Abstract
In the present study, an attempt has been made to identify and quantify, with a rigorous analytical approach, all possible sources of error involved in the estimation of the fluctuating velocity gradients \(\overline {\left( {\partial u_i /\partial x_j } \right)^2 } \) when a two-point laser Doppler velocimetry (LDV) technique is employed. Measurements were carried out in a grid-generated turbulence flow where the local dissipation rate can be calculated from the decay of kinetic energy. An assessment of the cumulative error determined through the analysis has been made by comparing the values of the spatial gradients directly measured with the gradient estimated from the decay of kinetic energy. The main sources of error were found to be related to the length of the two control volumes and to the fitting range, as well as the function used to interpolate the correlation coefficient when the Taylor length scale \(\left( {{\text{or }}\overline {\left( {\partial u_i /\partial x_j } \right)^2 } } \right)\) are estimated.
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Notes
The perfect symmetry of the correlation curve is caused by the fact that R ii (Δx i ) has been set equal to R ii (−Δx i ).
Abbreviations
- C f :
-
Calibration factor (m)
- D :
-
Grid wire diameter (mm)
- L :
-
Test section length (m)
- L ij :
-
Integral length scale in the ith direction of the u j velocity component (m)
- M :
-
Grid mesh size (m)
- N :
-
Number of points in the fitting range
- N tot :
-
Number of particles in coincidence
- R ii (Δx j ):
-
Correlation coefficient of the ith velocity component in the jth direction
- R mii :
-
Measured correlation coefficient of the ith velocity component
- R rii :
-
Real correlation coefficient of the ith velocity component
- Re :
-
Reynolds number
- Re M :
-
Reynolds number based on M
- t :
-
Particle arrival time (s)
- U i :
-
Velocity component in the i direction (ms−1)
- u i :
-
Turbulent velocity component in the i direction (ms−1)
- \(\overline {U_i }\) :
-
Mean velocity in the i direction (ms−1)
- W :
-
Width of the test section (m)
- x 0 :
-
Position where the two control volumes overlap completely (m)
- x i :
-
Coordinate in the i direction (m)
- x M :
-
Non-dimensionalised distance from the grid
- Δx j :
-
Displacement in the j direction (m)
- Δxmax:
-
Upper limit of the fitting range (m)
- Δxmin:
-
Lower limit of the fitting range (m)
- ε :
-
Viscous dissipation rate of turbulent kinetic energy (m2s−3)
- η :
-
Kolgomorov length scale (m)
- Λ:
-
Control volume length (m)
- λ ij :
-
Taylor length scale in the i direction of the u j velocity component (m)
- ν :
-
Kinematic viscosity (m2s−1)
- σ :
-
Grid solidity ratio
- τ w :
-
Coincidence window (s)
- HWA:
-
Hot wire anemometry
- LDA/LDV:
-
Laser Doppler anemometry/velocimetry
- PIV:
-
Particle image velocimetry
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Ducci, A., Yianneskis, M. Analysis of errors in the measurement of energy dissipation with two-point LDA. Exp Fluids 38, 449–460 (2005). https://doi.org/10.1007/s00348-004-0924-x
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DOI: https://doi.org/10.1007/s00348-004-0924-x