Mark A. Grosenbaugh

Drag reduction in fish-like locomotion

We present experimental force and power measurements demonstrating that the power required to propel an actively swimming, streamlined, fish-like body is significantly smaller than the power needed to tow the body straight and rigid at the same speed U . The data have been obtained through accurate force and motion measurements on a laboratory fish-like robotic mechanism, 1.2 m long, covered with a flexible skin and equipped with a tail fin, at Reynolds numbers up to 10 6 , with turbulence stimulation. The lateral motion of the body is in the form of a travelling wave with wavelength λ and varying amplitude along the length, smoothly increasing from the front to the tail end. A parametric investigation shows sensitivity of drag reduction to the non-dimensional frequency (Strouhal number), amplitude of body oscillation and wavelength λ, and angle of attack and phase angle of the tail fin. A necessary condition for drag reduction is that the phase speed of the body wave be greater than the forward speed U . Power estimates using an inviscid numerical scheme compare favourably with the experimental data. The method employs a boundary-integral method for arbitrary flexible body geometry and motions, while the wake shed from the fish-like form is modelled by an evolving desingularized dipole sheet.

The optimal control of a flexible hull robotic undersea vehicle propelled by an oscillating foil

Determining the optimal swimming motion for a flexible hull robotic undersea vehicle propelled by an oscillating foil is an acutely complex problem involving the vehicles body kinematics and the hydrodynamics of the surrounding water. The overall intractability of the hydrodynamics of a flexible body precludes a purely analytical solution. The immense size of the experimental variable space prevents a purely empirical one. In order to overcome both difficulties, we have developed a self-optimizing motion controller based on a genetic algorithm. This controller effectively uses evolutionary principles to exponentially optimize swimming performance.

An accurate four-quadrant nonlinear dynamical model for marine thrusters: theory and experimental validation

This paper reports two specific improvements in the finite-dimensional nonlinear dynamical modeling of marine thrusters. Previously reported four-quadrant models have employed thin airfoil theory considering only axial fluid flow and using sinusoidal lift/drag curves. First, we present a thruster model incorporating the effects of rotational fluid velocity and inertia on thruster response. Second, we report a novel method for experimentally determining nonsinusoidal lift/drag curves. The model parameters are identified using experimental thruster data (force, torque, and fluid velocity). The models are evaluated by comparing experimental performance data with numerical model simulations. The data indicates that thruster models incorporating both reported enhancements provide superior accuracy in both transient and steady-state responses.

Calculation of dynamic motions and tensions in towed underwater cables

A matrix method for mooring system analysis is extended to address the dynamic response of towed underwater systems. Key tools are equivalent linearization and small perturbation theory, and a pitching towfish model. Two examples of application of the technique are provided. The first studies a fundamental limitation to constrained passive heave compensation, while the second concerns the use of floated tethers as a means for dynamic decoupling. >

Robust control for underwater vehicle systems with time delays

A robust control scheme is presented for controlling systems with time delays. The scheme is based on the Smith controller and the LQG/LTR (linear quadratic Gaussian/loop transfer recovery) methodology. The methodology is applicable to underwater vehicle systems that exhibit time delays, including tethered vehicles that are positioned through the movements of a surface ship and autonomous vehicles that are controlled through an acoustic link. An example, using full-scale data from the tethered vehicle ARGO, demonstrates the developments. >

Using computational fluid dynamics to calculate the stimulus to the lateral line of a fish in still water

SUMMARY This paper presents the first computational fluid dynamics (CFD) simulations of viscous flow due to a small sphere vibrating near a fish, a configuration that is frequently used for experiments on dipole source localization by the lateral line. Both two-dimensional (2-D) and three-dimensional (3-D) meshes were constructed, reproducing a previously published account of a mottled sculpin approaching an artificial prey. Both the fish-body geometry and the sphere vibration were explicitly included in the simulations. For comparison purposes, calculations using potential flow theory (PFT) of a 3-D dipole without a fish body being present were also performed. Comparisons between the 2-D and 3-D CFD simulations showed that the 2-D calculations did not accurately represent the 3-D flow and therefore did not produce realistic results. The 3-D CFD simulations showed that the presence of the fish body perturbed the dipole source pressure field near the fish body, an effect that was obviously absent in the PFT calculations of the dipole alone. In spite of this discrepancy, the pressure-gradient patterns to the lateral line system calculated from 3-D CFD simulations and PFT were similar. Conversely, the velocity field, which acted on the superficial neuromasts (SNs), was altered by the oscillatory boundary layer that formed at the fishs skin due to the flow produced by the vibrating sphere (accounted for in CFD but not PFT). An analytical solution of an oscillatory boundary layer above a flat plate, which was validated with CFD, was used to represent the flow near the fishs skin and to calculate the detection thresholds of the SNs in terms of flow velocity and strain rate. These calculations show that the boundary layer effects can be important, especially when the height of the cupula is less than the oscillatory boundary layers Stokes viscous length scale.

Drag Forces and Flow-Induced Vibrations of a Long Vertical Tow Cable—Part I: Steady-State Towing Conditions

The analysis of data from a unique experiment on the quasistatics and vortex-induced dynamics of a long vertical tow cable is presented. The experiment consisted of measuring simultaneously the vortex-induced motions and the steady-state configuration of a tow cable. The measured steady-state configuration of the cable was used to calculate the cable’s average drag coefficient which ranged from 2.2 to 2.5. It was observed that the vortex-induced motion of the cable was modulated by a beat due to the presence of a shear current. This is confirmed here by spectral and time domain analysis of the acceleration records. The rms motion of the cable varies with depth, which indicates that the drag coefficient of the cable is spatially varying.

On the dynamics of oceanographic surface moorings

An analytical model is proposed for predicting the dynamics of instrumented oceanographic surface moorings made up of a combination of wire rope and compliant synthetic rope. The model simplifies the problem by treating only the vertical motion of the buoy and the longitudinal motion of the mooring line and attached instruments. It is demonstrated using full-scale experimental data and numerical simulations, that the simplified model captures all of the important dynamic effects and gives accurate predictions of the dynamic tension at the top of the mooring line. The model shows that the total mass and damping of the instruments and wire rope that make up the stiff upper half of the mooring are the major sources of the dynamic tension. Damping of the instruments becomes a significant factor in larger sea states, especially near the peak frequency of the wave spectrum. Elastic stretching of the wire and synthetic rope make up approximately 10% of the total response. This is based on a coefficient of friction equal to 0.003 which fits the experimental data best.

A simple model for heave-induced dynamic tension in catenary moorings

An empirical model for the dynamic tension due to vertical motions at the top of a catenary mooring is presented. The model is applicable for forcing at ocean wave frequencies (as opposed to slow drift frequencies). The model calculates the standard deviation of the tension as the sum of an inertial term proportional to heave acceleration and a drag term proportional to quadratic heave velocity. Using numerical simulations, the model is shown to capture coupling between inertia and damping effects. The proportionality parameters of the model are expressed as effective mass and drag constants times a linear function of the mean tension at the top of the mooring. Formulae are derived for calculating these constants in terms of the hydrodynamic and material properties of the mooring. Comparison of model results to measurements from an instrumented oceanographic mooring and simulations of a lazy wave riser under ranges of conditions yield maximum errors of 8 and 11% and root mean square errors of 2 and 3%, respectively. The greatest errors occur in situations where there is high mean tension and large dynamic forcing and when horizontal motion produces significant tension effects.

The hydrodynamic footprint of a benthic, sedentary fish in unidirectional flow.

Mottled sculpin (Cottus bairdi) are small, benthic fish that avoid being swept downstream by orienting their bodies upstream and extending their large pectoral fins laterally to generate negative lift. Digital particle image velocimetry was used to determine the effects of these behaviors on the spatial and temporal characteristics of the near-body flow field as a function of current velocity. Flow around the fishs head was typical for that around the leading end of a rigid body. Flow separated around the edges of pectoral fin, forming a wake similar to that observed for a flat plate perpendicular to the flow. A recirculation region formed behind the pectoral fin and extended caudally along the trunk to the approximate position of the caudal peduncle. In this region, the time-averaged velocity was approximately one order of magnitude lower than that in the freestream region and flow direction varied over time, resembling the periodic shedding of vortices from the edge of a flat plate. These results show that the mottled sculpin pectoral fin significantly alters the ambient flow noise in the vicinity of trunk lateral line sensors, while simultaneously creating a hydrodynamic footprint of the fishs presence that may be detected by the lateral line of nearby fish.