Showing papers by "Joaquim Peiró published in 2009"

A free-surface and blockage correction for tidal turbines

Abstract: The effects of free-surface proximity on the flow field around tidal stream turbines are modelled using actuator disc theory. Theoretical results are presented for a blocked configuration of tidal stream turbines such as a linear array that account for the proximity of the free surface and the seabed. The theoretical results are compared to open channel flow experimental results in which the flow field has been simulated using a porous disc and strip. These results are complemented by more detailed measurements of the performance of a model horizontal-axis turbine carried out in a water flume and a wind tunnel. The two sets of experiments represent highly blocked and effectively unblocked cases, respectively. The theoretical model of the effects of free-surface proximity provides a blockage correction for axial induction that can be incorporated in blade element momentum codes. The performance predictions obtained with such a code are in good agreement with the experimental results for C P and C T at low tip-speed ratios. The agreement weakens with increasing tip-speed ratio, as the wake of turbine enters a reversed flow state. A correction following the philosophy of Maskell is applied to C T in this region, which provides a better agreement.

Analysing the pattern of pulse waves in arterial networks: a time-domain study

Abstract: The mechanisms underlying the shape of pulse waves in the systemic arterial network are studied using the time-domain, one-dimensional (1-D) equations of blood flow in compliant vessels. The pulse waveform at an arbitrarylocationinthenetworkisinitiallyseparatedintoaperipheralcomponentthatdependsonthecardiacoutput, total compliance and total peripheral resistance of the network, and a conduit component governed by reflections at the junctions of the large conduit arteries and at the aortic valve. The dynamics of the conduit component are then analysed using a new algorithm that describes all the waves generated in the linear 1-D model network by a single wavefront starting at the root. This algorithm allows one to systematically follow all the waves arriving at the measuring site and identify all the reflection sites that these waves have visited. Application of this method to the pulse waves simulated using a 1-D model of the largest 55 systemic arteries in the human demonstrates that peripheral components make a larger contribution to aortic pressure waveforms than do the conduit components. Conduit components are closely related to the outflow from the left ventricle in early systole. Later in the cardiac cycle, they are the result of reflections at the arterial junctions and aortic valve. The number of reflected waves increases approximately as 3 m , with m being the number of reflection sites encountered. The pressure changes associated with these waves can be positive or negative but their absolute values tend to decrease exponentially. As a result, wave activity is minimal during late diastole, when the peripheral components of pressure and the flow are dominant,andaorticpressurestendtoaspace-independentvaluedeterminedbythecardiacoutput,totalcompliance and total peripheral resistance. The results also suggest that pulse-wave propagation is the mechanism by which the arterial system reaches the mean pressure dictated by the cardiac output and total resistance that is required to perfuse the microcirculation. The total compliance determines the rate at which this pressure is restored when the system has departed from its equilibrium state of steady oscillation. This study provides valuable information on

Reduced models of the cardiovascular system

Abstract: Due to the large number of vessels involved and the multitude of different length scales required to accurately represent the flow in the various regions of the cardiovascular system, simulations of the flow of blood in the system based on full 3D models (see Chapters 2 and 3) are beyond the capability of current computers and they will be for years to come. Moreover, the huge amount of data that would be generated by such simulations is costly to process and of difficult clinical interpretation.

Camber effects in the dynamic aeroelasticity of compliant airfoils

Abstract: This paper numerically investigates the effect of chordwise flexibility on the dynamic stability of compliant airfoils. A classical two-dimensional aeroelastic model is expanded with an additional degree of freedom to capture time-varying camber deformations, defined by a parabolic bending profile of the mean aerodynamic chord. Aerodynamic forces are obtained from unsteady thin airfoil theory and the corresponding compliant-airfoil inertia and stiffness from finite-element analysis. V–g and state-space stability methods have been implemented in order to compute flutter speeds. The study looks at physical realizations with an increasing number of degrees of freedom, starting with a camber-alone system. It is shown that single camber leads to flutter, which occurs at a constant reduced frequency and is due to the lock in between the shed wake and the camber motion. The different combinations of camber deformations with pitch and plunge motions are also studied, including parametric analyses of their aeroelastic stability characteristics. A number of situations are identified in which the flutter boundary of the compliant airfoil exhibits a significant dip with respect to the rigid airfoil models. These results can be used as a first estimation of the aeroelastic stability boundaries of membrane-wing micro air vehicles.

From image data to computational domains

Abstract: The advent of high-resolution imaging systems and powerful computational resources has made it possible to obtain information about the in vivo anatomy of blood vessels in a non invasive way. By employing this information as the domain definition for computational fluid dynamics, it is now possible to model hemodynamics in realistic geometric configurations on a subject-specific basis. Since geometry has a strong influence on hemodynamics, as will shown extensively in Chapter 5, the procedure used to model the geometry of a blood vessel from medical images plays a primary role in determining the reliability of haemodynamic predictions and, ultimately, their clinical significance.