Unsteady Aerodynamic Loads Research Articles

Numerical study on the trajectory of a long-range flexible rocket with large slenderness ratio

With the requirements of the speed, maneuverability and agility, the structure with large slenderness ratio is widely employed in the design of the modern rocket. The interaction among unsteady aerodynamic load, structural elastic response, as well as flight motion will be more significant. This paper focuses on the aeroelastic effects in the trajectory simulation of a rotary rocket with a large slenderness ratio. A calculation method based on computational fluid dynamics / computational structural dynamics / computational flight mechanics (CFD/CSD/CFM) coupling system is proposed, in which an in-house CFD/CSD strong-coupled program is connected with the six-degree-of-freedom flight mechanics' equations. The CFD/CSD coupling subsystem as well as CFD/CFM coupling subsystem are validated by aeroelastic dynamic stability analysis of AGARD wing and trajectory simulation of a spinning projectile, respectively. Then, the coupling algorithm is performed on a long-range trajectory simulation of an uncontrolled rotary flexible rocket. The unsteady aerodynamic loads, structural deformation, flight motions, as well as the varying thrust direction are obtained and analyzed. Through the Monte Carlo Experiment, the landing points dispersion of the elastic rocket is also calculated and compared with the rigid case. The results show that the aeroelastic effect can decrease the range and reduce the landing accuracy of the rocket. The proposed method can provide technical support for the correction of the firing table and the design of the control system of the rocket.

A Numerical Study of a Rotor Induced Flow Based on a Finite-State Dynamic Wake Model.

A Helicopter rotor undergoes unsteady aerodynamic loads ruled by the aeroelastic coupling between the elastic blades and the dynamic wake induced by rotary wings. Modeling the dynamic interaction between the structural and aerodynamic fields is a key point to understand aeroelastic phenomena associated with rotor stability, flow induced vibration and noise generation, among others. In this study, we address the Generalized Dynamic Wake Model, which describes the inflow velocity field at the rotor disk as a superposition of a finite number of induced flow states. It is a mature model that has been validated based on experimental data and numerically investigated from an eigenvalue problem formulation, whose eigenvalues and eigenvectors provide a deeper insight on the dynamic wake behavior. The paper extends the results presented in the literature to date in order to support physical interpretation of inflow states drawn from the finite-state wake model for flight conditions varying from hover to edgewise flight. The discussion of the wake model mathematical formulation is also oriented towards practical engineering applications to fill a gap in the literature.

Three-Dimensional Numerical Investigation of Hypersonic Projectile Launched by Railgun on Transitional Ballistics

Supersonic accelerators use sabots to suppress bore balloting. However, sabot separation induces flight deviation in projectiles owing to their interactions with fluids and shock waves. A seamless calculation of the acceleration phase in the tube and the full separation process using 3-D computational fluid dynamics coupled with rigid body dynamics is presented in this study to investigate the shock-wave interactions around a railgun-launched projectile. The railgun acceleration induced a normal shock wave propagating ahead of the projectile; when the projectile reached the tube end, a substantial expansion at the muzzle generated two shock waves, namely, a spherical precursor shock wave and a Mach disk, outside the tube. Aerodynamic forces on the projectile/sabot decreased to almost zero just after the tube exit, with the ambient fluid around the tube being faster than the projectile by the substantial expansion. After exiting, the projectile penetrated the precursor shock wave at ; this increased the aerodynamic forces acting on the projectile, initiated sabot separation, and generated multiple shock-wave interactions, which induce unsteady aerodynamic loads on the projectile.

Delamination effects on the unsteady aero-elastic behavior of composite wing by modal analysis

This article deals with the investigation of delamination effects on the flutter behavior of a composite wing subjected to unsteady aerodynamic loads by providing an integrated procedure using exact methods. For this purpose, an undamaged wing structure is modeled as a cantilevered Bernoulli–Euler beam with coupled bending–torsion and delaminated wing structure is modeled as three interconnected intact beams. The modal analysis of the intact and delaminated beams is performed and mode shapes are obtained based on the development of the dynamic stiffness matrix, a computer code and an iterative method. The fundamental modes of the intact and delaminated beam derived from the free vibration analysis are used in Galerkin’s method and the flutter and divergence speeds are obtained with aerodynamic forces applied. In fact, modified shape functions were developed to reflect the delamination effect in the aeroelastic analysis. Also, for the unsteady aeroelastic analysis, an iterative algorithm is developed. It was found that the delamination location and its length can have a positive or a negative influence on the flutter behavior of the wing which depends on the fiber angle.

Unsteady aero-elastic analysis of a composite wing containing an edge crack

This paper deals with the investigation of the influence of edge surface crack on the flutter behavior of unidirectional fiber-reinforced composite wing subjected to unsteady aerodynamic loads. The cracked wing is modeled as a two-interconnected Euler–Bernoulli beam at the crack location, where the related crack is modeled using continuity conditions and the local flexibility matrix concept. The components of this matrix are extracted from the theory of linear fracture mechanics. The formulation takes into account the effects of bending-torsion coupling due to the unbalanced laminates and offset of the center of gravity inherent to composite structures. The fundamental modes of intact and cracked cantilevered beams, derived from the free vibration analysis by the dynamic stiffness matrix, are used in Galerkin's method. The modified shape functions are developed to reflect the crack effect in the aeroelastic analysis, wherein an unsteady aerodynamic model based on strip theory is employed. By a combination of structure and aerodynamic formulations and using the modified mode shapes and applying a new iteration algorithm, the flutter boundary is determined. Also, the procedure has been evaluated by solving the number of problems available in the literature. Then effects of various parameters, such as the fiber angle, the crack location, the crack length, and the aerodynamic model, on the flutter boundary are analyzed and a detailed discussion will be addressed. It was found that these parameters can have a positive or negative influence on the flutter behavior of the structure.

An integrated measurement approach for the determination of the aerodynamic loads and structural motion for unsteady airfoils

The structural motion and unsteady aerodynamic loads of a pitching airfoil model that features an actuated trailing edge flap are determined experimentally using a single measurement and data processing system. This integrated approach provides an alternative to the coordinated use of multiple measurement systems for simultaneous position and flow field measurements in large-scale fluid–structure interaction experiments. The measurements in this study are performed with a robotic PIV system using Lagrangian particle tracking. Flow field measurements are obtained by seeding the flow with helium-filled soap bubbles, while the structural measurements are performed by tracking fiducial markers on the model surface. The unsteady position and flap deflection of the airfoil model are determined from the marker tracking data by fitting a rigid body model, that accounts for the motion degrees of freedom of the airfoil model, to the measurements. For the determination of the unsteady aerodynamic loads (lift and pitching moment) from the flow field measurements, two different approaches are evaluated, that are both based on unsteady potential flow and thin airfoil theory. These methods facilitate an efficient non-intrusive load determination on unsteady airfoils and produce results that are in good agreement with reference measurements from pressure transducers.

Experimental investigation of wind turbine wake and load dynamics during yaw maneuvers

Abstract. This article investigates the far wake response of a yawing upstream wind turbine and its impact on the global load variation in a downstream wind turbine. In order to represent misalignment and realignment scenarios, the upstream wind turbine was subjected to positive and negative yaw maneuvers. Yaw maneuvers could be used to voluntarily misalign wind turbines when wake steering control is targeted. The aim of this wind farm control strategy is to optimize the overall production of the wind farm and possibly its lifetime, by mitigating wake interactions. While wake flow and wind turbine load modifications during yaw maneuvers are usually described by quasi-static approaches, the present study aims at quantifying the main transient characteristics of these phenomena. Wind tunnel experiments were conducted in three different configurations, varying both scaling and flow conditions, in which the yaw maneuver was reproduced in a homogeneous turbulent flow at two different scales and in a more realistic flow such as a modeled atmospheric boundary layer. The effects of yaw control on the wake deviation were investigated by the use of stereo particle imaging velocimetry while the load variation on a downstream wind turbine was measured through an unsteady aerodynamic load balance. While overall results show a nondependence of the wake and load dynamics on the flow conditions and Reynolds scales, they highlight an influence of the yaw maneuver direction on their temporal dynamics.

Modeling dynamic loads on oscillating airfoils with emphasis on dynamic stall vortices

AbstractWe present a modified version of the ONERA dynamic stall model for improving the prediction of the unsteady forces and load overshoots generated by the shedding of dynamic stall vortices. The modifications include modeling the chord‐axis forces instead of the wind‐axis forces used originally. A novel approach for defining the onset of a dynamic stall is based on the behavior of the chordwise force without correlating the onset empirically. Overshoots in the unsteady aerodynamic loads caused by vortex shedding are modeled by sine‐shaped functions added to the normal force and moment. The onset and duration of these pulses are empirically described in the time domain for convenient use in time‐marching simulations. The modified dynamic stall model is calibrated using a genetic algorithm and compared to experimental data of different airfoils relevant to wind turbine applications. The results show an excellent correlation with the experimental data, particularly in deep dynamic stall, which are characterized by large fluctuations in the aerodynamic loads.

Dynamic Response of Offshore Wind Turbine on 3×3 Barge Array Floating Platform under Extreme Sea Conditions

Offshore wind farm construction is nowadays state of the art in the wind power generation technology. However, deep water areas with huge amount of wind energy require innovative floating platforms to arrange and install wind turbines in order to harness wind energy and generate electricity. The conventional floating offshore wind turbine system is typically in the state of force imbalance due to the unique sway characteristics caused by the unfixed foundation and the high center of gravity of the platform. Therefore, a floating wind farm for 3×3 barge array platforms with shared mooring system is presented here to increase stability for floating platform. The NREL 5 MW wind turbine and ITI Energy barge reference model is taken as a basis for this work. Furthermore, the unsteady aerodynamic load solution model of the floating wind turbine is established considering the tip loss, hub loss and dynamic stall correction based on the blade element momentum (BEM) theory. The second development of AQWA is realized by FORTRAN programming language, and aerodynamic — hydrodynamic — Mooring coupled dynamics model is established to realize the algorithm solution of the model. Finally, the 6 degrees of freedom (DOF) dynamic response of single barge platform and barge array under extreme sea condition considering the coupling effect of wind and wave were observed and investigated in detail. The research results validate the feasibility of establishing barge array floating wind farm, and provide theoretical basis for further research on new floating wind farm.

Controlling dynamic stall using vortex generators on a wind turbine airfoil

Vortex generators (VGs) have proven their capabilities in wind turbine applications to delay stall in steady flow conditions. However, their behaviour in unsteady conditions is insufficiently understood. This paper presents an experimental study that demonstrates the effect of VGs in unsteady flow, including controlling and suppressing the dynamic stall process. An airfoil, particularly designed for a vertical-axis wind turbine, has been tested in a wind tunnel in steady flow and unsteady flow caused by a sinusoidal pitching motion. The steady and unsteady pressure distributions, lift, drag and moment were measured for a range of cases. The cases vary in motion (mean angle of attack, frequency, amplitude) and VG configuration. VGs have shown to delay or even suppress dynamic stall depending on the VG configuration, with particularly important factors being VG height and VG mounting position. The VGs promote a later dynamic stall onset and reduce the hysteresis loop. As soon as the VG’s effectiveness vanishes, the configurations with VGs show a severe loss in normal coefficient, larger than in the case of the clear airfoil. However, the flow reattaches quicker and the airfoil recovers easier from the deep-stall conditions. The experimental results demonstrate that the use of VGs significantly changes the unsteady aerodynamic loads. This experimental database can serve for validation purposes to evaluate whether current modelling strategies in unsteady conditions are sufficient for blades equipped with VGs.

Nonlinear Longitudinal Aerodynamic Analysis at Low Speed Delta Jet Fighter

This study examines the application of the bifurcation theory to the reduced-order of longitudinal flight dynamic systems. An investigation of steady and unsteady longitudinal aerodynamic coefficients is carried out, including an extended range of pitch up and down reduced frequencies from 0 - 0.06 values. An aircraft with a general delta wing with a 1.5 aspect ratio is chosen for the present analysis. Different elevator control deflection from -28.6 to 0 degrees is used to determine the equilibrium points α and q. The unsteady aerodynamic load results increase the requirements for control power as well as thrust vectoring during pitch up than steady longitudinal one. The flow unsteadiness effect improves the vehicle stability during pitch up corresponding a loosing of it during pitch down. A positive pitch rate is required to point the nose of the fighter. This improvement in pointing will help in the close maneuvering missions.

Load control and unsteady aerodynamics for floating wind turbines

Unlike fixed-base offshore wind turbine, the soft floating platform introduces 6 more degrees of freedom of motions to the floating offshore wind turbine. This may cause much more complex inflow environment to the wind turbine rotors compared with fixed-base wind turbine. The wind seen locally on the blade changes due to the motions of the floating wind turbine platform which has a direct impact on the aerodynamic condition on the blade such as the angle of attack and the inflow velocity. Such unsteady aerodynamic effects may lead to high fluctuation of the loads and power output. The present work aims to study the high unsteady aerodynamic performance of the floating wind turbine under platform surge motion. The unsteady aerodynamic loads are predicted with a lifting surface method with a free wake model. A preview predict control algorithm is used as the pitch control strategy. A full scale U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) 5 MW floating wind turbine is chosen as the subject of the present study. The unsteady aerodynamic performance and instabilities have been discussed in detail under prescribed platform surge motions with different control targets. Both minimizing the power output and rotor thrust fluctuation are set as the control objectives respectively. The theory analysis and the simulation results indicate that the blade pitch control can effectively alleviate the variation of the rotor thrust under platform surge motions. Larger amplitude of the variation of blade pitch is needed to alleviate the variation of the wind turbine power and this leads to high rotor thrust fluctuation. It is also shown that negative damping can be achieved during the blade pitch control process and may lead the floating platform wind turbine system into unstable condition.

Dynamic stall of the wind turbine airfoil and blade undergoing pitch oscillations: A comparative study

Dynamic stall significantly causes the unsteady aerodynamic loads on horizontal axis wind turbines. However, many studies are about dynamic stall of 2D airfoils rather than 3D rotating blades. The 3D dynamic stall is therefore still poorly understood and also challenging to predict accurately. This paper presents comparative analyses of dynamic stall among the 2D airfoil, 3D non-rotating blade and 3D rotating blade undergoing sinusoidal pitch oscillations. The parameters of 2 radial locations and 11 pitch conditions are also studied. All 3D aerodynamic responses come from the NREL Phase VI experiment. URANS simulations are used to predict the 2D aerodynamic responses of NREL S809 airfoil. Rotational augmentation is found to make the key difference between 2D airfoil flow and 3D blade flow. Rotational augmentation effectively suppresses the extension of separated flow during the upstroke process, and significantly accelerates the flow reattachment during the downstroke process. The onset of dynamic stall is therefore delayed with the maximum lift coefficient increased by 46%. The aerodynamic hysteresis intensity is also greatly reduced by 61%. Increasing the mean angle of attack (AOA), AOA amplitude and reduced frequency can further delay the onset of dynamic stall. On the other hand, rotational augmentation may unexpectedly produce a negative aerodynamic pitch damping and cause stall flutter on the inboard blade. Increasing AOA amplitude could reduce this negative damping and improve the torsional aeroelastic stability. This work might deepen the understanding of 3D dynamic stall with rotational augmentation on wind turbines.

Non-intrusive determination of the unsteady surface pressure and aerodynamic loads on a pitching airfoil

The unsteady surface pressure distribution and aerodynamic loads on a pitching airfoil are determined non-intrusively using PIV measurements. An experimental test case is considered where the flow around the airfoil is mostly attached while the unsteady effects on the aerodynamic loads are significant. The surface pressure is calculated from the flow velocity measurements in the vicinity of the airfoil surface, that are obtained with a robotic PIV system, by using relations from unsteady potential flow and thin airfoil theory. The proposed approach is a robust and computationally efficient approach to obtain non-intrusive measurements of the unsteady surface pressure distribution and the aerodynamic loads, that are in good agreement with reference data from installed pressure transducer sensors.

Numerical simulation of the unsteady aerodynamic loads on the tail fin in the vortex breakdown flow

Numerical simulation of the unsteady aerodynamic loads on the tail fin in the vortex breakdown flow

Aeroelastic performance analysis of horizontal axis wind turbine (HAWT) swept blades

Over the past decades, the capacity of commercial wind turbines has risen from 50 kW with 10 to 15 m rotor diameter to 5 MW with more than 120 m rotor diameter. Conversely, increase in the rotor blade length causes substantial blade deflection in torsional, lead-lag and flap-wise directions which leads to unsteady aerodynamic load distributions. The structural safety and stability of the wind turbine will also be affected by the undesirable aerodynamic performance. It is important to analyze the aeroelastic performance of wind turbines under yaw conditions for large scale wind turbine design. Swept blade is a recent innovation to increase the power production of wind turbines while reducing the blade loads. There are insufficient studies in which the effect of yaw error on aeroelastic performance of Horizontal Axis Wind Turbine (HAWT) swept blade has been analysed yet. In this study, aeroelastic performance analysis was performed to investigate the effect of yaw angle on the aeroelastic performance of HAWT with swept blades. The same analysis was performed on HAWT unswept blade, so as to compare the results with that of HAWT swept blade. CFD analysis in ANSYS Fluent was chosen for aerodynamic analysis whereas FEA in ANSYS Static Structural was chosen for structural dynamic analysis. The aeroelastic performance analysis of wind-swept blades under yaw angles of 0°, 10°, 30° and 60° at wind speed of 10 m/s was performed. The results show that yaw error has negative effect on the aeroelastic performance of swept blade as well as unswept blade. It can be proven that swept blades have a better aeroelastic performance in terms of higher rotor power and lower deformation than unswept blades, i.e. higher stability of swept blades under yaw condition compared to unswept blades.

Numerical simulation of the unsteady aerodynamic loads on the tail fin in the vortex breakdown flow

This work presents numerical simulation results of the flow around an airbrake-tail fin system of an aircraft. In this research, airbrake location and deflection angles effects on the unsteady aero...

Analysis of rotor aerodynamic response during ramp collective pitch increase by CFD method

In order to study the aerodynamic response characteristics of the helicopter rotor during ramp collective pitch increase, the moving-embedded grid technique is employed for numerical simulation. The governing equations are modeled via Navier-Stokes equations, as well as one equation S-A turbulence model. In order to improve the precision of unsteady simulation of the rotor flowfield, the three-order scheme known as Roe-WENO scheme is employed for the spatial discretization of convective fluxes, and the implicit LU-SGS scheme is adopted for the temporal discretization. The flowfield and aerodynamic characteristics of the SA349/2 Gazelle helicopter rotor are computed for verification, and thereafter, the present method is used to simulate the transient aerodynamic response of the rotor under different collective pitch increment rates. The unsteady flowfield and aerodynamic characteristics of the rotor under ramp collective pitch increase are obtained and compared with the experimental data. The results show that the numerical method not only can accurately predict the unsteady aerodynamic loads of the rotor in steady state, but also is capable of effectively simulating the transient aerodynamic response of the rotor, characterized by overshoot and delay phenomenon, during ramp collective pitch increase. Finally, the opposite ramp decrease in collective pitch and the influence of pre-twist on aerodynamic response are analyzed. The result shows that the transient aerodynamic response of the rotor under ramp collective pitch increase and decrease present a certain of symmetry. The change in pre-twist of blades only affects the thrust coefficient in steady state, while have little influence on the transient maneuvering process of collective pitch.

Modeling and simulation of mass-actuated flexible aircraft for roll control

Moving mass control (MMC) is a flight control technique that provides several unique advantages over conventional aerodynamic control surfaces, particularly for near space vehicles. In this study, we propose an MMC strategy for roll control of low-speed flexible HALE aircraft. Two internal moving masses are used to produce rolling moment rather than conventional ailerons. The complete equations of motion of the flexible aircraft carrying moving mass are first derived in terms of quasi-coordinates. A simplified model integrated with unsteady aerodynamic loads is obtained for roll control. The effect of moving mass positions on rigid-body motion and aeroelasticity couplings are analyzed. The feasibility of the mass-actuation concept for roll control of the flexible aircraft is primarily proven. The roll control ability and aeroelastic characteristics can be improved through the reasonable design of the moving mass system. The dynamic behavior of the system, while the mass travels along the wing, is simulated, implying that the elastic motion induces the delay effect in the roll motion of the flexible model compared with the rigid model. A roll rate control system is designed and analyzed to demonstrate the feasibility of using moving masses in real-time flight control.

두 프롭 간의 이격 거리에 대한 제자리 비행 시 공기역학적 상호작용 수치 해석

Aerodynamic interactions according to the separation distance between two props were investigated using the computational fluid dynamics methodology. Numerical analysis was performed for two shapes, two-bladed props and three-bladed props. Unsteady aerodynamic load occurred due to the interaction of the two props, and the fluctuation of unsteady load increased dramatically as the separation distance decreased. The thrust was compared with the experimental results of other researchers, and as the separation distance increased, the tendency to approach the thrust of a single prop was found to be consistent. It was also found that the torque increased than that of a single prop, but the effect of the separation distance was relatively insignificant. In the comparison of the flow field, it was found that the aerodynamic interaction of the two props has an asymmetric velocity field unlike the case of a single prop.