Abstract
This paper details the influence of the magnitude of imposed inflow fluctuations on Large Eddy Simulations of a spatially developing turbulent mixing layer originating from laminar boundary layers. The fluctuations are physically-correlated, and produced by an inflow generation technique. The imposed high-speed side boundary layer fluctuation magnitude is varied from a low-level, up to a magnitude sufficiently high that the boundary layer can be considered, in a mean sense, as nominally laminar. Cross-plane flow visualisation shows that each simulation contains streamwise vortices in the laminar and turbulent regions of the mixing layer. Statistical analysis of the secondary shear stress reveals that mixing layers originating from boundary layers with low-level fluctuations contain a spatially stationary streamwise structure. Increasing the high-speed side boundary layer fluctuation magnitude leads to a weakening of this stationary streamwise structure, or its removal from the flow entirely. The mixing layer growth rate reduces with increasing initial fluctuation level. These findings are discussed in terms of the available experimental data on mixing layers, and recommendations for both future experimental and numerical research into the mixing layer are made.
Highlights
The use of numerical simulation techniques such as Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) to compute turbulent flows is becoming more widespread thanks to ever increasing computing power
Regardless of the root cause, the above investigations show that the mixing layer displays a hypersensitivity to its initial conditions, and their effects persist to Reynolds numbers that are in excess of those found in flows of practical engineering interest
All three simulations show reasonable agreement with the reference data, Case RRM-L slightly over-predicts the magnitude of the fluctuations towards the outer edges of the mixing layer
Summary
The use of numerical simulation techniques such as Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) to compute turbulent flows is becoming more widespread thanks to ever increasing computing power. For the spatially-developing mixing layer flow, it is expected that these simulation methods will assist in solving the outstanding problems that persist in the field, in spite of seventy years of extensive research. Over this period of time, the mixing layer that forms between two merging parallel streams of fluid has proven to be a remarkably challenging flow configuration. Explanations for the discrepancies in observed growth rates include the laminar or turbulent state of the separating high-speed side boundary layer [2, 3], and even whether the measured flows could be considered as truly fully-developed [4]. Regardless of the root cause, the above investigations show that the mixing layer displays a hypersensitivity to its initial conditions, and their effects persist to Reynolds numbers (based on the mixing layer visual thickness and velocity difference across it) that are in excess of those found in flows of practical engineering interest
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