Axel E. Larreteguy

A transient shear stress model for the analysis of laminar water-hammer problems

A transient shear stress model for the solution of water-hammer problems for laminar flow in pipes is presented. The model is based on the polynomial expansion of the radial profiles of axial velocities, and the solution of the resulting set of equations by the method of characteristics. This approach, as compared to the usual quasi-steady model (which can be regarded as a particular case of the new method), allows for a better representation of the shear stress at the wall during the pressure transients. The present model can be included with only minor modifications into any existing code for solving water-hammer problemirthat uses the characteristics method and the quasi-steady model. The test of the new model against experimental results of Holmboe and Rouleau, given in [8], and mathematical models and numerical simulations of other authors |5](8| show that it is less cpu and memory demanding, and is able of obtaining comparable results.

Sensitivity analysis of finite volume simulations of a breaking dam problem

Purpose – The present work is a numerical study of a breaking dam problem. The purpose of this paper is to assess the effect of turbulence and surface tension models in the prediction of the interface position in a long-term analysis. Additionally, dimensional effects are analyzed by carrying out both 2D and 3D simulations. Design/methodology/approach – Finite volume simulations performed with the different models are compared between them and contrasted with numerical results computed using other numerical techniques and experimental data. Findings – The reported numerical results are in general in good agreement with experimental results available in the literature. They are also consistent with numerical solutions of other authors obtained using different numerical techniques. The results show that the laminar simulations exhibit strong mesh size dependency, while the turbulence models seem to help in producing mesh-independent solutions. Surface tension modeling does not seem to play a relevant role i...

A Particle-Center-Averaged Two-Fluid Model for Wall-Bounded Bubbly Flows

The accurate prediction of void fraction profiles near solid walls using the two-fluid model is still an unresolved issue. These profiles result from the combination of two factors: a) forces acting on the bubbles, and, b) geometric constraints imposed by their shape. We propose herein a two-fluid model that involves a particle-center-averaging procedure for the disperse phase, and a reinterpretation and postprocessing of the results obtained. This center-averaged approach averages the disperse phase (bubbles) based on a particle center indicator function, while using the standard phase indicator averaging for the continuous phase (liquid). The solution fields obtained are then postprocessed to introduce the geometry of the bubbles in order to recover the values that should be representative of measured fields. The key idea here is to separate the geometric aspect from the dynamic aspect of the problem into two independent, successive steps. The new model may be easily incorporated into existing two-fluid model codes. Results obtained with the new model showing agreement with experimental data (Moursali et al, 1995) are also presented.Copyright

Influence of arterial geometry on a model for growth rate of atheromas

Atherosclerosis is a disease that affects medium and large size arteries and it can partially or totally obstruct blood flow through them. The lack of blood supply to the heart or the brain can cause an infarct or a stroke with fatal consequences or permanent effects. This disease involves the proliferation of cells and the accumulation of fat, cholesterol, cell debris, calcium and other substances in the artery wall. Such accumulation results in the formation of atherosclerotic plaques called atheromas, which may cause the obstruction of the blood flow. Cardiovascular diseases, among which atherosclerosis is the most frequent, are the first cause of death in developed countries. The published works in the subject suggest that hemodynamic forces on arterial walls have influence on the localization, initial development and growth rate of atheromas. This paper presents a model for this growth rate, and explores the influence of the bifurcation angle on the blood flow patterns and on the predictions of the model in a simplified carotid artery. The choice of the carotid bifurcation as the subject for this study obeys the fact that atheromas in this artery are often responsible for strokes. Our model predicts a larger initial growth rate in the external walls of the bifurcation and smaller growth area and lower growth rates as the bifurcation angle is increased. The reason for this seems to be the appearance of helical flow patterns as the angle is increased.

Study of 3D sloshing in a vertical cylindrical tank

Moving liquid-gas interfaces appear frequently in both natural processes and engineering applications. In the case of partially filled tanks, for instance, the accurate description of the free surface transient behavior during transportation or earthquakes is of paramount importance for structural stability analyses. This work presents new experimental data of sloshing at laboratory scale in a vertical cylindrical tank with different filling levels, along with numerical simulations of selected cases using an open source finite volume application. Maximum and minimum experimental wave heights, measured with ultrasonic sensors, are reported for several non-resonant cases during the periodic steady state regime, along with snapshots of a video recorded near-resonance case. For the numerical simulations, a suitable mesh was designed based on a mesh convergence analysis focused on the simulated velocity profiles at the tank wall. A slight nonlinear behavior is detected in the experimental wave patterns, expressed as non-symmetrical minimum and maximum wave heights. The near-resonance case, in turn, shows a highly three-dimensional behavior of the free surface and a rotational effect. The numerical results obtained for the non-resonant cases show good overall agreement with the experiments, although the non-linear behavior is not accurately modelled. The evolution of the highly distorted free surface in the near-resonance case is well captured by the simulation, along with the observed rotational effect.

Linear Analysis of Density-Wave Oscillations in a Parallel Heated Channel

A one-dimensional analytical model has been developed to be used for the linear analysis of density-wave oscillations in a parallel heated channel. The heated channel is divided into a single-phase and a two-phase region. The two-phase region is represented by the drift-flux model. The model accounts for phasic slip and subcooled boiling. The localized friction at the exit of the heated channel is treated considering the two-phase mixture. The exact equation for the total channel pressure drop is perturbed around the steady state. The stability characteristics of the heated channel are investigated using the Nyquist criterion. The marginal stability boundary (MSB) is determined in the two-dimensional thermodynamic equilibrium space parameters, the subcooled boiling number and the phase change number. The predictions of the model agree well with experimental data published in open literature. (1) The effect of thermal equilibrium, equal velocity (homogeneous) model increases the channel steam quality which leads to decrease in localized exit loss coefficient and finally stabilizes the system. (2) The effect of thermal equilibrium, non-equal velocity (drift-flux) model decreases the channel steam quality than the homogeneous model and finally destabilizes the system. (3) The effect of thermal non-equilibrium, equal velocity model (a) decreases the channel steam quality from high subcooled boiling number to a critical value and destabilizes the system, (b) increases the channel steam quality from the critical value to low subcooled boiling number and stabilizes the system. (4) The effect of thermal non-equilibrium, non-equal velocity model (a) decreases the channel steam quality from high subcooled boiling number to a critical value and destabilizes the system, (b) increases the channel steam quality from the critical value to low subcooled boiling number and stabilizes the system and the frequency of oscillation is in good agreement with the experimental data than the other models.Copyright

The Effect of the Treatment of Localised Friction in Two-Phase Mixtures on the Stability of Parallel Channels

A one-dimensional analytical model has been developed to be used for the linear analysis of density-wave oscillations in a parallel heated channel. The heated channel is divided into a single-phase and a two-phase region. The two-phase region is represented by the homogeneous model. The localised friction at the channel exit is treated considering the two-phase mixture. The exact equation for the total channel pressure drop is perturbed around the steady state. The stability characteristics of the heated channel are investigated using the Nyquist criterion. The marginal stability boundary (MSB) is determined in the two-dimensional thermodynamic equilibrium space parameters, the subcooled boiling number and the phase change number. The predictions of the model are compared with experimental results published in open literature. The results indicate a more stable system with (1) low system pressure, (2) high inlet restriction, (3) low outlet restriction, and (4) high inlet velocity. The results show that the model agrees well with the available experimental data. In particular, the results show the significance of correcting the localised friction due to the presence of the two-phase mixture in the two-phase region: explicit inclusion of the two-phase localised friction improves the agreement with experimental results. This effect is more important for high heating power and high inlet subcooling.Copyright