Three-dimensional trajectory tracking control of an underactuated autonomous underwater vehicle with a robust adaptive algorithm

This paper investigates the problem of three-dimensional trajectory tracking control for an underactuated autonomous underwater vehicle in the presence of uncertain disturbances. The concept of virtual velocity control is adopted and desired velocities are designed using the backstepping method. Then, the trajectory tracking problem is transformed into a stabilization problem of virtual velocity errors. Dynamic control laws are developed based on non-singular terminal sliding mode control to stabilize virtual velocity errors, and adaptive laws are introduced to deal with parameter perturbation and current disturbances. The stability of the closed-loop control system is analyzed based on Lyapunov stability theory. Two sets of typical simulations are carried out to verify the effectiveness and robustness of the trajectory tracking control algorithm under uncertain disturbances.

Tracking vortex surfaces frozen in the virtual velocity in non-ideal flows

We demonstrate that, if a globally smooth virtual circulation-preserving velocity exists, Kelvin’s and Helmholtz’s theorems can be extended to some non-ideal flows which are viscous, baroclinic or with non-conservative body forces. Then we track vortex surfaces frozen in the virtual velocity in the non-ideal flows, based on the evolution of a vortex-surface field (VSF). For a flow with a viscous-like diffusion term normal to the vorticity, we obtain an explicit virtual velocity to accurately track vortex surfaces in time. This modified flow is dissipative but prohibits reconnection of vortex lines. If a globally smooth virtual velocity does not exist, an approximate virtual velocity can still facilitate the tracking of vortex surfaces in non-ideal flows. In a magnetohydrodynamic Taylor–Green flow, we find that the conservation of vorticity flux is significantly improved in the VSF evolution convected by the approximate virtual velocity instead of the physical velocity, and the spurious vortex deformation induced by the Lorentz force is eliminated.

A Walking-in-Place Method for Virtual Reality Using Position and Orientation Tracking

People are interested in traveling in an infinite virtual environment, but no standard navigation method exists yet in Virtual Reality (VR). The Walking-In-Place (WIP) technique is a navigation method that simulates movement to enable immersive travel with less simulator sickness in VR. However, attaching the sensor to the body is troublesome. A previously introduced method that performed WIP using an Inertial Measurement Unit (IMU) helped address this problem. That method does not require placement of additional sensors on the body. That study proved, through evaluation, the acceptable performance of WIP. However, this method has limitations, including a high step-recognition rate when the user does various body motions within the tracking area. Previous works also did not evaluate WIP step recognition accuracy. In this paper, we propose a novel WIP method using position and orientation tracking, which are provided in the most PC-based VR HMDs. Our method also does not require additional sensors on the body and is more stable than the IMU-based method for non-WIP motions. We evaluated our method with nine subjects and found that the WIP step accuracy was 99.32% regardless of head tilt, and the error rate was 0% for squat motion, which is a motion prone to error. We distinguish jog-in-place as “intentional motion” and others as “unintentional motion”. This shows that our method correctly recognizes only jog-in-place. We also apply the saw-tooth function virtual velocity to our method in a mathematical way. Natural navigation is possible when the virtual velocity approach is applied to the WIP method. Our method is useful for various applications which requires jogging.

Bedload transport in a steep alpine stream: Assessment of sediment mobility and virtual velocity using the bedload tracking

The bedload transport is challenging to analyze in field, consequently, several assumptions about it were made basing on laboratory researches or on short-term field studies. During the last decades several monitoring methods were developed to assess the bedload transport in the fluvial systems. The aim of this work is to investigate the transport of the coarse sediment material in a steep alpine stream, using the bedload tracking. The Rio Cordon is a typical alpine channel, located in the northeast of Italy. It is characterized by a rough streambed with a prevalent boulder-cascade and step pool morphology. Since 2011, 250 clasts equipped with Passive Integrated Transponders (PIT) were installed in the main channel, to analyze their mobility along a reach 320 m long. From November 2012 to August 2015, the transport induced by a range of hydraulic forcing between 0.44 m3 s-1 and 2.10 m3 s-1 was assessed by 10 PIT-surveys. First, the mobility expressed by the tracers was analyzed, observing marked differences in terms of travel distance. Then, the average recovery rate achieved during the tracer inventories (Rr > 70%) permitted to define the threshold discharge for each grain size class analyzed and, then, to assess the virtual velocity experienced by the tracers.

Aerodynamic Inverse Blade Design of Axial Compressors in Three-Dimensional Flow Using a Commercial CFD Program

An aerodynamic inverse design method is developed for the simulation of three-dimensional viscous flow over blades, it is implemented into a commercial CFD program, namely ANSYS-CFX, and it is applied to the design of a transonic compressor stage. The implementation is validated for Rotor 37; it is then assessed in the redesign of Stage 67 stator. One set of design choices is to prescribe a target blade pressure loading and blade thickness distributions and a stacking line from hub to tip. The blade walls are assumed to be moving with a virtual velocity that would asymptotically drive the blade to the shape that would correspond to the specified target pressure distribution. This virtual velocity distribution is computed from the difference between the computed and the target pressure distributions. This inverse design approach is fully consistent with the viscous flow assumption and is independent of the CFD approach taken. The Arbitrary Lagrangian-Eulerian formulation of the unsteady Reynolds-Averaged Navier Stokes equations is solved in a time accurate fashion with the blade motion being the source of unsteadiness. At each time step, the blade shape is modified and dynamic meshing is used to remesh the fluid flow domain. To demonstrate the ability of this approach, it is applied to redesign the stator of a transonic axial fan, Stage 67, to improve its performance.