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Book Free Wake Analysis of Helicopter Rotor Flow Fields Using a Coupled Vorticity Embedding Fractional Step Method

Download or read book Free Wake Analysis of Helicopter Rotor Flow Fields Using a Coupled Vorticity Embedding Fractional Step Method written by and published by . This book was released on 2000 with total page 68 pages. Available in PDF, EPUB and Kindle. Book excerpt: A hybrid CFD (Computational Fluid Dynamics) method combining a potential-based Vorticity Embedding method with an Euler solver is presented and validated for isolated rotor flows. First, a formulation of Vorticity Embedding is developed and validated for Cartesian grids. This formulation is utilized together with simple lifting lines to represent an isolated rotor in hover. The results for an isolated rotor are then compared with experiment. The method is then hybridized by coupling the Vorticity Embedding grid containing the rotor to a region containing a ground plane where a standard finite difference Euler solver is utilized. initial results from this hybrid scheme are presented including a demonstration of the code's potential to calculate the ground vortex roll-up generated by a rotor in ground effect in the presence of a cross-wind.

Book Free Wake Analysis of Helicopter Rotor Blades in Hover Using a Finite Volume Technique

Download or read book Free Wake Analysis of Helicopter Rotor Blades in Hover Using a Finite Volume Technique written by Krishnamurthi Ramachandran and published by . This book was released on 1987 with total page 170 pages. Available in PDF, EPUB and Kindle. Book excerpt: In the computation of helicopter rotor flow fields, wake effects can be very important since each blade passes close to the wake produced by the preceding ones, causing a large local effect. Also, the vortical flow from the wake of a number of blade passages causes a large global effect. Extensive work has been done on helicopter rotor flows using integral methods to follow the vortex wake. These generally cannot treat compressibility effects. Also, they have difficulty attaining stable solutions, particularly in hover, where a large number of interacting vortex sheets must be treated. A new method has been developed which like integral methods does not constrain or spread the wake. Also, like finite difference methods, it can treat compressibility effects. This method has been developed into a computer program for the computation of rotor flow fields in hover with free wakes. The method utilizes a finite volume potential flow technique. The basic approach involves modifying the potential flow wake treatment so that, within the numerical approximation, the momentum is conserved there as in the rest of the flow field. The internal structure of the vortex is not solved for, but is modeled and spread over several grid points. The wake position and vorticity strength are computed so that the momentum over the wake is balanced in an integral sense. Results computed by this approach for the circulation and wake geometry are compared with experimentally measured data. Cases treated include subsonic and transonic flows, high and low aspect ratios, and two and four-bladed rotor configurations. (SDW).

Book Aerodynamic Analysis of Helicopter Rotors Using a Higher Order Free Wake Method

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.

Book Rotor Design Optimization Using a Free Wake Analysis

Download or read book Rotor Design Optimization Using a Free Wake Analysis written by and published by . This book was released on 1993 with total page 138 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Book Free Wake Analysis of Hover Performance Using a New Influence Coefficient Method

Download or read book Free Wake Analysis of Hover Performance Using a New Influence Coefficient Method written by National Aeronautics and Space Adm Nasa and published by . This book was released on 2018-11-07 with total page 106 pages. Available in PDF, EPUB and Kindle. Book excerpt: A new approach to the prediction of helicopter rotor performance using a free wake analysis was developed. This new method uses a relaxation process that does not suffer from the convergence problems associated with previous time marching simulations. This wake relaxation procedure was coupled to a vortex-lattice, lifting surface loads analysis to produce a novel, self contained performance prediction code: EHPIC (Evaluation of Helicopter Performance using Influence Coefficients). The major technical features of the EHPIC code are described and a substantial amount of background information on the capabilities and proper operation of the code is supplied. Sample problems were undertaken to demonstrate the robustness and flexibility of the basic approach. Also, a performance correlation study was carried out to establish the breadth of applicability of the code, with very favorable results. Quackenbush, Todd R. and Bliss, Donald B. and Ong, Ching Cho and Ching, Cho Ong Unspecified Center COMPUTER PROGRAMS; FREE FLOW; HELICOPTER PERFORMANCE; HELICOPTER WAKES; HOVERING STABILITY; INFLUENCE COEFFICIENT; COMPUTERIZED SIMULATION; CONVERGENCE; LOADS (FORCES); PERFORMANCE PREDICTION; RELAXATION METHOD (MATHEMATICS); ROBUSTNESS (MATHEMATICS); TIME MARCHING...

Book Special Opportunities in Helicopter Aerodynamics

Download or read book Special Opportunities in Helicopter Aerodynamics written by W. J. McCroskey and published by . This book was released on 1983 with total page 38 pages. Available in PDF, EPUB and Kindle. Book excerpt: Aerodynamic research relating to modern helicopters includes the study of three-dimensional, unsteady, nonlinear flow fields. A selective review is made of some of the phenomenon that hamper the development of satisfactory engineering prediction techniques, but which provides a rich source of research opportunities: flow separations, compressibility effects, complex vortical wakes, and aerodynamic interference between components. Several examples of work in progress are given, including dynamic stall alleviation, the development of computational methods for transonic flow, rotor-wake predictions, and blade-vortex interactions. (Author).

Book Free Wake Computation of Helicopter Rotor Flowfield for General Flight Regimes

Download or read book Free Wake Computation of Helicopter Rotor Flowfield for General Flight Regimes written by and published by . This book was released on 1992 with total page 79 pages. Available in PDF, EPUB and Kindle. Book excerpt: A CFD based computer code, HELIX II has been developed for computing helicopter rotor flowfield in hover and in forward flight. In order to predict the vibratory airloads in forward flight and the performance in hover the effect of shed wake has to be accurately accounted for. This has been accomplished by an unique vorticity embedding procedure. The wake is convected in a Lagrangian sense and does not suffer from numerical dissipation. The effect of this convected wake is included in the Eulerian using Clebsch representation. The unsteady full potential equation is solved in the Eulerian grid with the embedded vortical velocity accounting for the influence of wake. The wake evolves as a part of the solution. Dynamic grid modifications are performed at each time step to account for blade motion and deformation. These modifications are confined to regions near the blade using suitable blending functions. In addition a vorticity confinement method is developed which involves adding a term to the momentum conservation equation of fluid dynamics. This modification is very useful in the numerical solution of flow problems involving vortical regions. The Euler equations solved with this extra term can convect vortices without Numerical dissipation. Unsteady airloads, Lagrangian wake convection, Eulerian grid, Vorticity Embedding, Clebsch representation, Vorticity confinement, Euler Equation solution without dissipation.

Book Enhancements to a New Free Wake Hover Analysis

Download or read book Enhancements to a New Free Wake Hover Analysis written by and published by . This book was released on 1989 with total page 46 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Book A Method for Assessing the Impact of Wake Vortices of USAF Operations

Download or read book A Method for Assessing the Impact of Wake Vortices of USAF Operations written by George Kurylowich and published by . This book was released on 1979 with total page 114 pages. Available in PDF, EPUB and Kindle. Book excerpt: Experience as a consultant to the Safety Office at Norton AFB led to compiling the engineering tools presented so that this report can be used by engineering personnel to investigate future incidents/accidents and existing USAF operations that are impacted by the vortical wake hazard. The approach presented is amenable to easy hand computations. Mixed airplane/helicopter operations can be assessed, since the engineering tools to determine the location and strength of the rotor downwash field behind a helicopter are presented. Finally, a simplified mathematical model is given to represent this hazard for use in USAF simulators, to make pilots aware of the problems associated with operating in wake-contaminated airspace.

Book A New Methodology for Free Wake Analysis Using Curved Vortex Elements

Download or read book A New Methodology for Free Wake Analysis Using Curved Vortex Elements written by Donald B. Bliss and published by . This book was released on 1987 with total page 144 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Book Zonal Procedure for Predicting the Hover Performance of a Helicopter Rotor

Download or read book Zonal Procedure for Predicting the Hover Performance of a Helicopter Rotor written by Marvin Allyn Moulton and published by . This book was released on 1998 with total page 268 pages. Available in PDF, EPUB and Kindle. Book excerpt: