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At least 613 records · Page 34Linked to original sources
PubMed1983

Response of a human head-neck model to transient saggital plane loading.

A reproducible, synthetic replica of the human head and neck system utilizing a water-filled cadaver skull mounted on fiberglass-reinforced resin vertebrae and supported by passive silicone rubber muscle and ligament elements has been constructed. Contact force, head accelerations, disk pressures and intracranial pressure responses to conditions of transient saggital plane pendulum loading have been ascertained. Front and rear head impacts as well as excitation of the entire system through its rigid base were effected. Input energies ranging from 0.0089 N m to 4.63 N m were employed resulting in head accelerations up to 180 g.

Acceleration↗
PubMed1983

Comparison of steady and pulsatile flow in a double branching arterial model.

Data are presented to compare fluid flow parameters for steady flow with those for time-varying flow in a simplified two branch model which simulates the region of the abdominal aorta near the celiac and superior mesenteric branches of the dog. Measurements in the model included laser doppler anemometry velocity profiles during steady flow, sinusoidal flow with a superimposed mean flow (referred to as simple oscillatory flow) and arterial pulsatile flow. Shear rate measurements were made by an electrochemical technique during steady flow. Flow visualization studies were done during steady and pulsatile flow. Fluid flow effects in the simplified model during steady flow showed many similarities to the results from previous steady flow studies in a canine aortic cast. Shear rates in the region of the proximal (first, or celiac) branch were independent of flow rates in the distal (second, or mesenteric) branch, but the shear pattern within the proximal branch changed significantly as flow in the proximal branch increased. Shear rates on the proximal flow divider (leading edge into the distal branch) depended primarily on the flow rate to the proximal branch, but not on flow to the distal branch. At certain daughter branch flow ratios (approximately 2:1, proximal to distal), flow separation was promoted at the outer wall of the second branch, but flow separation did not occur in the first branch. In contrast to the canine aortic case results, flow separation was never detected on the distal (mesenteric) flow divider of the simplified model. This observation reflects the subtle effects of geometry on flow since the mesenteric flow divider in the canine cast protrudes into the main flow whereas the distal flow divider in the simplified model does not. There were distinct differences in the flow phenomena between steady, simple oscillatory and arterial pulsatile flow. Peak shear rates during pulsatile flow were as much as 10--100 times greater than steady flow shear rates at comparable mean flow rates. Particularly noteworthy for the pulsatile flow with a Womersley parameter of sixteen were very blunt velocity profiles throughout systole, and the absence of flow separation or reversal in those regions of the model that exhibited flow separation during steady flow. The shape of the waveform influences the nature of the flow during time-varying flows. Future studies of fluid dynamics in model systems must consider the pulsatile nature of the flow if a true interpretation of arterial flow phenomena is to be made.

Aorta, Abdominal↗
PubMed1985

Experimental study of physiological pulsatile flow past valve prostheses in a model of human aorta--I. Caged ball valves.

Pulsatile flow development past a caged ball valve in a model human aorta was studied using laser Doppler anemometry. Velocity profiles measured in the ascending aorta and in the mid-arch region were strongly influenced by the geometry of the valve at the root of the aorta. Velocity profiles distal to the valve were asymmetric with jet-like flow in the peripheral region having larger velocity magnitudes towards the left lateral wall. In early diastole, a streamwise vortex motion was observed throughout the model aorta with fluid moving towards the downstream direction along the left lateral wall and reversed flow along the right lateral wall. With the caged ball valve at the root of the aorta, no reversed flow was observed along the inner wall of curvature in the mid-arch region.

Aortic Valve↗
PubMed1985

Experimental study of physiological pulsatile flow past valve prosthesis in a model of human aorta--II. Tilting disc valves and the effect of orientation.

In Part II of this two paper sequence, pulsatile flow development past a tilting disc valve in a model human aorta has been studied using quantitative laser Doppler techniques. The valve was mounted in three different orientations with respect to the aortic root in this study. Under pulsatile flow, the region of flow reversal induced near the wall of the minor flow orifice extends to more than one tissue annulus diameter downstream from the valve into the ascending aorta. In a plane perpendicular to the tilt axis, a bi-helical secondary flow is induced distal to the valve. This secondary flow is further compounded by the multiple curvatures in the aorta. Hence the valve orientation affects the velocity profiles as far downstream as the mid-arch region as well as in the brachio-cephalic arterial branch. In the mid-arch region, a flow reversal along the entire cross-section is observed in early diastole for all the three orientations of the disc valve.

Aorta↗
PubMed1985

On the flow field distortions due to the aortic arch twist.

The aortic arch has, on average, an angle of twist of 15 degrees. The purpose of the research conducted was to ascertain if this twist angle has any effect on the flow field of the aortic arch and the distribution of the flow amongst the branches. It was found that the blood flow distribution is practically independent of the angle of twist of the aortic arch. However, the destination of a fluid particle located at a specific point in the cross-section at the entrance to the aortic arch does depend on the angle of twist.

Aorta, Thoracic↗
PubMed1985

Velocity profiles in the wake of two prosthetic heart valves using a new cardiovascular simulator.

In this paper we present a study of the post valvular flow field on a new cardiovascular simulator including an elastic model of the aortic arch. Transverse and vertical two-dimensional velocity measurements are performed with an ultrasonic velocimeter. Two prosthetic heart valves are tested in the aortic position. The behaviour of the velocity vectors patterns during one pulsatile cycle is one of the most striking features of the flow.

Aorta, Thoracic↗
PubMed1987

A model of multicomponent cardiac fibre.

In order to simulate the contraction of a cardiac myofibre, a multicomponent fibre model has been developed. This model is composed of a series of segments which are activated in succession. Each segment is represented by the Hill's three component model of the sarcomere. The contractile element behaviour is described by the Huxley's theory and the time dependence agrees with the activation factor proposed by Julian for skeletal muscle, and modified by Wong for cardiac muscle. The two elastic elements have non-linear exponential characteristics. The isometric contraction of the multicomponent fibre has been simulated by means of a computer program. The results show the tension generated by the fibre, the propagation of the contraction along the fibre and the different contribution of each segment depending on its position inside the fibre.

Biomechanical Phenomena↗
PubMed1987

Motion patterns of pedestrian surrogates in simulated vehicle-pedestrian collisions.

In about 80-85% of all vehicle-pedestrian collisions, a pedestrian is hit by the frontal area of a vehicle. Thereby, an enormous variety of spatial motion patterns of the impacted pedestrian is observed. The analysis of injury mechanisms and injury prevention measures depends largely on a sufficient knowledge of the relevant impact-induced motions. Accordingly, the investigation and classification of these motions is an important task in accident biomechanics. To this end, about 150 collision experiments with the aid of a catapult and several types of pedestrian surrogates were performed. Also, extensive use of a mathematical gross motion analysis model was made. The experimental impacts were analysed with the aid of automated high-speed cinephotogrammetry and acceleration measurements while the mathematical model was validated and calibrated for a number of impact configurations chosen strategically. Results of the experimental and theoretical impact simulations are presented which are related to the observed motion characteristics. In particular, a method is discussed which allows the assessment of the sensitivity of a given motion with respect to the impact parameters. An attempt is made to classify these parameters according to the significance of their influence, whereby pedestrian-related and vehicle-related parameters are discerned. It was found, that such a classification is necessary and possible and results are shown.

Acceleration↗
PubMed1987

Laser Doppler anemometer measurements of pulsatile flow in a model carotid bifurcation.

Hemodynamics at the human carotid bifurcation is important to the understanding of atherosclerotic plaque initiation and progression as well as to the diagnosis of clinically important disease. Laser Doppler anemometry was performed in a large scale model of an average human carotid. Pulsatile waveforms and physiologic flow divisions were incorporated. Disturbance levels and shear stresses were computed from ensemble averages of the velocity waveform measurements. Flow in the common carotid was laminar and symmetric. Flow patterns in the sinus, however, were complex and varied considerably during the cycle. Strong helical patterns and outer wall flow separation waxed and waned during each systole. The changing flow patterns resulted in an oscillatory shear stress at the outer wall ranging from -13 to 9 dyn cm-2 during systole with a time-averaged mean of only -0.5 dyn cm-2. This contrasts markedly with an inner wall shear stress range of 17-50, (mean 26) dyn cm-2. The region of transient separation was confined to the carotid sinus outer wall with no reverse velocities detected in the distal internal carotid. Notable disturbance velocities were also time-dependent, occurring only during the deceleration phase of systole and the beginning of diastole. The present pulsatile flow studies have aided in identifying hemodynamic conditions which correlate with early intimal thickening and predict the physiologic level of flow disturbances in the bulb of undiseased internal carotid arteries.

Blood Flow Velocity↗