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NASA Technical Reports Server (NTRS) 19970040814: The Development of a Tool for Semi-Automated Generation of Structured and Unstructured Grids about Isolated Rotorcraft Blades PDF

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Preview NASA Technical Reports Server (NTRS) 19970040814: The Development of a Tool for Semi-Automated Generation of Structured and Unstructured Grids about Isolated Rotorcraft Blades

/i THE DEVELOPMENT OF A TOOL FOR SEMI-AUTOMATED GENERAT|ON OF STRUCTURED AND UNSTRUCTURED GRIDS ABOUT ISOLATED ROTORCRAFT BLADES* By Ramakrishnan Shanmugasundaram, Javier A. Garriz and Jamshid A. Samareh, Computer Sciences Corporation Hampton, VA ABSTRACT The grid generation used to model rotorcraft configurations for Computational Fluid Dynamics (CFD) analysis is highly complicated and time consuming. The highly complex geometry and irregular shapes encountered in entire rotorcraft configurations are typically modeled using overset grids. Another promising approach is to utilize unstructured grid methods. With either approach the majority of time is spent manually setting up the topology. For less complicated geometries such as isolated rotor blades, less time is obviously required. This paper discusses the capabilities of a tool called Rotor blade Optimized Topology Organizer and Renderer(ROTOR) being developed to quickly generate block structured grids and unstructured tetrahedral grids about isolated blades. The key algorithm uses individual airfoil sections to construct a Non- Uniform Rational B-Spline(NURBS) surface representation of the rotor blade. This continuous surface definition can bequeried to define the block topology used in constructing a structured mesh around the rotor blade. Alternatively, the surface definition can be used to define the surface patches and grid cell spacing requirements for generating unstructured surface and volume grids. Presently, the primary output for ROTOR is block structured grids using O-H and H-H topologies suitable for full-potential solvers. This paper will discuss the present capabilities of the tool and highlight future work. INTRODUCTION Regardless of whether a structured The high degree of geometric complexity or unstructured approach is chosen to typically encountered in full rotorcrafl generate the grid(s), users are still required configurations is such that even the to manually set up the blocking/topology for generation of grids for inviscid flow multi-block structured grids or surface simulations requires a large amount of user patching for unstructured grids. Far less intervention. Overset grids have emerged as intervention is obviously required when the method of choice for modeling entire modeling an isolated rotor blade or configurations [1], driven in equal parts by propeller, as is done when performing the simplification of generating grids for computational studies of rotorcrafl airfoils individual rotorcrafi components and and blades[4]. This paper discusses the advances in flow solver algorithms. The capabilities of a tool being developed with flow solver advances include all the pre- the intention of allowing the user to quickly processing that overset methods require, for generate block structured and unstructured example, determining the interpolation tetrahedral grids about isolated blades. stencils. Another approach that holds At the heart of the tool being promise for rotorcmft and indeed any developed is an algorithm which uses the complex shape is utilizing unstructured grid individual airfoil sections defining the blade techniques. Figure !, for instance, depicts and global blade characteristics such as the triangular surface mesh on a simplified twist, and sweep as input and constructs a AH-64 Apache helicopter fuselage. Recent Non-Uniform Rational B-Spline (NURBS) improvements to the basic "advancing front" surface representation of the blade. This algorithm used to produce the mesh shown "geometry definition" can then be queried to in Figure 1have resulted in the capability to obtain the mesh points describing the now generate viscous unstructured meshes, with continuous surface, and a block structured or without anisotropic stretching to keep the mesh constructed around it. Alternatively, total number of cells reasonable [2]. Such the geometry definition can be queried to capability has already been demonstrated output surface patches and grid cell spacing with favorable results, although mainly in specifications for use in generating fixed-wing applications to this point. unstructured surface and volume meshes. The common surface representation can just *presented at Technical Specialists' Meeting for Rotorcrafi Acoustics and Aerodynamics Williamsburg, Virginia; October 28-30, 1997 aseasilybeused to write out input for linear behind the ROTOR tool is use of pre- (panel) codes for preliminary design/quick existing tools and reusability. "check-out" of the blade aerodynamics. The paper will demonstrate the Presently, the tool is being written such that present capabilities of the tool from input it's primary output will be block structured sections to volume grid on sample blade grids using O-H and H-H topologies geometries. Future work and possible suitable for use in full-potential solvers such applications will also be discussed. as FPR and FPX[3]. As such, an approximate wake shape model will be METHODOLOGY implemented depending on whether the grid The ROTOR tool has three integral is designed for calculations in helicopter or components. The first component converts propeller modes. Outputs for other codes the input blade characteristics to a NURBS and/or grid generators will follow as time surface representation. The surface definition permits. A schematic of the simple process may be used to generate a block structured described above is shown in Figure 2. volume grid or an unstructured volume grid. Figure 3 shows the input blade airfoil The second component organizes the sections for a sample case, while Figure 4 topology. The tool uses a grid command shows the NURBS surface representation language to define the topology. A built from the input sections. command set has been implemented to The capability to generate grids define an O-H structured block topology with little or no user intervention could, of surrounding an isolated rotor blade defined course, also be exploited to advantage in a by a single NURBS surface. The third design-and-optimization environment, where component is the volume grid generator. changes in design variables need to be The ROTOR tool writes out a command set reflected in the grid to be used by the to be used by the Coordinate and Sensitivity analysis code(s). Calculator for Multi-Disciplinary Design When using typical grid generators, Optimization(CSCMDO) [5] to generate the the grid topology is defined interactively volume grid. CSCMDO is specifically around the geometry. The user defines the designed to modify existing volume grids topology and generates the volume grid. within a design optimization loop. Each Often, a significant portion of time is spent component is a separate code bound together refining the grid to resolve regions of by a UNIX script. Eventually, the first and interest. If the blade is redefined, the second components are expected to be joined topology must be modified and the complete so that maintenance of two separate codes volume grid regenerated. Similarly, if the will no longer be necessary. volume grid is refined, the sequence of steps used to define the original volume grid must Rotor Definition be repeated. Codes exist that exploit the fact The input to the tool is the rotor that when modeling the flow field definition and the grid generation surrounding isolated rotor blades, the grid parameters. The rotor is defined as a series topology is normally simple and fixed. of airfoil sections at given span stations. Codes such as GRGEN take as input the Each section is defined by a shape and airfoil section, blade characteristics, and grid various properties. The various properties generation parameters and automatically include chord, thickness ratio, twist, sweep, generate a single block structured volume and center of rotation. A global rotor grid to be used in full-potential flow codes. parameter of dihedral at the root may be Oftentimes, these grid generators lack specified. Alter applying each of the adequate control of the block interior spacing properties to each airfoil section, a set of to resolve regions of interest. It would be three dimensional curves are created. The useful to modify the volume grid locally. NURBS surface is created by interpolating However, since the grid topology is not each point on the curve and lofting a surface available to the user, it is very difficult to between the curves. The resultant surface make the necessary changes. The ROTOR quality is highly dependent upon the point tool allows the user to build the topology as distribution of each curve as well as the a sequence of commands. The resulting spanwise distance between span stations. At topology is then passed to a volume grid present a separate utility based on the generator. The main design philosophy routines found in the GridTool[6] code actually performs the creation of the NURBS surfacferomdiscretceurvedata.Thisutility surface definition is expected to be read in as writes out the surfacedefinitionin an IGES file. Eventually, the functionality IGES(InitiaGl raphicEsxchangSetandard) of defining a surface by a series of discrete format.TheNURBSsurfacceannowbe curves and generating the NURBS surface usedto generatean unstructuredor representation is expected to be added to the structuregdrid. main ROTOR code. At that time, improvements to surface interpolation Block Structured Volume routines will be made. Points are defined as The ROTOR tool is primarily raw coordinate values, a parametric position designed to generate a block structured grid on asurface, or as an offset from a previously by defining the topology using an ASCII defined point. Curves are defined by as a input deck. The ROTOR tool command set series of two or more points. A point defines geometric and topological entities in previously defined within the database may order to define the topology for a block be referenced. Surface curves have a special structured grid. Commands exist to define reference and are defined by specifying the surfaces, points, curves, blocks, edges and endpoints on the surface. A special function patches. The commands are defined in a allows the creation of an arc by specifying hierarchy as shown in Figure 5 by first three points. defining a configuration, followed by the Once the geometry is defined, the geometry, and finally the topology. The topological entities such as blocks, edges, command hierarchy is necessary so that and patches must be defined. As per the dependent entities may be defined at their entity hierarchy, the blocks are defined first. instantiation instead of waiting for the At a minimum, a block is expected to definition of the parent entity. For example, describe the dimension and the default if a point is evaluated on a specified surface, spacing at the comers. When a block is the surface must be defined before the added to the configuration, the six faces of definition of the point. The ROTOR tool the blocks are automatically created. By was designed so that additional commands default, these faces are referenced by the tags could be easily added to the tool. Efforts Kmin, Kmax, Imin, Imax, Jmin, and Jmax were made to keep the command set concise describing the relationship of the face to the and simple. Currently, there are two main computational domain. Optionally, the commands Create and Add...To. These are faces within a block may be renamed to a used to create a configuration and to add more meaningful tag using the FaceNames entities to the database. There are additional command. The edges and patches are added data identifiers to specify the type data used to the appropriate block. An edge is defined to define the object attributes. Figure 6 by a shape, a grid point distribution, and an shows a portion of the input deck used to application region within the governing define a zero twist rotor blade. There are block. The edge is associated on a specified five additional functions called Evaluate, face within the governing block. The shape Offset, Arc3pt, Reference and Conic3pt. of an edge is defined by specifying a list of Respectively, these functions are used to pre-existing curves and the valid parametric evaluate points on a surface, define a point range within each curve. Since edges are by offsetting an existing point, create an arc, directional entities, one must be careful in reference a point to be used as an endpoint to ordering the parametric range. a curve and create aconic. These are used to For example, consider the edge define points by setting dependencies to curve definition for Kmin%EDGE1 within existing points within the database. The Figure 6. The edge EDGE1 is defined as input deck consists of a series of commands part of the Kmin face of block OHBLOCK. defining each object. The edge shape is defined by two curves A configuration is first defined CURVE8 and CURVE9. The edges start at within the input deck. A configuration may the beginning of CURVE8(t=-0), and have a title associated with it. Multiple continues to the end of CURVE8(t=l). It configurations within a single input deck are continues to the beginning of CURVE9(t=0) possible. Atter defining the configuration, and finally ends at the ending of geometry and topology may be added to the CURVE9(t=l). Alternatively, consider the configuration. Surfaces are the first entities shape definition of Jmin%EDGE2. The defined within the input deck, followed by shape is defined by CURVEl5 and points, and finally curves. Currently, the CURVE l. However, the edge direction startsatthebeginninogfCURVEI5 (t=0) generate volume grids for isolated rotor andendsatthebeginningof CURVEI blades using full-potential solvers, it should (t=0). be straight forward to place the tool within a The distributionof grid points design optimization cycle. Also, CSCMDO alongtheedgeisobtainebdyspecifyintghe has some additional capabilities such as spacingat eachof the endpoints. A elliptic smoothing of faces and grid hyperbolictangentfunctionis usedto deformations that could benefit the ROTOR determintheedistributioonfinteriorpoints tool. Currently, CSCMDO uses a discrete constrainebdytheendpoinstpacing.The point representation of surfaces which is applicatiornegiondescribewsherewithin expected to be changed to a NURBS thegoverninbglockthattheedgeisdefined. representation in the near future. The grid Apatchisdefinedbyspecifyintghe deformations within CSCMDO are used to faceofthegoverninbglock,andfouredges. update an existing volume grid to conform Currentlyth,efouredgemsustbecomprised to a perturbation of the surface. It may be ofsurfacceurvefsromthesamesurfaceT.he possible to utilize this technology to model shapoefapatchwill bedeterminebdythis the rotor wake in fixed wing or hover mode. surfaceT.heapplicatiorengionforthepatch Experience has shown that the flow specifietshecomputatioranngewithinthe solver FPX is particularly sensitive at the blockthatthepatchaffects.It will be trailing edge of the rotor blade. Figure 7 determinedby the regionwithin the compares the face of grid which was governingblockwhereeachoftheedges elliptically smoothed using only Thomas- havebeendefined. Middlecoff formulation to same grid that was Once the geometryand the also smoothed using a combination of topologyis createdth,ecodegeneratethse LaPlace and Sorenson smoothing. The first necessafrilyestobeusedbythevolumegrid grid resulted in a divergent solution using generatoTr.hesurfacdeefinitionalongwith FPX. The second grid resulted in a thediscreteedgeinformatiofonrall12edges convergent solution using FPX. However, ofeachblockiswrittenout. Also,asetof the second grid required quite a bit of user- commandtso generattehe volumegrid interaction to obtain. Consequently, further withinCSCMDOisalsogenerated. study to quantify what FPX considers as a valid grid is needed. Therefore, it is likely Volume Grid Generation that some additional routines are needed to The ROTOR tool uses CSCMDO manage the smoothing within CSCMDO in to generate the volume grid. The ROTOR order to satisfy FPX requirements. tool is expected to generate a grid suitable Presently, the volume grid generated by the for using in the full-potential code FPX. ROTOR is algebraic only. The initial step is to create an algebraic structured volume grid. Afterwards, block CONCLUDING REMARKS faces can be elliptically smoothed in order to As a proof of concept, an O-H obtain the necessary orthogonality and topology was created for an untwisted V22 desired grid smoothness for FPX. Tiltrotor rotor blade. An algebraic volume CSCMDO is a general multi-block three grid was generated. Using the input deck dimensional volume grid generator which is with some minor modifications, a grid specifically designed for Multidisciplinary around a twisted V22 rotor blade was Design Optimization (MDO). The volume created. Thus far, the ROTOR tool has grids are generated and controlled by using shown some promising results. It can ASCII user input deck[5]. Within the generate a NURBS surface representation ROTOR tool, CSCMDO is automatically from discrete airfoil definitions. The run by the main script completely continuous surface representation of the rotor transparent to the user. The main advantage blade can be used to generate a structure of using CSCMDO as the volume grid block or unstructured volume grid. The generator isreuse of technology. CSCMDO primary purpose of the ROTOR tool is to has been successful in generating volume generate a block structured volume grid to multi-block structured volume grids for be used with full-potential solvers such as some time now. Another advantage is that FPX. It has been demonstrated that the CSCMDO was specifically designed to fit ROTOR tool can generate an algebraic grid within a design optimization loop. Since with no user intervention beyond setting the ROTOR tool is being designed to up the initial topology input deck. Howevesre,veraislsuenseedtoberesolved 3. Bridgeman, J.O., Pilchard, D., and beforetheobjectiveofusingtheresultant Caradonna, F.X., "The Development of a volumegridwithinFPXisreached. CFD Potential Method for the Analysis of Tilt-Rotors," presented at the AHS FUTUREWORK Technical Specialists Meeting on Rotorcrafl The scopeof the project that Acoustics and Fluid Dynamics, initiated this paper is to obtain a tool that Philadelphia, PA, October 15-17, 1991. can be used to generate a volume grid about an isolated rotor and be used by the full- 4. McCroskey, W. J.,"Some Recent potential code FPX. Another aspect of the Applications of Navier-Stokes Codes to project to generate a surface representation Rotorcrait," presented at the Fifth that can be used in both unstructured and Symposium on Numerical and Physical structured grid analysis. In order to reach Aspects of Aerodynamic Flows, Long these goals, additional work in resolving Beach, CA, January 13-16, 1992. some of the issues relating to grid smoothness, aspect ratio, and orthogonality 5. Jones, W.T. and Jamshid Samareh- off the rotor surface need to be resolved. Abolhassani, "A Grid Generation System Additionally, the use of surface fitting as for Multi-disciplinahrY Design Optimization," opposed to surface interpolation in order to Proceedings of 12 AIAA Computational create the NURBS surface representation of Fluid Dynamics Conference, AIAA-95-1689, the isolated rotor blade is also being San Diego, CA, June 20, 1995, pp. 9. investigated. As a final phase of the project, an interactive Graphical User Inlerface (GUI) 6. Samareh, Jamshid, "GridTool: A Surface is to be developed in order to help set up the Modeling and Grid Generation Too," topology more easily. Proceedings of the Workshop on Surface Modeling, Grid Generation, and Related ACKNOWLEDGMENTS Issues in CFD Solutions, NASA CP-3291, The authors would like to thank May 9- I1, 1995. Dr. H. Jones and C. Burley of the Fluid Mechanics and Acoustics Division at NASA-Langley Research Center for FIGURES initiating this project. The first author I) Unstructured surface mesh around Apache would like to thank William Jones of Helicopter Computer Sciences Corporation for his suggestions, and the use of CSCMDO 2) Schematic of ROTOR tool process which serves as the structured volume grid generator for the ROTOR tool. The first 3) Image showing input curve sections author would also like to thank Carl Rogers defining a BERP rotor blade for his suggestions and expertise in quantifying the grid quality measures that 4) Image showing the NURBS stwface result in a valid grid for FPX. obtained by ROTOR using input curves defining the BERP rotor REFERENCES 5) Diagram showing the entity definition hierarchy 1. Meakin, R., "Moving Body Overset Grid Methods for Complete Aircraft Tiltrotor 6) Text showing portions of a sample Simulations," AIAA-93-3350-CP, 1lth ROTOR input file used to describe a rotor. Computational Fluid Dynamics Conference, pp. 576-588, July 1993. 7) Image comparing two grids which were given as input to FPX full-potential flow 2. Pirzadeh, S., "Progress Toward A User- solver Oriented Viscous Unstructured Grid Generator," AIAA-96-0031, 34th Aerospace Sciences Meeting and Exhibit, January 1996. UNSTRUCTURED GRID ON AH - 64 APACHE HELICOPTER 249087 NODES, 1407114 TETRAHEDRA 16125 BOUNDARY NODES (32250 BOUNDARY FACES) Figure 1: Unstructured surface mesh around Apache helicopter Figure 2: Schematic of ROTOR tool process i • Figure 3: Image showing input curve sections defining BERP rotor blade \ \ Figure 4: Image showing the NURBS surface obtained by ROTOR using input curves defining the BERP rotor Figure 5: Diagram showing the ROTOR entity definition hierarchy Create Config XVI5 { Title Case1: Zero twist blade ) Add Surface ROTOR To XVI5 { RaadIGES Rotor. igs ) Add Point Pointl To XVI5 { XYZ( -5.0, 0.0, 0.0) ) Add Point Polntll To XV15 { Evaluate Surface ROTOR At { _/V(ul = 0.5, u2 = 0.0} } } Add Curve CURVE1 To XVI5 { Arc3Pt { Point2 Pointl6 Pointl } } Add Block OHBLOCK TO XV15 { Dimension(101, 49, 37) Spacing { .01 .01 .01 .01 .1 .1 .1 .I } } .°o Add Patch Kmin_Patchl TO OHBLOCK { Define Shape ROTOR With { Restriction { 0.0 1.0 0.0 1.0 ) } ApplyToBlock { 1 101 1 37 } ) Figure 6: Shows portions of a sample ROTOR input file used to describe a rotor Elliptically smoothed grid divergent FPX solution User smoothed grid convergent FPX solution I ,_"-x_J'Y__3-- Figure 7: Image comparing two grids which were given as input to FPX full-potential flow solver 10

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