What is it about?
This study isolates periodic processes in the partially turbulent flow around a wing based on data from numerical simulation. When laminar flow separates from a wing, a shear layer forms above the reversed flow. Within the shear layer vortices form by the Kelvin-Helmholtz instability. These vortices then break up into turbulence and under certain conditions the flow re-attaches to the wall. Since the vortices form at a certain frequency, the whole flow around such a wing is dominated by this periodicity. Structures in the flow are identified by phase averaging and their development is observed. It is shown that the vortices result from disturbances far upstream of the separation and that they remain coherent throughout the turbulent flow.
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Why is it important?
The investigated case represents for example wings of small to medium-sized aerial vehicles, which are prone to laminar flow separation. Unlike the separation of turbulent wing flow, this does not necessarily lead to a wing stall but has an impact on the aerodynamic efficiency. It is crucial to identify conditions of separation early in the design process employing numerical flow simulation. In order to build reliable models for these challenging simulations, understanding of the underlying flow physics is essential. The present study contributes to the understanding and identifies details within the flow field.
Perspectives
For me, the work perfectly represents the dilemma in engineering between accurary and usefulness of data. For higher accuracy we need to resolve the turbulent flow ending up with a vast amount of data, which is very accurate but features too much information to draw conclusions. Only after extracting flow features in various ways, we can learn about the underlying physical principles of the flow.
Eike Tangermann
Universitat der Bundeswehr Munchen
Read the Original
This page is a summary of: Coherent Flow Structures Downstream of a Laminar Separation Bubble - a Numerical Investigation, January 2024, American Institute of Aeronautics and Astronautics (AIAA),
DOI: 10.2514/6.2024-2197.
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Resources
Phase averaged view to the vortex shedding
By phase averaging coherent structures can be identified from turbulent flow with embedded periodic processes. Spanwise tubes from pressure drop in vortex cores, front plane shows vertical velocity, back plane indicates Reynolds shear stress u'v'.
Flow Around the Airfoil
Large Eddy simulation of the flow around a NACA0018 airfoil at Re=80000 and 4° angle of attack. Laminar separation occurs on upper and lower side. Flow separation marked blue, Q isosurface coloured by streamwise velocity in red-white, green surfaces indicate the pressure drop within shear layer vortices.
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