arXiv subjects

Vasudevan Mukund

Publications and source records attributed to Vasudevan Mukund.

4 recordsLinked to original sources

arXiv2026

Discontinuous transition to shear flow turbulence

Depending on the type of flow, the transition to turbulence can take one of two forms: either turbulence arises from a sequence of instabilities or from the spatial proliferation of transiently chaotic domains, a process analogous to directed percolation. The former scenario is commonly referred to as a supercritical transition and frequently encountered in flows destabilized by body forces, whereas the latter subcritical transition is common in shear flows. Both cases are inherently continuous in a sense that the transformation from ordered laminar to fully turbulent fluid motion is only accomplished gradually with flow speed. Here we show that these established transition types do not account for the more general setting of shear flows subject to body forces. The combination of the two continuous scenarios leads to the attenuation of spatial coupling; with increasing forcing amplitude, the transition becomes increasingly sharp and eventually discontinuous. We argue that the suppression of laminar-turbulent coexistence and the approach towards a discontinuous phase transition potentially apply to a broad range of situations including flows subject to, for example, buoyancy, centrifugal or electromagnetic forces.

physics.flu-dyn↗

arXiv2025

Aging and memory of transitional turbulence

The recent classification of the onset of turbulence as a directed percolation (DP) phase transition has been applied to all major shear flows including pipe, channel, Couette and boundary layer flows. A cornerstone of the DP analogy is the memoryless (Poisson) property of turbulent sites. We here show that, for the classic case of channel flow, neither the decay nor the proliferation of turbulent stripes is memoryless. As demonstrated by a standard analysis of the respective survival curves, isolated channel stripes, in the immediate vicinity of the critical point, age. Consequently, the one to one mapping between turbulent stripes and active DP-sites is not fulfilled in this low Reynolds number regime. In addition, the interpretation of turbulence as a chaotic saddle with supertransient properties, the basis of recent theoretical progress, does not apply to individual localized stripes. The discrepancy between channel flow and the transition models established for pipe and Couette flow, illustrates that seemingly minor geometrical differences between flows can give rise to instabilities and growth mechanisms that fundamentally alter the nature of the transition to turbulence.

physics.flu-dyn↗

arXiv2017

The critical point of the transition to turbulence in pipe flow

Reynolds proposed that after sufficiently long times, the flow in a pipe should settle to a steady condition: below a critical Reynolds number, flows should (regardless of initial conditions) always return to laminar, while above, eddying motion should persist. As shown, even in pipes several thousand diameters long, the spatio-temporal intermittent flow patterns observed at the end of the pipe strongly depend on the initial conditions, with no indication of an approach to a (statistical) steady state. Exploiting the fact that turbulent puffs do not age, we continuously recreate the puff sequence exiting the pipe at the entrance, thus introducing periodic boundary conditions for the flow pattern. This procedure allows us to study the evolution of the flow patterns for arbitrary long times. We find that after times in excess of $10^7$ advective time units, a statistical steady state is reached. Though the resulting flows remain spatio-temporally intermittent, puff splitting and decay rates eventually reach a balance so that the turbulent fraction fluctuates around a well defined level which only depends on $Re$. We find that at lower $Re$ (here 2020), flows eventually always laminarize, while for higher $Re$ ($>=2060$) turbulence persists. The critical point for pipe flow hence lies in the interval $2020<Re_c<2060$, which is in good agreement with the recently proposed value of $Re_c=2040$. The latter estimate was based on single puff statistics and entirely neglected puff interactions. Unlike typical contact processes where such interactions strongly affect the critical point, in pipe flow it is only marginally influenced. Interactions on the other hand, are responsible for the approach to the statistical steady state. As shown, they strongly affect the resulting flow patterns, where they cause `puff clustering', with the clusters traveling across the puff pattern in a wave like fashion.

physics.flu-dyn↗

arXiv2015

The rise of fully turbulent flow

Over a century of research into the origin of turbulence in wallbounded shear flows has resulted in a puzzling picture in which turbulence appears in a variety of different states competing with laminar background flow. At slightly higher speeds the situation changes distinctly and the entire flow is turbulent. Neither the origin of the different states encountered during transition, nor their front dynamics, let alone the transformation to full turbulence could be explained to date. Combining experiments, theory and computer simulations here we uncover the bifurcation scenario organising the route to fully turbulent pipe flow and explain the front dynamics of the different states encountered in the process. Key to resolving this problem is the interpretation of the flow as a bistable system with nonlinear propagation (advection) of turbulent fronts. These findings bridge the gap between our understanding of the onset of turbulence and fully turbulent flows.

physics.flu-dyn↗