3,152 research outputs found
Predicting wind turbine blade loads using vorticity transport and RANS methodologies
Two computational methods, one based on the solution of the vorticity transport equa-
tion, and a second based on the solution of the Reynolds-Averaged Navier-Stokes equa-
tions, have been used to simulate the aerodynamic performance of a horizontal axis wind
turbine. Comparisons have been made against data obtained during Phase VI of the
NREL Unsteady Aerodynamics Experimental and against existing numerical data for a
range of wind conditions. The Reynolds-Averaged Navier-Stokes method demonstrates
the potential to predict accurately the flow around the blades and the distribution of aero-
dynamic loads developed on them. The Vorticity Transport Model possesses a consid-
erable advantage in those situtations where the accurate, but computationally efficient,
modelling of the structure of the wake and the associated induced velocity is critical,
but where the prediction of blade loads can be achieved with sufficient accuracy using
a lifting-line model augmented by incorporating a semi-empirical stall delay model. The
largest benefits can be extracted when the two methods are used to complement each
other in order to understand better the physical mechanisms governing the aerodynamic
performance of wind turbines
Aerodynamic Response of a Hovering Rotor to Ramp Changes in Pitch Input
Under transient conditions, a helicopter rotor generates a complex, time-dependent pattern of shed and
trailed vorticity in its wake that has profound eects on its loading. To examine these eects, the response
of a two-bladed hovering rotor to a ramp change in collective pitch is investigated using three dierent
computational approaches. Solutions obtained using a Compressible Reynolds Averaged Navier{Stokes ap-
proach are compared to results obtained from lifting-line theory coupled to an Eulerian Vorticity Transport
Model, and from a simple single-state dynamic in
ow model. The dierent numerical approaches yield
very similar predictions of the thrust response of the rotor to ramp changes in collective pitch, as long as
the ramp rates are small. This suggests that the basic underlying
ow physics is properly represented by all
the approaches. For more rapid ramp rates, an additional delay in the aerodynamic response of the rotor,
that is related to the nite extent of the wake during its early history, is predicted by the Navier{Stokes
and Vorticity Transport approaches. Even though the evolution of the wake of the rotor is strongly three
dimensional and highly unsteady, the predictions of the Navier{Stokes and lifting-line models agree very
closely as long as the blades of the rotor do not stall. In the pre-stall regime, a quasi two-dimensional
representation of the blade aerodynamics thus appears adequate for predicting the performance of such
systems even under highly transient conditions. When
ow separation occurs, the resulting three dimen-
sionality of the blade aerodynamics forces the predictions of the Navier{Stokes and lifting-line approaches
to diverge, however. The characterization of the wake interactions and stall propagation mechanisms that
are presented in this study oers some insight into the fundamental
uid dynamic mechanisms that govern
the transient aerodynamic response of a rotor to control inputs, and provides some quantication of the
limits of applicability of some popular current approaches to rotor aerodynamic analysis
Wake dynamics and rotor-fuselage aerodynamic interactions
The unsteady loads experienced by a helicopter are known to be strongly influenced by aerodynamic interactions between the rotor and fuselage; these unsteady loads can lead to deficiencies in handling qualities and unacceptable vibratory characteristics of the rotorcraft. This work uses a vorticity-based computational model to study the governing processes that underpin this aerodynamic interaction and aims to provide greater understanding of the wake dynamics in the presence of a fuselage, as well as an appreciation of how the geometry of the wake affects the loading on the fuselage. The well-known experiments using NASA's ROBIN fuselage are used to assess the accuracy of the computations. Comparisons of calculations against results from smoke visualization experiments are used to demonstrate the ability of the model to reproduce accurately the geometry of the rotor wake, and comparisons with inflow data from the experiments show the method to capture well the velocity field near to the rotor. The fuselage model is able to predict accurately the unsteady fuselage loading that is induced by blade passage and also by the inviscid interaction between the main rotor wake and fuselage
A rational approach to comparing the performance of coaxial and conventional rotors
The merit, in terms of its efficiency and performance, of the twin, contrarotating coaxial rotor configuration over the more conventional single rotor system has long been a point of contention. Previously published comparisons yield seemingly inconsistent and conflicting conclusions. In this paper, the basis for a fair, like-for-like comparison of the performance of coaxial and single rotor systems is discussed. A comparison between experimentally measured data and numerical predictions of rotor performance obtained using the vorticity transport model shows that a computational approach can be used reliably to decompose the power consumption into induced and profile constituents. These comparisons show that a somewhat stronger similarity in geometry needs to be enforced between the two types of rotor system than previously suggested in order that the systems be directly comparable. If the equivalent single rotor system is constructed to have the same disk area, blade geometry, and total number of blades as that of the coaxial rotor, then the geometric differences between the two systems are confined to the defining characteristics of the two types of rotor system, in other words to the vertical separation between the rotor blades and their relative direction of rotation. The differences in aerodynamic performance between a coaxial rotor and an equivalent single rotor defined in this way then arise solely as a result of the differences in the detailed interaction between the blades and their wakes that arise within the two types of system. Using this form of comparison, the articulated coaxial system is shown to consume marginally less induced power than the equivalent single rotor system. The difference is small enough, however, to be obscured if the profile drag of the blades is overtly sensitive to operating condition, as for instance might be the case at low Reynolds number
Main rotor-tail rotor intraction and its implications for helicopter directional control
Aerodynamic interference between the main and tail rotor can have a strong negative influence on the flight mechanics of a conventional helicopter. Significant unsteadiness in the tail rotor loading is encountered under certain flight conditions, but the character of the unsteadiness can depend on the direction of rotation of the tail rotor. Numerical simulations, using Brown's vorticity transport model, of the aerodynamic interaction between the main and tail rotors of a helicopter are presented for a range of forward and lateral flight trajectories. Distinct differences are predicted in the behavior of the system in left and right sideward flight that are consistent with flight experience that the greatest fluctuations in loading or control input are required in left sideways flight (for a counterclockwise rotating main rotor). These fluctuations are generally more extreme for a system with tail rotor rotating top-forward than top-aft. Differences are also exposed in the character of the lateral excitation of the system as forward flight speed is varied. The observed behavior appears to originate in the disruption of the tail rotor wake that is induced by its entrainment into the wake of the main rotor. The extent of the disruption is dependent on flight condition, and the unsteadiness of the process depends on the direction of rotation of the tail rotor. In intermediate-speed forward flight and right sideward flight, the free stream delays the entrainment of the tail rotor wake far enough downstream for the perturbations to the rotor loading to be slight. Conversely, in left sideward and quartering flight, the free stream confines the entrainment process close to the rotors, where it causes significant unsteadiness in the loads produced by the system
Simulating wind turbine interactions using the vorticity transport equations
The aerodynamic interactions that can occur within a wind farm result in the constituent turbines generating a lower power output than would be possible if each of the turbines were operated in isolation. Tightening of the constraints on the siting of wind farms is likely to increase the scale of the problem in the future. The aerodynamic performance ofturbine rotors and the mechanisms that couple the fluid dynamics of multiple rotors can be understood best by simplifying the problem and considering the interaction between only two rotors. The aerodynamic interaction between two rotors in both axial and yawed wind conditions has been simulated using the Vorticity Transport Model. The aerodynamic interaction is a function of the tip speed ratio, the separation between the rotors, and the angle of yaw to the incident wind. The simulations show that the momentum deficit at a turbine operating within the wake developed by the rotor of a second turbine can limitsubstantially the mean power coefficient that can be developed by the turbine rotor. In addition, the significant unsteadiness in the aerodynamic loading on the rotor blades that results from the inherent asymmetry of the interaction, particularly in certain configurations and wind conditions, has considerable implications for the fatigue life of the blade structure and rotor hub. The Vorticity Transport Model enables the simulation the wakedynamics within wind farms and the subsequent aerodynamic interaction to be evaluated over a broad range of wind farm configurations and operating conditions
Aeroacoustics of a coaxial rotor in level flight
The aeroacoustic characteristics of a coaxial system with teetering rotors in level forward °ight are com-
pared to those of an equivalent articulated single rotor with the same solidity. A lifting line representation
of the blade aerodynamics is coupled to Brown's Vorticity Transport Model to simulate the aerodynam-
ics of the rotor systems. The acoustic ¯eld is determined using the Ffowcs Williams-Hawkings equation.
Acoustic analysis shows that the principal contribution to noise radiated by both the coaxial and equivalent
single rotor systems is at the fundamental blade passage frequency, but that the coaxial rotor generates
higher sound pressure levels (by 10 dB for the evaluated con¯gurations) than the equivalent single rotor
at all °ight speeds. The sources of blade vortex interaction (BVI) noise are investigated and the principal
BVI events are identi¯ed. For the coaxial rotor, the most intense impulsive noise is seen to be generated
by the inter-rotor BVI on the advancing side of the lower rotor. The impulsive noise that is generated by
blade vortex interactions for the equivalent single rotor reduces in amplitude as the strength of BVI events
on the rotor decreases with forward speed. Conversely, the BVI noise of the coaxial rotor intensi¯es with
increasing °ight speed due to the increasing strength of the interaction between the wake of the upper rotor
and the blades of the lower rotor. The impulsive noise due to BVI for the coaxial rotor is found to be higher
by 20{35 dB compared to the equivalent single rotor. The overall and impulsive noise characteristics of
the coaxial system are found to be weakly sensitive to changes in rotor separation and the relative phasing
of the rotors
Aeroacoustic analysis of main rotor and tail rotor interaction
The increased restrictions placed on helicopter noise levels over recent decades have encouraged
manufacturers to better understand tail rotor noise and its aerodynamic sources. A
generic single main rotor and tail rotor helicopter has been simulated in high speed forward,
and quartering, flight using the Vorticity Transport Model. The unsteady loads developed on
the tail rotor blades and the resulting acoustic noise propagation have been computed. The
sound propagation from isolated tail rotors with top-aft and top-forward senses of rotation in
high speed forward flight results in impulsive sound being directed downward from the former
and upward from the latter. The principal source of tail rotor noise in high speed forward
flight is a periodic blade-vortex interaction between the tail rotor blades. The effect of aerodynamic
interaction on tail rotor noise is highly dependent on the flight speed and trajectory,
such that the noise produced as a result of interaction is, for the particular helicopter geometry
simulated here, greater in quartering flight than in high speed forward flight. The sound pressure
produced by periodic impulsive loads in high speed forward flight and the high frequency
sound generated in quartering flight is sensitive to the scales to which the vortical features
within the wake, and the radial and azimuthal distributions of blade loading, are resolved
The influence of blade curvature and helical blade twist on the performance of a vertical-axis wind turbine
Accurate aerodynamic modeling of vertical-axis wind turbines poses a significant challenge, but is essential if the performance of such turbines is to be predicted reliably. The rotation of the turbine induces large variations in the angle of attack of its blades that canmanifest as dynamic stall. In addition, interactions between the blades of the turbine and the wake that they produce can exacerbate dynamic stall and result in impulsive changes to the aerodynamic loading on the blades. The Vorticity Transport Model has been used to simulate the aerodynamic performance and wake dynamics of vertical-axis wind turbines with straight-bladed, curved-bladed and helically twisted configuration. It is known that vertical-axis wind turbines with either straight or curved blades deliver torque to their shaft that fluctuates at the blade passage frequency of the rotor. In contrast, a rotor with helically twisted blades delivers a relatively steady torque to the shaft. In the present paper, the interactions between helically twisted blades and the vortices within their wake are shown to result in localized perturbations to the aerodynamic loading on the rotor that can disrupt the otherwise relatively smooth power output that is predicted by simplistic aerodynamic tools that do not model the wake to sufficient fidelity. Furthermore, vertical-axis wind turbines with curved blades are shown to be somewhat more susceptible to local dynamic stall than turbines with straight blades
The flow physics of helicopter brownout
The formation of the dust cloud that is associated with low-level helicopter operations in desert environments has been simulated using the Vorticity Transport Model together with a coupled model to represent the entrainment and subsequent transport of particulate matter through the flow. A simple thin-layer theory, supported by simulations performed using the more physically-representative numerical model, is used to explain the formation of characteristic sheet- and filament-like structures in the dust cloud in terms of the interactions between individual vortical filaments and the ground. In parts of the flow, for instance near the ground vortex that is formed under the leading edge of the rotor when in forward flight, the dust cloud becomes more space-filling than sheet-like in character, and the theory suggests that this is a result of the dust distribution having been processed by multiple vortices over a significant period of time. The distribution of the regions on the ground plane from which significant entrainment of dust into the flow takes place is shown to be influenced strongly by the unstable nature of the vortical structures within the flow. It is suggested that the effect of this vortical instability, when integrated over the timescales that are characteristic of the formation of the dust cloud, is to de-sensitize the gross characteristics of the dustcloud to the details of the wake structure at its inception on the rotor blades. This suggests that the formation of the brownout cloud may be relatively insensitive to the detailed design of the blades of the rotors and may thus be influenced only by less subtle characteristics of the helicopter system
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