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In this paper, the behavior of planar rigid-body mechanical systems due to the dynamic interaction of multiple revolute clearance joints is numerically studied. One revolute clearance joint in a multibody mechanical system is characterized by three motions which are: the continuous contact, the free-flight, and the impact motion modes. Therefore, a mechanical system with n -number of revolute clearance joints will be characterized by 3 n motions. A slider-crank mechanism is used as a demonstrative example to study the nine simultaneous motion modes at two revolute clearance joints together with their effects on the dynamic performance of the system. The normal and the frictional forces in the revolute clearance joints are respectively modeled using the Lankarani–Nikravesh contact-force and LuGre friction models. The developed computational algorithm is implemented as a MATLAB code and is found to capture the dynamic behavior of the mechanism due to the motions in the revolute clearance joints. This study has shown that clearance joints in a multibody mechanical system have a strong dynamic interaction. The motion mode in one revolute clearance joint will determine the motion mode in the other clearance joints, and this will consequently affect the dynamic behavior of the system. Therefore, in order to capture accurately the dynamic behavior of a multi-body system, all the joints in it should be modeled as clearance joints.

This paper develops an improved simulation model for the gas metal arc welding process, incorporating the arc root conduction zone, the transistorized power supply, and its PID-controller parameters. The model is applicable to short-circuiting, free-flight, and active control transfer modes. For the short-circuiting transfer mode, the molten metal bridging electrode tip and workpiece is modelled with a parabolic wall profile, dependent on its height. This approach significantly simplifies and accurately addresses numerical issues and singularities in calculating the principal radii. Furthermore, the pressure due to the electromagnetic force is determined, and the average velocity of the transferred liquid metal is estimated using Bernoulli’s equation. In the free-flight transfer mode, based on the dynamic force balance model, the arc root zone is considered thanks to the relationship between the arc current and the current emission density. Finally, the proposed model is simulated using MATLAB/SIMULINK, and the simulation results are discussed and compared with those in the specialized literature, showing a great accordance.

The high-power density and good scaling properties of electric motors enable new propulsion arrangements and aircraft configurations. This results in distributed propulsion systems allowing to make use of aerodynamic interaction effects between individual propellers and the wing of the aircraft, improving flight performance and thus reducing in-flight emissions. In order to systematically analyze these effects, an unmanned research platform was designed and built at the University of Stuttgart. As the aircraft is being used as a testbed for various flight performance studies in the field of distributed electric propulsion, a methodology for precise identification of its performance characteristics is required. One of the main challenges is the determination of the total drag of the aircraft to be able to identify an exact drag and lift polar in flight. For this purpose, an on-board measurement system was developed which allows for precise determination of the thrust of the aircraft which equals the total aerodynamic drag in steady, horizontal flight. The system has been tested and validated in flight using the unmanned free-flight test platform. The article provides an overview of the measuring system installed, discusses its functionality and shows results of the flight tests carried out.

Gas metal arc (GMA) welding requires improved process stability, higher quality and efficiency, and quantitative control of the heat input and deposition. These requirements can be achieved by appropriately controlling the metal transfer phenomenon. However, this control method has primarily been applied to short-circuit transfer, and very few examples of its application to free-flight transfer exist. Therefore, the effect of wire feed control on free-flight transfer remains unclear. In this study, the influence of wire feed control on the free-flight transfer phenomenon in the GMA welding process using an aluminum wire electrode was investigated through experimental observations, and free-flight transfer control was attempted. It was observed the free-flight transfer phenomenon, particularly globular transfer, under low-current conditions with controlled wire feeding under various feed conditions, using wire feed–retract speeds and cycles as parameters. The observation results revealed two patterns with different timings of droplet detachment under long- and short-period conditions. Furthermore, the observation of the droplet detachment motions revealed that the inertia caused by the acceleration or deceleration of the feed speed acts on the droplet. Moreover, the difference between the two transfer patterns is primarily caused by the inertia acting on the droplet before and after switching the wire feed–retract direction and the size of the droplet at that time. Based on this, free-flight transfer can be stabilized by reconfiguring the feed conditions.

This paper provides a study of free-flight air traffic behaviour in increasingly constrained airspace environments. Traffic assumes three different free-flight operational constructs with airspace constraints considered as restricted (no-fly) regions. Simulations combine path planning and Monte Carlo techniques to qualitatively analyse emergent traffic behaviour and quantitatively assess spatial–temporal airspace conflict as the airspace constraints vary. Findings indicate that airspace constraints have a much stronger influence on aircraft behaviour than the free-flight operational construct, with any benefits of free flight rapidly diminishing as the airspace becomes more constrained. We conclude that structured traffic route (or network) designs and associated risk modelling approaches should be considered for safe and efficient traffic management of highly constrained and congested (or dense) airspace. This work therefore provides evidence to inform new airspace design and management initiatives, including low-altitude uncrewed traffic.

Parallel sidewalls are the standard bounding walls in wind tunnels when making a wind tunnel model for free-flight condition. The consequence of confinement in wind tunnel tests, known as wall-interference, is one of the main sources of uncertainty in experimental aerodynamics, limiting the realizability of free-flight conditions. Although this has been an issue when designing transonic wind tunnels and/or in cases with large blockage ratios, even subsonic wind tunnels at low-blockage-ratios might require wall corrections if a good representation of free-flight conditions is intended. In order to avoid the cumbersome streamlining methods especially for subsonic wind tunnels, a sensitivity analysis is conducted in order to investigate the effect of inclined sidewalls as a reduced-order wall insert in the airfoil plane. This problem is investigated via Reynolds-averaged Navier–Stokes (RANS) simulations, and a NACA4412 wing at the angles of attack between 0 and 11 degrees at a moderate Reynolds number (400 k) is considered. The simulations are validated with well-resolved large-eddy simulation (LES) results and experimental wind tunnel data. Firstly, the wall-interference contribution in aerodynamic forces, as well as the local pressure coefficients, are assessed. Furthermore, the isolated effect of confinement is analyzed independent of the boundary-layer growth. Secondly, wall-alignment is modified as a calibration parameter in order to reduce wall-interference based on the aforementioned assessment. In the outlined method, we propose the use of linear inserts to account for the effect of wind tunnel walls, which are experimentally simple to realize. The use of these inserts in subsonic wind tunnels with moderate blockage ratio leads to very good agreement between free-flight and wind tunnel data, while this approach benefits from simple manufacturing and experimental realization.

The stick-free flight stability is an old-fashioned and non-progressive issue; nevertheless, it is still existent and of significant importance to the design of aircraft whose control system is reversible. The existence of the problem necessitates a deep assessment of stick-free flight stability throughout the aircraft design. Up to now, this problem has been addressed using either analytical approaches, which are only related to the static stability evaluation, or performing flight tests. In this study, the problem is handled in its entirety, from static and dynamic flight stability assessment to design criteria with a comprehensive perspective. Moreover, it is also exhibited that, contrary to what has been generally proposed in the literature, the limitation of the problem of stick-free flight stability through static stability assessment is far from being the main challenge. As a brief scope, the derivation of the control surface dynamics, a stick-free trim algorithm, and assessment rationale of the stick-free static and dynamic flight stability using a simulation approach are proposed. As a consequence, the aim is to set a broad understanding for designers related to this phenomenon and add adjunct design criteria in the design optimization process by approaching it from a modeling, simulation, and flight test perspective.

Understanding the aerodynamic coefficients of meteoroid fragments, deorbiting space debris, or launch vehicle stages through atmospheric reentry is essential for ground risk assessments. In high enthalpy flow, surface roughness is a crucial factor affecting the aerodynamic coefficient. In this work, the effect of surface roughness on drag coefficient is investigated experimentally within a Ludwieg tube at a Mach 4 test condition. The test model is a sphere with a 5 mm diameter. Three different types of surface roughness are considered using a pre-heating process. Shadowgraph technique was used to visualize the flow features and model behavior. Based on the acquired high-speed images, the drag coefficients were obtained using an image tracking technique. Results show that the drag coefficient decreases with corresponding increases in surface roughness for the given flow condition, implying the importance of surface roughness effect in ground risk assessments.

Based on the linear displacement motion similarity and angular displacement motion similarity, the similarity law for free flight test of light store separation from aircraft is deduced. The problem that the ideal test model is too light to be processed and the free flight separation test in wind tunnel is harder to achieve due to the model scaling is solved. The new similarity law was simplified reasonably, and the relationship equation between separation velocity and model mass was obtained. The wind-loaded state is simulated by computational fluid dynamics, and the real separation data of aircraft, the separation data of previous test methods, and the simulation data of different mass and different separation velocity trajectories of this similarity law are obtained. The improved effect of the new similarity law is verified by data comparison. The results show that by properly increasing the mass of wind tunnel test model, the difficulty of wind tunnel test can be greatly reduced while the separation trajectory is similar, and the angular displacement error can be guaranteed to be within acceptable range. In order to ensure better consistency between flight and free flight wind tunnel test, it solves the problem that similarity law can only be designed but cannot be realized.

Flow visualizations have been performed on a free flying, flapping-wing micro air vehicle (MAV), using a large-scale particle image velocimetry (PIV) approach. The PIV method involves the use of helium-filled soap bubbles (HFSB) as tracer particles. HFSB scatter light with much higher intensity than regular seeding particles, comparable to that reflected off the flexible flapping wings. This enables flow field visualization to be achieved close to the flapping wings, in contrast to previous PIV experiments with regular seeding. Unlike previous tethered wind tunnel measurements, in which the vehicle is fixed relative to the measurement setup, the MAV is now flown through the measurement area. In this way, the experiment captures the flow field of the MAV in free flight, allowing the true nature of the flow representative of actual flight to be appreciated. Measurements were performed for two different orientations of the light sheet with respect to the flight direction. In the first configuration, the light sheet is parallel to the flight direction, and visualizes a streamwise plane that intersects the MAV wings at a specific spanwise position. In the second configuration, the illumination plane is normal to the flight direction, and visualizes the flow as the MAV passes through the light sheet.