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 Experiments in Fluids   [SJR: 1.033]   [H-I: 62]   [8 followers]  Follow         Hybrid journal (It can contain Open Access articles)    ISSN (Print) 1432-1114 - ISSN (Online) 0723-4864    Published by Springer-Verlag  [2300 journals]
• Vortex-enhanced mixing through active and passive flow control methods
• Abstract: Abstract This study aims to understand the underlying physics of vortex-enhanced mixing through active and passive flow control methods. To find a best flow control method that enhances turbulent mixing through the generation of streamwise vortices, an experimental investigation was carried out to compare active and passive flow control methods of an incompressible axisymmetric jet. For active flow control, the lip of the circular jet was equipped with a single small flap deflected away from the jet stream at an angle of 30° to the jet axis. The flap incorporated a flow control slot through which steady and oscillatory suction were implemented. The active flow control methods require power input to the suction devices. For passive flow control, the lip of the circular jet was equipped with a single small delta tab deflected into the jet stream at an angle of 30° to the jet axis. The chord lengths of the flap and delta tab were one-sixth of the jet diameter. The momentum of jet increased in the case of active flow control by entraining the ambient fluid, whereas momentum decreased in the case of passive flow control. The effect of steady suction saturated for volumetric suction coefficient values greater than 0.82 %. The strength of streamwise vortices generated by the flap were greater than those generated by the delta tab. Steady suction produced positive pressures just downstream of the flow control slot in the central portion of the flap and negative pressures at the flap edges. Oscillatory suction was highly dependent on dimensionless frequency (F +) based on the distance from the flow control slot to the flap trailing edge; the pressures on the central portion of the flap increased for F + ≤ 0.11 and then decreased for greater F +; finally attained negative pressures at F + = 0.44. The increase in jet momentum and turbulence intensity, combined with the induced streamwise vorticity, makes steady suction a potential concept for increasing propulsion efficiency through vortex-enhanced mixing. The flow control methods modify the jet flow, which in turn would alter the jet noise spectra.
PubDate: 2015-02-26

• An experimental and theoretical investigation of spray characteristics of
impinging jets in impact wave regime
• Abstract: Abstract The current study focuses on experimentally and theoretically improving the characterization of the drop size and drop velocity for like-on-like doublet impinging jets. The experimental measurements were made using phase Doppler anemometry (PDA) at jet Weber numbers We j corresponding to the impact wave regime of impinging jet atomization. A more suitable dynamic range was used for PDA measurements compared to the literature, resulting in more accurate experimental measurements for drop diameters and velocities. There is some disagreement in the literature regarding the ability of linear stability analysis to accurately predict drop diameters in the impact wave regime. This work seeks to provide some clarity. It was discovered that the assumed uniform jet velocity profile was a contributing factor for deviation between diameter predictions based on models in the literature and experimental measurements. Analytical expressions that depend on parameters based on the assumed jet velocity profile are presented in this work. Predictions based on the parabolic and 1/7th power law turbulent profiles were considered and show better agreement with the experimental measurements compared to predictions based on the previous models. Experimental mean drop velocity measurements were compared with predictions from a force balance analysis, and it was observed that the assumed jet velocity profile also influences the predicted velocities, with the turbulent profile agreeing best with the experimental mean velocity. It is concluded that the assumed jet velocity profile has a predominant effect on drop diameter and velocity predictions.
PubDate: 2015-02-20

• Improvement in spatial resolution of background-oriented schlieren
technique by introducing a telecentric optical system and its application
to supersonic flow
• Abstract: Abstract A telecentric optical system is applied to the background-oriented schlieren (BOS) technique to improve accuracy, overcoming the drawbacks of conventional diverging light observation. This paper describes the optical arrangement and formula for telecentric BOS measurement and presents measurement results obtained by the colored-grid background-oriented schlieren technique to confirm the theoretical prediction. The application of the new approach for a large-scale supersonic wind tunnel test is reported.
PubDate: 2015-02-19

• Experiments on the effect of laminar–turbulent transition on the
SWBLI in H2K at Mach 6
• Abstract: Abstract This paper presents the results of the experiments performed in the hypersonic wind tunnel H2K in the framework of the ESA technology research project “laminar to turbulent transition in hypersonic flows”. The investigations include the free boundary-layer transition on a flat plate as well as the influence of a shock wave–boundary layer interaction on the transition. The shock is created by a wedge with a small angle of attack resulting in a moderate shock intensity. The experiments were performed at Mach 6.0, at three different unit Reynolds numbers and with a translational displacement of the shock generator. Besides the optical methods—Schlieren photography and infrared thermography—several intrusive sensors were used. High-speed measurements were carried out using PCB and atomic layer thermo pile sensors. Kulite sensors were used for low- and mid-speed pressure measurements. The data analysis includes the comparison of the absolute values, the frequency spectra and wavelets and their distributions in time and space.
PubDate: 2015-02-19

• A novel approach for reconstructing pressure from PIV velocity
measurements
• Abstract: Abstract The purpose of this work is to develop an innovative procedure for reconstructing the pressure field from PIV velocity measurements of unsteady, incompressible flows. The proposed technique is based on a generalization of the Glowinski–Pironneau method for the uncoupled solution of the incompressible Navier–Stokes equations written in primitive variables and exploits a finite element discretization of the measurement grid. By virtue of the underlying mathematical formulation, the method is stable and more accurate than the other techniques proposed so far in the literature. The method is first applied to an exact solution of the Navier–Stokes equations, showing second-order convergence of the $$L^{\infty }$$ error for the pressure variable. The robustness of the method with respect to stochastic perturbations in the velocity field is then tested and the results compared with other techniques proposed in the literature. Finally, the proposed technique is applied to both phase-averaged and time-resolved PIV velocity measurements of the flow around a pitching airfoil employed to investigate the dynamic stall. The reconstructed pressure is compared with direct pressure measurements, showing very encouraging results.
PubDate: 2015-02-15

• Three-dimensional flow structures and unsteady forces on pitching and
surging revolving flat plates
• Abstract: Abstract Tomographic particle image velocimetry was used to explore the evolution of three-dimensional flow structures of revolving low-aspect-ratio flat plates in combination with force measurements at a Reynolds number of 10,000. Two motion kinematics are compared that result in the same terminal condition (revolution with constant angular velocity and $$45^{\circ }$$ angle of attack) but differ in the motion during the buildup phase: pitching while revolving at a constant angular velocity; or surging with a constant acceleration at a fixed angle of attack. Comparison of force histories shows that the pitching wing generates considerably higher forces during the buildup phase which is also predicted by a quasi-steady model quite accurately. The difference in the buildup phases affects the force histories until six chords of travel after the end of buildup phase. In both cases, a vortex system that is comprised of a leading-edge vortex (LEV), a tip vortex and a trailing edge vortex is formed during the initial period of the motion. The LEV lifts off, forms an arch-shaped structure and bursts into substructures, which occur at slightly different phases of the motions, such that the revolving–surging wing flow evolution precedes that of the revolving–pitching wing. The delay is shown to be in accordance with the behavior of the spanwise flow which is affected by the interaction between the tip vortex and revolving dynamics. Further analysis shows that the enhanced force generation of the revolving–pitching wing during the pitch-up phase originates from: (1) increased magnitude and growth rate of the LEV circulation; (2) relatively favorable position and trajectory of the LEV and the starting vortex; and (3) generation of bound circulation during the pitching motion, whereas that of the revolving–surging wing is negligible in the acceleration phase.
PubDate: 2015-02-15

• Investigation of coherent structures generated by acoustic tube in
turbulent flow separation control
• Abstract: Abstract An acoustic tube was designed in order to control the turbulent flow separation downstream of a backward-facing step. The Reynolds number based on the free-stream velocity and the step height was Re h = 2.0 × 104. As an active flow control device, the acoustic tube generated periodic pressure perturbations at a frequency of f a = 100 Hz, which was close to the most amplified frequency of the shedding instability of the turbulent shear layer. Spanwise vortices rolled up due to the perturbations. 2D–2C particle image velocimetry was used to measure separated shear layer and the reattachment area downstream of the BFS. The flow control results show that the acoustic tube can suppress recirculation regions behind the step and reduce the reattachment length by 43.7 %. The roll-up and pairing processes of the vortices lead to an increase in the total Reynolds shear stress. The coherent structures are extracted by proper orthogonal decomposition and represented by two pairs of modes, of which the coherence is analyzed by the corresponding coefficients. Both the primary and secondary series of vortices are reconstructed as traveling waves with the fundamental frequency f a and the overtone frequency 2f a, respectively.
PubDate: 2015-02-15

• Pressure-field extraction on unstructured flow data using a Voronoi
tessellation-based networking algorithm: a proof-of-principle study
• Abstract: Abstract A novel technique is described for pressure extraction from Lagrangian particle-tracking data. The technique uses a Poisson solver to extract the pressure field on a network of data nodes, which is constructed using the Voronoi tessellation and the Delaunay triangulation. The technique is demonstrated on two cases: synthetic Lagrangian data generated for the analytical case of Hill’s spherical vortex, and the flow in the wake behind a NACA 0012 which was impulsively accelerated to $$Re = 7{,}500$$ . The experimental data were collected using four-camera, three-dimensional particle-tracking velocimetry. For both the analytical case and the experimental case, the dependence of pressure-field error or sensitivity on the normalized spatial particle density was found to follow similar power-law relationships. It was shown that in order to resolve the salient flow structures from experimental data, the required particle density was an order of magnitude greater than for the analytical case. Furthermore, additional sub-structures continued to be identified in the experimental data as the particle density was increased. The increased density requirements of the experimental data were assumed to be due to a combination of phase-averaging error and the presence of turbulent coherent structures in the flow. Additionally, the computational requirements of the technique were assessed. It was found that in the current implementation, the computational requirements are slightly nonlinear with respect to the number of particles. However, the technique will remain feasible even as advancements in particle-tracking techniques in the future increase the density of Lagrangian data.
PubDate: 2015-02-13

• Proper orthogonal decomposition based outlier correction for PIV data
• Abstract: Abstract Particle image velocimetry (PIV) is a powerful tool to study complex flows quantitatively. Post-processing of PIV data is necessary for outlier correction (OC) because of the image noise. Traditional methods detect and correct spurious vectors, respectively, using local statistical models. A new method proposed in this paper iteratively detects and replaces outliers using proper orthogonal decomposition (POD), which can dynamically approximate the original pure velocity field. The new algorithm, named as POD-OC, reconstructs a reference velocity field using low-order POD modes to detect outliers and uses that reference field for OC as well. Compared with the method of normalized median test, POD-OC is more efficient for detecting clustered outliers. It is also more accurate than other common interpolation approaches on outlier fixing. A novel block POD-OC is also designed for post-processing on an instantaneous velocity field, which overcomes the limit that POD can only be applied on a dataset with a large number of instantaneous fields.
PubDate: 2015-02-12

• On the use of helium-filled soap bubbles for large-scale tomographic PIV
in wind tunnel experiments
• Abstract: Abstract The flow-tracing fidelity of sub-millimetre diameter helium-filled soap bubbles (HFSB) for low-speed aerodynamics is studied. The main interest of using HFSB in relation to micron-size droplets is the large amount of scattered light, enabling larger-scale three-dimensional experiments by tomographic PIV. The assessment of aerodynamic behaviour closely follows the method proposed in the early work of Kerho and Bragg (Exp Fluids 50:929–948, 1994) who evaluated the tracer trajectories around the stagnation region at the leading edge of an airfoil. The conclusions of the latter investigation differ from the present work, which concludes sub-millimetre HFSB do represent a valid alternative for quantitative velocimetry in wind tunnel aerodynamic experiments. The flow stagnating ahead of a circular cylinder of 25 mm diameter is considered at speeds up to 30 m/s. The tracers are injected in the free-stream and high-speed PIV, and PTV are used to obtain the velocity field distribution. A qualitative assessment based on streamlines is followed by acceleration and slip velocity measurements using PIV experiments with fog droplets as a term of reference. The tracing fidelity is controlled by the flow rates of helium, liquid soap and air in HFSB production. A characteristic time response, defined as the ratio of slip velocity and the fluid acceleration, is obtained. The feasibility of performing time-resolved tomographic PIV measurements over large volumes in aerodynamic wind tunnels is also studied. The flow past a 5-cm-diameter cylinder is measured over a volume of 20 × 20 × 12 cm3 at a rate of 2 kHz. The achieved seeding density of <0.01 ppp enables resolving the Kármán vortices, whereas turbulent sub-structures cannot be captured.
PubDate: 2015-02-11

water entry problems through particle image velocimetry
• Abstract: Abstract Predicting the hydrodynamic loading during water impact is of fundamental importance for the design of offshore and aerospace structures. Here, we experimentally characterize the 3D hydrodynamic loading on a rigid wedge vertically impacting a quiescent water surface. Planar particle image velocimetry is used to measure the velocity field on several planes, along the width and the length of the impacting wedge. Such data are ultimately utilized to estimate the 3D velocity field in the whole fluid domain, where the pressure field is reconstructed from the solution of the incompressible Navier–Stokes equations. Experimental results confirm that the velocity field is nearly 2D at the mid-span of the wedge, while the axial velocity along the length of the wedge becomes significant in the proximity of the edges. The variation of the fluid flow along the length of the wedge regulates the hydrodynamic loading experienced during the impact. Specifically, the hydrodynamic loading is maximized at the mid-span of the wedge and considerably decreases toward the edges. The method proposed in this study can find application in several areas of experimental fluid mechanics, where the analysis of unsteady 3D fluid–structure interactions is of interest.
PubDate: 2015-02-10

• Flow field in the wake of a bluff body driven through a steady
recirculating flow
• Abstract: Abstract The wake produced by a bluff body driven through a steady recirculating flow is studied experimentally in a water facility using particle image velocimetry. The bluff body has a rectangular cross section of height, $$H$$ , and width, $$D$$ , such that the aspect ratio, AR = H/D, is equal to 3. The motion of the bluff body is uniform and rectilinear, and corresponds to a Reynolds number based on width, Re D  = 9,600. The recirculating flow is confined within a hemicylindrical enclosure and is generated by planar jets emanating from slots of width, $$h$$ , such that $$Re_h=500$$ . Under these conditions, experiments are performed in a closed-loop facility that enables complete optical access to the near-wake. Velocity fields are obtained up to a distance of $$13D$$ downstream of the moving body. Data include a selection of phase-averaged velocity fields representative of the wake for a baseline case (no recirculation) and an interaction case (with recirculation). Results indicate that the transient downwash flow typically observed in wakes behind finite bodies of small aspect ratio is significantly perturbed by the recirculating flow. The wake is displaced from the ground plane and exhibits a shorter recirculation zone downstream of the body. In summary, it was found that the interaction between a bluff body wake and a recirculating flow pattern alters profoundly the dynamics of the wake, which has implications on scalar transport in the wake.
PubDate: 2015-02-08

• Corrections on LIFPA velocity measurements in microchannel with moderate
velocity fluctuations
• Abstract: Abstract Laser-induced fluorescence photobleaching anemometer (LIFPA) has been developed in order to measure velocity fluctuations of the unsteady micro electrokinetic turbulent flows in microfluidics. The statistical errors of LIFPA measurement, because of 3-D flows and Taylor’s hypothesis (compared with local Taylor’s hypothesis Pinton and Labbé in J Phys II 4:1461–1468, 1994), are theoretically estimated and compared to hot-wire anemometer (HWA) measurement that is used for conventional turbulence measurement. The correction factor in the direction parallel to the laser beam is estimated, and the influence of directional correction factors of LIFPA is also investigated. It is found that in our investigation, the error due to Taylor’s hypothesis is negligible. The influence of 3-D flows on the first derivative variance of velocity fluctuations in LIFPA is smaller than that in HWA measurement.
PubDate: 2015-02-08

• Mixing enhancement of an axisymmetric jet using flaplets with zero
mass-flux excitation
• Abstract: Abstract A novel active control concept aimed at mixing enhancement of an axisymmetric incompressible jet was investigated experimentally. The lip of the jet was equipped with evenly distributed small flaps, or flaplets, deflected away from the stream at an angle of 30°. Controlled attachment of the jet’s boundary layer to the flaps was achieved by introducing zero mass-flux perturbations through control slots located at the base of the flaps, yielding a radial deflection of the shear layer. As a result, pairs of strong streamwise vortices of a finite length were periodically generated and shed in phase with the control signal. At a Strouhal number of 0.3 based on the nozzle diameter, the perturbations also regulated the shedding of spanwise vortex rings. Hot-wire measurements in the vicinity of the flaplets as well as phase-averaged stereoscopic PIV measurements at various streamwise locations were employed to elucidate the mechanism of controlled attachment and to map the evolution of the coherent structures. The strength of axial vorticity was strongly dependent upon the control frequency. A semiempirical framework adopted to quantify the overall effect of control predicted a significant increase in mixing in the region close to the nozzle.
PubDate: 2015-02-08

• Physics-based scaling laws for confined and unconfined transverse jets
• Abstract: Abstract An experimental study was conducted to explore the mixing properties of single and multiple confined transverse jets. A new physics-based scaling law variable was developed based on unconfined transverse jet trajectories. This variable accounts for both entrainment and drag momentum transport mechanisms that cause the jet deflection. The utility of this parameter under confined conditions was considered. It was observed that this new scaling parameter does correlate both qualitative and quantitative measures of the mean mixture properties, in particular prior to any jet–wall interactions. It was found that no local optimum mixing condition was present for two and three jets. For six jets, the behavior changed dramatically, with the emergence of a local optimum mixing state that is consistent with previous data collected for gas turbine geometries (Holdeman in Prog Energy Combust Sci 19:31–70, 1993). It is apparent that the local optimum observed for six jets involves jet penetration to a finite radial position while spreading in the cross plane, leading to the jets blending together resulting in a highly uniform mean mixture fraction distribution. When the number of jets is three or less, this blending process cannot occur due to the excessive distance between the jets. Jet impaction at the pipe center facilitates mixing for two and three jets, while degrading uniformity for six jets.
PubDate: 2015-02-06

• Spatial correlation of measured unsteady surface pressure behind a
backward-facing step
• Abstract: Abstract The spatial correlations of the unsteady surface pressure for a backward-facing step were studied experimentally. The measurements were acquired using an array of surface pressure sensors within an anechoic wind tunnel which was designed to minimize acoustic contamination. The maximum Reynolds number based on the step height was 59,200. The spatial characteristics of the surface pressure were studied in two parts. First, a linear array of microphones oriented in the streamwise direction was used to obtain the evolution of the pressure spectra, length scale, and phase speed. Second, an array oriented in the spanwise direction was used to provide the coherence and integral length scales in that direction. The correlation length functions were found to vary with downstream location in the separated region behind the step. After reattachment, the integral scales of the surface pressure continued to increase in magnitude as far as 18 step heights downstream of the step.
PubDate: 2015-02-06

• Experimental investigations and large-eddy simulation of low-swirl
combustion in a lean premixed multi-nozzle combustor
• Abstract: Abstract This paper presents laser diagnostic experiments and large-eddy simulations (LES) of low-swirl lean premixed methane/air flames in a multi-nozzle combustor including five nozzles with the same structure. OH planar laser-induced fluorescence is used to observe flame shapes and identify main reaction zones. NOx and CO emissions are also recorded during the experiment. The flows and flames are studied at different equivalence ratios ranging from 0.5 to 0.8, while the bulk inlet velocity is fixed at 6.2 m/s. Results show that the neighboring swirling flows interact with each other, generating a highly turbulent interacting zone where intensive reactions take place. The flame is stabilized above the nozzle rim, and its liftoff height decreases with increasing equivalence ratio. The center flow is confined and distorted by the neighboring flows, resulting in instabilities of the center flame. Mean OH radical images reveal that the center nozzle flame is extinguished when equivalence ratio is equals to 0.5, which is successfully predicted by LES. In addition, NOx emissions show log-linear dependency on the adiabatic flame temperature, while the CO emissions remain lower than 10 ppm. NOx emissions for multi-nozzle flame are less sensitive to the flame temperature than that for single nozzle. These results demonstrate that the low-swirl multi-nozzle concept is a promising solution to achieve stable combustion with ultra-low emissions in gas turbines.
PubDate: 2015-02-01

• Effects of pitching phase angle and amplitude on a two-dimensional
flapping wing in hovering mode
• Abstract: Abstract This paper reports on a fundamental investigation of the effects of pitching phase angle (ϕ) and pitching amplitude (α A) on the aerodynamics of a two-dimensional (2D) flapping wing executing simple harmonic motion in hovering mode. A force sensor and digital particle image velocimetry were employed to obtain the time-dependent aerodynamic forces acting on the wing and the associated flow structures, respectively. Pitching phase angle ranging from 0° to 360° at three different pitching amplitudes, that is, 30°, 45° and 60°, was studied. Our experimental results revealed that the largest lift and lift/drag ratio were achieved under the condition of advanced pitching (ϕ > 90°). However, further increasing ϕ beyond a certain value would not enhance the average lift any more. In contrast, the delayed pitching (ϕ < 90°) would cause the average lift to decrease and generally the averaged drag to increase, compared to the normal pitching (ϕ = 90°), overall reducing the lift/drag ratio greatly. Our experimental results also supported the findings of Lua et al. (J Exp Fluids 51:177–195, 2011) that there are two kinds of wing–wake interactions, and they can either enhance or reduce lift on the wing depending on the wing motion and the timing of the reverse stroke. Our results show that wing–wake interaction of the first kind normally has an adverse effect on lift generation when the wing is undergoing delayed pitching but has a positive effect on the lift when the wing is undergoing advanced pitching motion. When the ϕ became larger, the second kind of wing–wake interaction, that is, sliding of the leading edge vortex under the wing, will cause the concurrent fall in lift and drag.
PubDate: 2015-02-01

• Temperature measurements in an axisymmetric methane–air flame using
Talbot images
• Abstract: Abstract The paper discusses the principles of optical testing of transparent objects using the Talbot images method and its applicability to diagnostic of flames. The experimental study was performed for premixed methane–air flame formed by an axisymmetric nozzle. The local deflection angles of the probe radiation were determined from measurements of the relative displacements of intensity maxima of the Talbot image which is caused by passing of light through the flame. The Abel integral equation was solved to reconstruct the refractive index distribution in the flame. Calculation of the temperature field from the refractive index data was based on neglecting the spatial variation of the component composition. Inaccuracy of the calculations was evaluated by comparing the results with the thermocouple measurements. The results demonstrate that the Talbot images method can be used to measure the temperature distribution in axisymmetric reacting gas flows with high spatial resolution.
PubDate: 2015-01-31

• Observations and measurements on unsteady cavitating flows using a
simultaneous sampling approach
• Abstract: Abstract The main purpose of this study was to shed light on the unsteady cavitating flow and corresponding wall-pressure fluctuation characteristic. A simultaneous sampling technique is used to synchronize the observations of cavitation images and the measurements of wall-pressure signals in a convergent–divergent channel. The results show that, with the decreasing cavitation number, cavitating flows in a convergent–divergent channel display several types of cavitation behavior, such as cavitation inception, sheet cavitation, and sheet/cloud cavitation. The intensity of the pressure fluctuation increases with the decrease in cavitation number. However, with decreasing cavitation number, the dominant frequency of the unsteady pressure fluctuation decreases significantly, and for sheet/cloud cavitation, the dominant frequency of pressure fluctuation is consistent with that of global cavitation area fluctuation. A typical quasi-periodic sheet/cavitation development cycle is characterized by three stages such as: (1) the growth of attached cavity, (2) the shedding of attached cavity, and (3) the development and collapse of detached cavities. In the stage one, the magnitude of pressure fluctuations under the attached cavity is small; however, it is large in the closure region of attached cavity, especially when attached cavity obtains its maximum length. In the stage two, the attached cavity begins to shed and some small detached cavities are observed, and small local pressure fluctuations with higher frequency are detected. In the stage three, a large detached cavity is formed in the rear of attached cavity. When the detached cavity collapses rapidly in the downstream region, pressure pulses with the magnitude of the order of several atmospheres are detected. The propagation speeds of pressure pulses in different cavitation regions are found to be related with the bubble density in the flow field. It is also found that the pressure impulse in the region covered by attached cavity is much lower than that in the attached cavity closure area.
PubDate: 2015-01-31

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