The decomposition of apparent stresses in disturbed pulsatile flow in the presence of large scale organized structures

Flow disturbance phenomena that occur in unsteady-in-the-mean flows (i.e. pulsatile or oscillating) at moderate Reynolds numbers are analyzed in both the time domain and the frequency domain. The analysis utilizes variable decomposition into a time-varying underlying waveform and flow disturbances which are composed of large scale organized structures and random fluctuations. A practical technique which incorporates time domain phase conditioning, trend removal, and frequency domain matched filtering, is presented and examined using simulated data of known statistical behavior. The applicability of the method is shown by the decomposition of the simulated data and the technique is then applied to experimental data obtained in pulsatile flow through a constricted tube by means of a laser Doppler anemometer. The cross-sectional area reduction at the constriction throat was 90%. The Womersley parameter in the experiments was 5.3 and the Reynolds number based on the average flow rate per cycle was 300 with a minimum/maximum value of 55/600 based on the instantaneous flow rate. Measurements were taken in the flow region downstream of the constriction throat which included several interesting flow disturbance phenomena. The results of the decomposed flow phenomena demonstrate the significant role of large scale organized structures in such flows. This is particularly important when analyzing blood flow in the large arteries in the presence of severe stenosis or behind prosthetic devices in an attempt to estimate the 'turbulent' stress which act on cellular elements. Estimation of the apparent stress tensor is of importance in an effort to elucidate the mechanical factors which influence the durability of red blood cells under abnormal conditions.     

A numerical and experimental investigation of transitional pulsatile flow in a stenosed channel

In the present paper, a closely coupled numerical and experimental investigation of pulsatile flow in a prototypical stenotic site is presented. Detailed laser Doppler velocimetry measurements upstream of the stenosis are used to guide the specification of velocity boundary conditions at the inflow plane in a series of direct numerical simulations (DNSs). Comparisons of the velocity statistics between the experiments and DNS in the post-stenotic area demonstrate the great importance of accurate inflow conditions, and the sensitivity of the post-stenotic flow to the disturbance environment upstream. In general, the results highlight a borderline turbulent flow that sequentially undergoes transition to turbulence and relaminarization. Before the peak mass flow rate, the strong confined jet that forms just downstream of the stenosis becomes unstable, forcing a role-up and subsequent breakdown of the shear layer. In addition, the large-scale structures originating from the shear layer are observed to perturb the near wall flow, creating packets of near wall hairpin vortices.     

On the discrimination between symmetric pentapeptides

Patchornik has pointed out that identification of the dipeptides AC, GC, GA, and AG is insufficient information to allow one to distinguish between the pentapeptide sequences ACGAG and AGACG. The present study proposes a probabilistic approach which will sometimes allow one to distinguish between the parent pentapeptides when all the dipeptides have been determined quantitatively. For the case when the free amino acids have also been quantitatively determined, necessary and sufficient conditions are established for unambiguous discrimination.     

Estimation of turbulent shear stresses in pulsatile flow immediately downstream of two artificial aortic valves in vitro

Measuring turbulent shear stresses is of major importance in artificial heart valve evaluation. Bi- and unidirectional fluid velocity measurements enable calculation of Reynolds shear stress ( ) and Reynolds normal stress ( ). τ is important due to the relation to hemolysis and thrombus formation, but σ is the only obtainable parameter in vivo. Therefore, determination of a correlation factor between τ and σ is pertinent.

In a pulsatile flow model, laser Doppler (LDA) and hot-film (HFA) anemometry were used for simultaneous bi- and unidirectional fluid velocity measurements downstream of a Hall Kaster and a Hancock Porcine aortic valve. Velocities were registered in two flow field locations and at four cardiac outputs. The velocity signals were subjected to analog signal processing prior to digital turbulence analysis, as a basis for calculation of τ and σ.

A correlation factor of 0.5 with a correlation coefficient of 0.97 was found between the maximum Reynolds shear stress and Reynolds normal stress, implying . In vitro estimation of turbulent shear stresses downstream of artificial aortic valves, based on the axial velocity component alone, seems possible.     

2-Deoxy-D-glucose accumulation in adipocytes: apparent transport discrimination between 2-deoxy-D-glucose and 3-O-methyl-D-glucose

The uphill accumulation of free 2-deoxy-D-glucose and 2-deoxy-D-glucose 6-phosphate in rat adipocytes was found not to affect the steady-state distribution of 3-O-methyl-D-glucose although both hexoses share a common transport pathway. This observation argues against a possible effect of 2-deoxy-D-glucose phosphate on the equilibrating nature of the carrier. The results are discussed in light of hypotheses advanced to explain free 2-deoxy-D-glucose accumulation in a variety of cells.     

Utility of random amplified polymorphic DNA in the discrimination between Candida albicans and Candida dubliniensis

Candida dubliniensisis a recently described species closely related to Candida albicans. Since the discrimination between both species by conventional mycological methods is not easy, many researchers have been trying DNA-related techniques in order to identify C. dubliniensis correctly. In this study, we propose the use of the random amplification of polymorphic DNA (RAPD) with a commercialized short primer which discriminates between both species. This oligonucleotide, AB1-12, allowed also separating C. albicans isolates into four different genotypes. These genotypes were different from the unique genotype observed in C. dubliniensis.     

Estimation of turbulent shear stresses in pulsatile flow immediately downstream of two artificial aortic valves in vitro

Measuring turbulent shear stresses is of major importance in artificial heart valve evaluation. Bi- and unidirectional fluid velocity measurements enable calculation of Reynolds shear stress ( ) and Reynolds normal stress ( ). τ is important due to the relation to hemolysis and thrombus formation, but σ is the only obtainable parameter in vivo. Therefore, determination of a correlation factor between τ and σ is pertinent.In a pulsatile flow model, laser Doppler (LDA) and hot-film (HFA) anemometry were used for simultaneous bi- and unidirectional fluid velocity measurements downstream of a Hall Kaster and a Hancock Porcine aortic valve. Velocities were registered in two flow field locations and at four cardiac outputs. The velocity signals were subjected to analog signal processing prior to digital turbulence analysis, as a basis for calculation of τ and σ.A correlation factor of 0.5 with a correlation coefficient of 0.97 was found between the maximum Reynolds shear stress and Reynolds normal stress, implying . In vitro estimation of turbulent shear stresses downstream of artificial aortic valves, based on the axial velocity component alone, seems possible.     

On the molecular discrimination between adenine and guanine by proteins

The molecular recognition and discrimination of adenine and guanine ligand moieties in complexes with proteins have been studied using empirical observations on carefully selected crystal structures. The distribution of protein folds that bind these purines has been found to differ significantly from that across the whole PDB, but the most populated architectures and folds are also the most common in three genomes from the three different domains of life. The protein environments around the two nucleic acid bases were significantly different, in terms of the propensities of amino acid residues to be in the binding site, as well as their propensities to form hydrogen bonds to the bases. Plots of the distribution of protein atoms around the two purines clearly show different clustering of hydrogen bond donors and acceptors opposite complimentary acceptors and donors in the rings, with hydrophobic areas below and above the rings. However, the clustering pattern is fuzzy, reflecting the variety of ways that proteins have evolved to recognise the same molecular moiety. Furthermore, an analysis of the conservation of residues in the protein chains binding guanine shows that residues in contact with the base are in general better conserved than the rest of the chain.     

Hypoxic pulmonary vasoconstriction during steady and pulsatile flow in ferrets

We examined the effects of hypoxia and pulsatile flow on the pressure-flow relationships in the isolated perfused lungs of Fitch ferrets. When perfused by autologous blood from a pump providing a steady flow of 60 ml/min, the mean pulmonary arterial pressure rose from 14.6 to 31.3 Torr when alveolar PO2 was reduced from 122 to 46 Torr. This hypoxic pressor response was characterized by a 10.1-Torr increase in the pressure-axis intercept of the extrapolated pressure-flow curves and an increase in the slope of these curves from 130 to 240 Torr X l-1 X min. With pulsatile perfusion from a piston-type pump, mean pulmonary arterial pressure increased from 17.5 to 36.3 Torr at the same mean flow. This hypoxic pressor response was also characterized by increases in the intercept pressure and slope of the pressure-flow curves. When airway pressure was raised during hypoxia, the intercept pressure increased further to 25 +/- 1 Torr with a further increase in vascular resistance to 360 Torr X l-1 X min. Thus, in contrast to the dog lung, in the ferret lung pulsatile perfusion does not result in lower perfusion pressures during hypoxia when compared with similar mean levels of steady flow. Since the effects of high airway pressure and hypoxia are additive, they appear to act at or near the same site in elevating perfusion pressure.     

Ventilatory effects of high and pulsatile blood flow in the pulmonary circulation

The mechanism of ventilatory stimulation that accompanies increases in cardiac output is unknown. Previous studies addressing this issue have been inconclusive. However, only steady pulmonary blood flow was used. The effect of flow pulsatility merits consideration, because increasing cardiac output raises not only mean pulmonary arterial pressure but also pulse pressure; mechanoreceptors with an important dynamic component to their responses may cause a response to pulsatile, but not steady, flow. Studies were done on anesthetized cats (n = 4) and dogs (n = 4). The right pulmonary artery was cannulated within the pericardium, and systemic blood was pumped from the left atrium to the right pulmonary artery. The right pulmonary circulation was perfused at different levels of flow, which was either steady or pulsatile. Steady-state flow of up to 150 ml.kg-1.min-1 (270 ml.kg-1.min-1 when corrected for the proportion of lung tissue perfused) did not affect breathing pattern. When high pulmonary flow was made pulsatile (pulse pressure approximately 23 mmHg), breath duration decreased from 3.7 +/- 0.72 to 3.4 +/- 0.81 (SD) s (P less than 0.01), representing a change in frequency of only 9%. There was no change in peak inspiratory activity. It was concluded that pulmonary vascular mechanoreceptors are not likely to contribute significantly to the increase in ventilation in association with increases in cardiac output.     

Disorder distal to modeled stenoses in steady and pulsatile flow

Measurements of the velocity and energy spectra were made in the distal region of modeled stenoses in a rigid tube with both steady and pulsatile water flows. Reynolds numbers of 318–2540 and a pulsatile flow frequency parameter of 15 were employed. The effects of the degree of stenosis, the stenosis geometry and the presence or absence of the downstream confining wall on the development of flow disturbances were investigated. Visualization of the distal flow patterns in stenotic and free jets illustrated the existence of complex fields which included vortex shedding, highly turbulent regions, and recirculation zones. Significant flow disorder was created by a mild stenosis in pulsatile, but not in steady, flow. Nondimensionalization employing the stenosis diameter and flow velocity in the throat of the constriction correlates the vortex shedding frequency and energy spectra within a limited postestenotic region.     

Rheological discrimination between native, rigid and aggregated red blood cells in oscillatory flow

The viscoelastic behaviour of hardened or aggregated red blood cells is compared with the flow pattern of native red blood cells, all suspended in buffer solution at a hematocrit of 45%. The rheological properties are investigated under oscillatory shear at the constant frequency of 2Hz. Variation of the amplitude covers a range of shear-rates from 0.5/s to 200/s. It can be seen that rigidification of the red cells by treatment with glutardialdehyde leads to changes of the flow properties in the range of shear-rates above 10/s, whereas aggregate formation due to addition of dextran distinctly alters the flow properties in the range of shear-rates below 10/s.     

Post-stenotic core flow behavior in pulsatile flow and its effects on wall shear stress

Arteries of several species, including man, tend to adjust their diameters such that the mean wall shear stress is in the range of 10-20 dynes cm-2. Additionally, intimal thickening in the human carotid bifurcation correlates well with the reciprocal of wall shear stress as determined in model studies. The correlation indicates that wherever the local mean wall shear stress exceeds approximately 10 dynes cm-2, the artery tends to be spared from intimal thickening. However, it is not known whether mean shear stress, i.e. the time-averaged value, or the instantaneous shear stress is the appropriate correlative variable. Each of these variables suggests different mechanisms for the reaction of the artery wall to its hemodynamic environment. It is therefore important to devise means by which the effects of mean shear and pulsatile shear can be separated in the study of atherogenesis. The present investigation examines the post-stenotic flow field in Plexiglas models under pulsatile conditions approximating those in the aortas of the cynomolgus monkey, an animal often employed in atherogenesis research. Behavior of the core flow and its effects on wall shear stress are studied for stenoses of 75 and 90% area reductions using laser velocimetry. The results show that the post-stenotic field contains regions in which the mean wall shear stress is low, but the pulsatile excursions are large.(ABSTRACT TRUNCATED AT 250 WORDS)     

An LDA and flow visualization study of pulsatile flow in an aortic bifurcation model

The structure of pulsatile flow in a rigid aortic bifurcation model was studied by means of a flow visualization technique and three-dimensional laser-Doppler anemometry. The model was made of glass, having the same shape as that of the average human aortic bifurcation. It was installed into a mock circulatory loop that generated physiological pulsatile flow. Flow separation was observed during accelerated and decelerated flow periods. Double helical flow existed inside the flow separation in the early accelerated flow period. In the decelerated flow period, disturbed flow appeared behind the separation zone. Flow was strongly disturbed during the back flow period, and then was gradually stabilized in the forward flow period. The flow separation and the disturbances released from the flow separation zone greatly influenced near-wall velocities along the lateral wall. The wave form of the near-wall velocity in the flow separation zone was much different from that observed in the aortic portion and behind the separation zone; for example, the magnitude of the negative peak velocity in the direction of the tube axis was larger than that of the positive one, and mean velocity in a cycle was very low. This abnormal phasic change of the near-wall velocity may be associated with atherogenesis. The three-dimensional velocity measurement is very useful for the detailed analysis of near-wall velocity patterns.     

Microcontinuum approach to the pulsatile flow in tubes with and without longitudinal vibration

A fully developed pulsatile flow in a circular rigid tube is analysed by a microcontinuum approach. Solutions for radial variation of axial velocity and cell rotational velocity across the tube are obtained using the momentum integral method. Simplified forms of the solutions are presented for the relevant physiological data. Marked deviations in the results are observed when compared to a Newtonian fluid model. It is interesting to see that there is sufficient reduction in the mass flow rate, phase lag and friction due to the micropolar character of the fluid.     

Coronary venous flow and O2 saturation during transitional phases between various cardiac rates.

The instantaneous and continuous interrelationship between coronary blood flow and coronary venous O2 saturation was determined during transient periods following abrupt rate change in the electrically paced canine heart. Through a catheter in the coronary sinus, O2 saturation was continuously monitored using a fibreoptics technique, and venous flow was measured with an electromagnetic flowmeter. Various patterns of change in flow and O2 saturation were observed depending both on the absolute values of the cardiac rates as well as on the relative difference between them during changes from one rate to another. Whereas elevation of coronary flow was monophasic when the magnitude of heart rate change was below 75 beats per minute, a drop in flow was observed preceeding its elevation when the difference was greater. At high rates further increase in rate caused either no alteration or led to a monophasic drop in flow during the transitional period. Changes in O2 saturation were observed only when heart rate difference exceeded 60 beats per minute. Between 60-90 beats per minute O2 saturation remained steady except during the transient rate elevation, ending in a lower steady state O2 saturation. The results indicate that both O2 saturation and coronary flow change with heart rate initially because of mechanical consequence of the increased rate on the myocardium, and later according to its new metabolic needs also manifested by changed O2 extraction.     

Wall shear stress distribution in the human carotid siphon during pulsatile flow

Wall shear stress distribution in the carotid siphon, which is a multiple curved segment of the internal carotid artery, is investigated numerically under physiological flow conditions. The computer simulation of flow through the model segment is based on the time-dependent, three-dimensional Navier-Stokes equations, solved numerically with a finite element method. The study shows the behavior of the wall shear stress-vector field and identifies the zones of high and low wall shear stress values during the cardiac cycle.     

Effect of non-Newtonian and pulsatile blood flow on mass transport in the human aorta

To investigate the effects of both non-Newtonian behavior and the pulsation of blood flow on the distributions of luminal surface LDL concentration and oxygen flux along the wall of the human aorta, we numerically compared a non-Newtonian model with the Newtonian one under both steady flow and in vivo pulsatile flow conditions using a human aorta model constructed from MRI images. The results showed that under steady flow conditions, although the shear thinning non-Newtonian nature of blood could elevate wall shear stress (WSS) in most regions of the aorta, especially areas with low WSS, it had little effect on luminal surface LDL concentration (c(w)) in most regions of the aorta. Nevertheless, it could significantly enhance c(w) in areas with high luminal surface LDL concentration through the shear dependent diffusivity of LDLs. For oxygen transport, the shear thinning non-Newtonian nature of blood could slightly reduce oxygen flux in most regions of the aorta, but this effect became much more apparent in areas with already low oxygen flux. The pulsation of blood flow could significantly reduce c(w) and enhance oxygen flux in these disturbed places. In most other regions of the aorta, the oxygen flux was also significantly higher than that for the steady flow simulation. In conclusion, the shear shining non-Newtonian nature of blood has little effect on LDL and oxygen transport in most regions of the aorta, but in the atherogenic-prone areas where luminal surface LDL concentration is high and oxygen flux is low, its effect is apparent. Similar is for the effect of pulsatile flow on the transport of LDLs. But, the pulsation of blood flow can apparently affect oxygen flux in the aorta, especially in areas with low oxygen flux.