Prediction of Turbulent Shear Stresses through Dysfunctional Bileaflet Mechanical Heart Valves using Computational Fluid Dynamics

There are more than 300,000 heart valves implanted annually worldwide with about 50% of them being mechanical valves. The heart valve replacement is often a common treatment for severe valvular disease. However, valves may dysfunction leading to adverse hemodynamic conditions. The current computational study investigated the flow around a bileaflet mechanical heart valve at different leaflet dysfunction levels of 0%, 50%, and 100%, and documented the relevant flow characteristics such as vortical structures and turbulent shear stresses. Studying the flow characteristics through these valves during their normal operation and dysfunction can lead to better understanding of their performance, possibly improved designs, and help identify conditions that may increase the potential risk of blood cell damage. Results suggested that maximum flow velocities increased with dysfunction from 2.05 to 4.49 ms-1 which were accompanied by growing eddies and velocity fluctuations. These fluctuations led to higher turbulent shear stresses from 90 to 800 N.m-2 as dysfunctionality increased. These stress values exceeded the thresholds corresponding to elevated risk of hemolysis and platelet activation. The regions of elevated stresses were concentrated around and downstream of the functional leaflet where high jet velocity and stronger helical structures existed

Break-Up of Aerosol Agglomerates in Highly Turbulent Gas Flow

Agglomerate aerosols in a turbulent flow may be subjected to very high turbulent shear rates which through the generation of lift and drag can overcome the adhesive forces binding the constituents of an agglomerate together and cause it to break-up. This paper presents an analysis of the experimental measurements of the breakup of agglomerates between 0.1-10μm in size, in a turbulent pipe flow followed by an expansion zone with a Reynolds numbers in the range 105 to 107. The analysis shows that even in wall bounded turbulence, the high turbulent shear stresses associated with the small scales of turbulence in the core can be the main source of breakup preceding any break-up that may occur by impaction at the wall. More importantly from these results, a computationally fast and efficient solution is obtained for the General Dynamic Equation (GDE) for agglomerate transport and breakup in highly turbulent flow. Furthermore the solution for the evolution of the aerosol size distribution is consistent with the experimental results. In the turbulent pipe flow section, the agglomerates are exposed continuously to turbulent shear stresses and experience more longer term breakup than in the expansion zone (following the pipe flow) where the exposure time is much less and break-up occurs instantaneously under the action of very high local turbulent shear stresses. The validity of certain approximations made in the model is considered. In particular, the inertia of the agglomerates characterised by a Stokes Number from 0.001 for the smallest particles up to 10 for 10μm particles and the fluctuations of the turbulent shear stresses are important physical phenomena which are not accounted for in the mode

Large Eddy simulations of the ceiling jet induced by the impingement of a turbulent air plume

In this paper, a sensitivity study is performed with FireFOAM 2.2.x for a hot air jet plume impinging onto a flat horizontal ceiling. The plume evolution and the induced ceiling flow are considered. The influence of the level of turbulence imposed at the inlet, in terms of intensity and eddy length scale, is discussed. Also, the effect of the turbulence model constant is examined. For the case considered, the best results are obtained when no sub-grid scale (SGS) model is used. If a SGS model is used, the level of turbulence at the inlet and the choice of the turbulence model constant are shown to have a significant effect on the prediction of plume's spreading and the ceiling flow velocity. The eddy length scale at the inflow does not have significant impact on the results. Comparisons with the available experimental data indicate that FireFOAM is capable of predicting the mean velocity-field well. In the near field region, an under-estimation of the turbulent velocity fluctuations is observed, whereas reasonably good agreement is obtained in the far field

Turbulent shear stresses in hydraulic jumps and decelerating surges

In an open channel, a sudden rise in water level induces a positive surge. Once fully-developed, the surge becomes a hydraulic jump in translation. Herein unsteady turbulent shear stresses were measured during the translation of a fully-developed positive surge. New investigations were conducted in a large rectangular channel (12 m long, 0.5 m wide) and measurements were performed using acoustic Doppler velocimetry with a high temporal and spatial resolution (200 Hz sampling rate). Horizontal and adverse bed slope configurations were tested. In the latter, the surge decelerated until it became a stationary hydraulic jump

Turbulent shear stresses in hydraulic jumps, bores and decelerating surges

In an open channel, a sudden rise in water level induces a positive surge, or bore, that may develop as a hydraulic jump in translation. When the surge propagates against an adverse slope, it decelerates until it becomes a stationary hydraulic jump. Both hydraulic jumps and decelerating surges induce some intense turbulent mixing and have some major impact on the sediment transport in natural systems. Herein, a physical investigation was conducted in a relatively large rectangular channel. Hydraulic jumps and surges were generated by the rapid closure of a gate at the channel downstream end. The turbulent shear stresses were measured with high temporal and spatial resolution (200 Hz sampling rate) in the jump flow. A comparison between the stationary hydraulic jump, hydraulic jump in translation and decelerating surge measurements showed some marked differences in terms of turbulent mixing. The results highlighted some intense mixing beneath the jump front and roller for all configurations. The levels of turbulent stresses were one to two orders of magnitude larger than a critical threshold for sediment motion. The findings provide some insights into the hydraulic jump migration processes in mobile bed channels, and the complex transformation from a moving jump into a stationary jump. Copyright (C) 2010 John Wiley & Sons, Ltd

Turbulence measurements in a swirling confined jet flowfield using a triple hot-wire probe

An axisymmetric swirling confined jet flowfield, similar to that encountered in gas turbine combustors was investigated using a triple hot-wire probe. The raw data from the three sensors were digitized using ADC's and stored on a Tektronix 4051 computer. The data were further reduced on the computer to obtain time-series for the three instantaneous velocity components in the flowfield. The time-mean velocities and the turbulence quantities were deduced. Qualification experiments were performed and where possible results compared with independent measurements. The major qualification experiments involved measurements performed in a non-swirling flow compared with conventional X-wire measurements. In the swirling flowfield, advantages of the triple wire technique over the previously used multi-position single hot-wire method are noted. The measurements obtained provide a data base with which the predictions of turbulence models in a recirculating swirling flowfield can be evaluated

Modeling Interface Motion Of Combustion (MINOC). A computer code for two-dimensional, unsteady turbulent combustion

A computer code for calculating the flow field and flame propagation in a turbulent combustion tunnel is described. The model used in the analysis is the random vortex model, which allows the turbulent field to evolve as a fundamental solution to the Navier-Stokes equations without averaging or closure modeling. The program was used to study the flow field in a model combustor, formed by a rearward-facing step in a channel, in terms of the vorticity field, the turbulent shear stresses, the flame contours, and the concentration field. Results for the vorticity field reveal the formation of large-scale eddy structures in the turbulent flow downstream from the step. The concentration field contours indicate that most burning occurred around the outer edges of the large eddies of the shear layer

Duct flow nonuniformities: Effect of struts in SSME HGM 2+

This study consists of an analysis of flow through the Space Shuttle Main Engine (SSME) Hot Gas Manifold (HGM) for the purpose of understanding and quantifying the flow environment and, in particular, the flow through a region of structural supports located between the inner and outer walls of the HGM. The primary task of the study, as defined by NASA-MSFC, is to assess and develop the computational capability for analyzing detailed three-dimensional flow through the HGM support strut region to be incorporated into a full fuelside HGM analysis. Secondarily, computed results are to be compared with available experimental results