Vortex Wake Analysis Of A Single Bladed Hovering Rotor And A Comparison With Experimental Data

Author : Tenzin Choephel
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (971 download)

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Download or read book Aerodynamic Analysis of Helicopter Rotors Using a Higher-Order, Free-Wake Method written by Tenzin Choephel and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The wake has a strong influence on the aerodynamics of a helicopter rotor, andthe accurate prediction of its geometry and the resulting induced velocity fieldis extremely important for rotorcraft aerodynamic analysis. In this thesis, a newhigher-order, free-wake method for rotor aerodynamic analysis is presented. Themethod uses elements of distributed vorticity to model the lifting surfaces and theassociated wake. The use of such higher-order spanwise elements ensures higherresolution compared to traditional filament-based free-wake analysis and does notrequire explicit vortex core modeling with a user-specified core size. Since themethod uses a full-span, singularity-free, relaxed wake, it can resolve the effect ofon-blade, partial-span devices.The free-wake method is validated in both hover and forward flight againstmeasured data from well-documented experiments. In hover, the blade spanwiselift coefficients predicted by the free-wake analysis correlate well with the measureddata from the classic Caradonna-Tung model rotor experiment. The figure of merit,which is a measure of rotor efficiency, predicted by the present method is comparedto that from the experiments conducted by Knight and Hefner, and the correlationsare found to be quite good given the level of fidelity of this method, which is basedon potential flow theory. Rotor downwash, which is one of the most importantconsiderations in rotor aerodynamic analysis, is predicted very well by the free-wakemethod when compared to measured data from a full-scale rotor test performedby Boatwright. These correlation studies provide a lot of promise as to the abilityof the method in predicting the challenging aerodynamics of a helicopter rotor inhover.Validation studies are also performed to assess the accuracy of the free-wakeanalysis in predicting downwash distribution in forward flight. Comparison ofnumerical predictions with experimental data requires the rotor to be trimmed tothe conditions recorded in the experiment. To ensure this, the free-wake programis coupled with RCAS, a comprehensive helicopter analysis code developed by theiiiUS Army, in order to take advantage of its robust trim algorithm, among othercapabilities. The coupling is achieved through what is called a loose-couplingmethodology, whereby data is exchanged at the end of each coupling iteration orconverged rotor revolution. The downwash distributions predicted by the presentmethod are compared to measured data for a model rotor at various advancedratios and thrust levels taken at the U.S. Army/NASA Langley Research Center(LaRC) facility. The free-wake method not only captures the important phenomenaobserved in the experiments but the results also correlate well with the measureddata both in terms of magnitude and distribution. However, exceptions exist atthe highest advance ratio, where other methods also demonstrate poor correlations.The results from this free-wake analysis are also compared to predictions by otherexisting methods such as the University of Maryland free-wake method (UMDFW)and the vortex transport method (VTM). The present method yields resultscomparable to the ones obtained using VTM, both of which correlate better withmeasurements than does UMD-FW.Sensitivity studies show that the blade panel density and azimuthal time-stepsize do not have a significant influence on the solution fidelity. In addition, free-wakeanalyses with the azimuthal time-step size of = 3o demonstrate the robustnessof the method even with small time-steps, which is important for certain problemsincluding rotor acoustics.The thesis concludes with a discussion on the capability of the new free-wakemethod in resolving on-blade, partial-span devices. An analysis of a 2-bladed rotorwith a partial-span deflection is performed and the resulting changes in sectionalloadings, downwash distribution, wake geometry and aggregate performance parametersare highlighted to demonstrate its potential as a tool for future rotorcraftanalysis.