Mechanical Energy Analysis Research Articles

The coupling of interfacial instabilities and the stabilization of two-layer annular flows

In this paper the stability of annular pressure-driven parallel flows of two liquids sandwiching a free cylindrical interface is considered. For small to moderate Reynolds numbers, the interface is susceptible to capillary and interfacial wave instabilities, the latter instability caused by a jump in viscosity at the interface. It is shown that favorable velocity profiles in both liquids may stabilize capillary breakup of the interface and suppress the axisymmetric interfacial wave instability. A long-wave analysis leads to the physical mechanism responsible for stabilization of capillary breakup. This physical mechanism is a generalization of that by which capillary breakup is stabilized by interfacial shear in an annular film of a single liquid. Stabilization of intermediate wavelengths is studied with a mechanical energy analysis, which leads to a description of the energetic processes at work.

An angular velocity profile in cycling derived from mechanical energy analysis

The contributions of this article are twofold. One is a procedure for determining the angular velocity profile in seated cycling that maintains the total mechanical energy of both legs constant. A five-bar linkage model (thigh, shank, foot, crank and frame) of seated (fixed hip) cycling served for the derivation of the equations to compute potential and kinetic energies of the leg segments over a complete crank cycle. With experimentally collected pedal angle data as input, these equations were used to compute the total combined mechanical energy (sum of potential and kinetic energies of the segments of both legs) for constant angular velocity pedalling at 90 rpm. Total energy varied indicating the presence of internal work. Motivated by a desire to test the hypothesis that reducing internal work in cycling will reduce energy expenditure, a procedure was developed for determining the angular velocity profile that eliminated any change in total energy. Using data recorded from five subjects, this procedure was used to determine a reference profile for an average equivalent cadence of 90 rpm. The pahse of this profile is such that highest and lowest angular velocities occur when the cranks are near vertical and horizontal respectively. The second contribution is the testing of the hypothesis that the reference angular velocity profile serves to effectively reduce internal work for the subjects whose data were used to develop this profile over the range of pedalling rates (80–100 rpm) naturally preferred. In this range, the internal work was decreased a minimum of 48% relative to the internal work associated with constant angular velocity pedalling. The acceptance of this hypothesis has relevance to the protocol for future experiments which explore the effect of reduced internal work on energy expenditure in cycling.

Mechanical energy of walking of stroke patients

The mechanical energy costs of walking have been studied in ten stroke patients with hemiplegia. A two-dimensional sagittal plane cinematographic analysis of two strides of the subjects' normal walking was undertaken, yielding continuous information about the mechanical energy costs of the whole body and each of its parts, about the energy types involved, and the amounts of energy conservation. The large head, arms, and trunk (HAT) were found to dominate the total pattern. Three major disturbances were seen. In contrast to normal subjects who show energy-conserving negatively correlated potential and kinetic energy curves for the HAT, the subjects who demonstrated the first disturbance showed gross irregularity of the curves, with almost no opportunity for exchange between energy types. In a second disturbance the curves of the HAT showed some energy-conserving portions, but levels of kinetic energy curves were low, resulting in little energy exchange. In the third disturbance, some exchange was evident, but the pattern was dominated by potential energy changes in the form of a single large rise and fall, coinciding with swing phase of the affected leg. Each of these disturbances would require a different approach to treatment. Although mechanical energy analyses do not reflect certain energy costs, such as the effort required to hold the body up against the pull of gravity and that used in contracting antagonist muscles, they could be of considerable assistance in pinpointing costly variations in energy patterns during walking and in determining appropriate treatment procedures.

Differences in Body Segment Energy Utilization between World-Class and Recreational Cross-Country Skiers

Differences in the utilization of body segment movements between world-class and recreational cross-country skiers which result in a longer stride of the elite were studied using mechanical energy analyses. Nine world-class racers and six recreational skiers (novices) were filmed, the latter while they executed their fastest possible stable diagonal stride on a level track, and the former during competition. A 15-member linked segment model was digitized, the coordinate data filtered at 4.5 Hz and body segment energy curves; mechanical work output and mechanical energy transfers were calculated using the method described by Pierrynowski, Winter, and Norman (1980). The elite skiers exhibited larger exchanges between potential and kinetic energy in all segments during swing phases and all but the upper arm segment during pushing phases. Step-wise discriminant function analysis showed significant differences in the swinging foot, pushing foot, and pushing shank. The differences appear to be largely attributable to the higher leg swings of the experts, who prolong the glide and enhance step length, probably at a relatively lower metabolic cost by exploiting gravity to augment muscular force by generating pendulum-like movements.

Mechanical efficiency of positive work in running at different speeds.

To investigate the possible role of elastic potentiation on mechanical efficiency, three male marathon runners were filmed while running on a treadmill at various steady-state speeds ranging from 7.0-22.0 km X h-1. Kinematic and mechanical energy analyses were performed from the film. Expired air was collected for energy expenditure determination. The analysis disclosed that during contact on the treadmill the knee and ankle joints initially had a phase of negative (flexion) angular velocity, followed by a positive velocity. In the hip joint the stretch-shortening cycle of the extensor muscles occurred primarily during the flight phase. The mean vertical and horizontal forces of the negative and positive phases of the contact period increased linearly with the increase in the running speed. The calculated mechanical efficiency of positive work was high but relatively constant (55.1 +/- 12.7%) across all speeds. The absolute contribution of the extra work, which comes from the stored elastic energy to the positive work, increased with running speed; however, its relative value (0.61 +/- 0.09 J X min-1 X kg-1) remained constant at all measured speeds. It is suggested, therefore, that when the flight phase is included in the mechanical energy calculations, the measured efficiency for the positive work reaches a high but constant value in running at low-to-moderate speeds.

Mechanical energy analyses of the human during local carriage on a treadmill.

Variations of the mechanical energy levels of the body segments, as affected by different backpack loads were investigated, to determine if mechanical energy analyses rather than metabolic measures could be used to evaluate load carriage devices. Six male subjects were required to walk upon a level treadmill at 5·54km/hour while they carried five loads ranging from 15·16 to 33·85kg, in a specially designed backpack. Cinefilm was taken, digitized and filtered. Kinematic data, energy levels of the segments of a 15 member linked-segment model and overall work indices were calculated. Expired air was collected and metabolic cost determined. Results showed that about 1/3 of the mechanical work done by all body segments represents energy exchanges within segments and another 1/3 between segments, the major increase in the mechanical work done was in the load itself and virtually no alternations in gait pattern were made by the subjects to accommodate heavier loads. Some undesirable exchanges of energy were evid...