A.A. van Steenhoven

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.

In vitro closing behaviour of Björk-Shiley, St Jude and Hancock heart valve prostheses in relation to the in vivo recorded aortic valve closure

To compare prosthetic valve behaviour with that of the natural aortic valve, experiments were performed in vitro as well as in vivo. In a mock circulation system, cinematographic high-speed recordings of the valvular behaviour were made for Björk-Shiley, St Jude and Hancock heart valve prostheses. Simultaneously with the film recording, the aortic flow and the left ventricular and aortic pressures were measured. The closing behaviour of the natural aortic valve was recorded in in vivo experiments following the same measuring technique. Comparison of the film frames with the aortic flow signal revealed that the mechanical prostheses mainly close due to the back flow in the early phase of diastole; they close only for 5% of their cross-sectional area during systolic ejection. The Hancock bioprosthesis closes already for 45% during the flow deceleration phase of systole, which is however significantly less than the corresponding closure of the aortic valve as recorded in vivo (74%). The change in fluid viscosity from 3.10(-3) Ns/m2 to 1.10(-3) Ns/m2 does not affect the closing behaviour of the mechanical prostheses. Reducing the peak value of systolic aortic flow by a factor two decreases the cross-sectional area of the prosthetic valves at peak systole. When the natural aortic valve closing mechanism is applied to a disc-valve prosthesis, a partial (30%) systolic valve closure is found.

The heat flux method for producing burner stabilized adiabatic flames: an evaluation with cars thermometry

Flat adiabatic stretchless methane/air flames at atmospheric pressure are investigated with CARS thermometry in a folded BOXCARS configuration. The flames are stabilized on a perforated-plate burner by adjusting the flow until zero net heat flux to the burner is created. Vertical temperature profiles are measured in flames with equivalence ratios of 0·80, 0·90, 1·00, and 1·10, up to a height of 35·0  mm above the burner. One horizontal profile is measured in the flame with π = 0·80. The measurements are compared with calculations of free flames, based on the GRI 2·11 reaction mechanism. A simple model is

Heat transfer predictions for micro-/nanochannels at the atomistic level using combined molecular dynamics and Monte Carlo techniques

The thermal behavior of a gas confined between two parallel walls is investigated. Wall effects such as hydrophobic or hydrophilic wall interactions are studied, and the effect on the heat flux and other characteristic parameters such as density and temperature is shown. For a dilute gas, the dependence on gas-wall interactions of the temperature profile between the walls for the incident and reflected molecules is obtained using molecular dynamics (MD). From these profiles, the effective accommodation coefficients for different interactions and different mass fluid/wall ratio are derived. We show that Monte Carlo (MC) with Maxwell boundary conditions based on the accommodation coefficient gives good results for heat flux predictions when compared with pure molecular dynamics simulations. We use these effective coefficients to compute the heat flux predictions for a dense gas using MD and MC with Maxwell-like boundary conditions.

The influence of the wall temperature on the development of heat transfer and secondary flow in a coiled heat exchanger

Abstract In the present study the development of mixed convective flow is studied in a helically coiled heat exchanger with an axially varying wall temperature for Re = 500, Pr = 5 and δ = 1 14 and compared to the constant wall temperature boundary condition. In the method used the parabolized equations are solved using a finite difference discretization scheme. The influence of buoyancy forces is analyzed on heat transfer and secondary flow. For all Grashof numbers studied it appears that both heat transfer, quantified by the Nusselt number, and secondary flow, quantified by the relative kinetic energy, exhibit a wavy behaviour in axial direction. For higher Grashof numbers, however, this phenomenon diminishes for the case with an axially varying wall temperature due to stabilizing stratification effects.

Design optimization for fast heat-transfer gauges

Heat flux gauges at a flat plate are designed suited to measure the heat transfer in transition boundary layers. The measuring technique employed (cold thin films) is convenient for transient experimental facilities (such as a Ludwieg tube). The development of a well defined and optimized sensor design is discussed.

Molecular Dynamics and Monte Carlo Simulations for Heat Transfer in Micro and Nano-channels

There is a tendency to cool mechanical and electrical components by microchannels. When the channel size decreases, the continuum approach starts to fail and particle based methods should be used. In this paper, a dense gas in micro and nano-channels is modelled by molecular dynamics and Monte Carlo simulations. It is shown that in the limit situation both methods yield the same solution. Molecular dynamics is an accurate but computational expensive method. The Monte Carlo method is more efficient, but is less accurate near the boundaries. Therefore a new coupling algorithm for molecular dynamics and Monte Carlo is introduced in which the advantages of both methods are used.

The Role of the Trapped Sinus Vortex in Aortic Valve Closure

The aortic valve is one of the four valves controlling the fluid motion through the heart. It is positioned at the outlet of the left ventricle, which pumps blood into the aorta. This valve is shown diagrammatically in figure 1. It has three leaflets (cusps) and behind each leaflet there is a cavity, the sinus of Valsalva. The leaflets are very thin (0.1–0.3 mm), non-muscular and very flexible in the axial direction.

Numerical analysis of an optical method to determine temperature profiles in hot glass melts

In the present study a technique is developed to determine temperature profiles in hot glass melts, using intensity measurements performed at various wavelengths in the infrared spectrum. To that end an analytical model is developed which describes the internal energy transfer in a glass layer and the spectral intensity emerging from the glass layer. The spectral intensity so calculated is confronted with a measured spectral intensity to reconstruct the temperature profile in the glass layer. Because the temperature reconstruction from the measured spectral intensity is an ill posed inverse problem, Tikhonov regularization and the L-curve method are used to determine a meaningful temperature distribution in the glass layer. In order to gain insight into the accuracy of the temperature measurement method a sensitivity analysis is made using numerical simulations. The influence of random noise on the measurement signal and systematic errors in the properties is investigated. Furthermore, the temperature reconstruction of linear, logarithmic and parabolic temperature distributions is analysed.

Properties of a Dense Hard-Sphere Gas Near the Walls of a Microchannel

A mathematical model has been developed to characterize the effect of packing of molecules of a hard-sphere dense gas near the hard walls of a microchannel. Analytical techniques, Monte Carlo (MC) methods and Molecular Dynamics (MD) simulation methods have been used to characterize the influence of the characteristic parameters such as number density, reduced density, length of the system and molecular diameter on the equilibrium properties of the gas near the hard walls of the microchannel. The height and the position of the density oscillation peaks near the wall are characterized. Comparisons between MD and MC results for particles having different diameter are presented. For the same size of the particles and moderately dense gas, MC and MD results are similar, differences in the density profiles being limited only to the oscillatory region. For different particle sizes, MD and MC results are limited to a short distance near the wall for long size systems and moderately dense fluids. The effect of the boundary (particle size) on the simulation results is increasing with η (reduced density) and it is very small in case of a dilute gas. For small η and small particle size (R) relative to length of the system L, the height of the oscillations peaks is slowly increasing with R/L, and for high densities is always decreasing with R/L. The position of these peaks depends only on the size of the particles and when R is much smaller than L, it shows a small dependence on L.Copyright