Accuracy and precision of time-averaged flow as measured by nontriggered 2D phase-contrast MR angiography,a phantom evaluation

The purpose of this study was to assess the accuracy and precision of time-averaged flow as measured by nontriggered 2D PC. Mono-, bi-, and triphasic flow patterns, modelling waveforms encountered in the human vascular system, were generated by a computer-controlled flow system. Time-averaged flow velocity was measured by conventional 2D cardiac-triggered cine PC and by nontriggered 2D PC for different settings of the excitation flip angle and the velocity sensitivity. Accuracy and precision were determined by repeating the measurements (N = 6) and comparing the results against precisely known calibration values. Measurements revealed waveform-specific deviations between triggered and nontriggered acquisitions that depended on the velocity sensitivity and, more strongly, on the flip angle of the nontriggered experiment. This confirmed the theoretically predicted predominance of amplitude over phase effects. Systematic errors could be reduced by decreasing the flip angle and the velocity sensitivity, although at the expense of signal-to-noise, so that additional signal averaging was required to maintain a specified precision. The attainable accuracy appeared to be acceptable only for waveforms with a relatively low pulsatility index. The study demonstrates the feasibility of accurate and precise nontriggered velocity measurements for weakly pulsatile flow and indicates a route towards improving the reliability for highly pulsatile flow.     

Simultaneous velocity and temperature measurements in gaseous flow fields using the VENOM technique

We present an initial demonstration of simultaneous velocity and temperature mapping in gaseous flow fields using a new nitric oxide planar laser-induced fluorescence-based method. The vibrationally excited NO monitoring (VENOM) technique is an extension of two-component velocimetry using vibrationally excited NO generated from the photodissociation of seeded NO(2) [Appl. Opt. 48, 4414 (2009)], where the two sequential fluorescence images are obtained probing two different rotational states to provide both velocity and temperature maps. Comparisons to computational fluid dynamics simulations show that the initial VENOM measurements provide good velocity and temperature maps in the relatively high-density regions of the flow, where the rms uncertainties are approximately 5% for velocity and 9% for temperature.     

Simultaneous measurement of temperature and velocity maps by inversion recovery tagging method

A new method, called the inversion recovery (IR) tagging method, for simultaneous measurement of temperature and velocity maps of flowing fluid has been developed. The present method employs a set of tagging pulses which acts as an inversion pulse of the conventional IR method, based on the temperature dependence of the spin-lattice relaxation of water proton in a fluid, and has the advantage of being able to compensate the reduction of the NMR signal intensity due to flow motion and to reduce the total time to measure these maps. First, the accuracy of the temperature measurement of stagnant doped water in a differentially heated cell using the conventional IR method, as the basic sequence of the IR tagging method, has been evaluated. The accuracy was within 10% of the temperature difference DeltaT = 17.2 degrees C and the measurable temperature resolution was within +/-0.5 degrees C. Then temperature and velocity maps of the flowing doped-water through a cooled pipe were measured simultaneously by the IR tagging method, and the accuracy of temperature measurement was evaluated. The accuracy obtained using the present method was within 15% of the temperature difference DeltaT = 15 degrees C.     

A laser Doppler velocimeter for pulsatile water flow measurements in resonant hydraulic circuits

Described is a laser Doppler velocimeter designed to measure a small oscillatory velocity component superposed to the steady water flow of a resonant hydraulic circuit in fully developed turbulent conditions. A frequency tracker with lock-in analysis of the velocity output is used. The rms amplitude of the oscillatory velocity component is measured with an accuracy of 1.5 mm s^-1, and the mean velocity is about 3 ms^-1. The resonance behaviour of the velocity amplitude measured at a fixed point of the circuit is reported for the exciting frequency range 2–5 Hz.     

Phase-resolved Doppler optical coherence tomography--limitations and improvements

We describe how the simple phase difference averaging causes a systematic bias in the velocity estimation obtained by phase-resolved Fourier domain optical coherence tomography (FdOCT). The magnitude of this bias depends on the signal-to-noise ratio as well as proximity of the measured velocity to the limits of the velocity range. We demonstrate the proper way of data processing, which enables obtaining velocity values free of this error. We validate the improved technique by measurements of flow velocity in glass capillaries, in human retinal vessels, and we compare the results with those obtained by standard phase-resolved FdOCT.     

A comparison of 2D and 3D PIV measurements in an oblique jet

A stereo PIV (SPIV) acquisition and analysis system was developed to measure three velocity components in planar flow fields. The analysis software is based on a third order mapping function method. The system was calibrated by imaging a square grid in three measurement planes with two Kodak Megaplus cameras oriented at 30 to the bisector between them. The camera images were dewarped into real coordinates by employing a set of transform matrices computed for each calibration plane. Bias and rms errors were determined by comparing displacements measured directly with displacements estimated from the dewarping and recombination algorithm. The bias errors in the directions parallel with the measurement plane were negligible while the bias in thez direction was about 0.6 pixel. The rms errors, 0.2–0.3 pixels, were largest in thez direction. These errors were thought to result from limitations in the calibration method. The SPIV system was tested in a two-dimensional oblique jet with Reynolds number of 1800. The three dimensional results were taken in a vertical (x, y) plane parallel with the jet span. The SPIV results were compared with LDV data and two-dimensional PIV data obtained in a vertical (y, z) plane of the same jet. The SPIV measurements yielded accurate values for the in-plane mean and rms velocity components. The measured out-of-plane mean component was underestimated due to the bias error mentioned above. The rms component was accurate in part of the field but overestimated in another part due to local variations in rms error. It is expected that in the future, the out-of-plane errors can be minimized by improving the calibration and transformation procedures.     

Quantitative magnetic resonance flow and diffusion imaging in porous media

Quantitative flow and diffusion measurements have been made for water in model porous media, using magnetic resonance micro-imaging methods. The samples consisted of compacted glass beads of various sizes down to 1 mm diameter. Typical flow and diffusion images exhibited a spatial resolution of 117 μm × 117 μm and velocities in the range 1–2 mm/s. Comparison of volume flow rates calculated from the flow velocity maps with values measured directly yielded good agreement in all cases. There was also good agreement between the mean diffusion coefficient of water calculated from the diffusion maps and the bulk diffusion coefficient for pure water at the same temperature. In addition, the mean diffusion coefficient did not depend on the pore sizes in the bead diameter range of 1–3 mm. Our results also show that partial volume effects can be compensated by appropriate thresholding of the images prior to the final Fourier transformation in the flow-encoding dimension.     

Velocity encoding and velocity compensation for multi-spoke RF excitation

PurposeTo investigate velocity encoded and velocity compensated variants of multi-spoke RF pulses that can be used for flip-angle homogenization at ultra-high fields (UHF). Attention is paid to the velocity encoding for each individual spoke pulse and to displacement artifacts that arise in Fourier transform imaging in the presence of flow.Theory and methodsA gradient waveform design for multi-spoke excitation providing an algorithm for minimal TE was proposed that allows two different encodings. Such schemes were compared to an encoding approach that applies an established scheme to multi-spoke excitations. The impact on image quality and quantitative velocity maps was evaluated in phantoms using single- and two-spoke excitations. Additional validation measurements were obtained in-vivo at 7 T.ResultsPhantom experiments showed that keeping the first gradient moment constant for all k-space lines eliminates any displacements in phase-encoding and slice-selection direction for all spoke pulses but leads to artifacts for non-zero velocity components along readout direction. Introducing variable but well-defined first gradient moments in the phase-encoding direction creates displacements along the velocity vector and thus minimizes velocity-induced geometrical distortions. Phase-resolved mean volume flow in the ascending and descending aorta obtained from two-spoke excitation showed excellent agreement with single-spoke excitation over the cardiac cycle (mean difference 0.8 ± 16.2 ml/s).ConclusionsThe use of single- and multi-spoke RF pulses for flow quantification at 7 T with controlled displacement artifacts has been successfully demonstrated. The presented techniques form the basis for correct velocity quantification and compensation not only for conventional but also for multi-spoke RF pulses allowing in-plane B₁⁺ homogenization using parallel transmission at UHF.     

Comparison between measured and calculated parameters for the acoustical characterization of small classrooms

This paper presents a comparison between measured and calculated acoustical parameters in eight high school classrooms. The mid frequency unoccupied and occupied reverberation times and the 1 kHz sound propagation (SP) of the reverberant and total speech levels in occupied classrooms were compared with analytical and numerical predictions. The ODEON 6.5 code and the Sabine formula gave the most accurate results for reverberation time in the empty classrooms with overall relative differences of 8.1% and 9.7%, respectively. With students present, the Eyring and Sabine formulas and Hodgson’s empirical model resulted to be the most accurate with relative differences of 11.1%, 13.2% and 13.6%, respectively. The reverberant speech levels decrease with increasing distance from the source at rates varying from −1.21 to −2.62 dB/distance doubling, and the Hodgson model fits the slope values quite well. The best predictions of the SP of the reverberant and total speech levels are shown, in order of accuracy, for the ODEON code, the Barron and Lee theory and the classical diffuse field theory. Lower rms errors were found when the measured total acoustic absorptions were used. The lowest rms error of 1.4 dB for the SP of the total speech level were found for both the ODEON code and the Barron and Lee theory.     

径向浓淡旋流煤粉燃烧器气流湍流特性的冷态试验研究

本文利用ＩＦＡ３００型一维热膜风速仪系统对径向浓淡旋流煤粉燃烧器单相冷态湍流流场进行了试验研究,测量了流场内不同位置瞬时速度的数值,同时测量了脉动速度均方根、湍流度、平坦因子和偏斜因子在流场内的分布,得到了新型旋流燃烧器气流湍流特性参数的分布规律,可以用于研究径向浓淡旋流煤粉燃烧器的燃烧特性。     

Estimating the direct-to-reverberant energy ratio from the coherence between coincident pressure and particle velocity

An analytical expression for the relationship between the direct-to-reverberant energy ratio (DRR) and the coherence estimation function between coincident pressure and particle velocity component is derived. The analytical solution is first validated with simulated room impulse responses and then used to estimate the DRR in five octave bands for several receiver positions measured in a total of 11 rooms of vastly different sizes and acoustic characteristics. The accuracy is evaluated by comparison with the DRR estimated directly from the room impulse response. The difference is typically 5 dB. For two rooms, the variation of the DRR estimate with source-to-receiver position is also shown. The method is blind in the sense that it is virtually independent of the signal generated by a single sound source.     

Measurement of Mach probe on plasma flow velocity in highly collisional plasma jet

A normalized plasma flow velocity in highly collisional plasma formed by a microwave plasma jet, which is dimensionless unit for plasma flow velocity/ion acoustic velocity, was measured by the parallel Mach probe. To deduce the normalized plasma flow velocity under highly collisional plasma conditions, the collisional model of a Mach probe was proposed. In addition, neutral gas flow velocity which assumed to be plasma flow velocity was calculated by the turbulent model. The results for the two different models were compared with those for the collsionless models of the Mach probe. The turbulent model produced 2–4 times reduced values than by measurements with collsionless models. The measured results with the collisional model were shown as approximately 100–250% lower than those for collsionless models. They were obtained to be in good agreement with difference rate of 10–30% when compared to those for the turbulent model.     

Hemodynamics of aneurysm intervention with different stents

An ideal cerebral aneurysm model with different stent forms is established. By using the single-relaxation-time (SRT) lattice Boltzmann method (LBM) to solve the flow field, the blood flow characteristics in the aneurysm under different conditions are studied numerically. The intra-arterial stenting of saccular aneurysms at different Reynolds numbers and the feasibility of new stenting forms such as double stenting and variable-spacing stenting in the aneurysms are explored. The hemodynamic factors such as velocity distribution and wall shear stress (WSS) in the aneurysm are analyzed. Numerical results show that the risk of aneurysm rupture is mainly centralized at the right corner of the aneurysm. Intervention of stents in the aneurysm can effectively reduce the intra-aneurysmal velocity and WSS, and decrease the danger of aneurysm rupture during strenuous exercise or emotional excitement. At the same time, the intervention of a double stent and the stent shape with a dense anterior part in the aneurysm has certain advantages in preventing aneurysm rupture. The intra-aneurysmal mean velocity reduction can reach 90.39% and 80.29% after the intervention of the double stent and the anterior densified stent respectively.     

Pressure measurement in supersonic air flow by differential absorptive laser-induced thermal acoustics

Nonintrusive, off-body flow barometry in Mach 2 airflow has been demonstrated in a large-scale supersonic wind tunnel using seedless laser-induced thermal acoustics (LITA). The static pressure of the gas flow is determined with a novel differential absorption measurement of the ultrasonic sound produced by the LITA pump process. Simultaneously, the streamwise velocity and static gas temperature of the same spatially resolved sample volume were measured with this nonresonant time-averaged LITA technique. Mach number, temperature, and pressure have 0.2%, 0.4%, and 4% rms agreement, respectively, in comparison with known free-stream conditions.     

Non-coherent noise reduction in digital holography based on root mean square technique

In this paper, the root mean square (rms) technique is applied in order to reduce the non-coherent noise in phase-contrast image. The proposed technique is applied to a sample of 200 μm step height nominally. The recorded off-axis interferograms generated from two different wavelengths are processed to obtain an object wave (amplitude and phase) for each wavelength separately. The independent phase maps are subtracted and a phase map for the beat-wavelength is obtained and converted to height map. The rms values of 10 pixels profiles from the obtained height map are calculated automatically to show the three-dimensional (3D) profile. The experimental results show that the non-coherent noise is reduced by the order of 90% when the rms technique is applied and the uncertainty in measurement has been found to be of the order of 1.5 μm. The proposed technique can provide a simple and real solution for measuring 3D objects having high abrupt height difference.     

Velocity and diffusion imaging in dynamic NMR microscopy

The imposition of resolution gradients in a pulsed-gradient spin-echo (PGSE) NMR sequence induces motionally dependent phase and amplitude modulation in the image, a technique which we have termed dynamic NMR microscopy. Fourier analysis of this modulation gives a dynamic displacement profile for each pixel which can then be analyzed to obtain velocity and diffusion maps. The application of this method at high spatial resolution is motivated by a desire to measure vascular flow in living plants and variations in molecular self-diffusion under the influence of velocity shear in narrow capillaries. The theory of dynamic NMR microscopy is presented and potential artifacts discussed, including the effect of slice selection gradients, PGSE gradient nonuniformity, and specific problems associated with the measurement of self-diffusion in the presence of velocity gradients. It is demonstrated that a double-echo PGSE pulse sequence can be used to restore coherent phase shifts associated with steady-state flow, and examples of self-diffusion maps and signed velocity maps from sequences of phase-encoded images obtained by projection reconstruction are given. This method has been applied at 20,um transverse resolution in laminar capillary flow.     

Velocity measurements in a turbulent round jet using PTV: the effect of tracer particles

The effects of different tracer particles used in the PTV technique on the velocity field are investigated. The measurements are taken in a water round jet at a Reynolds number equal to about 40000, a flow field widely examined in the literature, in which strong velocity differences are encountered. The interest is focused onto particles with different density. Although the mean and rms values are almost unaffected, both measurements obtained with light and heavy particles (in comparison to the density of the fluid) reveal a modified velocity field. The moments of velocity differences (structure functions) point out that such a modification is felt within each range of flow scales. Particles with density almost equal to that of the fluid reproduce the flow behaviour over all the scales.     

On-Line Measurement of Mass Flow of Pneumatically Conveyed Solids

CSIRO has developed an ultrasonic attenuation and transit time technique and a microwave transmission technique for on-line determination of the mass flow of pneumatically conveyed solids. In a comparative plant trial on a single burner line at Bayswater power station, the ultrasonic technique determined the coal density and velocity with rms errors of 3.9 and 5.9% relative respectively; the microwave technique successfully determined the coal density with an rms error of 6.2% relative, but was unable to accurately determine solids flow velocity at the low coal densities typical of the trial. In a plant trial at a direct smelter, the microwave technique successfully determined the mass flow of pneumatically conveyed iron ore fines feeding the smelter with an rms error of 10.2% relative.     

Solid-fluid transition in a granular shear flow

The rheology of a granular shear flow is studied in a quasi-2D rotating cylinder. Measurements are carried out near the midpoint along the length of the surface flowing layer where the flow is steady and nonaccelerating. Streakline photography and image analysis are used to obtain particle velocities and positions. Different particle sizes and rotational speeds are considered. We find a sharp transition in the apparent viscosity (eta) variation with rms velocity (u). Below the transition depth we find that the rms velocity decreases with depth and eta proportional to u(-1.5) for all the different cases studied. The material approaches an amorphous solidlike state deep in the layer. The velocity distribution is Maxwellian above the transition point and a Poisson velocity distribution is obtained deep in the layer. The results indicate a sharp transition from a fluid to a fluid + solid state with decreasing rms velocity.     

气液两相流速度及粒径分布激光干涉测量方法的研究

为了实现对气液两相流的粒子粒径、空间分布及其速度测量。对激光干涉气液两相流测量技术（ILIDS）进行了深入研究,该技术是一种应州于气液两相流测量的新技术,其主要优点是不干扰流场和颗粒粒径、位置测量精度高。基于该技术所开发的图像自动处理方法可以利用普通粒子成像测量技术系统拍摄气液两相流的激光散射干涉图像。并利用图像卷积定位、傅里叶变换频率分析及其图像互相关测速等图像处理手段从干涉图像中自动提取粒子的位置、直径和速度信息。为了验证该方法的测量精度,对喷嘴生成的气水两相流进行了测量实验,得到了喷嘴出口处不同区域的粒径、速度矢量的空间分布,并将测得的速度矢量与用粒子成像测量技术方法测得的结果进行对比,证明两种方法测量的平均速度差别仅为0．38％。