Post-Processing Methods of PIV Instantaneous Flow Fields for Unsteady Flows in Turbomachines

Cavazzini, Giovanna; Pavesi, Giorgio; Dupont, Patrick; Bois, Gérard

doi:10.5772/37273

Cited by 4 publications

(3 citation statements)

References 27 publications

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“…8a and 8c have been subjected to filtering and "cleaning" methods such as background subtraction and mask application at the different stages of post-processing of PIV results to increase the signal to noise ratio and to avoid misinterpretation of flow separation phenomenon. However, these techniques are not able to remove the outliers in the results due to the dependency of the removal criteria on the choice of threshold values (Westerweel & Scarano, 2005;Cavazzini et al, 2012). The most common technique to overcome this problem, and to enable a quantitative interpretations of flow separation in the wake region, is to average the instantaneous PIV flow fields.…”

Section: Flow Over the Hump -Unactuated Casementioning

confidence: 99%

Application of synthetic jet arrays for flow separation control on a circular “hump” model

Jafari

Jaworski

Rona

2022

Experimental Thermal and Fluid Science

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Section: Flow Over the Hump -Unactuated Casementioning

confidence: 99%

Application of synthetic jet arrays for flow separation control on a circular “hump” model

Jafari

Jaworski

Rona

2022

Experimental Thermal and Fluid Science

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“…Besides the challenges above, selecting an appropriate approach to identify flow behavior is essential. Following the periodicity of the flow in turbomachinery, the mean and phase averaging of the results can indicate the flow's nature [5], [6]. However, ensuring the repeatability of the obtained results remains a challenge.…”

Section: Introductionmentioning

confidence: 99%

“…However, ensuring the repeatability of the obtained results remains a challenge. The Gaussian distribution of velocity on points, as advocated by Cavazinni et al, represents repeatability [5], albeit this approach is not always appropriate due to excessive turbulence, particularly in off-design operating conditions of hydraulic turbines. Another approach is to verify repeatability with a confidence interval on certain specific lines for specific velocity components [2], although generalizing the results of one line to the entire plane is dubious.…”

Section: Introductionmentioning

confidence: 99%

PIV measurements in the draft tube of a down-scale propeller turbine: uncertainty analysis

Sotoudeh

Shiraghaee

Andersson

et al. 2022

IOP Conf. Ser.: Earth Environ. Sci.

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In this study, the flow in the conical section of the draft tube of a propeller turbine has been investigated at the best efficiency point and part-load operating conditions using 2D and stereoscopic 3D particle image velocimetry. Since the flow in the turbine is periodic, it is necessary to study the mean flow field rather than the instantaneous one to identify the flow characteristics from a statistical standpoint. However, the statistical convergence of the obtained mean velocity is questionable. Thus, the current work proposes a methodology for investigating the convergence of mean velocity profiles based on the central limit theorem. The methodology is applied to the best efficiency point and part-load results. The results show that 3D PIV results have lower uncertainty than 2D PIV results because measuring the tangential velocity component affects uncertainty, only measured in 3D PIV. The uncertainty difference is more significant, especially in part-load operation, due to the presence of the rotating vortex rope, and therefore a more accurate measurement is necessary to produce a reliable mean flow field. Furthermore, the convergence of the mean velocity profile is faster, with lower uncertainty for best efficiency point results since, at the part-load condition, the tangential velocity component of the flow is higher. In addition, the converged mean velocity profiles show a backflow region with minor rotation in the center, surrounded by a high rotational axial flow during the part-load operation of the turbine.

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Converged statistics for time-resolved measurements in low-speed axial fans using high-frequency response probes

Oro

Díaz

Lastra

et al. 2014

Experimental Thermal and Fluid Science

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Fast-response probes in multistage turbomachinery are typically used to measure unsteady flows and turbulence in a number of traverse locations throughout the machine (rotor-stator interregions, inlet and outlet sections, tip clearance gaps…). When used intensively, they provide complete information of time-resolved flow structures, including wake patterns, wake mixing, wake-wake and rotor-wake interactions or turbulence transport in 2D planes and even 3D pictures if the raw signals are post-processed accurately. The segregation between deterministic, unsteady features and turbulent scales is essential to understand the unsteady mechanisms responsible for the energy transfer and requires an accurate selection of the sampling frequencies and the total length of the measured traces to assure a valid statistical reduction. Similar considerations must be made if they are filtered in a frequency basis (for example, filtering low-scale turbulence or extracting only BPF components), employing welldesigned periodograms or power spectra with minimum scatter and large periods of time integration. This work presents the effect of number of periods (ensembles), resolution in which the averaged periods are reconstructed and turbulence intensity on the experimental accuracy of ensemble-averaged measurements in low-speed axial fans using fast-response probes. In particular, the statistical analysis is established in terms of convergence (residuals) between timeresolved traces retrieved using different sampling frequencies and number of total samples. The possible effects of three-dimensionality, the measured regions (hub, tip, midspan) or the sensibility to turbulence levels is also explored. A technique to quantify the convergence of the phase-locked averaging (PLA) processes is applied to a low-speed axial fan, with twin configurations of rotor-stator and stator-rotor arrangements. As a starting point, a concise survey of usual practices employed by other authors in the literature for axial fans and compressors is firstly reviewed, in order to identify fundamental parameters and values typically adopted to guarantee convergence. Finally, typical requirements are given as a function of the variable analyzed, the wake pattern to be described or the global disorder of the flow structures inside axial flow fans.

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Post-Processing Methods of PIV Instantaneous Flow Fields for Unsteady Flows in Turbomachines

Cited by 4 publications

References 27 publications

Application of synthetic jet arrays for flow separation control on a circular “hump” model

Application of synthetic jet arrays for flow separation control on a circular “hump” model

PIV measurements in the draft tube of a down-scale propeller turbine: uncertainty analysis

Converged statistics for time-resolved measurements in low-speed axial fans using high-frequency response probes

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