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

Effect of rotor stiffness and lift offset on the aeroacoustics of a coaxial rotor in level flight

The acoustic characteristics of a twin contra-rotating coaxial rotor configuration with significant flapwise stiffness are investigated in steady forward flight. The Vorticity Transport Model is used to simulate the aerodynamics of the rotor system and the acoustic field is determined using the Ffowcs Williams-Hawkings equation implemented using the Farassat-1A formulation. Increasing the hub stiffness alters the strengths of the blade vortex interactions, particularly those between the upper and lower rotors, and affects the intensity and directivity of the blade vortex interaction noise produced by the system. The inter-rotor blade vortex interaction on the advancing side of the lower rotor is the principal source of the most intensively focused noise that is generated by a conventionally articulated coaxial rotor system. For stiffened coaxial rotors, this particular inter-rotor blade vortex interaction is weakened as a result of a broad redistribution in lateral loading, yielding a reduction in the intensity of the noise that is produced by this interaction. The spanwise distribution of loading on the rotors of a stiffened coaxial system can be modified further by altering the lateral partition of lift (or lift offset). It is shown that decreasing the lift offset has the effect of counteracting the redistribution of loading due to flapwise stiffness and hence increases the blade vortex interaction noise as well as the power consumed by the rotor. Conversely, a reduction in both the power consumption and the blade vortex interaction noise is observed if the lift offset is increased, with the maximum benefit of lift offset being achieved at high speed. The computational results suggest that the noise from the dominant inter-rotor blade vortex interaction can be ameliorated through the use of lift offset control on stiffened coaxial systems, to the extent that the noise produced by this interaction can be made to be comparable to that produced by the other, weaker interactions between the two rotors of the system

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 propeller-augmented compound coaxial helicopter

The aerodynamic and acoustic characteristics of a generic hingeless coaxial helicopter with a tail-mounted propulsor and stabiliser have been simulated using Brown's Vorticity Transport Model. This has been done to investigate the ability of models of this type to capture the aerodynamic interactions that are generated between the various components of realistic, complex helicopter configurations. Simulations reveal the aerodynamic environment of the coaxial main rotor of the configuration to be dominated by internal interactions that lead to high vibration and noise. The wake of the main rotor is predicted to interact strongly with the tailplane, particularly at low forward speed, to produce a strong nose-up pitching moment that must be countered by significant longitudinal cyclic input to the main rotor. The wake from the main rotor is ingested directly into the tail propulsor over a broad range of forward speeds, where it produces significant vibratory excitation of the system as well as broadband noise. The numerical calculations also suggest the possibility that poor scheduling of the partition of the propulsive force between the main rotor and propulsor as a function of forward speed may yield a situation where the propulsor produces little thrust but high vibration as a result of this interaction. Although many of the predicted effects might be ameliorated or eliminated entirely by more careful or considered design, the model captures many of the aerodynamic interactions, and the resultant effects on the loading on the system, that might be expected to characterise the dynamics of such a vehicle. It is suggested that the use of such numerical techniques might eventually allow the various aeromechanical problems that often beset new designs to be circumvented - hopefully well before they manifest on the prototype or production aircraft

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

Intra-abdominal Esophageal Duplication Cyst in an Adult

Esophageal duplication cysts are congenital anomalies of the foregut that are rarely found in the abdomen. An accurate preoperative diagnosis is not always possible, so the definitive diagnosis can be made by histologic examination of the surgical specimen. We experienced a case of Intra-abdominal esophageal duplication cyst in a 52-year-old female, who initially presented with an esophageal submucosal tumor on upper gastrointestinal endoscopy. She did not have any gastrointestinal symptoms. Barium esophagography, chest computed tomography scan and endoscopic ultrasonography demonstrated the cystic lesion in the intra-abdominal esophagus. Transhiatal enucleation of the lesion was performed successfully via the abdominal approach with no postoperative complications. Histologic study showed that the cyst wall contained a two-layered muscle coat and the surface of the lumen was lined by pseudo-ciliated columnar epithelium. The patient has been doing well without any complaints for 3 months of follow-up period

Small intestinal model for electrically propelled capsule endoscopy

The aim of this research is to propose a small intestine model for electrically propelled capsule endoscopy. The electrical stimulus can cause contraction of the small intestine and propel the capsule along the lumen. The proposed model considered the drag and friction from the small intestine using a thin walled model and Stokes' drag equation. Further, contraction force from the small intestine was modeled by using regression analysis. From the proposed model, the acceleration and velocity of various exterior shapes of capsule were calculated, and two exterior shapes of capsules were proposed based on the internal volume of the capsules. The proposed capsules were fabricated and animal experiments were conducted. One of the proposed capsules showed an average (SD) velocity in forward direction of 2.91 ± 0.99 mm/s and 2.23 ± 0.78 mm/s in the backward direction, which was 5.2 times faster than that obtained in previous research. The proposed model can predict locomotion of the capsule based on various exterior shapes of the capsule

The Bilirubin Level is Negatively Correlated with the Incidence of Hypertension in Normotensive Korean Population

Reactive oxygen species have been known to be an important factor in the pathogenesis of hypertension. Bilirubin, one of the metabolites of heme degraded by heme oxygenase, is a potent anti-oxidant. We verified the effect of serum bilirubin level on the incidence of hypertension in normotensive subjects. We grouped 1,208 normotensive subjects by the criterion of the highest quintile value of serum bilirubin, 1.1 mg/dL. The incidence of hypertension was higher in group 1 with bilirubin less than 1.1 mg/dL than in group 2 with bilirubin 1.1 mg/dL or more (186/908 vs. 43/300, p=0.018). The relative risk for hypertension was 0.71 (95% confidence interval, 0.51-0.99), p=0.048 in group 2 compared to group 1 by Cox's proportional hazard model. Among the groups stratified by gender, smoking, and liver function status, the group 2 showed a lower risk of hypertension in females and in non-smokers. In conclusion, a mild increase within the physiological range of serum bilirubin concentration was negatively correlated with the incidence of hypertension. The effect of bilirubin on the development of hypertension was more evident in females and in non-smokers