Accurate models for the prediction of ship airwake flowfields are critical to the development of realistic flight simulation tools for aircraft carrier launch and recovery operations. The accurate computation of the ship airwake can be very challenging due to the complexity of the ship geometry, the size of and difficulty in generating a suitable computational mesh, and the large range of length and time scales present in the unsteady flowfield. The present paper investigates the sensitivity of the airwake solution to several modeling parameters, including geometric complexity and the resolution of boundary layers, with the aim of determining the level of fidelity required to obtain an accurate solution.Results are compared to wind tunnel experimental measurements. The results of these studies show that, in general, a majority of the airwake flow features are characterized by bluff body shedding from the larger geometric entities that comprise the ship geometry. Depending on the requirements and intended use of the solution, a certain tradeoff can be reached between solution turn-around/grid generation time and solution accuracy.
This study presents the development of computationally efficient coupling of NavierStokes Computational Fluid Dynamics (CFD) with a helicopter flight dynamics model with the ultimate goal of real-time simulation of airwake effects in the helicopter/ship Dynamic Interface (DI). The flight dynamics model is free to move within a computational domain, where the main rotor forces are converted to source terms in the momentum equations of the CFD solution using an actuator disk model. Simultaneously, the CFD solver calculates induced velocities that are fed back to the simulation and affect the aerodynamic loads in the flight dynamics. The CFD solver models the inflow, ground effect and interactional aerodynamics in the flight dynamics simulation, and these calculations can be coupled with the solution of the external flow (e.g., ship airwake effects). The simulation framework for fully-coupled pilot-in-the-loop (PIL) flight dynamics/CFD is demonstrated for a simplified shedding wake. Initial tests were performed with 0.38 million structured grid cells running on 352 processors and showed near-real-time performance. Improvements to the coupling interface are described that allow the simulation run at near-real-time execution speeds on currently available computing platforms. Improvements in computing hardware are expected to allow real-time simulations.
The safe and reliable operation of high pressure test stands for rocket engine and component testing places an increased emphasis on the performance of control valves and flow metering devices. In this paper, we will present a series of high fidelity computational analyses of systems ranging from cryogenic control valves and pressure regulator systems to cavitating venturis that are used to support rocket engine and component testing at NASA Stennis Space Center. A generalized multi-element framework with sub-models for grid adaption, grid movement and multi-phase flow dynamics has been used to carry out the simulations. Such a framework provides the flexibility of resolving the structural and functional complexities that are typically associated with valve-based high pressure feed systems and have been difficult to deal with traditional CFD methods. Our simulations revealed a rich variety of flow phenomena such as secondary flow patterns, hydrodynamic instabilities, fluctuating vapor pockets etc. In the paper, we will discuss performance losses related to cryogenic control valves, and provide insight into the physics of the dominant multi-phase fluid transport phenomena that are responsible for the "choking like" behavior in cryogenic control elements. Additionally, we will provide detailed analyses of the modal instability that is observed in the operation of the dome pressure regulator valve. Such instabilities are usually not localized and manifest themselves as a system wide phenomena leading to an undesirable chatter at high flow conditions.
Validation of CRUNCH CFD® for problems of relevance to modeling coupled VSTOL-Ship Airwake flow fields is presented. The basic numerical framework consists of an edge based multi-element unstructured Navier-Stokes solver. The governing equations are solved using an upwind biased MUSCL scheme and an implicit scheme is used for time marching. Validation of the spatial and temporal aspects of the numerical framework are presented here, with a special emphasis on bluff body shedding problems as these are of particular relevance to the VSTOL-Ship airwake problem. Results from simulations of several unit problems are presented.
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