Stephanie E. Forrester

Gas-inducing impeller design and performance characteristics

A theoretical and experimental study on the design and performance characteristics of gas-inducing impellers is presented. In particular, the model developed by Evans et al. (1991, A.I.Ch.E. Spring National Meeting, Houston, TX, Paper 33e) is critically reviewed and, as a result, improvements to the kinetic energy pressure loss analysis and to the initial conditions are proposed. In addition, the model is successfully extended to account for multiple gas outlet orifice on each blade. Experimental measurements of the power consumption, rate of gas induction, mass transfer coefficient and detached bubble size for a partially optimised, 0.154 m diameter, six-bladed concave gas-inducing impeller are presented. A significant increase in the induced gas rate is observed by adding more outlet orifices to each blade. The principal advantage of using multiple orifices is that similar size bubbles are produced, compared to a single orifice, but larger interfacial areas are generated; the aerated power input is only slightly reduced from its ungassed value. Mass transfer coefficients, kLa, of the order of 0.02 s−1 are attainable for a single outlet orifice on each blade; kLa is significantly increased by using multiple orifices. The dimensionless bubble size distributions, ddgm independent of the impeller speed over the range 4–8 rps, and can be successfully represented by a log-normal distribution.

Bubble formation from cylindrical, flat and concave sections exposed to a strong liquid cross-flow

The results of an experimental study on gas bubble formation from a submerged orifice on a cylindrical, flat or concave blade section, which is exposed to a strong liquid cross-flow, are presented. The effects of the gas velocity in the orifice (3–45 m s-1), the liquid cross-flow velocity (0.5–4 m s-1) and the blade configuration on the mode of bubble formation and the detached bubble size are investigated using high-speed flash photography. The results show that when the orifice is positioned in, or close to, an unseparated flow region the predominant bubbling mode is jetting. In contrast, when the orifice is positioned within the wake region behind the blade, the bubbles are generated individually at the orifice. For a fixed gas velocity, the detached bubble size decreases significantly with increasing liquid velocity, by approximately 50%, between 1 and 3 m s-1; at liquid velocities greater than 3 m s-1, the bubble size decreases more slowly. For a fixed liquid velocity, the bubble size increases approximately linearly with increasing gas velocity through the orifice.

Neuromuscular function in healthy occlusion

This study aimed to measure neuromuscular function for the masticatory muscles under a range of occlusal conditions in healthy, dentate adults. Forty-one subjects conducted maximum voluntary clenches under nine different occlusal loading conditions encompassing bilateral posterior teeth contacts with the mandible in different positions, anterior teeth contacts and unilateral posterior teeth contacts. Surface electromyography was recorded bilaterally from the anterior temporalis, superficial masseter, sternocleidomastoid, anterior digastric and trapezius muscles. Clench condition had a significant effect on muscle function (P = 0.0000) with the maximum function obtained for occlusions with bilateral posterior contacts and the mandible in a stable centric position. The remaining contact points and moving the mandible to a protruded position, whilst keeping posterior contacts, resulted in significantly lower muscle activities. Clench condition also had a significant effect on the per cent overlap, anterior-posterior and torque coefficients (P = 0.0000-0.0024), which describe the degree of symmetry in these muscle activities. Bilateral posterior contact conditions had significantly greater symmetry in muscle activities than anterior contact conditions. Activity in the sternocleidomastoid, anterior digastric and trapezius was consistently low for all clench conditions, i.e. <20% of the maximum voluntary contraction level. In conclusion, during maximum voluntary clenches in a healthy population, maximum masticatory muscle activity requires bilateral posterior contacts and the mandible to be in a stable centric position, whilst with anterior teeth contacts, both the muscle activity and the degree of symmetry in muscle activity are significantly reduced.

The need for muscle co-contraction prior to a landing

In landings from a flight phase the mass centre of an athlete experiences rapid decelerations. This study investigated the extent to which co-contraction is beneficial or necessary in drop landings, using both experimental data and computer simulations. High speed video and force recordings were made of an elite martial artist performing drop landings onto a force plate from heights of 1.2, 1.5 and 1.8m. Matching simulations of these landings were produced using a planar 8-segment torque-driven subject-specific computer simulation model. It was found that there was substantial co-activation of joint flexor and extensor torques at touchdown in all three landings. Optimisations were carried out to determine whether landings could be effected without any co-contraction at touchdown. The model was not capable of landing from higher than 1.05m with no initial flexor or extensor activations. Due to the force-velocity properties of muscle, co-contraction with net zero joint torque at touchdown leads to increased extensor torque and decreased flexor torque as joint flexion velocity increases. The same considerations apply in any activity where rapid changes in net joint torque are required, as for example in jumps from a running approach.

Comparing different approaches for determining joint torque parameters from isovelocity dynamometer measurements

Strength, or maximum joint torque, is a fundamental factor governing human movement, and is regularly assessed for clinical and rehabilitative purposes as well as for research into human performance. This study aimed to identify the most appropriate protocol for fitting a maximum voluntary torque function to experimental joint torque data. Three participants performed maximum isometric and concentric-eccentric knee extension trials on an isovelocity dynamometer and a separate experimental protocol was used to estimate maximum knee extension angular velocity. A nine parameter maximum voluntary torque function, which included angle, angular velocity and neural inhibition effects, was fitted to the experimental torque data and three aspects of this fitting protocol were investigated. Using an independent experimental estimate of maximum knee extension angular velocity gave lower variability in the high concentric velocity region of the maximum torque function compared to using dynamometer measurements alone. A weighted root mean square difference (RMSD) score function, that forced the majority (73-92%) of experimental data beneath the maximum torque function, was found to best account for the one-sided noise in experimental torques resulting from sub-maximal effort by the participants. The suggested protocol (an appropriately weighted RMSD score function and an independent estimate of maximum knee extension angular velocity) gave a weighted RMSD of between 11 and 13 Nm (4-5% of maximum isometric torque). It is recommended that this protocol be used in generating maximum voluntary joint torque functions in all torque-based modelling of dynamic human movement.

Modelling the increased gas capacity of self-inducing impellers

Abstract In many gas—liquid mixing processes occuring in stirred vessels, the rate of reaction is limited by mass transfer between the gas and liquid phases, e.g. acetic acid fermentation. An important aspect in the design of such devices is to increase the rate of reaction by improving the mass transfer performance. This study examines the effect of increasing the number of gas outlet orifices on each blade on the gas induction rate and mass transfer performance of a concave-bladed self-inducing impeller. Increasing the number of orifices is found to significantly increase the rate of gas induction, however, this effect diminishes as the number of orifices increases. Volumes of liquid per unit volume of gas per minute (VVMs) of up to 0.6 can be achieved using all four orifices on each blade at an impeller speed of 10 rps; this may be compared to a value of 0.25 obtained under similar conditions for a single orifice. Furthermore, this method of increasing the gas capacity is not at the expense of generating bigger bubbles, and thus large gas—liquid interfacial areas are maintained. This is reflected in experimental measurements of the mass transfer coefficient; the value ofkLa for a given specific power input approximately doubles in moving from one to four orifices per blade. Mass transfer coefficients of up to 0.055s−1 (at a specific power input of 1.7 kW/m3) are attainable for the coalescing air—water system. An existing model for gas-inducing impeller design (Evans., 1991a) is extended to include the effect of multiple orifices. The revised model gives acceptable predictions of the induced gas rate, to within 20% of the experimental data, over the full range of measurements.

The effect of running velocity on footstrike angle – A curve-clustering approach

Despite a large number of studies that have considered footstrike pattern, relatively little is known about how runners alter their footstrike pattern with running velocity. The purpose of this study was to determine how footstrike pattern, defined by footstrike angle (FSA), is affected by running velocity in recreational athletes. One hundred and two recreational athletes ran on a treadmill at up to ten set velocities ranging from 2.2-6.1 ms(-1). Footstrike angle (positive rearfoot strike, negative forefoot strike), as well as stride frequency, normalised stride length, ground contact time and duty factor, were obtained from sagittal plane high speed video captured at 240 Hz. A probabilistic curve-clustering method was applied to the FSA data of all participants. The curve-clustering analysis identified three distinct and approximately equally sized groups of behaviour: (1) small/negative FSA throughout; (2) large positive FSA at low velocities (≤ 4 ms(-1)) transitioning to a smaller FSA at higher velocities (≥ 5 ms(-1)); (3) large positive FSA throughout. As expected, stride frequency was higher, while normalised stride length, ground contact time and duty factor were all lower for Cluster 1 compared to Cluster 3 across all velocities; Cluster 2 typically displayed intermediate values. These three clusters of FSA - velocity behaviour, and in particular the two differing trends observed in runners with a large positive FSAs at lower velocities, can provide a novel and relevant means of grouping athletes for further assessment of their running biomechanics.

Predicting maximum eccentric strength from surface EMG measurements

The origin of the well-documented discrepancy between maximum voluntary and in vitro tetanic eccentric strength has yet to be fully understood. This study aimed to determine whether surface EMG measurements can be used to reproduce the in vitro tetanic force-velocity relationship from maximum voluntary contractions. Five subjects performed maximal knee extensions over a range of eccentric and concentric velocities on an isovelocity dynamometer whilst EMG from the quadriceps were recorded. Maximum voluntary (MVC) force-length-velocity data were estimated from the dynamometer measurements and a muscle model. Normalised amplitude-length-velocity data were obtained from the EMG signals. Dividing the MVC forces by the normalised amplitudes generated EMG corrected force-length-velocity data. The goodness of fit of the in vitro tetanic force-velocity function to the MVC and EMG corrected forces was assessed. Based on a number of comparative scores the in vitro tetanic force-velocity function provided a significantly better fit to the EMG corrected forces compared to the MVC forces (p< or =0.05), Furthermore, the EMG corrected forces generated realistic in vitro tetanic force-velocity profiles. A 58+/-19% increase in maximum eccentric strength is theoretically achievable through eliminating neural factors. In conclusion, EMG amplitude can be used to estimate in vitro tetanic forces from maximal in vivo force measurements, supporting neural factors as the major contributor to the difference between in vitro and in vivo maximal force.

Running Technique Is An Important Component Of Running Economy And Performance

Despite an intuitive relationship between technique and both running economy (RE) and performance, and the diverse techniques used by runners to achieve forward locomotion, the objective importance of overall technique and the key components therein remain to be elucidated. Purpose This study aimed to determine the relationship between individual and combined kinematic measures of technique with both RE and performance. Methods Ninety-seven endurance runners (47 females) of diverse competitive standards performed a discontinuous protocol of incremental treadmill running (4-min stages, 1-km·h−1 increments). Measurements included three-dimensional full-body kinematics, respiratory gases to determine energy cost, and velocity of lactate turn point. Five categories of kinematic measures (vertical oscillation, braking, posture, stride parameters, and lower limb angles) and locomotory energy cost (LEc) were averaged across 10–12 km·h−1 (the highest common velocity < velocity of lactate turn point). Performance was measured as seasons best (SB) time converted to a sex-specific z-score. Results Numerous kinematic variables were correlated with RE and performance (LEc, 19 variables; SB time, 11 variables). Regression analysis found three variables (pelvis vertical oscillation during ground contact normalized to height, minimum knee joint angle during ground contact, and minimum horizontal pelvis velocity) explained 39% of LEc variability. In addition, four variables (minimum horizontal pelvis velocity, shank touchdown angle, duty factor, and trunk forward lean) combined to explain 31% of the variability in performance (SB time). Conclusions This study provides novel and robust evidence that technique explains a substantial proportion of the variance in RE and performance. We recommend that runners and coaches are attentive to specific aspects of stride parameters and lower limb angles in part to optimize pelvis movement, and ultimately enhance performance.

Application of an industrial robot in the sports domain: simulating the ground contact phase of running

Abstract Mechanical devices currently used to test sports equipment are limited to one or two degrees of freedom and cannot replicate complete human movements. The purpose of this study was to investigate the capabilities of a six-degrees-of-freedom industrial robot (iRobot) to replicate the ground contact phase of human running. The objectives were as follows: to quantify the repeatability of the iRobot system; to assess the ability of the system to replicate heelstrike running and forefoot running. High-speed video and force plate data were collected for a single-subject heelstrike running and forefoot running. The iRobot was programmed to replicate the two footstrikes and then to perform 500 cycles of each. System kinematics and ground contact forces were recorded every tenth cycle. The kinematic repeatability of the iRobot was extremely good (less than 2 mm mean standard deviation in all marker trajectories). The peak vertical ground reaction forces showed systemic trends specific to the footstrike; heelstrike 3 per cent decrease and forefoot 19 per cent increase over the 500 cycles. iRobot replication of the footstrikes met with some success, particularly for the forefoot running. The iRobot generated highly repeatable kinematics and demonstrated potential for applications within the footwear industry. A number of improvements to the system were identified which could further improve its ability to replicate human running.