Biomedical subjects

J D Janssen

Publications and source records attributed to J D Janssen.

30 records · Page 2Linked to original sources

PubMed1994

Analysis of the flow in stenosed carotid artery bifurcation models--hydrogen-bubble visualisation.

This paper deals with the effect of geometric changes of mild stenoses on large-scale flow disturbances in the carotid artery bifurcation. Hydrogen-bubble visualisation experiments have been performed in Plexiglas models of a non-stenosed and a 25% stenosed carotid artery bifurcation. The flow conditions approximate physiological flow. The experiments show that shortly after the onset of the diastolic phase vortex formation occurs in the plane of symmetry. This vortex formation is found in a shear layer, which is formed in the carotid sinus. The shear layer is located between a region with low shear rates at the non-divider wall and a region with high shear rates at the divider wall. In order to gain insight into the parameters that are important with respect to the stability of the shear layer, experiments have been performed in which the influence of the shape of the flow pulse, the Reynolds number (Re), the Womersley parameter (alpha) and the flow division ratio (gamma) on the flow phenomena is studied. From these experiments it appears that the flow phenomena in the carotid artery bifurcation are significantly influenced by Re, alpha the systolic acceleration (sa) and deceleration (sd) and the duration of the peak-systolic flow (Tmax). With these results a simplified flow pulse is chosen, with which the experiments in the non-stenosed and the 25% stenosed bifurcation are performed. Comparison of the hydrogen-bubble profiles in the 0 and 25% stenosed models with similar flow conditions shows that the geometric change of the 25% stenosis only slightly influences the flow phenomena. The most striking influences are found in the stability of the shear layer. Quantitative experiments by means of laser Doppler anemometry measurements and numerical computations are needed to analyse the influence of the stenosis of the flow field more accurately.

Acceleration↗

PubMed1994

Assessment of energy expenditure for physical activity using a triaxial accelerometer.

A triaxial accelerometer was used to evaluate the relationship between energy expenditure due to physical activity (EEact) and body acceleration during different types of activity. In a laboratory experiment, 11 male subjects performed sedentary activities and walked on a motor driven treadmill (3-7 km.h-1). EEact was calculated from total energy expenditure (EEtot), as measured by indirect calorimetry, and sleeping metabolic rate (SMR): EEact = EEtot--SMR. Body accelerations were measured with a triaxial accelerometer at the low back. Special attention was paid to the analysis of unidirectional and three-directional accelerometer output. During sedentary activities a linear relationship between EEact and the sum of the integrals of the absolute value of accelerometer output from all three measurement directions (IAAtot) was found (r = 0.82, P < 0.001, Sy,x = 0.22 W.kg-1). During walking EEact was highly correlated with the integral of absolute accelerometer output in antero-posterior direction (IAAx; r = 0.96, P < 0.001, Sy,x = 0.53 W.kg-1). When all examined activities were included in a regression analysis, a strong linear relationship between EEact and IAAtot was found (r = 0.95, P < 0.001, Sy,x = 0.70 W.kg-1). Using this relationship, EEact during sedentary activities as well as EEact during walking could be estimated with an accuracy of about 15%. Although sedentary activities and walking represent a large part of normal daily physical activity, the validity and usefulness of the triaxial accelerometer--measuring IAAtot--to predict EEact in daily life must be studied under free-living conditions.

Acceleration↗

PubMed1990

A numerical analysis of steady flow in a three-dimensional model of the carotid artery bifurcation.

A finite element approximation of steady flow in a rigid three-dimensional model of the carotid artery bifurcation is presented. A Reynolds number of 640 and a flow division ratio of about 50/50, simulating systolic flow, was used. To limit the CPU- and I/O-times needed for solving the systems of equations, a mesh-generator was developed, which gives full control over the number of elements into which the bifurcation is divided. A mini-supercomputer, based on parallel and vector processing techniques, was used to solve the system of equations. The numerical results of axial and secondary flow compare favorably with those obtained from previously performed laser-Doppler velocity measurements. Also, the influence of the Reynolds number, the flow division ratio, and the bifurcation angle on axial and secondary flow in the carotid sinus were studied in the three-dimensional model. The influence of the interventions is limited to a relatively small variation in the region with reversed axial flow, more or less pronounced C-shaped axial velocity contours, and increasing or decreasing axial velocity maxima.

Blood Flow Velocity↗

PubMed1990

A two-dimensional numerical analysis of unsteady flow in the carotid artery bifurcation. A comparison with three-dimensional in-vitro measurements and the influence of minor stenoses.

In the present study a two-dimensional finite element model for incompressible Newtonian flow is applicated to the modelling of carotid artery flow. In earlier studies, the numerical model was validated experimentally for several flow configurations. In general the pulsatile flow is characterized by reversed flow regions at the non-divider side walls of both the internal and external carotid arteries. The unsteadiness of the flow is associated with rather complex spatial and temporal velocity distributions and leads to temporal variations of the location and length of the reversed flow regions. As a consequence, pronounced spatial and temporal variations in the wall shear stresses are found. At the non-divider side walls, wall shear stresses are relatively low and exhibits an oscillatory behaviour in space and time. At the divider side walls, wall shear stresses are relatively high and approximately follow the flow rate distribution in time. The aim of this study is not only to present two-dimensional calculations but also to compare the calculated two-dimensional velocity profiles with those from three-dimensional experiments. It is observed that in the common carotid artery and in the proximal parts of the internal and external carotid arteries, the two-dimensional numerical model provides valuable information with respect to the three-dimensional configuration. In the more distal parts of especially the internal carotid artery, deviations are found between the two-dimensional numerical and three-dimensional experimental model. These deviations can mainly be attributed to the neglect of the secondary velocity distribution in the two-dimensional model. In the two-dimensional numerical model the influence of a minor stenosis in the internal carotid artery is hardly distinguishable from a minor geometrical variation without stenosis. Full three-dimensional analyses of the influence of minor stenoses are needed to prove numerically whether in-vivo measurements of the axial velocity distribution are useful in the detection of minor stenoses.

Arterial Occlusive Diseases↗

PubMed1990

Experimental and numerical analysis of carotid artery blood flow.

To obtain more insight into the complex blood flow patterns in the carotid artery bifurcation numerical analyses, limited to Newtonian incompressible flows in rigid-wall geometries, have been carried out combined with laser-Doppler velocity measurements. The steady and unsteady flow in two-dimensional models of a stenosis and of the carotid artery bifurcation were analysed. As a first step to a three-dimensional analysis, the steady flow development in a curved tube was investigated.

Blood Flow Velocity↗

PubMed1989

An experimental setup for the measurement of forces on a human cadaveric foot during inversion.

An experimental setup was developed for statically measuring seven vertical and three horizontal reaction forces on the foot. In the setup, the leg can be simultaneously loaded (1) by a vertical force, (2) by an externally applied axial moment, and (3) by simulated muscle forces. The foot is free to invert under influence of the external loads. Statical analysis and test experiments were used for evaluation. The setup can be used in combination with Roentgen photogrammetry to measure bone positions simultaneously with forces.

Cadaver↗

PubMed1988

A mechanical analysis of the closed Hancock heart valve prosthesis.

In order to obtain mechanical specifications for the design of an artificial leaflet valve prosthesis, a geometrically non-linear numerical model is developed of a closed Hancock leaflet valve prosthesis. In this model, the fibre reinforcement of the leaflet and the viscoelastic properties of frame and leaflets are incorporated. The calculations are primarily restricted to 1/6 part of the valve and a time varying pressure load is applied. The calculations are verified experimentally by measuring the commissure displacements and leaflet centre displacement of a Hancock valve. The numerically obtained commissure displacements are found to be linearly dependent on the pressure load, and the slope of the curves is hardly dependent on loading type and loading velocity. Experimentally a difference is found between the three commissure displacements, which is also predicted numerically using a simplified asymmetric total valve model. Besides, experimentally a clear dependency of commissure displacements on frame size is found. For the leaflet centre displacement, a qualitative agreement exists between numerical prediction and experimental result, although the numerical predicted values are systematically higher. The numerically obtained stress distributions revealed that the maximum von Mises intensity in the membranes occurs in the vicinity of the commissure in the free leaflet area (0.2 N mm-2). Wrinkling of the membranes may occur in the coaptation area near the leaflet suspension. The maximum fibre stress is found near the aortic ring in the fibres which form the boundaries of the coaptation area (0.64 N mm-2). These locations seem to correlate with some common regions of tissue valve failure.

Computer Simulation↗

PubMed1987

A numerical and experimental analysis of the flow field in a two-dimensional model of the human carotid artery bifurcation.

Axial velocities were measured in an enlarged, two-dimensional, rigid model of the carotid artery bifurcation by means of a laser-Doppler anemometer, under both steady and unsteady flow conditions. Also a numerical model was developed, based on the finite element approximation of the Navier-Stokes and continuity equations. From this study it appeared that the numerically predicted velocities agree well with the experimentally obtained values. Besides, the bifurcation hardly influenced the upstream flow in the main branch (common carotid artery), high velocity gradients were observed at the divider walls of the daughter branches (internal and external carotid arteries) and large zones with reversed flow were present near the nondivider walls of these branches. For steady flow the maximal diameter of this zone at the entrance of the internal carotid artery (carotid sinus) was about 25% of the local diameter of this branch. For unsteady flow this zone was absent during the initial phase of flow acceleration and maximal at the end of flow deceleration with a maximal diameter of about 50% of the local diameter of the carotid sinus.

Carotid Arteries↗

PubMed1983

The mechanical properties of porcine aortic valve tissues.

In uniaxial tensile experiments in vitro mechanical properties of the different parts of porcine aortic valves, i.e. the leaflets, the sinus wall and the aortic wall, have been dealt with. Tissue strips cut in different directions were investigated. The collagen bundles in the leaflets show a stiffening effect and cause a marked anisotropy: within the physiological range of strains the largest slopes of the stress-strain curves of leaflet specimens in the bundle direction are a factor of about 20 larger than those of specimens taken along the perpendicular direction. For the sinus and aortic tissues, these values are 50-200 times smaller than those obtained from the leaflet specimens in the bundle direction. Two aspects of viscoelastic behaviour were examined: the strain rate sensitivity of the stress-strain curves and the relaxation behaviour. The stress-strain curves of the different valve parts appeared to be rather insensitive to the strain rate: the most pronounced sensitivity observed in our experiments, was a doubling of the stress at the same strain caused by a hundredfold increase of the strain rate. In analyzing the relaxation behaviour, use was made of the relaxation model proposed by Fung (1972, in Biomechanics, its Foundations and Objectives; Fung, Perrone and Anliker. Prentice Hall). In the leaflets, about 45% stress relaxation was found whereas this amounted to 30% in the sinus and aortic walls. Predictions based upon the model indicate that on cyclic loading the larger viscous losses have to be expected in the leaflets.

Animals↗

PubMedDate not supplied

A numerical-experimental method for a mechanical characterization of biological materials.

For the determination of material parameters, it is a common practice to extract specimens with well-defined geometries. The design of the samples and the choice of the applied load are meant to lead to a homogeneous stress and strain distribution in a part of the sample. When applied to biological materials, this raises a number of problems: homogeneous strains cannot be obtained because the materials have inhomogeneous properties, and the manufacturing of samples is hard or sometimes impossible. In this technical note a different approach is presented, based on the use of a digital image technique for the measurement of nonhomogeneous strain distributions, finite element modeling and the use of a minimum-variance estimator. The method is tested by means of experiments on an orthotropic elastic membrane of a woven and calendered textile. Five parameters are identified using the experimental data of one single experiment.

Algorithms↗

PubMedDate not supplied

Wall shear stress in backward-facing step flow of a red blood cell suspension.

An experimental investigation of the wall shear stress distribution downstream of a backward-facing step is carried out. The wall shear stress distribution was determined by measuring the deformation of a gel layer, attached to the wall downstream of the step. Speckle pattern interferometry was applied to measure the deformation of the gel layer. The measured deformation, combined with the properties of the gel layer, served as an input for a finite element solid mechanics computation to determine the stress distribution in the gel layer. The wall shear stress, required to generate the measured deformation of the gel layer, was determined from these computations. A Newtonian buffer solution and a non-Newtonian red blood cell suspension were used as measuring fluids. The deformation of the gel layer was determined for a Newtonian buffer solution to evaluate the method and to obtain the properties of the gel layer. Subsequently, the wall shear stress distribution for the non-Newtonian red blood cell suspension was determined for three different flow rates. The inelastic non-Newtonian Carreau-Yasuda model served as constitutive model for the red blood cell suspension. Using this model, the velocity and wall shear stress distribution were computed by means of a finite element fluid mechanics computation. From the comparison between the numerical and the experimental results, it can be concluded that wall shear stresses, induced by the red blood cell suspension, can be modeled accurately by employing a Carreau-Yasuda model.

Blood Vessels↗

PubMedDate not supplied

Numerical analysis of steady generalized Newtonian blood flow in a 2D model of the carotid artery bifurcation.

The stationary flow of blood in a two-dimensional model of the bifurcation of the human carotid artery is simulated numerically using a finite element method. The Reynolds number is taken as equal to 300, corresponding to the value during the end-diastolic phase of the heart cycle. As constitutive equations, the Newtonian model and the non-Newtonian power-law and Casson models are used. The chosen model parameters corresponded with blood. The flow in this geometry is determined by the branching of the artery and the existence of a reversed flow area in the internal carotid artery. From the results of this problem, we conclude that the general flow structure is not influenced by the generalized (non-)Newtonian models. However, there are differences that cannot be neglected. First, the generalized Newtonian models result in axial and secondary velocity profiles that have 5-10% lower maximum values compared to the Newtonian model. Second, the pressure has higher values in the case of the generalized Newtonian models, especially in the internal carotid artery where these models give maximal 25% higher pressure values. Third, along the divider wall, the wall shear stresses are lower for the generalized Newtonian models; near the apex, this difference is maximal 40% in case of the power-law model. The generalized Newtonian models give higher wall shear stresses along the non-divider wall than the Newtonian model, the maximum difference being 5%. And fourth, in the internal carotid artery the reversed flow area is 10% reduced by the generalized Newtonian models. In general, the differences are more pronounced in the case of the power-law model.

Carotid Arteries↗