is provided. The research was primarily concerned with the elucidation of the fluid flow mechanisms induced by the bumps and the impact of those mechanisms on airfoil performance, on both symmetric and cambered profiles. The bumps on the leading edge influenced the aerodynamic performance of the airfoil, and the lift curves were found to feature an early recovery in post-stall for the symmetric profile with an additional gain in lift for the cambered profile. The bumps drove the fluid dynamic on the suction side of the airfoil, which in turn resulted in the capability to control the separation at the trailing edge in coincidence with the peak of the sinusoid at the leading edge.

Dynamic Stall Measurements and Computations for a VR-12 Airfoil with a Variable Droop Leading Edge

Abstract: High density-altitude operations of helicopters with advanced performance and maneuver capabilities have lead to fundamental research on active high-lift system concepts for rotor blades. The requirement for this type of system was to improve the sectional lift-to-drag ratio by alleviating dynamic stall on the retreating blade while simultaneously reducing the transonic drag rise of the advancing blade. Both measured and computational results showed that a Variable Droop Leading Edge (VDLE) airfoil is a viable concept for application to a rotor high-lift system. Results are presented for a series of 2D compressible dynamic stall wind tunnel tests with supporting CFD results for selected test cases. These measurements and computations show a dramatic decrease in the drag and pitching moment associated with severe dynamic stall when the VDLE concept is applied to the Boeing VR-12 airfoil. Test results also show an elimination of the negative pitch damping observed in the baseline moment hysteresis curves.

Effect of landform on aerodynamic performance of high-speed trains in cutting under cross wind

Abstract: The effects of the different landforms of the cutting leeward on the aerodynamic performance of high-speed trains were analyzed based on the three-dimensional, steady, and incompressible Navier-Stokes equation and k-ɛ double-equation turbulent model. Results show that aerodynamic forces increase with the cutting leeward slope decreasing. The maximum adding value of lateral force, lift force, and overturning moment are 147%, 44.3%, and 107%, respectively, when the slope varies from 0.67 to −0.67, and the changes in the cutting leeward landform have more effects on the aerodynamic performance when the train is running in the line No. 2 than in the line No. 1. The aerodynamic forces, except the resistance force, sharply increase with the slope depth decreasing. By comparing the circumstance of the cutting depth H= −8 m with that of H=8 m, the resistance force, lateral force, lift force, and overturning moment increase by 26.0%, 251%, 67.3% and 177%, respectively. With the wind angle increasing, the resistance force is nonmonotonic, whereas other forces continuously rise. Under three special landforms, the changes in the law of aerodynamic forces with the wind angle are almost similar to one another.

Experimental investigation of dynamic stall performance for the EDI-M109 and EDI-M112 airfoils

Abstract: An experimental investigation of the dynamic performance of two new rotor blade airfoils was undertaken in a transonic wind tunnel. The EDI-M109 and EDI-M112 airfoils were tested at 0.3

A Hybrid Airfoil Design Method for Icing Wind Tunnel Tests

Abstract: .Modern commercial aircraft wings are far too large to be tested full-scale in existing icing wind tunnels and ice accretion scaling methods are not practical for large scale factors. Thus the use of hybrid scaling techniques, maintaining full-scale leading-edges and redesigned aft sections, is an attractive option for generating full-scale leading-edge ice accretions. The advantage lies in utilizing reduced chord models that minimize blockage effects in the icing tunnels. The present work discusses the design of hybrid airfoils with large scale factors that match the ice shapes of the full-scale airfoils predicted by LEWICE. Assessments of the effects of scale factor, extent of the full-scale leading-edge, nose droop angle, zero-angle of attack pitching moment coefficient (Cm0), and droplet size are also presented. Hybrid or truncated airfoils are shown to produce ice shapes accurately, even at angles of attack different from the design angle of attack with the proper application of either flap, adjusted test angle of attack, or both. Further results suggest that hybrid circulation does not need to match full-scale circulation in order to match ice shapes, resulting in decreased loading for higher scale factor hybrid airfoils. Matching the flowfield around the hybrid airfoil to the full-scale flowfield provided a superior method for predicting ice shape agreement, stagnation point location being a first order and suction peak magnitude a second order parameter. This goal can be accomplished by varying the aft geometry, through Cm0 and nose droop angle.

Influence of rotation on dynamic stall

Abstract: A computational investigation of the effect of rotation on two-dimensional (2D) deep dynamic stall has been undertaken, showing that the effect of rotation is to reduce the severity of the pitching moment peak and cause earlier reattachment of the flow. A generic single blade rotor geometry was investigated, which had a pitching oscillation around the quarter chord axis while in hover, causing angle-driven dynamic stall. The results at the midpoint of the blade have the same Mach number (0.31), Reynolds number (1.15×106), and pitching motion (α =13◦±7◦) as a dynamic stall test case for which significant experimental wind tunnel data and 2D computations exist. The rotating blade is compared with 2D computations and computations using the same blade without rotation at Mach 0.31 and with the same pitching motion. All test cases involve geometries propagating into undisturbed flow with no downwash. The three-dimensional (3D) grid computed without rotation had lower lift at the reference section than for a 2D computation with the same geometric angle of attack time history, and the lift overshoot classically observed for Spalart–Allmaras turbulence models during 2D dynamic stall was significantly reduced in the 3D case. Rotation reduced the strength of the dynamic stall vortex, which reduced the accompanying pitching moment peak by 25%.

Realistic Leading-Edge Roughness Effects on Airfoil Performance.

Abstract: Wind farm operators observe power production decay over time, with the exact cause unknown and difficult to quantify. A likely explanation is blade surface roughness, as wind turbines are continuously subjected to environmental hazards. Difficulty arises in understanding and quantifying performance degradation. Historically, wind turbine airfoil families were designed for the lift to be insensitive to roughness by simulating roughness with 2D trip strips. Despite this, roughness is still shown to negatively affect airfoil lift performance. Experiments have also illustrated that random-distributed roughness is not properly simulated by trip strips. Therefore, to better understand how real roughness effects performance, field measurements of turbine-blade roughness were made and simulated on an airfoil section in a wind tunnel. This data will serve to validate and calibrate a one-equation, computational roughness amplification model that interacts with the Langtry-Menter transition model. The observed roughness contains 2D steps, heavy 2D erosion, pitting, insects, and repairs. Of these observations, 2D steps from paint chips were characterized and recreated for this particular wind tunnel entry. The model was tested at chord Reynolds numbers up to 3.6 × 10. Measurements of lift, drag, and pitching moment were made with and without roughness contamination. Transition location was acquired with infrared thermography and a hotfilm array. The paint roughness yields a consistent increase in drag compared to the clean configuration. Numerical simulations are only compared to the clean configuration and match well to lift, drag, and transition for Rec = 1.6 × 10. However, drag is overpredicted at Rec = 3.2 × 10.

Effect of Pivot Point on Aerodynamic Force and Vortical Structure of Pitching Flat Plate Wings

Abstract: This paper reports aerodynamic forces and 2D PIV velocity measurements on a rectangular wing with an effective aspect ratio 4 during a pitch ramp-hold wing motion at reduced frequencies from 0.022 to 0.39. The parameter space in terms of the Stokes number and the Reynolds number is used to correlate pitch rate and free stream velocity effects. The maximum angle of attack is 45 degrees and pivot locations, such as leading edge pivot, midchord pivot, and trailing edge pivot, are considered. The evolution of aerodynamic forces as functions of convective time, pitching time, and angle of attack are discussed. Noncirculatory effects are observed for reduced frequency higher than 0.13. For leading edge pivot axis non-circulatory effects produce a positive force spike at the start of rotation and a negative spike at the end of rotation. The opposite is found for trailing edge pivot. For midchord pivot the non-circulatory force spikes are not present. Rotation rate effects are observed during the constant rate part of the motion for reduced frequency higher than 0.065. For leading edge pivot axis non-circulatory effects and rotation rate effects combine to produce very large lift and drag coefficients. Incipient vortex shedding is observed at high reduced frequency during the hold phase for convective times in the range 4-20. For reduced frequency lower than 0.022 the flow is quasi steady and lifting line theory provides good estimates of lift for angle of attack below the steady stall angle. Although significant leading edge suction was measured at the final steady state condition, projection of the normal force in the drag direction is the main contribution to the drag. PIV measurements show the flow topology for different pivot axes. For leading edge pivot axis a starting vortex is formed at the trailing edge which promotes development of the LEV and enhances aerodynamic forces. For mid-chord pivot axis starting vortices are formed at the leading and trailing edges. For trailing edge pivot axis the starting vortex is formed at the leading edge only. Formation of a starting vortex at the leading edge delays development of the LEV and inhibits aerodynamic force generation.

Determination of aerodynamic forces on stationary/moving vehicle-bridge deck system under crosswinds using computational fluid dynamics

Abstract: To investigate the safety of road vehicles running on a long-span bridge under high crosswinds, the aerodynamic forces of a moving vehicle-bridge deck system need to be determined. This paper reports the use of computational fluid dynamics (CFD) to simulate the aerodynamic forces on coupled vehicle-bridge deck systems under crosswinds in terms of aerodynamic coefficients. The aerodynamic coefficients of a stationary vehicle-deck system were simulated and then compared with wind tunnel results. A relative velocity method was then used to obtain the aerodynamic forces of a moving vehicle-deck system. The moving effects on the aerodynamic forces on the vehicle-deck system were evaluated. The results show that the movement of the vehicle on the first lane of the bridge deck does affect the aerodynamic coefficients of the bridge deck but has only slight effects on the aerodynamic coefficients of the vehicle if the relative yaw angle to the vehicle is taken into account.

Analysis of NASA Common Research Model Dynamic Data in JAXA Wind Tunnel Tests

Abstract: The JAXA 2m x 2m Transonic Wind Tunnel (JTWT) conducted tests for 80% scaled NASA Common Research Model (CRM). The dynamic data including buffet measurement with strain gauges and dynamic pressure sensors were acquired at 50,000 Hz sampling rate. A prediction of buffet phenomenon is one of important factors to design aircraft. If buffet phenomenon occurs, dynamic bending and torsion moment are measured with wing-root strain gauges. Spectrum analyses are being executed for the strain gauges and dynamic pressure data. It is expected to observe dynamic flow separation at points around where the relation with lift coefficient data and pitching moment coefficient is non-linear. This paper describes overview of the tests and the analysis data.

Pitch Control of a Micro Air Vehicle with Micropressure Sensors

Abstract: Maintaining stable flight of micro aerial vehicles is challenging, especially in complex, low-Reynolds-number flight environments while considering wind gust disturbance, flow separation, and flow reattachment. To date, most micro aerial vehicles use vision, inertial measurement units, and/or global positioning systems as their primary sensing and navigation devices; however, actual flow conditions over the aircraft wing surfaces cannot be captured directly. In this paper, a biologically inspired micro aerial vehicle pitch control system is designed using distributed pressure information. The pressure information on the wing surfaces of a micro aerial vehicle is directly measured by a array of digital barometric micropressure sensors and is then used to calculate the aerodynamic forces, center of pressure, pitching moment, etc. A new pitch motion model that can capture the pressure information is derived from the control perspective. A nonlinear controller is also designed to achieve accurate pitch contro...

Real-Time Unsteady Loads Measurements Using Hot-Film Sensors

Abstract: Several flight-critical aerodynamic problems such as buffet, flutter, stall, and wing rock are strongly affected or caused by abrupt changes in unsteady aerodynamic loads and moments. Advanced sensing and flow diagnostic techniques have made possible simultaneous identification and tracking, in real-time, of the critical surface, viscosity-related aerodynamic phenomena under both steady and unsteady flight conditions. The wind tunnel study reported here correlates surface hot-film measurements of leading edge stagnation point and separation point, with unsteady aerodynamic loads on a NACA 0015 airfoil. Lift predicted from the correlation model matches lift obtained from pressure sensors for an airfoil undergoing harmonic pitchup and pitchdown motions. An analytical model was developed that demonstrates expected stall trends for pitchup and pitchdown motions. This report demonstrates an ability to obtain unsteady aerodynamic loads in real-time, which could lead to advances in air vehicle safety, performance, ride-quality, control, and health management.

Airfoil Dynamic Stall and Rotorcraft Maneuverability

Abstract: The loading of an airfoil during dynamic stall is examined in terms of the augmented lift and the associated penalties in pitching moment and drag. It is shown that once stall occurs and a leading-edge vortex is shed from the airfoil there is a unique relationship between the augmented lift, the negative pitching moment, and the increase in drag. This relationship, referred to here as the dynamic stall function, shows limited sensitivity to effects such as the airfoil section profile and Mach number, and appears to be independent of such parameters as Reynolds number, reduced frequency, and blade sweep. For single-element airfoils there is little that can be done to improve rotorcraft maneuverability except to provide good static C(l(max)) characteristics and the chord or blade number that is required to provide the necessary rotor thrust. However, multi-element airfoils or airfoils with variable geometry features can provide augmented lift in some cases that exceeds that available from a single-element airfoil. The dynamic stall function is shown to be a useful tool for the evaluation of both measured and calculated dynamic stall characteristics of single element, multi-element, and variable geometry airfoils.

Trim Analysis of a Moving-mass Actuated Airplane in Steady Turn

Abstract: This paper is to follow up on a prior research work on moving-mass actuated airplane without conventional control surfaces. The prior work studied the feasibility of trimming the airplane in a straight level ight by moving an internal mass longitudinally to generate pitching moment. The current research is to study the feasibility of trimming the airplane in steady level turn by two moving masses; one longitudinally and the other one laterally.

Experimental Investigation of Air Jets for the Control of Compressible Dynamic Stall

Abstract: The experimental investigation of constant blowing air jets as fluidic control devices for helicopter dynamic stall control is described. A carbon fiber airfoil of constant OA209 cross section was fitted with a pneumatic system to deliver dry compressed air as jets for flow control at total pressures of up to 10 bar. The experiment used porthole jets of radius 1% chord, positioned at 10% chord and with spacing 6.7% chord. The positive dynamic stall control effects were demonstrated at Mach 0.3, 0.4, and 0.5 for deep dynamic stall test cases with the best test cases reducing the pitching moment peak after the main stall by 83% while increasing the mean lift over one pitching cycle by 30%. The conclusions from the experiments are supported by three-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) computations of the pitching airfoil with flow control using the DLR-TAU code.

On the acoustic radiation of a pitching airfoil

Abstract: We examine the acoustic far field of a thin elastic airfoil, immersed in low-Mach non-uniform stream flow, and actuated by small-amplitude sinusoidal pitching motion. The near-field fluid-structure interaction problem is analyzed using potential thin-airfoil theory, combined with a discrete vortex model to describe the evolution of airfoil trailing edge wake. The leading order dipole-sound signature of the system is investigated using Powell-Howe acoustic analogy. Compared with a pitching rigid airfoil, the results demonstrate a two-fold effect of structure elasticity on airfoil acoustic field: at actuation frequencies close to the system least stable eigenfrequency, elasticity amplifies airfoil motion amplitude and associated sound levels; however, at frequencies distant from this eigenfrequency, structure elasticity acts to absorb system kinetic energy and reduce acoustic radiation. In the latter case, and with increasing pitching frequency ωp, a rigid-airfoil setup becomes significantly noisier than an...

Controlled transitory stall on a pitching airfoil using pulsed actuation

Abstract: Transitory separation control of a static and pitching 2-D airfoil is investigated in wind tunnel experiments using pulsed actuation on time scales that are an order of magnitude shorter than the characteristic convective time scale T conv. Actuation is provided by momentary [O(0.05T conv)] pulsed jets that are generated by a spanwise array of combustion-based actuators integrated in the center segment of the airfoil. The flow field in the center plane above the airfoil and in its near wake is computed from high-resolution PIV measurements in multiple overlapping cross-stream frames that are obtained phase-locked to the actuation and allow for tracking of vorticity concentrations. A single actuation pulse leads to a strong transitory increase in the circulation about the airfoil that is manifested by a partial collapse of the separated flow domain and is accompanied by the shedding of a large-scale clockwise vortex, and the attachment and accumulation of the surface vorticity layer behind it. The slow relaxation of the flow following termination of pulsed actuation returns the airfoil to full stall within 10T conv. It is shown that repetition of actuation pulses within T conv can increase the streamwise extent of the attached flow domain, and the trapped vorticity leads to a substantial increase in the peak transitory circulation before the flow separates again when the actuation is terminated. The coupling of the pulsed actuation to the airfoil’s motion enhances the actuation’s control authority. Single pulse can significantly increase the lift over most of the oscillation cycle both at post-stall and at angles of attack that are below stall. Several actuation pulses distributed during the pitch oscillation cycle can momentarily extend the accumulation of vorticity and thus increase the transitory and cycle-averaged lift, and improve the airfoil’s pitch stability.

The Practical Calculation of the Aerodynamic Characteristics of Slender Finned Vehicles

Abstract: The basic objective of this thesis is to provide a practical method of computing the aerodynamic characteristics of slender finned vehicles such as sounding rockets, high speed bombs, and guided missiles. The aerodynamic characteristics considered are the normal force coefficient derivative, c(sub N(sub alpha)); center of pressure, bar-X; roll forcing moment coefficient derivative, c(sub l(sub delta)); roll damping moment coefficient derivative, c(sub l(sub p)); pitch damping moment coefficient derivative, c(sub mq); and the drag coefficient, c (sub D). Equations are determined for both subsonic and supersonic flow. No attempts is made to analyze the transonic region. The general configuration to which the relations are applicable is a slender axisymmetric body having three or four fins.

Effect of the Model-Sidewall Connection for a Static Airfoil Experiment

Abstract: It has been shown that the gap between an airfoil and the windtunnel wall can have a significant effect on the three-dimensional near-wall flow topology for a static angle of attack, without being detectable at the midline of an airfoil model. Understanding the interaction between model and sidewall is necessary to improve the accuracy of wind-tunnel airfoil testing and reduce the sidewall interference. The measurements in this technical note show that care must be exercised, when comparing full three-dimensional modeling of the wind tunnel and model with experiments, not to draw the wrong conclusion from a good fit between computation and experiment. In this case, a good agreement can be gained by CFD modeling that produces the wrong flow topology, but the agreement will not necessarily remain good if the test conditions change.

Method for determining aerodynamic parameters of model free flight tests

Abstract: A method for determining aerodynamic parameters of model free flight tests is used in model free flight tests including a wind tunnel free flight test, an atmospheric free flight test and the like. Under the situation that a model used in wind tunnel free flight or atmospheric free flight moves in a plane, the method of polynomial time is adopted to match the linear displacement of the centroid of the model in the horizontal direction and the vertical direction and the shooting recording observing value, changing along with time, of the angle of pitch of the model, then, the time-dependent changing rule of the linear displacement, the linear velocity and the linear acceleration of the centroid of the model and the angle displacement, the angle velocity and the angle acceleration of the angle of pitch is obtained, and accordingly the time-dependent changing rule of the resistance coefficient, the lift coefficient and the pitch moment coefficient in the process of model flying is obtained. The application conditions and the range of a data processing method of the model free flight tests are expanded, and the test recorded data can be processed in wide application conditions and ranges to obtain the aerodynamic parameters and the movement rule of the model.

Role of Synthetic Jet Frequency & Orientation in Dynamic Stall Vorticity Creation

Abstract: Vorticity creation and its evolution play an important role in the formation of large dynamic stall vortices which cause large excursions in lift, drag and pitching moment on rapidly pitching and oscillating airfoils. While synthetic jets have shown potential in controlling dynamic stall vortices by sustaining lift increases without corresponding drag and pitching moment, their effect on the vorticity creation remains unknown. To address this, high fidelity computational fluid dynamics simulations were conducted to investigate how synthetic jet frequency and orientation alter the dynamic stall leading-edge boundary vorticity flux. Changes in baseline boundary vorticity flux were related to corresponding flow fields to delineate the role of synthetic jets in vorticity creation. Slot orientation was found to play a greater role in altering the amount of vorticity diffused into the flow than actuation frequency. Dynamic stall vortex sizes were found to be directly proportional to the vorticity diffused from the leading-edge and could therefore be effectively manipulated using synthetic jets. The study provides a better understanding of the dynamic stall vorticity creation process and its control using synthetic jets.

Swallow tail airfoil

Abstract: Aerodynamic element having a cross section in an airflow direction with a trailing edge (10c). The aerodynamic element (10) further comprises a non-symmetrical swallow tail shaped cavity body (1, 2) attached to the trailing edge (10c) of the aerodynamic element (10).The aerodynamic element (10) is e.g. applied in a rotor blade for a wind turbine.