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Extraction of coherent structures in a rotating turbulent flow experiment
Jori E Ruppert-Felsot1, Olivier Praud, Eran Sharon
1Center for Nonlinear Dynamics and Department of Physics, The University of Texas at Austin, Austin, Texas 78712, USA.
Summary
The discrete wavelet transform (DWT) and discrete wavelet packet transform (DWPT) efficiently separate coherent structures from turbulent rotating fluid dynamics. These methods represent the flow using only 3% of coefficients, preserving key properties.
Area of Science:
- Fluid Dynamics
- Turbulence Research
- Wavelet Analysis
Background:
- Turbulent rotating fluids exhibit complex dynamics with long-lived coherent vortices.
- Understanding the behavior of these structures is crucial for fluid dynamics research.
- Traditional methods may struggle to efficiently capture the dynamics of coherent structures.
Purpose of the Study:
- To investigate the use of discrete wavelet transform (DWT) and discrete wavelet packet transform (DWPT) for analyzing coherent structures in turbulent rotating fluids.
- To compare the efficiency of DWT/DWPT with other methods like Fourier transform and JPEG compression.
- To determine the minimal data representation required to capture essential flow properties.
Main Methods:
- Generating three-dimensional turbulence in a rotating tank, which evolves into two-dimensional turbulence with height.
- Utilizing particle image velocimetry (PIV) to measure the quasi-2D flow.
- Applying DWT and DWPT to the vorticity field to separate coherent and incoherent components by coefficient thresholding.
- Comparing results with Fourier transform, JPEG compression, and the Okubo-Weiss criterion.
Main Results:
- DWT and DWPT effectively separate coherent vortices from the incoherent background in turbulent rotating fluid.
- These wavelet methods are significantly more efficient than Fourier-based approaches.
- A small fraction (approx. 3%) of large-amplitude DWT/DWPT coefficients captures the coherent component, preserving vorticity PDF, transport, and correlations.
- The incoherent component, represented by small-amplitude coefficients, has near-Gaussian vorticity PDF and minimal impact on transport.
Conclusions:
- DWT and DWPT provide an efficient and effective means to analyze and represent coherent structures in turbulent rotating flows.
- Wavelet-based methods allow for significant data compression while retaining crucial flow dynamics.
- This approach offers a promising avenue for simplified simulation and description of such complex fluid systems.