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Publisher: Springer-Verlag   (Total: 2210 journals)

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Dynamic Games and Applications     Hybrid Journal   (Followers: 2)
Dysphagia     Hybrid Journal   (Followers: 218, SJR: 0.684, h-index: 46)
e & i Elektrotechnik und Informationstechnik     Hybrid Journal   (Followers: 8, SJR: 0.146, h-index: 8)
e-Neuroforum     Hybrid Journal  
Early Childhood Education J.     Hybrid Journal   (Followers: 13, SJR: 0.367, h-index: 12)
Earth Science Informatics     Hybrid Journal   (Followers: 3, SJR: 0.245, h-index: 5)
Earth, Moon, and Planets     Hybrid Journal   (Followers: 5, SJR: 0.436, h-index: 28)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7, SJR: 0.433, h-index: 17)
Earthquake Science     Hybrid Journal   (Followers: 8, SJR: 0.486, h-index: 7)
East Asia     Hybrid Journal   (Followers: 7, SJR: 0.165, h-index: 9)
Eating and Weight Disorders - Studies on Anorexia, Bulimia and Obesity     Hybrid Journal   (Followers: 8, SJR: 0.289, h-index: 23)
EcoHealth     Hybrid Journal   (Followers: 1, SJR: 0.651, h-index: 22)
Ecological Research     Hybrid Journal   (Followers: 8, SJR: 0.698, h-index: 38)
Economic Botany     Hybrid Journal   (Followers: 9, SJR: 0.666, h-index: 40)
Economic Bulletin     Hybrid Journal   (Followers: 4)
Economic Change and Restructuring     Hybrid Journal   (Followers: 1, SJR: 0.263, h-index: 6)
Economic Theory     Hybrid Journal   (Followers: 8, SJR: 1.857, h-index: 31)
Economic Theory Bulletin     Hybrid Journal   (Followers: 1)
Economics of Governance     Hybrid Journal   (Followers: 2, SJR: 0.367, h-index: 12)
Ecosystems     Hybrid Journal   (Followers: 19, SJR: 1.793, h-index: 83)
Ecotoxicology     Hybrid Journal   (Followers: 10, SJR: 1.041, h-index: 53)
Education and Information Technologies     Hybrid Journal   (Followers: 216, SJR: 0.207, h-index: 15)
Educational Assessment, Evaluation and Accountability     Hybrid Journal   (Followers: 16, SJR: 0.519, h-index: 14)
Educational Psychology Review     Hybrid Journal   (Followers: 15, SJR: 1.781, h-index: 52)
Educational Research for Policy and Practice     Hybrid Journal   (Followers: 7, SJR: 0.211, h-index: 8)
Educational Studies in Mathematics     Hybrid Journal   (Followers: 10, SJR: 0.946, h-index: 27)
Educational Technology Research and Development     Partially Free   (Followers: 205, SJR: 1.124, h-index: 45)
Electrical Engineering     Hybrid Journal   (Followers: 12, SJR: 0.352, h-index: 17)
Electrocatalysis     Hybrid Journal   (SJR: 0.542, h-index: 7)
Electronic Commerce Research     Hybrid Journal   (Followers: 4, SJR: 0.636, h-index: 14)
Electronic Markets     Hybrid Journal   (Followers: 6, SJR: 0.326, h-index: 5)
Electronic Materials Letters     Hybrid Journal   (Followers: 3, SJR: 0.566, h-index: 11)
Elemente der Mathematik     Hybrid Journal   (Followers: 1)
Emergency Radiology     Hybrid Journal   (Followers: 4, SJR: 0.446, h-index: 22)
Empirica     Hybrid Journal   (Followers: 3, SJR: 0.185, h-index: 12)
Empirical Economics     Hybrid Journal   (Followers: 8, SJR: 0.5, h-index: 29)
Empirical Software Engineering     Hybrid Journal   (Followers: 4, SJR: 2.319, h-index: 33)
Employee Responsibilities and Rights J.     Hybrid Journal   (Followers: 2, SJR: 0.21, h-index: 13)
Endocrine     Hybrid Journal   (Followers: 5, SJR: 0.659, h-index: 55)
Endocrine Pathology     Hybrid Journal   (Followers: 2, SJR: 0.555, h-index: 27)
Energy Efficiency     Hybrid Journal   (Followers: 11, SJR: 1.056, h-index: 10)
Energy Systems     Hybrid Journal   (Followers: 10, SJR: 0.589, h-index: 5)
Engineering With Computers     Hybrid Journal   (Followers: 5, SJR: 0.497, h-index: 26)
Entomological Review     Hybrid Journal   (Followers: 3, SJR: 0.128, h-index: 5)
Environment Systems & Decisions     Hybrid Journal   (Followers: 2)
Environment, Development and Sustainability     Hybrid Journal   (Followers: 28, SJR: 0.319, h-index: 26)
Environmental and Ecological Statistics     Hybrid Journal   (Followers: 5, SJR: 0.389, h-index: 29)
Environmental and Resource Economics     Hybrid Journal   (Followers: 17, SJR: 1.651, h-index: 46)
Environmental Biology of Fishes     Hybrid Journal   (Followers: 4, SJR: 0.486, h-index: 53)
Environmental Chemistry Letters     Hybrid Journal   (Followers: 2, SJR: 0.664, h-index: 22)
Environmental Earth Sciences     Hybrid Journal   (Followers: 10, SJR: 0.601, h-index: 55)
Environmental Economics and Policy Studies     Hybrid Journal   (Followers: 6, SJR: 0.35, h-index: 3)
Environmental Evidence     Open Access  
Environmental Fluid Mechanics     Hybrid Journal   (Followers: 2, SJR: 0.732, h-index: 23)
Environmental Geochemistry and Health     Hybrid Journal   (Followers: 2, SJR: 0.909, h-index: 32)
Environmental Geology     Hybrid Journal   (Followers: 11)
Environmental Health and Preventive Medicine     Hybrid Journal   (Followers: 2, SJR: 0.388, h-index: 14)
Environmental Management     Hybrid Journal   (Followers: 32, SJR: 0.773, h-index: 60)
Environmental Modeling & Assessment     Hybrid Journal   (Followers: 10, SJR: 0.413, h-index: 27)
Environmental Monitoring and Assessment     Hybrid Journal   (Followers: 8, SJR: 0.671, h-index: 46)
Environmental Science and Pollution Research     Hybrid Journal   (Followers: 14, SJR: 0.878, h-index: 42)
Epidemiologic Perspectives & Innovations     Open Access   (Followers: 2, SJR: 1.002, h-index: 14)
Epileptic Disorders     Hybrid Journal   (Followers: 1, SJR: 0.669, h-index: 34)
EPJ A - Hadrons and Nuclei     Hybrid Journal   (Followers: 1, SJR: 1.435, h-index: 58)
EPJ B - Condensed Matter and Complex Systems     Hybrid Journal   (Followers: 3, SJR: 0.749, h-index: 85)
EPJ direct     Hybrid Journal  
EPJ E - Soft Matter and Biological Physics     Hybrid Journal   (Followers: 1, SJR: 0.661, h-index: 57)
EPMA J.     Open Access   (SJR: 0.161, h-index: 4)
ERA-Forum     Hybrid Journal   (Followers: 2, SJR: 0.13, h-index: 2)
Erkenntnis     Hybrid Journal   (Followers: 11, SJR: 0.62, h-index: 14)
Erwerbs-Obstbau     Hybrid Journal   (SJR: 0.173, h-index: 8)
Esophagus     Hybrid Journal   (SJR: 0.268, h-index: 9)
Estuaries and Coasts     Hybrid Journal   (Followers: 4, SJR: 1.111, h-index: 61)
Ethical Theory and Moral Practice     Hybrid Journal   (Followers: 7, SJR: 0.278, h-index: 8)
Ethics and Information Technology     Hybrid Journal   (Followers: 208, SJR: 0.363, h-index: 20)
Ethik in der Medizin     Hybrid Journal   (SJR: 0.204, h-index: 6)
Euphytica     Hybrid Journal   (Followers: 7, SJR: 0.709, h-index: 57)
Eurasian Soil Science     Hybrid Journal   (Followers: 2, SJR: 0.271, h-index: 10)
EURO J. of Transportation and Logistics     Hybrid Journal   (Followers: 4)
EURO J. on Computational Optimization     Hybrid Journal  
EURO J. on Decision Processes     Hybrid Journal  
Europaisches J. fur Minderheitenfragen     Hybrid Journal  
European Actuarial J.     Hybrid Journal   (Followers: 3)
European Archives of Oto-Rhino-Laryngology     Hybrid Journal   (Followers: 4, SJR: 0.737, h-index: 37)
European Archives of Paediatric Dentistry     Hybrid Journal   (Followers: 1, SJR: 0.446, h-index: 12)
European Archives of Psychiatry and Clinical Neuroscience     Hybrid Journal   (Followers: 2, SJR: 1.334, h-index: 62)
European Biophysics J.     Hybrid Journal   (Followers: 4, SJR: 0.979, h-index: 53)
European Child & Adolescent Psychiatry     Hybrid Journal   (Followers: 5, SJR: 1.269, h-index: 51)
European Clinics in Obstetrics and Gynaecology     Hybrid Journal   (Followers: 5)
European Food Research and Technology     Hybrid Journal   (Followers: 8, SJR: 0.773, h-index: 49)
European J. for Education Law and Policy     Hybrid Journal   (Followers: 6)
European J. for Philosophy of Science     Partially Free   (Followers: 4, SJR: 0.165, h-index: 2)
European J. of Ageing     Hybrid Journal   (Followers: 10, SJR: 0.49, h-index: 17)
European J. of Applied Physiology     Hybrid Journal   (Followers: 8, SJR: 1.044, h-index: 74)
European J. of Clinical Microbiology & Infectious Diseases     Hybrid Journal   (Followers: 11, SJR: 0.958, h-index: 74)
European J. of Clinical Pharmacology     Hybrid Journal   (Followers: 9, SJR: 0.916, h-index: 69)
European J. of Dermatology     Hybrid Journal   (Followers: 7)
European J. of Drug Metabolism and Pharmacokinetics     Hybrid Journal   (Followers: 7, SJR: 0.24, h-index: 25)
European J. of Epidemiology     Hybrid Journal   (Followers: 17, SJR: 1.946, h-index: 60)
European J. of Forest Research     Hybrid Journal   (Followers: 3, SJR: 0.864, h-index: 25)

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Journal Cover Experiments in Fluids     [SJR: 1.033]   [H-I: 62]
   [8 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  [2210 journals]
  • Length and time for development of laminar flow in tubes following a step
           increase of volume flux
    • Abstract: Laminar flows starting up from rest in round tubes are relevant to numerous industrial and biomedical applications. The two most common types are flows driven by an abruptly imposed constant pressure gradient or by an abruptly imposed constant volume flux. Analytical solutions are available for transient, fully developed flows, wherein streamwise development over the entrance length is absent (Szymanski in J de Mathématiques Pures et Appliquées 11:67–107, 1932; Andersson and Tiseth in Chem Eng Commun 112(1):121–133, 1992, respectively). They represent the transient responses of flows in tubes that are very long compared with the entrance length, a condition that is seldom satisfied in biomedical tube networks. This study establishes the entrance (development) length and development time of starting laminar flow in a round tube of finite length driven by a piston pump that produces a step change from zero flow to a constant volume flux for Reynolds numbers between 500 and 3,000. The flows are examined experimentally, using stereographic particle image velocimetry and computationally using computational fluid dynamics, and are then compared with the known analytical solutions for fully developed flow conditions in infinitely long tubes. Results show that step function volume flux start-up flows reach steady state and fully developed flow five times more quickly than those driven by a step function pressure gradient, a 500 % change when compared with existing estimates. Based on these results, we present new, simple guidelines for achieving experimental flows that are fully developed in space and time in realistic (finite) tube geometries. To a first approximation, the time to achieve steady spatially developing flow is nearly equal to the time needed to achieve steady, fully developed flow. Conversely, the entrance length needed to achieve fully developed transient flow is approximately equal to the length needed to achieve fully developed steady flow. Beyond this level of description, the numerical results reveal interaction between the effects of space and time development and nonlinear Reynolds number effects.
      PubDate: 2015-01-14
       
  • Flow dynamics of multi-lateral jets injection into a round pipe flow
    • Abstract: Controlling the mixing field of turbulent jets is an important approach in optimizing practical combustion systems. The use of multi-lateral jets upstream from the nozzle exit to control mixing fields is one particular method. Existing studies have investigated jets into a confined cross-flow (JICCF) for dilution mixing, but there is a paucity of data available on the fundamentals for turbulent mixing capabilities of JICCF. The current study investigates the flow structures and Primary Reynolds number mixing characteristics within a round pipe flow modified by four equi-spaced, lateral side injectors. Experiments are conducted in a primary water jet flow that is modified with smaller jets located one central (axial) jet diameter upstream of the nozzle exit. Flow structures and mixing within the nozzle are non-intrusively characterized using simultaneous planar optical techniques. Planar laser-induced fluorescence is used to measure the scalar mixing of the side and axial jet streams, and particle imaging velocimetry is used to measure the planar velocities. Several cases are investigated with variable primary flow to explore the influence of cross-flow velocity on the induced mixing structures within the nozzle. By varying the momentum ratio, three characteristic flow modes are identified within the primary flow, namely streaming mode, impinging mode, and backflow mode. The impact of these modes on the flow and scalar fields is presented and discussed.
      PubDate: 2015-01-13
       
  • Motion of an isolated liquid plug inside a capillary tube: effect of
           contact angle hysteresis
    • Abstract: Dynamics of a single, small and isolated partially wetting liquid plug (of known length L and wettability), placed at rest inside a long, dry, circular capillary tube (D = 1.5 mm), and subsequently quasi-statically pushed from one end by applying air pressure, the other end being kept exposed to atmosphere, are reported. The air pressure first overcomes the ‘static’ friction manifested by the three-phase contact line at the advancing and receding menisci, and then, the plug motion gets initiated, eventually leading to a terminal velocity (Ca ~ 2.8 × 10−5), when pressure force balances net frictional resistance due to viscous and surface forces. It is seen that, under steady motion, the curvature profiles of the advancing and receding menisci of liquid plug, respectively, remain the same, independent of the plug length. Steady-state pressure drop is dominated by the contribution due to contact angle hysteresis, which is also independent of the plug length. Increasing the system wettability drastically decreased the contact angle hysteresis and the associated net pressure drop.
      PubDate: 2015-01-13
       
  • Wing–vortex interaction: unraveling the flowfield of a hovering
           rotor
    • Abstract: This paper focuses on one of the most prominent flow features of the hovering rotor wake, the close interaction of the tip vortex with a following blade. Such vortex interactions are fundamental determinants of rotor performance, loads, and noise. Yet, they are not completely understood, largely due to the lack of sufficiently comprehensive experimental data. The present study aims to perform such comprehensive measurements, not on hovering helicopter rotors (which hugely magnifies test complexity) but using fixed-wing models in controlled wind tunnel tests. The experiments were designed to measure, in considerable detail, the aerodynamic loading resulting from a vortex interacting with a semi-span wing, as well as the wake resulting from that interaction. The goal of the present study is to answer fundamental questions such as (a) the influence of a vortex passing below a wing on the lift, drag, tip vortex, and the wake of that wing and (b) the strength of the forming tip vortex and its relation to the wing loading and/or the tip loading. This paper presents detailed wing surface pressure measurements that result from the interaction of the wing with an interacting vortex trailing from an upstream wing. The data show large lift distribution changes for a range of wing–vortex interactions including the effects of close encounter with the vortex core. Significant asymmetry in the vortex-induced lift loading was observed, with the increase in wing sectional lift outboard of the interacting vortex (closer to the tip) being much smaller than the corresponding decrease inboard of the vortex.
      PubDate: 2015-01-13
       
  • Flow quality experiment in a tandem nozzle wind tunnel at Mach 3
    • Abstract: In this study, the disturbance characterization and flow quality improvement of a newly designed Tandem Nozzle Mach 3 Wind Tunnel are presented. Firstly, a combined modal analysis is conducted to characterize the freestream disturbances with initial set-up of the settling chamber by using a Pitot probe and a hot-wire anemometry. Then, disturbance reduction in the supersonic wind tunnel is investigated by inserting various damping materials into the settling chamber, while a Pitot probe instrumented with Kulite sensor is employed to monitor the variation of the Pitot pressure fluctuation in the test section. Eventually, an optimized configuration of the settling chamber is determined by a combination of certain damping materials. Afterward, the freestream disturbances are re-characterized with the optimized set-up of the settling chamber, and the disturbance level is found to be significantly reduced. Through this study, valuable experience has been acquired for the disturbance reduction in tandem nozzle type supersonic wind tunnel for the first time, which enhances the feasibility of extending the operation range of conventional hypersonic Ludwieg tubes.
      PubDate: 2015-01-13
       
  • A direct measure of the frequency response of hot-wire anemometers:
           temporal resolution issues in wall-bounded turbulence
    • Abstract: Based on the need to characterise the accuracy of hot-wire anemometry (HWA) in high Reynolds number wall-bounded turbulence, we here propose a novel direct method for testing the frequency response of various systems to very high frequency velocity fluctuations (up to 50 kHz). A fully developed turbulent pipe flow is exploited to provide the input velocity perturbations. Utilising the unique capabilities of the Princeton Superpipe, it is possible to explore a variety of turbulent pipe flows at matched Reynolds numbers, but with turbulent energy in different frequency ranges. Assuming Reynolds number similarity, any differences between the appropriately scaled energy spectra for these flows should be indicative of measurement error. Having established the accuracy of this testing procedure, the response of several anemometer and probe combinations is tested. While these tests do not provide a direct or definitive comparison between different anemometers (owing to non-optimal tuning in each case), they do provide useful examples of potential frequency responses that could be encountered in HWA experiments. These results are subsequently used to predict error arising from HWA response for measurements in wall-bounded turbulent flows. For current technology, based on the results obtained here, the frequency response of under- or over-damped HWA systems can only be considered approximately flat up to 5–7 kHz. For flows with substantial turbulent energy in frequencies above this range, errors in measured turbulence quantities due to temporal resolution are increasingly likely.
      PubDate: 2015-01-13
       
  • Structure and dynamics of the wake of a reacting jet injected into a
           swirling, vitiated crossflow in a staged combustion system
    • Abstract: Secondary injection of the fuel, also referred to as staged combustion, is being studied by gas turbine manufacturers as a means of increasing the power output of the gas turbine systems with minimal contribution to NO x emission. A reacting jet issuing into a swirling, vitiated crossflow operating at gas turbine relevant conditions was investigated as a means of secondary injection. In this study, the flow field of the reacting jet was investigated using high repetition rate (HRR) (5 kHz), two-component particle imaging velocimetry and OH-PLIF. In applications similar to the one currently studied in this work, viz. secondary injection of fuel in a gas turbine combustor, rapid mixing and chemical reaction in the near field of jet injection are desirable. Based on our analysis, it is hypothesized that the shear layer and wake field vortices play a significant role in stabilizing a steady reaction front within the near wake region of the jet. Premixed jets composed of natural gas and air were injected through an extended nozzle into the vitiated flow downstream of a low-swirl burner that produced the vitiated, swirled flow. The jet-to-crossflow momentum flux ratio was varied to study the corresponding effect on the flow field. The time-averaged flow field shows a steady wake vortex very similar to that seen in the wake of a cylindrical bluff body which helps to stabilize the reaction zone within the wake of the jet. The HRR data acquisition also provided temporally resolved information on the transient structure of the wake flow associated with the reacting jet in crossflow.
      PubDate: 2015-01-13
       
  • High-speed particle image velocimetry for the efficient measurement of
           turbulence statistics
    • Abstract: A high-frame-rate camera and a continuous-wave laser are used to capture long particle image sequences exceeding 100,000 consecutive frames at framing frequencies up to 20 kHz. The electronic shutter of the high-speed CMOS camera is reduced to \(10\,\upmu\) s to prevent excessive particle image streaking. The combination of large image number and high frame rate is possible by limiting the field of view to a narrow strip, primarily to capture temporally resolved profiles of velocity and derived quantities, such as vorticity as well as higher order statistics. Multi-frame PIV processing algorithms are employed to improve the dynamic range of recovered PIV data. The recovered data are temporally well resolved and provide sufficient samples for statistical convergence of the fluctuating velocity components. The measurement technique is demonstrated on a spatially developing turbulent boundary layer inside a small wind tunnel with \(Re_\delta = 4{,}800,\, Re_\tau = 240\) and \(Re_\theta = 515\) . The chosen magnification permits a reliable estimation of the mean velocity profile down to a few wall units and yields statistical information such as the Reynolds stress components and probability density functions. By means of single-line correlation, it is further possible to extract the near-wall velocity profile in the viscous sublayer, both time-averaged as well as instantaneous, which permits the estimation the wall shear rate \(\dot{\gamma }\) and along with it the shear stress \(\tau _w\) and friction velocity \(u_\tau\) . These data are then used for the calculation of space-time correlation maps of wall shear stress and velocity.
      PubDate: 2015-01-13
       
  • Frequency-lock reactive control of a separated flow enabled by visual
           sensors
    • Abstract: In this study, a physical-based algorithm is used to control the separated flow downstream of a backward-facing step. It is well known that the spatial extent of the re-circulation bubble can be minimized when acting on the shear layer at the shedding frequency. Using this information in a closed-loop algorithm is not straightforward because of the difficulty of measuring in real time the shedding frequency accurately without disturbing the flow. A novel real-time analysis of 2D velocity fields uses vortex detection to dynamically measure the shedding frequency. Actuation (pulsed jet) is then locked on this frequency. If flow characteristics stray too far from a set point, shedding frequency is updated. The present work demonstrates the efficacy and robustness of this approach in reducing re-circulation, while Reynolds number is randomly varied between 1,400 and 2,400.
      PubDate: 2015-01-13
       
  • Characterization of the turbulent bistable flow regime of a 2 D bluff body
           wake disturbed by a small control cylinder
    • Abstract: A small control cylinder placed in a turbulent wake of a much larger 2D bluff body can cause a significant increase in drag fluctuations. These fluctuations occur on timescales longer than the timescales of the vortex shedding. The critical positions of the control cylinder are highly localized. Ensemble averages of PIV acquisitions and pressure measurements at the base of the bluff body reveal a bistable wake regime. Long duration hot-wire measurements are used to characterize the states and the transition process. The results show that a stochastic process is responsible for the transitions between the two stable states.
      PubDate: 2015-01-11
       
  • Stability of the laminar boundary-layer flow behind a roughness element
    • Abstract: Roughness elements in laminar boundary layers generate both high shear layers and streaky structures. Because these phenomena interact, it is difficult to precisely ascertain the dominant instability mechanisms. With the goal of explicating such interactions, we study the stability of a laminar boundary layer subject to a single roughness element at a Reynolds number subcritical of bypass transition. Our work involves two parts: bi-global linear stability theory (LST) analysis and corroborating experimental measurements. Linear stability analysis of a flat-plate boundary layer perturbed by streamwise streaks reveals the presence of several unstable modes. Of the dominant two modes, one exhibits spanwise symmetry and the other is antisymmetric. These modes are termed ‘varicose’ and ‘sinuous,’ respectively. Corroborating experiments were conducted in the laminar water channel of the University of Stuttgart. By simultaneously traversing two hot-film probes, we are able to confirm the presence of both eigenmodes predicted by LST and to extract relevant data for each: eigenvalues, eigenfunctions, growth rates and phase distributions. The main part of the experiments has been performed under ‘natural’ conditions, i.e., in the absence of external forcing. As the amplitude of the sinuous part of the results is much smaller than the varicose one and hence affected by measurement noise, a case with asymmetric external forcing is presented as well. Despite some deficiencies of the setup, it is possible to enhance the sinuous mode with respect to the unforced case and to confirm its existence as an eigenmode of the flow.
      PubDate: 2015-01-11
       
  • Active and passive flow control on a precessing jet
    • Abstract: A precessing jet nozzle with water as the working fluid is investigated under passive and active flow control. The actuation effectiveness of 12 microjets around the nozzle inlet for active control of the precessing jet is the focus of this work. Passive control is also applied by modifying the geometry of the precessing jet either by adding a center body near the chamber exit or varying the chamber length. The flow behavior under control is studied using pressure measurement at the chamber exit plane to monitor jet precession. The pressure data are analyzed using a phase plane representation to determine the motion of the jets high-velocity region in the chamber exit plane. The standard deviation of the phase of the triggered pressure data is used for stability analysis. This analysis results in a phase diagram in terms of Reynolds number and actuation frequency. Active control can be utilized over a range of actuation frequencies (and corresponding Strouhal numbers) to control precession direction and stability which can be further enhanced with passive control mechanisms. However, the flow follows the actuation with the lowest variation when the active actuation matches with the natural Strouhal number \((0.002<St<0.006)\) of the nozzle jet flow.
      PubDate: 2015-01-11
       
  • Sensor-based estimation of the velocity in the wake of a low-aspect-ratio
           pyramid
    • Abstract: We perform a sensor-based velocity field estimation in the wake of a wall-mounted pyramid from experimental data. The velocity field is measured with time-resolved stereoscopic PIV, and the sensors monitor local surface pressure. Starting point is the extended proper orthogonal decomposition technique. Key enablers for the spatio-temporal resolution of the strongly modulated shedding are (1) the exploitation of cross-correlation between velocity field and pressure, (2) time-delayed sensor signals, (3) symmetry considerations and (4) guaranteed orthonormality of the velocity expansion modes. The combined filtering operations are shown to yield a near-optimal flow estimation from pressure signals. The residual of the estimated coherent kinetic energy is about 30–50 % smaller, and the mean-field paraboloid is better rendered than with the previously proposed estimation methods.
      PubDate: 2015-01-11
       
  • Laser-induced incandescence measurements in a fired diesel engine at
           3 kHz
    • Abstract: Laser-induced incandescence (LII) was performed at 3 kHz in an optically accessible cylinder of a fired diesel engine using a commercially available diode-pumped solid-state laser and an intensified CMOS camera. The resulting images, acquired every 3° of crank angle, enabled the spatiotemporal tracking of soot structures during the expansion/exhaust stroke of the engine cycle. The image sequences demonstrate that soot tends to form in thin sheets that propagate and interact with the in-cylinder flow. These sheets tend to align parallel to the central axis of the cylinder and are frequently wrapped into conical spirals by aerodynamic swirl. Most of the soot is observed well away from the cylinder walls. Quantitative soot measurements were beyond the scope of this study but the results demonstrate the practical utility of using kHz-rate LII to acquire ensemble-averaged statistical data with high crank angle resolution over a complete engine cycle. Based on semi-quantitative measures of soot distribution, it was possible to identify soot dynamics related to incomplete charge exchange. This study shows that long-duration, multi-kHz acquisition rate LII measurements are viable in a fired diesel engine with currently available laser and camera technology, albeit only in the expansion and exhaust phase of the cycle at present. Furthermore, such measurements yield useful insight into soot dynamics and therefore constitute an important new tool for the development and optimization of diesel engine technology.
      PubDate: 2015-01-10
       
  • A PIV measurement of the natural transition of a natural convection
           boundary layer
    • Abstract: A natural convection boundary layer undergoing natural transition to turbulence is investigated by particle image velocimetry (PIV) measurements for the first time. The boundary layer adjacent to an isothermally heated plate immersed in water is subjected to environmental noise, the transition of which is therefore regarded as a natural transition. Distinct stripe-like flow structures characterising the natural transition of the natural convection boundary layer are visualised for the first time by the PIV experiments. The spatial wavenumber and the frequency of the flow structures are further quantified by applying a two-dimensional Fourier transform to time series of the flow properties. A distinct transitional regime in the boundary layer is noted, in which the three-dimensionality of the flow grows exponentially in the streamwise direction. The characteristic frequency of the natural convection boundary layer is also obtained from the PIV measurements, which agrees well with a theoretical prediction reported previously.
      PubDate: 2015-01-10
       
  • Optical flow for incompressible turbulence motion estimation
    • Abstract: We propose in this paper a new formulation of optical flow dedicated to 2D incompressible turbulent flows. It consists in minimizing an objective function constituted by an observation term and a regularization one. The observation term is based on the transport equation of the passive scalar field. For non-fully resolved scalar images, we propose to use the mixed model in large eddy simulation to determine the interaction between large scales and unresolved ones. The regularization term is based on the continuity equation of 2D incompressible flows. Compared to prototypical method, this regularizer preserves more vortex structures by eliminating constraints over the vorticity field. The evaluation of the proposed formulation is done over synthetic and experimental images, and the improvements in term of estimation are discussed.
      PubDate: 2015-01-10
       
  • PIV investigations on optical magnification and small scales in the
           near-field of an orifice jet
    • Abstract: Among turbulent jets, the orifice jet is still far from being fully understood especially due to the phenomenon of “vena contracta”, which takes place in the near field. In this work, the attention is focused to large-scale statistics (mean field and higher statistical moments of the velocity) and to small-scale statistics (velocity derivative statistical moments) of such a jet, with a special focus onto the effect of optical magnification on particle image velocimetry measurements. In order to investigate this effect on the measurements, framed areas of different size have been considered by changing the magnification factor through the object plane distance. The obtained results show that a limit on the choice of this parameter exists for a proper description of both small and large scales.
      PubDate: 2015-01-10
       
  • A Lagrangian study of liquid flow in a reverse-flow hydrocyclone using
           positron emission particle tracking
    • Abstract: In this study, the liquid flow in a reverse-flow hydrocyclone is studied experimentally using the Lagrangian approach. Resin beads with densities that are close to the density of the liquid in which they move, i.e., neutral-density particles, are used to model a fluid element in the highly turbulent flow in a hydrocyclone separator and tracked using PEPT with a temporal resolution of up to 0.5 ms. A method of producing neutral-density particles for PEPT was developed. The data processing algorithm was improved for the extra challenging tracking conditions that were encountered. The components of velocity, which reveal the detailed velocity field of the fluid, were calculated from the positions of the tracers. Various noise-removal methods, again to cope with the challenging tracking conditions, were applied and discussed.
      PubDate: 2015-01-10
       
  • Three-dimensional investigation of liquid slug Taylor flow inside a
           micro-capillary using holographic velocimetry
    • Abstract: Digital holography is an optical technique which is capable of providing instantaneous three-components of fluid flow velocity in three-dimensions (3D-3C) using a single camera. Digital holographic microscopy has been implemented in the present study to analyze liquid slug Taylor flow in a micro-channel of cross-sectional dimensions of 1,000 × 1,000 µm2. The working fluids are water (liquid) and air (gas), with superficial velocities of liquid, U L = 0.6 mm/s and gas, U G = 1.2 mm/s, respectively. The corresponding Capillary number, Ca = 0.035 × 10−3 and Bond number, Bo = 0.144. The holographic velocimetry technique has been implemented and appropriately validated by comparing the velocity profile from present experiment with that from analytical velocity profile for single-phase flow. Complete flow field results, i.e., u-, v- and w-components of velocity inside the liquid slug volume, i.e., in both streamwise (x–y) and cross-stream (y–z) planes are presented. The present experiments on liquid slug Taylor flow show strong cross-stream velocity near the advancing and receding meniscus due to higher capillary pressure. The stream traces show converging and diverging radial flow in the cross-stream plane near the receding and advancing meniscus, respectively. Two three-dimensional recirculation bubbles are observed inside the liquid slug. Overall, this paper reports the complex three-dimensional flow field inside a liquid slug Taylor flow from the 3D-3C flow field measurements.
      PubDate: 2015-01-10
       
  • Loads and pressure evaluation of the flow around a flapping wing from
           instantaneous 3D velocity measurements
    • Abstract: The flow around a flapping wing is characterized by an unsteady evolution of three-dimensional vortices, which are one of the main sources of loads. The difficulty in directly measuring such low forces by means of sensors and the need of the characterization of the evolution of the flow have lead to the evaluation of loads using the integral form of the momentum equation. This paper describes methods for evaluating instantaneous loads and three-dimensional pressure fields using 3D3C velocity fields only. An evaluation of the accuracy of these methods using DNS velocity fields is presented. Loads and pressure fields are then calculated using scanning tomography PIV velocity fields, around a NACA 0012 airfoil for a flapping motion in a water tank at a Reynolds number of 1,000. The results suggest a sufficient accuracy of calculated pressure fields for a global analysis of the topology of the flow and for the evaluation of loads by integrating the calculated pressure field over the surface of the wing.
      PubDate: 2015-01-10
       
 
 
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