Antonio Cenedese

Effect of the prosthetic mitral valve on vortex dynamics and turbulence of the left ventricular flow

Mechanical heart valves implanted in mitral position have a great effect on the ventricular flow. Changes include alteration of the dynamics of the vortical structures generated during the diastole and the onset of turbulence, possibly affecting the efficiency of the heart pump or causing blood cell damage. Modifications to the hemodynamics in the left ventricle, when the inflow through the mitral orifice is altered, were investigated in vitro using a silicone rubber, flexible ventricle model. Velocity fields were measured in space and time by means of an image analysis technique: feature tracking. Three series of experiments were performed: one with a top hat inflow velocity profile (schematically resembling physiological conditions), and two with mechanical prosthetic valves of different design, mounted in mitral position—one monoleaflet and the other bileaflet. In each series of runs, two different cardiac outputs have been examined by changing the stroke volume. The flow was investigated in terms of phase averaged velocity field and second order moments of turbulent fluctuations. Results show that the modifications in the transmitral flow change deeply the interaction between the coherent structures generated during the first phase of the diastole and the incoming jet during the second diastolic phase. Top hat inflow gives the coherent structures which are optimal, among the compared cases, for the systolic function. The flow generated by the bileaflet valve preserves most of the beneficial features of the top hat inflow, whereas the monoleaflet valve generates a strong jet which discourages the permanence of large coherent structures at the end of the diastole. Moreover, the average shear rate magnitudes induced by the smoother flow pattern of the case of top hat inflow are nearly halved in comparison with the values measured with the mechanical valves. Finally, analysis of the turbulence statistics shows that the monoleaflet valves yield higher turbulence intensity in comparison with the bileaflet and, with top hat inflow, there is not a complete transition to turbulence.

Quasi-two-dimensional decaying turbulence subject to the β effect

Freely decaying quasi-2D turbulence under the influence of a meridional variation of the Coriolis parameter f (β effect) is experimentally and numerically modelled. The experimental flow is generated in a rotating electromagnetic cell where the variation of f is approximated by a nearly equivalent topographical effect. In the presence of a high β effect, the initial disordered vorticity field evolves to form a weak polar anticyclonic circulation surrounded by a cyclonic zonal jet demonstrating the preferential transfer of energy towards zonal motions. In agreement with theoretical predictions, the energy spectrum becomes peaked near the Rhines wave number with a steep fall-off beyond, indicating the presence of a soft barrier to the energy transfer towards larger scales. DNS substantially confirmed the experimental observations.

Comparison among feature tracking and more consolidated velocimetry image analysis techniques in a fully developed turbulent channel flow

The presence of a large number of software codes for image analysis suggests the need for testing the suitability and accuracy of the algorithms developed. One of the possible approaches is testing these systems with experiments of well-known flow properties. Alternatively, tests can be performed by analysing synthetically generated images. The advantage of the latter approach is that there is no need to set up an experiment and the flow field is known in detail. This paper provides some insights into the relationship between results on both real and synthetic images in a turbulent channel flow. We focus on comparing performances of feature tracking, a novel image analysis technique, particle image velocimetry and particle tracking velocimetry. The three techniques have been used to explore first- and second-order statistics. The results are compared to direct numerical simulations of turbulent flow in a channel (Kim J, Moin P and Moser R 1987 Turbulence in channel flow at low Reynolds number J. Fluid Mech. 177 133–66). Feature tracking performances are rather good, even in its purely translational motion model implementation. No constraints on tracer density have to be introduced. More than 3000 velocity vectors per frame were reconstructed. Resulting accuracy and resolution are always comparable to those achieved by the other techniques.

High-Resolution Numerical Modeling of Thermally Driven Slope Winds in a Valley with Strong Capping

Abstract The complete day–night cycle of the circulation over a slope under simplified idealized boundary conditions is investigated by means of large-eddy simulations (LES). The thermal forcing is given with a time-varying law for the surface temperature. A surface layer parameterization based on the Monin–Obukhov similarity theory is used as a wall layer model. The domain geometry is symmetric, having an infinitely long straight valley in the y direction. Since the depth of the katabatic flow in midlatitude climates is limited to 5–30 m, the authors introduced a vertically stretched grid to obtain a finer mesh near the ground. The length scale for the calculation of eddy viscosities is modified to take into account the grid anisotropy. A preintegration of 24 h is made to obtain a capping inversion over the valley. Results show that the model is able to reproduce microscale circulation dynamics driven by thermal forcing over sloping terrain. The diurnal growth of the convective boundary layer leading to ...

Experimental testing of Taylor's hypothesis by L.D.A. in highly turbulent flow

A new configuration for the transmitting optics of a laser Doppler anemometer has been developed in order to measure the velocity at two different points at the same time. From the simultaneous measurements at two points along the mean flow direction it is possible to evaluate the spatial correlations and to compare them with the temporal correlation to verify the validity limits of Taylors hypothesis also known as the frozen turbulence hypothesis. The transfer function between the velocity signals at two different points has been introduced to better explain the differences between Taylors hypothesis and non frozen flow. The analysis is carried out in a flow with high turbulence levels.

Recognition of partially overlapped particle images using the Kohonen neural network

A neural network is proposed for the recognition of partially overlapped particle images in the analysis of Particle Tracking Velocimetry (PTV) frames. The Kohonen neural network is an approximation to an optimum classifier. In this work it allows single particle images to be distinguished from overlapped particle images by shape analysis: it classifies 99.1% of the spots correctly (in test images). If a spot has an almost circular shape, the barycenter co-ordinates are extracted. If the spot shape is far from being circular, it is believed to be a particle overlap, and a procedure to find more centroids is activated.The particle recognizer based on the Kohonen neural network is tested on both multi-exposed and single-exposure images at high particle density, and compared to a particle recognizer that did not consider the partial overlap. The management of overlapped particles causes the neural network to produce a big improvement in the number of barycenters that can be extracted from these images. The practical consequence is that the seeding density in PTV can be increased, so as to improve the spatial resolution of the technique in the velocity field calculation.

Lagrangian statistics and transilient matrix measurements by PTV in a convective boundary layer

The most common velocity measurement techniques, based on image analysis, calculate the velocity by cross-correlation of a portion of the digitized images, and give a Eulerian description of the investigated field. Particle tracking velocimetry (PTV), based on the recognition of trajectories of seeding particles, only furnishes a Eulerian description provided the trajectories are shorter than the characteristic scale of the phenomenon. If particles are tracked for a longer time, a Lagrangian description is obtained. Consequently, in order to successfully evaluate Lagrangian statistics, a long series of single-exposed images has to be acquired. PTV has been used to examine the pollutant dispersion in a laboratory simulation of the convective boundary layer of the atmosphere. The convective layer has been simulated by a water tank heated from below, where the atmospheric thermal stratification has been reproduced. Though the phenomenon was observed to be steady in the Eulerian reference frame, the same did not occur in the Lagrangian reference frame. From the analysis of particle motion, it is possible to determine the characteristic time scale of the turbulence and to describe the different behaviour of hot updraughts and cold downdraughts. The pollutant dispersion is described in detail by the transilient matrix representing the probability of transition of a particle from one level to another of the convective layer. From the information given by this matrix, it is possible, in principle, to estimate the concentration fields, due to a variety of concentrated and distributed pollutant sources.

Spatial?temporal improvements of a two-frame particle-tracking algorithm

A novel algorithm for particle-tracking velocimetry is proposed and tested with both synthetic and real images. It uses nearest-neighbour cluster matching which performs better than fixed area approaches in terms of spatial adaptivity. The algorithm includes several temporal multi-frame improvements, i.e. extrapolation of the expected particle positions in subsequent frames and the frame-gap technique. To further improve the tracking algorithm performances, the particle identification procedure was modified with respect to the traditional background subtraction, local thresholding and grey level weighted averaging by using the optical flow equation. The local maximum of grey levels around each feature extracted is identified and the barycentres of the particle associated with it are calculated by using Gaussian fitting. The novel algorithm works well with several seeding densities, both homogeneously and inhomogeneously distributed. The multi-frame approach substantially improves the average trajectory length and the number of long trajectories in images with and without noise. The number of barycentres correctly identified by employing the feature extraction is significantly larger than when traditional techniques are used, which in turn increases the number of velocity vectors, allowing a better characterization of the flow field under investigation.

Using optical flow equation for particle detection and velocity prediction in particle tracking

Abstract A new algorithm of particle identification suitable for particle tracking technique in fluid mechanics is proposed and tested with synthetic images specifically developed with different particle parameters. The new approach is based on the solution of the optical flow equation via a sum-of-squared-difference method. Particles are detected through the identification of corner features, where image intensity gradients are not null in two orthogonal directions. It is thus possible to identify low intensity and overlapped particles. Furthermore, the feature selection criterion is optimal by construction because it is based on the optical flow solution and therefore a good feature is the one that can be tracked well. This leads to the second advantage of the method, which is the possibility to obtain the local velocity, given by the approximate solution of the optical flow equation, that can be used as a predictor for the subsequent particle pairing step. The proposed algorithm is tested using synthetically generated and experimental images and demonstrates its ability to detect a great number of particles with high reliability in different cases analysed.

Experimental study of wind flow over the model of a valley

Abstract Fundamental studies of ground-level wind flows for the case of a hill-valley configuration were performed in a boundary layer wind tunnel. The investigations, carried out for four models, were focused on the mean flow and the analysis of the turbulent fluctuations providing the information necesary for the determination of diffusion characteristics. The results obtained for three simplified models of a valley, allowed features of the local flows occurring in a real terrain model of much more complicated geometry, to be explained.