Dynamic Inflow Models Research Articles

Performance and Loads of a Wing-Offset Compound Helicopter

Analysis of a hingeless rotor with a single wing on the retreating side for lift compounding was conducted. The goals included validation of performance and load predictions with wind tunnel test data, study of the impact of different aerodynamic inflow models, and understanding of benefits by lift compounding with a single wing on the retreating side. The three primary test cases include collective sweeps of the isolated rotor, and the rotor with the wing, at two different incidence angles. The comprehensive analysis was able to accurately predict the performance and blade structural loads of both the isolated rotor and rotor plus wing configurations. Overprediction of propulsive force leads to underprediction of lift-to-drag ratio in several cases. The normal bending moments were well captured for all cases, while the chord bending moment predictions had a phase offset from the test data, but magnitude and harmonics were captured. Comparing inflow models found that dynamic inflow and vortex wake (prescribed and free) models provided similar results. At these advance ratios, prescribed and free wake models showed almost no differences. Additionally, the vortex particle method showed an overprediction of thrust and greater rotorto-wing aerodynamic interference compared to test data. The addition of the wing on the retreating side provided dual benefits of increasing maximum lift-to-drag ratio and reduction of structural loads for a given total thrust. These effects are a result of both lift share between the rotor and wing, and lift offset, the rotor carries a roll moment to balance the wing's roll moment.

Advances and Modern Applications of Frequency-Domain Aircraft and Rotorcraft System Identification

This paper provides an overview of advances and applications of aircraft and rotorcraft flight dynamics system identification at the U.S. Army Combat Capabilities Development Command Aviation & Missile Center over the past 20 years. An overarching topic is how frequency-domain system identification has evolved to aid modeling work of modern aircraft and rotorcraft configurations. The paper starts with a brief overview of the frequency-response method for system identification. Subsequently, the paper highlights and demonstrates several new advances using flight-test data from a conventional helicopter, fixed-wing aircraft, advanced coaxial compound rotorcraft, and a small multirotor unmanned aerial vehicle. The first advancement is the use of identification results to improve simulation model fidelity. The next advancement is the Joint Input-Output Method for identification of aircraft with highly correlated inputs. Next, the paper discusses the advancement of flexible constraints, which provide the ability to directly determine physical parameters from flight-test data. The paper then discusses a methodology to identify lower-order linearized dynamic inflow models from higher-order free wake aerodynamic models. Finally, the paper discusses model stitching, which is the technique of combining or stitching together individual linear models and trim data for discrete flight conditions to produce a continuous, full flight-envelope real-time stitched simulation model.

A comparison of dynamic inflow models for the blade element momentum method

Abstract. With the increase in rotor sizes, the implementation of innovative pitch control strategies, and the first floating solutions entering the market, the importance of unsteady aerodynamic phenomena in the operation of modern offshore wind turbines has increased significantly. Including aerodynamic unsteadiness in blade element momentum (BEM) methods used to simulate wind turbine design envelopes requires specific sub-models. One of them is the dynamic inflow model, which attempts to reproduce the effects of the unsteady wake evolution on the rotor plane induction. Although several models have been proposed, the lack of a consistent and comprehensive comparison makes their relative performance in the simulation of large rotors still uncertain. More importantly, different dynamic inflow model predictions have never been compared for a standard fatigue load case, and thus it is not clear what their impact on the design loads estimated with BEM is. The present study contributes to filling these gaps by implementing all the main dynamic inflow models in a single solver and comparing their relative performance on a 220 m diameter offshore rotor design. Results are compared for simple prescribed blade pitch time histories in uniform inflow conditions first, verifying the predictions against a high-fidelity free-vortex-wake model and showing the benefit of new two-constant models. Then the effect of shed vorticity is investigated in detail, revealing its major contribution to the observed differences between BEM and free-vortex results. Finally, the simulation of a standard fatigue load case prescribing the same blade pitch and rotor speed time histories reveals that including a dynamic inflow model in BEM tends to increase the fatigue load predictions compared to a quasi-steady BEM approach, while the relative differences among the models are limited.

Applicability of Dynamic Inflow Models of HAWT in Yawed Flow Conditions

Horizontal axis wind turbines (HAWTs) experience yaw misalignments due to the physical limitations of yaw controllers and various novel active yaw controls. Moreover, the motion of floating offshore wind turbines (FOWTs) accelerates yaw misalignment. The blade element momentum (BEM) method is widely used due to its computational efficiency for the design of HAWTs. Momentum theory, the basis of BEM, assumes steady flow and uniform induction field at the disc. Those assumptions are relaxed by engineering models to capture yaw and unsteady effects. Current yaw engineering models, however, are inaccurate since they do not capture the asymmetric wake expansion effect. Dynamic inflow models have been developed for non-yawed flow. Furthermore, the AVATAR project shows that BEM using fully coupled engineering models, the current yaw, dynamic inflow and various engineering models, suffers from significant deficiencies. This purpose of this paper, therefore, is to investigate dynamic effects for yawed flow, and determine if current dynamic inflow models are applicable in yawed conditions. The Glauert’s modified momentum theory is applied to dynamic inflow models to couple the two models. Among all coupled models, Øye, Yu PWVM and Yu FWVM DIM can capture asymmetric trends. However, the results show the significant deficiencies in phase delay on the actuator disc.

Experimental analysis of the dynamic inflow effect due to coherent gusts

Abstract. The dynamic inflow effect describes the unsteady aerodynamic response to fast changes in rotor loading due to the inertia of the wake. Fast changes in turbine loading due to pitch actuation or rotor speed transients lead to load overshoots. The phenomenon is suspected to be also relevant for gust situations; however, this was never shown, and thus the actual load response is also unknown. The paper's objectives are to prove and explain the dynamic inflow effect due to gusts, and compare and subsequently improve a typical dynamic inflow engineering model to the measurements. An active grid is used to impress a 1.8 m diameter model turbine with rotor uniform gusts of the wind tunnel flow. The influence attributed to the dynamic inflow effect is isolated from the comparison of two experimental cases. Firstly, dynamic measurements of loads and radially resolved axial velocities in the rotor plane during a gust situation are performed. Secondly, corresponding quantities are linearly interpolated for the gust wind speed from lookup tables with steady operational points. Furthermore, simulations with a typical blade element momentum code and a higher-fidelity free-vortex wake model are performed. Both the experiment and higher-fidelity model show a dynamic inflow effect due to gusts in the loads and axial velocities. An amplification of induced velocities causes reduced load amplitudes. Consequently, fatigue loading would be lower. This amplification originates from wake inertia. It is influenced by the coherent gust pushed through the rotor like a turbulent box. The wake is superimposed on that coherent gust box, and thus the inertia of the wake and consequently also the flow in the rotor plane is affected. Contemporary dynamic inflow models inherently assume a constant wind velocity. They filter the induced velocity and thus cannot predict the observed amplification of the induced velocity. The commonly used Øye engineering model predicts increased gust load amplitudes and thus higher fatigue loads. With an extra filter term on the quasi-steady wind velocity, the qualitative behaviour observed experimentally and numerically can be caught. In conclusion, these new experimental findings on dynamic inflow due to gusts and improvements to the Øye model enable improvements in wind turbine design by less conservative fatigue loads.

Dynamic inflow and unsteady aerodynamics models for modal and stability analyses in OpenFAST

This work presents two aerodynamic models that are used to perform modal and stability analyses of wind turbines. Including aerodynamic effects in stability analyses is important to capture aerodynamic inertia, stiffness, and damping. The two models consist of a dynamic inflow model based on Øye’s model and an unsteady aerodynamics model based on a simplified Beddoes-Leishman dynamic stall model. Both models are written in a continuous-time state-space form, which is the form needed to perform linearizations within OpenFAST. In this article, we present the new models, perform some simple verifications against HAWC2 results, and then perform stability analyses. We investigate the contribution of the aerodynamic effects on the turbine modes, particularly the flapwise modes, which are the most affected. Because of the development presented in this work, Campbell diagrams accounting for dynamic stall and dynamic wake effects were generated for the first time using OpenFAST.

Fidelity Enhancement of a Multirotor Dynamic Inflow Model via System Identification

Multirotor analytical dynamic inflow models in the literature, such as pressure potential superposition inflow model or velocity potential superposition inflow model (VPSIM), have been shown to capture the fundamental inflow interference effects between the rotors. Some of the differences in inflow predictions seen between these analytic models and high-fidelity wake models are attributed to missing real flow effects such as wake distortion, contraction, decay, swirl, etc. As such, correction terms are needed in the analytically derived multirotor finite-state inflow models, because of the potential flow and rigid wake assumptions they are based on, in order to capture some of the missing real flow effects in them. This paper develops a systematic methodology for arriving at the needed correction terms in the VPSIM through comparisons of its inflow predictions with those of a viscous vortex particle model (VVPM). Also, a procedure is developed to assess the relative importance of individual real flow effects and the associated corrections needed for improving the overall fidelity of the VPSIM. The developed methodology is applied to the Harrington coaxial rotor using its geometric and aerodynamic data from the literature. It is shown that the addition of swirl coupling correction terms to the VPSIM significantly improves its correlations with the VVPM. Further, it is shown that the required corrections are reasonably insensitive to thrust sharing ratio conditions between the rotors.

Dynamic inflow model for a floating horizontal axis wind turbine in surge motion

Abstract. Floating offshore wind turbines may experience large surge motions, which can cause blade–vortex interaction if they are similar to or faster than the local wind speed. Previous research hypothesized that this blade–vortex interaction phenomenon represented a turbulent wake state or even a vortex ring state, rendering the actuator disc momentum theory and the blade element momentum theory invalid. This hypothesis is challenged, and we show that the actuator disc momentum theory is valid and accurate in predicting the induction at the actuator in surge, even for large and fast motions. To accomplish this, we develop a dynamic inflow model that simulates the vorticity–velocity system and the effect of motion. The model's predictions are compared to other authors' results, a semi-free-wake vortex ring model, other dynamic inflow models, and CFD simulations of an actuator disc in surge. The results show that surge motion and rotor–wake interaction do not result in a turbulent wake or vortex ring state and that the application of actuator disc momentum theory and blade element momentum theory is valid and accurate when applied correctly in an inertial reference frame. In all cases, the results show excellent agreement with the higher-fidelity simulations. The proposed dynamic inflow model includes a modified Glauert correction for highly loaded streamtubes and is accurate and simple enough to be easily implemented in most blade element momentum models.

Experimental analysis of radially resolved dynamic inflow effects due to pitch steps

Abstract. The dynamic inflow effect denotes the unsteady aerodynamic response to fast changes in rotor loading due to a gradual adaption of the wake. This does lead to load overshoots. The objective of the paper was to increase the understanding of that effect based on pitch step experiments on a 1.8 m diameter model wind turbine, which are performed in the large open jet wind tunnel of ForWind – University of Oldenburg. The flow in the rotor plane is measured with a 2D laser Doppler anemometer, and the dynamic wake induction factor transients in axial and tangential direction are extracted. Further, integral load measurements with strain gauges and hot-wire measurements in the near and close far wake are performed. The results show a clear gradual decay of the axial induction factors after a pitch step, giving the first direct experimental evidence of dynamic inflow due to pitch steps. Two engineering models are fitted to the induction factor transients to further investigate the relevant time constants of the dynamic inflow process. The radial dependency of the axial induction time constants as well as the dependency on the pitch direction is discussed. It is confirmed that the nature of the dynamic inflow decay is better described by two rather than only one time constant. The dynamic changes in wake radius are connected to the radial dependency of the axial induction transients. In conclusion, the comparative discussion of inductions, wake deployment and loads facilitate an improved physical understanding of the dynamic inflow process for wind turbines. Furthermore, these measurements provide a new detailed validation case for dynamic inflow models and other types of simulations.

Comparison of a radially resolved dynamic inflow pitch step experiment to mid-fidelity simulations and BEM

In this paper a detailed comparison of the experimental and numerical results of a scaled wind turbine model in a wind tunnel subjected to fast pitching steps leading to the so-called dynamic inflow effect is presented. We compare results of an Actuator Line LES tool, a vortex code and four engineering models, to the experiment. We perform one and two time constant model analysis of axial wake induction and investigate the overshooting of integral loads. Our results show, that the effect is captured better by the two time constant models than by the one time constant models. Also the experiment and mid-fidelity simulations are best described by a two time constant fit. We identify the best dynamic inflow model to be the 0ye model. Different possibilities for the improvement of dynamic inflow models are discussed.

Effect of platform disturbance on the performance of offshore wind turbine under pitch control

AbstractThe aerodynamic performance of offshore floating wind turbines (OFWTs) is more complicated than onshore wind turbines due to 6‐degree of freedom (DOF) motion of the floating platform. In the current study, the aerodynamic analysis of a horizontal‐axis floating offshore wind turbine is performed with the aim of studying the effects of floating platform movement on the aerodynamic characteristics of the turbine in the presence of a pitch angle control system. The National Renewable Energy Laboratory (NREL) 5‐MW offshore wind turbine is selected as the baseline wind turbine. For this sake, the unsteady blade element momentum method with dynamic stall and dynamic inflow models have been employed to obtain the unsteady aerodynamic loads. The baseline pitch angle control system is assumed to be coupled with the aerodynamic model to maintain the rated condition of the wind turbine and also to approach a closer model of wind turbine. In case of pitching motion input, the reduction of mean power coefficient for tip speed ratios (TSRs) less that 7 is expected by an amount of 16% to 20% at pitch amplitude of 2° and frequency of 0.1 Hz. For high TSRs, the trend is reverse with respect to fixed‐platform case. The mean thrust coefficient is reduced for almost all range of TSRs with maximum loss of 37%. Moreover, the mean control pitch angle that is an index of control system effort is increased. The results also represent the importance of considering the pitch control system for aerodynamic analysis of disturbed OFWT.

The need for dynamic inflow models for vertical axis wind turbines

This paper investigates the need for dynamic inflow models for vertical axis wind turbines (VAWTs). The approach is two-fold. First, dynamic inflow is realised by dynamic thrust on an actuator disk in OpenFOAM. The induction phase shift and amplitude showed a significant dependency on the streamwise location. Second, a reference turbine in surging motion is studied using an actuator line OpenFOAM model as reference and an actuator cylinder model (with and without dynamic inflow model). The Larsen and Madsen dynamic inflow model is able to capture the overall behaviour in dynamic inflow conditions, however, it may be improved in the most upwind and downwind location. This study indicates that the modelling of VAWTs in dynamic inflow conditions may be enhanced by improving the dynamic inflow models.

Experimental investigation of dynamic inflow effects with a scaled wind turbine in a controlled wind tunnel environment

Dynamic inflow is a phenomenon that leads to a load overshooting, for example by fast pitch angle changes. Established engineering models for this effect are known to still have deficits. Advanced control mechanisms for turbines like collective pitch control in order to alleviate tower loads or individual pitch control increase the need for accurate dynamic inflow models. We performed fast pitching step experiments with our MoWiTO 1.8 (Model Wind Turbine Oldenburg 1.8 m) in the WindLab wind tunnel of the University of Oldenburg and investigated the load overshooting of blade loads, torque and thrust as well as velocity transients in the near wake. Dynamic inflow phenomena were clearly visible and their non-dimensional time constants matched existing full scale and lab size experiments. The transient process of the different measured loads and flow in the near wake is disussed in the paper to gain a deeper understanding of the underlying physical phenomena.

Modal analysis of finite-state dynamic inflow for rotary wing systems

In this paper, the finite-state helicopter rotor inflow modes have been studied based on eigenanalysis. The inflow velocity mode shapes with node lines have been displayed with various skew angles. The eigenvalues are highly coupled especially for higher skew angles, and the mode shapes change significantly for different angles. The changing of eigenvalues with different harmonic numbers is also exhibited in the tables for axial flow of both the Peters–He and Morillo dynamic inflow models. An easy way to estimate the eigenvalues of the Peters–He inflow model is also established.

Validation of engineering dynamic inflow models by experimental and numerical approaches

The state of the art engineering dynamic inflow models of Pitt-Peters, Øye and ECN have been used to correct Blade Element Momentum theory for unsteady load prediction of a wind turbine for two decades. However, their accuracy is unknown. This paper is to benchmark the performance of these engineering models by experimental and numerical methods. The experimental load and flow measurements of an unsteady actuator disc were performed in the Open Jet Facility at Delft University of Technology. The unsteady load was generated by a ramp-type variation of porosity of the disc. A Reynolds Averaged Navier-Stokes (RANS) model, a Free Wake Vortex Ring (FWVR) model and a Vortex Tube Model (VTM) simulate the same transient load changes. The velocity field obtained from the experimental and numerical methods are compared with the engineering dynamic inflow models. Velocity comparison aft the disc between the experimental and numerical methods shows the numerical models of RANS and FWVR model are capable to predict the velocity transient behaviour during transient disc loading. Velocity comparison at the disc between the engineering models and the numerical methods further shows that the engineering models predict much faster velocity decay, which implies the need for more advanced or better tuned dynamic inflow models.

Verifying the Blade Element Momentum Method in unsteady, radially varied, axisymmetric loading using a vortex ring model

AbstractAlthough the Blade Element Momentum method has been derived for the steady conditions, it is used for unsteady conditions by using corrections of engineering dynamic inflow models. Its applicability in these cases is not yet fully verified. In this paper, the validity of the assumptions of quasi‐steady state and annuli independence of the blade element momentum theory for unsteady, radially varied, axi‐symmetric load cases is investigated. Firstly, a free wake model that combines a vortex ring model with a semi‐infinite cylindrical vortex tube was developed and applied to an actuator disc in three load cases: (i) steady uniform and radially varied, (ii) two types of unsteady uniform load and (iii) unsteady radially varied load. Results from the three cases were compared with Momentum Theory and also with two widely used engineering dynamic inflow models—the Pitt‐Peters and the Øye for the unsteady load cases. For unsteady load, the free wake vortex ring model predicts different hysteresis loops of the velocity at the disc or local annuli, and different aerodynamic work from the engineering dynamic inflow models. Given that the free wake vortex ring model is more physically representative, the results indicate that the engineering dynamic inflow models should be improved for unsteady loaded rotor, especially for radially varied unsteady loads. © 2016 The Authors. Wind Energy Published by John Wiley & Sons, Ltd.

Effect of wind turbine surge motion on rotor thrust and induced velocity

ABSTRACTOffshore wind turbines on floating platforms will experience larger motions than comparable bottom fixed wind turbines—for which the majority of industry standard design codes have been developed and validated. In this paper, the effect of a periodic surge motion on the integrated loads and induced velocity on a wind turbine rotor is investigated. Specifically, the performance of blade element momentum theory with a quasisteady wake as well as two widely used engineering dynamic inflow models is evaluated. A moving actuator disc model is used as reference, since the dynamics associated with the wake will be inherently included in the solution of the associated fluid dynamic problem. Through analysis of integrated rotor loads, induced velocities and aerodynamic damping, it is concluded that typical surge motions are sufficiently slow to not affect the wake dynamics predicted by engineering models significantly. Copyright © 2012 John Wiley & Sons, Ltd.

Calculation of Unsteady Load on Wind Turbine Blades with Nonzero Yaw Error Using Dynamic Inflow Models

Calculation of Unsteady Load on Wind Turbine Blades with Nonzero Yaw Error Using Dynamic Inflow Models

Hingeless-Rotor Aeromechanical Stability in Axial and Forward Flight With Wake Dynamics

A finite-state wake model is used to investigate aeromechanical stability of hingeless-rotor helicopters in the ground-contact, hover and trimmed-night conditions. The investigation covers three items: (1) the convergence of the damping with increasing number of wake harmonics for the lag regressing, and body pitch and roll modes; (2) a parametric study of the damping over a range of thrust level, advance ratio and number of blades; and (3) correlations, primarily with the damping and frequency measurements of these lag and body modes. The convergence and parametric studies are conducted in the hover and trimmed-flight conditions; they include predictions from the widely used dynamic inflow model. The correlations are conducted in the ground-contact conditions and include predictions from the dynamic inflow and vortex models; recently, this vortex model is proposed for the axial-flight conditions and is used to investigate the coupled free vibrations of rotor flapping and body modes. The convergence and parametric studies show that a finite-state wake model that goes well beyond the dynamic inflow model is required for fairly converged damping, Moreover, the correlations from the finite-state wake, dynamic inflow and vortex models are generally satisfactory.

Theoretical Flap‐Lag Damping with Various Dynamic Inflow Models

Theoretical Flap‐Lag Damping with Various Dynamic Inflow Models