Analysis of Tail Rotor Orthogonal Blade Vortex Interaction. Dept Report 0301

During helicopter operation, the wake of the main rotor can interact with the main rotor blades and the tail rotor blades. The focus of the present research Is the interaction of the main rotor tip vortices with the tail rotor blades. Helicopters are particularly susceptible to this type of blade vortex interaction during low angle descent and climb. As a result of these interactions high decibel noise is emitted and control degradation may occur. The aim of the present research is to indicially model the blade vortex interaction for the limiting orthogonal case. It is hoped that the research will provide a greater understanding of the complex interactions leading, in the long term, to the potential development of more environmentally appealing civil aircraft and increased component life. In this report initial analysis of pressure data from the orthogonal interactions of a vortex with a stationary blade, collected in the University of Glasgow 2.64 m by 2.04 m wind tunnel is presented. This analysis involved removing high frequency noise and random freestream turbulence effects from the data before examining the impulsive change in normal force during the initial stages of the interaction. It has been established that the impulsive response is most severe in the vicinity of the vortex core centreline where the axial core flow in the vortex is the dominant parameter. With increasing distance from the core, the severity of the response becomes dependent on the sense of rotation of the vortex. Finally, the future direction of the work towards development of a robust indicial model of the phenomenon is discussed

Analysis of Tail Rotor Orthogonal Blade Vortex Interaction. Dept Report 0301

During helicopter operation, the wake of the main rotor can interact with the main rotor blades and the tail rotor blades. The focus of the present research Is the interaction of the main rotor tip vortices with the tail rotor blades. Helicopters are particularly susceptible to this type of blade vortex interaction during low angle descent and climb. As a result of these interactions high decibel noise is emitted and control degradation may occur. The aim of the present research is to indicially model the blade vortex interaction for the limiting orthogonal case. It is hoped that the research will provide a greater understanding of the complex interactions leading, in the long term, to the potential development of more environmentally appealing civil aircraft and increased component life. In this report initial analysis of pressure data from the orthogonal interactions of a vortex with a stationary blade, collected in the University of Glasgow 2.64 m by 2.04 m wind tunnel is presented. This analysis involved removing high frequency noise and random freestream turbulence effects from the data before examining the impulsive change in normal force during the initial stages of the interaction. It has been established that the impulsive response is most severe in the vicinity of the vortex core centreline where the axial core flow in the vortex is the dominant parameter. With increasing distance from the core, the severity of the response becomes dependent on the sense of rotation of the vortex. Finally, the future direction of the work towards development of a robust indicial model of the phenomenon is discussed

Analysis and indicial modelling of helicopter tail rotor orthogonal blade vortex interaction

This study builds on previous experimental investigations of orthogonal tail rotor blade vortex interaction, and investigates semi-empirical modelling of the phenomenon. In the early stages of the work, weaknesses were identified in the published experimental data that limited their use as a modelling correlation source. For this reason, a detailed analysis of data from the experimental study of Wang et al. (2002) was conducted to allow the validation of semi-empirical modelling strategies applied to orthogonal blade vortex interaction. Indicial modelling was identified as a suitable modelling strategy due to its computational efficiency and also its current use in the helicopter industry. The long-term intention is that the subsequent integration of the orthogonal blade vortex interaction model into a full helicopter aerodynamic model will enable a more complete simulation that considers orthogonal blade vortex interaction during the design stages of a helicopter's development. A number of modelling strategies were considered during this study. Initial models were based on the Kussner function for an aerofoil encountering an upgust. The orthogonal interaction was captured by representing the axial core flow of the tip vortex as an upgust in the shape of a Lamb vortex that engulfed the entire vortex. This resulted in a markedly greater lift response compared to experimental data because the chordwise distribution of axial velocities due to the interacting tip vortex were not properly represented. The modelling approach was then improved to account for this distribution. This produced good agreement with the experimental data, where the vortex centre interacted with the blade. The predicted response was found to be symmetric about the vortex centre, which was in contrast to the asymmetry found in the experimental data. The asymmetry was investigated using a two-dimensional panel method simulation of the orthogonal interaction. It was hoped that the asymmetry could be accounted for by the rotational flow of the tip vortex, however, the panel method demonstrated that there was insufficient rotational flow to account for the magnitude of the asymmetry found in the experimental data. To investigate this asymmetry further a numerical simulation of the wind tunnel experiment was used. This simulation was inviscid and featured a three-dimensional source panel method to represent the wind tunnel walls, a lifting line calculation for the blade of the vortex generator, and a free-wake solution for the wake of the vortex generator. The simulation had previously been found to simulate the experimental wake shape well. The axial velocities predicted by this model at the location of the installed interacting blade were extracted and used as an input into the indicial model. The indicial model then reproduced the asymmetric lift response found in the experimental data; however, the magnitude of the measured lift response was not well represented. This difference may be associated with the inviscid nature of the numerical simulation or flapping of the vortex generator blade observed during the experiment. As a first step towards understanding the difference, the angle of incidence of the vortex generator was reduced in the numerical simulation until the circulation over the interacting blade matched the experimentally measured value. This resulted in a closer agreement in the magnitude of the blade vortex interaction response for ail spanwise locations. The differences between the prediction based on the prescribed Lamb type axial flow distribution and the indicial prediction based on the horizontal cross flow velocities extracted from the numerical simulation, indicate that the shape of the wake and its corresponding induced flow influence the interaction response. The prescribed indicial prediction features a sharper drop off in lift response compared to the indicial prediction forced by the simulated velocities. This can be attributed to the curved wake shape and the trailing vorticity sheet in the numerical simulation, which represent real features of the experiment. (Abstract shortened by ProQuest.)

The development and validation of a scoring tool to predict the operative duration of elective laparoscopic cholecystectomy

Background: The ability to accurately predict operative duration has the potential to optimise theatre efficiency and utilisation, thus reducing costs and increasing staff and patient satisfaction. With laparoscopic cholecystectomy being one of the most commonly performed procedures worldwide, a tool to predict operative duration could be extremely beneficial to healthcare organisations. Methods: Data collected from the CholeS study on patients undergoing cholecystectomy in UK and Irish hospitals between 04/2014 and 05/2014 were used to study operative duration. A multivariable binary logistic regression model was produced in order to identify significant independent predictors of long (> 90 min) operations. The resulting model was converted to a risk score, which was subsequently validated on second cohort of patients using ROC curves. Results: After exclusions, data were available for 7227 patients in the derivation (CholeS) cohort. The median operative duration was 60 min (interquartile range 45–85), with 17.7% of operations lasting longer than 90 min. Ten factors were found to be significant independent predictors of operative durations > 90 min, including ASA, age, previous surgical admissions, BMI, gallbladder wall thickness and CBD diameter. A risk score was then produced from these factors, and applied to a cohort of 2405 patients from a tertiary centre for external validation. This returned an area under the ROC curve of 0.708 (SE = 0.013, p  90 min increasing more than eightfold from 5.1 to 41.8% in the extremes of the score. Conclusion: The scoring tool produced in this study was found to be significantly predictive of long operative durations on validation in an external cohort. As such, the tool may have the potential to enable organisations to better organise theatre lists and deliver greater efficiencies in care