A mechanistic model of separation bubble

TL;DR

This paper develops a low-dimensional, mechanistic model of separated flow bubbles that captures bifurcation and hysteresis phenomena, advancing understanding and control of complex flow separation dynamics.

Contribution

It introduces a novel, physically-based model that reflects key nonlinear behaviors like bifurcation and hysteresis, unlike previous linear or POD-based models.

Findings

Uncovered physical mechanisms for hysteresis in separation bubbles.

Predicted finite amplitude instability of the separation bubble.

Revealed the low-dimensional nature of complex flow phenomena.

Abstract

This work uncovers the low-dimensional nature the complex dynamics of actuated separated flows. Namely, motivated by the problem of model-based predictive control of separated flows, we identify the requirements on a model-based observer and the key variables and propose a prototype model in the case of thick airfoils as motivated by practical applications. The approach in this paper differs fundamentally from the logic behind known models, which are either linear or based on POD-truncations and are unable to reflect even the crucial bifurcation and hysteresis inherent in separation phenomena. This new look at the problem naturally leads to several important implications, such as, firstly, uncovering the physical mechanisms for hysteresis, secondly, predicting a finite amplitude instability of the bubble, and thirdly to new issues to be studied theoretically and tested experimentally. More importantly, by employing systematic reasoning, the low-dimensional nature of these complex phenomena at the coarse level is revealed.