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 Experiments in Fluids   [SJR: 1.596]   [H-I: 69]   [8 followers]  Follow         Hybrid journal (It can contain Open Access articles)    ISSN (Print) 1432-1114 - ISSN (Online) 0723-4864    Published by Springer-Verlag  [2292 journals]
• Generation of vertical gusts in a transonic wind tunnel
• Abstract: Abstract This article reports on the qualification of a gust generator device in a transonic wind tunnel. A vanning apparatus has been installed in the contraction of the S3Ch transonic wind tunnel at the ONERA Meudon center in order to generate up and down air movements in the test section. The apparatus has been tested in a range of Strouhal number based on frequency and vane chord up to 0.15 and in a range of Mach number between 0.3 and 0.73. The amplitude of the gusts has been characterized by a fast-response two-hole pressure probe and phase-averaged PIV. The system delivers vertical velocity amplitude of 0.5 % of the freestream velocity at transonic speeds. For a constant vane oscillation angle, the gust strength is found to increase with the Strouhal and the Mach numbers. The gust exhibit a satisfying uniformity and a quasi-sinusoidal waveform. A simple dynamic point vortex model of the oscillating vanes and of the downstream wake has been developed in order to (1) compare the experimental results and (2) enrich the description of the flow induced by the gusts. In particular, the model is used to analyze the detrimental effect of the upper and lower walls. This simple unsteady model gives a valuable prediction of the amplitude of the gust obtained in the tunnel and the workable frequency range permitted by the present apparatus.
PubDate: 2015-07-02

• Effects of an upstream tetrahedron on the circular cylinder–flat
plate juncture flow
• Abstract: Abstract A technique of installing a tetrahedron at the upstream corner of the circular cylinder–flat plate juncture is developed to control the characteristic horseshoe vortices appearing in the natural juncture flow. The Reynolds numbers based on the cylinder diameter are within the range of 500–2900. The flow patterns and time-averaged velocity fields in the vertical symmetry plane and a horizontal plane near the flat plate of the natural and tetrahedron-controlled juncture flows are examined by using the laser-assisted particle flow visualization method and particle image velocimetry in a towing water tank. The flow approaching the circular cylinder–flat plate juncture can induce a characteristic horseshoe vortical flow consisting of a single vortex, dual vortex, or triple vortex. These horseshoe vortices appearing in the natural case may be changed to a characteristic mode of vortical flow, reverse flow, or forward flow when a tetrahedron is installed at the upstream corner of the juncture. The appearance of the vortical flow, reverse flow, or forward flow mode depends on the geometric parameters of normalized axial length, expansion angle, and tilt angle as well as the flow parameter of the Reynolds number. The vortical flow mode appears at small axial length of tetrahedron. The forward flow mode appears at the large axial length of tetrahedron. When the forward flow mode appears, the boundary-layer upstream of the circular cylinder does not separate. Therefore, the horseshoe vortices induced in the natural juncture flow disappear. The data bank consists of the design parameters of axial length, tilt angle, and expansion angle of the tetrahedron, which is provided as a figure.
PubDate: 2015-07-02

• Heat release and UV–Vis radiation in non-premixed
hydrogen–oxygen flames
• Abstract: Abstract The relationship between the volumetric heat release rate and radiation of non-premixed hydrogen–oxygen flames at atmospheric and elevated pressure is investigated. Both the radiation of the excited hydroxyl radical ( $${\hbox {OH}^*}$$ ) and the continuous blue radiation are considered. To physically interpret radiation and heat release, the phenomena are first analyzed within laminar flames following a hybrid approach: a pressurized jet flame experiment is set up to correctly measure the $${\hbox {OH}^*}$$ and blue radiation. The heat release rate is obtained from a complementary CFD simulation. Radiation and heat release are clearly uncorrelated for changes in pressure. Spatially, radiation and heat release occur at separate locations. To further scrutinize the laminar flame structure, non-premixed counterflow flame simulations are performed. By considering statistical ensembles of flamelets, these findings are transferred onto turbulent flames. As before, no general direct proportionality between radiation and heat release rate is observed because of flame straining. A technique for correcting these effects is applied, and its potential is evaluated. The impact of self-absorption of $${\hbox {OH}^*}$$ radiation at elevated pressures on its interpretation is discussed.
PubDate: 2015-06-30

• Impaction of spray droplets on leaves: influence of formulation and leaf
character on shatter, bounce and adhesion
• Abstract: Abstract This paper combines experimental data with simple mathematical models to investigate the influence of spray formulation type and leaf character (wettability) on shatter, bounce and adhesion of droplets impacting with cotton, rice and wheat leaves. Impaction criteria that allow for different angles of the leaf surface and the droplet impact trajectory are presented; their predictions are based on whether combinations of droplet size and velocity lie above or below bounce and shatter boundaries. In the experimental component, real leaves are used, with all their inherent natural variability. Further, commercial agricultural spray nozzles are employed, resulting in a range of droplet characteristics. Given this natural variability, there is broad agreement between the data and predictions. As predicted, the shatter of droplets was found to increase as droplet size and velocity increased, and the surface became harder to wet. Bouncing of droplets occurred most frequently on hard-to-wet surfaces with high-surface-tension mixtures. On the other hand, a number of small droplets with low impact velocity were observed to bounce when predicted to lie well within the adhering regime. We believe this discrepancy between the predictions and experimental data could be due to air layer effects that were not taken into account in the current bounce equations. Other discrepancies between experiment and theory are thought to be due to the current assumption of a dry impact surface, whereas, in practice, the leaf surfaces became increasingly covered with fluid throughout the spray test runs.
PubDate: 2015-06-30

• A new method for measuring turbulent heat fluxes using PIV and
fast-response cold-wires
• Abstract: Abstract A new method for measuring turbulent heat fluxes using a combination of particle image velocimetry and a nanoscale fast-response cold-wire is tested by examining a rough-wall turbulent boundary layer subject to weakly stable stratification. The method has the advantages of simple calibration and setup, as well as providing spatial correlations of velocity and temperature and their associated integral length scales. The accuracy of using Taylor’s hypothesis when employing a large field of view is investigated. Heat flux, velocity–temperature correlation coefficients and turbulent Prandtl number profiles, as well as spatial velocity and temperature correlations, are presented.
PubDate: 2015-06-26

• On the universality of inertial energy in the log layer of turbulent
boundary layer and pipe flows
• Abstract: Abstract Recent experiments in high Reynolds number pipe flow have shown the apparent obfuscation of the $$k_x^{-1}$$ behaviour in spectra of streamwise velocity fluctuations (Rosenberg et al. in J Fluid Mech 731:46–63, 2013). These data are further analysed here from the perspective of the $$\log r$$ behaviour in second-order structure functions, which have been suggested as a more robust diagnostic to assess scaling behaviour. A detailed comparison between pipe flows and boundary layers at friction Reynolds numbers of $${{Re}}_\tau \approx$$  5000–20,000 reveals subtle differences. In particular, the $$\log r$$ slope of the pipe flow structure function decreases with increasing wall distance, departing from the expected $$2A_1$$ slope in a manner that is different to boundary layers. Here, $$A_1 \approx 1.25$$ , the slope of the log law in the streamwise turbulence intensity profile at high Reynolds numbers. Nevertheless, the structure functions for both flows recover the $$2A_1$$ slope in the log layer sufficiently close to the wall, provided the Reynolds number is also high enough to remain in the log layer. This universality is further confirmed in very high Reynolds number data from measurements in the neutrally stratified atmospheric surface layer. A simple model that accounts for the ‘crowding’ effect near the pipe axis is proposed in order to interpret the aforementioned differences.
PubDate: 2015-06-25

• Laser-induced micro-jetting from armored droplets
• Abstract: Abstract We present findings from an experimental study of laser-induced cavitation within a liquid drop coated with a granular material, commonly referred to as “armored droplets” or “liquid marbles.” The cavitation event follows the formation of plasma after a nanosecond laser pulse. Using ultra-high-speed imaging up to 320,610 fps, we investigate the extremely rapid dynamics following the cavitation, which manifests itself in the form of a plethora of micro-jets emanating simultaneously from the spaces between particles on the surface of the drop. These fine jets break up into droplets with a relatively narrow diameter range, on the order of 10 μm.
PubDate: 2015-06-23

• Nanoscale sensing devices for turbulence measurements
• Abstract: Abstract A collection of nanoscale sensing devices developed specifically for high-frequency turbulence measurements is presented. The new sensors are all derived from the nanoscale thermal anemometry probe (NSTAP), which uses a free-standing platinum wire as active sensing element. Each sensor is designed and fabricated to measure a specific quantity and can be customized for special applications. In addition to the original NSTAP (for single-component velocity measurement), the new sensors include the T-NSTAP (for temperature measurement), the x-NSTAP (for two-component velocity measurement), and the q-NSTAP (for humidity measurement). This article provides a summary of the NSTAP family including details of design and fabrication as well as presentation of flow measurements using these sensors. Also, a custom-made constant-temperature anemometer that allows proper operation of the NSTAP sensors will be introduced.
PubDate: 2015-06-20

• Green water velocity due to breaking wave impingement on a tension leg
platform
• Abstract: Abstract The present study employed the image-based bubble image velocimetry (BIV) technique to investigate the flow kinematics of a plunging breaking wave impinging on a geometry-simplified, unrestrained tension leg platform (TLP). A high-speed camera was used to record images for the BIV velocity determination for both fluid and structure velocities. The plunging breaker was generated using a wave focusing method, and repeated measurements were acquired to calculate the mean flow and turbulence intensity using ensemble averaging. BIV measurements were performed on two perpendicular planes: side view and top view. The flow measurements were compared with those of a similar experiment on a fixed structure by Ryu et al. (Exp Fluids 43(4):555–567, 2007a). The maximum velocity occurred in the run-up stage with a magnitude reaching 2.8C with C being the phase speed of the breaking wave. The dominant velocities for three distinct phases—impingement, run-up, and overtopping—are very close to those found on the fixed structure. Turbulence intensity was also examined, and the ratio among the three components was quantified. Ryu et al. (Appl Ocean Res 29(3):128–136, 2007b) reported that Ritter’s dam-break flow solution agrees surprisingly well with the measured green water velocity on the fixed structure to a certain degree. The present study followed the same approach and confirmed that Ritter’s solution is also in excellent agreement with the green water velocity on the unrestrained TLP model. Based on the self-similar behavior, the prediction equation proposed by Ryu et al. (2007a) was used to model the green water velocity distribution. The results show that the prediction equation is applicable to not only a fixed structure, but also the unrestrained TLP for green water velocity caused by extreme waves.
PubDate: 2015-06-20

• Transformation of flow structure on a rotating wing due to variation of
• Abstract: Abstract The flow structure on a rotating wing (rectangular plate) is characterized over a range of travel distance at different radii of gyration. Travel distance is defined as the length of the arc subtended by the radius of gyration. Stereoscopic particle image velocimetry is employed to determine the volumetric flow structure, in the form of three-dimensional surfaces of the q-criterion, helical density, and downwash velocity. These representations are complemented by sectional patterns of vorticity and tangential velocity. An increase in the radius of gyration reduces the influence of rotation on the flow structure. At small radius of gyration, a coherent leading-edge vortex develops rapidly and then persists over a range of travel distance. At moderate radius of gyration, this leading-edge vortex is replaced by an arch vortex, which develops relatively slowly over a larger travel distance, and is eventually swept into the wake of the wing. The foregoing classes of vortical structures are associated with distinctive patterns of: helical density, which represents the axial vorticity flux through the three-dimensional vortex system; downwash related to the strengths of the components of the vortex system; and tangential velocity associated with the extent of reverse flow, or stall.
PubDate: 2015-06-20

• Investigation of wave phenomena on a blunt airfoil with straight and
serrated trailing edges
• Abstract: Abstract An investigation of pressure waves in compressible subsonic and transonic flow around a generic airfoil is performed in a modified shock tube. New comprehensive results are presented on pressure waves in compressible flow. For the first time, the influence of trailing edge serration will be examined in terms of the reduction in pressure wave amplitude. A generic airfoil is tested in two main configurations, one with blunt trailing edges and the other one with serrated trailing edges in a Mach number range from 0.6 to 0.8 and at chord Reynolds numbers of 1 × 106 < Re c < 5 ×106. The flow of the blunt trailing edge is characterized by a regular vortex street in the wake creating a regular pattern of upstream-moving pressure waves along the airfoil. The observed pressure waves lead to strong pressure fluctuations within the local flow field. A reduction in the trailing edge thickness leads to a proportional increase in the frequency of the vortex street in the wake as well as the frequency of the waves deduced from constant Strouhal number. By serrating the trailing edge, the formation of vortices in the wake is disturbed. Therefore, also the upstream-moving waves are influenced and reduced in their strength resulting in a steadier flow. An increasing length of the saw tooth enhances the three dimensionality of the structures in the wake and causes a strong decrease in the wave amplitude.
PubDate: 2015-06-19

• Quantitative wake analysis of a freely swimming fish using 3D synthetic
aperture PIV
• Abstract: Abstract Synthetic aperture PIV (SAPIV) is used to quantitatively analyze the wake behind a giant danio (Danio aequipinnatus) swimming freely in a seeded quiescent tank. The experiment is designed with minimal constraints on animal behavior to ensure that natural swimming occurs. The fish exhibits forward swimming and turning behaviors at speeds between 0.9 and 1.5 body lengths/second. Results show clearly isolated and linked vortex rings in the wake structure, as well as the thrust jet coming off of a visual hull reconstruction of the fish body. As a benchmark for quantitative analysis of volumetric PIV data, the vortex circulation and impulse are computed using methods consistent with those applied to planar PIV data. Volumetric momentum analysis frameworks are discussed for linked and asymmetric vortex structures, laying a foundation for further volumetric studies of swimming hydrodynamics with SAPIV. Additionally, a novel weighted refocusing method is presented as an improvement to SAPIV reconstruction.
PubDate: 2015-06-17

• Measurements of the dynamics of thermal plumes in turbulent mixed
convection based on combined PIT and PIV
• Abstract: Abstract The dynamics of thermal plumes and their abundance is investigated in mixed convection in a cuboidal sample with respect to the characteristic numbers. The parameter range spans $$Ra=1.0{-}3.2\times 10^8$$ , $$Re=0.5{-}1.7\times 10^4$$ and $$Ar=1.1{-}7.6$$ . Combined particle image thermography and particle image velocimetry is conducted in a horizontal layer close above the bottom thermal boundary layer. This combination of measurement techniques, using thermochromic liquid crystals as tracer particles, which is novel for air flows, allows for simultaneous measurement of temperature and velocity fields. Details of the measurement technique are published in Schmeling et al. (Meas Sci Technol 25:035302, 2014). The fingerprints of sheet-like plumes and those of the stems of mushroom-like plumes are visible in the instantaneous temperature fields. A study of temperature PDFs reveals that the distributions can be well described by a sum of two Gaussian distributions. Analysing the ratio of the probabilities $$P_2/P_1$$ reveals a sudden change at a critical Ra c  ≈ 2.3 × 108. Here, $$P_1$$ denotes the abundance of fluid temperatures imprinted by the bulk flow, while $$P_2$$ represses the abundance of temperatures ascribed to warm thermal plumes. Accordingly, $$P_2/P_1$$ is a measure for the plume fraction in the measurement plane. The change occurs in the $$Ar$$ regime $$2.7\,\lesssim\, Ar\,\lesssim\, 3.3$$ , in which the interaction of buoyancy-induced large-scale circulations with the wall jet of the incoming air results in an instability reported already by Schmeling et al. (Exp Fluids 54:1517, 2013). A combined evaluation of the temperature and velocity fields reveals a change in the horizontal heat fluxes at $$Ar\approx 2.7{-}3$$ . Furthermore, the total amount of heat transported in x direction within the measurement layer increases with $$Ra$$ in bulk-dominated regions, while it stays almost constant for plume-dominated ones.
PubDate: 2015-06-16

• Influence of solidification on the impact of supercooled water drops onto
cold surfaces
• Abstract: Abstract This study presents an experimental investigation of the impact of a supercooled drop onto hydrophilic and superhydrophobic substrates. The aim is to better understand the process of airframe icing caused by supercooled large droplets, which has been recently identified as a severe hazard in aviation. The Weber number and Reynolds number of the impinging drop ranged from 200 to 300 and from 2600 to 5800, respectively. Drop impact, spreading, and rebound were observed using a high-speed video system. The maximum spreading diameter of an impacting drop on hydrophilic surfaces was measured. The temperature effect on this parameter was only minor for a wide range of the drop and substrate temperatures. However, ice/water mixtures emerged when both the drop and substrate temperatures were below 0 °C. Similarly, drop rebound on superhydrophobic substrates was significantly hindered by solidification when supercooled drop impacted onto substrates below the freezing point. The minimum receding diameter and the speed of ice accretion on the substrate were measured for various wall temperatures. Both parameters increased almost linearly with decreasing wall temperature, but eventually leveled off beyond a certain substrate temperature. The rate of ice formation on the substrate was significantly higher than the growth rate of free ice dendrites, implying that multiple nucleation sites were present.
PubDate: 2015-06-13

• Interfacial shear stress measurement using high spatial resolution
multiphase PIV
• Abstract: Abstract In multiphase flows, form drag and viscous shear stress transfer momentum between phases. For numerous environmental and man-made flows, it is of primary importance to predict this transfer at a liquid–gas interface. In its general expression, interfacial shear stress involves local velocity gradients as well as surface velocity, curvature, and surface tension gradients. It is therefore a challenging quantity to measure experimentally or compute numerically. In fact, no experimental work to date has been able to directly resolve all the terms contributing to the shear stress in the case of curved and moving surfaces. In an attempt to fully resolve the interface shear stress when surface tension gradients are negligible, high-resolution particle image velocimetry (PIV) data are acquired simultaneously on both sides of a water–air interface. The flow consists of a well-conditioned uniform and homogeneous water jet discharging in quiescent air, which exhibits two-dimensional surface waves as a result of a shear layer instability below the surface. PIV provides velocity fields in both phases, while planar laser-induced fluorescence is used to track the interface and obtain its curvature. To compute the interfacial shear stress from the data, several processing schemes are proposed and compared, using liquid and/or gas phase data. Vorticity at the surface, which relates to the shear stress through the dynamic boundary condition at the surface, is also computed and provides additional strategies for estimating the shear. The various schemes are in agreement within the experimental uncertainties, validating the methodology for experimentally resolving this demanding quantity.
PubDate: 2015-06-13

• Aerodynamics of dynamic wing flexion in translating wings
• Abstract: Abstract We conducted a systematic experimental study to investigate the aerodynamic effects of active trailing-edge flexion on a high-aspect-ratio wing translating from rest at a high angle of attack. We varied the timing and speed of the trailing-edge flexion and measured the resulting aerodynamic effects using a combination of direct force measurements and two-dimensional PIV flow measurements. The results indicated that the force and flow characteristics depend strongly on the timing of flexion, but relatively weakly on its speed. This is because the force and vortical flow structure are more sensitive to the timing of flexion relative to the shedding of starting vortex and leading-edge vortex. When the trailing-edge flexion occurred slightly before the starting vortex was shed, the lift production was greatly improved with the instantaneous peak lift increased by 54 % and averaged lift increased by 21 % compared with the pre-flexed case where the trailing-edge flexed before wing translation. However, when the trailing-edge flexed during or slightly after the leading-edge vortex shedding, the lift was significantly reduced by the disturbed development of leading-edge vortex. The force measurement results also imply that the trailing-edge flexion prior to wing translation does not augment lift but increases drag, thus resulting in a lower lift–drag ratio as compared to the case of flat wing.
PubDate: 2015-06-11

• Experimental study of a three-dimensional cylinder–filament system
• Abstract: Abstract This experimental study reports on the behavior of a filament attached to the rear of a three-dimensional cylinder. The axis of the cylinder is placed normal to a uniform incoming flow, and the filament is free to move in the cylinder wake. The mean position of the filament is studied as a function of the filament length L. It is found that for long (L/D  $$>$$  6.5, where D is the cylinder diameter) and short (L/D  $$<$$  2) filaments, the mean position of the filament tends to align with the incoming flow, whereas for intermediate filament lengths (2  $$<$$  L/D  $$<$$  6.5), the filament lies down on the cylinder and tends to align with the cylinder axis. The underlying mechanism of the bifurcations is discussed and related to buckling and inverted-pendulum-like instabilities.
PubDate: 2015-06-09

• An investigation of channel flow with a smooth air–water interface
• Abstract: Abstract Experiments and numerical simulation are used to investigate fully developed laminar and turbulent channel flow with an air–water interface as the lower boundary condition. Laser Doppler velocimetry measurements of streamwise and wall-normal velocity components are made over a range of Reynolds number based upon channel height and bulk velocity from 1100 to 4300, which encompasses the laminar, transitional and low Reynolds numbers turbulent regimes. The results show that the airflow statistics near the stationary wall are not significantly altered by the air–water moving interface and reflect those found in channel flows. The mean statistics on the water interface side largely exhibit results similar to simulated Poiseuille–Couette flow (PCF) with a solid moving wall. For second-order statistics, however, the simulation and experimental results show some discrepancies near the moving water surface, suggesting that a full two-phase simulation is required. A momentum and energy transport tubes analysis is investigated for laminar and turbulent PCFs. This analysis builds upon the classical notion of a streamtube and indicates that part of the energy from the pressure gradient is transported towards the stationary wall and is dissipated as heat inside the energy tubes, while the remainder is transmitted to the moving wall. For the experiments, the airflow energy is transmitted towards the water to overcome the drag force and drive the water forward; therefore, the amount of energy transferred to the water is higher than the energy transferred to a solid moving wall.
PubDate: 2015-06-09

• Particle imaging through planar shock waves and associated velocimetry
errors
• Abstract: Abstract When imaging particles through a shock wave, the resulting particle image appears blurred and at the wrong location, which is referred to as a position error. Particle image doublets are observed if only part of the light scattered by a particle is deflected or reflected by the shock. These optical distortions are due to the jump in the refractive index that occurs over the shock. Within the context of popular particle-based velocimetry techniques, such as particle image velocimetry and particle tracking velocimetry, the position error propagates into an error in the measured velocity. These particle image distortions and associated errors are assessed and quantified in this paper for the case of planar shocks by means of a light ray tracing approach and by experiments. The errors are shown to be most sensitive to the angle between the viewing direction and the plane of the shock. Increasing this angle to modest values (~5°) is a particularly effective way to decrease the relative velocity error. Looking at the shock from the high-density side is recommended when the accurate determination of the particle response to the shock wave is desired.
PubDate: 2015-06-06

• The effect of aspect ratio on the wake of the Ahmed body
• Abstract: Abstract This paper seeks to further elucidate the wake of the Ahmed body by investigating how the time-averaged flow structures vary with frontal aspect ratio. High-resolution particle image velocimetry results are provided for eight different width Ahmed geometries at $$\text {Re}_{\sqrt{FA}}=3\times 10^4$$ . It is shown that the narrower the body, the greater the downwash over the back slant, meaning the flow remains more attached. At a critical aspect ratio ( $$=1.9$$ ), the flow downstream changes. The separation over the back slant is shown to be affected by the , and this in turn has a significant effect on the circulation in the c-pillar vortices.
PubDate: 2015-06-04

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