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### Dynamics Of The Spatio-Temporal Wave-Front As Unique Precursor Of Flow Transition

**Speaker:** Dr. Swagata Bhaumik

**Date & time:** 31st Oct 2020 @ 11:00 am (New Delhi)

30th Oct: 10:30 pm (Los Angeles); 31st Oct 2020 @ 6:30 am (Berlin), 1:30 pm (Beijing), 2:30 pm (Tokyo)

**Abstract of the lecture**

Regardless of numerous theoretical, numerical and experimental efforts, several aspects of laminar to turbulent transition in fluid flows still remain ambiguous. One way of studying the flow transition is by treating it as a dynamical system where transition is caused by amplification of instability waves, which are mathematically, posed as the eigen-solution of the linearized governing equations describing evolution of small disturbances over the base flow. Instability waves are triggered inside the boundary layer by natural or imposed external perturbations through a process known as receptivity, a term receptivity first coined by Morkovin (Springer, 1993). Conventionally flow instability is studied by treating it either as temporal or spatial problem disregarding the spatio-temporal evolution of physical disturbances. Linearized spatio-temporal analysis of incompressible boundary layer by Bromwich contour integral method predicted the existence of the spatio-temporal wave-packet (STWP) (Sengupta et al. Phys. Rev. Lett., 96, 2006), which exhibits growth even for flows, which are stable following linearized stability analysis. Subsequently, the STWP has been established as the prime-mover for flow transition by high-accuracy direct numerical simulation (DNS) of the receptivity of the low-speed incompressible flows [1-6] to monochromatic definitive wall-excitation (started impulsively or gradually). These showed how STWP triggers flow transition and generation of turbulent spots, which later merge together to create fully developed turbulent flow. 3D receptivity results reported in [5,6] showed that both K- and H- route of flow transition can be induces by the growth of STWP due to single monochromatic excitation frequency.

**Related publications**

- T. K. Sengupta, S. Bhaumik, Onset of turbulence from the receptivity stage of fluid flows, Physical Review Letters 107(15) (2011) 154501.
- T. K. Sengupta, S. Bhaumik, Y. G. Bhumkar, Direct numerical simulation of two-dimensional wall-bounded turbulent flows from receptivity stage, Physical Review E 85(2) (2012) 026308.
- S. Bhaumik, T. K. Sengupta, Precursor of transition to turbulence: Spatiotemporal wave front, Physical Review E 89(4) (2014) 043018.
- S. Bhaumik, T. K. Sengupta, A new velocity-vorticity formulation for direct numerical simulation of 3D transitional and turbulent flows, Journal of Computational Physics 284 (2015) 230–260.
- P. Sharma, T. K. Sengupta, S. Bhaumik, Three-dimensional transition of zero-pressure-gradient boundary layer by impulsively and nonimpulsively started harmonic wall excitation, Physical Review E 98 (2018) 053106.
- S. Bhaumik, T. K. Sengupta, Z. A. Shabab, Receptivity to harmonic excitation following nonimpulsive start for boundary-layer flows, AIAA Journal (2017) 3233–3238.

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