7145 2 Ii O<12I5

SOME RECENT APPLICATIONS OF NAVIER-STOKES CODES TO ROTORCRAFr WJ. McCroskey N 9 3 - 7145 2 f SeniorResearchScientist ii o<12i5 U.S.Army AeroflightdynamicsDircctoram(AVSCOM) andNASA Ames ResearchCenter MoffettField,California ABSTRACT Many operational limitations of helicopters and other rotary-wing aircraft are due to nonlinear aerodynamic phenomena, including unsteady, three-dimensional transonic and separated flow near the surfaces and highly vortical flow in the wakes of rotating blades. Modern computational fluid dynamics (CFD) technology offers new tools to study and simulate these complex flows. However, existing Euler and Navier-Stokes codes have to be modified significantly for rotorcraft applications, and the enormous computational requirements presently limit their use in routine design applications. Nevertheless, the Euler/Navier-Stokes technology is progressing in anticipation of future supercomputers that will enable meaningful calculations to be made for complete rotorcraft configurations. L INTRODUCTION advanced CFD techniques to fixed-wing aircraft suggest that improved predictive tools are highly desirable for all The flow fields of helicopters and other rotorcraft provide a rich variety of challenging problems in applied flight vehicles. Such methodology should help to reduce the risks and testing requirementsfor new designs, and it aerodynamics.Much of theflowneartherotating blades isnonlinear,three-dimensional,and oftenunsteady,with should enable engineers to increase rotorcraft periodicregionsof transonicflownearthebladetips,and performance, efficiency, and maneuverability while withdynamic stallpocketsinboard.The bladesalsoshed reducing noise, vibrations, and detectability. complex vorticalwakes, and detrimentalaerodynamic In the following section, a very bfid description is interactionsoftenarisebetween themajor rotatingand given of the methods that are currently being developed nonrotatingcomponents. and used by the author and his coworkcrs. In subsequent Inrecentyears,CFD methodsforisolated,nonlinear sections, some representative examples of recent results piecesof theoverallproblem have been developedto are given, along with descriptions of some of the major issues that remain unresolved. complement the mixture of analyticaland empirical aerodynamictheories,wind tunneldata,and designcharts traditionallyusedby helicopterengineers.References1,4, n. NUMERICAL METHODS for example, describe some of these modern developments,and the presentauthorhighlightedthe Two differentapproaches are currentlybeing activitiesof some of hiscolleaguesin Rd. 5, as they pursuedto solvethecompressibleEuleror Reynolds- existedin 1988.Thispaperrepresentsa briefreviewand Averaged Thin-Layer Navier-Stokesequations for updateof thoseeffortsdescribedin Ref.5, withsome aerodynamic configurations that consist of rotating lifting additionaldiscussionof thechallengesthatremaintoour surfaces and nonrotating airframe components. The first long-term goal of obtaining complete numerical approach employs several finite-difference, implicit simulationsof realisticrotorcraftflow fieldswithas few approximate-factorization algorithms, with algebraic physical approximations as possible.It must be turbulence modeling, to solve the governing equations in emphasizedthatno attemptis made in thispaperto strong conservation-law form [11] on either single- or rewew the bulk of research underway in this field; only multiple-block, body-conforming structured grids. The the work of the author's immediate colleagues is single-block code is called TURNS (TransonicUnsteady described herein. Thus, more activities are neglected than Rotor Navier Stokes), which is described in Ref. 12 and included. which borrows heavily from Obayashi's work [13]. This As can be gleaned from Refs. I-5, many different numerical scheme uses upwind-biased flux-difference levels of sophistication' of CFD technology can be and splitting, an LU-SGS implicit operator, higher-order MUSCL-type limiting, and, in me case of unsteady are being gainfully applied to practical rotorcraft problems. The approach followed here, which is by no calculations, Newton sub-iterations at each time step. means unique (cf. Refs. 4, 6-10), is to develop, adapt, and The current multi-block implicit code is called apply advanced Euler/Reynolds-Averaged Navier-Stokes OVERFLOW, a new code being developed by Dr. Pieter methodology to the specialneeds of futurerotorcraft.Buning at the Ames Research Center, using elements of While the near-termutilityand practicalityof this F3D [14] and ARC3D [15]. OVERFLOW currently has a approach may be open to debate, the historyof two-factor, block tridia_onal option with upwind supercomputer growth and the contributionsof differencing in the su'eamwlse direction, as in F3D, and a three-factor, scalar pentadiagonal option with central differencing, as in ARC3D. OVERFLOW uses the PresentedattheFifthSymposium¢m NumericalandPhysical Chimera overset-grid scheme [16] to subdivide the AspectsofAercxlynamicFlows,Long Beach,California,13-16 January1992. computational domain into subdomains, some of which agreement with experiments, but the computed peak may move relative to others. suction levels on the upper surface of the wing very near the tip were not as high as the measured ones, Fig. 3. Solution-adaptive grids are not currently used in the More recently, Strawn [17] found similar discrepancies in three-dimensional versions of either TURNS or his unstructured-grid calculations. In addition, Strawn OVER bLOW, although this technology will be pursued in found the calculated peak velocities in the vortex itself to the future. TURNS has been used to compute the be substantially less than recently reported by McAlister aerodynamics and acoustics of isolated rotor blades, and and Takahashi [21], although Strawn's computed vortex OVERFLOW has been applied to fuselages and wing- dissipated very little as it convected downstream. In body combinations. The Chimera overset-grid method is unpublished work, Srinivasan has re-computed this currently being combined with TURNS to improve the resolution of vortical structures in the wakes of rotor problem on finer grids with his structured-grid code, with similar results. Thus the cause of the discrepancy remains blades, and OVERFLOW with Chimera is beginning to be undetermined, but it appears that even finer grids and/or applied to rotor-body interactions. better turbulence models may be required to resolve this Body-conforming C-O or C-H structured grid issue. This, in turn, has important implications for the topologies are normally used in TURNS and problem of computing rotor blade-vortex interaction OVERFLOW, with from approximately 100,000 to accurately, as described below. The tip-vortex formation 1,000,000 grid points. Figure 1 shows a typical C-H grid problem is currently being studied computationally by Dr. for aerodynamic calculations. The grid lines are nearly Jennifer Dacles-Mariani at the Ames Research Center, in orthogonal at the surface, and the grid spacing normally conjunction with a new experimental program [22] starts at about 0.00002 - 0.00005 chord in the normal designed to shed further light on the physics of this direction. Figure 2 illustrates the distortion of the grid in phenomenon. the H, or spanwise, direction that is typically used for Despitetheselimitations,theexistingNavier-Stokes acoustic calculations. For these applications, it is technologycan provideusefulinformationabout the important to align the grid lines with linear characteristics performanceof modern bladetips.For example,Duque beyond the tip of the rotor, in order to capture the low- [23,24]has studiedthe unusualBritishExperimental level radiating sound waves. Rotor Program (BERP) helicopter blade under The second approach uses a solution-adaptive nonrotatingconditions,and he has computed the unstructured-grid subdivision scheme developed by complex separationpaRerns that develop at high Strawn [17], incorporating the explicit upwind finite- incioence.Figure4 shows some of theseresultsat 10w volume flow solver of Barth [18]. The code contains an Mach number,incomparisonwithexperimentaloil-fiow efficient edge data structure for computational domains patterns.He also obtainedexcellentagreement with comprised of arbitrary polyhedra, which are subdivided in measuredpressuredistributionseverywhereexceptinthe regions of high flowfield gradients to improve the immediatevicinityofthetip. solution. Barth's method uses a reconstruction scheme in each control volume that is exact for linear variations, and Hover the reconstructed polynomials are flux-limited in regions The axialsymmetry and nominally steady-state of flow discontinuities. The explicit time operator is a conditionsin blade-fixedcoordinatesmake hovering four-stage Runge-Kutta method with local time stepping flightof isolatedrotorbladesan attractiveS_g poifft for steady problems. The motivation for this adaptive-grid for developingrotorcraftcodes.Forerunnersof the approach is to convect vortical flow regions with minimal TURNS codewere developedand appliedtothisprob!_ numerical dissipation, but it shows promise in convecting by Srinlvasan,etal[25]andChen,etal[26];theseefforts low-level acoustic waves, as well. Thus far, only the Euler were summarizedinRef.5.Reference25 showed some of equations have been solved by this method. thebasicdifferencesbetween rotatingand nonrotating The adaptive-grid calculations normally start on a blades,buttheinducedflowdue tothey_ic__ wake was coarse structured

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