Simulation of a Large-Eddy-Break-Up Device (Lebu) in a Moderate Reynolds Number Turbulent Boundary Layer

Abstract

A well-resolved large eddy simulation (LES) of a large-eddy breakup (LEBU) device in a spatially evolving turbulent boundary layer is performed with, Reynolds number, based on free-stream velocity and momentumloss thickness, of Re θ ≈ 4300. The implementation of the LEBU is via an immersed boundary method. The LEBU is positioned at a wall-normal distance of 0.8δ (δ denoting the local boundary layer thickness at the location of the LEBU) from the wall. The LEBU acts to delay the growth of the turbulent boundary layer and produces global skin friction reduction beyond 180δ downstream of the LEBU, with a peak local skin friction reduction of approximately 12%. However, no net drag reduction is found when accounting for the device drag of the LEBU in accordance with the towing tank experiments by Sahlin et al. (Phys. Fluids 31, 2814. Further investigation is performed on the interactions of high and low momentum bulges with the LEBU and the corresponding output is analysed, showing a 'break-up' of these large momentum bulges downstream of the LEBU. In addition, results from the spanwise energy spectra show consistent reduction in energy at spanwise length scales for λ + z > 1000 independent of streamwise and wall-normal location when compared to the corresponding turbulent boundary layer without LEBU.

Experimental investigation of turbulent boundary layers at high Reynolds number with uniform blowing, part I: statistics

“…Persisting effect of LEBU devices were found to be active upto 160δ downstream, velocity deficit of the wake region downstream of a LEBU diminishes gradually with streamwise distance. Similar to the study from Corke et al (1981), results from Anders (1989), Spalart et al (2006) and Chin et al (2017), no net reduction of friction drag was confirmed. Therefore, further literature survey on the method have shown very little/no net reduction of the skin friction drag so far.…”

“…Although, their finding was very interesting as LEBU devices can effectively break-up the larger eddies into smaller ones but streamwise turbulence production is very quickly recovered as opposed by the findings from Corke et al (1981). Recently, Chin et al (2017) performed Large Eddy Simulation (LES) of a spatially developed zero-pressure-gradient TBL within the range of Re θ = 500 ∼ 4300. Persisting effect of LEBU devices were found to be active upto 160δ downstream, velocity deficit of the wake region downstream of a LEBU diminishes gradually with streamwise distance.…”