Study on longitudinal stability of ducted vertical take-off and landing fixed-wing UAV

The longitudinal flight stability of the ducted vertical take-off and landing fixed-wing UAV during the flight state of hovering and transition is studied. Firstly, based on the Blade-Element Momentum Theory (BEMT) and experimental data, a coaxial dual-rotor ducted aerodynamic model and a thrust ducted aerodynamic model based on characteristic cross-section calculations are established. The model parameters are identified according to the experimental data. Secondly, a UAV flight dynamics model with thrust duct deflection is established according to the six-degree-of-freedom equations. Finally, the case UAV was used to solve the longitudinal balance and stability analysis of hovering and transition state with the established model method, and compared with the hovering experimental results. The results show that the UAV flight dynamics model combined with the ducted dynamic model established in the article can accurately describe the longitudinal flight stability characteristics of this type of aircraft.

Introduction to the Special Issue on 3D Methodologies in Mediterranean Archaeology

This issue brings together several papers originally presented at the 2019 annual meeting of the Archaeological Institute of America in a panel entitled, “Three-Dimensional Archaeology Comes of Age.” This collection takes stock of a decade’s worth of groundbreaking transformation in archaeological practices with a focus on the ancient Mediterranean. Over this time, a subtle transition has occurred in which contentious debates over the value and practicality of 3D tools, such as photogrammetry, 3D scanning, 3D reconstruction, and virtual reality (VR), have given way to an emergent consensus that these constitute a new and important class of recording and heuristic instruments. Rather than seek to cover this fundamental shift in a comprehensive matter, this issue presents a characteristic cross-section of current archaeological research-based on three-dimensional computational methodologies. The content cuts across some 3,000 years of Mediterranean archaeology, from the Aegean Bronze Age to the later Roman Empire, underlining the discipline-wide impact of this methodological revolution. It seeks to shed light on how digital tools are transforming not just the ways we record data, but the very questions archaeologists ask of this information and how this will shape methodological and analytical trends in the next decade and beyond.

Characterization of Measured VIV for Free Spanning Pipelines

As the search for oil and gas takes place on increasing water depths, the traditional production systems based on fixed or floating platforms becomes increasingly costly. A good alternative to the traditional production systems are subsea to shore solutions, where pipelines transport the product to a land terminal for processing. One of the main challenges of documenting pipeline integrity in subsea to shore solutions is related to fatigue induced by vortex induced vibrations (VIV). Rough and uneven seabed introduce free spans that can be short or long, have small or large clearance and can have a large variety of boundary conditions at the shoulders. Hence, there is a need for methods that can give reliable estimate for fatigue damage under all realistic conditions. This paper investigates the measured VIV for a free spanning pipeline model. The model has L/D ratios from 72 to 350 and includes both single and multi-spans. It is exposed to uniform current and is free to vibrate in both cross-flow (CF) and in-line (IL) direction. Bending strains are measured in CF and IL direction at ten locations along the pipe. The observed cross-sectional trajectories are analyzed, with particular focus on the complex interaction between CF and IL VIV, and the significance of higher order response components. The purpose of this investigation is to understand the behaviour of the free spanning pipelines with respect to response amplitudes, frequency and modal composition, and also to identify characteristic cross section trajectories for further investigation. This work is a continuation of the earlier published results by Nielsen et al. [1] and Soreide et al. [2].