Time-averaged Velocity Vectors Research Articles

Upstream velocity fields induced by frontal jet injection in a square cylinder

The flow characteristics around the frontal face of a square cylinder with a frontal jet injection were experimentally studied in a closed-loop wind tunnel using the PIV technique. Four characteristic flow modes were identified based on the injection ratio: swinging jet, deflected oscillating jet, deflection jet, and penetrating jet. At low injection ratio, the flow mode denoted as the swinging jet is characterised by two recirculation regions and a four-way saddle point. At moderate injection ratio, the flow mode termed deflected oscillating jet is characterised by a recirculation region and a counterclockwise rotating vortex within the recirculation region. At moderately high injection ratio, the flow mode is denoted as deflection jet, where a clockwise rotating vortex appears in the flow field. At high injection ratio, the flow mode is termed as penetrating jet, with no vortex formation in the flow field. The strength of the turbulence intensities increases considerably with high injection ratios. The time histories of instantaneous velocity for non-injection case, swinging jet, and deflected oscillating jet modes show periodic oscillations, whereas those of deflection jet and penetrating jet modes do not show periodic oscillations. The study presents and discusses the time-averaged velocity vectors, and streamlines, vorticity contours, velocity properties, velocity time histories, and power spectral density function under different flow modes.

Experimental and Numerical Investigation of the Control of the Flow Structure on Surface Modified Airfoils

In this study, experimental and numerical flow analysis was performed on three different blade profiles with a chord length of 165 mm using passive flow control method. The first of the airfoil is the standard NACA 0018 profile. The second airfoil type has a NACA 0018 profile with a gap in the suction surface. The last airfoil is the NACA 0018 profile which is 66% of the trailing edge cut from the chord length. All airfoil profiles were analyzed at the Reynolds number, Re=2x104, and angles of attack α=0o, 5o, 10o, 12o and 14o in both experiment and numerical studies. The experiments were carried out using the Particle Image Velocimetry (PIV) method in a closed-loop open water channel, and the time-averaged velocity vectors, streamlines, and vorticity contours of the flow field were obtained. Subsequently, numerical analyses were performed using the ANSYS Fluent package program, one of the Computational Fluid Dynamics (CFD) programs used frequently in the literature. The streamlines and pressure contours of the airfoil profiles have been compared visually at the same Re and different angles of attack. In addition, according to the angle of attack of the airfoil profiles, lift coefficient CL, drag coefficient CD, and the ratio of lift coefficient to drag coefficient CL/CD graphs were presented. It has been shown that the gap on the airfoil at high attack angles caused changes in lift (up to 0.7) and drag (up to 0.15). These features can allow these models to be used for different purposes in the aerodynamics field.

Numerical simulations of a subsonic round jet with transverse acoustic and mechanical forcing

The results of numerical experiments on the bifurcation effects for a free round jet caused by acoustic or vibrational excitations are presented. The three-dimensional distributions of the instantaneous and time-averaged velocity vector components have been obtained and analyzed. The effects of Strouhal and Reynolds numbers on the jet splitting angle, as well as those of the amplitude and type of jet perturbations is discussed in such a comparative parametric study.

Experimental flow analysis of vertical axis turbine for power generation in open channel system

In this experimental study, flow structure over a three bladed vertical axis Darrieus turbine is investigated experimentally for Reynold Number, Re=9000 in an open channel. Rigidity ratio (σ), angle of attack, optimum values of blade chord length (CL), turbine radius (R) and height of turbine blades (H) are used as approximately 1.1 and α=-2°, CL=5 cm, R=13.75 cm and H=20 cm, respectively. According to the values, ¼ scale model of a vertical axis Darrieus turbine has been used in order to perform experimental flow analysis. In experimental flow analysis, two different azimuth angles, θ=270° and θ=240° are used to examine the flow characteristics in the Particle İmage Velocimetry (PIV) experiments. As a result of the experiments, time averaged velocity vectors , averaged streamlines , and averaged vortex fields , are obtained and interpreted time averaged velocity vectors , averaged streamlines , and averaged vortex fields , changes with the turbine blade positions. As a result, it is seen that turbine azimuthal angle affetcs the flow structure and streamline focal points, positive and negative vortex are formed in the flow region behind the turbine. Only one focal point at θ=240° is occurred while two focal points and one stagnation point are occurred at the angle of θ=270°.

Time-averaged flow characteristics and mixing properties of double-concentric jets

We examined the flow behaviors and mixing characteristics of double-concentric jets using laser-assisted smoke flow visualization method to analyze typical flow patterns and binary boundary detection technique to investigate jet spread width. Time-averaged velocity vectors, streamline patterns, velocity distributions, turbulence properties, and vorticity contours were analyzed using Particle Image Velocimetry (PIV). Topological flow patterns were analyzed to interpret the vortical flow structures. Mixing properties were investigated using a tracer-gas concentration detection method. Four characteristic modes were observed: annular flow dominated mode, transition mode, central jet dominated mode-low shear, and central jet dominated mode-high shear. The jets’ mixing properties were enhanced by two major phenomena: the merging of annular flow and central jet at the centerline and the large turbulence fluctuations produced in the flow field. The merging of the jets induced stagnation points on the central axis in the annular flow dominated mode, which caused reverse flow on the central axis and drastic turbulence fluctuations of the near field region. When the central jet penetrated the recirculation region in the other three modes, the stagnation points on the central axis and the reverse flow vanished. Therefore, the mixing behaviors were prominently enhanced in the annular flow dominated mode.

Effect of annular flow pulsation on flow and mixing characteristics of double concentric jets at low central jet Reynolds number

This study investigated the effects of annulus pulsation on the flow characteristics and mixing properties of double-concentric jets at a low central jet Reynolds number. The central jet Reynolds number was fixed to Rec = 313. Annulus pulsation was induced by employing a solenoid valve. Streak pictures of instantaneous smoke flow patterns and long-exposure images of jet flows were observed using the laser-light-sheet-assisted smoke flow visualization method. A binary edge-detection technique was used to compute the jet spread widths. Particle image velocimetry (PIV) was utilized to examine the time-averaged velocity vectors, streamline patterns, velocity properties, turbulence intensity distributions, and vorticity contours. A radial dispersion improvement index was defined to quantitatively estimate the mixing property using the tracer-gas concentration detection technique. When the annular flow was pulsated with high amplitudes, it merged toward the central axis owing to an increase in radial momentum. Stagnation points were intermittently witnessed on the central axis. Large jet spread width exhibited, especially at low annulus Reynolds numbers. Turbulence intensities increased drastically in the flow field by the axial elongation of the recirculation region due to the annulus pulsations. The axial elongation of the recirculation region and the intermittent stagnation points led to effective dispersion of the central jet into the annular flow. The radial dispersion improvement index revealed that the mixing capability between the jets could be enhanced up to 80% in the recirculation region by applying annulus pulsations.

Droplet mobilization at the walls of a microfluidic channel

The mechanism of dynamic wetting and the fluid dynamics during the onset of droplet mobilization driven by a microchannel flow are not clearly understood. In this work, we use microparticle tracking velocimetry to visualize the velocity distribution inside the droplet both prior to and during mobilization. Time-averaged and instantaneous velocity vectors are determined using fluorescent microscopy for various capillary numbers. A circulating flow exists inside the droplet at a subcritical capillary number, in which case the droplet is pinned to the channel walls. When the capillary number exceeds a critical value, droplet mobilization occurs, and this process can be divided into two stages. In the first stage, the location of the internal circulation vortex center moves to the rear of the droplet and the droplet deforms, but the contact lines at the top walls remain fixed. In the second stage, the droplet rolls along the solid wall, with fixed contact angles keeping the vortex center in the rear part of the droplet. The critical capillary number for the droplet mobilization is larger for the droplet fluid with a larger viscosity. A force-balance model of the droplet, considering the effect of fluid properties, is formulated to explain the experimental trends of advancing and receding contact angles with the capillary number. Numerical simulations on internal circulations for the pinned droplet indicate that the reversed flow rate, when normalized by the inlet flow rate and the kinematic viscosity ratio of the wetting and nonwetting phases, is independent of the capillary number and the droplet composition.

Experimental Investigation of Flow Structures Around side-by-side Yawed Cylinders in Shallow Water

The aim of this experimental study is to investigate the flow behaviour around two equally yawed side-by-side cylinders in shallow water. Time averaged velocity vector fields, Reynolds shear stress distrubutions and streamline patterns were obtained using Particle Image Velocimetry (PIV) technique. The gap ratio between the cylinders were in the range of G/D=0.25-1.25 with an increment of 0.25 where G is the distance between the cylinders and D is the cylinder diameter. Five different yaw angles of cylinders were employed during the experiment. The results showed that the yaw angle, α had an important effect on the flow structures of the downstream of the cylinders. Reynolds shear stress and vortex structures were decreased, the intensity of the jet like flow were significantly attenuated for the gap ratios of G/D=0.25, 0.50 and 0.75.

Flow dynamics in the vicinity of a gravel embedded vertical retaining wall: conditions corresponding to the initial stages of local erosion

The present study examines the turbulent flow characteristics in the vicinity of the leading edge of a vertical retaining wall structure embedded in a gravel bed. Conditions before and during the initial stages of local scour development were considered. To that end, three laboratory experiments were carried out: one with fixed boundaries as a reference case, a second one with an erodible bed, but immobile channel bank, and a third one characterized by an erodible channel bed and bank. Instantaneous and time-averaged flow features, and turbulence statistics associated with the three-component velocity fields, obtained with a stereoscopic particle image velocimetry system, were analyzed. Water surface examination determined that channel bank erosion alters flow behavior upstream of the retaining wall. This change is manifested through more frequent formation of eddies near the top corner of the junction between the upstream face of the protrusion and the channel bank. Time-averaged and instantaneous velocity vector fields demonstrate the presence of two counter-rotating vortices in the experiments with the fixed channel bank, one located near the junction of retaining wall and the channel bed and one away from it generated as a result of a strong downflow. The latter motion represents a section of a horseshoe vortex. During the channel bank erosion, however, these vortices are not as significant and persistent. The results also indicate that the coherency and magnitude of the streamwise vorticity field diminish as a result of scour. However, it was confirmed that local turbulent kinetic energy and shear stresses increase in the presence of scour due to enhanced local flow separation.

Flow comparison around horizontal single and tandem cylinders at different immersion elevations

Flow characteristic around a horizontal cylinder and tandem horizontal cylinders were compared in shallow water of which height was kept constant as hw = 60 mm in this study. The other parameters, taken constant, were free stream velocity and diameter of circular cylinders of which values were U = 167 mm/s and D = 30 mm, respectively. To evaluate the time-averaged and instantaneous velocity vector area in the wake zone at Reynolds number; ReD = 5000 basis on the diameter of cylinder, Particle Image Velocimetry (PIV) was operated. The gap (L) between the tandem cylinders was enhanced from 0 until 90 mm through 15 mm enhancements and it was enhanced from 90 up to 150 mm through 30 mm enhancements in order to observe effect of the gap on flow characteristics. Moreover, another important parameter was the submersion level that ranged from 7.5 until 60 mm through 7.5 mm increments (hD/D = 0.25–2) during the experiments. Five hundred instantaneous images were used in order to obtain Reynolds stress correlation, corresponding streamline topology and the mean velocity vector field. The investigation represented that the alternative cylinder attenuates the dimension of the wake zone in accordance with L/D = 0 location. The wake zone obviously happens between tandem cylinders at the starting of L/D = 1 location. When the space between tandem cylinders deepens the dimension of the wake zone rises.

CFD–DEM simulation of a pseudo-two-dimensional spouted bed comprising coarse particles

In the present study, computational fluid dynamics (CFD) and the discrete element method (DEM) are used in conjunction with the Eulerian–Lagrangian method to simulate a pseudo-two-dimensional spouted bed comprising coarse 6-mm particles. The open-source OpenFOAM code is used to solve the governing equations. The predicted vertical particle velocity along the bed axis, particle velocity profiles in the radial direction, power spectral density, time-averaged particle velocity vectors, bed pressure drop, and solid flow pattern are evaluated and compared with existing experimental data. Good agreement is found between the CFD–DEM results and the measured data. It is also shown that the present CFD–DEM model accurately predicts the particle flow pattern throughout the bed. It is found that the drag force, solid stresses, and gravity play important roles in the CFD–DEM simulation of a spouted bed comprising coarse particles.

Flow measurement around a cylindrical body by attaching flexible plate: A PIV approach

In this study, flow structures and downstream wake behind a flexible splitter plate attached to a circular cylinder have been qualitatively and quantitatively investigated at three different Reynolds numbers of 2500, 5000 and 7500. The Particle Image Velocimetry (PIV) technique was implemented to examine the flow field downstream of the bluff body in deep water. Laser sheet illuminated the region 5 mm below the top of the cylinder. All results were compared with a bare cylinder case to analyze differences in flow characteristics. Time-averaged velocity vector field <V>, vorticity contours<ω>, turbulent kinetic energy, TKE, rms of streamwise, urms, transverse, vrms velocity components were acquired and also frequency, f of vortex shedding was carried out using single point spectral analysis from PIV data to indicate flow structures. It was observed that the flexible splitter plate had a considerable effect on vortex shedding and fluctuations were prominently weakened by the use of control material.

Experimental Investigation of Flow Structures around a Torpedo-like Geometry Placed in a Boundary Layer Flow

Defense applications for both under oceans and seas, particularly underwater vehicles have been considered in this research. With this aim, flow characteristics around a torpedo-like geometry under the effect of the boundary layer flow over a smooth flat plate have been experimentally examined by using PIV technique. All of the experiments have been done for Re=20000 and Re=40000 based on the length (L) of the geometry as a characteristic length. As a result, time-averaged streamwise velocity components , velocity vectors , streamline topologies and Reynolds stress correlations in the wake region of the torpedo-like geometry have been acquired in the range of 0 ≤ G/D ≤ 1.5. Here, G is the space between the bottom point of the geometry and flat plate surface; D stands for the diameter of the geometry. It is found that at the smallest value of G/D=0.25, jet-like flow occurs between the plate and the model which causes a powerful scouring. As the gap ratio is increased to G/D=0.5 and G/D=1.0, the jet-like flow diminishes slightly and then the flow structure in the wake region becomes similar to the uniform incoming flow condition for G/D=1.50. Due to the effect of the jet-like flow and boundary layer flow, time-averaged flow patterns present asymmetrical distributions which are clearly shown a bigger size focus close to the plate in streamline topology. Reynolds stress patterns form more powerful viscous forces in the boundary layer flow due to the occurrence of eddy vortices and viscosity effect. It is observed from the aforementioned flow patterns that interaction between the flow structure, the model and boundary layer flow yields very complex structure. In order to decrease the energetic flow in this condition, passive or active flow control method can be integrated on the torpedo-like geometry.

Numerical simulation on the powder propellant pickup characteristics of feeding system at high pressure

A numerical study of powder propellant pickup progress at high pressure was presented in this paper by using two-fluid model with kinetic theory of granular flow in the computational fluid dynamics software package ANSYS/Fluent. Simulations were conducted to evaluate the effects of initial pressure, initial powder packing rate and mean particle diameter on the flow characteristics in terms of velocity vector distribution, granular temperature, pressure drop, particle velocity and volume. The numerical results of pressure drop were also compared with experiments to verify the TFM model. The simulated results show that the pressure drop value increases as the initial pressure increases, and the granular temperature under the conditions of different initial pressures and packing rates is almost the same in the area of throttling orifice plate. While there is an appropriate value for particle size and packing rate to form a “core-annulus” structure in powder box, and the time-averaged velocity vector distribution of solid phase is inordinate.

Hydrodynamics studies of a pseudo 2D rectangular spouted bed by CFD

A Eulerian–Eulerian two-fluid model in conjunction with the kinetic theory of granular flows (KTGF) was used for simulation of a pseudo 2D rectangular spouted bed. The thin thickness of the rectangular spouted bed was included in the analysis. To save the computational time, symmetry framework was used in all simulations. Comparisons were made for particle velocity distributions along the bed axis and in the radial direction, as well as, the time-averaged particle velocity vectors for 2D and 3D modeling. Different values of the specularity and particle–wall restitution coefficients, which are important parameters of the solid wall boundary condition, were used in these simulations. The simulation results showed that the 3D computational fluid dynamic (CFD) model is more sensitive to the specularity coefficient and the particle–wall restitution coefficient compared to the 2D model. In addition, the 3D model was more significantly affected by the variation of the specularity coefficient when compared with changes in the particle–wall restitution coefficient. It was also found that the 3D model gives more accurate results in comparison with the 2D Cartesian frame for spouted bed with low thickness.

Characterizing Liquid Redistribution in a Biphasic Vibrating Vocal Fold Using Finite Element Analysis

Vocal fold tissue is biphasic and consists of a solid extracellular matrix skeleton swelled with interstitial fluid. Interactions between the liquid and solid impact the material properties and stress response of the tissue. The objective of this study was to model the movement of liquid during vocal fold vibration and to estimate the volume of liquid accumulation and stress experienced by the tissue near the anterior-posterior midline, where benign lesions are observed to form. A three-dimensional biphasic finite element model of a single vocal fold was built to solve for the liquid velocity, pore pressure, and von Mises stress during and just after vibration using the commercial finite element software COMSOL Multiphysics (Version 4.3a, 2013, Structural Mechanics and Subsurface Flow Modules). Vibration was induced by applying direct load pressures to the subglottal and intraglottal surfaces. Pressure ranges, frequency, and material parameters were chosen based on those reported in the literature. Postprocessing included liquid velocity, pore pressure, and von Mises stress calculations as well as the frequency-stress and amplitude-stress relationships. Resulting time-averaged velocity vectors during vibration indicated liquid movement toward the midline of the fold, as well as upward movement in the inferior-superior direction. Pore pressure and von Misses stresses were higher in this region just after vibration. A linear relationship was found between the amplitude and pore pressure, whereas a nonlinear relationship was found between the frequency and pore pressure. Although this study had certain computational simplifications, it is the first biphasic finite element model to use a realistic geometry and demonstrate the ability to characterize liquid movement due to vibration. Results indicate that there is a significant amount of liquid that accumulates at the midline; however, the role of this accumulation still requires investigation. Further investigation of these mechanical factors may lend insight into the mechanism of benign lesion formation.

Investigation of flow structure around a horizontal cylinder at different elevations in shallow water

Flow characteristics around a horizontal circular cylinder were investigated in shallow water. The diameter of circular cylinder, the height of shallow water and free stream velocity were kept constant during the experiments as D=30mm, hw=60mm and U=167mm/s, respectively. Particle Image Velocimetry (PIV) was used to measure instantaneous and time-averaged velocity vector fields in the near wake region. In order to investigate the effect of immersion, the cylinder was placed at eight different elevations (hD) between bottom and free surface (from 7.5 to 60mm with 7.5mm increments). Instantaneous and time-averaged velocity vector field, corresponding streamline patterns and Reynolds stress correlation were used to analyze the behavior of the flow downstream of horizontal cylinder. The mean velocity vector field, corresponding streamline topology and Reynolds stress correlation were obtained using 500 instantaneous images. As the immersion level ratio (hD/D) increases, the magnitude of jet-like flow velocity goes up ranged from hD/D=0.25–1. Time averaged flow characteristics show that there is a difference between primary and developing circulation bubble depending on direction of rotation. This occurrence causes the entrainment and this stimulates the momentum transfer between the core and wake flow region for hD/D=1 and 2 cases.

Investigation of Effect of Boundary Layer on Flow Structure Around a Cylinder with a Strip

In this study, the flow characteristic of the circular cylinder was placed vertically in channel which has dimensions as 8000 mm, 1000 mm, 750 mm, lenght, width and height repectively, was investigated. The cylinder was located in boundary layer with a diameter of 60 mm (D) and a elastic stripwhich has a 1400 N/mm 2 modulus of elasticity vinyl PVC transperent film was attached behind the cylinder. Lenght of the strip (L) was 240 mm L/D=4. The Reynolds number was fixed at Re=7500. The time-averaged and instantaneous velocity vector maps, vorticity contours, Reynold shear and normal stresses, turbulent kinetic energy and frequency of shedding were obtained using the particle image velocimetry (PIV) technique. It was found that the elastic plate which exists behind the cylinder has a slight influence on the flow structure of the wake-boundary layer interaction. Values of turbulent kinetic energy, streamwise Reynold stress, transverse Reynold stresses were decreased by attaching strip.

Comparison of Flow Structures in the Downstream Region of a Cylinder with Flexible Strip

The present study investigates the details of flow structure to downstream of a circular cylinder mounted on a flat surface, in successive plan-view plane both in the boundary layer and up level region. The behavior of the flow in the wake of the bare cylinder and attached a flexible strip which has a 1400 N/mm 2 modulus of elasticity vinyl PVC transperent film. The length of strip 240 mm (L/D=4) is investigated using Particle Image Velocimetry (PIV) technique for Reynolds numbers based on the cylinder diameter of 2500. The flow data downstream of the cylinder are presented using time-averaged velocity vector map, Vavg, streamline patterns, � avg, vorticity contours, � avg, and Reynolds stress correlations, u ' u ' avg, v ' v ' avg, u ' v ' avg and rms velocity values. The locations of the peak values of Reynolds stress correlations and other data are also presented in both bare cylinder and attached body in order to determine the regions under high fluctuations. Another L/D ratios will be investigated in other experiments.

Vorticity and Circulation of Horseshoe Vortex in Equilibrium Scour Holes at Different Piers

The present work deals with an experimental investigation on clear water local scour and horseshoe vortex flow field of cohesionless bed sediment around an equilateral triangular pier (side facing the approaching flow). The turbulence field within the equilibrium scour hole is experimentally measured in a laboratory tilting flume using an Acoustic Doppler Velocimeter. The experiment is conducted for the approaching flow having undisturbed flow depth (=0.125 m) greater than the pier width and the depth-averaged approaching flow velocity (=0.247 m/s) about 68 % of the critical velocity of the uniform bed sand of median diameter of 0.825 m. The triangular pier width is 0.11 m. The outcomes of triangular pier have been compared with the square pier (side facing the approaching flow) and circular pier obtained from previous study. The distribution of time-averaged velocity vectors, contours of the vorticities and turbulent kinetic energies at different azimuthal planes (0° i.e. at the upstream axis of symmetry, 45° and 90°) are presented. Vector plots of the flow field at azimuthal planes reveal the characteristics of the horseshoe vortex flow. The circulation of the horseshoe vortex is determined by using Stokes theorem and forward difference technique. An attempt has also been made to correlate the horseshoe vortex strength with the effective obstruction width of pier for equilibrium scour hole at different azimuthal angles. The presented data would be useful for the development and validation of turbulence field models that can be used to find out the strength of the horseshoe vortex.