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HYDRODYNAMICS AND ACOUSTICS
2021 ◊ Volume 2 (92) ◊ Issue 2 ◊ p. 149-175
V. G. Kuzmenko *
* Institute of Hydromechanics of NAS of Ukraine, Kyiv, Ukraine
Simulating the separated turbulent flow. Evolution of the energy of coherent structures end their sizes
Gidrodin. akust. 2021, 2(2):149-175
TEXT LANGUAGE: Russian
ABSTRACT
The paper deals with the numerical study of a non-stationary three-dimensional turbulent flow of an incompressible fluid over a rectangular two-dimensional barrier. For the boundary layer, a hybrid LES/URANS approach is used. A finite-difference method with second-order approximation accuracy was applied for near-wall models. The ratio of the height of the barrier to its length was 4, the Reynolds number for the obstacle was Re=10500, and the Reynolds number for the turbulent boundary layer at the "inlet" Reδ=10500. The number of grid nodes used was 1601 x 101 x 141 = 22799841. The coherent structures were identified by the Q-criterion with monitoring {Qsi} threshold values for the entire calculation area. Q-isosurfaces, integral energy characteristics, and cross-sectional area of organized vortex formations were studied by numerical modeling. For the computational zone with a longitudinal size of about 80 obstacle heights, coherent structures with different scales and configurations were found. The highest values of the turbulent energy of coherent structures occur in the zone of the separated flow reattachment and its recovery. Significant amounts of turbulent energy are still observed far behind the barrier, and its maxima are close to the local energy maximum above the obstacle. A new technique for processing numerical data for the evolution of random and coherent formations of various scales has been developed. It provides determining of limiting values of the integral turbulent energy characteristics when distinguishing different types of eddies over a long time interval. A complex nonlinear relationship between the coherence parameter Q, turbulent energy, vortex sizes, and their integral characteristics is revealed in various sections along the flow corresponding to the turbulent boundary layer, separation, recirculation zone, reattachment, and recovery.
KEY WORDS
turbulent boundary layer, obstacle, numerical method, coherent structures, identification criterion, evolution