Fully Turbulent Wakes at Low Reynolds Numbers: the Case of the Thin Flat Plate

TL;DR

This study demonstrates that the wake flow behind a thin flat plate becomes turbulent at a surprisingly low Reynolds number of 400, with flow characteristics similar to higher Reynolds number flows, challenging traditional transition understanding.

Contribution

The paper provides the first detailed comparison showing turbulence in wake flows at low Reynolds numbers for a flat plate, revealing differences from canonical wake flows and suggesting new transition mechanisms.

Findings

Wake flow at Re=400 exhibits turbulence features similar to higher Re flows.

Flow at Re=150 lacks turbulence characteristics and differs significantly.

Transition mechanisms in flat plate wakes differ from those in canonical cylinders.

Abstract

We consider the wake flow past a thin two-dimensional flat plate normal to the uniform stream and demonstrate that this flow is turbulent already at a relatively low Reynolds number of $Re = 400$. This is achieved by performing a careful comparison of the results of a DNS of this flow with experimental measurements of wake flows in the same geometric configuration at the Reynolds numbers of $Re=12500, 19700$. This comparison reveals that the distribution of several key quantities, including the mean velocity, Reynolds stresses and different effects contributing to the transport of the turbulent kinetic energy, are, up to measurement uncertainty, the same in these flows. Moreover, the wake flow at $Re = 400$ also features energy spectra characteristic of turbulent flows with intermittency detected in the distributions of the fluctuating strain and rotation rates. In contrast, these features are absent from the results of the DNS of the wake flow at $Re = 150$ where the distribution of the key quantities is also fundamentally different. These results show that the path to transition to turbulence in the wake past a thin flat plate is different from that in the wakes of canonical (i.e., circular or square) cylinders. We also identify possible physical mechanisms that may be responsible for these differences.