Mechanical Energy Flow Research Articles

A Review of Adaptive Change in Musculoskeletal Impedance during Space Flight and Associated Implications for Postflight Head Movement Control

We present a review of converging sources of evidence which suggest that the differences between loading histories experienced in 1-g and weightlessness are sufficient to stimulate adaptation in mechanical impedance of the musculoskeletal system. As a consequence of this adaptive change we argue that we should observe changes in the ability to attenuate force transmission through the musculoskeletal system both during and after space flight. By focusing attention on the relation between human sensorimotor activity and support surfaces, the importance of controlling mechanical energy flow through the musculoskeletal system is demonstrated. The implications of such control are discussed in light of visual-vestibular function in the specific context of head and gaze control during postflight locomotion. Evidence from locomotory biomechanics, visual-vestibular function, ergonomic evaluations of human vibration, and specific investigations of locomotion and head and gaze control after space flight, is considered.

Active and Reactive Structural Energy Flow

Vibration of linear structures having internal material damping is considered. Energy supplied to such structures by external dynamic loads and mechanical energy flow inside these structures are investigated. The paper aims at presenting and discussing relations which are useful when studying energy transmission through mechanical systems. Special attention is paid to the case of stationary harmonic vibration, but general stationary vibration is also discussed. Continuity equations for the active and reactive mechanical intensities are derived for a solid continuum model. Limits for the active and reactive power supplied to a structure in terms of structural and loading properties are also established. Numerical examples with discrete, continuous and discretized structures demonstrate the results obtained.

MECHANICAL ENERGETICS OF KICKING 277

Fast unloaded movements like striking, throwing and kicking are typically performed in a proximo-distal sequence: Initially proximal segments accelerate while distal segments lag behind, then proximal segments decelerate while distal segments accelerate. To understand the dynamic nature of these movement patterns previous research has mainly used kinetic analyses. The results from these studies are ambiguous concerning the proximal segment deceleration. In kicking, for instance, some ascribe the thigh deceleration to active hip extensor muscles, while others ascribe it to reaction forces from the moving shank. The purpose of the present study was to utilize a different analytical approach based on mechanical energetics to explain the cause of thigh deceleration observed during kicking. Energetics, i.e. analysis of mechanical energy flow between the segments of the moving limbs, has previously mainly been used to gain insight regarding the mechanical efficiency of cyclical movements like gait or cycling. However, by applying this method to kicking we provide an alternative and intuitively pleasing way of explaining the dynamics of fast unloaded movements. Seventeen skilled taekwon-do practitioners were high-speed filmed (200 frames per second) while performing a high frontal kick. From the kinematic data, joint moments and joint reaction forces were calculated by use of inverse dynamics. Kinematic and kinetic data were combined to yield instantaneous muscle and joint power (energy transfer) for the hip and knee. Instantaneous power were then time integrated to yield work done on the thigh and shank from start of movement until the instant of maximal shank angular velocity. The results showed that despite a large energy inflow from the hip flexor muscles to the thigh (normalized to 100%), the thigh decelerated to zero velocity due to energy outflow to the shank through the knee extensor muscles (34%) and the knee joint (50%). The energy left over in the thigh (16%) was in potential form. In conclusion, no support was found for active thigh deceleration by hip extensor muscles.

An approximate analysis of waves in layered piezoelectric plates from an interdigital source transducer

This paper presents an approximate analysis for predicting the generation of elastic waves in multi-layered piezoelectric materials, when these waves are generated using interdigital transducers (IDTs). Each IDT finger is assumed to act as a source of plane (partial) waves propagating in all directions, and the condition for efficient wave generation is taken to be when the waves from adjacent pairs add in phase. The width of the IDT fingers is not explicitly taken into account and neither are the finite dimensions of the IDT. The conditions for surface or guided wave propagation between IDT transmit and receive pairs are then determined by satisfying the boundary conditions on the surfaces and at the interfaces of the substrate, using the partial waves already determined. The outer surfaces of the substrate may be either metallized or free. The model thus predicts discrete frequencies of operation of IDT devices, corresponding to the generation of surface or guided waves. No `width' is attached to these frequencies, because of the neglect of IDT finger width and of overall IDT dimensions. Wave profiles within the piezoelectric substrate are calculated by summation of the partial waves, to depict the depth-dependence of the mechanical displacement, electric potential and energy flow. This information is used to distinguish surface waves from guided waves and to show the depth of penetration of the surface waves. The paper is essentially divided into three parts. The model formulation is presented, computational methods and associated difficulties are discussed, and sample model predictions for Rayleigh and SH wave devices in ST cut quartz are compared with experiment, showing good agreement.

Mechanical energy flow models of rods and beams

It has been proposed that the flow of mechanical energy through a structural/acoustic system may be modeled in a manner similar to that of flow of thermal energy in a heat conduction problem [1]. If this hypothesis is true, it would result in relatively efficient numerical models of structure-borne energy in large built-up structures. Fewer parameters are required to approximate the energy solution than are required to model the characteristic wave behavior of structural vibration by using traditional displacement formulations. The energy flow hypothesis is tested in this investigation for both longitudinal vibration in rods and transverse flexural vibrations of beams. The rod is shown to behave approximately according to the thermal energy flow analogy. However, the beam solutions behave significantly differently than predicted by the thermal analogy unless locally space averaged energy and power are considered. Several techniques for coupling dissimilar rods and beams are also discussed. Illustrations of the solution accuracy of the methods are included.

Instability in a cavitating centrifugal pump. 2nd Report, Delivery of mechanical energy during oscillation.

Mechanical energy delivery during low-cycle system oscillation was studied by measuring the fluctuating pressure and flow rates at the inlet and outlet of a pump. By studying the relationship between the mechanical energy delivery versus flow patterns and observing the inlet and the inside of the impeller subject to cavitation, it was found that one of the causes of low-cycle system oscillation is the effect of cross sectional area reduction upstream of the impeller by the cavitation reverse flow. This is because a dynamic difference in phase between the cavitation volume versus inlet flow rate of the pump is a consequence of the above effect.

Energization of plasma in a weak magnetic field and structure of the geomagnetic tail

A ‘two-fluid’ plasma is described as a single continuum characterised by the generalised tensor of mechanical pressure and generalised vector of flow of mechanical energy. Plasma energization due to the transfer of mechanical energy inside the plasma body is emphasised and the energization of plasma by conversion of the electromagnetic energy into the mechanical energy is discussed. Two kinds of conversion associated with the convection electric field −(1/c)V×B and with the deviationE * of the total electric field from −(1/c)V×B are distinguished. TheV×B-field is related to the work done upon the plasma, while theE *-field is related to the plasma heating. Plasma motions with scale length larger than the Debye distance, taking place in the central part of the Earth-plasma sheet, are considered. The change of energy of any element of plasma is due mainly to the transfer of mechanical energy across the element's boundary; the EM-field is not strong enough to make a significant contribution. The work done by the internal loads is the main source of mechanical energy in the configurations in which the physical quantities do not vary along the current lines. The rates of change of the kinetic and internal energies are comparable. The transfer of mechanical energy is the principal source of the kinetic energy also in the general case when the physical quantities vary along the current lines. Conversion of the EM energy into mechanical energy is the main source of the internal energy in this case. In the tail plasma located outside the central part of the plasma sheet, conversion of the EM-energy into mechanical energy, which is due to the work done by the EM-force, takes place. The tail plasma is likely to undergo a two-phase energization process: first, it is accelerated and later, when it approaches the ‘neutral’ sheet, it is heated.

Radiation From an Oscillating Magnetic Dipole in a Streaming Plasma

The electromagnetic field of an oscillating magnetic dipole is calculated, assuming that the dipole is immersed in a cold, streaming plasma. The amplitude of the magnetic dipole moment is assumed to be sufficiently weak that the linearized cold‐plasma equations may be used to describe the response of the plasma.The resulting field of the dipole is rather different from the field that would result if the plasma were not streaming. In particular, a longitudinal electrostatic field appears as a consequence of the plasma's motion. The far field of the dipole is such that the Poynting vector is not purely radial, but is tilted against the direction of the zeroth‐order plasma flow.An outward flow of mechanical energy is associated with the electrostatic field. However the mechanical energy flow is negligible for streaming velocities small compared with the velocity of light. The force necessary to hold the dipole in place is also calculated. This force vanishes when the dipole axis is parallel to the streaming direction, as does the longitudinal electric field.

Methods for studying energy costs and energy flow during human locomotion.

Methods are described for studying the metabolic cost of increased and diminished gravitational work done by the human subject during normal locomotion at various speeds and slopes on the treadmill. It is shown that the energy expenditure is a linear function of the gravitational work and, as long as the gait is of a smooth and natural character, appears to be dependent upon the true vertical lift per step multiplied by the number of steps per minute. The true vertical lift is defined as the lift resulting from muscle action, as contrasted with components due to treadmill motion. Methods are also described for recording the vertical and translational motions of the torso during a single step, and for analyzing the flow of mechanical energy into and out of the torso during each phase of the walking cycle. Implications for calculation of efficiency are briefly discussed.

Mechanical Energy Flow in Crystal Lattices

A detailed study of the mechanical energy transported through a completely general crystal lattice in the classical temperature range is presented. Both a mechanical "Poynting" vector and the corresponding mechanical "Poynting" theorem are established for an arbitrary, 3-dimensional, anisotropic, anharmonic crystal lattice, containing many atoms per cell, inclusive of substitutional impurities, and possessing an arbitrary range of atomic interactions. The extension of the energy-flow procedures to a quantum treatment is discussed, and the resulting quantum-mechanical form of the mechanical "Poynting" vector is given. In addition, a mechanical energy theorem, linking the average mechanical energy flow to the group velocity for the restricted case of a perfectly periodic and harmonic but otherwise completely general, crystal is derived for the classical temperature range.

Utility of Variable-Displacement Oil-Pressure Pumps for Hot-Pressing in Plywood Operations

Abstract Variable-displacement pumps or generators form the primary part of a complete hydraulic energy transformer, the secondary part of which is a hydraulic motor. Hydraulic transformers of fluid drives are becoming more and more useful for all kinds of modern production machinery, such as presses, machine tools, automotive engines, cranes, etc., because of certain advantages offered along the line of the modern power-transmission problems. The flow of chemical energy occurs in the form of a fuel and air compound, into an internal-combustion engine, where it will be transformed to thermal and partly mechanical energy. The mechanical energy flows further in periodic impulses to the crankshaft of the engine in the form of a mechanical direct-current energy, where through the flywheel of the shaft it will be somewhat smoothed out, so that in a mechanical-electrical energy transformer it will be transformed to direct-current or alternating-current energy. These currents finally reach the electromotor of the shop, which operates the drive shafts of mechanical or hydraulic energy transmission apparatus, transmission-belt shafts, gear transmissions of machine tools, electromotors of press pumps, etc.