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1.
The two-dimensional space–time turbulence statistics of free shear jet flows in the form of the two-point velocity correlation tensor are important for aeroacoustic noise source modelling based on the acoustic analogy approach. This paper presents a direct application of the Point-Referenced Global Correlation (PRGC) technique to measure the components of this correlation tensor for a sector of the flow field in two jet configurations. The PRGC approach combines single point and global measurement techniques and enables two-point space–time correlations over a region of the flow to be obtained. The technique is applied to a single stream jet and a co-axial coplanar jet at a Mach number of 0.24 using commercial Laser Doppler velocimetry (LDV) and low-speed particle image velocimetry (PIV) systems. Results for the one-dimensional correlations are shown to compare well with two-point measurements. The results for the two-dimensional space–time correlations are presented and the characteristics for both configurations discussed. 相似文献
2.
A method to extract whole-field spatio-temporal correlations by combining global and single-point measurement techniques of different time resolutions is proposed. For fluid mechanics applications, the emphasis is on the combination of low repetition rate particle image velocimetry (PIV) results with experimental data obtained at largely higher sampling frequencies. The experimental feasibility of the procedure is established from results obtained in the wake of a cylinder, using PIV and constant temperature hot wire anemometry (CTA). The method is then applied to examine the shear layer in the core of a round subsonic jet using PIV and laser Doppler velocimetry (LDV). The accuracy of the cross-correlation functions is compared to the auto- and cross-correlation functions obtained from series of LDV and CTA measurements. 相似文献
3.
Astigmatism or wavefront deformation, microscopic particle tracking velocimetry (A-μPTV) (Chen et al. in Exp Fluids 47:849–863,
2009; Cierpka et al. in Meas Sci Technol 21:045401, 2010b) is a method to determine the complete 3D3C velocity field in micro-fluidic devices with a single camera. By using an intrinsic
calibration procedure that enables a robust and precise calibration on the basis of the measured data itself (Cierpka et al.
in Meas Sci Technol 22:015401, doi:, 2011), accurate results without errors due to spatial averaging or bias due to the depth of correlation can be obtained. This
method takes all image aberrations into account, allows for the use of the whole CCD sensor, and is easy to apply without
expert knowledge. In this paper, a comparative study is presented to assess the uncertainties of two state-of-the-art methods
for 3C3D velocity field measurements in microscopic flows: stereoscopic micro-particle image velocimetry (S-μPIV) and astigmatism
micro-particle tracking velocimetry (A-μPTV). First, the main parameters affecting all methods’ measurement uncertainty are
identified, described, and quantified. Second, the test case of the flow over a backward-facing step is analyzed using all
methods. For comparison, standard 2D2C μPIV measurements and numerical flow simulations are shown as well. Advantages and
disadvantages of both methods are discussed. 相似文献
4.
T. Astarita 《Experiments in fluids》2006,40(6):977-987
As testified by a previous article (Astarita and Cardone in Exp Fluids 38:233–243, 2005), a critical point that can influence significantly the accuracy of image deformation methods (IDM) for particle image velocimetry (PIV) is the interpolation scheme (IS) used in the reconstruction of deformed images. In the cited paper the effect of noise has been neglected and for this reason in this follow-up paper the influence of the IS, in the presence of noise, on both accuracy and spatial resolution is studied. Performance assessment is conducted using synthetic images with particles of Gaussian shape and with constant and sinusoidal displacement fields. Both the local and the top hat moving average approaches are investigated and the modulation transfer function, the total and bias errors have been used to evaluate the performances of IDMs for PIV applications. The results show that, when a high noise level is present in the images, the influence of the IS is less relevant than what was shown by Astarita and Cardone (Exp Fluids 38:233–243, 2005). 相似文献
5.
The turbulence structure near a wall is a very active subject of research and a key to the understanding and modeling of this
flow. Many researchers have worked on this subject since the fifties Hama et al. (J Appl Phys 28:388–394, 1957). One way to study this organization consists of computing the spatial two-point correlations. Stanislas et al. (C R Acad
Sci Paris 327(2b):55–61, 1999) and Kahler (Exp Fluids 36:114–130, 2004) showed that double spatial correlations can be computed from stereoscopic particle image velocimetry (SPIV) fields and can
lead to a better understanding of the turbulent flow organization. The limitation is that the correlation is only computed
in the PIV plane. The idea of the present paper is to propose a new method based on a specific stereoscopic PIV experiment
that allows the computation of the full 3D spatial correlation tensor. The results obtained are validated by comparison with
2D computation from SPIV. They are in very good agreement with the results of Ganapthisubramani et al. (J Fluid Mech 524:57–80,
2005a). 相似文献
6.
In the present paper, the interaction mechanisms of the vortices shed by a single-screw propeller with a rudder installed
in its wake are addressed; in particular, following the works by Felli et al. (Exp Fluids 6(1):1–11, 2006a, Exp Fluids 46(1):147–1641, 2009a, Proceedings of the 8th international symposium on particle image velocimetry: Piv09, Melbourne, 2009b), the attention is focused on the analysis of the evolution, instability, breakdown and recovering mechanisms of the propeller
tip and hub vortices during the interaction with the rudder. To investigate these mechanisms in detail, a wide experimental
activity consisting in time-resolved visualizations, velocity measurements by particle image velocimetry (PIV) and laser Doppler
velocimetry (LDV) along horizontal chordwise, vertical chordwise and transversal sections of the wake have been performed
in the Cavitation Tunnel of the Italian Navy. Collected data allows to investigate the major flow features that distinguish
the flow field around a rudder operating in the wake of a propeller, as, for example, the spiral breakdown of the vortex filaments,
the rejoining mechanism of the tip vortices behind the rudder and the mechanisms governing the different spanwise misalignment
of the vortex filaments in the pressure and suction sides of the appendage. 相似文献
7.
The technical basis and system set-up of a dual-plane stereoscopic particle image velocimetry (PIV) system, which can obtain
the flow velocity (all three components) fields at two spatially separated planes simultaneously, is summarized. The simultaneous
measurements were achieved by using two sets of double-pulsed Nd:Yag lasers with additional optics to illuminate the objective
fluid flow with two orthogonally linearly polarized laser sheets at two spatially separated planes, as proposed by Kaehler
and Kompenhans in 1999. The light scattered by the tracer particles illuminated by laser sheets with orthogonal linear polarization
were separated by using polarizing beam-splitter cubes, then recorded by high-resolution CCD cameras. A three-dimensional
in-situ calibration procedure was used to determine the relationships between the 2-D image planes and three-dimensional object
fields for both position mapping and velocity three-component reconstruction. Unlike conventional two-component PIV systems
or single-plane stereoscopic PIV systems, which can only get one-component of vorticity vectors, the present dual-plane stereoscopic
PIV system can provide all the three components of the vorticity vectors and various auto-correlation and cross-correlation
coefficients of flow variables instantaneously and simultaneously. The present dual-plane stereoscopic PIV system was applied
to measure an air jet mixing flow exhausted from a lobed nozzle. Various vortex structures in the lobed jet mixing flow were
revealed quantitatively and instantaneously. In order to evaluate the measurement accuracy of the present dual-plane stereoscopic
PIV system, the measurement results were compared with the simultaneous measurement results of a laser Doppler velocimetry
(LDV) system. It was found that both the instantaneous data and ensemble-averaged values of the stereoscopic PIV measurement
results and the LDV measurement results agree well. For the ensemble-averaged values of the out-of-plane velocity component
at comparison points, the differences between the stereoscopic PIV and LDV measurement results were found to be less than
2%.
Received: 18 April 2000/Accepted: 2 February 2001 相似文献
8.
Mark F. Reeder Jim W. Crafton Jordi Estevadeordal Joseph DeLapp Charles McNiel Don Peltier Tina Reynolds 《Experiments in fluids》2010,48(5):889-900
Flow visualization, particle image velocimetry (PIV), and laser Doppler velocimetry (LDV) are among the most useful tools
available for experimental aerodynamics studies. Implementation of these techniques, however, requires that seed material
be introduced into the flow. The undesirable qualities of the seeding material often prevent the use of flow visualization
and velocimetry techniques in many test environments. This is particularly true for large-scale, closed-circuit tunnels where
facility operators must weigh the risks of facility contamination, sensor damage, and safety concerns that might result from
the introduction of seed particles. Identification of a practical clean seeding material that minimizes or eliminates these
concerns would enable flow visualization and velocimetry techniques to be deployed in these facilities. Here, we demonstrate
two seeding systems that have the potential to provide such a solution. The first system is a new concept which uses liquid
carbon dioxide that can be made to form discrete particles as it expands from a high-pressure tank. PIV measurements are demonstrated
in several flows, including supersonic and subsonic tunnels, using these residue-free seed particles. The second system utilizes
a combination of steam and liquid nitrogen to produce an aerosol or fog that serves as flow seeding. Water- or steam-based
seeding has been previously demonstrated for flow visualization in subsonic tunnels; here however, we utilize this seed material
for PIV and LDV measurements as well as for flow visualization in a large supersonic tunnel. 相似文献
9.
μPIV is a widely accepted tool for making accurate measurements in microscale flows. The particles that are used to seed the
flow, due to their small size, undergo Brownian motion which adds a random noise component to the measurements. Brownian motion
introduces an undesirable error in the velocity measurements, but also contains valuable temperature information. A PIV algorithm
which detects both the location and broadening of the correlation peak can measure velocity as well as temperature simultaneously
using the same set of images. The approach presented in this work eliminates the use of the calibration constant used in the
literature (Hohreiter et al. in Meas Sci Technol 13(7):1072–1078, 2002), making the method system-independent, and reducing the uncertainty involved in the technique. The temperature in a stationary
fluid was experimentally measured using this technique and compared to that obtained using the particle tracking thermometry
method and a novel method, low image density PIV. The method of cross-correlation PIV was modified to measure the temperature
of a moving fluid. A standard epi-fluorescence μPIV system was used for all the measurements. The experiments were conducted
using spherical fluorescent polystyrene-latex particles suspended in water. Temperatures ranging from 20 to 80°C were measured.
This method allows simultaneous non-intrusive temperature and velocity measurements in integrated cooling systems and lab-on-a-chip
devices. 相似文献
10.
The purpose of the current paper is to describe an experimental study of the spatially correlated precision errors associated
with particle image velocimetry (PIV) measurements made in turbulent flows. A free jet was used as the base flow for the study.
The precision error of time-averaged statistics of the mean and turbulent flowfield is governed by the probability distribution
function of the various quantities and the finite sample size of the data sets. Spatial measurements that are separated by
a distance that is shorter than the size of the large turbulent scales will not be independent, resulting in a correlated
precision error. The characteristics of the precision error for various statistics will be described. It is found that mean
vorticity has a correlated precision error that is limited to a much smaller length scale. The results demonstrate the importance
of understanding the role of error correlation in the interpretation of PIV data. 相似文献
11.
An experimental analysis of the precessing vortex core (PVC) instability in a free swirling jet of air at ambient pressure
and temperature is performed by means of laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Two parametric
studies are considered, varying the swirl parameter and the Reynolds number. The range of parameters considered allowed to
study conditions of strong precession as well as the inception and settlement of the instability. Mean velocity and standard
deviation profiles, power spectral density functions and probability density functions for the axial and tangential velocity
components are presented. Average as well as instantaneous PIV maps are considered in the characterization of the flowfield
structure and detection of the instantaneous position of the vortex center. Joint analysis of velocity PDFs and power spectra
shows that the PVC contribution to the global statistics of the velocity field can be properly separated from the contribution
of the true flow turbulence, giving additional insight to the physics of the precession phenomenon. The results obtained in
the explored range of conditions indicate that the true turbulence intensity is not dependent on the swirl parameter.
An erratum to this article can be found at 相似文献
12.
A turbulent plane jet impinging nearby and far from a flat plate 总被引:1,自引:0,他引:1
Plane air jets presenting an impact find applications in many industrial devices. They can be found in installations of heating,
cooling or drying, cleaning, pulverization, or containment of polluted environments. Other applications can be found in the
ventilation of buildings. The correct design of these kinds of installations requires thorough knowledge of the structure
of the jet from the cinematic point of view. With this intention a test bench with variable geometry was developed. Then,
using laser Doppler velocimetry (LDV) and particle image velocimetry (PIV), it is possible to analyze the development of the
jet for various geometrical and cinematic configurations. It appears that the development of the jet is independent of the
Reynolds number, and the velocity decrease in the developed and impinging zones can be characterized by using very simple
laws. Furthermore, by PIV visualization of the impinging zone, it has been possible to highlight the causes of mass transfer
through the jet.
Received: 19 January 2000 / Accepted: 14 May 2001 相似文献
13.
Imaging laser Doppler velocimetry (ILDV) is a novel flow measurement technique, which enables the measurement of the velocity
in an imaging plane. It is an evolution of heterodyne Doppler global velocimetry (HDGV) and may be regarded as the planar
extension of the classical dual-beam laser Doppler velocimetry (LDV) by crossing light sheets in the flow instead of focused
laser beams. Seeding particles within the flow are illuminated from two different directions, and the light scattered from
the moving particles exhibits a frequency shift due to the Doppler effect. The frequency shift depends on the direction of
the illumination and the velocity of the particle. The superposition of the two different frequency-shifted signals on the
detector creates interference and leads to an amplitude modulated signal wherein the modulation frequency depends on the velocity
of the particle. This signal is detected using either a high-speed camera or alternatively a smart pixel imaging array. This
detector array performs a quadrature detection on each pixel with a maximum demodulation frequency of 250 kHz. To demonstrate
the feasibility of the technique, two experiments are presented: The first experiment compares the measured velocity distribution
of a free jet using ILDV performed with the smart pixel detector array and a high-speed camera with a reference measurement
using PIV. The second experiment shows an advanced setup using two smart pixel detector arrays to measure the velocity distribution
on a rotating disk, demonstrating the potential of the technique for high-velocity flow measurements. 相似文献
14.
The spatial resolution of correlation particle image velocimetry (PIV) is a frequently addressed issue that still raises scientific
interest. In conventional non-iterative PIV, the spatial resolution limits are of common knowledge (Willert and Gharib (1991) Exp Fluids 10:181–193; Raffel et al. (1998) ISBN 3-540-63683-8, Springer, Berlin Heidelberg New York, among others). On the contrary, those advanced iterative multipass
methods that use image distortion techniques or multigrid techniques present a more complex scenario. One of the concepts
that raises more debate is the limiting effect of the interrogation window size. This paper focuses on the subject, trying
to clarify key points. The results indicate that iterative algorithms using an appropriate weighting function eliminate the
window size from the ensemble of spatial resolution limits. 相似文献
15.
PIV error correction 总被引:15,自引:9,他引:6
D. P. Hart 《Experiments in fluids》2000,29(1):13-22
A non-post-interrogation method of reducing subpixel errors and eliminating spurious vectors from particle image velocimetry
(PIV) results is presented. Unlike methods that rely on the accuracy or similarity of neighboring vectors, errors are eliminated
before correlation information is discarded using available spatial and/or temporal data. Anomalies are removed from the data
set through direct element-by-element comparison of the correlation tables calculated from adjacent regions. The result is
a processing technique that yields a symmetric correlation profile representing the velocity at the boundary of the combined
regions. This correlation based correction (CBC) technique greatly improves subpixel accuracy, and is highly robust to out-of-boundary
particle motion, particle overlap, unmatched particle correlations, and electronic and optical imaging noise.
Received: 22 June 1999/Accepted: 21 September 1999 相似文献
16.
Large polymer filaments can form when drag reducing polymers are injected through wall slots. The presence of these structures enhances the performance of the drag reducing function by mechanisms which are not understood. This paper shows how particle image velocimetry (PIV) techniques can be used to study changes in the configuration of the injected polymer and in the structure of the velocity field with increasing drag reduction. The filaments are found to behave as solid bodies which break up in high shear regions close to a boundary. The breakup process provides an explanation of why the filaments are not observed close to a wall and offers the possibility of providing a heterogeneous distribution of small aggregates of polymers which could be more effective than uniformly distributed molecules as suggested by Hoyer and Gyr (J Non-Newton Fluid Mech 65:221–240, 1996; J Fluids Eng 120:818–823, 1998), Dunlop and Cox (Phys Fluids 20:203–213, 1977) and Vlachogiannis et al. (Phys Fluid 15:3786–3794, 2004). PIV measurements show dramatic qualitative changes in the velocity patterns at maximum drag reduction. 相似文献
17.
A phase discrimination method for two-phase PIV is presented that is capable of simultaneously separating the two phases from
time-resolved stereoscopic PIV images taken in a particle-laden jet. The technique developed expands on previous work done
by Khalitov and Longmire (Exp Fluids 32:252–268, 2002), where by means of image processing techniques, a raw two-phase PIV image can be separated into two single-phase images
according to particle size and intensity distributions. The technique is expanded through the use of three new image processing
algorithms to separate particles of similar size (up to an order of magnitude better than published work) for fields of view
much larger than previously considered. It also addresses the known problem of noisy background images produced by high-speed
CMOS cameras, which makes the particle detection and separation from the noisy background difficult, through the use of a
novel fast Fourier transform background filter. 相似文献
18.
Measurement of fully-developed turbulent pipe flow with digital particle image velocimetry 总被引:6,自引:0,他引:6
J. Westerweel A. A. Draad J. G. Th. van der Hoeven J. van Oord 《Experiments in fluids》1996,20(3):165-177
A new and unique high-resolution image acquisition system for digital particle image velocimetry (DPIV) in turbulent flows is used for the measurement of fully-developed turbulent pipe flow at a Reynolds number of 5300. The flow conditions of the pipe flow match those of a direct numerical simulation (DNS) and of measurements with conventional (viz., photographic) PIV and with laser-Doppler velocimetry (LDV). This experiment allows a direct and detailed comparison of the conventional and digital implementations of the PIV method for a non-trivial unsteady flow. The results for the turbulence statistics and power spectra show that the level of accuracy for DPIV is comparable to that of conventional PIV, despite a considerable difference in the interrogation pixel resolution, i.e. 32 × 32 (DPIV) versus 256 × 256 (PIV). This result is in agreement with an earlier analytical prediction for the measurement accuracy. One of the advantages of DPIV over conventional PIV is that the interrogation of the DPIV images takes only a fraction of the time needed for the interrogation of the PIV photographs. 相似文献
19.
Mathieu Legrand José Nogueira Shigeru Tachibana Antonio Lecuona Sara Nauri 《Experiments in fluids》2011,51(4):861-870
This paper proposes a method to sort experimental snapshots of a periodic flow using information from the first three POD
coefficients. Even in presence of turbulence, phase-average flow fields are reconstructed with this novel technique. The main
objective is to identify and track traveling coherent structures in these pseudo periodic flows. This provides a tool for
shedding light on flow dynamics and allows for dynamical contents comparison, instead of using mean statistics or traditional
point-based correlation techniques. To evaluate the performance of the technique, apart from a laminar test on the relative
strength of the POD modes, four additional tests have been performed. In the first of these tests, time-resolved PIV measurements
of a turbulent flow with an externally forced main frequency allows to compare real phase-locked average data with reconstructed
phase obtained using the technique proposed in the paper. The reconstruction technique is then applied to a set of non-forced,
non time-resolved Stereo PIV measurements in an atmospheric burner, under combustion conditions. Besides checking that the
reconstruction on different planes matches, there is no indication of the magnitude of the error for the proposed technique.
In order to obtain some data regarding this aspect, two additional tests are performed on simulated non-externally forced
laminar flows with the addition of a digital filter resembling turbulence (Klein et al. in J Comput Phys 186:652–665, 2003). With this information, the limitation of the technique applicability to periodic flows including turbulence or secondary
frequency features is further discussed on the basis of the relative strength of the Proper Orthogonal Decomposition (POD)
modes. The discussion offered indicates coherence between the reconstructed results and those obtained in the simulations.
In addition, it allows defining a threshold parameter that indicates when the proposed technique is suitable or not. For those
researchers interested on the background and possible generalizations of the technique, part I of this work (Legrand et al.
in Exp Fluid (submitted in 2010) 2011) offers the mathematic fundamentals of the general space–time reconstruction technique using POD coefficients. Noteworthy,
the involved computational time is relatively small: all the reconstructions have been performed in the order of minutes. 相似文献
20.
《Journal of Fluids and Structures》2007,23(2):191-206
This paper presents the measurements of the flow in the space between an enclosed corotating disk pair using particle image velocimetry (PIV) and laser doppler velocimetry (LDV). LDV gives the time history of velocity for time-domain analysis, while PIV provides the spatial distribution of the instantaneous velocity. A flow visualization technique displaying the concentration distribution of seeding particles was also employed to visualize the flow patterns. Experiments were conducted on the interdisk midplane with a Reynolds number of 5.25×105. Based on the LDV measured rotating frequency of the vortices around the hub, the phase-resolved PIV measurements were achieved, and a rotating reference coordinate system was employed to represent the flow patterns. The phase-resolved measurements reveal that the circumferential flow velocity oscillates periodically in both the inner and outer regions but in opposite trends. Based on the phase averaged data, the contributions of the periodic and random motions to the Reynolds stresses were evaluated, and the spatial distributions of the periodic Reynolds stresses were displayed. It is found that, the local rotation of the fluid induced by the deformation of the inner region contribute to a significant portion of the momentum transport. 相似文献