N15A-T002 TITLE: Improved Turbulence Modelling Across Disparate Length Scales for Naval Computational Fluid Dynamics Applications

TECHNOLOGY AREAS: Air Platform

OBJECTIVE: Develop improved numerical methods for predicting the unsteady turbulent flows that dominate naval aviation.

DESCRIPTION: Highly unsteady vorticity-dominated turbulent flows are significant drivers of aircraft performance; moreover, operating in the naval environment adds additional complexities that are truly unique and directly impact safety and mission success. For example, approaching and landing on the back of a pitching and heaving small-deck ship is one the most challenging tasks faced by rotary-wing naval aviators. In addition to the difficulties related to ship motion and weather, the airwake associated with the ship, which contains a complicated set of turbulent vortical structures of varying length scales, can persist for many ship lengths downwind. The interaction of this ship airwake with the approaching aircraft directly impacts aircraft aeromechanics through complicated fluid dynamics and fluid-structural dynamics interactions.

In recent years, advancements in computational fluid dynamics (CFD) methods (large eddy simulation [LES], detached eddy simulation [DES], unsteady Reynolds-averaged Navier-Stokes [URANS] and their variation) have demonstrated their ability to predict rotorcraft aeromechanics (Ref 1 and 2) and ship airwakes near to the deck (Ref 3 and 4). However, there are vastly different lengths and time scales associated with rotor tip vortices (measured in inches) and the vortical structures shed from the ship (measured in multiple feet). Predicting fully-resolved (spatially and temporally) rotorcraft-ship airwake interactions in the far wake using these CFD methods will be unlikely for many years to come without major simplifications of the physics. It is exactly this issue that the proposed effort seeks to address. The Navy solicits innovative methods for addressing these disparate length scales and maintaining the necessary accuracy at minimal computational effort.

For example, it has been shown that vorticity-velocity formulations of the Navier-Stokes equations can maintain vorticity on coarser grids than conventional pressure, velocity, and density CFD formulations (Ref 5 and 6). Furthermore, the addition of numerical viscosity to stabilize traditional numerical methods not only diffuses the types of vortical structures prevalent in the wakes of ships and rotorcraft over time, but may also temporally smooth out the inherent instabilities in the underlying physics associated with the cascade of energy from large- to small-scale turbulent structures. The objective of this effort is to develop efficient turbulence modeling by understanding some of the assumptions that have been made in the development of contemporary CFD solvers. The following questions should be addressed in this effort: the role of symmetric vs. non-symmetric assumptions in the viscous stress tensor represented on a finite size mesh, how to efficiently model the growth of small-scale turbulent structures without refining down to Kolmogorov�s scale, and understanding the tradeoffs associated with vorticity-velocity versus traditional CFD formulations. It should be noted that the aforementioned observations on the vorticity-velocity formulation are used for illustrative purposes and are not meant to imply an approach to this topic.

PHASE I: Formulate and define a theoretical basis for turbulence modeling. Demonstrate feasibility through the prediction of unsteady vorticity-dominated flows through comparison of the new approach to existing turbulence models. Key metrics include changes in length scales that can now be resolved (versus being modeled) and computational complexity of the algorithm, including changes in floating point operations.

PHASE II: Develop new CFD analysis on classical turbulence problems and problems of interest to the Navy. Demonstrate prototype turbulence modeling software capable of predicting: ship airwake turbulence during aircraft launch and recovery; wing low speed, high lift flows with extended flap systems; aircraft-to-aircraft formation flight interactional aerodynamics; rotorcraft downwash/wake ground interference; and transonic shock / boundary layer interactions.

PHASE III: Commercialize and transition the technology for improved predictions of ship airwake, dynamic interface, rotorwash, formation flight and rotorcraft performance. This will involve a detailed verification and validation effort along with a demonstration of application capability in a production-type and widely used tool. This phase would likely include integration within the US Department of Defense (DoD) primary tools being developed within the Computational Research and Engineering Acquisition Tools and Environments (CREATE-AV) program. The details of working with the CREATE-AV team will be developed through close coordination with the NAVAIR engineers who are assigned to the CREATE-AV quality assurance team.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: While the Navy has a very unique environment with regards to shipboard launch and recovery, this technology would also be useful for other complex wake interactions. Improvement to CFD codes resulting from the work would be applicable to interactions ranging from wind gust interactions between city buildings to the complex flow field emanating from the nose of a race car and interacting with the rear wing. Anything that CFD is used on in the commercial world could benefit from this work.

REFERENCES:
1. Lim, J.W., Wissink, A., Jayaraman, B., & Dimanlig, A. (2012). Helios adaptive mesh refinement for HART II rotor wake simulations. Paper presented at the 68th Annual Forum of the American Helicopter Society, Fort Worth, TX. Retrieved from: http://arc.aiaa.org/doi/abs/10.2514/6.2012-713

2. Biedron, R.T. & Lee-Rausch, E.M. (2011). Computation of UH-60A airloads using CFD/CSD coupling on unstructured meshes. Paper presented at the 67th Annual Forum of the American Helicopter Society, Virginia Beach, VA. Retrieved from: http://ntrs.nasa.gov/search.jsp?R=20110011257

3. Quon, E.W., Cross, P.A., Smith, M.J., Rosenfeld, N.C., & Whitehouse, G.R. (2014). Investigation of ship airwakes using a hybrid computational methodology. Paper presented at the 70th Annual Forum of the American Helicopter Society, Montreal, Canada.

4. Polsky, S.A. and Miklosovic, D.S. (2011). CFD study of bluff body wake from a hangar with comparison to experimental data. Paper presented at the 29th AIAA Applied Aerodynamics Conference, Honolulu, HI, AIAA-2011-3351. Retrieved from: http://arc.aiaa.org/doi/abs/10.2514/6.2011-3351

5. Whitehouse, G.R. & Boschitsch, A.H. (2013). Towards the next generation of grid-based vorticity-velocity solvers for general rotorcraft flow analysis. Paper presented at the 69th Annual Forum of the American Helicopter Society, Phoenix, AZ.

6. Harris, R.E., Sheta, E.S., Noack, R.W., & Sankaran, V. (2012). Rotorcraft flow modeling using hybrid vorticity transport and Navier-Stokes method. Paper presented at the 50th AIAA Aerospace Sciences Meeting, Nashville, TN, AIAA-2012-1102.

KEYWORDS: computational fluid dynamics; Turbulence models; vorticity-velocity formulation; helicopter/ship airwake interaction; Large Eddy Simulation; unsteady Reynolds-Averaged Navier-Stokes

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