In-plane Velocity Research Articles

Numerical Analysis of Flow Phenomena in a Residual Heat Removal Pump

The hydraulic performances as well as the cavitation phenomena in a scaled residual heat removal pump were investigated by experimental and numerical methods, respectively. In particular, a 3D numerical model of cavitation was adopted to simulate the internal cavitating flow through the model pump. The hydraulic performances of the model pump predicted by numerical simulations were in good agreement with the corresponding experimental data. The main generation and evolution of attached cavitation throughout the blade channels at different cavitating conditions have been investigated using the vapor fraction ISO surface and in-plane velocity vectors. Results show that the low static pressure at the impeller inlet is the main reason for leading edge cavitation by correlation analysis of static pressure on the midspan of impeller. Cavitation proved to occur over a wide range of flow rates, producing a characteristic creeping shape of the head-drop curve and developing in the form of nonaxisymmetric cavities at design flow rate. Moreover, the occurrence of these cavities, attached to the suction surface of blades, was found to depend on the NPSHA value. Numerical and experimental results in this paper can provide better understanding of the origin of leading edge cavitation in residual heat removal pumps.

Experimental investigation of interactions of three co-planar and converging jets

This experimental study focused on the structures of the flow produced by a novel configuration of three co-planar circular and neutrally buoyant pipe jets. The configuration presented in this paper was symmetrical about the central jet, with both side jets set at an inclined angle of 30° towards the central jet. These three jets discharged into a tank of still receiving water. The flow fields were captured using a particle image velocimetry system, and the complex flow patterns on two orthogonal planes (in-plane and normal to the central jet flow direction) were analysed and characterised in terms of velocity and vorticity distributions. Based on the findings of the study, it was deduced that the centreline velocity of the combined jet exhibited a preserved core before a longitudinal distance of 12D (D is the jet diameter) from the pipe exit and then decayed linearly until 17D. After 17D, the in-plane velocity profile followed a Gaussian distribution, while the profile in the normal plane portrayed a double-peak distribution. Finally, the dynamic interactions among the three converging jets were illustrated through a time series of instantaneous flow fields.

Quantitative velocity measurement in thin-gap Poiseuille flows

Quantitative in-plane velocity measurement by means of particle image velocimetry (PIV) within thin-gap devices subject to a large depth of focus and Poiseuille flow conditions across the gap is investigated. The primary obstacles to a reliable quantitative measurement are due to the effects of the inherent wall-normal velocity gradient and the inertial migration of particles in the wall-normal direction. Specifically, in the simplest case of no particle migration, the PIV correlation peak is broadened due to velocity variations within the interrogation region, and the result is expected to predict the maximum centerline velocity. The current work demonstrates, however, that there is an inevitable underestimation of the peak velocity due to the convolution of the fluid displacement probability distribution function (PDF) by the particle image size that introduces a biased error typically up to 33 % of the centerline velocity for all but the smallest particle images and largest displacements. Due to the low signal-to-noise ratio caused by the velocity gradient, the probability of a valid estimate is significantly impaired, demanding an unrealistically high concentration of tracer particles. In addition, inertial particle migration within the channel introduces a selective sampling of the velocity PDF, causing a second correlation peak to emerge as the particles rapidly move away from the wall, making a reliable measurement troublesome. In later times, the particles reach their equilibrium position and hence sample only a single velocity value, presenting conditions similar to traditional PIV interrogations, with the correlation function reduced to a single symmetric peak. A practical procedure is proposed to make PIV quantitative by manipulating the particles to their equilibrium position prior to performing measurements. A demonstration of a reliable PIV measurement under appropriate working conditions is discussed for diffusive Rayleigh–Bénard convection in a Hele-Shaw cell.

Improved pulse wave velocity wave front detection by template-matching

Background Pressure waves distributed through the aorta by left ventricular contraction propagate at a rate known as the pulse wave velocity (PWV). PWV is a measure of aortic stiffness, and increased PWV has been correlated with increased coronary artery disease risk. PWV can be measured by velocity-encoded magnetic resonance (PCMR), with velocity profiles collected at a minimum of two locations along the vessel. The commonly used simple linear regression method computes PWV from detected wavefronts in each profile and distance along the aorta. Recognizing the limitation imposed on wave front detection by simple maximum impulse by noise in the signal producing local maxima, we designed and tested a template-matching scheme to improve PWV estimation. Methods In vivo CMR was performed on a 3T scanner (Verio, Siemens Healthcare) in perimenopausal women with a variable number of atherosclerosis risk factors but no evident atherosclerotic disease. The derivation cohort consisted of 12 scans (4 patients over 3 time points) and validation cohort consisted of 139 scans PC-MR was acquired in an oblique sagittal plane through the descending aorta with in-plane velocity encoding in the head-foot direction and prospective ECG triggering (TE/ TR2.1/9.15 ms, Venc 150 cm/s, flip angle 15°). Distance from the base of the aorta and velocity were collected along the centerline of the aorta. Wave detection was performed at each cross section by the following scheme. A generalized logistic function f(a,t) = -Vmax +2Vmax/[1+exp{-a(t-c)}], c = tmax+ln(1/.99-1)/a, the center point, where Vmax is the maximum velocity, t is time, tmax is the time of the observed Vmax, and a was optimized to maximize the goodness of fit between the template and observed data between c and tmax. An example fit varying the control parameter, a, is shown in Figure 1. Pulse wave velocity was then calculated using simple linear regression over the (c, distance) pairs collected along the aorta. A one-tailed Student t-test with a of .05 was used to test for a significant increase in goodness of fit.

INTERLOCKING RESONANCE PATTERNS IN GALAXY DISKS

We have developed a method for finding dynamical resonances in disk galaxies using the change in sense of the radial component of the in-plane velocity at a resonance radius. Using simulations we show that we would expect to find these changes at corotation radii, with a weaker effect at the ILR and a small effect at the OLR. The method works well with observations at high spectral and angular resolutions, and is suited to the analysis of 2D velocity fields in Ha from Fabry-Perot spectroscopy, (though it is also applicable to fields in 21cm emission from HI, or to CO emission lines). The observations are mainly from the GHASP spectrometer data base, taken on the 1.93m telescope at Haute Provence, plus some data from the GHaFaS spectrometer on the 4.2m WHT at La Palma. We find clear indications of resonance effects in the disk velocity fields of virtually all of the 104 galaxies. The number of resonance radii detected ranges from one to seven, with a median of four. We have derived the resonance curves: Omega, Omega+/-kappa/2, Omega+/-kappa/4 against radius for all the galaxies, and used them to derive the ILR, the OLR, and the two 4:1 resonances for each corotation in each galaxy. This led us to discover a pattern in over 70% of the sample: given two pattern speeds, say Omega1 and Omega2, the OLR of Omega1 coincides with the corotation of Omega2, and the inner 4:1 resonance of Omega2 coincides with the corotation of Omega1. Although the second coincidence has been predicted, a prediction of this double coincidence is not found in the literature. This pattern is found once in 42 of the galaxies, twice in a further 26, three times in five, and even four times in one galaxy. We also compute the ratio of corotation radius to bar length where we have good enough image quality, finding a mean value of 1.3, and a shallow increase towards later type galaxies.

Two-perspective fluorescence analysis of droplets creeping down a tilted plate

Many experimental techniques have been developed and applied to investigate hydrodynamics of liquid films and single droplets on solid substrates. A simple but reliable measurement technique has been recently proposed to quantify thickness and apparent contact angles of droplets and rivulets (Hagemeier et al. Exp Fluids 52(2):361–374, 2012). However, this technique leads to ambiguities for any contact angle exceeding 90°. An improved version has thus been derived to solve the most important issues associated with the original method. For this purpose, top and sideways two-perspective images are acquired simultaneously, both relying on fluorescence. Analyzing the data obtained from both views, a correlation between fluorescence intensity and droplet shape can be derived. Furthermore, advancing and receding contact angles can be determined in this manner. A new and particularly important feature of the improved technique is the estimation of the contact line velocity at various locations, all around the moving droplet. The in-plane velocity components show a clear dependency on the Bond number and on the position around the droplet circumference.

Secondary Flow in Peripheral Vascular Prosthetic Grafts Using Vector Doppler Imaging

Prosthetic grafts are used for the treatment of peripheral arterial disease. Re-stenosis in the distal anastomosis of these grafts is a common reason for graft occlusion. The role of local hemodynamics in development of neo-intimal hyperplasia is well known. A new graft design has been proposed for the induction of optimized spiral flow in the host vessel. The secondary flow motions induced by this graft were compared with those of a control device. Both types of grafts were connected with vessel mimic and positioned in ultrasound flow phantoms with identical geometry. Constant flow rates were applied. Data collected in the cross-sectional view distal from the graft outflow and dual-beam vector Doppler was applied to create 2-D velocity maps. A single-spiral flow pattern was found for the flow-modified graft, and double or triple spirals for the control graft. In-plane maximum velocity was greater for the flow-modified graft than for the control device.

Laser-enabled experimental wavefield reconstruction in two-dimensional phononic crystals

Abstract During the past two decades, noteworthy experimental investigations have been conducted on wave propagation in phononic crystals, with special emphasis on crystals for acoustic wave control, consisting of the repetition of cylindrical or spherical elements in a fluid medium. On the other hand, the experimental characterization of the elastic wave phenomena observed in the solid microstructure of phononic crystals designed for elastic wave control has been quite sparse and limited in scope. The related literature focuses mostly on steady-state analyses that aim at highlighting filtering properties, and are limited to out-of-plane measurements. The scope of this work is to address these limitations and provide a detailed experimental characterization of the transient wave phenomena observed in the cores of lattice-like phononic crystals. This is achieved using a 3D Scanning Laser Vibrometer, which allows measuring the in-plane velocity of material points belonging to the lattice topology. This approach is tested against the benchmark case of a regular honeycomb lattice. Specifically, the objective is to demonstrate the directional and dispersive nature of the S-mode at relatively low frequencies and characterize the P-mode below and above its veering frequency. The experimental results are compared against numerical simulations and unit cell Bloch analysis to highlight similarities and differences between the true response of finite crystals and the infinite lattice approximation. This study also intends to highlight the advantages of three-dimensional laser vibrometry as a tool for the characterization of complex structural materials, while carefully exposing some limitations of this methodology.

Interferometric Sensor System for Blade Vibration Measurements in Turbomachine Applications

In order to improve the safety, lifetime, and energy efficiency of turbomachines, the dynamic behavior of the rotor has to be analyzed. Blade vibrations have to be monitored during operation to optimize the rotor design and to validate numerical models. However, measuring the vibration amplitude and frequency of the blades is a challenging task for metrology, since the blades to be measured are rotating quickly, and noncontact measurements are demanded. To solve this problem, we present a measurement system consisting of four laser Doppler sensors that have been mounted around the circumference of the rotor. These sensors measure simultaneously and contactlessly the in-plane velocity and the out-of-plane position of laterally moving objects. By analyzing the variation of the blade tip velocities, the vibration amplitude and frequency of the blades were estimated. Blade vibration measurements down to amplitudes of only 20 μm in tangential direction have been carried out. We achieved a standard uncertainty of approximately 400 nm for these experiments.

Short‐term effects of a standardized glucose load on region‐specific aortic pulse wave velocity assessed by MRI

To assess the short-term effects of a standardized oral glucose load on regional aortic pulse wave velocity (PWV) using two-directional in-plane velocity encoded MRI. A randomized, controlled intervention was performed in 16 male subjects (mean ± standard deviation: age: 59±7 years, body mass index: 28±3 kg/m2) with impaired fasting glucose. The intervention consisted of an oral glucose load (75 grams of carbohydrates in 300 mL water) at 1 study day and water (300 mL) at the other study day. PWV was measured using multislice two-directional in-plane velocity-encoded MRI. PWV in the proximal aorta at 1 h post-glucose load decreased compared with PWV 1-h post-water (delta PWV: -1.0±2.6 m/s versus 0.6±2.0 m/s, P=0.02). Eight responding subjects showed a significant decrease in PWV of the proximal aorta after the glucose load and had a decreased waist circumference (P=0.037) compared with nonresponders, being one of the major criteria of the metabolic syndrome. There was no significant change in PWV of the distal aorta at 1 h post-load comparing both intervention groups. A standardized oral glucose load induces a decrease of the proximal, but not of the distal, aortic PWV. Regional response of aortic PWV may be associated with features of the metabolic syndrome.

Secondary Flow Loss Reduction Through Blowing for a High-Lift Front-Loaded Low Pressure Turbine Cascade

Efforts to increase individual blade loading in the low pressure turbine have resulted in blade geometries optimized for midspan performance. Many researchers have shown that increased blade loading and a front-loaded pressure distribution each separately contribute to increased losses in the endwall region. A detailed investigation of the baseline endwall flow of the L2F profile, which is a high-lift front loaded profile, is performed. In-plane velocity vectors and total pressure loss maps are obtained in five planes oriented normal to the blade surface for three Reynolds numbers. A row of pitched and skewed jets are introduced near the endwall on the suction surface of the blade. The flow control method is evaluated for four momentum coefficients at the high Reynolds number, with a maximum reduction of 42% in the area averaged total pressure loss coefficient. The same blade is also fitted with midspan vortex-generator jets and is tested at a Reynolds number of 20,000, resulting in a 21% reduction in the area averaged total pressure loss.

Turbulent linewidths as a diagnostic of self-gravity in protostellar discs

We use smoothed particle hydrodynamics simulations of massive protostellar discs to investigate the predicted broadening of molecular lines from discs in which self-gravity is the dominant source of angular momentum transport. The simulations include radiative transfer, and span a range of disc-to-star mass ratios between 0.25 and 1.5. Subtracting off the mean azimuthal flow velocity, we compute the distribution of the in-plane and perpendicular peculiar velocity due to large scale structure and turbulence induced by self-gravity. For the lower mass discs, we show that the characteristic peculiar velocities scale with the square root of the effective turbulent viscosity parameter, as expected from local turbulent-disc theory. The derived velocities are anisotropic, with substantially larger in-plane than perpendicular values. As the disc mass is increased, the validity of the locally determined turbulence approximation breaks down, and this is accompanied by anomalously large in-plane broadening. There is also a high variance due to the importance of low-m spiral modes. For low-mass discs, the magnitude of in-plane broadening is, to leading order, equal to the predictions from local disc theory and cannot constrain the source of turbulence. However, combining our results with prior evaluations of turbulent broadening expected in discs where the magnetorotational instability (MRI) is active, we argue that self-gravity may be distinguishable from the MRI in these systems if it is possible to measure the anisotropy of the peculiar velocity field with disc inclination. Furthermore, for large mass discs, the dominant contribution of large-scale modes is a distinguishing characteristic of self-gravitating turbulence versus MRI driven turbulence.

Investigation of the 3D flow field in an IC engine using tomographic PIV

In-cylinder measurements of the instantaneous 3D flow field are required for further understanding of turbulent mixing and combustion processes in internal combustion (IC) engines as well provide valuable data for the development of large eddy simulation (LES) models for combustion simulation. Tomographic particle image velocimetry (TPIV) is applied within a motored direct-injection spark-ignition (DISI) optical engine to measure the instantaneous volumetric flow field at selected crank-angle degrees (CAD) during the intake and compression stroke. Measurements were conducted using a double-pulse Nd:YAG laser with an average pulse energy of 375mJ for illumination and four CCD cameras arranged circularly around the optically accessible cylinder. A volume of 47×35×4mm3 was imaged located centrally between the intake valves. The uncertainty of the 3D velocity data was addressed using the principle of mass conservation for the inlet flow. A precision within 9% was found for the calculation of the velocity gradient tensor and is comparable to experiments in generic configurations, indicating that TPIV measurements within a motored IC engine are feasible. Based on the volumetric velocity data, the average structure of the flow field was analyzed showing a clear orientation of the average velocity, which changes during the different phases of the cycle. The 3D turbulent kinetic energy (TKE) is computed and compared to the 2D-TKE determined from in-plane velocity components and reveals distinct regions of large out-of-plane velocity fluctuations. Results from volumetric velocimetry furthermore provides insight into the occurrence and orientation of vortical structures that here are identified by using the Q-criterion. Differences between flow structures during intake and compression strokes are discussed on basis of ensemble-averaged and individual cycle velocity maps as well as the full vorticity vector.

Polynomial element velocimetry (PEV): a technique for continuous in-plane velocity and velocity gradient measurements for low Reynolds number flows

Particle image velocimetry (PIV) selects the maximum of the cross-correlation map to represent the modal displacement, and a wealth of information stored in the cross-correlation is discarded. We introduce a novel method, termed polynomial element velocimetry (PEV), which results in continuous velocity and velocity gradient measurements. PEV utilizes the extra information stored in the cross-correlation to determine continuous velocity measurements with low levels of measurement noise. In contrast to PIV, the continuous nature of velocity measurements facilitates the direct determination of the velocity gradient. The PEV method is applied to two laboratory flows: flow in a channel and flow behind a circular cylinder at Reynolds number, Re = 30, and is shown to greatly reduce the noise in the measurements. In addition, the accuracy of PEV is validated using two computer-generated synthetic flows: parabolic flow in a channel and flow past a square cylinder at Re = 30. In these cases, PEV is shown to reduce the velocity measurement error by up to 45% and the vorticity estimation error by up to 77% when compared to PIV. A key benefit of the PEV method is that it is capable of calculating continuous measures for flow gradient with greatly reduced bias errors. In particular, PEV provides a more accurate measurement of the vorticity near interfaces such as a cylinder wall or channel wall where PIV methods only provide measurement data at half the sampling window size from the wall. Since PEV utilizes the entire shape of the cross-correlation map to determine a local map for the underlying velocity, minimal random error is transmitted to the estimated flow gradient. This feature of the PEV method makes it optimal for flows where flow gradients are well defined and there are insufficient pixels to fully resolve structures in the flow using PIV.

Multi-frame pyramid correlation for time-resolved PIV

A novel technique is introduced to increase the precision and robustness of time-resolved particle image velocimetry (TR-PIV) measurements. The innovative element of the technique is the linear combination of the correlation signal computed at different separation time intervals. The domain of the correlation signal resulting from different temporal separations is matched via homothetic transformation prior to the averaging of the correlation maps. The resulting ensemble-averaged correlation function features a significantly higher signal-to-noise ratio and a more precise velocity estimation due to the evaluation of a larger particle image displacement. The method relies on a local optimization of the observation time between snapshots taking into account the local out-of-plane motion, continuum deformation due to in-plane velocity gradient and acceleration errors. The performance of the pyramid correlation algorithm is assessed on a synthetically generated image sequence reproducing a three-dimensional Batchelor vortex; experiments conducted in air and water flows are used to assess the performance on time-resolved PIV image sequences. The numerical assessment demonstrates the effectiveness of the pyramid correlation technique in reducing both random and bias errors by a factor 3 and one order of magnitude, respectively. The experimental assessment yields a significant increase of signal strength indicating enhanced measurement robustness. Moreover, the amplitude of noisy fluctuations is considerably attenuated and higher precision is obtained for the evaluation of time-resolved velocity and acceleration.

Kinematics of Roller Migration in the Planetary Roller Screw Mechanism

This paper develops a kinematic model to predict the axial migration of the rollers relative to the nut in the planetary roller screw mechanism (PRSM). This axial migration is an undesirable phenomenon that can cause binding and eventually lead to the destruction of the mechanism. It is shown that this migration is due to slip at the nut–roller interface, which is caused by a pitch mismatch between the spur-ring gear and the effective nut– roller helical gear pairs. This pitch circle mismatch can be due to manufacturing errors, deformations of the mechanism due to loading, and uncertainty in the radii of contact between the components. This paper derives the angle through which slip occurs and the subsequent axial migration of the roller. It is shown that this roller migration does not affect the overall lead of the PRSM. In addition, the general orbital mechanics, in-plane slip velocity at the nut–roller interface, and the axial slip velocities at the nut–roller and the screw–roller interfaces are also derived. Finally, an example problem is developed using a range of pitch mismatch values for the given roller screw dimensions, and the axial migration and slip velocities are determined.

Structures of turbulent open-channel flow in the presence of an air–water interface

Direct numerical simulations (DNSs) of open-channel flows in the presence of an air–water interface were performed to examine the effects of interface deformation on the turbulence structures. In the water-driven turbulence, flows characterized by either of the two Froude numbers (Fr = 0.2 and 0.8) were examined and compared. A coupled level-set and volume-of-fluid (CLSVOF) method was employed to track the interface. The mean and root-mean-square of the air–water interface elevation varied rapidly with the spanwise distance as Fr increased. The air that contacted the water was entrained into the turbulent flow. At high Fr, all turbulent normal stresses on the air side of the interface were high near the sidewall. Two-point correlations between the streamwise vortex and the velocity fluctuations provided structural information about the near-wall streaky structures and the inner secondary flows in the cross-stream plane. Linear stochastic estimates of the conditionally averaged flow field showed that the inner secondary flow consisted of not only in-plane velocity components but also streamwise velocity components.

How to optimize intracardiac blood flow tracking by echocardiographic particle image velocimetry? Exploring the influence of data acquisition using computer-generated data sets

Echocardiographic particle image velocimetry (EPIV) has been used for tracking contrast-enhanced intracavitary blood flow. Little is known, however, how basic imaging parameters (line density, frame rate, contrast bubble density) affect the quality of such tracking results. Our study aimed at investigating this by using simulated echo data sets. A computational three-dimensional (3D) blood flow field of the left ventricle (LV) was built using Fluent 12.1 (ANSYS Inc., USA). Then, the 3D motion of contrast microbubbles was simulated and 2D B-mode image loops were obtained (f = 4.5 MHz; 50 sector angle) and analysed using flow tracking software (Omega Flow, Siemens, USA). Vorticity and the resulting in-plane velocity vector field was calculated at different frame rates (227, 113, 76, and 57 fps) and bubble densities (100, 63, 36, 19, 10, and 3 bubbles/mL) and compared with the ground truth known from the computational LV flow model. The normal distribution of the amplitude error and angle error histograms confirmed the overall good performance of the tracking method. In the standard deviation analysis of error histograms, tracked velocity amplitudes correlated best with the ground truth at 10 bubbles/mL and 227 fps (45.81 ± 3.43%, P < 0.05), while the best performance of flow direction estimates was at 10 bubbles/mL and 76 fps (25.41 ± 1.22°, P < 0.05). The correlation of estimated and true vorticity tended to grow with increasing frame rate and was optimal at 19 bubbles/mL and 113 fps (r = 0.79 ± 0.02). To achieve accurate vorticity measurements, frame rate acquisitions as 113 fps and contrast bubble density of 19 bubbles/mL are needed.

Novel Dynamic Rotor and Blade Deformation and Vibration Monitoring Technique

Monitoring rotor deformations and vibrations dynamically is an important task for improving both the safety and the lifetime as well as the energy efficiency of motors and turbo machines. However, due to the high rotor speed encountered in particular at turbo machines, this requires concurrently high measurement rate and high accuracy, which is hardly possible to achieve with currently available measurement techniques. To solve this problem, in this paper, we present a novel nonincremental interferometric optical sensor that measures simultaneously the in-plane velocity and the out-of-plane position of laterally moving objects with micrometer precision and concurrently with microsecond temporal resolution. It will be shown that this sensor exhibits the outstanding feature that its measurement uncertainty is generally independent of the object velocity, which enables precise deformation and vibration measurements also at high rotor speed. Moreover, this sensor does not require an in situ calibration and it allows a direct measurement of blade velocity variations in contrast to blade tip timing systems. For application under harsh environmental conditions such as high temperatures, a robust and miniaturized fiber-optic sensor setup was developed. To demonstrate the capability of this sensor, measurements of tip clearance changes and rotor blade vibrations at varying operating conditions of a transonic centrifugal compressor test rig at blade tip velocities up to 600 m/s are presented among others.

Proton acceleration in antiparallel collisionless magnetic reconnection: Kinetic mechanisms behind the fluid dynamics

[1] This paper investigates the proton kinetic mechanisms leading to the formation of plasma jets in antiparallel magnetic reconnection. In particular, the interaction of the protons with the Hall electric field in the proton non-ideal region is discussed. The study, based on a two-dimensional hybrid simulation, details the important role of the proton pressure force in the acceleration process and its role in maintaining open and steady the proton outflow channel. When no fluid closure is assumed, it is found that this force arises from a strong anisotropy in velocity space which comes from kinetic effect. By analyzing the distribution functions and the individual particle dynamics, it is shown that the mixing of protons bouncing in a divergent electrostatic potential well associated to the Hall effect statistically couples the two in-plane velocity components of the particles. This coupling results, from the macroscopic point of view, in off-diagonal components of the pressure tensor.