Abstract: Abstract
This work investigates the effect of selected linear image processing methods on the depth of correlation (DoC) in micro particle image velocimetry using a single camera. In practice, band-pass and high-pass filters (background subtraction) are commonly applied to micro particle image velocimetry images. This work provides analytical models describing the effect of the parameters of low-, high-, and band-pass filters on the DoC and verifies the models by experiments. Furthermore, we propose a scheme that allows computing the weighting function and DoC for more complicated cases numerically. PubDate: 2014-09-19

Abstract: Abstract
An experimental study has been conducted to investigate the flow around two identical square cylinders in tandem arrangement and placed near a plane wall at a Reynolds number of 6,300. The inter-cylinder spacing ratio was varied from S
* = 0.5 to 6, and the cylinder-to-wall gap ratio from G
* = 0.25 to 2. Totally, 42 cases were considered to systematically examine the effects of wall proximity and the mutual interference between the two cylinders in the normalized gap–spacing (G
*–S
*) plane. The flow fields were captured using digital particle image velocimetry, in conjunction with measurements of the fluid forces (drag and lift) acting on the downstream cylinder using a piezoelectric load cell. The results show that the flow is highly dependent on the combined values of G
* and S
*. Categories relating to G
* could be broadly classified as small-gap regime (G
* < 0.5) at which periodic vortex shedding from the cylinders is suppressed, intermediate-gap regime (0.5 < G
* < 1) where vortex shedding occurs but is under the influence of the wall proximity, and large-gap regime (G
* > 1) where the wall effects become negligible. Similarly, the flow interference between the two cylinders can be divided into three basic categories as a function of S
*, namely, shielding regime at S
* < 1, reattachment regime at 1 < S
* < 3, and impinging regime at S
* > 3. Variations of force coefficients, amplitude spectra, Strouhal numbers, and Reynolds shear stress with G
* and S
* are presented to characterize the different flow regimes. PubDate: 2014-09-19

Abstract: Abstract
We present an experimental approach for estimating finite-time Lyapunov exponent fields (FTLEs) in three-dimensional multi-component or multi-phase flows. From time-resolved sequences of particle images, we directly compute the flow map and coherent structures, while avoiding and outperforming the computationally costly numerical integration. Performing this operation independently on each flow component enables the determination of three-dimensional Lagrangian coherent structures (LCSs) without any bias from the other components. The locations of concurrent LCSs for different flow elements (e.g., passive tracers, inertial particles, bubbles, or active particles) can provide new insight into the interpenetrating FTLE structure in complex flows. PubDate: 2014-09-17

Abstract: Abstract
A novel technique to obtain simultaneous velocity and concentration measurements is applied to the Richtmyer–Meshkov instability. After acceleration by a Mach 2.2 shock wave, the interface between the two gases develops into a turbulent mixing layer. A time-separated pair of acetone planar laser-induced fluorescence images are processed to yield concentration and, through application of the Advection-Corrected Correlation Image Velocimetry technique, velocity fields. This is the first application of this technique to shock-accelerated flows. We show that when applied to numerical simulations, this technique reproduces the velocity field to a similar quality as particle image velocimetry. When applied to the turbulent mixing layer of the experiments, information about the Reynolds number and anisotropy of the flow is obtained. PubDate: 2014-09-16

Abstract: Abstract
Echo particle image velocimetry (Echo PIV) presents itself as an attractive in vivo flow quantification technique to traditional approaches. Promising results have been acquired; however, limited quantification and validation is available for post-stenotic flows. We focus here on the comprehensive evaluation of in vitro downstream stenotic flow quantified by Echo PIV and validated in relation to digital particle image velocimetry (DPIV). A Newtonian blood analog was circulated through a closed flow loop and quantified immediately downstream of a 50 % axisymmetric blockage at two Reynolds numbers (Re) using time-averaged Echo PIV and DPIV. Centerline velocities were in good agreement at all Re; however, Echo PIV measurements presented with elevated standard deviation (SD) at all measurements points. SD was improved using increased line density (LD); however, frame rate or field of view (FOV) is compromised. Radial velocity profiles showed close agreement with DPIV with the largest disparity in the shear layer and near-wall recirculation. Downstream recirculation zones were resolved by Echo PIV at both Re; however, magnitude and spatial coverage was reduced compared to DPIV that coincided with reduced contrast agent penetration beyond the shear layer. Our findings support the use of increased LD at a cost to FOV and highlight reduced microbubble penetration beyond the shear layer. High local SD at near-wall measurements suggests that further refinement is required before proceeding to in vivo quantification studies of wall shear stress in complex flow environments. PubDate: 2014-09-16

Abstract: Abstract
Fluid–structure interaction phenomena are extremely important when laminar flows through elastic vessels such as in biomedical flow problems are considered. In general, such elastic vessels are curved which is why an elastic 180° bend at a curvature ratio
\(\delta = D/D_{\rm C} = 0.\bar{2}\)
defines the reference geometry in this study. It is the purpose of this study to compare the results with the steady flow through a 180° rigid pipe bend and to quantify the impact of the fluid–structure interaction on the overall flow pattern and the vessel deformation at oscillating fully developed entrance flow. The findings comprise velocity, pressure, and structure deformation measurements. The vessel dilatation amplitude was varied between 3.75 % and 7 % of the vessel diameter at Dean De and Womersley number Wo ranges of
\(327\,\le\,De\,\le\,350\)
and
\(7\,\le\,Wo\,\le\,8.\)
The flow is investigated by time-resolved stereoscopic particle-image velocimetry in five radial cross sections located in the elastic 180° bend and in the inlet pipes. The unsteady static vessel pressure is measured synchronously at these cross sections. The comparison of the steady with the unsteady flow field shows a strong change in the axial and secondary velocity distributions at periods of transition between the centrifugal forces and the unsteady inertia forces dominated regimes. These changes are characterized by asymmetric fluctuations of the centers of the counter-rotating vortex pair. The investigation of the impact of the structure deformation amplitude on these fluctuations reveals a significant attenuation at high deformation amplitudes. The spatial motion of the elastic vessel due to the forces applied by the flow exhibits amplitudes up to 15 % of the vessel diameter. Considering the fluid–structure interaction, an amplification of the volume flux amplitude by a factor of 2.1 at the vessel outlet and phase lags up to 30° occur. The static pressure distribution is characterized by a pronounced asymmetry between forward and backward flow with a 40 % higher peak magnitude at backward flow and phase lags of 35°. The results evidence that a strong distortion of the velocity distribution in the bend, which is caused by the oscillating nature of the flow, is reduced as a result of the fluid–structure interaction. PubDate: 2014-09-16

Abstract: Abstract
A novel point Doppler velocimeter (pDV) based upon the Doppler global velocimetry principle is presented, which is capable of three-component velocity vector measurements at 100 kHz mean rates over extended time periods. In this implementation, two laser beams are multiplexed to illuminate the flow over alternating time windows, providing for a reduction in the number of sensors required. The implications of this multiplexing paradigm coupled with the fundamental limits set by the optical absorption filter are examined in detail, and uncertainties are predicted via instrumentation modeling and representative synthetic flow data. The results indicate that the multiplexing pDV instrument provides the required temporal and velocity resolution for turbulent shear flows at velocities of nominally 500 m/s. As a demonstration and validation of this time-resolved technique, statistics of three-velocity component measurements in a cold, supersonic, over-expanded jet at jet exit Mach number M
j
= 1.4 (design Mach number M
d
= 1.65) are presented. Time resolution up to 250 kHz and instantaneous velocity uncertainties between 6.6 and 11.1 m/s were obtained. Comparisons of mean pDV data with laser Doppler velocimetry data are consistent with uncertainty predictions for the technique. The ultimate value of the instrument is exhibited in the analysis of Reynolds stress spectra in the screeching jet, exposing the spatial development of motions at the harmonics of the screech tone, variable phase-coordinated shock motions, and growth of turbulent fluctuations in the developing shear layer of the jet. From the data presented, the screech tone phenomenon is suspected to be linked to the production of radial–azimuthal shear stresses in extended regions beyond the potential core. PubDate: 2014-09-13

Abstract: Abstract
The finite-time Lyapunov exponent (FTLE) is a popular tool to extract characteristic features of flows that cannot be revealed by other criteria. However, even if the computational cost of computing particle trajectories in space and time has been reduced and optimized in a considerable number of works, the main challenge probably consists in increasing the spatial resolution of Lagrangian coherent structures locally, i.e., where the FTLE field reaches its maximum values. On the other hand, most of experimental data are obtained in planes so that the FTLE field is computed without the out-of-plane particle movements. To investigate which physics the FTLE can capture in flows that are highly three dimensional, the criterion is computed from high-speed stereoscopic particle image velocimetry measurements of the pulsatile flow that develops behind a bi-leaflet mechanical heart valve. The similitude is based on the Womersley number, and experiments are performed for a lower Reynolds number than the physiologic value to obtain sufficiently resolved data in space and time. It is found that the vortex shedding is well captured and that its development can be decomposed into four successive phases. The longest phase occurs near the peak flow rate and exhibits a break of symmetry similar to the one appearing in the wakes of two side-by-side cylinders in the regime when the separation between the cylinders is of the order of their diameter. Specifically, a vortex street with alternating vortex sheddings is observed in a narrow wake behind one of the leaflets, whereas single large vortices develop inside a wide wake downstream of the other leaflet. It appears that these patterns are difficult or even impossible to discern with classical Eulerian vortex identification techniques. The Strouhal numbers of vortex-shedding frequencies, obtained from continuous wavelet transforms and based on the apparent height of the leaflets, are also close to those found in the flow behind two cylinders. By invoking the Taylor hypothesis, an approximate three-dimensional reconstruction of the flow can be obtained and a three-dimensional FTLE field is deduced, which provides a very detailed view of the vortex structures that form and develop in the wakes of the leaflets. PubDate: 2014-09-04

Abstract: Abstract
Spectral methods are ubiquitous in the analysis of dynamically evolving fluid flows. However, tools like Fourier transformation and dynamic mode decomposition (DMD) require data that satisfy the Nyquist–Shannon sampling criterion. In many fluid flow experiments, such data are impossible to acquire. We propose a new approach that combines ideas from DMD and compressed sensing to accommodate sub-Nyquist-rate sampling. Given a vector-valued signal, we take measurements randomly in time (at a sub-Nyquist rate) and project the data onto a low-dimensional subspace. We then use compressed sensing to identify the dominant frequencies in the signal and their corresponding modes. We demonstrate this method using two examples, analyzing both an artificially constructed dataset and particle image velocimetry data from the flow past a cylinder. In each case, our method correctly identifies the characteristic frequencies and oscillatory modes dominating the signal, proving it to be a capable tool for spectral analysis using sub-Nyquist-rate sampling. PubDate: 2014-09-04

Abstract: Abstract
Experiments on a rotating wing in a liquid-filled tank were conducted to determine the minimum required tip clearance to produce data free from wall effects. A rotating wing fixed at an angle of attack of 45° was revolved for two revolutions at Reynolds numbers between 120 and 10,000. Tip clearance was varied from 0.5 to 5 chords by varying wing size, while also varying wing speed to hold Reynolds number constant. Force measurements on the wing, as well as dye flow visualization and particle image velocimetry of the entire tank, were conducted. Tip clearances of 0.5–7 chords were also tested computationally. Results of all measurements show large tip effects for 0.5 chords of tip clearance, and no wall effects for 5 chords of tip clearance at all Reynolds numbers tested. The 3 chord clearance case showed negligible wall effects in both the particle image velocimetry and dye flow visualization for all Reynolds numbers observed. The forces on the 3 chord tip clearance wing indicate wall effects appearing in the second revolution for Reynolds numbers of >1,000. A tip clearance of 5 chords is deemed to be free of wall effects for experiments limited to two wing revolutions within the range of tested Reynolds numbers. PubDate: 2014-09-02

Abstract: Abstract
The investigation of flows at high Reynolds number is of great interest for the theory of turbulence, in that the large and the small scales of turbulence show a clear separation. But, as the Reynolds number of the flow increases, the size of the Kolmogorov length scale (
\(\eta\)
) drops almost proportionally. Aiming at achieving the adequate spatial resolution in the central region of a self-similar round jet at high Reynolds numbers (
\(Re_\lambda \approx 350\)
), a long-range μPIV system was applied. A vector spacing of
\(1.5 \eta\)
was achieved, where the Kolmogorov length scale was estimated to be
\(55\,\upmu {\rm m}\)
. The resulting velocity fields were used to characterize the small-scale flow structures in this jet. The autocorrelation maps of vorticity and
\(\lambda _{\rm ci}\)
(the imaginary part of the eigenvalue of the reduced velocity gradient tensor) reveal that the structures of intense vorticity have a characteristic diameter of approximately
\(10 \eta\)
. From the autocorrelation map of the reduced (2D) rate of dissipation, it is inferred that the regions of intense dissipation tend to organize in the form of sheets with a characteristic thickness of approximately
\(10 \eta\)
. The regions of intense dissipation have the tendency to appear in the vicinity of intense vortices. Furthermore, the joint pdf of the two invariants of the reduced velocity gradient tensor exhibits the characteristic teapot-shape. These results, based on a statistical analysis of the data, are in agreement with previous numerical and experimental studies at lower Reynolds number, which validates the suitability of long-range μPIV for characterizing turbulent flow structures at high Reynolds number. PubDate: 2014-08-28

Abstract: Abstract
The aerodynamic interaction of a stream-wise vortex impacting on a NACA 23012 oscillating airfoil was investigated using stereo particle image velocimetry. The experimental rig enabled the study of the aerodynamic effects due to the blade pitching motion in the interaction with the vortex. The experimental study focused on the light dynamic stall regime, which represents a typical condition of the retreating blade of a helicopter in forward flight. Particle image velocimetry was applied to a measurement volume close to the airfoil upper surface in order to obtain the three-dimensional interacting flow field. In particular, the experimental results show that during the airfoil downstroke motion, the vortex impact triggers the stall of the local blade section, indicating that detrimental effects on the blade performance can be introduced by perpendicular vortex interactions. PubDate: 2014-08-26

Abstract: Abstract
The influence of leading edge modification on the time-averaged and instantaneous flow around a fan airfoil is investigated by particle image velocimetry (PIV), schlieren imaging and high-speed shock shadowgraphs in a transonic cascade windtunnel. In addition to a global characterization of the time-averaged flow using PIV, the instantaneous passage shock position was extracted from single-shot PIV measurements by matching the tracer velocity across the normal shock with an exponential fit. The instantaneous shock positions are assigned to a probability density distribution in order to obtain the average position and the range of fluctuations of the eroded and reference leading edge. The profiles are used to estimate the response time of the particles to the normal shock which was found to be in the sub-microsecond range. Averaged PIV measurements and the probability density of shock position from both geometries are obtained at near stall and choked conditions. In order to extract the frequency range of the shock motion, the shadow of the shock wave was tracked using high-speed shadowgraphy. The paper also provides details on the experimental implementation such as a specifically designed light-sheet probe. PubDate: 2014-08-24

Abstract: Abstract
Control of the dynamic stall process of a NACA 0015 airfoil undergoing periodic pitching motion is investigated experimentally at the NASA Ames compressible dynamic stall facility. Multiple microjet nozzles distributed uniformly in the first 12 % chord from the airfoil’s leading edge are used for the dynamic stall control. Point diffraction interferometry technique is used to characterize the control effectiveness, both qualitatively and quantitatively. The microjet control has been found to be very effective in suppressing both the emergence of the dynamic stall vortex and the associated massive flow separation at the entire operating range of angles of attack. At the high Mach number (M = 0.4), the use of microjets appears to eliminate the shock structures that are responsible for triggering the shock-induced separation, establishing the fact that the use of microjets is effective in controlling dynamic stall with a strong compressibility effect. In general, microjet control has an overall positive effect in terms of maintaining leading edge suction pressure and preventing flow separation. PubDate: 2014-08-23

Abstract: Abstract
Astigmatic particle tracking velocimetry (APTV) has been developed in the last years to measure the three-dimensional displacement of tracer particles using a single-camera view. The measurement principle relies on an astigmatic optical system that provides aberrated particle images with a characteristic elliptical shape univocally related to the corresponding particle depth position. Because of the precision of this method, this concept is well established for measuring and controlling the distance between a CD/DVD and the reading head. The optical arrangement of an APTV system essentially consists of a primary stigmatic optics (e.g., a microscope, or a camera objective) and an astigmatic optics, typically a cylindrical lens placed in front of the camera sensor. This paper focuses on the uncertainty of APTV in the depth direction. First, an approximated analytical model is derived and experimentally validated. From the model, a set of three non-dimensional parameters that are the most significant in the optimization of the APTV performance are identified. Finally, the effect of different parameter settings and calibration approaches are studied systematically using numerical Monte Carlo simulations. The results allow for the derivation of general criteria to minimize the overall error in APTV measurements and provide the basis for reliable uncertainty estimation for a wide range of applications. PubDate: 2014-08-22

Abstract: Abstract
This paper investigates the formation and evolution of the unsteady three-dimensional wake structures generated by the flapping wings of the DelFly II micro aerial vehicle in forward flight configuration. Time-resolved stereoscopic particle image velocimetry (Stereo-PIV) measurements were carried out at several spanwise-aligned planes in the wake, so as to allow a reconstruction of the temporal development of the wake of the flapping wings throughout the complete flapping cycle. Simultaneous thrust-force measurements were performed to explore the relation between the wake formation and the aerodynamic force generation mechanisms. The three-dimensional wake configuration was subsequently reconstructed from the planar PIV measurements by two different approaches: (1) a spatiotemporal wake reconstruction obtained by convecting the time-resolved, three-component velocity field data of a single measurement plane with the free-stream velocity; (2) for selected phases in the flapping cycle a direct three-dimensional spatial wake reconstruction is interpolated from the data of the different measurement planes, using a Kriging regression technique. Comparing the results derived from both methods in terms of the behavior of the wake formations, their phase and orientation indicate that the spatiotemporal reconstruction method allows to characterize the general three-dimensional structure of the wake, but that the spatial reconstruction method can reveal more details due to higher streamwise resolution. Comparison of the wake reconstructions for different values of the reduced frequency allows assessing the impact of the flapping frequency on the formation and interaction characteristics of the vortical structures. For low values of the reduced frequency, it is observed that the vortex structure formation of instroke and outstroke is relatively independent of each other, but that increasing interaction occurs at higher reduced frequencies. It is further shown that there is a phase lag in the appearance of the structures for increasing flapping frequency, which is in correlation with the generation of the forces. Comparison of thrust generated during the instroke and the outstroke phases of the flapping motion in conjunction with the development of the wake structures indicates that wing–wing interaction at the start of outstroke (peel motion) becomes a dominant feature for reduced frequencies greater than 0.62. PubDate: 2014-08-21

Abstract: Abstract
An experimental investigation of a passively controlled open cavity with
a length to depth ratio of six and freestream Mach number of 1.4 was conducted to investigate the mechanisms responsible for the observed surface pressure reductions. The passive control comes from placing a spanwise aligned cylinder in the boundary layer near the leading edge of the cavity. The two control configurations were isolated from previous experiments of the fluctuating surface pressure and correspond to a larger diameter rod near the top of the boundary layer and a smaller diameter rod placed near the wall. These were further analyzed using particle image velocimetry in an attempt to elicit the responsible mechanism for the flow control. The use of two-point statistics revealed the wall normal turbulent velocity correlation’s evolution became elongated in the wall normal direction. This suggests that the shear layer may be less-organized and consists of smaller-scale structures. The disturbance of the feedback receptivity loop is clearly demonstrated for the controlled configurations evidenced by weakened correlation signals between the aft wall sensor and positions on the cavity floor. The presence of the rod is shown to decrease the mean shear gradient, more effectively for the large rod placed at the top of the boundary layer, throughout the shear layer. The efficacy of the control leads to an initially thicker shear layer which spreads more rapidly and is clearly demonstrated by vorticity growth rates, mean, and turbulent flowfield statistics. PubDate: 2014-08-20

Abstract: Abstract
Dynamic stall on a pitching OA209 airfoil in a wind tunnel is investigated at Mach 0.3 and 0.5 using high-speed pressure-sensitive paint (PSP) and pressure measurements. At Mach 0.3, the dynamic stall vortex was observed to propagate faster at the airfoil midline than at the wind-tunnel wall, resulting in a “bowed” vortex shape. At Mach 0.5, shock-induced stall was observed, with initial separation under the shock foot and subsequent expansion of the separated region upstream, downstream and along the breadth of the airfoil. No dynamic stall vortex could be observed at Mach 0.5. The investigation of flow control by blowing showed the potential advantages of PSP over pressure transducers for a complex three-dimensional flow. PubDate: 2014-08-20

Abstract: Abstract
We investigated the effect of sound irradiated from loudspeakers on the flow of preheated air in the combustion chamber of a swirl burner. The temporally periodic pattern of the flow generated by the sound was detected by fast particle image velocimetry (PIV), with a repetition rate that was adapted to the observation of 12 phase angles of the irradiated monochromatic sound. The strong observed movement of the air is related to the movement by the sound itself, as determined by the pressure measurements with microphones. The PIV measurements reveal also a nonlinear interaction between the irradiated sound and the precession of the vortex core. The accuracy of the sound measurements was tested by determining in quiescent air the acoustic velocity by microphones and as well by PIV; good agreement was obtained thereby. Numerical calculations, using large eddy simulation and accounting for the sound forcing by variation in the mass flow at the inlet of the computational domain, approximately reproduce some of the experimental results. PubDate: 2014-08-20

Abstract: Abstract
The unsteady flow field past a backward-facing step in a rectangular duct is investigated by adopting time-resolved particle image velocimetry (PIV) in the Reynolds number range of 2,640–9,880 based on step height and the inlet average velocity. The PIV realizations are subjected to post-processing techniques, namely, proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). At low Reynolds numbers, the second spatial POD modes indicate the presence of the shear layer mode, whereas this feature shifts to higher modes at higher Reynolds numbers. The corresponding temporal modes are Fourier-transformed to obtain the dominant frequency, whose Strouhal number corroborates the above observation. Short-time windows in the transverse velocity component along the shear layer are selected to investigate the temporal stability of the flow field by DMD to quantify the growth rate of the shear layer mode. The higher harmonics of this mode are also observed to grow, albeit at lesser rate. By relating to POD analysis, the most energetic structures were found to correspond to the unstable modes. The correlation between these unstable DMD modes and the Fourier-filtered flow fields for the same frequencies indicate better match for the lower operating Reynolds number case as compared to higher ones. The spatial stability analysis demonstrates the growth of the shear layer vortices, which is combined with the temporal stability analysis to evaluate the phase velocity of the identified shear layer structures. The calculated phase velocity magnitude of the shear layer is found to be reasonably below the local velocity as expected. PubDate: 2014-08-19