The phenomenon of "brownout" is characterized by a large cloud of sediment or dust that is formed around a rotorcraft when it takes off or lands in arid or dusty environments. To further understand the physics of brownout, a laboratory-scale rotor hovering in water was tested over a ground plane covered with a mobile sediment bed. The sensitivity of the dual-phase flow environment to changes in the values of the similarity parameters that potentially govern the fluid dynamics of the rotor flow and the transport of sediment was explored. First, dye flow visualization was performed to study the general evolution of the rotor flow and its interaction with the ground plane. Then, dual-phase flow visualization was used to expose the details of the processes that mobilize and uplift loose particles from the sediment bed. It was shown using the flow visualization that the trailed vortices from the rotor blades were a primary contributor to the mobilization and suspension of sediment. Particle image velocimetry (PIV) was also used to obtain quantitative measurements of the flow velocities found in the rotor wake and near the ground plane. It is then discussed as to why the steady flow assumptions used in the usual definitions of the classical similarity parameters governing sediment transport are not as applicable to the dual-phase flows produced by a rotor operating over a mobile sediment bed. A Buckingham-Π analysis was performed to determine a set of new similarity parameters that potentially better reflect the dual-phase flow characteristics relevant to sediment mobilization and suspension by a rotor wake, including the characteristics of the tip vortices. Sixteen new similarity parameters were initially determined, five of which selected as having particular relevance. Specifically, these new similarity parameters were: 1. The mobile inertia ratio; 2. The stationary inertia ratio, 3. The terminal-swirl velocity ratio; 4. The threshold-swirl velocity ratio; 5. The terminal/threshold-swirl velocity ratio. The values of these similarity parameters were determined using the PIV measurements, and were all found to correlate to the quantity of sediment mobilized and uplifted by the rotor. The terminal/threshold-swirl velocity ratio is proposed as the potentially most important similarity parameter for further characterizing the brownout phenomenon.
Regime Recognition (RR) is a key enabling technology for a Usage Based Maintenance (UBM) program. If the estimated aircraft usage spectrum provided by such an RR algorithm is sufficiently accurate, then component life expenditure can be calculated as a function of the duration and number of occurrences of each flight regime. In general, production RR algorithms with the required accuracy have not been fielded, in part because of the challenges associated with Verification and Validation (V&V). In particular, the distinction between transient (event-based) regimes and steady-state (duration-based) regimes is an important consideration when defining performance metrics in a V&V process. While duration-based regimes lend themselves well to traditional analysis metrics that employ a confusion matrix, event-based regimes lack the error properties that make such analysis methods possible. In this paper we present analysis methods that address this deficiency and allow the use of well understood metrics derived from the traditional confusion matrix.
The Structures Division at the Naval Air Systems Command (NAVAIR) continues to support capital investment in enabling technologies for sustainment of our aircraft which will lower total life cycle costs, ensure safety, and increase operational readiness. This paper presents a general overview of the major improvements which have been made in the area of Structural Health and Usage Management (SHUM), including: usage severity monitoring via regime recognition (RR), gross weight and center of gravity (GW/CG) estimation, local/global damage detection, environmental effects monitoring, damage alleviation, prognostication, and individual asset/component tracking (IAT). Advances in structural analyses have been made in the accuracy of predicted rotorcraft loads using coupled rotor and fuselage interactions. Innovative approaches to fatigue testing at both the component and full scale airframe levels will allow for more accurate introduction of vibratory loading content from operation, reveal failure modes, and improve fatigue life predictions. Additive manufacturing (AM) of fly-away aircraft parts and the standardization of cold spray repair applications present unique qualification challenges and benefits to the warfighter.
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