Showing papers by "Roger E. A. Arndt published in 2014"

Planing avoidance control for supercavitating vehicles

Abstract: Underwater vehicles traveling inside a bubble or supercavity enable the reduction of drag and increase of speed; however, planing forces generated when the vehicle aft end pierces the bubble can lead to oscillatory motion and instability. In this paper, a framework for the synthesis of planing avoidance controllers is presented and the trade-off between tracking performance and planing avoidance is investigated. We propose mathematical models of the supercavity and planing forces, based on experimental data, that are used to construct a nonlinear model of the vehicle dynamics and a simplified version suitable for analysis and controller synthesis. A planing metric, based on the simplified vehicle dynamics, is used within the controller synthesis to avoid planing. Simulations of vehicle maneuvers demonstrate that including planing avoidance as a control objective, reduces planing and increases the bounds of tracking commands for which the vehicle remains stable.

Measurements in the wake of a ventilated hydrofoil: a step towards improved turbine aeration techniques

Abstract: The purpose of this study is to develop the necessary algorithms to determine the bubble size distribution and velocity in the wake of a ventilated or cavitating hydrofoil utilizing background illumination. A simplified experiment was carried out to validate the automatic bubble detection algorithm at the Saint Anthony Falls Laboratory (SAFL) of the University of Minnesota. The experiment was conducted in the SAFL high-speed water tunnel. First, particle shadow velocimetry (PSV) images of a bubbly flow were collected. Bubbles were identified in the images using an edge detection method based on the Canny algorithm. The utilized algorithm was designed to detect partly overlapping bubbles and reconstruct missing parts. After all images were analyzed, the bubble velocity was determined by applying a tracking algorithm. This study has shown that the algorithm enables reliable analysis of irregularly shaped bubbles even when bubbles are highly overlapped in the wake of the ventilated hydrofoil. It is expected that this technique can be used to determine the bubble velocity field as well as the bubble size distributions.

Application of a shadow image velocimetry to a ventilated hydrofoil wake

Abstract: A study is being carried out at the Saint Anthony Falls Laboratory (SAFL) of the University of Minnesota to develop the necessary algorithms to determine the velocity deficit and bubble size distribution in the bubbly wake behind a ventilated or cavitating hydrofoil. This is done by utilizing background illumination of the bubbly wake. Shadows of the bubbles, cast on a bright background, were collected by a high-speed video camera. For detecting bubble shadows, the image was segmented using an edge detection technique such as Canny algorithm. Then, a curvature profile method was employed to distinguish individual bubbles within a cluster of highly overlapping bubbles. The utilized algorithm was made to detect partly overlapping bubbles and reconstruct the missing parts. The movement of recognized individual bubbles was tracked on a two-dimensional plane within a flow volume. To suppress only high frequency background variations generated by out-of-focus bubbles, a Gaussian low-pass filter was applied in this study. To obtain quantitative results, the wake of a ventilated NACA0015 hydrofoil was next investigated by applying a shadowgraphy technique and the described bubble detection algorithm. Results regarding bubbles’ appearance, their velocity, and void fraction are presented. It was found that most of the bubbles were shaped like a circle and with a radius of 0.22 mm and the bubble’s velocity was dependent of its size. Local void fraction was further analyzed and a possible problem was discussed in the paper.

Modeling and simulation of supercavity with inertial force in the horizontal curvilinear motion

Abstract: To make a curvilinear motion in the horizontal plane is one of the most contents for realizing the maneuverability of the supercavitating vehicle. It is significant to achieve the controllability and maneuverability of the vehicle in three dimensions both theoretically and practically on research. Models of angle of attack, gravity and inertial force effects on the supercavity in the horizontal curvilinear motion are established, respectively. The supercavity is simulated based on these models in combination with Logvinovich model and the unsteady gas-leakage rate model at the given ventilation rate, and the effect of the inertial force on it is analyzed numerically. Results show that the maximum deviation of the center line of the cross section of supercavity towards the outward normal direction of its trajectory increases as the cavitation number or curvature radius decrease and always occur in the tail because of the increase of inertial effects along the axis of supercavity from the cavitator when other models and flow parameters are constant for the given trajectory curvature. For the variable curvature, the supercavity sheds due to its instability caused by the time-varying angle of attack. The deviation increases along the length of supercavity if the curvature remains the same sign.

Flow structure interaction around an axial-flow hydrokinetic turbine: Experiments and CFD simulations

Abstract: We carry out large-eddy simulation of turbulent flow past a complete hydrokinetic turbine mounted on the bed of a straight rectangular open channel. The complex turbine geometry, including the rotor and all stationary components, is handled by employing the curvilinear immersed boundary (CURVIB) method [1], and velocity boundary conditions near all solid surfaces are reconstructed using a wall model based on solving the simplified boundary layer equations [2]. In this study we attempt to directly resolve flow-blade interactions without introducing turbine parameterization methods. The computed wake profiles of velocities and turbulent stresses agree well with the experimentally measured values.

Turbulence within variable-size wind turbine arrays

Abstract: A wind tunnel experiment was performed to study turbulence processes within a model wind turbine array of 3 by 8 model wind turbines of alternating sizes placed aligned with the mean flow. The model wind farm was placed in a boundary layer developed over both smooth and rough surfaces under neutrally stratified conditions. Turbulence statistics, TKE budget terms, and the spectral structure of the turbulence generated within and above the wind farm reveal relevant information about the processes modulating the turbulent energy transfer from the boundary layer to the turbines. The results of the experiment suggest that heterogeneity in turbine size within a wind farm introduce complex flow interactions not seen in a homogeneous farm, and may have positive effects on turbulent loading on the turbines and turbulent exchange with the atmosphere. In general, large scale motions are heavily dampened behind the first row of turbines but a portion of such structures are generated far inside the wind farm, and the scale of the most energetic eddy motions was relatively consistent at different elevations. Overall, the experiment revealed the possibility that heterogeneity of wind turbine size within wind farms have the potential to change the overall potential to harvest energy from the wind, and alter the economics of a project.