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Experiments in Fluids    [5 followers]  Follow
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ISSN (Print) 1432-1114 - ISSN (Online) 0723-4864
Published by Springer-Verlag  [2180 journals]   [SJR: 1.033]   [H-I: 62]
• Influence of turbulence on the drag of solid discs and turbine simulators in a water current
• Abstract: Abstract Laboratory experiments have been used to investigate the effects of turbulence on the drag of both solid discs and porous disc turbine simulators. These discs were introduced to turbulent flows, in a gravity-fed water flume, with various levels of turbulence intensity and integral length scales. The turbulence was generated using three different grid configurations, which produced intensities and scales comparable with previous wind tunnel studies. The drag measurements were taken with discs of two different diameters and porosities with and without the upstream grids. The experimental results have demonstrated that the drag coefficients, of all the discs tested, are significantly dependent on both the turbulence intensity and integral length scale. For small integral length scales, relative to the disc, the drag coefficients converged for turbulence intensities greater than 13 %, with an increase of around 20 % in drag coefficient over the low-intensity case. Experiments with turbulence intensities of 10 % demonstrated minimum drag coefficients when the integral length scale-to-disc diameter ratio was around 50 %. Significant variations in the drag coefficient of circular bluff bodies are therefore expected when operating in turbulent flows with different characteristics.
PubDate: 2013-12-03

• An inexpensive and versatile technique for wide frequency range surface pressure measurements: an application for the study of turbulent buffeting of a square cylinder
• Abstract: Abstract This work presents the development of an inexpensive measurement technique based on miniature microphones for the measurement of pressure fluctuations in a wide frequency range, starting from infrasound up to several kilohertz. Special emphasis has been put on achieving accurate calibration of the system at very low frequencies and good agreement with reference measurements have been achieved at frequencies as low as 1 Hz, therefore opening new low-budget research possibilities in many fields of fluid mechanics. The measurement technique proposed is specially indicated when the number of simultaneous pressure measurements is high since the sensors used are inexpensive, contrarily to common research equipment. One particular area in which this technique results useful is bluff-body aerodynamics. As an example of the potential of the technique, the structural response of a finite-square cylinder immersed in a turbulent flow is studied.
PubDate: 2013-11-28

• PIV experiments in rough-wall, laminar-to-turbulent, oscillatory boundary-layer flows
• Abstract: Abstract Exploratory measurements of oscillatory boundary layers were conducted over a smooth and two different rough beds spanning the laminar, transitional and turbulent flow regimes using a multi-camera 2D-PIV system in a small oscillatory-flow tunnel (Admiraal et al. in J Hydraul Res 44(4):437–450, 2006). Results show how the phase lag between bed shear stress and free-stream velocity is better defined when the integral of the momentum equation is used to estimate the bed shear stress. Observed differences in bed shear stress and phase lag between bed shear stress and free-stream velocity are highly sensitive to the definition of the bed position (y = b). The underestimation of turbulent stresses close to the wall is found to explain such differences when using the addition of Reynolds and viscous stresses to define both the bed shear stress and the phase lag. Regardless of the flow regime, in all experiments, boundary-layer thickness reached its maximum value at a phase near the flow reversal at the wall. Friction factors in smooth walls are better estimated using a theoretical equation first proposed by Batchelor (An introduction to fluid dynamics. Cambridge University Press, Cambridge, 1967) while the more recent empirical predictor of Pedocchi and Garcia (J Hydraul Res 47(4):438–444, 2009a) was found to be appropriate for estimating friction coefficients in the laminar-to-turbulent transition regime.
PubDate: 2013-11-28

• Extensive study of temperature dissipation measurements on the centerline of a turbulent round jet based on the $\overline{\theta^{2}}/2$ budget
• Abstract: Abstract The temperature dissipation rate inferred from the balance of $\overline{\theta^{2}}/2$ budget is used for the purpose of studying different methods employed to directly measure dissipation. The terms involved in the budget equation of temperature variance are measured with laser Doppler velocimetry and cold-wire thermometry used simultaneously. This study focuses on the centerline of a turbulent round jet, in the far field, at high Reynolds number (x/D = 30, Re D  = 1.5 × 105 and Re λ  = 548). Particular attention is devoted to statistical convergence of second- and third-order moments of velocity and temperature fluctuations. Temperature dissipation obtained by Taylor’s hypothesis and radial temperature derivative spectra confirm local isotropy. A high level of low wave number content is reported for the longitudinal derivative spectra, probably due to transverse mode spectral aliasing and noise contamination for small wire separation. A parallel is drawn between finite difference formulations and the behavior of the autocorrelation coefficient for small wire separations. The temperature dissipation estimates found are close to the budget reference value, but spectral analysis cast doubts on the validity of the streamwise derivative obtained with a pair of probes.
PubDate: 2013-11-28

• Measurement and decomposition of periodic flow structures downstream of a test turbine
• Abstract: Abstract This paper shows the experimental methodology which has been employed for the simultaneous aerodynamic and aeroacoustic analysis of a test turbine by means of a fast-response aerodynamic pressure probe. Two different phase references are used to resolve the fluctuations in time of the velocity and the pressure of phenomena occurring at different and unrelated periodicity. The phase-averaged fields are further post-processed by decomposing them into spatial-temporal Fourier coefficients. The deterministic unsteadiness is then simply represented by a limited number of Fourier coefficients, the physical meaning of which is discussed in details. In particular, this approach has been used to distinguish between propagating and non-propagating noise sources.
PubDate: 2013-11-28

• Conditional entrainment statistics of inertial particles across shearless turbulent interfaces
• Abstract: Abstract In order to gain insight into droplet behavior at the edge of clouds, a laboratory experiment has been carried out to study the conditioned statistics of inertial sub-Kolmogorov particles in shearless turbulent–non-turbulent and turbulent–turbulent mixing layers. The water droplets were injected from the homogeneous turbulence side of the flow, and their velocity and size distribution profiles were measured by a combined LDV/PDPA technique. The fluid velocity field was measured using hot-wire anemometry in the droplet-free flow. A conditioning method with the stream-wise velocity chosen as a turbulence detector function was used to identify the turbulent regions in the mixing layer. The particle concentration profiles, mass fluxes and small-scale clustering were compared for the conditioned and unconditioned cases. Previously we demonstrated (Gerashchenko et al. in J Fluid Mech 668:293–303, 2011; Good et al. in J Fluid Mech 694:371–398, 2012) that in this flow, the overall inertial particle transport is dominated by large-scale intermittent motion corresponding to turbulent bursts penetrating from one side of the mixing interface to the other, and that the particle concentration, to a large extent, preserves its homogeneous turbulence (injection side) values inside of turbulent bursts in the mixing layer. In the present work, we show that the conditioned concentration is higher for the turbulent–non-turbulent than for the turbulent–turbulent interface due to higher averaged burst widths for the latter case. This trend is opposite to that for the unconditioned concentration profiles. The unconditioned particle mass flux peaks approximately in the middle of the layer and is more pronounced for the turbulent–turbulent interface. The conditioned particle mass flux monotonically increases across the layer and is higher for the turbulent–turbulent interface. The small-scale turbulent clustering (less than 10 Kolmogorov scales) quantified by the particle radial distribution function is well preserved inside of bursts. Large-scale clustering (10–500 Kolmogorov scales) caused by the burst activity is observed for the unconditioned cases. Particles with large Stokes number are less sensitive to large-scale clustering than those with small Stokes number.
PubDate: 2013-11-21

• Streamwise velocity statistics in turbulent boundary layers that spatially develop to high Reynolds number
• Abstract: Abstract Well-resolved measurements of the streamwise velocity in zero pressure gradient turbulent boundary layers are presented for friction Reynolds numbers up to 19,670. Distinct from most studies, the present boundary layers undergo nearly a decade increase in Reynolds number solely owing to streamwise development. The profiles of the mean and variance of the streamwise velocity exhibit logarithmic behavior in accord with other recently reported findings at high Reynolds number. The inner and mid-layer peaks of the variance profile are evidenced to increase at different rates with increasing Reynolds number. A number of statistical features are shown to correlate with the position where the viscous force in the mean momentum equation loses leading order importance, or similarly, where the mean effect of turbulent inertia changes sign from positive to negative. The near-wall peak region in the 2-D spectrogram of the fluctuations is captured down to wall-normal positions near the edge of the viscous sublayer at all Reynolds numbers. The spatial extent of this near-wall peak region is approximately invariant under inner normalization, while its large wavelength portion is seen to increase in scale in accord with the position of the mid-layer peak, which resides at a streamwise wavelength that scales with the boundary layer thickness.
PubDate: 2013-11-10

• Turbulent boundary-layer structure of flows over freshwater biofilms
• Abstract: Abstract The structure of the turbulent boundary-layer for flows over freshwater biofilms dominated by the diatom Tabellaria flocculosa was investigated. Biofilms were grown on large test plates under flow conditions in an Australian hydropower canal for periods up to 12 months. Velocity-profile measurements were obtained using LDV in a recirculating water tunnel for biofouled, smooth and artificially sandgrain roughened surfaces over a momentum thickness Reynolds number range of 3,000–8,000. Significant increases in skin friction coefficient of up to 160 % were measured over smooth-wall values. The effective roughnesses of the biofilms, k s, were significantly higher than their physical roughness measured using novel photogrammetry techniques and consisted of the physical roughness and a component due to the vibration of the biofilm mat. The biofilms displayed a k-type roughness function, and a logarithmic relationship was found between the roughness function and roughness Reynolds number based on the maximum peak-to-valley height of the biofilm, R t. The structure of the boundary layer adhered to Townsend’s wall-similarity hypothesis even though the scale separation between the effective roughness height and the boundary-layer thickness was small. The biofouled velocity-defect profiles collapsed with smooth and sandgrain profiles in the outer region of the boundary layer. The Reynolds stresses and quadrant analysis also collapsed in the outer region of the boundary layer.
PubDate: 2013-11-07

• Evaluation of the pressure field on a rigid body entering a quiescent fluid through particle image velocimetry
• Abstract: Abstract The objective of this work is to verify the accuracy of indirect pressure measurement from particle image velocimetry in water entry problems. The pressure is evaluated by solving the incompressible Navier–Stokes equations, whose kinematic components are estimated from particle image velocimetry. We focus on the water entry of a rigid wedge, for which we explore variations of the entry velocity. Experimental results are verified through comparison with well-established analytical formulations based on potential flow theory. Our findings demonstrate the feasibility of accurately reconstructing the hydrodynamic pressure field over the entire duration of the impact. Along with a thorough experimental validation of the method, we also offer insight into experimentally relevant factors, such as the maximum resolved fluid velocity and the required spatial integration area.
PubDate: 2013-11-07

• Instantaneous volumic concentration and velocity measurements of a jet in crossflow for the evaluation of the entrainment
• Abstract: Abstract A new method employing instantaneous 3D coupled measurements of planar laser-induced fluorescence and particle Image velocimetry is developed to compute 3D fluid fluxes. The three-dimensional fields of concentration and velocity are used for identifying the vortex structures and for the evaluation of the fluxes at the interface between a jet exiting in a crossflow. This approach is helpful for the 3D estimation of the momentum and scalar transport at the turbulent/non-turbulent interface and will help understanding the mixing process.
PubDate: 2013-11-06

• Evolution of coherent structures in turbulent boundary layers based on moving tomographic PIV
• Abstract: Abstract A moving tomographic particle image velocimetry method was designed and implemented to measure temporal evolution of velocity fields in three-dimensional volumes and to track coherent structures within a turbulent boundary layer with Re τ ≈ 2,410. The evolution of hairpin structures and eddy packets was examined at two locations in the logarithmic region: z + = 100–300 and 300–500. Meandering, merging and breaking of long slow regions associated with packets were observed. The meandering of long slow regions at both wall-normal locations was tracked using cross-correlation between neighboring time steps. It was found that long slow regions could persist within the logarithmic region over a travel distance of 15δ corresponding to a time period 24.3δ/U ∞ (t + > 2,300) and that the packet regions could travel stably forward in the streamwise direction while maintaining fixed spanwise inclinations in the range 0°–10°.
PubDate: 2013-11-06

• Subsonic jet pressure fluctuation characterization by tomographic laser interferometry
• Abstract: Abstract This paper describes the application of a nonconventional experimental technique based on optical interferometry for the characterization of aeroacoustic sources. The specific test case studied is a turbulent subsonic jet. Traditional experimental methods exploited for the measurement of aerodynamic velocity fields are laser Doppler anemometer and particle image velocimetry which have an important drawback due to the fact that they can measure only if the flow is seeded with tracer particles. The technique proposed, by exploiting a laser Doppler interferometer and a tomographic algorithm for 3D field reconstruction, overcomes the problem of the flow seeding since it allows directly measuring the flow pressure fluctuation due to the flow turbulence. A laser Doppler interferometer indeed is sensitive to the density oscillation within the medium traversed by the laser beam even though it integrates the density oscillation along the entire path traveled by the laser. Consequently, the 3D distribution of the flow density fluctuation can be recovered only by exploiting a tomographic reconstruction algorithm applied to several projections. Finally, the flow pressure fluctuation can be inferred from the flow density measured, which comprehends both the aerodynamic pressure related to the turbulence and the sound pressure due to the propagation of the acoustic waves into the far field. In relation to the test case studied in this paper, e.g., the turbulent subsonic jet, the method allows a complete aeroacoustic characterization of the flow field since it measures both the aerodynamic “cause” of the noise, such as the vortex shedding, and the acoustic “effect” of it, i.e., the sound propagation in the 3D space. The performances and the uncertainty have been evaluated and discussed, and the technique has been experimentally validated.
PubDate: 2013-10-31

• Scalar gradient trajectory measurements using high-frequency cinematographic planar Rayleigh scattering
• Abstract: Abstract In this work, we perform an experimental investigation into statistics based on scalar gradient trajectories in a turbulent jet flow, which have been suggested as an alternative means to analyze turbulent flow fields by Wang and Peters (J Fluid Mech 554:457–475, 2006, 608:113–138, 2008). Although there are several numerical simulations and theoretical works that investigate the statistics along gradient trajectories, only few experiments in this area have been reported. To this end, high-frequency cinematographic planar Rayleigh scattering imaging is performed at different axial locations of a turbulent propane jet issuing into a CO2 coflow at nozzle-based Reynolds numbers Re 0 = 3,000–8,600. Taylor’s hypothesis is invoked to obtain a three-dimensional reconstruction of the scalar field in which then the corresponding scalar gradient trajectories can be computed. These are then used to examine the local structure of the mixture fraction with a focus on the scalar turbulent/non-turbulent interface. The latter is a layer that is located between the fully turbulent part of the jet and the outer flow. Using scalar gradient trajectories, we partition the turbulent scalar field into these three regions according to an approach developed by Mellado et al. (J Fluid Mech 626:333–365, 2009). Based on the latter, we investigate the probability to find the respective regions as a function of the radial distance to the centerline, which turns out to reveal the meandering nature of the scalar T/NT interface layer as well as its impact on the local structure of the turbulent scalar field.
PubDate: 2013-10-31

• Analysis of flow and density oscillations in a swirl-stabilized flame employing highly resolving optical measurement techniques
• Abstract: Abstract Modern aircraft engines operate with a reduced core air mass flow, which is challenging regarding an efficient and most of all stable combustion of fuel. A variable geometry burner investigated here allows a stable lean combustion with lower air mass flow rate than with a fixed geometry. In order to optimize such burners further, the occurring flame instabilities have to be investigated. This requires optical measurement techniques with a high measurement rate and an insensitivity regarding flame glow. Concerning flow velocity measurements, the frequency modulated Doppler global velocimetry (FM-DGV) fulfills these demands. In the swirl-stabilized flame of the variable geometry burner, spectra up to 2.5 kHz of the flow velocity field were obtained with FM-DGV. For example, a resonance peak at about 255 Hz was identified in the swirled flame, which also occurs in complementing density measurements by laser interferometric vibrometry. The combined analysis of velocity and density oscillations offer new insights into the physics of flame flows.
PubDate: 2013-10-30

• Wave patterns generated by an axisymmetric obstacle in a two-layer flow
• Abstract: Abstract Gravity waves generated by a moving obstacle in a two-layer stratified fluid are investigated. The experimental configuration is three-dimensional with an axisymmetric obstacle which is towed in one of the two layers. The experimental method used in the present study is based on a stereoscopic technique allowing the 3D reconstruction of the interface between the two layers. Investigation into the wave pattern as a function of the Froude number, Fr, based on the relative density of the fluid layers and the velocity of the towed obstacle is presented. Specific attention is paid to the transcritical regime for which Fr is close to one. Potential energy trapped in the wave field patterns is also extracted from the experimental results and is analyzed as a function of both the Froude number, Fr, and the transcritical similarity parameter $\varGamma$ . In particular, a remarkable increase in the potential energy around Fr = 1 is observed and a scaling allowing to assemble data resulting from different experimental parameters is proposed.
PubDate: 2013-10-30

• Tomographic PIV measurements in a turbulent lifted jet flame
• Abstract: Abstract Measurements of instantaneous volumetric flow fields are required for an improved understanding of turbulent flames. In non-reacting flows, tomographic particle image velocimetry (TPIV) is an established method for three-dimensional (3D) flow measurements. In flames, the reconstruction of the particles location becomes challenging due to a locally varying index of refraction causing beam-steering. This work presents TPIV measurements within a turbulent lifted non-premixed methane jet flame. Solid seeding particles were used to provide the 3D flow field in the vicinity of the flame base, including unburned and burned regions. Four cameras were arranged in a horizontal plane around the jet flame. Following an iterative volumetric self-calibration procedure, the remaining disparity caused by the flame was less than 0.2 pixels. Comparisons with conventional two-component PIV in terms of mean and rms values provided additional confidence in the TPIV measurements.
PubDate: 2013-10-29

• Plane jet excited by disturbances with spanwise phase variations
• Abstract: Abstract Evolution of the near-field structures of a plane jet excited by temporal periodic disturbances with spanwise phase variations was investigated with stereoscopic particle image velocimetry. The three-dimensional vorticity distributions were reconstructed by using Taylor’s frozen field hypothesis. When ϕ, the temporal phase difference of disturbances in the spanwise direction was π; chain-link-fence type structures were formed. The $\Uplambda$ vortices in the chain-link-fence structures were then distorted into an $\Upomega$ shape, and the head of the vortex was detached and reconnects to form a vortex ring, or reconnects to the adjacent V-shaped vortices to form an A-shaped vortex. After the reconnection stage, the flow field was occupied by uniformly distributed fine scale eddies. Here, the overall turbulent kinetic energy and shear stress were suppressed, and the jet width was narrower than that of the unexcited case and other forced cases. In the case of ϕ = π/2, spanwise rollers and rib structures were formed near the nozzle exit after the first vortex pairing. However, further vortex pairing did not occur downstream, and the rate at which the jet widened was reduced.
PubDate: 2013-10-29

• Hybrid transition control approach for plasma actuators
• Abstract: Abstract This work reports on the development of a novel hybrid transition control method for single DBD plasma actuators. The experiments have been carried out on a natural laminar flow airfoil in a wind tunnel and combine two methods previously used for transition control purposes with DBD plasma actuators: boundary-layer stabilization by quasi-steady wall-parallel momentum addition, and active wave cancelation by linear superposition utilizing modulated momentum injection. For this purpose, the modulated body force is controlled using an improved extremum seeking controller based on an extended Kalman filter. Combining the two methods in a single actuator has advantages. Applied to 2-D Tollmien–Schlichting waves, the achievable transition delay in hybrid mode is significantly larger than the isolated effects, while the energy consumption remains almost unchanged compared to the case of continuous actuation. For a Reynolds number of $Re=0.94 \cdot 10^6$ , a transition delay of ${\Updelta}x/c=0.242$ could be observed.
PubDate: 2013-10-25

• Transmitted light microscopy for visualizing the turbulent primary breakup of a microscale liquid jet
• Abstract: Abstract Aiming at a maximum spatial resolution and a minimum motion blur, a new simple double-imaging transmitted light microscopy technique is developed in this work enabling a fundamental investigation of primary breakup of a microscale liquid jet. Contrary to conventional far-field visualization techniques, the working distance is minimized to increase the numerical aperture. The resulting images provide information about shapes, length scales and velocities of primary liquid structures. The method is applied to an optically dense spray leaving a 109-μm diesel nozzle at various injection pressures under atmospheric conditions. A phenomenological study on the temporal spray evolution is done with focus on droplet and ligament formation. Different breakup processes are identified and described. It is found that the jet is characterized by long ligaments parallel or angular to the inner jet region. These ligaments result from collapsing films developing at the spray edge. A significant influence of outlet velocity variation on shape and velocity of these ligaments is observed. The experimental results prove that a transmitted light microscopy technique with reduced working distance is an appropriate tool for a better understanding of primary breakup for small-scaled diesel nozzles and a valuable complement to highly complex measurement techniques.
PubDate: 2013-10-24

• Three-dimensional structure of the flow inside the left ventricle of the human heart
• Abstract: Abstract The laboratory models of the human heart left ventricle developed in the last decades gave a valuable contribution to the comprehension of the role of the fluid dynamics in the cardiac function and to support the interpretation of the data obtained in vivo. Nevertheless, some questions are still opened and new ones stem from the continuous improvements in the diagnostic imaging techniques. Many of these unresolved issues are related to the three-dimensional structure of the left ventricular flow during the cardiac cycle. In this paper, we investigated in detail this aspect using a laboratory model. The ventricle was simulated by a flexible sack varying its volume in time according to a physiologically shaped law. Velocities measured during several cycles on series of parallel planes, taken from two orthogonal points of view, were combined together in order to reconstruct the phase-averaged, three-dimensional velocity field. During the diastole, three main steps are recognized in the evolution of the vortical structures: (1) straight propagation in the direction of the long axis of a vortex ring originated from the mitral orifice; (2) asymmetric development of the vortex ring on an inclined plane; and (3) single vortex formation. The analysis of three-dimensional data gives the experimental evidence of the reorganization of the flow in a single vortex persisting until the end of the diastole. This flow pattern seems to optimize the cardiac function since it directs velocity towards the aortic valve just before the systole and minimizes the fraction of blood residing within the ventricle for more cycles.
PubDate: 2013-10-24

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