Megha Singh

Experimental analysis of the influence of stenotic geometry on steady flow.

We studied the flow behavior under steady flow conditions in four models of cylindrical stenoses at Reynolds numbers from 150 to 920. The flow upstream of the constrictions was always fully developed. The constriction ratios of the rigid tubes (D) to the stenoses (d) were d/D = 0.273; 0.505; 0.548; 0.786. The pressure drop at various locations in the stenotic models was measured with water manometers. The flow was visualized with a photoelasticity apparatus using an aqueous birefringent solution. We also studied the flow behavior at pulsatile flow in a dog aorta with a constriction of 71%. The flow through stenotic geometries depends on the Reynolds number of the flow generated in the tube and the constriction ratio d/D. At low d/D ratios, (with the increased constriction), the flow separation zones (recirculation zones, so-called reattachment length) and flow disturbances increased with larger Reynolds numbers. At lower values, eddies were generated. At high Re, eddies were observed in the pre-stenotic regions. The pressure drop is a function of the length and internal diameter of the stenosis, respective ratio of stenosis to the main vessel and the Reynolds numbers. At low Re-numbers and low d/D, distinct recirculation zones were found close to the stenosis. The flow is laminar in the distal areas. Further experiments under steady and unsteady flow conditions in a dog aorta model with a constriction of 71% showed similar effects. High velocity fluctuations downstream of the stenosis were found in the dog aorta. A videotape demonstrates these results.

Erythrocytes sedimentation profiles under gravitational field as determined by He-Ne laser. VII. Influence of dextrans, albumin and saline on cellular aggregation and sedimentation rate

The erythrocytes sedimentation profiles (ESP) of normal blood and of blood mixed with saline, albumin (7%), and various molecular weight dextrans of different concentrations, at various height and widths of the sample holder are determined. These observations show that the sedimentation characteristics of the erythrocytes depend on the influence of these substitutes on the plasma and cellular constituents. The normalised aggregation and the sedimentation rate, as determined from these profiles, show that the dextran 40 and dextran 70 retard the erythrocytes sedimentation, for high molecular weight it is similar to that of normal blood and is the maximum for saline. This change for high molecular weight dextrans could be attributed to the enhanced aggregation tendency of erythrocytes and for saline to the enhanced sedimentation due to decrease in the viscosity and density of suspending medium. The influence of the various concentrations of dextrans on these parameters has been determined.

Analysis of erythrocyte aggregation mechanism in presence of dextran and magnetic field by ultrasound scattering in blood

Aggregation of erythrocytes is studied as function of time during their sedimentation process under the gravitational field. The method is based on ultrasonic scattering from the various blood samples in presence and absence of inhomogeneous magnetic field (IMF) and dextran 70 (10%). The experimental arrangement is consisting of ultrasonic transmitter and receiver probes placed in mutually perpendicular direction intersecting at the sampling volume of blood located at the centre of the blood column. The temporal kinetic process is represented in terms of histograms of amplitudes and number of scattering fluctuations related to the size and motion of aggregates. The results show that the application of IMF influences the aggregation and sedimentation of erythrocytes. The aggregates thus formed sediment faster than that of control sample. The aggregate formation and their movements in presence of dextran 70 are slower than that of normal blood which may be attributed to the enhanced suspending medium viscosity and their interaction with erythrocytes.

Mechanism of erythrocyte aggregate formation in presence of magnetic field and dextrans as analyzed by laser light scattering.

The mechanism of erythrocyte aggregation has been studied in normal plasma, dextran 40 and dextran 70 suspensions in presence and absence of magnetic field at a concentration of 5 percent by laser light scattering. The inhomogeneous magnetic field enhances the aggregating tendency of normal erythrocytes. The growth of aggregates due to dextran 70 is enhanced in presence of magnetic field. On the other hand the disaggregating effect of dextran 40 is reduced due to this field. The induced changes due to magnetic field during the development of erythrocyte aggregates in these media are determined.