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 Experiments in Fluids   [SJR: 1.596]   [H-I: 69]   [6 followers]  Follow         Hybrid journal (It can contain Open Access articles)    ISSN (Print) 1432-1114 - ISSN (Online) 0723-4864    Published by Springer-Verlag  [2302 journals]
• Toluene-based planar laser-induced fluorescence imaging of temperature in
hypersonic flows
• Abstract: Abstract Planar laser-induced fluorescence imaging is carried out in a hypersonic gun tunnel at a freestream Mach number of 8.9 and Reynolds number of $$47.4 \times 10^6\,\hbox {m}^{-1}$$ ( $$N_2$$ is the test gas). The fluorescence of toluene $$(C_7H_8)$$ is correlated with the red shift of the emission spectra with increasing temperature. A two-colour approach is used where, following an excitation at 266 nm, emission spectra at two different bands are captured in separate runs using two different filters. Two different flow fields are investigated using this method: (i) hypersonic flow past a blunt nose, which is characterised by a bow shock with strong entropy effects, and (ii) an attached shock-wave/boundary-layer interaction induced by a flare located further downstream on the same blunt cylinder body. Measurements from as low as the freestream temperature of $$68.3$$ K all the way up to $$380$$ K $$(T_{\infty }-5.6T_{\infty })$$ are obtained. The uncertainty at the higher temperature level is approximately $$\pm 15$$  %, while at the low end of the temperature, an additional $$\pm 15$$  % uncertainty is expected. Application of the technique is further challenged at high temperatures due to the exponentially reduced fluorescence quantum yields and the occurrence of toluene pyrolysis near the stagnation region ( $$T_\mathrm{o}=1150$$  K). Overall, results are found to be within $$10$$  % agreement with the expected distributions, thus demonstrating suitability of the technique for hypersonic flow thermometry applications in low-enthalpy facilities.
PubDate: 2015-05-27

• Three-dimensional inspiratory flow in the upper and central human airways
• Abstract: Abstract The steady inspiratory flow through an anatomically accurate model of the human airways was studied experimentally at a regime relevant to deep inspiration for aerosol drug delivery. Magnetic resonance velocimetry was used to obtain the three-component, mean velocity field. A strong, single-sided streamwise swirl was found in the trachea and persists up to the first bifurcation. There, the swirl and the asymmetric anatomy impact both the streamwise momentum distribution and the secondary flows in the main bronchi, with large differences compared to what is found in idealized branching tubes. In further generations, the streamwise velocity never recovers a symmetric profile and the relative intensity of the secondary flows remains strong. Overall, the results suggest that, in real human airways, both streamwise dispersion (due to streamwise gradients) and lateral dispersion (due to secondary flows) are very effective transport mechanisms. Neglecting the extrathoracic airways and idealizing the bronchial tree may lead to qualitatively different conclusions.
PubDate: 2015-05-27

• Autonomous spatially adaptive sampling in experiments based on curvature,
statistical error and sample spacing with applications in LDA measurements

• Abstract: Abstract Spatially varying signals are typically sampled by collecting uniformly spaced samples irrespective of the signal content. For signals with inhomogeneous information content, this leads to unnecessarily dense sampling in regions of low interest or insufficient sample density at important features, or both. A new adaptive sampling technique is presented directing sample collection in proportion to local information content, capturing adequately the short-period features while sparsely sampling less dynamic regions. The proposed method incorporates a data-adapted sampling strategy on the basis of signal curvature, sample space-filling, variable experimental uncertainty and iterative improvement. Numerical assessment has indicated a reduction in the number of samples required to achieve a predefined uncertainty level overall while improving local accuracy for important features. The potential of the proposed method has been further demonstrated on the basis of Laser Doppler Anemometry experiments examining the wake behind a NACA0012 airfoil and the boundary layer characterisation of a flat plate.
PubDate: 2015-05-27

• Optical measurements of gas bubbles in oil behind a cavitating
micro-orifice flow
• Abstract: Abstract In hydraulic systems, it is common for air release to occur behind valves or throttles in the form of bubbles. These air bubbles can affect the behavior and the performance of these systems to a substantial extent. In the paper, gas release in a liquid flow behind an orifice is analyzed by optical methods for various operation points. The bubbles are observed with a digital camera, and a detection algorithm based on the Hough transformation is used to determine their number and size. The appearance of gas bubbles is very sensitive to the inlet and outlet pressure of the orifice. Gas bubbles are only observed if choking cavitation occurs. An empirical relationship between an adjusted cavitation number and the appearance of gas release is presented. It is assumed that the observed bubbles contain mostly air. With the applied pressure differences, up to 30 % of the dissolved air was degassed in the form of bubbles.
PubDate: 2015-05-23

• High-resolution PIV measurements of a transitional shock
wave–boundary layer interaction
• Abstract: Abstract This study investigates the effects of boundary layer transition on an oblique shock wave reflection. The Mach number was 1.7, the unit Reynolds number was 35 × 106 m−1, and the pressure ratio over the interaction was 1.35. Particle image velocimetry is used as the main flow diagnostics tool, supported by oil-flow and Schlieren visualizations. At these conditions, the thickness of the laminar boundary layer is only 0.2 mm, and seeding proved to be problematic as practically no seeding was recorded in the lower 40 % of the boundary layer. The top 60 % could, however, still be resolved with good accuracy and is found to be in good agreement with the compressible Blasius solution. Due to the effects of turbulent mixing, the near-wall seeding deficiency disappears when the boundary layer transitions to a turbulent state. This allowed the seeding distribution to be used as an indicator for the state of the boundary layer, permitting to obtain an approximate intermittency distribution for the boundary layer transition region. This knowledge was then used for positioning the oblique shock wave in the laminar, transitional (50 % intermittency) or turbulent region of the boundary layer. Separation is only recorded for the laminar and transitional interactions. For the laminar interaction, a large separation bubble is found, with a streamwise length of 96 $$\delta_{i,0}^*$$ . The incoming boundary layer is lifted over the separation bubble and remains in a laminar state up to the impingement point of the shock wave. After the shock, transition starts and a turbulent profile is reached approximately 80–90 $$\delta_{i,0}^*$$ downstream of the shock. Under the same shock conditions, the transitional interaction displays a smaller separation bubble (43 $$\delta_{i,0}^*$$ ), and transition is found to be accelerated over the separation bubble.
PubDate: 2015-05-22

• Experimental assessment of spanwise-oscillating dielectric electroactive
surfaces for turbulent drag reduction in an air channel flow
• Abstract: Abstract In the present work, wall oscillations for turbulent skin friction drag reduction are realized in an air turbulent duct flow by means of spanwise-oscillating active surfaces based on dielectric electroactive polymers. The actuator system produces spanwise wall velocity oscillations of 820 mm/s semi-amplitude at its resonance frequency of 65 Hz while consuming an active power of a few 100 mW. The actuators achieved a maximum integral drag reduction of 2.4 %. The maximum net power saving, budget of the power benefit and cost of the control, was measured for the first time with wall oscillations. Though negative, the net power saving is order of magnitudes higher than what has been estimated in previous studies. Two new direct numerical simulations of turbulent channel flow show that the finite size of the actuator only partially explains the lower values of integral drag reduction typically achieved in laboratory experiments compared to numerical simulations.
PubDate: 2015-05-15

• Dynamic hysteresis control of lift on a pitching wing
• Abstract: Abstract Dynamic hysteresis appearing in the lift force during pitching maneuvers is distinctly different from conventional static hysteresis. The size and shape of dynamic hysteresis loops are dependent on the degree of flow attachment, the dimensionless pitching frequency, and two time delays associated with the flow separation process. A linearized version of the Goman–Khrabrov model is derived and shown to capture the dynamic hysteresis characteristics when the pitching amplitude is small. Closed-loop control using a linearized version of the Goman–Khrabrov model is demonstrated, which incorporates a disturbance model into the feed-forward controller. The controller is shown to reduce the dynamic hysteresis during periodic pitching, step-up and step-down maneuvers, and quasi-random pitching maneuvers.
PubDate: 2015-05-15

• PIV/PLIF experiments of jet mixing in a model of a rotary kiln
• Abstract: Abstract The jet mixing in a downscaled, isothermal model of a rotary kiln is investigated experimentally through simultaneous particle image velocimetry and planar laser-induced fluorescence measurements. The kiln is modeled as a cylinder with three inlets in one end, two semicircular-shaped inlets for what is called the secondary fluid divided by a wall in between, called the back plate, where the burner nozzle is located. The scaling of the burner nozzle between real kiln and model and the corresponding jet flow through it is determined by the Craya–Curtet parameter. Three momentum flux ratios of the secondary fluid are investigated, and the interaction with the burner jet is scrutinized. It is found that the burner jet characteristics, its mixing with the secondary fluid and the resulting flow field surrounding the jet are dependent on the momentum flux ratio. A particular result is that stable shear layers give a more even mixing as compared to a case with shear layers subjected to a more prominent vortex shedding.
PubDate: 2015-05-15

• Corrections to the direct spectral estimation for laser Doppler data
• Abstract: Abstract An algorithm for estimating the power spectral density and the correlation function of laser Doppler-generated data sets is introduced. The algorithm is of the type of direct spectral estimators including weighting of individual velocity values to correct statistical biases caused by the correlation of instantaneous data rate and velocity values. It is extended by the forward–backward inter-arrival time weighting, the correction of the wraparound error, that of dead-time influences, and an error due to the removal of estimated block mean values. A temporal limitation of the correlation function as an alternative to the block averaging allows the block lengths to be chosen in a wide range with less necessities for compromises between systematic and random errors.
PubDate: 2015-05-14

• Laser diagnostics of pulverized coal combustion in O 2 /N 2 and O 2 /CO 2
conditions: velocity and scalar field measurements
• Abstract: Abstract Optical diagnostic techniques are applied to a 21 kW laboratory-scale pulverized coal–methane burner to map the reaction zone during combustion, in mixtures with varying fractions of O2, N2 and CO2. Simultaneous Mie scatter and OH planar laser-induced fluorescence (PLIF) measurements have been carried out to study the effect of the oxidizer/diluent concentrations as well as the coal-loading rate. The spatial distribution of soot is captured using laser-induced incandescence (LII). Additionally, velocity profiles at selected axial locations are measured using the pairwise two-dimensional laser Doppler velocimetry technique. The OH PLIF images capture the reaction zones of pilot methane–air flames and the variation of the coal flame structure under various O2/CO2 compositions. Coal particles devolatilize immediately upon crossing the flame interface, so that the Mie scatter signal almost vanishes. Increasing coal-loading rates leads to higher reaction rates and shorter flames. LII measurements show that soot is formed primarily in the wake of remaining coal particles in the product regions. Finally, differences in the mean and RMS velocity field are explained by the combined action of thermal expansion and the changes in particle diameter between reacting and non-reacting flows.
PubDate: 2015-05-13

• Turbulent Taylor–Couette flow over riblets: drag reduction and the
effect of bulk fluid rotation
• Abstract: Abstract A Taylor–Couette facility was used to measure the drag reduction of a riblet surface on the inner cylinder. The drag on the surfaces of the inner and outer cylinders is determined from the measured torque when the cylinders are in exact counter-rotation. The three velocity components in the instantaneous flow field were obtained by tomographic PIV and indicate that the friction coefficients are strongly influenced by the flow regimes and structures. The riblet surface changes the friction at the inner-cylinder wall, which generates an average bulk fluid rotation. A simple model is proposed to distinguish drag changes due to the rotation effect and the riblet effect, as a function of the measured drag change $$\Delta \tau _w/\tau _{w,0}$$ and shear Reynolds number $$Re_{\rm s}$$ . An uncorrected maximum drag reduction of 5.3 % was found at $$Re_{\rm s}=4.7 \times 10^4$$ that corresponds to riblet spacing Reynolds number $$s^+=14$$ . For these conditions, the model predicts an azimuthal bulk velocity shift of 1.4 %, which is confirmed by PIV measurements. This shift indicates a drag change due to a rotation effect of −1.9 %, resulting in a net maximum drag reduction of 3.4 %. The results correspond well with earlier reported results and demonstrate that the Taylor–Couette facility is a suitable and accurate measurement tool to characterize the drag performance of surfaces.
PubDate: 2015-05-12

• Sound source mechanisms in under-expanded impinging jets
• Abstract: Abstract Experiments on the aeroacoustics of an under-expanded supersonic jet impinging on a flat plate are presented and thoroughly discussed. A wide range of nozzle pressure ratios and of nozzle-to-plate distances has been analyzed with particular attention to the behavior of the discrete component of the noise. The investigation has been carried out by means of acoustic, particle image velocimetry and wall pressure measurements. The analysis of the relationship between the acoustic data and the fluid dynamic fields allows to examine the different source mechanisms of the discrete component of the noise and to evaluate the link between the jet flow structure and the acoustic tone features. Specifically, two ranges of nozzle pressure ratio have been observed showing different acoustic behaviors, characterized by distinct mechanisms of discrete noise generation. These regions are separated by a range of nozzle pressure ratios where impinging tones are not observed. The present experimental data extend previously published results, improving the analysis of the connection between fluid dynamic and acoustic fields and leading to a better comprehension of the impinging tone source mechanisms.
PubDate: 2015-05-12

• Multiphase flow of miscible liquids: jets and drops
• Abstract: Abstract Drops and jets of liquids that are miscible with the surrounding bulk liquid are present in many processes from cleaning surfaces with the aid of liquid soaps to the creation of biocompatible implants for drug delivery. Although the interactions of immiscible drops and jets show similarities to miscible systems, the small, transient interfacial tension associated with miscible systems create distinct outcomes such as intricate droplet shapes and breakup resistant jets. Experiments have been conducted to understand several basic multiphase flow problems involving miscible liquids. Using high-speed imaging of the morphological evolution of the flows, we have been able to show that these processes are controlled by interfacial tensions. Further multiphase flows include investigating miscible jets, which allow the creation of fibers from inelastic materials that are otherwise difficult to process due to capillary breakup. This work shows that stabilization from the diminishing interfacial tensions of the miscible jets allows various elongated morphologies to be formed.
PubDate: 2015-05-12

• Analysis and characterization of ramp flow separation
• Abstract: Abstract Two-dimensional separation on 25°-ramp is studied experimentally in closed-loop wind tunnel. The analysis of the flow conditions and the separation is based on wall pressure distribution, velocity fields obtained by 2D-2C PIV measurements, and velocity profiles obtained by 2C LDV measurements. The boundary layer upstream and downstream the ramp is characterized. To control the longitudinal pressure gradient, a trailing edge flap is used and the flap angle effects are studied. When the flap angle is equal to +7.5°, the flow around leading edge is symmetric, but the separation is 3D. For the case where the flap is not slanted (0°), the separation is 2D. Proper orthogonal decomposition is used to characterize the separation unsteadiness.
PubDate: 2015-05-09

• Spray features in the near field of a flow-blurring injector investigated
by high-speed visualization and time-resolved PIV
• Abstract: Abstract In a flow-blurring (FB) injector, atomizing air stagnates and bifurcates at the gap upstream of the injector orifice. A small portion of the air penetrates into the liquid supply line to create a turbulent two-phase flow. Pressure drop across the injector orifice causes air bubbles to expand and burst thereby disintegrating the surrounding liquid into a fine spray. In previous studies, we have demonstrated clean and stable combustion of alternative liquid fuels, such as biodiesel, straight vegetable oil and glycerol by using the FB injector without requiring fuel pre-processing or combustor hardware modification. In this study, high-speed visualization and time-resolved particle image velocimetry (PIV) techniques are employed to investigate the FB spray in the near field of the injector to delineate the underlying mechanisms of atomization. Experiments are performed using water as the liquid and air as the atomizing gas for air to liquid mass ratio of 2.0. Flow visualization at the injector exit focused on a field of view with physical dimensions of 2.3 mm × 1.4 mm at spatial resolution of 7.16 µm per pixel, exposure time of 1 µs, and image acquisition rate of 100 k frames per second. Image sequences illustrate mostly fine droplets indicating that the primary breakup by FB atomization likely occurs within the injector itself. A few larger droplets appearing mainly at the injector periphery undergo secondary breakup by Rayleigh–Taylor instabilities. Time-resolved PIV is applied to quantify the droplet dynamics in the injector near field. Plots of instantaneous, mean, and root-mean-square droplet velocities are presented to reveal the secondary breakup process. Results show that the secondary atomization to produce fine and stable spray is complete within a few diameters from the injector exit. These superior characteristics of the FB injector are attractive to achieve clean combustion of different fuels in practical systems.
PubDate: 2015-05-08

• Quantification of the effect of surface heating on shock wave modification
by a plasma actuator in a low-density supersonic flow over a flat plate
• Abstract: Abstract This paper describes experimental and numerical investigations focused on the shock wave modification induced by a dc glow discharge. The model is a flat plate in a Mach 2 air flow, equipped with a plasma actuator composed of two electrodes. A weakly ionized plasma was created above the plate by generating a glow discharge with a negative dc potential applied to the upstream electrode. The natural flow exhibited a shock wave with a hyperbolic shape. Pitot measurements and ICCD images of the modified flow revealed that when the discharge was ignited, the shock wave angle increased with the discharge current. The spatial distribution of the surface temperature was measured with an IR camera. The surface temperature increased with the current and decreased along the model. The temperature distribution was reproduced experimentally by placing a heating element instead of the active electrode, and numerically by modifying the boundary condition at the model surface. For the same surface temperature, experimental investigations showed that the shock wave angle was lower with the heating element than for the case with the discharge switched on. The results show that surface heating is responsible for roughly 50 % of the shock wave angle increase, meaning that purely plasma effects must also be considered to fully explain the flow modifications observed.
PubDate: 2015-05-07

• Development of a sensitive experimental set-up for LIF fuel wall film
measurements in a pressure vessel
• Abstract: Abstract This paper focusses on fundamental investigations of fuel wall films, which are formed when the spray impinges on the piston or cylinder walls. To reproduce the wide range of operating conditions within homogeneously charged gasoline direct-injection engines, it is necessary to use a film thickness measurement method, which can be applied inside a high-pressure, high-temperature vessel. Hence, we developed a method based on laser-induced fluorescence that reaches: a precision better than 1 µm, a geometric resolution of 31 µm and a practical applicability for wall film thicknesses smaller 80 µm. To obtain accurate film thickness results, we provide a detailed description of the selection of the surrogate fuel isooctane with 3-pentanone as fluorescence tracer and the resulting assembly of the excitation source, beam expander, filters, camera and the essential image processing. Furthermore, advantages and disadvantages of other possible solutions are discussed. Earlier publications provide only little information about the accuracy of their calibration and measurement procedures. Therefore, we tested and compared three basic calibration methods to each other and provide an analysis of possible errors, such as the influence of the preferential evaporation of 3-pentanone. Finally, images of resulting wall films are presented, and practical considerations for the execution of the measurements like recording timings are discussed.
PubDate: 2015-05-06

• Investigation of impinging jet resonant modes using unsteady
pressure-sensitive paint measurements
• Abstract: Abstract At given nozzle to plate spacings, the flow field of high-speed impinging jets is known to be characterized by a resonance phenomenon. Large coherent structures that convect downstream and impinge on the surface create strong acoustic waves that interact with the inherently unstable shear layer at the nozzle exit. This feedback mechanism, driven by the coherent structures in the jet shear layer, can either be axisymmetric or helical in nature. Fast-response pressure-sensitive paint (PSP) is applied to the impingement surface to map the unsteady pressure distribution associated with these resonant modes. Phase-averaged results acquired at several kHz are obtained using a flush mounted unsteady pressure transducer on the impingement plate as a reference signal. Tests are conducted on a Mach 1.5 jet at nozzle to plate spacings of $$h/D_{j} = 4\, \text{ and}\, 4.5$$ . The resulting phase-averaged distribution reveals dramatically different flow fields at the corresponding impingement heights. The existence of a purely axisymmetric mode with a frequency of 6.3 kHz is identified at $$h/D_{j} = 4.5$$ and is characterized by concentric rings of higher/lower pressure that propagate radially with increasing phase. Two simultaneous modes are observed at $$h/D_{j} = 4$$ with one being a dominant symmetric mode at 7.1 kHz and the second a sub-dominant helical mode at 4.3 kHz. Complimentary phase-conditioned Schlieren images are also obtained visualizing the flow structures associated with each mode and are consistent with the PSP results.
PubDate: 2015-05-06

• Experimental characterization of the unsteady natural wake of the
full-scale square back Ahmed body: flow bi-stability and spectral analysis

• Abstract: Abstract In recent years, the increasing interest in reducing the aerodynamic drag of vehicles, such as station wagons, minivans or buses, has led research to focus on the characterization of square back bluff geometries. In this paper, the results of an extensive experimental campaign on the full-scale well-known body of Ahmed et al. (1984) are presented, for two height-based Reynolds numbers, $$Re_{\rm H} = 5.1 \times 10^5$$ and $$7.7 \times 10^5$$ . Eighty-one measurement points were used to map the base pressure field, while the wake topology was investigated by means of a series of ten 2D Particle Image Velocimetry planes. These measurements clearly show that the wake presents a bi-stable behavior, characterized by a random succession of switches between two well-defined mutually symmetric configurations, confirming the results from Grandemange et al. (J Fluid Mech 722:51–84, 2013b. doi:10.1017/jfm.2013.83) for the same model. For the presented results, the timescale of this phenomenon is of the order of $$800 \, V_{\infty} / H$$ . The sensitivity of the bi-stability to the yaw angle was also investigated, and considerations on how to take such a behavior into account in post-processing this kind of field are given. High-frequency measurements were also carried out with four piezoelectric transducers and a synchronized two-component hot-wire. The results show a low-frequency spectral activity: peaks at $$St_{\rm H} = 0.13$$ and 0.19, corresponding to vortex shedding modes, were found on the lateral base pressures and in the far wake, whereas a signature at $$St_{\rm H} = 0.08$$ was visible on the vertical base centerline and in the recirculation bubble shear layer. Correlation analysis and proper orthogonal decomposition confirm the interpretation of the latter mode as the pumping of the recirculation bubble.
PubDate: 2015-05-06

• A model-free noise removal for the interpolation method of correlation and
spectral estimation from laser Doppler data
• Abstract: Abstract A procedure to estimate and remove the contribution of data noise to the correlation function and the power spectral density calculated from laser Doppler data with the interpolation method is introduced. In comparison with earlier approaches, the new procedure is model free and, therefore, more objective. The new procedure is proven based on experimental data taken with a laser Doppler system, where the power spectral density obtained with the interpolation method is compared directly to data from a hot-wire reference measurement.
PubDate: 2015-05-06

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