Efficiency of Fish Propulsion

It is shown that the system efficiency of a self-propelled flexible body is ill-defined unless one considers the concept of quasi-propulsive efficiency, defined as the ratio of the power needed to tow a body in rigid-straight condition over the power it needs for self-propulsion, both measured for the same speed. Through examples we show that the quasi-propulsive efficiency is the only rational non-dimensional metric of the propulsive fitness of fish and fish-like mechanisms. Using two-dimensional viscous simulations and the concept of quasi-propulsive efficiency, we discuss the efficiency two-dimensional undulating foils. We show that low efficiencies, due to adverse body-propulsor hydrodynamic interactions, cannot be accounted for by the increase in friction drag

The impact of uncertainty shocks on the volatility of commodity prices

In this paper, we empirically examine the impact of uncertainty shocks on the volatility of commodity prices. Using alternative measures of economic uncertainty for the U.S. we estimate their effects on commodity price volatility by employing both VAR and OLS regression models. We find that the unobservable economic uncertainty measures of Jurado et al. (2015) have a significant and long-lasting positive impact on the volatility of commodity prices. Our results indicate that a positive shock in both macroeconomic and financial uncertainty leads to a persistent increase in the volatility of the broad commodity market index and of the individual commodity prices, with the macroeconomic effect being more significant. The impact is stronger in energy commodities compared to the agricultural and metals markets. In addition, our findings show that the measure of unpredictability of the macroeconomic environment has the most significant impact on the commodity price volatility when compared to the observable measures of economic uncertainty that have a rather small and transitory effect. Finally, we show that uncertainty in the macroeconomy is significantly reduced after the occurrence of large commodity market volatility episodes

Commodity price volatility and the economic uncertainty of pandemics

In this paper, we empirically investigate the impact of pandemics on commodity price volatility. In specific, we explore the impact of economic uncertainty related to global pandemics on the volatility of the SandP GSCI commodity index as well as on the sub-indexes of crude oil and gold. The results show that uncertainty related to pandemics have a strong negative impact on the volatility of commodity markets and especially on crude oil market, while the effect on gold market is positive but less significant. Our findings remain robust to a series of robustness checks

Robust Control For Underwater Vehicle Systems With Time Delays

Presented in this paper is a robust control scheme 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 undenvater 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 Woods Hole Oceanographic Institution’s tethered vehicle ARGO, demonstrates the developments

Forces on oscillating uniform and tapered cylinders in a crossflow

Forces are measured at both ends of rigid cylinders with span 60 cm, performing transverse oscillations within an oncoming stream of water, at Reynolds number Re ~3800. Forced harmonic motions and free vibrations of uniform and tapered cylinders are studied. To study free motions, a novel force-feedback control system has been developed, consisting of: (a) a force transducer, which measures forces on a section of a cylinder moving forward at constant speed; (b) a computer using the measured force signal to drive in real time a numerical simulation of an equivalent mass-dashpot-spring system; (c) a servomotor and linear table which impose, also in real time, the numerically calculated motion on the cylinder section. The apparatus allows very low equivalent system damping and strict control of the parametric values and structure of the equivalent system. Calculation of the cross-correlation coefficient between forces at the two ends of the uniform cylinder reveals five distinct regimes as a function of the nominal reduced velocity Vrn: two regimes, for low and high values of Vrn, and far away from the value of VrS corresponding to the Strouhal frequency, show small correlation; two regimes immediately adjacent to, but excluding, VrS show strong correlation, close to 1; surprisingly, there is a regime containing the Strouhal frequency, within which correlation is low. Free vibrations with a 40:1 tapered cylinder show that the regime of low correlation, containing the Strouhal frequency, stretches to higher reduced velocities, while lock-in starts at lower reduced velocities. When comparing the amplitude and phase of the lift coefficient measured for free and then for forced vibrations, we obtain close agreement, both for tapered and uniform cylinders. When comparing the cross-correlation coefficient however, we find that it is much higher in the forced oscillations, especially for the uniform cylinder. Hence, although the force magnitude and phase may be replicated well in forced vibrations, the correlation data suggest that differences exist between free and forced vibration cases

Vortical patterns behind a tapered cylinder oscillating transversely

Visualization studies of the flow behind an oscillating tapered cylinder are performed at Reynolds numbers from 400 to 1500. The cylinder has taper ratio 40:1 and is moving at constant forward speed U while being forced to oscillate harmonically in the transverse direction. It is shown that within the lock-in region and above a threshold amplitude, no cells form and, instead, a single frequency of response dominates the entire span. Within certain frequency ranges a single mode dominates in the wake, consisting of shedding along the entire span of either two vortices per cycle (`2S' mode), or four vortices per cycle (`2P' mode); but within specific parametric ranges a hybrid mode is observed, consisting of a `2S' pattern along the part of the span with the larger diameter and a `2P' pattern along the part of the span with the smaller diameter. A distinct vortex split connects the two patterns which are phaselocked and have the same frequency. The hybrid mode is periodic, unlike vortex dislocations, and the location of the vortex split remains stable and repeatable, within one to two diameters, depending on the amplitude and frequency of oscillation and the Reynolds number