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Experiments in Fluids

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ISSN (Print) 1432-1114 - ISSN (Online) 0723-4864
Published by Springer-Verlag

[2216 journals]

Aeroacoustic source analysis using time-resolved PIV in a free jet
- Abstract: Time-resolved particle image velocimetry (TR-PIV) has become a valuable tool for spatio-temporally resolved flow measurements. Current camera and laser technology has advanced such that time-domain events leading to sound generation can now be resolved over a reasonable spatial extent. This paper reports on the application of TR-PIV for the analysis of aeroacoustic sources in a free jet using the direct correlation between in-flow velocity fluctuations on the jet center-line and near-field pressure fluctuations. This correlation is considered both in the time domain and in the frequency domain (coherence), and the effect of TR-PIV errors on these estimates is considered by comparison to hot-wire anemometer measurements. In addition, a recently developed wavelet filtering technique is used to separate the acoustic and hydrodynamic components of recorded near-field pressure signals, enabling a gain in the signal-to-noise ratio. The results show that TR-PIV can recover the same time-domain correlation available from hot-wire and traditional PIV measurements, but that the frequency-domain estimates are corrupted by error, particularly at high frequencies. This result negates the principal benefit of using TR-PIV over PIV (the availability of coherence estimates). Despite this result, an analysis of the correlation signature gives evidence that large-scale, convecting, wave-like structures are associated with sound production, a result consistent with observations by many recent investigators. The analysis shows that in the presence of such large-scale structures, noise source localization based on the traditional correlation technique is ambiguous.
  PubDate: 2013-05-15
Laser induced phosphorescence imaging for the investigation of evaporating liquid flows
- Abstract: The phosphorescence properties of liquid and gaseous acetone, following excitation at 308 nm, are studied and utilized in order to overcome two main challenges of two-phase flow laser induced fluorescence imaging: the large fluorescence intensity disparity between the two phases and the ensuing effect of halation. This is achieved on account of the different phosphorescence decay rates of the liquid and vapour phases, which allow for a more favourable signal ratio to be obtained. The benefits of visualizing the phosphorescence emission, instead of the fluorescence, are demonstrated by droplet stream experiments set up in different bath gases, at 1 atm and 297 K. The liquid–vapour interface can be accurately located, while the vapour surrounding the droplets is clearly visualized without any halation interference. The vapour phase phosphorescence signal was calibrated in order to quantify the vapour concentration around an evaporating droplet stream, and the results are compared to laser induced fluorescence images collected in the present study and results found in the literature. The effect of halation in the fluorescence images is shown to extend as far as 10 droplet diameters away from the interface for 161 μm droplets, resulting in a significant overprediction of acetone vapour mole fractions in that region. The vapour profile obtained by laser induced phosphorescence (LIP) imaging agrees with data found in the literature, for which a halation correction on fluorescence images was successfully performed. The demonstrated LIP technique for simultaneous vapour and liquid phase visualisation is only applicable to oxygen-free environments, as even trace quantities of oxygen completely quench the vapour phase phosphorescence emission.
  PubDate: 2013-05-14
Study of the mechanisms for flame stabilization in gas turbine model combustors using kHz laser diagnostics
- Abstract: An image-processing routine was developed to autonomously identify and statistically characterize flame-kernel events, wherein OH (from a planar laser-induced fluorescence, PLIF, measurement) appears in the probe region away from the contiguous OH layer. This routine was applied to datasets from two gas turbine model combustors that consist of thousands of joint OH-velocity images from kHz framerate OH-PLIF and particle image velocimetry (PIV). Phase sorting of the kernel centroids with respect to the dominant fluid-dynamic structure of the combustors (a helical precessing vortex core, PVC) indicates through-plane transport of reacting fluid best explains their sudden appearance in the PLIF images. The concentration of flame-kernel events around the periphery of the mean location of the PVC indicates they are likely the result of wrinkling and/or breakup of the primary flame sheet associated with the passage of the PVC as it circumscribes the burner centerline. The prevailing through-plane velocity of the swirling flow-field transports these fragments into the imaging plane of the OH-PLIF system. The lack of flame-kernel events near the center of the PVC (in which there is lower strain and longer fluid-dynamic residence times) indicates that auto-ignition is not a likely explanation for these flame kernels in a majority of cases. The lack of flame-kernel centroid variation in one flame in which there is no PVC further supports this explanation.
  PubDate: 2013-05-14
Experimental investigation of liquid films in gravity-driven flows with a simple visualization technique
- Abstract: A visualization technique based on light absorption is used to monitor the thickness profile of a liquid film flowing on an inclined plane with high spatial and temporal resolutions. Surface waves are observed for a certain range of experimental parameters as expected from the classical stability analysis from Benjamin (J Fluid Mech 2:554–574, 1957). The liquid films are found systematically thicker than predicted by Nusselt (Z Ver Dtsch Ing 60:541, 1916) in the case of ideal viscous flows. We interpret this increase in thickness as a consequence of the propagation of waves on the films. The wave dynamics are in qualitative agreement with the asymptotic development from Anshus (Ind Eng Chem Fundam 11:502–508, 1972). Although the wavelength distribution is rather broad, space-time analysis indicates a well-defined phase velocity. Representing the wave velocity of a corrected Reynolds number allows to superimpose the experimental data into a single master curve.
  PubDate: 2013-05-14
Experimental investigation of the vortical activity in the close wake of a simplified military transport aircraft
- Abstract: This paper focuses on the experimental characterization of the vortex structures that develop in the aft fuselage region and in the wake of a simplified geometry of a military transport aircraft. It comes within the framework of the military applications of airflow influence on airdrop operations. This work relies on particle image velocimetry measurements combined with a vortex-tracking approach. Complex vortex dynamics is revealed, in terms of vortex positions, intensities, sizes, shapes and fluctuation levels, for both closed and opened cargo-door and ramp airdrop configurations.
  PubDate: 2013-05-11
Higher order multi-frame particle tracking velocimetry
- Abstract: Particle tracking velocimetry methods (PTV) have a great potential to enhance the spatial resolution compared to spatial correlation-based methods (PIV). In addition, they are not biased due to inhomogeneous seeding concentration or in-plane and out-of-plane gradients so that the measurement precision can be increased as well. The possibility to simultaneously measure the velocity with the temperature, ph-value, or pressure of the flow at the particle location by means of fluorescent particles is another advantage of PTV. However, at high seeding concentrations, the reliable particle pairing is challenging, and the measurement precision decreases rapidly due to overlapping particle images and wrong particle image pairing. In this paper, it is shown that the particle image information acquired at four or more time steps greatly enhances a reliable particle pairing even at high seeding concentrations. Furthermore, it is shown that the accuracy and precision can be increased by using vector reallocation and displacement estimation using a fit of the trajectory in the case of curved particle paths. The improvements increase the PTV working range as reliable and accurate measurements become possible at seeding concentrations typically used for PIV measurements.
  PubDate: 2013-05-11
Flow field measurements of pulverized coal combustion using optical diagnostic techniques
- Abstract: This paper demonstrates the application of laser Doppler velocimetry (LDV) and particle image velocimetry (PIV) techniques to a particle-laden reacting flow of pulverized coal. A laboratory-scale open-type annular burner is utilized to generate velocity profiles of coal particles and micrometric alumina particles. Pair-wise two-component LDV measurements and high-speed stereo PIV measurements provide three-dimensional velocity components of the flow field. A detailed comparison of velocities for alumina and coal particle seeding revealed differences attributed to the wide size distribution of coal particles. In addition, the non-spherical shape and high flame luminosity associated with coal particle combustion introduces noise to the Mie scatter images. The comparison of mean and RMS velocities measured by LDV and PIV techniques showed that PIV measurements are affected by the wide size distribution of coal particles, whereas LDV measurements become biased toward the velocity of small particles, as signals from large particles are rejected. This small-particle bias is also reflected in the spectral characteristics for both techniques, which are in good agreement within the range of frequencies accessible. PIV measurements showed an expected lack of response of large coal particles to the turbulence fluctuations. The overall good agreement between LDV and PIV measurements demonstrates the applicability of the high-speed PIV technique to a particle-laden, high luminosity coal flame while highlighting some of its limitations.
  PubDate: 2013-05-09
Preferential concentration of poly-dispersed droplets in stationary isotropic turbulence
- Abstract: The preferential concentration of poly-dispersed water droplets with a range of Sauter mean diameters between 25 and 95 μm has been studied experimentally in stationary homogeneous isotropic turbulence with four different intensities, characterized by turbulent Reynolds numbers based on Taylor microscale, of Re λ = 107, 145, 185 and 213. The image processing method of recorded scattered light intensity images from droplets is described and its ability to identify droplets is assessed in terms of image quality. The influence of image processing parameters on measured characteristics of droplet clustering is evaluated. The radial distribution function (RDF) and 2D Voronoï analysis quantified the magnitude of preferential droplet concentration and the results from both methods agreed well. RDF showed that the characteristic length scale of resulting droplet clusters varies between 20 and 30 times the Kolmogorov length scale over all the experimental conditions. It was found that the preferential concentration is more appropriately described by a Stokes number, based on various representative diameters, namely the arithmetic mean diameter, D 10, or the diameter, DN60 %, below which 60 % of the total droplet number in the spray is present, or the diameter, DV5 %, which carries 5 % of the total liquid volume in the spray. The magnitude of droplet preferential concentration was maximum when the proposed Stokes number was around unity for all experimental conditions. Little dependence of the magnitude of preferential concentration on turbulent Reynolds numbers was found, in contrast to the recent DNS findings (Tagawa et al. in J Fluid Mech 693:201–215, 2012).
  PubDate: 2013-05-09
Experimental study of entrainment and interface dynamics in a gravity current
- Abstract: The special case of entrainment in a stratified flow, relevant to many geophysical flows such as oceanic overflows, so far has not been studied experimentally in terms of small-scale aspects around the turbulent/non-turbulent interface. In view of the fact that existing engineering concepts perform unsatisfactorily in practice, a new gravity current facility was designed with the goal to gain understanding of how stratification affects interfacial physics. Here, we present the design of the new setup and give details on the turbulence enhancement in the inflow and the refractive index matching technique used. Validation measurements ensure that there is negligible backflow and an essentially irrotational flow outside the current. Measurements via particle image velocimetry of a flow with inflow Reynolds and Richardson numbers of $Re_0\approx \hbox{4,000}$ and Ri 0 = 0.22 are reported. An analysis in a laboratory frame agrees well with flow features reported in the literature, i.e., a streamwise invariant top-hat velocity scale and a Reynolds stress distribution are matched closely by a mixing length model. In a second step, the instantaneous interface position is determined based on a threshold on the normal enstrophy component. An investigation in a frame of reference conditioned on the interface position reveals a strong interfacial shear layer that is much more pronounced than the one observed in jet flows. Its thickness is about two times the Taylor microscale. The data moreover suggest the existence of a fairly strong interfacial density jump across the shear layer. The entrainment parameter is estimated at $E \approx 0.04$ congruently from the evaluations in laboratory and conditioned frame, respectively.
  PubDate: 2013-05-08
The capillary channel flow experiments on the International Space Station: experiment set-up and first results
- Abstract: This paper describes the experiments on flow rate limitation in open capillary channel flow that were performed on board the International Space Station in 2011. Free surfaces (gas–liquid interfaces) of open capillary channels balance the pressure difference between the flow of the liquid in the channel and the ambient gas by changing their curvature in accordance with the Young-Laplace equation. A critical flow rate of the liquid in the channel is exceeded when the curvature of the free surface is no longer able to balance the pressure difference and, consequently, the free surface collapses and gas is ingested into the liquid. This phenomenon was observed using the set-up described herein and critical flow rates are presented for steady flow over a range of channel lengths in three different cross-sectional geometries (parallel plates, groove, and wedge). All channel shapes displayed decreasing critical flow rates for increasing channel lengths. Bubble ingestion frequencies and bubble volumes are presented for gas ingestion at supercritical flow rates in the groove channel and in the wedge channel. At flow rates above the critical flow rate, bubble ingestion frequency appears to depend on the flow rate in a linear fashion, while bubble volume remains more or less constant. The performed experiments yield vast data sets on flow rate limitation in capillary channel flow in microgravity and can be utilised to validate numerical and analytical methods.
  PubDate: 2013-05-08
An experimental study of flow fields and wind loads on gable-roof building models in microburst-like wind
- Abstract: An experimental study was conducted to quantify the flow characteristics of microburst-like wind and to assess the resultant wind loads acting on low-rise, gable-roof buildings induced by violent microburst-like winds compared with those in conventional atmospheric boundary layer winds. The experimental work was conducted by using an impinging-jet-based microburst simulator in the Department of Aerospace Engineering, Iowa State University. Two gable-roof building models with the same base plan and mean roof height, but different roof angle, were mounted over a homogenous flat surface for a comparative study. In addition to measuring the surface pressure distributions to determine the resultant wind loads acting on the building models, a digital particle image velocimetry system was used to conduct flow field measurements to reveal the wake vortex and turbulence flow structures around the building models placed in the microburst-like wind. The effects of important parameters, such as the distance of the building from the center of the microburst, the roof angle of the building, and the orientation of the building with respect to radial outflow of the oncoming microburst-like wind, on the flow features such as the vortex structures and the surface pressure distributions around the building models as well as the resultant wind loads acting on the test models were assessed quantitatively. The measurement results reveal clearly that when the building models were mounted within the core region of the microburst-like wind, the surface pressure distributions on the building models were significantly higher than those predicted by ASCE 7-05 standard, thereby induced considerably greater downward aerodynamic forces acting on the building models. When the building models were mounted in the outflow region of the microburst-like wind, the measured pressure distributions around the building models were found to reach a good correlation with ASCE 7-05 standard gradually as the test models were moved far away from the center of the microburst-like wind. It was also found that both the radial and vertical components of the aerodynamic forces acting on the building models would reach their maximum values when the models were mounted approximately one jet diameter away from the center of the microburst-like wind, while the maximum pressure fluctuations on the test models were found to occur at further downstream locations. Roof angles of the building models were found to play an important role in determining the flow features around the building models and resultant wind loads acting on the test models. The flow field measurements were found to correlate with the measured surface pressure distributions and the resultant wind loads (i.e., aerodynamic forces) acting on the building models well to elucidate the underlying physics of flow-structure interactions between the microburst-like winds and the gable-roof buildings in order to provide more accurate prediction of the damage potentials of the microburst wind.
  PubDate: 2013-05-08
μPIV measurements of two-phase flows of an operated direct methanol fuel cell
- Abstract: In direct methanol fuel cells (DMFCs), two-phase flows appear in the channels of the anode side (CO2 bubbles in a liquid water–methanol environment) as well as of the cathode side (water droplets or films in an ambient air flow). CO2 bubbles or water droplets may almost completely fill the cross-section of a channel. The instantaneous effect of the formation of two-phase flows on the cell performance has not been investigated in detail, yet. In the current project, the micro particle image velocimetry (μPIV) technique is used to elucidate the corresponding flow phenomena on the anode as well as on the cathode side of a DMFC and to correlate those phenomena with the performance of the cell. A single-channel DMFC with optical access at the anode and the cathode side is constructed and assembled that allows for μPIV measurements at both sides as well as a detailed time-resolved cell voltage recording. The appearance and evolution of CO2 bubbles on the anode side is qualitatively and quantitatively investigated. The results clearly indicate that the cell power increases when the free cross-section area of the channel is decreased by huge bubbles. Methanol is forced into the porous gas diffusion layer (GDL) between the channels and the membrane is oxidized to CO2, and hence, the fuel consumption is increased and the cell performance rises. Eventually, a bubble forms a moving slug that effectively cleans the channel from CO2 bubbles on its way downstream. The blockage effect is eliminated; the methanol flow is not forced into the GDL anymore. The remaining amount of methanol in the GDL is oxidized. The cell power decreases until enough CO2 is produced to eventually form bubbles again and the process starts again. On the other hand under the investigated conditions, water on the cathode side only forms liquid films on the channels walls rather than channel-filling droplets. Instantaneous changes of the cell power due to liquid water formation could not be observed. The timescales of the two-phase flow on the cathode side are significantly larger than on the anode side. However, the μPIV measurements at the cathode side demonstrate the ability of feeding gas flows in microchannels with liquid tracer particles and the ability to measure in two-phase flows in such a configuration.
  PubDate: 2013-05-08
Two-photon microscopy with double-circle trajectories for in vivo cerebral blood flow measurements
- Abstract: Scanning microscopes normally use trajectories which produce full-frame images of an object at a low frame rate. Time-resolved measurements are possible if scans along a single line are repeated at a high rate. In conjunction with fluorescence labeling techniques, in vivo recording of blood flow in single capillaries is possible. The present work investigates scanning with double-circle trajectories to measure blood flow simultaneously in several vessels of a capillary network. With the trajectory centered near a bifurcation, a double circle crosses each vessel twice, creating a sensing gate for passing dark red blood cells in fluorescently labeled plasma. From the stack of scans repeated at 1,300 Hz, the time-resolved velocity is retrieved using an image correlation approach. Single bifurcation events can be identified from a few fluorescently labeled red blood cells. The applicability of the method for in vivo measurements is illustrated on the basis of two-photon laser scanning microscopy of the cerebral capillary network of mice. Its performance is assessed with synthetic data generated from a two-phase model for the perfusion in a capillary network. The calculation of velocities is found to be sufficiently robust for a wide range of conditions. The achievable limits depend significantly on the experimental conditions and are estimated to be in the 1 μm/s (velocity) and 0.1 s (time resolution) ranges, respectively. Some manual fine-tuning is required for optimal performance in terms of accuracy and time resolution. Further work may lead to improved reliability with which bifurcation events are identified in the algorithm and to include red blood cell flux and hematocrit measurements. With the capability for time-resolved measurements in all vessels of a bifurcation, double-circle scanning trajectories allow a detailed study of the dynamics in vascular networks.
  PubDate: 2013-05-08
Oscillations of the large-scale circulation in turbulent mixed convection in a closed rectangular cavity
- Abstract: Fluid temperature time series are recorded in turbulent mixed convection at specific locations inside a cuboidal convection cell. They reveal instabilities of the large-scale flow structures, which organise the heat transport in an intermediate range of Archimedes numbers, where buoyancy and inertia forces are of similar strength. The instabilities lead to periodic or spontaneous transitions between three and four convection rolls. Further, for either high Rayleigh or Reynolds numbers, for which the flow is either governed by buoyancy or by inertia forces, respectively, stable large-scale circulations (LSCs) develop. In the intermediate Ra–Re number regime, we ascribe the complex dynamics, visible as oscillation in the temperature time series, to the interaction of the pressure-driven wall jet at the ceiling with the buoyancy-driven LSCs. The maximal main oscillation frequency is about one order of magnitude smaller than the turnover frequencies of either the wall jet-induced circulation rolls or thermally induced LSCs. It is further shown that the periodic reconfigurations of the LSCs can be controlled by adjusting the inflow velocity, that is, the Reynolds number, to generate stable LSCs.
  PubDate: 2013-05-05
Effect on drag of the flow orientation at the base separation of a simplified blunt road vehicle
- Abstract: The separated flow past the square-back model used in the experiments of Ahmed et al. (1984) is controlled using flaps at the end of the top and bottom faces. A parametric study of the flow regarding the slant angle of the flaps is performed from pressure and force measurements as well as particle image velocimetry. When the bottom flap orientation is fixed, variations in the top slant angle indicate a quadratic dependence of drag versus lift. This relationship presents self-similarities when modifying the bottom flap angle. It is furthermore observed that the lift is an affine function of both slant angles and the drag is a second-order polynomial containing a coupling term between the two angles. The evolution of the drag, depending on both angles, is discussed. The contribution of the wake size, lift-induced drag as well as the local drag induced by the inclination of the flaps is interpreted.
  PubDate: 2013-05-05
Time-resolved PIV measurements of the flow field in a stenosed, compliant arterial model
- Abstract: Compliant (flexible) structures play an important role in several biological flows including the lungs, heart and arteries. Coronary heart disease is caused by a constriction in the artery due to a build-up of atherosclerotic plaque. This plaque is also of major concern in the carotid artery which supplies blood to the brain. Blood flow within these arteries is strongly influenced by the movement of the wall. To study these problems experimentally in vitro, especially using flow visualisation techniques, can be expensive due to the high-intensity and high-repetition rate light sources required. In this work, time-resolved particle image velocimetry using a relatively low-cost light-emitting diode illumination system was applied to the study of a compliant flow phantom representing a stenosed (constricted) carotid artery experiencing a physiologically realistic flow wave. Dynamic similarity between in vivo and in vitro conditions was ensured in phantom construction by matching the distensibility and the elastic wave propagation wavelength and in the fluid system through matching Reynolds (Re) and Womersley number (α) with a maximum, minimum and mean Re of 939, 379 and 632, respectively, and a α of 4.54. The stenosis had a symmetric constriction of 50 % by diameter (75 % by area). Once the flow rate reached a critical value, Kelvin–Helmholtz instabilities were observed to occur in the shear layer between the main jet exiting the stenosis and a reverse flow region that occurred at a radial distance of 0.34D from the axis of symmetry in the region on interest 0–2.5D longitudinally downstream from the stenosis exit. The instability had an axis-symmetric nature, but as peak flow rate was approached this symmetry breaks down producing instability in the flow field. The characteristics of the vortex train were sensitive not only to the instantaneous flow rate, but also to whether the flow was accelerating or decelerating globally.
  PubDate: 2013-05-03
Effect of strong anisotropy on the dissipative and non-dissipative regimes of the second-order structure function
- Abstract: We study the variations in second-order velocity structure functions (SFs) in the strongly anisotropic turbulent flow past a backward facing step. Time-resolved particle image velocimetry measurements were taken in a stationary turbulent flow past a backward facing step at Reynolds numbers 13,600, 9,000, and 5,500 based on the maximum velocity and step size. Large-scale anisotropic properties of the flow along with local small-scale turbulence characteristics were characterized in detail. Seven interrogation points distributed along points of different large-scale anisotropic characteristics systematically probed the influence of large-scale anisotropy on the second-order SFs. The velocity SFs at each interrogation point represent variance of velocity increments in the streamwise, transverse (wall normal), and the two principle directions of local deformation field. Logarithmic derivatives of the SFs captured the scale-dependent scaling characteristics at the small scales. Measurements revealed a strongly anisotropic large-scale flow with an intense turbulent free-shear layer downstream of the step. Comparison among second-order SFs reveals a mechanistic relationship between the mean flow deformation field, defined by the principle axis of deformation and the magnitude of eigenvalues, to the characteristic influence on SF scaling in the dissipative and non-dissipative scales. Specifically, we report that non-dissipative scaling between orthogonal directions does not differentially saturate if these directions are aligned with the principle axis of deformation. We also show that the relative root mean square of velocity components influences the level of exponent saturation in the dissipative scale regime.
  PubDate: 2013-05-01
High velocity impingement of single droplets on a dry smooth surface
- Abstract: The vertical impact of single, mono disperse water droplets on a dry smooth surface was studied experimentally by means of shadowgraphy. A glass substrate was mounted on a rotating wheel to obtain high impact velocities. The droplets were generated on demand. While the Ohnesorge number was kept constant, Weber number and Reynolds number were varied by adjusting the impact velocity. In all performed experiments, splashing was observed. The distinction of the different measurement series was done by the use of the Weber number. The different Weber numbers were, 3,500, 5,000 and 10,000. Phase-locked images were taken and the temporal evolution of the impact was reconstructed by means of the nondimensional impingement time. The outcome of the measurement was analysed by digital image processing to quantify the distribution of the diameter of the resulting secondary droplets in size and time as well as their velocity, and the total deposited mass fraction remaining on the surface after the impingement. In all cases, the greater part of the impinging primary droplet remained on the substrate.
  PubDate: 2013-04-27
Flow and heat transfer investigation behind trapezoidal rib using PIV and LCT measurements
- Abstract: The present work is an experimental investigation inside a rectangular duct for flow behind a trapezoidal type of rib with chamfering angle α (toward the direction of flow) at different Reynolds numbers. Chamfering angle α has been varied in between 0° and 20° with an increment of 5° and subsequently detailed fluid flow and heat transfer experiments have been performed at four different Reynolds numbers, that is, 9,400, 27,120, 44,600, and 61,480 (based on hydraulic diameter of the duct). In order to investigate the detailed fluid flow and heat transfer characteristics together, a distinct experimental setup has been designed while using 2-D particle image velocimetry and liquid crystal thermography, respectively. Flow investigations have been restricted within the streamwise location of x/e ≤ 11, while the region of interest for heat transfer measurement goes up to x/e ≤ 50. The emphasis is toward assessing and analyzing the potential impact of varying chamfering angle over the flow structures, and its subsequent effect on far downstream heat transfer enhancement, as well as its role in obviating the hot spots in the adjacent vicinity behind the chamfered rib turbulators. Transient heat transfer investigation has been performed for evaluating the surface heat transfer enhancement. Results are documented in terms of stream traces, mean and rms velocity fields, streamwise Reynolds stresses and vorticity distribution, and surface and spanwise averaged augmentation Nusselt numbers. The reattachment length has been identified for all of the configurations, and the turbulent characteristics have been discussed in reference to the reattaching shearing layer and its potential impact on the size of the recirculation bubble for different configurations and conditions. The result showed the successful impact of changing the trapezoidal angle α by manipulating the small-scale vortices at the leeward corner of the rib which helps in obviating the hot spots. Furthermore, the presence of large scale unsteady vortical structure within the shear layer has been confirmed, and it has been subsequently associated with heat transfer enhancement in the far downstream region.
  PubDate: 2013-04-24
Experimental investigations of a trailing edge noise feedback mechanism on a NACA 0012 airfoil
- Abstract: Discrete frequency tones in the trailing edge noise spectra of NACA 0012 airfoils are investigated with the Coherent Particle Velocity method. The Reynolds number and angle of attack range, in which these discrete frequency tones are present, are consistent with published results. The discrete tones are composed of a main tone and a set of regularly spaced side peaks resulting in a ladder-type structure for the dependency on the free stream velocity. The occurrence of this discrete frequency noise could be attributed to the presence of a laminar boundary layer on the pressure side opening up into a separation bubble near the trailing edge, which was visualized using oil flow. Wall pressure measurements close to the trailing edge revealed a strong spanwise and streamwise coherence of the flow structures inside this laminar separation bubble. The laminar vortex shedding frequencies inferred from the streamwise velocity fluctuations, which were evaluated from hot-wire measurements at the trailing edge, were seen to coincide with the discrete tone frequencies observed in the trailing edge noise spectra. Previous findings on discrete frequency tones for airfoils with laminar boundary layers up to the trailing edge hint at the existence of a global feedback loop. Hence, sound waves generated at the trailing edge feed back into the laminar boundary layer upstream by receptivity and are, then, convectively amplified downstream. The most dominant amplification of these disturbance modes is observed inside the laminar separation bubble. Therefore, the frequencies of the most pronounced tones in the trailing edge noise spectra are in the frequency range of the convectively most amplified disturbance modes. Modifying the receptivity behavior of the laminar boundary layer on the pressure side by means of very thin, two-dimensional roughness elements considerably changes the discrete tone frequencies. For roughness elements placed closer to the trailing edge, the main tone frequency was seen to decrease, while the frequency spacing in-between two successive tones increased. Based on the stability characteristics of the laminar boundary layer and the characteristics of the upstream traveling sound wave, a method for predicting the discrete tone frequencies was developed showing good agreement with the measured results. Hence, with a controlled modification of the laminar boundary layer receptivity behavior, the existence of the proposed feedback loop could be confirmed. At the same time, no significant influence of a second feedback loop previously proposed for the suction side of the NACA 0012 airfoil was observed neither by influencing the boundary layer with a receptivity–roughness element nor by tripping the boundary layer at the leading edge.
  PubDate: 2013-04-23