Towed Cable System Research Articles

A Flow Structure Interaction Method for Towed Cable System

Abstract: The ocean towed cable system is a classic example of fluid-structure interaction (FSI). This interaction can exhibit stability or oscillation between a highly deformable moving cable and the surrounding turbulent flow. However, in dynamic simulations of towed cable systems, a constant drag coefficient for an infinite circular cylinder is often used based on experimental data. An innovative fluid-structure interaction method is introduced to obtain accurate drag distribution along cable to couple with towed system dynamics. A modified nodal position finite element method (NPFEM) coupled with Reynolds-averaged Navier-Stokes (RANS) approach has been utilized to predict hydrodynamic forces along the cable. A data exchange algorithm has been developed specifically for fluid-structure interaction within the towed cable system where the cable profile is transferred to construct the flow domain while hydrodynamics is interpolated for NPFEM analysis. A topology partition around cable is applied. A multiblock grid is generated around cable. The simulation results of the fluid-structure interaction of the towing system are verified. This FSI scheme reveals how strongly hydrodynamics determine cable dynamics and induce vortex structure vibrations around a towed cable system. Parametrically controlled structured grid generation and their applicability for complex flow fields have also been discussed. Detailed descriptions of boundary layer separation evolution around spatially distributed cable are provided. This FSI scheme reveals a real strongly hydrodynamic determined cable dynamics and vortex structure induced vibrations around a towed cable system. The proposed method enhances predictive accuracy of the towed system dynamics response.

Parameters influence on maneuvered towed cable system dynamics

The dynamic response of a towed cable system to ship maneuver is parametrically simulated. Three dimensionless parameters influence on towed cable system maneuverability is investigated. They are ratio of total length to turning radius R/L, ratio of cable mass to vehicle mass σ, and ratio of mass unit length to hydrodynamic force w/r. An oscillatory motion of towed vehicle is found in simulation of spiral towed courses. Features of this oscillation in different spiral courses are compared. The sharp turns, gradual turns and their transient states of towed cable dynamics for different course directions are discussed extensively. According to the characters of transient states and horizontal trajectories evolution of maneuvered cable system, the dynamic behaviors can be divided into three situations in Fig. 8 turning maneuvers. The behavior of towed cable system during a zigzag turning course is simulated in the end. Two ingredients of heave motion are found during small ratio of turning radii to length in this course. The primary damp to initial turning becomes weak and the response to alternative turns plays a more and more important role. The damping properties of the transient behavior in different maneuvers show a periodical invariance to σ during some turning maneuvers.

Optimal Trajectory Generation Using Model Predictive Control for Aerially Towed Cable Systems

This paper studies trajectory generation for a mothership that tows a drogue using a flexible cable. The contributions of this paper include model validation for the towed cable system described by a lumped mass extensible cable using flight data, and optimal trajectory generation for the towed cable system with tension constraints using model predictive control. The optimization problem is formulated using a combination of the squared error and -norm objective functions. Different desired circular trajectories of the towed body are used to calculate optimal trajectories for the towing vehicle subject to performance limits and wind disturbances. Trajectory generation for transitions from straight and level flight into an orbit is also presented. The computational efficiency is demonstrated, which is essential for potential real-time applications. This paper gives a framework for specifying an arbitrary flight path for the towed body by optimizing the action of the towing vehicle subject to constraints.

Modeling and simulation for route-transition of mine-hunting and mine-sweeping by helicopter

Concerning the issue of route-transition by helicopter towing arming of mine-hunting and mine-sweeping,three patterns of route-transition were put forward.Through analyzing the swerve characteristics of helicopter and the real movement of underwater towed cable system,the kinematic model of swerving process was established,the complexity of fluid dynamics was avoided and the calculation rate was advanced.Finally,the correctness was testified by examples of three flight states by helicopter towing arming of mine-hunting and mine-sweeping.Through increasing velocity of helicopter,the underwater towed cable system was fast close to the beeline sea-lane,the result was in accordance with the real movement.The trajectory of swerving process of route-transition by helicopter towing arming of mine-hunting and mine-sweeping was truly reflected,and the result can be used for decision-making of route optimization and operation scheme.

An experimental investigation of the equilibrium and stability of long towed cable systems

The dynamics of towed objects in a fluid environment is of interest for many practical situations. We investigate experimentally the equilibrium and stability of the trajectory of a sphere towed at constant velocity at the tip of a cable with an unprecedented large length-to-diameter aspect ratio, exceeding 104. The towing configuration is artificially obtained by considering a steady cable (with one fixed end and a free end to which a sphere is eventually attached) in a low-turbulence wind tunnel. We consider three different configurations: (i) the cable towed by itself; (ii) a light millimetric towed sphere made of expanded polystyrene; and (iii) a denser millimetric towed sphere made of lead. The trajectory of the cable tip is monitored using high-speed Lagrangian tracking, which allows one to characterize the average position and the dynamical fluctuations of the towed object. We show that the mean equilibrium position is well predicted by a simple model including the aerodynamical forces acting along the cable and on the towed sphere (when present). Concerning stability issues, we find that the heavy lead particle is always towed in stable conditions (within the accessible range of velocities) with only very low energy oscillations related to a weak pendulum-like motion. In contrast, the free end and light sphere cases are shown to become unstable when the towing velocity exceeds a certain threshold. Spectral analysis shows a flutter-type instability for the sphere, with a dominant oscillatory motion, while the cable alone develops a divergence-type instability with random fluctuations.

Optimization of Circularly Towed Cable System in Crosswind

The dynamics of aerial tethers towed in circular paths by an orbiting aircraft have been studied extensively. Typically, the atmosphere is assumed to be stationary with respect to the ground so that stable steady-state solutions are possible for certain combinations of system parameters. This paper presents a numerical approach for minimizing the disturbance of a crosswind on the periodic solution of the cable tip using a combination of towpoint manipulation and tether reeling. A discrete lumped parameter cable representation is used to simulate the physics of the cable system. Periodic solutions are generated for a range of towpoint motions including elliptical orbits. The required angular velocity of the aircraft to maintain constant airspeed is derived. Numerical results show that steady-state wind effects can be compensated for by flying the aircraft in an elliptical orbit upwind of the target point in combination with reeling the cable or flying on an inclined orbit.

On improving the accuracy of simple aerial towed-cable system models

We outline a novel procedure for improving the accuracy of a simple one-link aerial towed-cable system model. The primary objective of this work is to provide a systematic framework for matching the dynamical motion of a simple aerial towed-cable system model with that of a correspondingly more complex model. The final outcome is to achieve a compromise between a model's representativeness and the ease with which it can be used for control purposes. By modifying the cable length and the payload drag coefficient of the simple model, the equilibrium position of the cable tip may be matched analytically to that of the more complex model. The modified cable length and drag coefficient are then used in dynamic simulations and shown to dramatically improve the accuracy of the simple model.

Optimal terrain-following for towed-aerial-cable sensors

Towed-aerial cable systems are often used for towing decoys from aircraft and for collecting electromagnetic data from low altitudes. Airborne cable systems are typically controlled by maneuvering the aircraft, which can limit the safe altitude of the towed-sensor package. In this paper, a real-time optimal control strategy for controlling a towed-cable system is proposed that uses cable winch control to control the altitude of the towed-body. The controller is based on a receding horizon control approach, where the aircraft senses the terrain profile ahead of the towed-body with a relatively short time horizon. This information is sent to the controller, which updates the cable winch reel acceleration to safely avoid colliding with the terrain, while keeping the towed-sensor close to the desired altitude for obtaining good measurements. The controller is developed for a simplified system model and implemented in a multibody cable system model that incorporates cable flexibility and elasticity.

Performance Optimization for a Towed Cable System with Attached Wind Sock

[article extract] Circularly towed-cable systems have been studied for several decades with the goal of stabilizing the cable tip in inertial space. When a cable is towed from an aircraft during a steady-banked turn, the cable end body tends to tow inside the path of the circle. As the cable length is increased and the cable end body weight to drag ratio is decreased, the cable tip approaches the center of the circle and ultimately appears to be nearly stationary when viewed from the inertial frame. The idea, proven successful in test flights, is to use this fundamental property of the system for pickup and delivery of payloads.

Trajectory Generation for a Towed Cable System Using Differential Flatness

In this paper we consider the problem of generating feasible motions for a towed cable, flight control system that has been proposed for use in remote sensor applications. Using the fact that the system is differentially flat, we illustrate how to construct feasible trajectories for the system and demonstrate the strengths and limitations of this approach. Simulations of the full dynamics are included to illustrate the proposed techniques. A significant limitation of the current approach is the numerical instability of the algorithm, resulting in the need for careful tuning of parameters to achieve convergence.