Evolution of isolated turbulent trailing vortices

In this work, the temporal evolution of a low swirl-number turbulent Batchelor vortex is studied using pseudospectral direct numerical simulations. The solution of the governing equations in the vorticity-velocity form allows for accurate application of boundary conditions. The physics of the evolution is investigated with an emphasis on the mechanisms that influence the transport of axial and angular momentum. Excitation of normal mode instabilities gives rise to coherent large scale helical structures inside the vortical core. The radial growth of these helical structures and the action of axial shear and differential rotation results in the creation of a polarized vortex layer. This vortex layer evolves into a series of hairpin-shaped structures that subsequently breakdown into elongated fine scale vortices. Ultimately, the radially outward propagation of these structures results in the relaxation of the flow towards a stable high-swirl configuration. Two conserved quantities, based on the deviation from the laminar solution, are derived and these prove to be useful in characterizing the polarized vortex layer and enhancing the understanding of the transport process. The generation and evolution of the Reynolds stresses is also addressed

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

High resolution computation of the aerodynamics and acoustics of blade vortex interaction

In the present work, high resolution CFD simulations have been performed on an idealised problem of the interaction of an independently generated vortex with a rotor blade, including a case where the vortex directly impacts on the blade. The resulting blade pressures and acoustics are comprehensively compared against experimental measurements. Two different modelling approaches are used: the first is to impose the vortex as a perturbation to the velocity field, and the second is to fully resolve the vortex formation, evolution and its interaction with the blade. For a case in which the vortex passes near the blade surface, the the fully resolved approach is confirmed to accurately preserve the vortex structure. The far field acoustic predictions offered by the fully resolved approach are seen to be very accurate and definite improvements are observed in the computed blade pressures and acoustics over the imposed vortex approach and other similar works in the literature. For a case in which the vortex axis passes through the blade, the shape and width of the acoustic pulse in the far field is accurately represented by the fully resolved approach, while the magnitude is slightly underpredicted. The improvement in prediction offered by the fully resolved approach is because this method allows for a more realistic representation of phenomena, such as dynamic change in vortex structure and trajectory due to the blade passage, that become important when the vortex miss-distance becomes small

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

Predicting blade vortex interaction, airloads and acoustics using the vorticity transport model

Interactions between the blades and vortical structures within the wake of a helicopter rotor are a significant source of impulsive loading and noise, particularly in descending flight. Advances in the prediction and understanding of such blade vortex interactions have been aided in recent years by the extensive experimental dataset made available through the HART test programme. Brown’s Vorticity Transport Model was used to predict the rotor blade loading, the resultant wake system and the acoustic noise radiation for the HART II rotor. The vorticity conserving properties of the Vorticity Transport Model allow the detailed wake features that are associated with blade vortex interactions to be resolved. The experimental airload data, in particular the higher harmonic loading associated with blade vortex interactions, is matched well by the computations. The computed vorticity distribution in the wake also shows good correlation with the experimentally measured vortex positions. Including a representation of the fuselage within the computation yields marked improvement in the prediction of the vortex positions compared to similar calculations with an isolated rotor. An accoustic analysis, based on a Ffowcs-Williams Hawkings approach, is able to predict accurately the locations of the sound pressure maxima and the upstream attenuation of the sound radiated by the rotor. The principal discrepancies in airload, vortex position and acoustic prediction are confined almost exclusively to the rear of the advancing side of the rotor and, if errors in measuring the blade deflection can be discounted, may be due to minor inaccuracies in modelling the roll-up of the wake

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

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