Towards Reconstruction of Complex Flow Fields Using Unit Flows

Abstract

Many complex turbulent flows in nature and engineering can be qualitatively regarded as being constituted of multiple simpler unit flows. The objective of this work is to characterize the coherent structures in such complex flows as a combination of constituent unitary flow structures for the purpose of reduced-order representation. While turbulence is clearly a non-linear phenomenon, we aim to establish the degree to which the optimally weighted superposition of unitary flow structures can represent the complex flow structures. The rationale for investigating such superposition stems from the fact that the large-scale coherent structures are generated by underlying flow instabilities that may be reasonably described using linear analysis. Clearly, the degree of validity of superposition will depend on the flow under consideration. In this work, we take the first step toward establishing a procedure for investigating superposition. Experimental data of single and triple tandem jets in crossflow are used to demonstrate the procedure. A composite triple tandem jet flow field is generated from optimal superposition of single jet data and compared against ‘true’ triple jet data. Direct comparisons between the true and composite fields are made for spatial, temporal, and kinetic energy content. The large-scale features (obtained from proper orthogonal decomposition or POD) of true and composite tandem jet wakes exhibit nearly 70% agreement in terms of modal eigenvector correlation. Corresponding eigenvalues reveal that the kinetic energy of the flow is also emulated with only a slight overprediction. Temporal frequency features are also examined in an effort to completely characterize POD modes. The proposed method serves as a foundation for more rigorous and robust dimensional reduction in complex flows based on unit flow modes.

Schlieren Flow Visualization and Analysis of Synthetic Jets

“…Year Frequency Jet Type Study Type (Hz) Kercher et al [6] 2003 1-1000 synthetic Travnicek and Tesar [8] 2003 106-692 synthetic smoke Smith and Swift [1] 2003 synthetic and Schlieren continuous Pavlova et al [9] 2006 420-1200 synthetic Arik [10] 2008 4500 synthetic Chaudhari et al [11] 2009 100-350 synthetic Bazdidi-Tehrani et al [12] 2011 16-400 synthetic Biden et al [13] 2012 unforced jet POD in crossflow Biden et al [14] 2012 forced jet POD in crossflow Ghaffari et al [15] 2016 20,000 ultrasonic microblower Ghadi et al [16] 2016 pulsed smoke impinging jet Albright and Solovitz [17] 2016 variable-diameter synthetic jet Firdaus et al [18] 2018 300-700 synthetic Solovitz et al [19] 2018 350-2000 synthetic PIV Viggiano et al [20] 2018 variable-density POD and PIV jet Kristo et al [21] 2021 crossflow jets POD and PIV Present work 2021 20-100 synthetic POD, smoke, Schlieren Pavlova et al [9] found that high-frequency jets removed heat more effectively, but that lower-frequency jets were better for larger separation distances. Many authors found that the optimal frequency for heat transfer was the resonance frequency for the device or the diaphragm [11,18,19].…”