3,104 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
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
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
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
Helicopter brownout - can it be modelled?
Significant progress has been made to date in modelling, computationally, the formation and development of the dust cloud that forms in the air surrounding the rotorcraft under brownout conditions. Modern computational methods are able to replicate not only the development of the dust cloud in appropriate operational scenarios, but also the sensitivity of the shape and density of the dust cloud to the detailed design of the rotorcraft. Results so far suggest that attempts to ameliorate brownout by aerodynamic means, for instance by modifying the rotor properties, will be frustrated to some extent by the inherent instability of the °flow field that is produced by the helicopter. Nonetheless, very recent advances in understanding the fundamental mechanisms that lead to the formation of the dust cloud may allow substantial progress to be made once certain elements of the basic physics of the problem are more fully understood and better quantified
The effect of rotor design on the fluid dynamics of helicopter brownout
Helicopters operating close to the ground in dusty environments tend to generate large clouds of dust in the surrounding air. These clouds can obscure the pilot's view of the ground and lead to a dangerous condition known as brownout. Given the intimate relationship between the induced flow feld around the rotor and the process through which the particulate matter becomes airborne and is subsequently transported, it has been speculated that the design of its rotor may influence the shape and size of the dust clouds that are produced by any particular type of helicopter. This paper presents a study of the influence of two key geometric properties of the rotor on the development of these dust clouds. A particle transport model is coupled to Brown's Vorticity Transport Model to represent the dynamics of the particulate-air system surrounding a generic helicopter rotor under various flight conditions. The number of blades on the rotor is altered, whilst keeping the solidity constant, thus altering the distribution of vorticity that is released onto the ground. In addition, the twist of the blades is varied in order to investigate the effect of the resultant changes in the distribution of induced downwash on the evolution of the dust cloud. The study suggests that, in general, the larger the number of blades, and the higher the blade twist, the less dense the dust clouds that are produced under brownout conditions. It appears thus that the characteristics of the dust clouds are indeed sensitive to the geometry of the rotor and hence that careful aerodynamic design may allow the severity of brownout to be ameliorated
Interactional aerodynamics and acoustics of a hingeless coaxial helicopter with an auxiliary propeller in forward flight
The aerodynamics and acoustics of a generic coaxial helicopter with a stiff main rotor system and a tail-
mounted propulsor are investigated using Brown's Vorticity Transport Model. In particular, the model
is used to capture the aerodynamic interactions that arise between the various components of the configuration. By comparing the aerodynamics of the full configuration of the helicopter to the aerodynamics
of various combinations of its sub-components, the influence of these aerodynamic interactions on the behaviour of the system can be isolated. Many of the interactions follow a simple relationship between cause
and effect. For instance, ingestion of the main rotor wake produces a direct effect on the unsteadiness
in the thrust produced by the propulsor. The causal relationship for other interdependencies within the
system are found to be more obscure. For instance, a dependence of the acoustic signature of the aircraft
on the tailplane design originates in the changes in loading on the main rotor that arise from the requirement to trim the load on the tailplane that is induced by its interaction with the main rotor wake. The
traditional approach to the analysis of interactional effects on the performance of the helicopter relies on
characterising the system in terms of a network of possible interactions between the separate components of
its configuration. This approach, although conceptually appealing, may obscure the closed-loop nature of
some of the aerodynamic interactions within the helicopter system. It is suggested that modern numerical
simulation techniques may be ready to supplant any overt reliance on this reductionist type approach and
hence may help to forestall future repetition of the long history of unforeseen, interaction-induced dynamic
problems that have arisen in various new helicopter designs
On the Treatment of Neutrino Oscillations Without Resort to Weak Eigenstates
We discuss neutrino oscillations in the framework of the quantum field theory
without introducing the concept of neutrino weak eigenstates. The external
particles are described by wave packets and the different mass eigenstate
neutrinos propagate between the production and detection interactions, which
are macroscopically localized in space-time. The time-averaged cross section,
which is the measurable quantity in the usual experimental setting, is
calculated. It is shown that only in the extremely relativistic limit the usual
quantum mechanical oscillation probability can be factored out of the cross
section.Comment: LaTeX-18pages, JHU-TIPAC-930011,DFTT 22/9
Applying rotorcraft modelling technology to renewable energy research
The perceived need to reduce mankind's impact on the global climate motivates towards a future society in which a significant proportion of its energy needs will be extracted from the winds and the tides of the planet. This paper shows several examples of the application of Brown's Vorticity Transport Model, originally developed to perform simulations of helicopter aeromechanics and wake dynamics, to the analysis of the performance of renewable energy devices and their possible impact on the environment. Prediction of the loading on wind turbines introduces significant additional challenges to such a model, including the need to account fully for the effects of radial flow on blade stall. The wake-mediated aerodynamic interactions that occur within a wind farm can reduce its power output significantly, but this problem is very similar to that where the aerodynamic unsteadiness of the coupled wake of the main and tail rotors of a helicopter can result in significantly increased pilot workload. The helicopter-related problem of brownout, encountered during operations in desert conditions, has its analogue in the entrainment of sediment into the wakes of tidal turbines. In both cases it may be possible to ameliorate the influence of the rotor on its environment by careful and well-informed design. Finally, calculations of the distortion and dispersal of the exhaust plumes of a helicopter by the wake of its rotor allow insight into how wind turbines might interfere with the dispersal of pollutants from nearby industrial sites. These examples show how cross-disciplinary information transfer between the rotorcraft field and the renewable energy community is helping to develop the technologies that will be required by our future society, as well as helping to understand the environmental issues that might need to be faced as these technologies become more prevalent
Simulations of the dispersion of reactive pollutants in a street canyon, considering different chemical mechanisms and micromixing
The Stochastic Fields (SF) or Field Monte Carlo method has been used to model the dispersion of reactive scalars in a street canyon, using a simple chemistry and the CBM-IV mechanism. SF is a Probability Density Function (PDF) method which allows both means and variances of the scalars to be calculated as well as considering the effect of segregation on reaction rates. It was found that the variance of reactive scalars such as NO2 was very high in the mixing region at roof-top level with rms values of the order of the mean values. The effect of segregation on major species such as O3 was found to be very small using either mechanism, however, some radical species in CBM-IV showed a significant difference. These were found to be the seven species with the fastest chemical timescales. The calculated photostationary state defect was also found to be in error when segregation is neglected
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