Numerical Investigations on Dynamic Stall Characteristics of a Finite Wing

The paper examines the dynamic stall characteristics of a finite wing with an aspect ratio of eight in order to explore the 3D effects on flow topology,aerodynamic characteristics,and pitching damping.Firstly,CFD methods are developed to calculate the aerodynamic characteristics of wings.The URANS equations are solved using a finite volume method,and the two-equation k-ωshear stress transport(SST)turbulence model is employed to account for viscosity effects.Secondly,the CFD methods are used to simulate the aerodynamic characteristics of both a static,rectangular wing and a pitching,tapered wing to verify their effectiveness and accuracy.The numerical results show good agreement with experimental data.Subsequently,the static and dynamic characteristics of the finite wing are computed and discussed.The results reveal significant 3D flow structures during both static and dynamic stalls,including wing tip vortices,arch vortices,Ω-type vortices,and ring vortices.These phenomena lead to differences in the aerodynamic characteristics of the finite wing compared with a 2D airfoil.Specifically,the finite wing has a smaller lift slope during attached-flow stages,higher stall angles,and more gradual stall behavior.Flow separation initially occurs in the middle spanwise section and gradually spreads to both ends.Regarding aerodynamic damping,the inboard sections mainly generate unstable loading.Furthermore,sections experiencing light stall have a higher tendency to produce negative damping compared with sections experiencing deep dynamic stall.

VIBRATION RESPONSE AND ITS CHARACTERISTICS OF CENTRIFUGAL COMPRESSOR WITH ROTATING STALL

Based on the beginning, propagating and ending mechanism of rotating-stallcell, the relation between the pressure history signal and the pressure distribution along rotorcircumference is proposed. The angular velocities of rotating-stall cell propagating are computedfrom time series picked by the pressure probes on a cross section. Self-relation calculatingfiltered the random noise of the pressure history data. The exciting load on rotor is computed byintegral of filtered pressure signal along rotor circumference. By Prohl-Myklestad method, dynamicalequations of rotor system are obtained. The dynamical response of rotor system is resolved by usingMatlab system. Further more, the situation of more than one of stall cells is discussed. Two casesrespectively from the natural gas compressor of some fertilizer plant and the CO_2 compressor ofsome nitrogenous fertilizer plant demonstrate that both methods of calculating the load exerted onrotor by pressure fluctuation and resolving the dynamic response of rotor are available and thecharacteristics of frequency spectrum of rotating stall are correct.

Experimental investigation of dynamic stall flow control using a microsecond-pulsed plasma actuator

To alleviate the performance deterioration caused by dynamic stall of a wind turbine airfoil,the flow control by a microsecond-pulsed dielectric barrier discharge(MP-DBD) actuator on the dynamic stall of a periodically pitching NACA0012 airfoil was investigated experimentally.Unsteady pressure measurements with high temporal accuracy were employed in this study,and the unsteady characteristics of the boundary layer were investigated by wavelet packet analysis and the moving root mean square method based on the acquired pressure.The experimental Mach number was 0.2,and the chord-based Reynolds number was 870 000.The dimensionless actuation frequencies F+ were chosen to be 0.5,1,2,and 3,respectively.For the light dynamic regime,the MP-DBD plasma actuator plays the role of suppressing flow separation from the trial edge and accelerating the flow reattachment due to the high-momentum freestream flow being entrained into the boundary layer.Meanwhile,actuation effects were promoted with the increasing dimensionless actuation frequency F+.The control effects of the deep dynamic stall were to delay the onset and reduce the strength of the dynamic stall vortex due to the accumulating vorticity near the leading edge being removed by the induced coherent vortex structures.The laminar fluctuation and Kelvin-Helmholtz(K-H) instabilities of transition and relaminarization were also mitigated by the MP-DBD actuation,and the alleviated K-H rolls led to the delay of the transition onset and earlier laminar reattachment,which improved the hysteresis effect of the dynamic stall.For the controlled cases of F+=2,and F+=3,the laminar fluctuation was replaced by relatively low frequency band disturbances corresponding to the harmonic responses of the MP-DBD actuation frequency.

Flow Field Improvement by Bowed Stator Stages in a Compressor with Different Axial Gaps Under Near Stall Condition

The outlet flow fields of a low-speed repeating-stage compressor with bowed stator stages are measured with five-hole probe under the near stall condition when the rotor/stator axial gap varies. The performances of the straight stator stages are investigated and compared to those of the bowed stator stages. The results show that using bowed stator stages could alleviate the flow separation at both upper and low corners of the suction surface and the endwalls, and decrease the losses along the flow passage as well as the outlet flow angle. As the rotor/stator axial gap decreases, although the diffusion capacity of the compressor increases obviously, the outlet flow field in the straight stator stages deteriorates quickly. By contrast, little changes occur in the bowed stator stages, indicating that as the rotor/stator axial gap decreases, improved performance is achieved in the bowed stator stages.

CHARACTERISTICS OF FAN STALLING BASED ON CORRELATED DIMENSIONS

Different from the previous qualitative analysis of linear systems in time and frequency domains, the method for describing nonlinear systems quantitatively is proposed based on correlated dimensions. Nonlinear dynamics theory is used to analyze the pressure data of a contrarotating axial flow fan. The delay time is 18 and the embedded dimension varies from 1 to 25 through phase-space reconstruction. In addition, the correlated dimensions are calculated before and after stalling. The results show that the correlated dimensions drop from 1. 428 before stalling to 1. 198 after stalling, so they are sensitive to the stalling signal of the fan and can be used as a characteristic quantity for the judging of the fan stalling.

SIMULATION OF ACTIVE CONTROL OF ROTATING STALL IN AXIAL COMPRESSOR

A theoretical system has been developed to simulate the control of rotating stall. Circulating disturbance was appended to the system to suppress the developing of rotating wave. The control system has slight influence on the systems critical B coefficient B cr . Self adaptive sector has been inserted into the proportion feed back control system to fit the nonlinear compressor system.

Stall Flutter Analysis of High-Aspect-Ratio Composite Wing

The stall flutter characters of high-aspect-ratio composite wing are investigated, and the effects of structure geometric nonlinearity and stiffness couple created by composite anisotropy on them also are discussed. Firstly, the high-aspect-ratio wing is modeled as a composite thin-walled closed section Euler beam whose displacement and rotation both could be of finite value, and the nonlinear dynamic equations is build up on it with all the effects of geometric nonlinearity, aerodynamic nonlinearity and anisotropy of material being considered. Then vibration equations are deduced through perturbing the dynamic equations at wing＇s equilibrium position, and coupled with unsteady stall aerodynamic model and ONERA model, to obtain the nonlinear stall flutter analysis equations of wing. Finally, the flutter stabilities with various wind speeds are determined by the harmonic balance method. With several exampies, the validity of the stall flutter model is proved, and the significant effects of geometric nonlinearity on the stall flutter various characters as wall as the effects of ply angle on the stall flutter speed and frequency also are discussed.

Wet Compression Performance of a Transonic Compressor Rotor at its Near Stall Point

In order to study the effects of wet compression on a transonic compressor,a full 3-D steady numerical simulation was carried out under varying conditions.Different injected water flow rates and droplet diameters were considered.The effect of wet compression on the shock,separated flow,pressure ratio,and efficiency was investigated.Additionally,the effect of wet compression on the tip clearance when the compressor runs in the near-stall and stall situations was emphasized.Analysis of the results shows that the range of stable operation is extended,and that the pressure ratio and inlet air flow rate are also increased at the near-stall point.In addition,it seems that there is an optimum size of the droplet diameter.

Dynamic stall control over an airfoil by NS-DBD actuation

The wind tunnel test was conducted with an NACA 0012 airfoil to explore the flow control effects on airfoil dynamic stall by NS-DBD plasma actuation. Firstly, light and deep dynamic stall states were set, based on the static stall characteristics of airfoil at a Reynolds number of 5.8 × 105. Then, the flow control effect of NS-DBD on dynamic stall was studied and the influence law of three typical reduced frequencies (k = 0.05, k = 0.05, and k = 0.15) was examined at various dimensionless actuation frequencies (F+ = 1, F+ = 2, and F+ = 3). For both light and deep dynamic stall states, NS-DBD had almost no effect on upstroke. However, the lift coefficients on downstroke were increased significantly and the flow control effect at F+ = 1 is the best. The flow control effect of the light stall state is more obvious than that of deep stall state under the same actuation conditions. For the same stall state, with the reduced frequency increasing, the control effect became worse. Based on the in being principles of flow separation control by NS-DBD, the mechanism of dynamic stall control was discussed and the influence of reduced frequency on the dynamic flow control was analyzed. Different from the static airfoil flow separation control, the separated angle of leading-edge shear layer for the airfoil in dynamic stall state is larger and flow control with dynamic oscillation is more difficult. The separated angle is closely related to the effective angle of attack, so the effect of dynamic stall control is greatly dependent on the history of angles of attack.

Experimental study on dynamic stall control based on AC-DBD actuation

To explore AC-DBD's ability in controlling dynamic stall,a practical SC-1095 airfoil of a helicopter was selected,and systematic wind tunnel experiments were carried out through direct aerodynamic measurements.The effectiveness of dynamic stall control under steady and unsteady actuation is verified.The influence of parameters such as constant actuation voltage,pulsed actuation voltage,pulsed actuation frequency and duty ratio on dynamic stall control effect is studied under the flow condition of k=0.15 above the airfoil,and the corresponding control mechanism is discussed.Steady actuation can effectively reduce the hysteresis loop area of dynamic lift,and control the peak drag and moment coefficient.For unsteady actuation,there is an optimal duty ratio DC=50%,which has the best effect in improving the lift and drag characteristics,and there is a threshold of pulsed actuation voltage in dynamic stall control.The optimal dimensionless frequency will not be found;different F+have different control advantages in different aerodynamic coefficients of different pitching stages.Unsteady actuation has obvious control advantages in improving the lift-drag characteristics and hysteresis,while steady actuation can better control the large nose-down moment.

Modeling three-dimensional dynamic stall

The dynamic stall process in three-dimensional （3D） cases on a rectangular wing undergoing a constant rate ramp-up motion is introduced to provide a qualitative analysis about the onset and development of the stall phenomenon. Based on the enhanced understanding of the mechanism of dynamic stalls, a 3D dynamic stall model is constructed with the emphasis of the onset, the growth, and the convection of the dynamic stall vortex on the 3D wing surface. The results show that this engineering dynamic stall model can simulate the 3D unsteady aerodynamic performance appropriately.

Active Stall Control System on NACA0012 by Using Synthetic Jet Actuator

Flow separation is typically an undesirable phenomenon, and boundary layer control is an important technique for the separation problems on airfoils. The synthetic jet actuator is considered as a promising candidate for flow control applications because of its compact nature and ability to generate momentum without the need for fluidic plumbing. In the present study, an active separation control system using synthetic jets is proposed and practically applied to the stall control of the NACA0012 airfoil in a wind tunnel test. In our proposed system, the flow conditions (stalled or unstalled) can be judged by calculating from two static pressure holes on the airfoil upper surface alone. The experimental results indicate that the maximum lift coefficient increases by 11% and the stall angle rises by 4°in contrast to the case under no control. It is confirmed that our proposed system can suppress the stall on the NACA0012 airfoil and that the aerodynamic performance of the airfoil can be enhanced. The proposed system can also be operated prior to the onset of stall. Therefore, separation control is always attained with no stall for all flow fields produced by changing the angle of attack that were examined.

Causal mechanism behind the stall delay by airfoil's pitching-up motion

Why the stall of an airfoil can be significantly delayed by its pitching-up motion？ Various attempts have been proposed to answer this question over the past half century, but none is satisfactory. In this letter we prove that a chain of vorticity-dynamics processes at accelerating boundary is fully responsible for the causal mechanism underlying this peculiar phenomenon. The local flow behavior is explained by a simple potential-flow model.

STUDY ON THE POST-STALL BEHAVIOR OF AN AXIAL-FLOW COMPRESSION SYSTEM

Post stall behaviors of a single stage compression system are studied theoretically and experimentally in this paper. A one dimensional nonlinear model, which is able to describe the dynamically post stall behaviors of the compression system, is applied to simulate the post stall behaviors digitally. The stall types, i.e. , rotating stall and surge, are determined. The variations of annular average parameters while the compression system goes into stall are also calculated exactly. The post stall behaviors are measured on the single stage compressor test rig. The measurement shows that rotating stall and surge appear under different conditions. On the basis of experiments, it is found that the post stall behaviors are influenced remarkably by some factors, such as rotation speeds, construction type and size of the exhaust duct. Good agreement between the simulation and experiments proves that this modeling technique is valid for simulating the post stall behaviors.

EXPERIMENTAL INVESTIGATION OF ROTATING STALL FOR A LOW-SPEED CENTRIFUGAL COMPRESSOR

Unsteady flows and rotating stall of a low-speed centrifugal compressor are investigated by measuring vaneless diffuser wall static pressure fluctuation and internal flow fields at different small flow fluxes. During the experiment, firstly the real time static pressure fluctuations on the vaneless diffuser shroud at different circumferential and radial position were acquired by high-frequency dynamic pressure transducers. Discrete Fourier transformation analysis and cross-correlation analysis were applied to the experimental results to ascertain the rotating stall beginning operation conditions and stall cells numbers and rotating speed. Secondly, the vaneless diffuser inlet flow angle distribution along diffuser width direction was acquired by single hotwire, which was compared with SENOO＇s analysis results. At last, the internal flow fields of the centrifugal compressor were investigated with a particle image velocimetry （PIV） system at different small flow fluxes. The flow field development of vaneless diffuser and blade flow passage are given at rotating stall conditions. The experiments enrich the understanding of rotating stall flow phenomenon of the low-speed centrifugal compressor and provide full experiment data for designing high performance centrifugal compressor.

Correction Model for Stall Delay on Rotating Blades

Stall delay is an important phenomenon on wind turbine blades.It makes the maximum aerodynamic load of a rotating blade much higher than the one predicted without a correction.A new stall delay model is established here based on the investigation of the flow around a rotating blade.The investigation results indicate that the shrink of separation vortex caused by centrifugal force is the major reason for the stall delay.Multiple factors related to the rotational effects are considered in the proposed model,such as rotating speed,inflow velocity,local chord length and radial position.Finally,the model are validated on the NREL PhaseⅣ blade and the results demonstrate that the accuracy of the aerodynamic prediction for the blade is significantly improved as the model is applied.

Optimum Duty Cycle of Unsteady Plasma Aerodynamic Actuation for NACA0015 Airfoil Stall Separation Control

Unsteady dielectric barrier discharge（DBD） plasma aerodynamic actuation technology is employed to suppress airfoil stall separation and the technical parameters are explored with wind tunnel experiments on an NACA0015 airfoil by measuring the surface pressure distribution of the airfoil.The performance of the DBD aerodynamic actuation for airfoil stall separation suppression is evaluated under DBD voltages from 2000 V to 4000 V and the duty cycles varied in the range of 0.1 to 1.0.It is found that higher lift coefficients and lower threshold voltages are achieved under the unsteady DBD aerodynamic actuation with the duty cycles less than 0.5as compared to that of the steady plasma actuation at the same free-stream speeds and attack angles,indicating a better flow control performance.By comparing the lift coefficients and the threshold voltages,an optimum duty cycle is determined as 0.25 by which the maximum lift coefficient and the minimum threshold voltage are obtained at the same free-stream speed and attack angle.The non-uniform DBD discharge with stronger discharge in the positive half cycle due to electrons deposition on the dielectric slabs and the suppression of opposite momentum transfer due to the intermittent discharge with cutoff of the negative half cycle are responsible for the observed optimum duty cycle.

Comparison of Rotating Stall Warning by Different Methods for Variable Speed Configurations in a Contra-Rotating Compressor

Stall in compressors can cause performance degradation and even lead to disasters.These unacceptable consequences can be avoided by timely monitoring stall inception and taking effective measures.This paper focused on the rotating stall warning in a low-speed axial contra-rotating compressor.Firstly,the stall disturbance characteristics under different speed configurations were analyzed.The results showed that as the speed ratio(RR)increased,the stall disturbance propagation speed based on the rear rotor speed gradually decreased.Subsequently,the standard deviation(SD)method,the cross-correlation(CC)method,and the discrete wavelet transform(DWT)method were employed to obtain the stall initiation moments of three different speed configurations.It was found that the SD and CC methods did not achieve significant stall warning results in all three speed configurations.Besides,the stall initiation moment obtained by the DWT method at RR=1.125 was one period after the stall had fully developed,which was unacceptable.Therefore,a stall warning method was developed in the present work based on the long short-term memory(LSTM)regression model.By applying the LSTM model,the predicted stall initiation moments of three speed configurations were at the 557th,518th,and 333rd revolution,which were44,2,and 74 revolutions ahead of stall onset moments,respectively.Furthermore,in scenarios where a minor disturbance preceded the stall,the stall warning effect of the LSTM was greatly improved in comparison with the aforementioned three methods.In contrast,when the pressure fluctuation before the stall was relatively small,the differences between the stall initiation moments predicted by these four methods were not significant.

Validation of Chaviaro Poulos and Hansen Stall Delay Model in the Case of Horizontal Axis Wind Turbine Operating in Yaw Conditions

Operating in natural wind field, the horizontal axis wind turbines are subject to cyclical variation of aerodynamic loads. This cyclical loads fluctuation is a result of two aerodynamic phenomenon: the first one is the advancing and retreating blade effect;the second one is related to the cyclical variation of induced velocity at the rotor plane. In these operating conditions, the correct prediction of this load variation is necessary to predict some important parameters linked to the fatigue and stability of free yawing turbines. The main objective of the present study is the evaluation of the azimuthal variation of normal force at different radial positions. To model the problem, the blade element momentum theory is used and wind turbine is supposed operate in yaw conditions. The aerodynamic coefficients are corrected using Chaviaropoulos and Hansen model to take into account the phenomenon of stall delay. A computer code was developed to obtain the numerical values and results are compared with measurements performed in the NASA Ames wind tunnel.

THE CANARD-WING INTERFERENCE AND STALL SAFETY OF LIGHT CANARD AIRCRAFT

In this article, the interference between canard and wing is discussed on the basis of wind-tunnel test, with a brief review of the development of aerodynamic configurations of AD-100 / 200 (two light canard aircraft designed by NAI) .The safety against stall of a canard is also discussed based upon flight test.The rule 'canard stall first'is introduced as an effective measure to improve safety against stall. It has been pointed out that the rule may threaten the flight safety at low-altitude. Finally,what a designer can do to remove the threat is presented.