Two-dimensional Velocity Measurements Research Articles

Detection of Subclinical Cardiac Involvement in Systemic Sclerosis by Echocardiographic Strain Imaging

Cardiac involvement is one of the major problems in systemic sclerosis (SSc). Subclinical cardiac involvement has a higher frequency than thought previously. In this study we investigated whether subclinical cardiac involvement can be detected by using echocardiographic strain imaging in SSc patients without pulmonary hypertension. Echocardiographic examinations were performed to 27 SSc patients and 26 healthy controls. Left ventricular strain parameters were obtained from apical views and average strain value was calculated from these measurements. There were no significant differences between patients and controls regarding two-dimensional (2D), conventional Doppler and tissue Doppler velocity measurements. Strain was reduced in 6 of 12 segments of the left ventricle (LV) and in 1 of 2 segments of the right ventricle (RV). Strain rate (SR) was reduced in 2 of 12 segments of the LV and 1 of 2 segments of the RV in SSc patients as compared to controls (P < 0.05 for all). These involvements did not match any particular coronary artery distribution. More important differences were detected by average strain and SR values of the LV between patients and controls (19.78 +/- 3.00% vs 23.41 +/- 2.73%, P < 0.001; 2.01 +/- 0.41 vs 2.23 +/- 0.27/sec, P = 0.026, respectively). Furthermore, carbon monoxide diffusion capacity (DLCO) in scleroderma patients significantly correlated with LV average strain (r = 0.59; P = 0.001). Evaluation of ventricular function by using echocardiographic strain imaging appears to be useful to detect subclinical cardiac involvement in SSc patients with normal standard echocardiographic and tissue Doppler velocity findings.

Nonlinear flow and heat transfer dynamics of impinging jets onto slightly-curved surfaces

The nonlinear flow and heat transfer characteristics for a slot-jet impinging on slightly-curved surfaces are experimentally studied here. The effects of curved surface geometry and jet Reynolds number on the jet velocity distribution and circumferential Nusselt numbers are examined. Two different slightly-curved surface geometries of convex and concave are used as target surfaces. The nozzle geometry is a rectangular slot, and the dimensionless nozzle-to-surface distance equals to L ∗ = 8. The constant heat fluxes are accordingly applied to the surfaces to obtain an impingement cooling by the air jet at ambient temperature. The measurements are made for the jet Reynolds numbers of Re = 8617, Re = 13 350 and Re = 15 415 for both curved surfaces. The velocity distributions of issuing jet from the nozzle exit to the target surface are obtained by a highly sensitive hot-wire anemometer. The T-type thermocouples are used to measure local temperatures of both the air jet and the plates. Two-dimensional velocity measurements show that the surfaces are remained out of the potential core region for all Re tested here. New correlations for local, stagnation point, and average Nusselt numbers as a function of jet Reynolds number and dimensionless circumferential distance are reported. The correlations reveal that the impinging cooling rate is higher with the concave surface and increase with increasing Re.

Two-Dimensional Velocity Measurement of Turbulent Flow in Smooth and Rib-Roughened Square Two-Pass Channels by Using Particle Image Velocimetry

This study experimentally deals with turbulent flow inside smooth and transverse-rib-roughened square two-pass channels in stationary condition. Details of the two-dimensional flow field were quantitatively measured by using a particle image velocimetry which has high spatial- and time-resolution. In the transverse rib case, reattachment and recirculation behind the rib, and small separation bubble in front of the rib were observed. High turbulence intensity in front of the rib as a result of highly fluctuating unsteady fluid motion was clearly captured. Furthermore vortices caused by the sharp turn were observed in the secondary flow.

Transition layer thickness at a fluid-porous interface

The length scale of the transition region between a porous layer and its overlying fluid layer is experimentally studied. The experimental setup consists of a rectangular channel, in which a fluid layer flows over a porous bed. Using particle image velocimetry and refractive index matching, two-dimensional velocity measurements in the interfacial region were performed. The thickness of this transition layer, defined by the height below the permeable interface up to which the velocity decreases to the Darcy scale, is measured and compared with the permeability and the matrix grain size. It was observed that the thickness of the transition zone, δ, is of the order of the grain diameter, and hence, much larger than the square root of the permeability as predicted by previous theoretical studies. The Reynolds number and the fluid height over the porous substrate were found to affect the gradient of the horizontal velocity component at the interfacial region while the length scale of the transition layer remains approximately unchanged. The effect of the porous matrix type has been investigated by utilizing spherical glass beads as well as granulates. Scaling the measured velocities by the interfacial velocity near the uppermost solid matrix resulted in a unique velocity distribution in the case of monodisperse glass beads, hinting that the interfacial velocity represents a proper scaling factor. However, for polydisperse granulate material deviation from this behavior was observed.

A Study of Impeller-Diffuser-Volute Interaction in a Centrifugal Fan

This paper reports velocity measurement data in the interaction region between the impeller and vaned diffuser and the results of numerical flow simulation of the whole machine (impeller, vaned diffuser and volute) of a single stage centrifugal fan. Two-dimensional instantaneous velocity measurement is done using particle image velocimetry (PIV). Numerical simulation of impeller-diffuser-volute interaction is performed using CFX-Tascflow commercial code. A frozen rotor simulation model is used for the steady calculation and a rotor-stator simulation model is used for the unsteady calculation using the steady results as an initial guess. The simulation results show that the separated flow regime near the diffuser hub extends to the volute. Comparison between the unsteady computation and those of measurement indicates that the rotor/stator model employed in the simulation predicts essential characteristics of unsteady flow in the centrifugal fan. However, quantitative agreement remains rather poor.

The entrainment behavior of a turbulent axisymmetric jet in a viscous host fluid

Although turbulent jets have been studied extensively, one configuration that has not received much attention is the viscosity-stratified jet, wherein a turbulent jet of lower viscosity issues into a density-matched host liquid of higher viscosity. We present experimental data for scalar dispersion and two-dimensional velocity measurements in the axial plane of a turbulent axisymmetric jet with a Reynolds number (Re) of 2,000 issuing into a viscous host liquid at viscosity ratios (m) ranging from 1 to 55. The presence of a strong viscosity discontinuity across the jet edge results in a significant decrease in the scalar spread rate. We attribute this to the rapid reduction in turbulence intensity and the suppression of large engulfing eddies at the jet edge. The velocity profile, on the other hand, indicates that the velocity width and mass flux reduce with increasing m up to about 20, but then increase for higher values of m. This non-monotonic variation is explained by the growing influence of viscous stress for m>20. The scalar spread rate, the velocity spread rate, the centerline velocity decay rate, and the jet mass flux are all minimized for m≈20 for Re=2,000.

Design and optimization of a flexible high-peak-power laser-to-fiber coupled illumination system used in digital particle image velocimetry

We present a study on the design and parameter optimization of a flexible high-peak-power fiber-optic laser delivery system using commercially available solid-core silica fibers and an experimental glass hollow waveguide (HW). The fiber-optic delivery system provides a flexible, safe, and easily and precisely positioned laser irradiation for many applications including uniform illumination for digital particle image velocimetry (DPIV). The delivery fibers, when coupled through a line-generating lens, produce a uniform thin laser sheet illumination for accurate and repeatable DPIV two-dimensional velocity measurements. We report experimental results on homogenizing the laser beam profile using various mode-mixing techniques. Furthermore, because a fundamental problem for fiber-optic-based high-peak-power laser delivery systems is the possible damage effects of the fiber material, we determine experimentally the peak power density damage threshold of various delivery fibers designed for the visible spectral range at a typical DPIV laser wavelength of 532 nm. In the case of solid-core silica delivery fibers using conventional lens-based laser-to-fiber coupling, the damage threshold varies from 3.7 GW/cm2 for a 100-μm-core-diameter high-temperature fiber to 3.9 GW/cm2 for a 200-μm-core-diameter high-power delivery fiber, with a total output laser energy delivered of at least 3–10 mJ for those respective fibers. Therefore, these fibers are marginally suitable for most macro-DPIV applications. However, to improve the high-power delivery capability for close-up micro-DPIV applications, we propose and validate an experimental fiber link with much higher laser power delivery capability than the solid-core fiber links. We use an uncoated grazing-incidence-based tapered glass funnel coupled to a glass HW with hollow air-core diameter of 700 μm, a low numerical aperture of 0.05, and a thin inside cladding of cyclic olefin polymer coating for optimum transmission at 532 nm. Because of the mode homogenizing effect and lower power density, the taper-waveguide laser delivery technique ensured high damage threshold for the delivery HW, and as a result, no damage occurred at the maximum measured input laser energy of 33 mJ used in this study.

Two-Dimensional Velocity Measurements With Self-Mixing Technique in Diode-Pumped Yb : Er Glass Laser

Self-mixing laser Doppler velocimeter based on a two beam geometry scheme is investigated using a single-frequency Yb:Er-doped phosphate glass laser. The geometry allows the observation of a beat mode with optical crosstalk between the two optical paths, giving two major improvements: 1) a precise positioning of the optical head compared to the scattering target; 2) the measurement of both the absolute value of the velocity and the orientation of the speed.

Particle image analysis of open-channel flow at a backward facing step having a trench

Characteristics of open-channel flow at a backward facing step with a trench section were experimentally investigated. The experiments were performed using a straight open channel flume by varying the trench length. Two-dimensional velocity measurements by the particle tracking velocimetry (PTV) were conducted at the trench section using a high-resolution video camera of 12 fps. Mean and turbulent flow properties along with the water surface profile were measured for various aspect ratios of the trench. It was also observed that the flow at the trench begins to oscillate periodically for a specific aspect ratio of the trench.

Properties of vortices in the self-similar turbulent jet

Instantaneous, two-dimensional velocity measurements were conducted in the axial plane of a self-similar turbulent axisymmetric jet. The velocity fields were high-pass filtered to expose the vortical structures. An automated method was used to identify the radial and axial coordinates of the vortex centers and rotational sense, and to measure their size, circulation, vorticity, and energy. New insights into turbulent jets are obtained by plotting statistical distributions for vortex properties as functions of Reynolds number and radial position. While the probability of finding a vortex is uniform up to the edge of the jet, the strongest eddies in the high-pass filtered field occur near the jet axis. The average circulation is directly proportional to the vortex size. The Reynolds number strongly affects the average vorticity, circulation, and energy of the eddies. However, the normalized curves show a good collapse implying that the jet is indeed self-similar. Results for the left and right half-planes of the jet are also presented. Interestingly, we find that contrary to customary drawings of jet flows, a substantial number of both clockwise and counter-clockwise rotating eddies exist on both sides of the jet axis, with almost equal numbers of oppositely rotating vortices close to the jet axis. Further, the disparity in the number of oppositely rotating eddies in each half-plane increases with the eddy size. Nevertheless, these results are consistent with the well-known radial vorticity distribution of axisymmetric jets.

Adaptive clutter filtering via blind source separation for two-dimensional ultrasonic blood velocity measurement.

A method for adaptive clutter rejection via blind source separation (BSS) using principal and independent component analyses is presented in application to blood velocity measurement in the carotid artery. In particular, the filtering method's efficacy for eliminating clutter and preserving lateral blood flow signal components is presented. The performance of IIR filters is compromised by shorth data ensembles (10 to 20 temporal samples) as implemented for color-flow and high frame-rate imaging due to initialization requirements. Further, the ultrasonic imaging system's transfer function maps axial wall and lateral blood motion to overlapping spectra. As such, frequency domain-based approaches to wall filtering are ineffective for distinguishing wall from blood motion signals. Rather than operating in the frequency domain. BSS performs clutter rejection by decomposing the input data ensemble into N constitutive source signals in time, where N is the ensemble length. Source signal energy coupled with respective signal depth and time course profiles reveal which source signals correspond to blood, noise and clutter components. Clutter components may then be removed without disruption of lateral blood flow information needed for two-dimensional blood velocity measurement. A simplistic data simulation is employed to offer an intuitive understanding of BSS methods for signal separation. The adaptive BSS filter is further demonstrated using a Field II simulation of blood flow through the carotid artery including tissue motion. BSS clutter filter performance is compared to the performance of FIR, IIR and polynomial regression clutter filters. Finally, the filter is employed for clinical application using a Siemens Elegra scanner, carotid artery data with lateral blood flow collected from healthy volunteers, and Speckle Tracking; velocity magnitude and angle profiles are shown. Once again, the BSS clutter filter is contrasted to FIR, IIR and polynomial regression clutter filters using clinical examples. Velocities computed with Speckle Tracking after BSS wall filtering are highest in the center of the artery and diminish to low velocities near the vessel walls, with velocity magnitudes consistent with physiological expectations. These results demonstrate that the BSS adaptive filter sufficiently suppresses wall motion signal for clinical lateral blood velocity measurement using data ensembles suitable for color-flow and high frame-rate imaging.

Simultaneous two-dimensional flow velocity and gas temperature measurements by use of a combined particle image velocimetry and filtered Rayleigh scattering technique

For the first time, to the best of our knowledge, two-dimensional instantaneous measurements of the flow velocity and the gas temperature have been performed in a turbulent flame with simultaneous use of particle image velocimetry and planar filtered Rayleigh scattering. These single-shot measurements provide simultaneous information on the local flame structure (curvature and temperature gradients) and on the local flow conditions (vortices, flow divergences, and strain rates). The applicability of the technique is demonstrated in a turbulent lean CH(4)-air V flame.

Analysis of the air flow in the radial engine cooling fan of a combat vehicle

Results of an optimizing procedure that led to better understanding of the sophisticated flow pattern and better aerodynamic characteristics of a modified inlet section of an engine cooling fan for a combat vehicle are presented in this paper. The highly turbulent air flow of the original cooling system configuration was the result of inadequate design and position of the cooling fan in the vehicle and especially its inlet configuration. Experimental analysis of the rotor flow velocity profiles was performed with a five-hole Pitot-Prandtl probe at the rotor exit section and by the computer-aided visualization of the turbulent flow within the baseline and new ‘flow improving’ coaxial leading vanes at the rotor inlet. The kinematic performance of the rotor inlet section was thus changed and was quantified by visualization and analysis of the measuring thread's agility in the air flow. Time-based averaging of the consecutive frames within a photograph shot of particular measuring threads determined the direction of the incoming flow, whereas the appropriate standard deviation of the thread's agility was estimated as the proportional variable that determined the intensity of turbulence in the incoming flow. This relatively simple method was subsequently tested and compared with the results of two-dimensional laser Doppler anemometry velocity measurements. It was also proved to be a suitable technique for flow analyses in complex and less accessible flow channels.

An experimental investigation of extinction of curved laminar hydrogen diffusion flames

The extinction limits of steady, curved, diluted H 2 (and D 2 ) counterflow diffusion flames have been measured as a function of curvature. Steady curved flames are established in the wake of a bluff-body, which is installed in one of the nozzles of a counterflow burner. The curvature is essentially varied by changing the gas throughput. The radius of flame curvature has been measured by planar laser-induced fluorescence (LIF) from OH, or OD, respectively. In addition, two-dimensional laser-based velocity measurements have been performed, in particular to determine the strain rate of the curved flames. A new flow tagging scheme, which is based on photodissociation and LIF detection of acetone, is proposed and applied for particle-free velocity measurements in the counterflow burner. Flame extinction turns out to be strongly influenced by the orientation of the curvature with regard to the H 2 (or O 2 ) streams and the radius of curvature. This can be explained by curvature and preferential diffusion effects. Furthermore, we investigated the influence of curvature and preferential diffusion separately by comparing the extinction limits of H 2 /N 2 −O 2 /N 2 flames with those from D 2 /N 2 −O 2 /N 2 and H 2 /He−O 2 /He flames as a function of curvature. The results can basically be explained by the corresponding binary diffusion coefficients.

The Ouchi illusion as an artifact of biased flow estimation

A pattern by Ouchi has the surprising property that small motions can cause illusory relative motion between the inset and background regions. The effect can be attained with small retinal motions or a slight jiggling of the paper and is robust over large changes in the patterns, frequencies and boundary shapes. In this paper, we explain that the cause of the illusion lies in the statistical difficulty of integrating local one-dimensional motion signals into two-dimensional image velocity measurements. The estimation of image velocity generally is biased, and for the particular spatial gradient distributions of the Ouchi pattern the bias is highly pronounced, giving rise to a large difference in the velocity estimates in the two regions. The computational model introduced to describe the statistical estimation of image velocity also accounts for the findings of psychophysical studies with variations of the Ouchi pattern and for various findings on the perception of moving plaids. The insight gained from this computational study challenges the current models used to explain biological vision systems and to construct robotic vision systems. Considering the statistical difficulties in image velocity estimation in conjunction with the problem of discontinuity detection in motion fields suggests that theoretically the process of optical flow computations should not be carried out in isolation but in conjunction with the higher level processes of 3D motion estimation, segmentation and shape computation.

Experimental and numerical study of pulsatile flows through stenosis:: Wall shear stress analysis

Different shapes of pulsatile flows through a model of stenosis are experimentally and numerically modeled to validate both methods and to determine the wall shear stress temporal evolution downstream from the stenosis. Two-dimensional velocity measurements are performed in a 75% severity stenosis using a pulsed Doppler ultrasonic velocimeter. Finite element package is employed for the transient numerical simulations. Polynomial method, based on the experimental velocity values, is proposed to determine the wall shear stress temporal evolution. There is a good agreement between the numerical and experimental results. The wall shear stress temporal analysis shows oscillating wall shear stress values during the cycle with high wall shear stress values at the throat of about 120 dyn/cm 2, and low values downstream from the stenosis of about −2.5 dyn/cm 2. The key result of the study is that the presence of the stenosis leads the artery to work in a direction which is opposite to the direction of a healthy artery.

Use of simultaneous IR temperature measurements and DPIV to investigate thermal plumes in a thick layer cooled from above

Simultaneous measurements of surface temperature and the underlying velocity field are presented for a thick horizontal layer evaporatively cooled from above. Previous studies have focused on either the temperature field at the cooled surface or, in a small number of cases, on point velocity measurements in the flow. The current investigation is, to the knowledge of the authors, the first to simultaneously and non-intrusively document both the surface temperature and underlying velocity field in this type of flow. An infrared (IR) sensing array was used to capture the instantaneous free surface temperature field while two-dimensional velocity measurements in planes either perpendicular or parallel to the free surface were acquired using digital particle image velocimetry (DPIV). Data from two cases are discussed.

Evolution and modelling of subgrid scales during rapid straining of turbulence

The response, evolution, and modelling of subgrid-scale (SGS) stresses during rapid straining of turbulence is studied experimentally. Nearly isotropic turbulence with low mean velocity and Rλ˜290 is generated in a water tank by means of spinning grids. Rapid straining (axisymmetric expansion) is achieved with two disks pushed towards each other at rates that for a while generate a constant strain rate. Time-resolved, two-dimensional velocity measurements are performed using cinematic PIV. The SGS stress is subdivided to a stress due to the mean distortion, a cross-term (the interaction between the mean and turbulence), and the turbulent SGS stress τ(T)ij. Analysis of the time evolution of τ(T)ij at various filter scales shows that all scales are more isotropic than the prediction of rapid distortion theory, with increasing isotropy as scales decrease. A priori tests show that rapid straining does not affect the high correlation and low square-error exhibited by the similarity model. Analysis of the evolution of total SGS energy dissipation reveals, surprisingly, that the Smagorinsky model with a constant coefficient (determined from isotropic turbulence data) underpredicts the dissipation during rapid straining. While the partial dissipation −〈τ(T)ijS˜ij〉 (due only to the turbulent part of the stress) is overpredicted by the Smagorinsky model, addition of the cross-terms reverses the trend. The similarity model with a constant coefficient appropriate for isotropic turbulence, on the other hand, overpredicts SGS dissipation. Owing to these opposite trends a linear combination of both models (mixed model) provides better prediction of SGS dissipation during rapid straining. However, the mixed model with coefficients determined from dissipation balance underpredicts the SGS stress.

An acoustic imaging system for the concurrent measurement of suspended sediment concentration and particle velocity

This paper presents work on a system providing concurrent concentration and two-dimensional (2-D) particle velocity measurements on suspended sediments in water. The paper shows how one-dimensional methods for independent measurement of sediment concentration and particle velocity can be combined and extended to provide the concurrent 2-D information. An experimental system is described in which suspensions of 0.5-mm diameter sand are illuminated by acoustic burst signals of 1-MHz frequency from two transducers. The relevant theory for determining sediment concentration and particle velocity is presented. The system geometry provides for the illumination of a 5-cm3 volume at ranges of up to 1.1 m, although longer ranges are possible. Results are presented for concurrent concentration and particle velocity measurements for a falling curtain of sediment, and these are shown to be consistent with predicted values. The measurements have sufficient resolution, and can be made sufficiently rapidly, to investigate short time-scale phenomena in hydraulics, such as the effect of wave dynamics on sediment transport. The method presented could form the basis of an instrument giving sequenced acoustic images of the transport phenomena occurring within an observed volume.

Experimental analysis of unsteady flows through a stenosis

In this paper we present a study of the post-stenotic velocity flow field corresponding to oscillatory, pulsatile and physiological flow waveforms. Two-dimensional velocity measurements are performed in a 75% severity stenosis using a pulsed Doppler ultrasonic velocimeter. Emphasis is placed on the analysis of the experimental velocity-profile patterns. It is recognized that, beyond the influence of the flow parameters such as the Reynolds number and the frequency parameter, velocity profiles (hence wall-shear stresses) highly depend on the flow waveform. In addition to this analysis, a model of the stenosis influence length is proposed in the case of oscillatory flow.