Impact of stratification mechanisms on turbulent characteristics of stable open-channel flows

TL;DR

This study uses direct numerical simulations to analyze how different stratification mechanisms affect turbulence in stable open-channel flows, revealing deviations from classical similarity theories and introducing a new external control parameter.

Contribution

It introduces a comprehensive analysis of combined stratification mechanisms in open-channel flows and evaluates the applicability of extended similarity theories under these conditions.

Findings

Significant deviations from Monin-Obukhov similarity occur with combined stratification.

The external stratification perturbation number influences flow behavior.

Extended similarity theory predicts shear well but less accurately for scalar gradients.

Abstract

Flow over a surface can be stratified by imposing a fixed mean vertical temperature (density) gradient profile throughout or via cooling at the surface. These distinct mechanisms can act simultaneously to establish a stable stratification in a flow. Here, we perform a series of direct numerical simulations of open-channel flows to study adaptation of a neutrally stratified turbulent flow under the combined or independent action of the aforementioned mechanisms. We force the fully developed flow with a constant mass flow rate. This flow forcing technique enables us to keep the bulk Reynolds number constant throughout our investigation and avoid complications arising from the acceleration of the bulk flow when a constant pressure gradient approach were to be adopted to force the flow instead. When both stratification mechanisms are active, the dimensionless stratification perturbation number emerges as an external flow control parameter, in addition to the Reynolds, Froude, and Prandtl numbers. We demonstrate that significant deviations from the Monin-Obukhov similarity formulation are possible when both types of stratification mechanisms are active within an otherwise weakly stable flow, even when the flux Richardson number is well below 0.2. An extended version of the similarity theory due to Zilitinkevich and Calanca shows promise in predicting the dimensionless shear for cases where both types of stratification mechanisms are active, but the extended theory is less accurate for gradients of scalar. The degree of deviation from neutral dimensionless shear as a function of the vertical coordinate emerges as a qualitative measure of the strength of stable stratification for all the cases investigated in this study.