Xikun Wang

Numerical simulation of patient-specific left ventricular model with both mitral and aortic valves by FSI approach

Intraventricular flow is important in understanding left ventricular function; however, relevant numerical simulations are limited, especially when heart valve function is taken into account. In this study, intraventricular flow in a patient-specific left ventricle has been modelled in two-dimension (2D) with both mitral and aortic valves integrated. The arbitrary Lagrangian-Eulerian (ALE) approach was employed to handle the large mesh deformation induced by the beating ventricular wall and moving leaflets. Ventricular wall deformation was predefined based on MRI data, while leaflet dynamics were predicted numerically by fluid-structure interaction (FSI). Comparisons of simulation results with in vitro and in vivo measurements reported in the literature demonstrated that numerical method in combination with MRI was able to predict qualitatively the patient-specific intraventricular flow. To the best of our knowledge, we are the first to simulate patient-specific ventricular flow taking into account both mitral and aortic valves.

Flow visualization and measurements of a square jet with mixing tabs

Abstract A square jet with mixing tabs was investigated using laser induced fluorescence and laser Doppler anemometer (LDA). The triangular tabs were tilted either downstream (i.e., at an inclination, θ=45°) or upstream (i.e., at θ=135°). With four tabs, the jet flow was bifurcated into a four-finger structure and there was a “mushroom” structure behind each tab for θ=135°. Secondary velocity vector measurements using LDA in the y–z plane for the θ=45° and 135° with tabs at x=0.25DH and 0.075DH respectively showed a vortex pair formed behind each tab. These streamwise vortices spread the jet fluids “outward” along the diagonal direction of the jet cross-section, resulting in the four-finger structure. At the two corners of each finger, a pair of streamwise vortices, which were the legs of weaker horseshoe vortices, were also found. For θ=135°, a third pair of streamwise vortices proposed by Reeder and Samimy [J. Fluid Mech. 311 (1996) 73] was observed at the tip of each tab, which explained the existence of the unique mushroom structure. LDA measurements of mean velocity contours on the two tabbed jets confirmed the flow visualization results that the jet core had bifurcated into a four-finger structure. Furthermore, with tabs at x=0.75DH and 1.25DH, the downstream tilted tab jet produced a larger cross-sectional area of the jet core than the jet with θ=135° and the jet without tab. Downstream tilted tabs were thus more efficient in mixing enhancement than upstream tilted tabs.

Application of potential theory to steady flow past two cylinders in tandem arrangement

The wake flow patterns associated with flow past a cylinder and a cylinder-pair in tandem configuration are revisited, compared, and evaluated with respect to the streamline patterns generated based on potential flow theory and superposition of various potential flow elements. The wakes, which are vortex shedding in the lee of the cylinder(s), are reproduced by placing pairs of equal but opposite circulation elements in the potential flow field. The strength of the circulation elements determines the size of the vortices produced. The streamline patterns of flow past a pair of unequal cylinders in tandem configuration provide an indirect means to establish the threshold condition for the wake transition from that of a single bluff body to alternating reattachment behavior. This threshold condition is found to be a function of the diameter ratio, (diameters and , ), spacing ratio, (centre-to-centre distance, , to cylinder diameter, ), and equivalent incident flow speed, A unique functional relationship (, , ) of this threshold condition is established.

Measurements of Fluctuation in Drag Acting on Rigid Cylinder Array in Open Channel Flow

AbstractIn this study, an array of rigid cylindrical rods was used to simulate emergent vegetation stems that were subject to unidirectional open channel flows. The instantaneous drag force experienced by the rods was measured with a load cell. In addition, the particle image velocimetry (PIV) technique was applied to sample the flow information in a horizontal plane, and wave gauges were used to record the fluctuation in the water-surface elevation. The results show that the drag fluctuation normalized by the mean value may reach as high as 133% when the Reynolds number (defined based on the stem diameter) varied in the range from 400 to 1,100. High fluctuations were also observed in the flow velocity and flow depth under similar flow conditions.

Bi-stable flow around tandem cylinders of different diameters at low Reynolds number

The flow around two circular cylinders in tandem arrangement with different diameters has been investigated numerically. The upstream to downstream cylinder diameter ratio was kept constant at a value of d/D=2/3 and the centre-to-centre distance was varied from 1.2D to 5D. The Reynolds number based on upstream cylinder diameter and free-stream velocity used in this study is equal to 200. Force coefficients and flow frequencies have been obtained from 2D fluid flow finite-element simulations. Single-wake shedding, reattachment and co-shedding regimes have been observed. The results show that, depending on initial perturbations, reattachment and co-shedding can be observed on a wide range of centre-to-centre distances (L/D[1.8, 3.8]), which defines the bi-stable flow range. The lower part of the bi-stable range is characterized by a lock-in phenomenon for which vortex-shedding from the upstream and downstream cylinders are synchronized when there are co-shedding flow conditions.

Experimental Study on the Flow Around Two Tandem Cylinders with Unequal Diameters

In this paper, flow around two circular cylinders in tandem arrangement with unequal diameters has been investigated using the particle image velocimetry technique (PIV) in a water channel. The upstream to downstream diameter ratio was kept constant at d/D = 2/3, the centre-to-centre distance was varied from 1.2D to 5D and the Reynolds number was varied from 1200 to 4800. The flow characteristics were analyzed through ensemble-averaged patterns of velocity, vorticity, normalized Reynolds stress contours and streamlines. Based on ensemble-averaged and instantaneous flow fields, different flow patterns, including single-wake-shedding at small spacing ratio, bi-stable flow behavior (alternating behavior of reattachment and vortex shedding) at intermediate spacing ratio and co-shedding pattern at large spacing ratio were observed. The effects of Reynolds number and the centre-to-centre spacing ratio on flow patterns and turbulent characteristics were also investigated. It was found that the diameter ratio appears to have a certain effect on the flow patterns at intermediate spacing ratios, where the reattachment of shear layer depends on the lateral width of the wake flow in the lee of the upstream cylinder. Extensive discussion on the distributions of Reynolds stress and turbulent kinetic energy was presented.

Experimental Studies of Vortex Structures in the Wake of a Cylinder With Helical Strakes

This work is experimental studies of the turbulent vortex structures in the wake of a fixed cylinder with helical strakes. The effect and mechanism of a triple helical strake system on suppression of vortex induced vibrations (VIV) are investigated experimentally in a water tunnel. Strakes with a length of 25D in pitch and 0.1D & 0.2D in height are fitted onto a rigid cylinder of diameter D = 12mm, and subjected to a transverse water flow. PIV techniques are used to measure the vector field to identify the vortex structures. The hot-film techniques are also employed to identify the shedding frequency. Measurements for the bare cylinders are also conducted for comparison. The mechanism of the VIV reduction by helical strakes is discussed in detail.Copyright

Hydrodynamics of trapezoidal embankment weirs

This paper presents the results of an experimental study over a 2-dimensional trapezoidal weir model with the upstream and downstream faces sloped at 1:2 (V:H). The flow field has been measured using Particle Image Velocimetry (PIV). Several hydrodynamic properties of the flow, including the different flow regimes, free surface profile, mean and instantaneous velocity fields, and discharge coefficient, are discussed and analyzed.

Environmental fluid dynamics-jet flow

Jet flow is a very important research subject in both fundamental fluid dynamics and engineering applications. Jet flow has the essences of fluid dynamics, such as free and wall-bounded shear flows, turbulent flow, eddy and large vortical structures and their stability and control, and so forth. This article serves as an overview of our past and ongoing research activities on various types of jet flow, with particular reference to their application in the field of environmental fluid dynamics. The research objectives, approach, results and their engineering implications of each topic will be presented.

Study of flow formed by three coplanar impinging pipe jets at inclination angles of 30° and 45°

This paper presents an experimental study of the interactions of three fully-submerged, coplanar impinging jets issued from long pipes. The jets were neutrally buoyant and were arranged symmetrically about the axis of a central jet, with two side jets set to intersect with the central jet at two inclination angles (30° and 45°) and three Reynolds numbers (4240, 6400 and 8000). Measurements of the flow fields were performed using particle image velocimetry to examine the flow structures in various planes, i.e., the jet axis plane (X–Y), the jet normal plane (X–Z) and the cross-sectional plane (Y–Z). This flow configuration results in pronounced interactions among the three jets, and hence better mixing than that of a canonical single pipe jet as illustrated by augmented centreline velocity decay, spreading rate and turbulence level. The jets at the inclination angle of 45° impinge and mix more rapidly than those at 30°. For each case, the flow is highly 3-dimensional, and jet development displayed several distinct regions (converging, merging and combining) along the streamwise direction. The expansion of flow in the X–Y plane is similar to the shape of a hyperbola with necking formed immediately downstream of the impinging point, whereas that in the X–Z plane assumes the shape of a parabola with an open rim exhibiting a pronounced velocity deficit in the central part of the combining flow. Self-similarity of streamwise mean velocity is explored in the combining region of the flow on the two planes of symmetry (X–Y and X–Z). Flow development in the combining region is dominated by large-scale vortical structures, including von Kárman-like spanwise vortices in the X–Y plane and secondary circulation in the form of two pairs of counter-rotating streamwise vortices in the Y–Z plane.