RELIABILITY OF LOW-COST PORTABLE FORCE PLATFORMS FOR MEASURING VERTICAL STIFFNESS DURING RUNNING

Ground reaction force (GRF) can provide useful information such as vertical stiffness (Kvert) to practitioners working with runners and sprinters, but high equipment costs are hindering applied research. Low-cost portable force platforms may be a useful alternative to traditional biomechanical equipment. Moderately trained runners (n = 9) completed overground running trials at various speeds (2.15-5.78 m/s), Kvert was determined, and a linear regression was used to characterize the relationship between Kvert and running speed. The results showed moderate to high correlation (r2 = 0.54 to 0.87). At 3.9 m/s (14 km/h), the widest regression model confidence interval was 4.4%, which shows this procedure likely provides adequate reliability. Future research should continue to investigate the use of low-cost portable force platforms for measuring running GRF

HEAD IMPACT FREQUENCY IN YOUTH AMERICAN FOOTBALL, AGES 9-13

The objective of this study was to quantify head impact frequency in youth American football players, ages 9-13. Kinematic data from head impacts were collected from 25 players on two teams (11.7 ± 1.2 years) using a helmet-mounted accelerometer system. A total of 4432 head impacts were recorded, including 1800 during 265 practice sessions and 2632 during 175 game sessions. Most peak linear acceleration magnitudes were less than 20 g in practices (60%) and games (55%). For impacts greater than or equal to 60 g, players sustained 122% more in games than practices. Both teams played in leagues that limited full-contact practice time, which likely contributed to lower impact frequencies in practices. Interventions to further reduce head impact frequency in youth football should include measures which affect in-game exposure

Experimental study of impulse turbines and permanent magnet alternators for pico-hydropower generation

Increasing access to modern forms of energy in developing countries is a crucial component to eliminating extreme poverty around the world. Pico-hydro schemes (less than 5-kW range) can provide environmentally sustainable electricity and mechanical power to rural communities, generally more cost-effectively than diesel/gasoline generators, wind turbines, or solar photovoltaic systems. The use of these types of systems has in the past and will continue in the future to have a large impact on rural, typically impoverished areas, allowing them the means for extended hours of productivity, new types of commerce, improved health care, and other services vital to building an economy. For this thesis, a laboratory-scale test fixture was constructed to test the operating performance characteristics of impulse turbines and electrical generators. Tests were carried out on a Pelton turbine, two Turgo turbines, and a permanent magnet alternator (PMA). The effect on turbine efficiency was determined for a number of parameters including: variations in speed ratio, jet misalignment and jet quality. Under the best conditions, the Turgo turbine efficiency was observed to be over 80% at a speed ratio of about 0.46, which is quite good for pico-hydro-scale turbines. The Pelton turbine was found to be less efficient with a peak of just over 70% at a speed ratio of about 0.43. The lower efficiency can be largely attributed to the rougher surface on the turbine blades. Both turbine types had lower than expected speed ratios which were likely caused by inefficiencies in the turbine. Tests of jet misalignment showed that moving the jet to the inside or outside edge of the buckets caused a drop in efficiency of around 10-20% as well as shifting the optimal speed ratio down 0.03 (6.5%). For the PMA, the generator efficiency peaked at just less than 70%. The data demonstrate the impact of impedance matching on generator efficiency for a PMA. Currently published literature provides few details specific to Turgo turbines or PMAs despite their suitability for pico-hydro. Nonetheless, Turgo turbines are currently available on the market (at least in the US) and offer a viable option for picohydro schemes, particularly in the medium head range. This thesis describes an experimental investigation of impulse turbines, including both Turgo and Pelton turbines, which will be useful for implementation of rural electrification projects. The results stress the importance of proper system design and installation in order to ensure a successful scheme

Augmenting the Space Domain Awareness Ground Architecture via Decision Analysis and Multi-Objective Optimization

Purpose — The US Government is challenged to maintain pace as the world’s de facto provider of space object cataloging data. Augmenting capabilities with nontraditional sensors present an expeditious and low-cost improvement. However, the large tradespace and unexplored system of systems performance requirements pose a challenge to successful capitalization. This paper aims to better define and assess the utility of augmentation via a multi-disiplinary study. Design/methodology/approach — Hypothetical telescope architectures are modeled and simulated on two separate days, then evaluated against performance measures and constraints using multi-objective optimization in a heuristic algorithm. Decision analysis and Pareto optimality identifies a set of high-performing architectures while preserving decision-maker design flexibility. Findings — Capacity, coverage and maximum time unobserved are recommended as key performance measures. A total of 187 out of 1017 architectures were identified as top performers. A total of 29% of the sensors considered are found in over 80% of the top architectures. Additional considerations further reduce the tradespace to 19 best choices which collect an average of 49–51 observations per space object with a 595–630 min average maximum time unobserved, providing redundant coverage of the Geosynchronous Orbit belt. This represents a three-fold increase in capacity and coverage and a 2 h (16%) decrease in the maximum time unobserved compared to the baseline government-only architecture as-modeled. Originality/value — This study validates the utility of an augmented network concept using a physics-based model and modern analytical techniques. It objectively responds to policy mandating cataloging improvements without relying solely on expert-derived point solutions

Head Impact Exposure in Youth Football: Elementary School Ages 9–12 Years and the Effect of Practice Structure

Head impact exposure in youth football has not been well-documented, despite children under the age of 14 accounting for 70% of all football players in the United States. The objective of this study was to quantify the head impact exposure of youth football players, age 9–12, for all practices and games over the course of single season. A total of 50 players (age = 11.0 ± 1.1 years) on three teams were equipped with helmet mounted accelerometer arrays, which monitored each impact players sustained during practices and games. During the season, 11,978 impacts were recorded for this age group. Players averaged 240 ± 147 impacts for the season with linear and rotational 95th percentile magnitudes of 43 ± 7 g and 2034 ± 361 rad/s(2). Overall, practice and game sessions involved similar impact frequencies and magnitudes. One of the three teams however, had substantially fewer impacts per practice and lower 95th percentile magnitudes in practices due to a concerted effort to limit contact in practices. The same team also participated in fewer practices, further reducing the number of impacts each player experienced in practice. Head impact exposures in games showed no statistical difference. While the acceleration magnitudes among 9–12 year old players tended to be lower than those reported for older players, some recorded high magnitude impacts were similar to those seen at the high school and college level. Head impact exposure in youth football may be appreciably reduced by limiting contact in practices. Further research is required to assess whether such a reduction in head impact exposure will result in a reduction in concussion incidence

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta