Re: DT-III aboard USS America
Posted: 11 Dec 2016, 03:58
This is the abstract & introduction from F-35B LHA Modelling PDF above/on previous page now.
Computational Analysis for Air/Ship Integration: 2nd Year Report
Susan A. Polsky US Naval Air Warfare Center, Aircraft Division (NAWCAD), Patuxent River, MD
"Abstract
This paper documents the accomplishments from the second year of a three-year Grand Challenge Project focusing on the application of computational fluid dynamics to predict coupled ship and aircraft aerodynamics. Unstructured chimera techniques were used to simulate the coupled ship and aircraft systems. Dynamic aircraft maneuvers were prescribed with the intention of building simulations with an auto-pilot-in-the-loop. All simulations were computed in a time-accurate fashion due to the unsteady nature of the flowfield, and used the commercial flow-solver Cobalt. Analyses for both vertical shipboard landings of the Joint Strike Fighter (JSF) and rotary-wing aircraft are discussed. Internal components of the JSF lift-fan were added to the model to increase the solution fidelity.
1. Introduction
While many aspects must be taken into consideration to ensure safe shipboard flight operations, a primary factor is evaluation of turbulent air-wake effects on aircraft performance and pilot workload. The air-wake is a product of wind passing over ship structures creating non-uniform, turbulent air flow. The US Navy conducts shipboard dynamic interface (DI) testing to evaluate ship air-wake effects on aircraft operations. These tests result in wind-over-deck (WOD) flight envelopes that prescribe in what wind conditions an aircraft can or cannot fly. The WOD flight envelopes are part of the operating procedures for all ship-based aircraft. Testing is required to generate WOD envelopes for each model of air vehicle operating from a given ship. The DI tests are performed at-sea, typically over the course of several weeks.
Application of computational fluid dynamics (CFD) methods to predict the turbulent ship air-wake has been studied in the past with considerable success[1–4]. This has resulted in the use of CFD as an analysis tool to “diagnose” air-wake structures that may impact air operations for both current and future ship designs. These diagnoses are accomplished by linking stored CFD-generated air-wake data with offline aircraft models, controlled by either a pilot model or some other autonomous controller. For this “one-way coupled” approach, the air-wake data is imposed on the aircraft model; however, the presence of the aircraft does not feed back into the air-wake data. While this approach has proven very useful, there are limitations to its applicability. When employing CFD data generated from a ship in isolation, the underlying assumption is that the presence of the aircraft will not affect the air-wake from the ship structures. In the case of a small uninhabited air vehicle (UAV), this is likely a valid assumption; however, for an aircraft that produces a large wake of its own (such as a helicopter), this assumption becomes less-and-less valid as the aircraft comes in closer proximity to ship structures. The interaction of the ship air-wake and the aircraft wake is generally referred to as ship/aircraft coupling. Aerodynamic coupling is a concern for both fixed-wing and rotary-wing shipboard operations.
As mentioned above, past research developed methods to accurately predict ship air-wake and laid the groundwork for prediction of coupled ship & aircraft predictions. Research executed in the 2006–2008 time-frame demonstrated the feasibility of modeling both stationary aircraft and aircraft with prescribed-motion immersed in ship air-wake. The aircraft types examined included fixed-wing (F-18) and rotary-wing (V-22, H-60). The present work builds upon past research in coupled ship/aircraft modeling through support from the Office of Naval Research “Coupled Aircraft Ship Simulation for Improved Acquisition” (CASSIA) program and the Joint Strike Fighter (JSF) program.
The goal of the CASSIA program is to understand the physical and numerical modeling deficiencies that prevent the application of current dynamic interface simulations for flight-envelope prediction. Analysis of an H-60 helicopter with a DDG (destroyer)-class ship was continued in the 2nd year (Figure 1). The motivation for the DDG/H-60 analysis is to understand where (in regards to proximity to a ship) aerodynamic coupling becomes important for rotary-wing vehicles. This knowledge, along with the coupled DDG/H-60 CFD data will be used to develop methods to account for aerodynamic coupling suitable for man-in-the-loop simulations (i.e., methods that run in real-time).
Analyses of the short takeoff vertical landing (STOVL) JSF on L-class US Navy ships (Figure 2) were also conducted. The JSF CFD analysis was required to prepare the ship for JSF testing. In particular, analyses were required to examine whether JSF outwash during vertical landings (VL) would damage critical (and costly) ship systems, either due to exposure to hot jet exhaust or due to the force of the jet exhaust. During an approach to a landing spot, the aircraft core nozzle passes near many large hull-mounted systems, such as radars and weaponry, and passes directly over the catwalk and flight deck. Although sub-scale experimental studies of the STOVL outwash field were conducted, these experiments were for zero ambient wind-speed with exhaust impingement on flat plates. It was recognized that the air-flow patterns would likely be affected by surrounding ship structures and prevailing wind-speed and direction. Therefore, the CFD analyses included the ship hull, island superstructure, and many of the larger ship systems in the area of concern (Figure 2). The CFD analyses were used to help determine whether steps should be taken to move or shield deck-edge equipment and replace it with instrumentation to gather data during flight-test at-sea. In the 2nd year of this project, short takeoff (STO) scenarios were also examined. In addition, the fidelity of the JSF model was significantly increased to include most of the internal lift-fan structure...."
Source: https://www.hpc.mil/images/hpcdocs/news ... _small.pdf (24.2Mb)