Search

Significant progress has been made over the years on the topic of hemorheology, not only in terms of the development of more accurate and sophisticated techniques, but also in terms of understanding the phenomena associated with blood components, their interactions and impact upon blood properties. The rheological properties of blood are strongly dependent on the interactions and mechanical properties of red blood cells, and a variation of these properties can bring further insight into the human health state and can be an important parameter in clinical diagnosis. In this article, we provide both a reference for hemorheological research and a resource regarding the fundamental concepts in hemorheology. This review is aimed at those starting in the field of hemodynamics, where blood rheology plays a significant role, but also at those in search of the most up-to-date findings (both qualitative and quantitative) in hemorheological measurements and novel techniques used in this context, including technical advances under more extreme conditions such as in large amplitude oscillatory shear flow or under extensional flow, which impose large deformations comparable to those found in the microcirculatory system and in diseased vessels. Given the impressive rate of increase in the available knowledge on blood flow, this review is also intended to identify areas where current knowledge is still incomplete, and which have the potential for new, exciting and useful research. We also discuss the most important parameters that can lead to an alteration of blood rheology, and which as a consequence can have a significant impact on the normal physiological behavior of blood.

The flow of viscoelastic fluids between parallel plates under the combined influence of electro-osmotic and pressure gradient forcings with asymmetric boundary conditions, by considering different zeta potentials at the walls, is investigated. The fluids are z-z symmetric electrolytes. The analytic solutions of the electrical potential, velocity distributions and streaming potential are based on the Debye-Huckel approximation for weak potential. The viscoelastic fluids used are modelled by the simplified Phan-Thien-Tanner constitutive equation, with linear kernel for the stress coefficient function, and the Finitely Extensible Nonlinear Elastic dumbbells model with a Peterlin approximation for the average spring force. The combined effects of fluid rheology, electrical double- layer thickness, ratio of the wall zeta potentials and ratio between the applied streamwise gradients of electrostatic potential and pressure on the fluid velocity and stress distributions are discussed.

The flow of Newtonian and viscoelastic fluids in a mixing-separating geometry that consists of two opposed channel flows interacting through a gap in the common separating wall is investigated. The flow in this type of geometry was studied experimentally by Cochrane et al. (Philos Trans R Soc Lond A301:163-181, 1981) using Newtonian and viscoelastic fluids at low Reynolds numbers (Re < 50). In this numerical study, by use of a finite-volume method, the effects of Deborah (De) and Reynolds numbers and gap size on the two-dimensional flow dynamics are assessed. The normalized gap size varies between 0 and 5, Re varies between 0 and 50 and De varies between 0 and the maximum attainable value. Due to the anti-symmetry of the fully developed inlet conditions and the symmetry of the flow geometry, the Newtonian creeping flow is anti-symmetric. Increasing the gap size of the separating walls leads to an increase of the reversed flow-rate ratio (R (r) ), which is defined as the ratio of the reversed and the total flow rate. For creeping flow of viscoelastic fluids, here described by the upper-convected Maxwell model, two distinct flow patterns are observed. Below a critical gap size, the reversed flow is slightly enhanced when the Deborah number increases. Further increase of De leads to a subsequent decrease in R (r) towards zero. For a supercritical gap size, increasing the Deborah number leads to a monotonic increase in R (r) .

Submitted by Jairo Amaro (jairo.amaro@sibi.ufrj.br) on 2019-07-16T15:17:56Z No. of bitstreams: 1 2008_FREIRE_BLM129-min.pdf: 692568 bytes, checksum: 4fc35aa6553a44b4bf2744b651f2d08d (MD5) Made available in DSpace on 2019-07-16T15:17:56Z (GMT). No. of bitstreams: 1 2008_FREIRE_BLM129-min.pdf: 692568 bytes, checksum: 4fc35aa6553a44b4bf2744b651f2d08d (MD5) Previous issue date: 2008-09-30 Indisponível. The present work investigates the lower boundary condition for flows over a steep, rough hill. Simple asymptotic arguments together with the mixing-length hypothesis are used to derive a local analytical solution that is tested against three different flow conditions. In all, 36 velocity profiles are compared with the proposed expression. The experiments were carried out in a water channel and velocity measurements were made through laser Doppler anemometry. The extent of separated flow was made to vary as a function of the roughness and the Reynolds number. The analysis includes regions of attached as well as separated flow. In particular, the solution of Stratford is studied at the points of separation and re-attachment and found to apply equally well in rough walls.

A modified second order viscoelastic constitutive equation is used to derive a k–l type turbulence closure to qualitatively assess the effects of elastic stresses on fully-developed channel flow. Specifically, the second order correction to the Newtonian constitutive equation gives rise to a new term in the momentum equation involving the time-averaged elastic shear stress and in the turbulent kinetic energy transport equation quantifying the interaction between the fluctuating elastic stress and rate of strain tensors, denoted by P w , for which a closure is developed and tested. This closure is based on arguments of isotropic turbulence and equilibrium in boundary layer flows and a priori P w could be either positive or negative. When P w is positive, it acts to reduce the production of turbulent kinetic energy and the turbulence model predictions qualitatively agree with direct numerical simulation (DNS) results obtained for more realistic viscoelastic fluid models with memory which exhibit drag reduction. In contrast, P w 0 but has also a small region of negative P w in the buffer layer, was also successfully tested. This second model for P w leads to predictions of drag reduction, in spite of the enhancement of turbulence production very close to the wall, but the equilibrium conditions in the inertial sub-layer were not strictly maintained.

Uniform steady flow of viscoelastic fluids past a cylinder placed between two moving parallel plates is investigated numerically with a finite-volume method. This configuration is equivalent to the steady settling of a cylinder in a viscoelastic fluid, and here, a 50% blockage ratio is considered. Five constitutive models are employed (UCM, Oldroyd-B, FENE-CR, PTT and Giesekus) to assess the effect of rheological properties on the flow kinematics and wake patterns. Simulations were carried out under creeping flow conditions, using very fine meshes, especially in the wake of the cylinder where large normal stresses are observed at high Deborah numbers. Some of the results are compared with numerical data from the literature, mainly in terms of a drag coefficient, and significant discrepancies are found, especially for the constant-viscosity constitutive models. Accurate solutions could be obtained up to maximum Deborah numbers clearly in excess of those reported in the literature, especially with the PTT and FENE-CR models. The existence or not of a negative wake is identified for each set of model parameters.

Near-wall data for the strongly perturbed flow in a neutrally stable boundary layer encountering a steep, smooth, two-dimensional hill are presented. Observations were made on the centerplane of a water channel at thirteen stations relative to the hill by laser Doppler anemometry. The large reverse flow region that is formed on the lee of the hill was particularly scrutinized through seven measuring stations. Results are presented for the mean and turbulent properties of the flow. Wall shear stress was evaluated through fitting procedures that resorted to the near wall behavior of the velocity profile. Logarithmic fits as well as predictions through the Reynolds stress profiles are also presented.

The present work investigates the role of different treatments of the lower boundary condition on the numerical prediction of flows over two-dimensional, smooth, steep hills. Four different law of the wall formulations are tested when a large recirculating region is formed on the lee side of the hill. Numerical implementa- tion of the near-wall functions was made through a finite elements code. The standard κ-� model was used to close the averaged Navier-Stokes equations. Results are vali- dated through original data obtained in a water tank. Measurements resorted to laser Doppler anemometry. The experiment provide detailed data for the characterization of the reverse flow in the region between the separation and the reattachment points, with emphasis on the near wall region. The experimental wall shear stress distribution is compared with the results provided by the different laws of the wall showing good agreement. The numerical predictions are shown to vary markedly between different formulations.

Measurements of the mean and turbulent flow characteristics of shear-thinning moderately elastic 0.1% and 0.2% xanthan gum aqueous solutions were carried out in a sudden expansion having a diameter ratio of 2. The inlet flow was turbulent and fully developed, and the results were compared with data for water in the same geometry and with previous published Newtonian and non-Newtonian data in a smaller expansion of diameter ratio equal to 1.538. An increase in expansion ratio led to an increase in the recirculation length and in the axial normal Reynolds stress at identical normalised locations, but the difference between Newtonian and non-Newtonian characteristics was less intense than in the smaller expansion. An extensive comparison of mean and turbulent flow characteristics was carried out in order to understand the variation of flow features.

A miniaturised fibre optic Laser-Doppler anemometer was used to carry out a detailed hydrodynamic investigation of the flow downstream of a sudden expansion with 0.1–0.2% by weight shear-thinning aqueous solutions of xanthan gum. Upstream of the sudden expansion the pipe flow was fully-developed and the xanthan gum solutions exhibited drag reduction with corresponding lower radial and tangential normal Reynolds stresses, but higher axial Reynolds stress near the wall and a flatter axial mean velocity profile in comparison with Newtonian flow. The recirculation bubble length was reduced by more than 20% relative to the high Reynolds number Newtonian flow, and this was attributed to the occurrence further upstream of high turbulence for the non-Newtonian solutions, because of advection of turbulence and earlier high turbulence production in the shear layer. Comparisons with the measurements of Escudier and Smith (1999) with similar fluids emphasized the dominating role of inlet turbulence. The present downstream turbulence field was less anisotropic, and had lower maximum axial Reynolds stresses (by 16%) but higher radial turbulence (20%) than theirs. They reported considerably longer recirculating bubble lengths than we do for similar non-Newtonian fluids and Reynolds numbers.

A Laser-Doppler anemometer and a pressure transducer were used to carry out detailed measurements of the mean and root mean square of the velocity and wall-pressure in an axisymmetric sudden expansion flow, with 0.4 and 0.5% by weight shear-thinning aqueous solutions of a low molecular weight polymer (6,000), after appropriate rheological characterisation. In spite of their very low molecular weight, these solutions still exhibited elongational elastic effects through drag reduction of up to 35% relative to Newtonian turbulent pipe flow, as shown by Pereira and Pinho (1994).