TECHNOLOGY AREAS: Air Platform, Materials/Processes

OBJECTIVE: Develop methodology and code suitable for the integration of more accurate rotor wake computations within current unstructured CFD computations.

DESCRIPTION: Current CFD rotor computations have notoriously dissipative rotor wakes. This premature numerical dissipation disallows rotor wake surface interaction problems from being accurately computed more than a couple of rotor wake revolutions from the tip path plane. There have been projects that address this issue with full overset rotor computations. Specifically these are the Hybrid CFD code, which integrates free wake computations with overset rotor motion, and the vortex tracking project, which attaches refined grids to the overset rotor blades to minimize wake dissipation. However, full overset rotor computations are expensive and in many cases not necessary. Additionally, current research in the development and enhancement of simplified actuator disk models as a substitute for full overset rotor computations do not address the modeling or dissipation of the rotor wake.

This topic specifically addresses the issue of modeling a rotor wake with simplified rotor models. The ultimate goal is to produce a method suitable for more efficient rotor/fuselage or rotor/ship interaction problems that will preserve the rotor wake away from the rotor plane in the computational regions of interest without losing the accuracy of unstructured CFD computations of the flow around the fuselage or ship structures. Portable library formats that may be integrated with various unstructured CFD codes are preferred.

PHASE I: Determine feasibility of the proposed methodology for the rotor model through initial testing. This must include an evaluation of an initial integration with unstructured CFD computations, and must address all other areas of technical risk associated with the proposed method. Efficiency of the proposed method must also be reported. Specific requirements for the end of Phase I are a measure of how far from the rotor the wake accuracy is preserved, and how well the wake is able to conform to the solid surfaces.

PHASE II: Full coupled code development and evaluation. Demonstration of code ability compared with available test data is expected, as is a report of efficiency compared with current overset CFD and actuator disk computations.

PHASE III: Final product development into a form suitable for transition.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Rotor CFD is presently used in government, industry, and academia. This is true of both overset rotor computations, and actuator disk computations. Thus an enhancement to these methods will be useful to all three organizations. The computations themselves may be used to predict many different rotor problems that need to address rotor vortex interactions. Including isolated rotor/fuselage problems and rotor/external structure problems. The external structures may include ships, buildings, or any structure that may presently be modeled using CFD.

REFERENCES:
1. Analysis of Rotor-Fuselage Interactions Using Various Rotor Models, David M. O’Brien, Jr. and Marilyn J. Smith, AIAA-2005-0468, January 2005.

2. Implementation of a Free-Vortex Wake Model in Real-Time Simulation of Rotorcraft, Joseph F. Horn; Derek O. Bridges; Daniel A. Wachspress; Sarma L. Rani, Journal of Aerospace Computing, Information, and Communication 2006 1542-9423 vol.3 no.3 (93-107).

3. Modeling Rotor Wake Dynamics with Viscous Vortex Particle Method, Chengjian He and Jinggen Zhao, 46th AIAA Aerospace Sciences Meeting and Exhibit, January 7-10, 2008 Reno, NV.

KEYWORDS: Rotor; Wake; Unstructured; CFD; Ship; Fuselage; Interaction