942 research outputs found
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
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
- …