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Diabetologia Notes de lecture     Hybrid Journal  
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Directieve therapie     Hybrid Journal  
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DNP - Der Neurologe und Psychiater     Full-text available via subscription  
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Dysphagia     Hybrid Journal   (Followers: 73, SJR: 0.822, h-index: 52)
e & i Elektrotechnik und Informationstechnik     Hybrid Journal   (Followers: 9, SJR: 0.279, h-index: 9)
e-Neuroforum     Hybrid Journal  
Early Childhood Education J.     Hybrid Journal   (Followers: 11, SJR: 0.466, h-index: 16)
Earth Science Informatics     Hybrid Journal   (Followers: 3, SJR: 0.282, h-index: 7)
Earth, Moon, and Planets     Hybrid Journal   (Followers: 8, SJR: 0.303, h-index: 29)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 5, SJR: 0.482, h-index: 21)
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East Asia     Hybrid Journal   (Followers: 9, SJR: 0.18, h-index: 9)
Eating and Weight Disorders - Studies on Anorexia, Bulimia and Obesity     Hybrid Journal   (Followers: 11, SJR: 0.362, h-index: 27)
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Elemente der Mathematik     Hybrid Journal   (Followers: 1)
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Empirica     Hybrid Journal   (Followers: 3, SJR: 0.319, h-index: 16)
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Empirical Software Engineering     Hybrid Journal   (Followers: 6, SJR: 1.285, h-index: 39)
Employee Responsibilities and Rights J.     Hybrid Journal   (Followers: 5, SJR: 0.361, h-index: 15)
Endocrine     Hybrid Journal   (Followers: 7, SJR: 0.878, h-index: 57)
Endocrine Pathology     Hybrid Journal   (Followers: 2, SJR: 0.638, h-index: 31)
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Energy Systems     Hybrid Journal   (Followers: 13, SJR: 1.176, h-index: 7)
Engineering With Computers     Hybrid Journal   (Followers: 6, SJR: 0.433, h-index: 30)
Entomological Review     Hybrid Journal   (Followers: 6, SJR: 0.144, h-index: 5)
Environment Systems & Decisions     Hybrid Journal   (Followers: 2)
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Environmental Biology of Fishes     Hybrid Journal   (Followers: 5, SJR: 0.725, h-index: 58)
Environmental Chemistry Letters     Hybrid Journal   (Followers: 2, SJR: 0.741, h-index: 28)
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Environmental Evidence     Open Access   (Followers: 1)
Environmental Fluid Mechanics     Hybrid Journal   (Followers: 3, SJR: 0.437, h-index: 24)
Environmental Geochemistry and Health     Hybrid Journal   (Followers: 3, SJR: 1.013, h-index: 36)
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Environmental Monitoring and Assessment     Hybrid Journal   (Followers: 23, SJR: 0.685, h-index: 52)
Environmental Science and Pollution Research     Hybrid Journal   (Followers: 13, SJR: 0.885, h-index: 46)
Epileptic Disorders     Hybrid Journal   (SJR: 0.608, h-index: 38)
EPJ A - Hadrons and Nuclei     Hybrid Journal   (SJR: 1.287, h-index: 63)
EPJ B - Condensed Matter and Complex Systems     Hybrid Journal   (Followers: 1, SJR: 0.731, h-index: 89)
EPJ direct     Hybrid Journal  
EPJ E - Soft Matter and Biological Physics     Hybrid Journal   (SJR: 0.641, h-index: 62)
EPMA J.     Open Access   (SJR: 0.284, h-index: 6)
ERA-Forum     Hybrid Journal   (Followers: 4, SJR: 0.128, h-index: 3)
Erkenntnis     Hybrid Journal   (Followers: 16, SJR: 0.621, h-index: 16)
Erwerbs-Obstbau     Hybrid Journal   (SJR: 0.206, h-index: 9)
Esophagus     Hybrid Journal   (SJR: 0.311, h-index: 10)

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Journal Cover Experiments in Fluids
  [SJR: 1.596]   [H-I: 69]   [7 followers]  Follow
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 1432-1114 - ISSN (Online) 0723-4864
   Published by Springer-Verlag Homepage  [2281 journals]
  • Non-intrusive calibration for three-dimensional particle imaging
    • Abstract: Abstract This letter introduces a non-intrusive calibration scheme for three-dimensional (3D) optical flow velocimetry techniques. For these 3D techniques, including tomographic PIV and 3D-PTV, calibration targets need to be imaged within the measurement volume at different depth positions. However, for domains with limited access and with small dimensions, it is difficult or impossible to place a calibration target. Therefore, a non-intrusive calibration approach is proposed to overcome these drawbacks, by employing light reflections of a continuous wave laser in the measurement domain. The laser is translated to different locations, yielding a set of calibration points, comprising the spatial coordinates of the light reflections and their corresponding sensor coordinates.
      PubDate: 2016-04-27
  • Shake-The-Box: Lagrangian particle tracking at high particle image
    • Abstract: Abstract A Lagrangian tracking method is introduced, which uses a prediction of the particle distribution for the subsequent time-step as a mean to seize the temporal domain. Errors introduced by the prediction process are corrected by an image matching technique (‘shaking’ the particle in space), followed by an iterative triangulation of particles newly entering the measurement domain. The scheme was termed ‘Shake-The-Box’ and previously characterized as ‘4D-PTV’ due to the strong interaction with the temporal dimension. Trajectories of tracer particles are identified at high spatial accuracy due to a nearly complete suppression of ghost particles; a temporal filtering scheme further improves on accuracy and allows for the extraction of local velocity and acceleration as derivatives of a continuous function. Exploiting the temporal information enables the processing of densely seeded flows (beyond 0.1 particles per pixel, ppp), which were previously reserved for tomographic PIV evaluations. While TOMO-PIV uses statistical means to evaluate the flow (building an ‘anonymous’ voxel space with subsequent spatial averaging of the velocity information using correlation), the Shake-The-Box approach is able to identify and track individual particles at numbers of tens or even hundreds of thousands per time-step. The method is outlined in detail, followed by descriptions of applications to synthetic and experimental data. The synthetic data evaluation reveals that STB is able to capture virtually all true particles, while effectively suppressing the formation of ghost particles. For the examined four-camera set-up particle image densities N I up to 0.125 ppp could be processed. For noise-free images, the attained accuracy is very high. The addition of synthetic noise reduces usable particle image density (N I ≤ 0.075 ppp for highly noisy images) and accuracy (still being significantly higher compared to tomographic reconstruction). The solutions remain virtually free of ghost particles. Processing an experimental data set on a transitional jet in water demonstrates the benefits of advanced Lagrangian evaluation in describing flow details—both on small scales (by the individual tracks) and on larger structures (using an interpolation onto an Eulerian grid). Comparisons to standard TOMO-PIV processing for synthetic and experimental evaluations show distinct benefits in local accuracy, completeness of the solution, ghost particle occurrence, spatial resolution, temporal coherence and computational effort.
      PubDate: 2016-04-27
  • Low hazard refractive index and density-matched fluid for quantitative
           imaging of concentrated suspensions of particles
    • Abstract: Abstract A novel refractive index and density-matched liquid–solid suspension system taking into account chemical hazard and health concerns was developed and characterized. The solid phase is made of PMMA spheres, the refractive index of which being adapted with a mixture of 2,2′-thiodiethanol and phosphate-buffered saline (PBS), while the density is adapted with a mixture of PBS and glycerol. The proposed chemicals present low hazard characteristics in comparison with former solutions. Data collected from density and refractive index measurements of the solid phase and of the different fluid constituents are used to define a specific ternary mixture adapted to commercial grade micron-size particles. The defined mixture is validated in a micron-sized granular flow experiment. The described method can be applied to other low-density solids.
      PubDate: 2016-04-27
  • Effects of surface-active impurities on the liquid bridge dynamics
    • Abstract: Abstract We examine experimentally the effects of surface-active impurities on the small-amplitude free oscillations of axisymmetric liquid bridges. The surface tension, oscillation frequency, and damping rate are measured at different instants from the free surface formation. The experiments with n-hexadecane and n-dodecane show that none of these interfacial quantities is significantly affected by the free surface age. The damping rates exceed by a \(\mathcal{O}(1)\) quantity their corresponding values for a clean free surface. This extra-damping can be modeled in terms of the monolayer shear viscosity exclusively. Similar values of this quantity are obtained for n-hexadecane and n-dodecane, although the impurity effects on the surface tension are much greater in the first case. We conducted experiments with deionized water liquid bridges to analyze the impurity effects on free surfaces with high elasticity numbers, where Marangoni convection is expected to increase the monolayer dissipation. In this case, the damping rates are up to three times as those of a clean free surface. The monolayer dissipative effects do not increase as the free surface ages, although the surface tension decreases considerably during this process. Similar impurity effects are observed when an anionic surfactant is dissolved in deionized water. For a fixed value of the liquid bridge slenderness, both the oscillation frequency and damping rate are functions of the liquid bridge volume exclusively, independently of the free surface age and the liquid-ambient combination. Extra-dissipation increases sharply as the liquid bridge volume decreases.
      PubDate: 2016-04-26
  • Transition along a finite-length cylinder in the presence of a thin
           boundary layer
    • Abstract: Abstract This work aims to investigate experimentally the transition of the aerodynamic forces on a cantilevered circular cylinder immersed in a thin boundary layer whose thickness is comparable to the cylinder diameter d. The aspect ratio H/d of the cylinder is 5, where H is the cylinder height. The Reynolds number Re, based on the freestream velocity (U ∞ ) and d, is varied from 0.68 × 105 to 6.12 × 105, covering the subcritical, critical and supercritical regimes. It has been found that the flow transition is non-uniform along the cylinder span, taking place at a smaller Re near the cylinder free end than near the base. Furthermore, the sectional drag coefficient of the cantilevered cylinder is smaller relative to that of a two-dimensional cylinder in the subcritical regime, but larger than the later in the supercritical regime. The sectional lift coefficient is not zero in the critical regime, with its maximum near the free end reaching almost four times of that near the base.
      PubDate: 2016-04-23
  • Quad-plane stereoscopic PIV for fine-scale structure measurements in
    • Abstract: Abstract The fine-scale structure in turbulence is investigated by quad-plane stereoscopic particle image velocimetry (QPSPIV). The quad-plane consists of two each of different polarizations and wavelengths, and it provides three velocity components at four independent parallel planes. Measurements have been undertaken in the developed region of a turbulent round jet with a spatial resolution sufficient to capture the small-scale structures. The advantage of the QPSPIV is presented in terms of the spectral response in the evaluation of the out-of-plane velocity gradient. The full velocity gradient tensor is computed with a fourth-order finite difference scheme in the out-of-plane direction as well as the in-plane directions. The turbulence quantities, such as the vorticity components, the energy dissipation rate and the second and third invariants of the velocity gradient tensor, are computed according to their faithful definitions. The coherent fine-scale eddies are extracted from the present QPSPIV data. The probability density functions of the diameter and the maximum azimuthal velocity of the extracted eddies exhibit their peak at approximately \(8\eta \) and \(1.5u_k\) , respectively, where \(\eta \) and \(u_k\) are the Kolmogorov length and velocity. These values agree well with the data in the literature. The phase-averaged distributions of turbulence quantities around the coherent fine-scale eddy indicate an apparent elliptic feature around the axis. Furthermore, the state of the strain rate exerting the eddy is quantified from the phase-averaged distributions of eigenvalues of the strain rate tensor and the alignment of the corresponding eigenvectors against the axis. The present study gives a solid experimental support of the coherent fine-scale structures in turbulence, and the technique can be applied to various flow fields and to the higher Reynolds number condition.
      PubDate: 2016-04-16
  • The effect of ambient pressure on ejecta sheets from free-surface ablation
    • Abstract: Abstract We present observations from an experimental study of the ablation of a free liquid surface promoted by a focused laser pulse, causing a rapid discharge of liquid in the form of a very thin conical-shaped sheet. In order to capture the dynamics, we employ a state-of-the-art ultra-high-speed video camera capable of capturing events at \(5 \times 10^{6}\) fps with shutter speeds down to 20 ns, whereby we were able to capture not only the ejecta sheet, but also the shock wave, emerging at speeds of up to 1.75 km/s, which is thus found to be hypersonic (Mach 5). Experiments were performed at a range of ambient pressures in order to study the effect of air drag on the evolution of the sheet, which was always observed to dome over, even at pressures as low as 3.8 kPa. At reduced pressures, the extended sheet evolution led to the formation of interference fringe patterns from which, by comparison with the opening speed of rupture, we were able to determine the ejecta thickness.
      PubDate: 2016-04-16
  • Simultaneous 10 kHz TPIV, OH PLIF, and CH 2 O PLIF measurements of
           turbulent flame structure and dynamics
    • Abstract: Abstract Simultaneous 10 kHz repetition-rate tomographic particle image velocimetry, hydroxyl planar laser-induced fluorescence (OH PLIF), and formaldehyde (CH \(_2\) O) PLIF were used to study the structure and dynamics of turbulent premixed flames. The flames investigated span from the classically defined corrugated flamelet regime to conditions at which broadened and/or broken flamelets are expected. Methods are presented for determining 3D flame topologies from the Mie scattering tomography and for tracking features through space and time using theoretical Lagrangian particles. Substantial broadening of the CH \(_2\) O region is observed with increasing turbulence intensity. However, OH production remains rapid, and the region of OH and CH \(_2\) O overlap remains thin. Local flame speeds exceeding three times the laminar flame speed are observed in regions of flame–flame interaction. Furthermore, a method of tracking fluid residence time within the CH \(_2\) O layer is presented and shows that residence time decreases at higher turbulence intensity despite the broader distribution of the CH \(_2\) O, indicating an increase in local reaction rate.
      PubDate: 2016-04-16
  • Free-stream static pressure measurements in the Longshot hypersonic wind
           tunnel and sensitivity analysis
    • Abstract: Abstract The performance of fast-response slender static pressure probes is evaluated in the short-duration, cold-gas, VKI Longshot hypersonic wind tunnel. Free-stream Mach numbers range between 9.5 and 12, and unit Reynolds numbers are within 3–10 × 106/m. Absolute pressure sensors are fitted within the probes, and an inexpensive calibration method, suited to low static pressure environments (200–1000 Pa), is described. Transfer functions relating the probe measurements p w to the free-stream static pressure p ∞ are established for the Longshot flow conditions based on numerical simulations. The pressure ratios p w /p ∞ are found to be close to unity for both laminar and turbulent boundary layers. Weak viscous effects characterized by small viscous interaction parameters \({\bar{\chi }}<1.5\) are confirmed experimentally for probe aspect ratios of L/D > 16.5 by installing multiple pressure sensors in a single probe. The effect of pressure orifice geometry is also evaluated experimentally and found to be negligible for either straight or chamfered holes, 0.6–1 mm in diameter. No sensitivity to probe angle of attack could be evidenced for α < 0.33°. Pressure measurements are compared to theoretical predictions assuming an isentropic nozzle flow expansion. Significant deviations from this ideal case and the Mach 14 contoured nozzle design are uncovered. Validation of the static pressure measurements is obtained by comparing shock wave locations on Schlieren photographs to numerical predictions using free-stream properties derived from the static pressure probes. While these results apply to the Longshot wind tunnel, the present methodology and sensitivity analysis can guide similar investigations for other hypersonic test facilities.
      PubDate: 2016-04-16
  • Krypton tagging velocimetry in a turbulent Mach 2.7 boundary layer
    • Abstract: Abstract The krypton tagging velocimetry (KTV) technique is applied to the turbulent boundary layer on the wall of the “Mach 3 Calibration Tunnel” at Arnold Engineering Development Complex (AEDC) White Oak. Profiles of velocity were measured with KTV and Pitot-pressure probes in the Mach 2.7 turbulent boundary layer comprised of 99 % \(\hbox {N}_{2}\) /1 % Kr at momentum-thickness Reynolds numbers of \({Re}_{\varTheta }= 800, 1400\) , and 2400. Agreement between the KTV- and Pitot-derived velocity profiles is excellent. The KTV and Pitot velocity data follow the law of the wall in the logarithmic region with application of the Van Driest I transformation. The velocity data are analyzed in the outer region of the boundary layer with the law of the wake and a velocity-defect law. KTV-derived streamwise velocity fluctuation measurements are reported and are consistent with data from the literature. To enable near-wall measurement with KTV ( \(y/\delta \approx \)  0.1–0.2), an 800-nm longpass filter was used to block the 760.2-nm read-laser pulse. With the longpass filter, the 819.0-nm emission from the re-excited Kr can be imaged to track the displacement of the metastable tracer without imaging the reflection and scatter from the read-laser off of solid surfaces. To operate the Mach 3 AEDC Calibration Tunnel at several discrete unit Reynolds numbers, a modification was required and is described herein.
      PubDate: 2016-04-16
  • Characterization of a highly efficient chevron-shaped anti-contamination
    • Abstract: Abstract This paper is devoted to the characterization of an optimized chevron-shaped anti-contamination device (ACD). This device can prevent efficiently the propagation of turbulence from the fuselage along the attachment line (hypothetical streamline that spreads the flow going to suction side and the one going to pressure side) of swept wings and enables the development of a new laminar boundary layer downstream. More specifically, the aim is to prevent boundary-layer transition along the attachment line by a contamination process. This process is characterized by the typical Reynolds number \(\overline{R}\) and the associated Poll’s criterion. Thus, ACD efficiency will be expressed in terms of \(\overline{R}\) values. Some experiments performed on a new numerically optimized ACD have shown its ability to prevent leading-edge contamination up to \(\overline{R}\) values close to the natural transition process of the laminar boundary layer along the attachment line. The corresponding stability analysis of the laminar boundary layer is made using the Görtler–Hämmerlin stability approach. The study is completed with the different transition processes that can occur downstream the attachment line, around the airfoil, especially with crossflow analysis.
      PubDate: 2016-04-13
  • The saturation of the fluorescence and its consequences for laser-induced
           fluorescence thermometry in liquid flows
    • Abstract: Abstract The temperature dependence of the fluorescence emission of certain organic dyes such as rhodamine B has been widely utilized for measuring the temperature in liquid flows. Measurements are generally based on two assumptions: The fluorescence signal is proportional to the intensity of the laser excitation, and the temperature sensitivity of the dye is not affected by the laser irradiance. In the ratiometric methods, these assumptions allow justifying that the influence of the laser intensity can be totally eliminated in the intensity ratio of two spectral bands of the fluorescence emission and thus that measurements can be taken with no biases under experimental conditions, where the laser propagation is disturbed by the flow. However, when pulsed lasers are used (mainly in planar LIF measurements), the peak irradiance usually compares or exceeds the saturation intensity of the dyes. The present study assesses the consequences of a saturation of the dye emission on temperature measurements. Tests among fluoresceins and rhodamines reveal that the saturation can be accompanied by a significant loss of temperature sensitivity. The dyes, for which this loss of sensitivity is observed, mainly owe their temperature dependence to the fluorescence quantum yield and have a fluorescence signal decreasing with the temperature. The couple fluorescein/sulforhodamine 640 is finally proposed for an implementation of the ratiometric method, since its relatively high temperature dependence (+3 %/ \({}^\circ {\mathrm{C}}\) ) is not altered at high laser irradiances. The possibility of measuring instantaneous temperature fields with this pair of dyes using a single laser shot is finally demonstrated on a turbulent heated jet injected into quiescent water.
      PubDate: 2016-04-13
  • Experimental characterization of turbulent subsonic
           transitional–open cavity flow
    • Abstract: Abstract Turbulent subsonic “transitional–open” cavity flow was investigated by wind-tunnel tests. The investigated cavity configuration had a length-to-depth ratio of 6.25 and a width-to-depth ratio of 2. The cavity was exposed to a free-stream Mach number of 0.40 and a Reynolds number (based on cavity depth) of \(1.6\times 10^6\) , with a turbulent incoming boundary layer. Measurements of velocity and wall pressures were taken simultaneously, which enabled the analysis of velocity–pressure cross-correlations. Special attention is paid to the shear layer that develops over the cavity and an emphasis is placed on the analysis of its characteristics and its stability. Application of linear hydrodynamic stability theory, together with examining velocity–pressure cross correlations, revealed that the behavior of the cavity shear layer is analogous to a free shear layer, approximately up to mid-length of the cavity, where further downstream nonlinear interactions occur.
      PubDate: 2016-04-13
  • More investigations in capillary fluidics using a drop tower
    • Abstract: Abstract A variety of contemplative demonstrations concerning intermediate-to-large length scale capillary fluidic phenomena were made possible by the brief weightless environment of a drop tower (Wollman and Weislogel in Exp Fluids 54(4):1, 2013). In that work, capillarity-driven flows leading to unique spontaneous droplet ejections, bubble ingestions, and multiphase flows were introduced and discussed. Such efforts are continued herein. The spontaneous droplet ejection phenomena (auto-ejection) is reviewed and demonstrated on earth as well as aboard the International Space Station. This technique is then applied to novel low-g droplet combustion where soot tube structures are created in the wakes of burning drops. A variety of new tests are presented that routinely demonstrate ‘puddle jumping,’ a process defined as the spontaneous recoil and ejection of large liquid drops from hydrophobic surfaces following the step reduction in ‘gravity’ characteristic of most drop towers. The inverse problem of ‘bubble jumping’ is also demonstrated for the case of hydrophilic surfaces. A variety of puddle jump demonstrations are presented in summary as a means of suggesting the further exploitation of drop towers to study such large length scale capillary phenomena.
      PubDate: 2016-03-30
  • Dynamic responses of asymmetric vortices over slender bodies to a rotating
           tip perturbation
    • Abstract: Abstract The dynamic responses of asymmetric vortices over a slender body to a rotating tip perturbation were investigated experimentally in a wind tunnel. A small rotating nose with an artificial micro-perturbation on the nose tip was driven by a servomotor with various rates to change azimuthal locations of tip perturbation. Wall pressures and spatial velocity fields were measured using pressure scanner and particle image velocimetry based on a phase-locked method. The results show that the spinning tip perturbation enables asymmetric vortices to exhibit significantly dynamic characteristics different from a case with a static perturbation. The orientations of asymmetric vortices and associated side forces show apparent phase delay that are enlarged with increasing rotational rates of the nose. The switching rates of asymmetric vortices among various orientations also increase with the rotational rates increasing, but asymmetry level of vortices is lowered, which reveals that the asymmetric vortices change requires an amount of time to switch from one orientation to another. The phase delays of vortices, however, are determined by the amount of time required for the propagation of disturbance waves along a body axis. As the rotational frequencies are sufficiently high, the orientations of vortices almost hold to be unchanged. The unchanged orientation of vortices is asymmetric, depending on the directions of rotation. The asymmetric vortices arising from high-frequency rotation of the nose are attributed to wall effects induced by the rotating nose with a finite length. In addition, there exist small intrinsic vortex oscillations which are superimposed on the average vortex structures with symmetric and asymmetric orientations for the cases of static and rotational tip perturbations.
      PubDate: 2016-03-30
  • Measurement of the droplets sizes of a flash boiling spray using an
           improved extended glare point velocimetry and sizing
    • Abstract: Abstract An improved extended glare point velocimetry and sizing (EGPVS) is proposed to investigate the droplets sizes of a flash boiling spray. When a spherical droplet with a relative refractive index from 1.16 to 1.41 is illuminated by two opposite laser sheets and a charge-coupled device camera is used to collect the s-polarization light at an observation angle of 90°, the intensities of the reflected lights are much stronger than the other order scattering lights. If the intensity of incident laser is controlled appropriately, two glare points from the reflected lights for the droplet are formed at the focused plane, while the intensities of the other order scattering lights are too weak to form any glare points. Then, the droplet diameter can be derived from the distance between the two glare points. In addition, the focused image is relative small, making it possible to measure dense spray. First, the characteristics of the improved EGPVS are discussed, and a series of standard particles are measured for validating this technique. Then, the technique is applied to investigate the droplets sizes of flash boiling spray. It is found that the minimum measurable diameter of droplets is 7.1 μm, and the relative error is less than 4.7 %. The droplet size distributions of spray are different at different stages. The Sauter mean diameter (SMD) of gasoline spray decreases gradually as the fuel temperature increases, which is different from that of a single-component fuel with a sharp decrease in SMD at the flash boiling stage.
      PubDate: 2016-03-30
  • Study of bubble-induced turbulence in upward laminar bubbly pipe flows
           measured with a two-phase particle image velocimetry
    • Abstract: Abstract In the present study, focusing on characterizing the bubble-induced agitation (turbulence), spatially varying flow statistics of gas and liquid phases in laminar upward bubbly flows (Reynolds number of 750) with varying mean void fraction are investigated using a two-phase high-speed particle image velocimetry. As the flow develops along the vertical direction, bubbles with small-to-moderate void fractions, which were intentionally distributed asymmetrically at the inlet, migrate fast and show symmetric distributions of wall or intermediate peaking. Meanwhile, the mean liquid velocity saturates relatively slowly to a flat distribution at the core region. Despite small void fractions considered, the bubbles generate a substantial turbulence, which increases with increasing mean void fraction. Interestingly, it is found that the mean vertical velocity, bubble-induced normal stress in radial direction, and Reynolds stress profiles match well with those of a single-phase turbulent flow at a moderate Reynolds number (e.g., 104), indicating the similarity between the bubble-induced turbulence and wall-shear-generated turbulence in a single-phase flow. Previously suggested scaling relations are confirmed such that the mean bubble rise velocity and bubble-induced normal stress (in both vertical and radial directions) scale with mean volume void fraction as a power of −0.1 and 0.4, respectively. Finally, based on the analysis of measured bubble dynamics (rise in an oscillating path), a theoretical model for two-phase turbulent (Reynolds) stress is proposed, which includes the contributions by the non-uniform distributions of local void fraction and relative bubble rise velocity, and is further validated with the present experimental data to show a good agreement with each other.
      PubDate: 2016-03-30
  • Pressure estimation from single-snapshot tomographic PIV in a turbulent
           boundary layer
    • Abstract: Abstract A method is proposed to determine the instantaneous pressure field from a single tomographic PIV velocity snapshot and is applied to a flat-plate turbulent boundary layer. The main concept behind the single-snapshot pressure evaluation method is to approximate the flow acceleration using the vorticity transport equation. The vorticity field calculated from the measured instantaneous velocity is advanced over a single integration time step using the vortex-in-cell (VIC) technique to update the vorticity field, after which the temporal derivative and material derivative of velocity are evaluated. The pressure in the measurement volume is subsequently evaluated by solving a Poisson equation. The procedure is validated considering data from a turbulent boundary layer experiment, obtained with time-resolved tomographic PIV at 10 kHz, where an independent surface pressure fluctuation measurement is made by a microphone. The cross-correlation coefficient of the surface pressure fluctuations calculated by the single-snapshot pressure method with respect to the microphone measurements is calculated and compared to that obtained using time-resolved pressure-from-PIV, which is regarded as benchmark. The single-snapshot procedure returns a cross-correlation comparable to the best result obtained by time-resolved PIV, which uses a nine-point time kernel. When the kernel of the time-resolved approach is reduced to three measurements, the single-snapshot method yields approximately 30 % higher correlation. Use of the method should be cautioned when the contributions to fluctuating pressure from outside the measurement volume are significant. The study illustrates the potential for simplifying the hardware configurations (e.g. high-speed PIV or dual PIV) required to determine instantaneous pressure from tomographic PIV.
      PubDate: 2016-03-18
  • Translational and rotational dynamics of a large buoyant sphere in
    • Abstract: Abstract We report experimental measurements of the translational and rotational dynamics of a large buoyant sphere in isotropic turbulence. We introduce an efficient method to simultaneously determine the position and (absolute) orientation of a spherical body from visual observation. The method employs a minimization algorithm to obtain the orientation from the 2D projection of a specific pattern drawn onto the surface of the sphere. This has the advantages that it does not require a database of reference images, is easily scalable using parallel processing, and enables accurate absolute orientation reference. Analysis of the sphere’s translational dynamics reveals clear differences between the streamwise and transverse directions. The translational autocorrelations and PDFs provide evidence for periodicity in the particle’s dynamics even under turbulent conditions. The angular autocorrelations show weak periodicity. The angular accelerations exhibit wide tails, however without a directional dependence.
      PubDate: 2016-03-16
  • Characterizing a burst leading-edge vortex on a rotating flat plate wing
    • Abstract: Abstract Identifying, characterizing, and tracking incoherent vortices in highly separated flows is of interest for the development of new low-order models for unsteady lift prediction. The current work examines several methods to identify vortex burst and characterize a burst leading-edge vortex. Time-resolved stereoscopic PIV was performed on a rotating flat plate wing at Re = 2500. The burst process was found to occur at mid-span and is characterized by axial flow reversal, the entrainment of opposite-sign vorticity, and a rapid expansion of vortex size. A POD analysis revealed that variations in certain mode coefficients are indicative of the flow state changes characteristics of burst. During burst, the leading-edge vortex evolves to a region of inhomogeneous vorticity distributed over a large area. Several methods of defining the vortex size and circulation are evaluated and a combination of these can be used to characterize the leading-edge vortex both pre- and post-burst.
      PubDate: 2016-03-16
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Edinburgh, EH14 4AS, UK
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