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Publisher: Springer-Verlag (Total: 2350 journals)

 Showing 1201 - 1400 of 2350 Journals sorted alphabetically J. of Clinical Geropsychology       (Followers: 1) J. of Clinical Immunology       (Followers: 14, SJR: 1.611, h-index: 3) J. of Clinical Monitoring and Computing       (Followers: 1, SJR: 0.712, h-index: 2) J. of Clinical Psychology in Medical Settings       (Followers: 14, SJR: 0.596, h-index: 2) J. of Cluster Science       (SJR: 0.332, h-index: 1) J. of Coal Science and Engineering (China) J. of Coastal Conservation       (Followers: 5, SJR: 0.393, h-index: 1) J. of Coatings Technology and Research       (Followers: 5, SJR: 0.415, h-index: 2) J. of Combinatorial Optimization       (Followers: 6, SJR: 0.579, h-index: 1) J. of Communications Technology and Electronics       (Followers: 2, SJR: 0.273, h-index: 1) J. of Community Genetics       (SJR: 0.687, h-index: 1) J. of Community Health       (Followers: 8, SJR: 0.784, h-index: 2) J. of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology     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 Experiments in FluidsJournal Prestige (SJR): 1.281Citation Impact (citeScore): 3Number of Followers: 13      Hybrid journal (It can contain Open Access articles) ISSN (Print) 1432-1114 - ISSN (Online) 0723-4864 Published by Springer-Verlag  [2350 journals]
• Experimental investigation of non-Newtonian droplet collisions: the role
of extensional viscosity
• Authors: Giulia Finotello; Shauvik De; Jeroen C. R. Vrouwenvelder; Johan T. Padding; Kay A. Buist; Alfred Jongsma; Fredrik Innings; J. A. M. Kuipers
Abstract: We investigate the collision behaviour of a shear thinning non-Newtonian fluid xanthan, by binary droplet collision experiments. Droplet collisions of non-Newtonian fluids are more complex than their Newtonian counterpart as the viscosity no longer remains constant during the collision process. Despite the complex collision dynamics, we are able to present a complete regime map based on non-dimensional Weber (We) number and impact parameter (B). We compare the collision outcomes of xanthan, glycerol and a milk concentrate at similar impact conditions. These experiments reveal very rich and complex collision morphologies for shear thinning xanthan solution, strikingly different from Newtonian droplet collisions. Unlike glycerol and milk, xanthan collisions show no reflexive separation even at very high We number. Instead of breakup, we observe disc-like shapes with an oscillating behaviour of the colliding droplets. A detailed analysis reveals that this outcome is related to increased viscous energy dissipation and extensional effects.
PubDate: 2018-06-08
DOI: 10.1007/s00348-018-2568-2
Issue No: Vol. 59, No. 7 (2018)

• Gas density field imaging in shock dominated flows using planar laser
scattering
• Authors: Joshua D. Pickles; Balachandra R. Mettu; Pramod K. Subbareddy; Venkateswaran Narayanaswamy
Abstract: Planar laser scattering (PLS) imaging of ice particulates present in a supersonic stream is demonstrated to measure 2D gas density fields of shock dominated flows in low enthalpy test facilities. The technique involves mapping the PLS signal to gas density using a calibration curve that accounts for the seed particulate size distribution change across the shock wave. The PLS technique is demonstrated in a shock boundary layer interaction generated by a sharp fin placed on a cylindrical surface in Mach 2.5 flow. The shock structure generated in this configuration has complicating effects from the finite height of the fin as well as the 3D relief offered by the cylindrical surface, which result in steep spatial gradients as well as a wide range of density jumps across different locations of the shock structure. Instantaneous and mean PLS fields delineated the inviscid, separation, and reattachment shock structures at various downstream locations. The inviscid shock assumed increasingly larger curvature with downstream distance; concomitantly, the separation shock wrapped around the cylinder and the separation shock foot missed the cylinder surface entirely. The density fields obtained from the PLS technique were evaluated using RANS simulations of the same flowfield. Comparisons between the computed and measured density fields showed excellent agreement over the entire measurable region that encompassed the flow processed by inviscid, separation, and reattachment shocks away from viscous regions. The PLS approach demonstrated in this work is also shown to be largely independent of the seed particulates, which lends the extension of this approach to a wide range of test facilities.
PubDate: 2018-06-05
DOI: 10.1007/s00348-018-2562-8
Issue No: Vol. 59, No. 7 (2018)

• Characterization of spray-induced turbulence using fluorescence PIV
• Authors: Dennis D. van der Voort; Nico J. Dam; Herman J. H. Clercx; Willem van de Water
Abstract: The strong shear induced by the injection of liquid sprays at high velocities induces turbulence in the surrounding medium. This, in turn, influences the motion of droplets as well as the mixing of air and vapor. Using fluorescence-based tracer particle image velocimetry, the velocity field surrounding 125–135 m/s sprays exiting a 200- $$\upmu$$ m nozzle is analyzed. For the first time, the small- and large-scale turbulence characteristics of the gas phase surrounding a spray has been measured simultaneously, using a large eddy model to determine the sub-grid scales. This further allows the calculation of the Stokes numbers of droplets, which indicates the influence of turbulence on their motion. The measurements lead to an estimate of the dissipation rate $$\epsilon \approx 35$$ m $$^{2}$$ s $$^{-3}$$ , a microscale Reynolds number Re $$_{\lambda } \approx$$ 170, and a Kolmogorov length scale of $$\eta \approx 10^{-4}$$ m. Using these dissipation rates to convert a droplet size distribution to a distribution of Stokes numbers, we show that only the large scale motion of turbulence disperses the droplet in the current case, but the small scales will grow in importance with increasing levels of atomization and ambient pressures.
PubDate: 2018-06-02
DOI: 10.1007/s00348-018-2561-9
Issue No: Vol. 59, No. 7 (2018)

• Effects of superhydrophobic surfaces on the flow around an NACA0012
hydrofoil at low Reynolds numbers
• Authors: Jungjin Lee; Hyunseok Kim; Hyungmin Park
Abstract: In the present study, the effects of superhydrophobic surface on the flow around an NACA0012 hydrofoil are experimentally investigated at low Reynolds number range of 0.2–1.0 $$\times 10^{4}$$ . The velocity fields were measured using two-dimensional digital particle image velocimetry in a water tunnel while varying the angle of attack from $$0^\circ$$ to $$20^\circ$$ . The spray-coating of hydrophobic nanoparticles was used to create superhydrophobic surfaces. Depending on the Reynolds number and angle of attack, we found that the effects of superhydrophobic surface show up differently, which is determined by the relative strength of turbulence caused by both the surface slip (influence of trapped air pockets) and roughness, compared to that of background (i.e., uncontrolled) flow. In general, the superhydrophobic surface imposes a little influence on the wake behind a hydrofoil when the angle of attack is very low (attached flow) or high (fully separated flow). At intermediate angles of attack (flow separates between the leading and trailing edges), however, it is measured that the flow over superhydrophobic surface has a stronger turbulence, and thus, the enhanced shear-layer instability forces the early vortex rollup in the wake and reduction of vortex formation length. Interestingly, there is a transitional range of angle of attack, in which this effect is reversed, and thus, the vortex rollup is slightly delayed. This trend can be explained based on the changes in the uncontrolled flow structures, and finally, we classify the effects of superhydrophobic surface in terms of Reynolds number and angle of attack, in the considered ranges of both.
PubDate: 2018-06-02
DOI: 10.1007/s00348-018-2564-6
Issue No: Vol. 59, No. 7 (2018)

• Coupling between premixed flame propagation and swirl flow during boundary
layer flashback
• Authors: Dominik Ebi; Rakesh Ranjan; Noel T. Clemens
Abstract: Flashback of premixed methane–air flames in the turbulent boundary layer of swirling flows is investigated experimentally. The premix section of the atmospheric model swirl combustor features an axial swirler with an attached center-body. Our previous work with this same configuration investigated the flame propagation during flashback using particle image velocimetry (PIV) with liquid droplets as seed particles that precluded making measurements in the burnt gases. The present study investigates the transient velocity field in the unburnt and burnt gas region by means of solid-particle seeding and high-speed stereoscopic PIV. The global axial and circumferential lab-frame flame propagation speed is obtained simultaneously based on high-speed chemiluminescence movies. By combining the PIV data with the global flame propagation speed, the quasi-instantaneous swirling motion of the velocity field is constructed on annular shells, which provides a more intuitive view on the complex three-dimensional flow–flame interaction. Previous works showed that flashback is led by flame tongues. We find that the important flow–flame interaction occurs on the far side of these flame tongues relative to the approach flow, which we henceforth refer to as the leading side. The leading side is found to propagate as a classical premixed flame front relative to the strongly modified approach flow field. The blockage imposed by flame tongues is not limited to the immediate vicinity of the flame base, but occurs along the entire leading side.
PubDate: 2018-06-02
DOI: 10.1007/s00348-018-2563-7
Issue No: Vol. 59, No. 7 (2018)

• Pulse-burst PIV of an impulsively started cylinder in a shock tube for Re
&gt; 10 5
• Authors: Justin L. Wagner; Edward P. DeMauro; Katya M. Casper; Steven J. Beresh; Kyle P. Lynch; Brian O. Pruett
Abstract: The impulsive start of a circular cylinder in a shock tube was characterized with time-resolved particle image velocimetry measurements (TR-PIV) at 50 kHz using a pulse-burst laser. Three Reynolds numbers Re of 1.07, 1.63 and 2.46 × 105 were studied adding insight into the transient process near the drag crisis. In all cases, vorticity was maximum in the first pair of vortices formed. In a fashion analogous to previous studies at Re ≤ 104, a single symmetric vortex pair was first shed from the cylinder at Re = 1.07 × 105 prior to the eventual transition to a von Kármán vortex street. In contrast, at Re ≥ 1.63 × 105, two or more symmetric vortex pairs were first shed. The non-dimensional time for the wake to begin to exhibit asymmetry was also found to be lower at the two higher Re. The time required to reach a fully antisymmetric wake (peak von Kármán shedding) was roughly five times the asymmetric onset time. Altogether, the study indicates a transformation in the impulsive wake structure and associated time scales to occur at Re near 1.6 × 105.
PubDate: 2018-05-25
DOI: 10.1007/s00348-018-2558-4
Issue No: Vol. 59, No. 6 (2018)

• Parametric investigation of friction drag reduction in turbulent flow over
a flexible wall undergoing spanwise transversal traveling waves
• Authors: Wenfeng Li; Dorothee Roggenkamp; Tobias Hecken; Wilhelm Jessen; Michael Klaas; Wolfgang Schröder
Abstract: Active friction drag reduction by spanwise transversal traveling surface waves is investigated experimentally in a fully developed zero-pressure gradient (ZPG) turbulent boundary layer (TBL). The spanwise transversal traveling wave of an aluminum surface is generated by an electromagnetic actuator system. A parametric study focusing on the influence of the wave amplitude (A+) and wave period (T+) is performed to analyze the impact of the wave parameters on drag reduction. Within the range of the parameters investigated, the maximum local drag reduction of 4.5% is found at A+ = 11.8 and T+ = 110. Furthermore, the TBL flows above the wave crest and trough are investigated by phase-locked PIV and µ-PTV measurements. The results evidence that the drag reduction effect is not only enhanced by increasing the amplitude, but also by reducing the period in the range of the current parameters. The turbulence statistics show that the velocity fluctuations and the Reynolds shear stresses in the streamwise and in the wall-normal direction are damped by the traveling surface wave motion in the near-wall region. The outer velocity distribution deviates from the inner scaling based on the actuated friction velocity, i.e., it possesses a slight tendency of a varying slope in the log region. The phase-locked measurements of the velocity profiles above the crest and the trough show that only above the crest the inner scaling property is valid. Above the moving surface a non-zero spanwise secondary flow is induced. The quadrant decomposition of the turbulent productions shows that the sweep and ejection events are weakened.
PubDate: 2018-05-24
DOI: 10.1007/s00348-018-2559-3
Issue No: Vol. 59, No. 6 (2018)

• Measurement of the translation and rotation of a sphere in fluid flow
• Authors: Diogo Barros; Ben Hiltbrand; Ellen K. Longmire
Abstract: The problem of determining the translation and rotation of a spherical particle moving in fluid flow is considered. Lagrangian tracking of markers printed over the surface of a sphere is employed to compute the center motion and the angular velocity of the solid body. The method initially calculates the sphere center from the 3D coordinates of the reconstructed markers, then finds the optimal rotation matrix that aligns a set of markers tracked at sequential time steps. The parameters involved in the experimental implementation of this procedure are discussed, and the associated uncertainty is estimated from numerical analysis. Finally, the proposed methodology is applied to characterize the motion of a large spherical particle released in a turbulent boundary layer developing in a water channel.
PubDate: 2018-05-23
DOI: 10.1007/s00348-018-2549-5
Issue No: Vol. 59, No. 6 (2018)

• Enhancing the hydrodynamic performance of a tapered swept-back wing
• Authors: Zhaoyu Wei; Lian Lian; Yisen Zhong
Abstract: The hydrodynamic benefit of implementing leading-edge (LE) tubercles on wings at very low Reynolds numbers (Res) has not been thoroughly elucidated to date, though their benefits at relatively higher Res are well-studied. Through wind tunnel testing at Re = 5.5 × 104, we found that the LE tubercles increase the lift at all pitch angles tested and slightly reduce the drag at a pitch angle of 4° < α < 10°, which finally results in a significant hydrodynamic performance enhancement at lower pitch angles. Flow visualization reveals that the hydrodynamic performance enhancement is due to the favourable attached flows downstream of the tubercle peaks. The attached flows are believed to be closely related to the downwash and momentum exchange within the boundary layers, which originate from surface and streamwise-aligned counter-rotating vortex pairs (CVPs).
PubDate: 2018-05-19
DOI: 10.1007/s00348-018-2557-5
Issue No: Vol. 59, No. 6 (2018)

• Comparison of 2c- and 3cLIF droplet temperature imaging
• Authors: Johannes Palmer; Manuel A. Reddemann; Valeri Kirsch; Reinhold Kneer
Abstract: This work presents “pulsed 2D-3cLIF-EET” as a measurement setup for micro-droplet internal temperature imaging. The setup relies on a third color channel that allows correcting spatially changing energy transfer rates between the two applied fluorescent dyes. First measurement results are compared with results of two slightly different versions of the recent “pulsed 2D-2cLIF-EET” method. Results reveal a higher temperature measurement accuracy of the recent 2cLIF setup. Average droplet temperature is determined by the 2cLIF setup with an uncertainty of less than 1 K and a spatial deviation of about 3.7 K. The new 3cLIF approach would become competitive, if the existing droplet size dependency is anticipated by an additional calibration and if the processing algorithm includes spatial measurement errors more appropriately.
PubDate: 2018-05-19
DOI: 10.1007/s00348-018-2545-9
Issue No: Vol. 59, No. 6 (2018)

• Estimation of perspective errors in 2D2C-PIV measurements for 3D
concentrated vortices
• Authors: Bao-Feng Ma; Hong-Gang Jiang
Abstract: Two-dimensional planar PIV (2D2C) is still extensively employed in flow measurement owing to its availability and reliability, although more advanced PIVs have been developed. It has long been recognized that there exist perspective errors in velocity fields when employing the 2D2C PIV to measure three-dimensional (3D) flows, the magnitude of which depends on out-of-plane velocity and geometric layouts of the PIV. For a variety of vortex flows, however, the results are commonly represented by vorticity fields, instead of velocity fields. The present study indicates that the perspective error in vorticity fields relies on gradients of the out-of-plane velocity along a measurement plane, instead of the out-of-plane velocity itself. More importantly, an estimation approach to the perspective error in 3D vortex measurements was proposed based on a theoretical vortex model and an analysis on physical characteristics of the vortices, in which the gradient of out-of-plane velocity is uniquely determined by the ratio of the maximum out-of-plane velocity to maximum swirling velocity of the vortex; meanwhile, the ratio has upper limits for naturally formed vortices. Therefore, if the ratio is imposed with the upper limits, the perspective error will only rely on the geometric layouts of PIV that are known in practical measurements. Using this approach, the upper limits of perspective errors of a concentrated vortex can be estimated for vorticity and other characteristic quantities of the vortex. In addition, the study indicates that the perspective errors in vortex location, vortex strength, and vortex radius can be all zero for axisymmetric vortices if they are calculated by proper methods. The dynamic mode decomposition on an oscillatory vortex indicates that the perspective errors of each DMD mode are also only dependent on the gradient of out-of-plane velocity if the modes are represented by vorticity.
PubDate: 2018-05-19
DOI: 10.1007/s00348-018-2556-6
Issue No: Vol. 59, No. 6 (2018)

• Droplet and multiphase effects in a shock-driven hydrodynamic instability
with reshock
• Authors: John B. Middlebrooks; Constantine G. Avgoustopoulos; Wolfgang J. Black; Roy C. Allen; Jacob A. McFarland
Abstract: Shock-driven multiphase instabilities (SDMI) are unique physical phenomena that have far-reaching applications in engineering and science such as high energy explosions, scramjet combustors, and supernovae events. The SDMI arises when a multiphase field is impulsively accelerated by a shock wave and evolves as a result of gradients in particle-gas momentum transfer. A new shock tube facility has been constructed to study the SDMI. Experiments were conducted to investigate liquid particle and multiphase effects in the SDMI. A multiphase cylindrical interface was created with water droplet laden air in our horizontal shock tube facility. The interface was accelerated by a Mach 1.66 shock wave, and its reflection from the end wall. The interface development was captured using laser illumination and a high-resolution CCD camera. Laser interferometry was used to determine the droplet size distribution. A particle filtration technique was used to determine mass loading within an interface and verify particle size distribution. The effects of particle number density, particle size, and a secondary acceleration (reshock) of the interface were noted. Particle number density effects were found comparable to Atwood number effects in the Richtmyer–Meshkov instability for small ( $$\sim 1.7~{\upmu }$$ m) droplets. Evaporation was observed to alter droplet sizes and number density, markedly after reshock. For large diameter droplets ( $$\sim 10.7~{\upmu }$$ m), diminished development was observed with larger droplets lagging far behind the interface. These lagging droplets were also observed to breakup after reshock into structured clusters of smaller droplets. Mixing width values were reported to quantify mixing effects seen in images.
PubDate: 2018-05-17
DOI: 10.1007/s00348-018-2547-7
Issue No: Vol. 59, No. 6 (2018)

• Kinematics and dynamics of green water on a fixed platform in a large wave
basin in focusing wave and random wave conditions
• Authors: Wei-Liang Chuang; Kuang-An Chang; Richard Mercier
Abstract: Green water kinematics and dynamics due to wave impingements on a simplified geometry, fixed platform were experimentally investigated in a large, deep-water wave basin. Both plane focusing waves and random waves were employed in the generation of green water. The focusing wave condition was designed to create two consecutive plunging breaking waves with one impinging on the frontal vertical wall of the fixed platform, referred as wall impingement, and the other directly impinging on the deck surface, referred as deck impingement. The random wave condition was generated using the JONSWAP spectrum with a significant wave height approximately equal to the freeboard. A total of 179 green water events were collected in the random wave condition. By examining the green water events in random waves, three different flow types are categorized: collapse of overtopping wave, fall of bulk water, and breaking wave crest. The aerated flow velocity was measured using bubble image velocimetry, while the void fraction was measured using fiber optic reflectometry. For the plane focusing wave condition, measurements of impact pressure were synchronized with the flow velocity and void fraction measurements. The relationship between the peak pressures and the pressure rise times is examined. For the high-intensity impact in the deck impingement events, the peak pressures are observed to be proportional to the aeration levels. The maximum horizontal velocities in the green water events in random waves are well represented by the lognormal distribution. Ritter’s solution is shown to quantitatively describe the green water velocity distributions under both the focusing wave condition and the random wave condition. A prediction equation for green water velocity distribution under random waves is proposed.
PubDate: 2018-05-17
DOI: 10.1007/s00348-018-2554-8
Issue No: Vol. 59, No. 6 (2018)

• 3D Measurements of coupled freestream turbulence and secondary flow
effects on film cooling
• Authors: David S. Ching; Haosen H. A. Xu; Christopher J. Elkins; John K. Eaton
Abstract: The effect of freestream turbulence on a single round film cooling hole is examined at two turbulence levels of 5 and 8% and compared to a baseline low freestream turbulence case. The hole is inclined at 30 $$^{\circ }$$ and has length to diameter ratio $$L/D=4$$ and unity blowing ratio. Turbulence is generated with grid upstream of the hole in the main channel. The three-dimensional, three-component mean velocity field is acquired with magnetic resonance velocimetry (MRV) and the three-dimensional temperature field is acquired with magnetic resonance thermometry (MRT). The 8% turbulence grid produces weak mean secondary flows in the mainstream (peak crossflow velocities are 7% of $$U_\mathrm{bulk}$$ ) which push the jet close to the wall and significantly change the adiabatic effectiveness distribution. By contrast, the 5% grid has a simpler structure and does not produce a measurable secondary flow structure. The grid turbulence causes little change to the temperature field, indicating that the turbulence generated in the shear layers around the jet dominates the freestream turbulence. The results suggest that secondary flows induced by complex turbulence generators may have caused some of the contradictory results in previous works.
PubDate: 2018-05-17
DOI: 10.1007/s00348-018-2555-7
Issue No: Vol. 59, No. 6 (2018)

• Tomographic PIV of flow through ordered thin porous media
• Authors: I. A. Sofia Larsson; T. Staffan Lundström; Henrik Lycksam
Abstract: Pressure-driven flow in a model of a thin porous medium is investigated using tomographic particle image velocimetry. The solid parts of the porous medium have the shape of vertical cylinders placed on equal interspatial distance from each other. The array of cylinders is confined between two parallel plates, meaning that the permeability is a function of the diameter and height of the cylinders, as well as their interspatial distance. Refractive index matching is applied to enable measurements without optical distortion and a dummy cell is used for the calibration of the measurements. The results reveal that the averaged flow field changes substantially as Reynolds number increases, and that the wakes formed downstream the cylinders contain complex, three-dimensional vortex structures hard to visualize with only planar measurements. An interesting observation is that the time-averaged velocity maximum changes position as Reynolds number increases. For low Reynolds number flow, the maximum is in the middle of the channel, while, for the higher Reynolds numbers investigated, two maxima appear closer to each bounding lower and upper wall.
PubDate: 2018-05-16
DOI: 10.1007/s00348-018-2548-6
Issue No: Vol. 59, No. 6 (2018)

• Real-time quantitative Schlieren imaging by fast Fourier demodulation of a
checkered backdrop
• Authors: Sander Wildeman
Abstract: A quantitative synthetic Schlieren imaging (SSI) method based on fast Fourier demodulation is presented. Instead of a random dot pattern (as usually employed in SSI), a 2D periodic pattern (such as a checkerboard) is used as a backdrop to the refractive object of interest. The range of validity and accuracy of this “Fast Checkerboard Demodulation” (FCD) method are assessed using both synthetic data and experimental recordings of patterns optically distorted by small waves on a water surface. It is found that the FCD method is at least as accurate as sophisticated, multi-stage, digital image correlation (DIC) or optical flow (OF) techniques used with random dot patterns, and it is significantly faster. Efficient, fully vectorized, implementations of both the FCD and DIC/OF schemes developed for this study are made available as open source Matlab scripts.
PubDate: 2018-05-16
DOI: 10.1007/s00348-018-2553-9
Issue No: Vol. 59, No. 6 (2018)

• Detection of small-amplitude periodic surface pressure fluctuation by
pressure-sensitive paint measurements using frequency-domain methods
• Authors: Takahiro Noda; Kazuyki Nakakita; Masaki Wakahara; Masaharu Kameda
Abstract: Image measurement using pressure-sensitive paint (PSP) is an effective tool for analyzing the unsteady pressure field on the surface of a body in a low-speed air flow, which is associated with wind noise. In this study, the surface pressure fluctuation due to the tonal trailing edge (TE) noise for a two-dimensional NACA 0012 airfoil was quantitatively detected using a porous anodized aluminum PSP (AA-PSP). The emission from the PSP upon illumination by a blue laser diode was captured using a 12-bit high-speed complementary metal-oxide-semiconductor (CMOS) camera. The intensities of the captured images were converted to pressures using a standard intensity-based method. Three image-processing methods based on the fast Fourier transform (FFT) were tested to determine their efficiency in improving the signal-to-noise ratio (SNR) of the unsteady PSP data. In addition to two fundamental FFT techniques (the full data and ensemble averaging FFTs), a technique using the coherent output power (COP), which involves the cross correlation between the PSP data and the signal measured using a pointwise sound-level meter, was tested. Preliminary tests indicated that random photon shot noise dominates the intensity fluctuations in the captured PSP emissions above 200 Hz. Pressure fluctuations associated with the TE noise, whose dominant frequency is approximately 940 Hz, were successfully measured by analyzing 40,960 sequential PSP images recorded at 10 kfps. Quantitative validation using the power spectrum indicates that the COP technique is the most effective method of identification of the pressure fluctuation directly related to TE noise. It is possible to distinguish power differences with a resolution of 10 Pa $$^2$$ (4 Pa in amplitude) when the COP was employed without use of another wind-off data. This resolution cannot be achieved by the ensemble averaging FFT because of an insufficient elimination of the background noise.
PubDate: 2018-05-15
DOI: 10.1007/s00348-018-2550-z
Issue No: Vol. 59, No. 6 (2018)

• Experimental investigation of the sloshing motion of the water free
surface in the draft tube of a Francis turbine operating in synchronous
condenser mode
• Authors: Elena Vagnoni; Arthur Favrel; Loïc Andolfatto; François Avellan
Abstract: Hydropower units may be required to operate in condenser mode to supply reactive power. In this operating mode, the water level in the turbine or pump-turbine is decreased below the runner by closing the guide vanes and injecting pressurized air. While operating in condenser mode the machine experiences power losses due to several air–water interaction phenomena which cause air losses. One of such phenomena is the sloshing motion of the water free surface below the runner in the draft tube cone of a Francis turbine. The objective of the present work is to experimentally investigate the sloshing motion of the water free surface in the draft tube cone of a reduced scale physical model of a Francis turbine operating in condenser mode. Images acquisition and simultaneous pressure fluctuation measurements are performed and an image processing method is developed to investigate amplitude and frequency of the sloshing motion of the free surface. It is found that this motion is excited at the natural frequency of the water volume and corresponds to the azimuthal wavenumber $$m = 1$$ of a rotating gravity wave. The amplitude of the motion is perturbed by wave breaking and it decreases by increasing the densimetric Froude number. The sloshing frequency slightly increases with respect to the natural frequency of the water volume by increasing the densimetric Froude number. Moreover, it results that this resonant phenomenon is not related to the torque perturbation.
PubDate: 2018-05-15
DOI: 10.1007/s00348-018-2552-x
Issue No: Vol. 59, No. 6 (2018)

• The effect of flow control on the wake dynamics of a rectangular bluff
body in ground proximity
• Abstract: The time-resolved flow field in the wake of a rectangular bluff body in ground proximity is examined through wind tunnel experiments. In addition to an extensive assessment of the baseline wake dynamics, the study also investigates the impact of passive (i.e., base flaps) and active (i.e., fluidic oscillators) flow control on the wake dynamics. The velocity field downstream of the model is acquired with a stereoscopic high-speed particle image velocimetry system at several streamwise and crosswise sections. Coherent wake structures are determined by conditional averaging, spectral analysis, and spectral proper orthogonal decomposition. The baseline flow field is dominated by a wake bi-stability that is characterized by a random shift between two stable wake states. The bi-stability is governed by the model’s aspect ratio and occurs in the vertical direction, because the model height is 1.35 times larger than its width. Higher frequency modes with less energy content as determined in the appropriate literature are identified and visualized. A coupling between these modes and the bi-stability is discussed. Flow control has a significant impact on the wake dynamics. When passive flow control is applied, the bi-stability of the wake is still present for a flap angle of $$20^\circ$$ . The higher frequency modes are still detectable but weakened. The turbulence intensity is significantly reduced when the flow attaches to the base flaps and the bi-stability is inhibited. When active flow control is applied, the higher baseline frequencies are suppressed in addition to the absence of the bi-stability. Solely the dominant mode at a Strouhal number of about 0.08 remains present for all flow control configurations. This mode is attributed to an alternating shear layer oscillation.
PubDate: 2018-05-30
DOI: 10.1007/s00348-018-2560-x

• Experimental analysis of transonic buffet on a 3D swept wing using
fast-response pressure-sensitive paint
• Abstract: Transonic buffeting phenomena on a three-dimensional swept wing were experimentally analyzed using a fast-response pressure-sensitive paint (PSP). The experiment was conducted using an 80%-scaled NASA Common Research Model in the Japan Aerospace Exploration Agency (JAXA) 2 m × 2 m Transonic Wind Tunnel at a Mach number of 0.85 and a chord Reynolds number of 1.54 × 106. The angle of attack was varied between 2.82° and 6.52°. The calculation of root-mean-square (RMS) pressure fluctuations and spectral analysis were performed on measured unsteady PSP images to analyze the phenomena under off-design buffet conditions. We found that two types of shock behavior exist. The first is a shock oscillation characterized by the presence of “buffet cells” formed at a bump Strouhal number St of 0.3–0.5, which is observed under all off-design conditions. This phenomenon arises at the mid-span wing and is propagated spanwise from inboard to outboard. The other is a large spatial amplitude shock oscillation characterized by low-frequency broadband components at St < 0.1, which appears at higher angles of attack (α ≥ 6.0°) and behaves more like two-dimensional buffet. The transition between these two shock behaviors correlates well with the rapid increase of the wing-root strain fluctuation RMS.
PubDate: 2018-05-30
DOI: 10.1007/s00348-018-2565-5

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