scholarly journals Extending the Frequency Limits of "Postage-Stamp PIV" to MHz Rates

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Steven Beresh ◽  
Russell Spillers ◽  
Melissa Soehnel ◽  
Seth Spitzer
Keyword(s):  
Interrogation Window ◽  
Window Size ◽  
Inertial Subrange ◽  
Postage Stamp ◽  
Effective Frequency ◽  
Acquisition Rate ◽  
Small Fields ◽  
Framing Rate ◽  
The Interrogation Window Size ◽  
Velocity Spectra

The effective frequency limits of postage-stamp PIV, in which a pulse-burst laser and very small fields of view combine to achieve high repetition rates, have been extended by increasing the PIV acquisition rate to very nearly MHz rates (990 kHz) by using a faster camera. Charge leaked through the camera shift register at these framing rates but this was shown not to bias the measurements. The increased framing rate provided oversampled data and enabled use of multi-frame correlation algorithms for a lower noise floor, increasing the effective frequency response to 240 kHz where the interrogation window size begins to spatially filter the data. The velocity spectra suggest turbulence power-law scaling in the inertial subrange steeper than the theoretical -5/3 scaling, attributed to an absence of isotropy.

Single Pixel Evaluation of Microchannel Flows

2005 ◽  
Author(s):  
Steve Wereley ◽  
Carl Meinhart ◽  
Lichuan Gui ◽  
Derek Tretheway ◽  
Arjun Sud
Keyword(s):  
Correlation Analysis ◽  
Particle Concentration ◽  
Interrogation Window ◽  
Window Size ◽  
Bias Error ◽  
Zero Bias ◽  
Effective Particle ◽  
The Interrogation Window Size ◽  
Error Behavior ◽  
Single Pixel

Recently a new μPIV interrogation algorithm has been proposed in which the interrogation window size is reduced to a single pixel. Such small interrogation window sizes are possible using correlation averaging to increase the effective particle concentration to levels required for correlation analysis to succeed. The random error exhibits the expected behavior of decreasing roughly in proportion to N−1/2 while the bias error exhibits unexpected peak-locking behavior with zero bias error at integer and half integer pixel displacements and maximal errors at one-quarter and three-quarter pixel displacements. Accompanying experiments show the potential of this technique but have not yet been sufficiently refined to confirm this unexpected bias error behavior.

Microscale temperature and velocity spectra in the atmospheric boundary layer

1977 ◽  
Vol 83 (3) ◽  
pp. 547-567 ◽  
Author(s):  
R. M. Williams ◽  
C. A. Paulson
Keyword(s):  
Reynolds Number ◽  
Prandtl Number ◽  
Temperature Fluctuations ◽  
Inertial Subrange ◽  
Velocity Fluctuations ◽  
One Dimensional ◽  
Temperature Spectra ◽  
The Mean ◽  
Velocity Spectra ◽  
The One

High-frequency fluctuations in temperature and velocity were measured at a height of 2 m above a harvested, nearly level field of rye grass. Conditions were both stably and unstably stratified. Reynolds numbers ranged from 370000 to 740000. Measurements of velocity were made with a hot-wire anemometer and measurements of temperature with a platinum resistance element which had a diameter of 0[sdot ]5 μm and a length of 1 mm. Thirteen runs ranging in length from 78 to 238 s were analysed.Spectra of velocity fluctuations are consistent with previously reported universal forms. Spectra of temperature, however, exhibit an increase in slope with increasing wavenumber as the maximum in the one-dimensional dissipation spectrum is approached. The peak of the one-dimensional dissipation spectrum for temperature fluctuations occurs at a higher wavenumber than that of simultaneous spectra of the dissipation of velocity fluctuations. It is suggested that the change in slope of the temperature spectra and the dissimilarity between temperature and velocity spectra may be due to spatial dissimilarity in the dissipation of temperature and velocity fluctuations. The temperature spectra are compared with a theoretical prediction for fluids with large Prandtl number, due to Batchelor (1959). Even though air has a Prandtl number of 0[sdot ]7, the observations are in qualitative agreement with predictions of the theory. The non-dimensional wavenumber at which the increase in slope occurs is about 0[sdot ]02, in good agreement with observations in the ocean reported by Grantet al. (1968).For the two runs for which the stratification was stable, the normalized spectra of the temperature derivative fall on average slightly below the mean of the spectra of the remaining runs in the range in which the slope is approximately one-third. Hence the Reynolds number may not have always been sufficiently high to satisfy completely the conditions for an inertial subrange.Universal inertial-subrange constants were directly evaluated from one-dimensional dissipation spectra and found to be 0[sdot ]54 and 1[sdot ]00 for velocity and temperature, respectively. The constant for velocity is consistent with previously reported values, while the value for temperature differs from some of the previous direct estimates but is only 20% greater than the mean of the indirect estimates. This discrepancy may be explained by the neglect in the indirect estimates of the divergence terms in the conservation equation for the variance of temperature fluctuations. There is weak evidence that the one-dimensional constant, and hence the temperature spectra, may depend upon the turbulence Reynolds number, which varied from 1200 to 4300 in the observations reported.

The spectrum of temperature fluctuations in turbulent flow

1968 ◽  
Vol 34 (3) ◽  
pp. 423-442 ◽  
Author(s):  
H. L. Grant ◽  
B. A. Hughes ◽  
W. M. Vogel ◽  
A. Moilliet
Keyword(s):  
Power Spectra ◽  
Temperature Fluctuations ◽  
Inertial Subrange ◽  
Velocity Fluctuations ◽  
One Dimensional ◽  
Open Sea ◽  
Spectrum Function ◽  
Velocity Spectra ◽  
The One ◽  
Rate Of Dissipation

Temperature and velocity fluctuations have been recorded in the open sea and in a tidal channel, and power spectra have been determined from the records. The one-dimensional spectra of temperature fluctuations are found to have an inertial subrange. At larger wave-numbers the data can be fitted by Batchelor's spectrum function for the viscous-convective range. The spectra are inconsistent with the form proposed by Pao for the viscous-convective range.Estimates are given for the constants in Batchelor's spectrum function, but these depend upon knowledge of the rate of dissipation of kinetic energy, which is determined from the velocity spectra. There is doubt about the validity of some of the velocity spectra, and in other cases there is reason to suspect that the turbulence is not locally isotropic.

scholarly journals Scaling properties of velocity and temperature spectra above the surface friction layer in a convective atmospheric boundary layer

2007 ◽  
Vol 14 (3) ◽  
pp. 257-271 ◽  
Author(s):  
K. G. McNaughton ◽  
R. J. Clement ◽  
J. B. Moncrieff
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Surface Friction ◽  
Inertial Subrange ◽  
Turbulence Energy ◽  
Convective Boundary ◽  
Friction Layer ◽  
Thermal Structures ◽  
Temperature Spectra ◽  
Velocity Spectra

Abstract. We report velocity and temperature spectra measured at nine levels from 1.42 meters up to 25.7 m over a smooth playa in Western Utah. Data are from highly convective conditions when the magnitude of the Obukhov length (our proxy for the depth of the surface friction layer) was less than 2 m. Our results are somewhat similar to the results reported from the Minnesota experiment of Kaimal et al. (1976), but show significant differences in detail. Our velocity spectra show no evidence of buoyant production of kinetic energy at at the scale of the thermal structures. We interpret our velocity spectra to be the result of outer eddies interacting with the ground, not "local free convection". We observe that velocity spectra represent the spectral distribution of the kinetic energy of the turbulence, so we use energy scales based on total turbulence energy in the convective boundary layer (CBL) to collapse our spectra. For the horizontal velocity spectra this scale is (zi εo)2/3, where zi is inversion height and εo is the dissipation rate in the bulk CBL. This scale functionally replaces the Deardorff convective velocity scale. Vertical motions are blocked by the ground, so the outer eddies most effective in creating vertical motions come from the inertial subrange of the outer turbulence. We deduce that the appropriate scale for the peak region of the vertical velocity spectra is (z εo)2/3 where z is height above ground. Deviations from perfect spectral collapse under these scalings at large and small wavenumbers are explained in terms of the energy transport and the eddy structures of the flow. We find that the peaks of the temperature spectra collapse when wavenumbers are scaled using (z1/2 zi1/2). That is, the lengths of the thermal structures depend on both the lengths of the transporting eddies, ~9z, and the progressive aggregation of the plumes with height into the larger-scale structures of the CBL. This aggregation depends, in top-down fashion, on zi. The whole system is therefore highly organized, with even the smallest structures conforming to the overall requirements of the whole flow.

Analysis and Simulation of Meteorological Wind Fields Based on Wavelet Transform

2014 ◽  
Vol 490-491 ◽  
pp. 1228-1236 ◽  
Author(s):  
Fo Rong Jin ◽  
Wei Rong Wang
Keyword(s):  
Wavelet Transform ◽  
Wavelet Coefficient ◽  
Low Frequency ◽  
Inertial Subrange ◽  
Target Velocity ◽  
Wind Fields ◽  
Transform Method ◽  
Time Frequency ◽  
Velocity Spectra ◽  
Energy Relations

In this work, we examined the non Gauss distribution characteristic and evolution law of the wavelet coefficient of a gust using wavelet transform; according to the time-frequency characteristic, the wavelet transform coefficients and the energy relations of the target velocity spectra are derived; the wavelet coefficient is generated using the cascade model reflecting the turbulent intermittent; the unsteady gust artificial generation method is established based on inverse wavelet transform; and the arbitrary unsteady fluctuation law can be generated by regulating the coefficient of low frequency. The results show that: the natural gust is in good agreement with Karman wind speed spectrum, meets the turbulence-5 / 3 law in the inertial subrange, and exhibits the nature of intermittence and local self-similarity; the artificial wind sequence based on the inverse wavelet transform method shows similar turbulence statistics with natural gust, with which, the effectiveness of the method is confirmed.

Theory of the pressure–strain rate. Part 2. Diagonal elements

1982 ◽  
Vol 116 ◽  
pp. 1-29 ◽  
Author(s):  
J. Weinstock
Keyword(s):  
Reynolds Number ◽  
Shear Flow ◽  
Strain Rate ◽  
Inertial Subrange ◽  
Turbulent Shear ◽  
Large Reynolds Number ◽  
Simple Shear Flow ◽  
Stress Anisotropy ◽  
Zero Moment ◽  
Velocity Spectra

A theoretical calculation is made of (the diagonal elements of) pressure-strain-rate calculation ρ0−1〈p[∇u + (∇u)T]〉 for a simple turbulent shear flow. This calculation parallels a previous calculation of the off-diagonal element. The calculation is described as follows. (1) Beginning with the Navier-Stokes equation, an expression for the (diagonal) pressure-strain-rate term is derived analytically in terms of measurable quantities (velocity spectra) - this derivation makes use of a cumulant discard. (2) It is proved that, to lowest order in the spectral anisotropy, the diagonal pressure-strain-rate term is linearly proportional to the diagonal Reynolds-stress elements. (3) A formula is derived for the proportionality constants (Rotta constants) in terms of arbitrary spectra. (4) This formula is used to calculate theoretically the numerical value of Rotta's constant Cii for models of velocity spectra (the variation of Cii with variations of spectral shapes and of Reynolds number are also determined). (5) Deficiencies and limitations of Rotta's model are identified and discussed.It is found that Rotta's expression for 2ρ0−1〈p∂ui/∂i〉 is only valid for special spectra. Surprisingly large deviations of Rotta's expression from theory are found for a more complex spectra thought to be typical of simple shear flow. In addition, it is found that Cxz is intrinsically and quantitatively different from Cii because the latter depends importantly on the large-wavenumber part of the spectrum (the inertial subrange) whereas the former does not. The numerical ratio Czz/Cxz is calculated theoretically and shown to be about 2 for the zero-moment model. It is concluded that a linear term in the stress anisotropy as proposed by Rotta does not always exist. The deviation of Rotta's model from theory is understood by distinguishing between the spectral anisotropy and the stress anisotropy.For the zero-moment spectral model, where the Rotta relation is valid, it is found that Cii varies significantly with large Reynolds number but is rather insensitive to the large-wavelength part of the spectrum.

An Improved Particle Image Velocimetry Algorithm for Velocity Measurement of Oil-Water Two-Phase Flow

2014 ◽  
Vol 602-605 ◽  
pp. 1654-1659 ◽  
Author(s):  
Ling Fu Kong ◽  
Wei Hang Kong ◽  
Ying Wei Li ◽  
Cong Zhang ◽  
Sheng Xu Du
Keyword(s):  
Two Phase Flow ◽  
Interrogation Window ◽  
Window Size ◽  
Phase Flow ◽  
Velocity Measurements ◽  
Two Phase ◽  
Horizontal Pipe ◽  
Rectangular Window ◽  
Image Velocimetry ◽  
Oil Water

In this paper, an improved PIV algorithm is proposed for the velocity field of oil-water two-phase dispersed flow in horizontal pipe. In the proposed PIV algorithm, interrogation windows are overlapped by 50% in all iterations other than just overlapped in the final iteration. And what’s still different is that the interrogation window can also be a rectangular window just as [64 64; 64 64; 32 32; 32 32; 32 16;]. What’s more, if any element of the final interrogation window is different from the penultimate iteration, there is going to be another interrogation with the last interrogation window size, which can reduce the false vectors. Experimental results show that velocity measurements of oil-water two-phase flow can be realized by this advanced PIV algorithm with high accuracy. At the same time, it provides the basis for further studying the application of PIV in velocity measurements of oil-water two-phase flow.

scholarly journals Effect of the boundary conditions, temporal, and spatial resolution on the pressure from PIV for an oscillating flow

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Nazmus Sakib ◽  
Alexander Mychkovsky ◽  
James Wiswall ◽  
Randy Samaroo ◽  
Barton Smith
Keyword(s):  
Boundary Conditions ◽  
Spatial Resolution ◽  
Temporal Resolution ◽  
Pressure Field ◽  
Interrogation Window ◽  
Window Size ◽  
Synthetic Jet ◽  
Frequency Noise ◽  
Time Resolved ◽  
Temporal Derivative

The pressure field of an impinging synthetic jet has been computed from time-resolved, three-dimensional, three-component (3D-3C) particle image velocimetry (PIV) velocity field data using a Poisson equationbased pressure solver. The pressure solver used in this work can take advantage of the temporal derivative of the pressure to enhance the temporal coherence of the calculated pressure field for time-resolved velocity data. The reconstructed pressure field shows sensitivity to the implementation of the boundary conditions, as well as to the spatial and temporal resolution of the PIV data. The pressure from a 3D Poisson solver that does not consider the temporal derivative of the pressure shows high random error. Invoking the temporal derivative of the pressure eliminates this high-frequency noise, however, the calculated pressure exhibits an unphysical temporal drift. This temporal drift is affected by both the temporal resolution of the PIV data and the spatial resolution of the PIV vector field, which was systematically evaluated by downsampling the instantaneous data and increasing the interrogation window size. It was observed that decreasing the temporal resolution increased the drift, while decreasing the spatial resolution decreased the drift.

scholarly journals Design of experiments: a statistical tool for PIV uncertainty quantification

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Sagar Adatrao ◽  
Andrea Sciacchitano ◽  
Simone van der Velden ◽  
Mark-Jan van der Meulen ◽  
Marc Cruellas Bordes
Keyword(s):  
Design Of Experiments ◽  
Uncertainty Quantification ◽  
Interrogation Window ◽  
Window Size ◽  
Measured Quantity ◽  
Test Case ◽  
Statistical Tool ◽  
Total Uncertainty ◽  
Systematic Uncertainties ◽  
The Individual

A statistical tool called Design of Experiments (DOE) is introduced for uncertainty quantification in particle image velocimetry (PIV). DOE allows to quantify the total uncertainty as well as the systematic uncertainties arising from various experimental factors. The approach is based on measuring a quantity (e.g. time-averaged velocity from PIV) several times by varying the levels of the experimental factors which are known to affect the value of the measured quantity. In this way, using Analysis of Variances (ANOVA), the total variance in the measured quantity can be computed and hence the total uncertainty. Moreover, the analysis provides the individual variances for each of the experimental factors leading to the estimation of the systematic uncertainties from each factor and their contribution to the total uncertainty. The methodology is assessed for an experimental test case of the flow at the outlet of a ducted Boundary Layer Ingesting (BLI) propulsor to quantify the total uncertainty in time-averaged velocity from stereoscopic PIV measurements as well as the constituent systematic uncertainties due to the experimental factors, namely, camera aperture, inter-frame time separation, interrogation window size and stereoscopic camera angle.

The Turbulent Structure of the Marine Atmospheric Boundary Layer During and Before a Cold Front

2021 ◽  
Author(s):  
Jian Huang ◽  
Zhongshui Zou ◽  
Qingcun Zeng ◽  
Peiliang Li ◽  
Jinbao Song ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Cold Front ◽  
Ocean Surface ◽  
Turbulent Structure ◽  
Inertial Subrange ◽  
Quadrant Analysis ◽  
Marine Atmospheric Boundary Layer ◽  
Velocity Spectra ◽  
Velocity Variances

AbstractThe turbulent structure within the marine atmospheric boundary layer is investigated based on four levels of observations at a fixed marine platform. During and before a cold front, the ocean surface is dominated by wind sea and swell waves, respectively, affording the opportunity to test the theory recently proposed in laboratory experiments or for flat land surfaces. The results reveal that the velocity spectra follow a k-1 law within the intermediate wavenumber (k) range immediately below inertial subrange during the cold front. A logarithmic height dependence of the horizontal velocity variances is also observed above the height of 20 m, while the vertical velocity variances increase with increasing height following a power law of 2/3. These features confirm the Attached Eddy Model and the “top-down model” of turbulence over the ocean surface. However, the behavior of velocity variances under swell conditions is much different from those during the cold front, although a remarkable k-1 law can be observed in the velocity spectra. The quadrant analysis of the momentum flux also shows a significantly different result for the two conditions.

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