On the effects of non-homogeneity on small scale turbulence

by Md Kamruzzaman

Institution: University of Newcastle
Year: 2016
Keywords: grid turbulence; turbulent shear and shearless mixing layer; turbulent boundary layer
Posted: 02/05/2017
Record ID: 2091434
Full text PDF: http://hdl.handle.net/1959.13/1316847


Research Doctorate - Doctor of Philosophy (PhD) The experimental results presented in this thesis were obtained by using hot-wire anemometry with single- and X-wire probes. The experiments were carried out at low to moderate Reynolds numbers (6 ≤ Rλ ≤ 100) in grid turbulence and turbulent boundary layers to investigate the effects of non-homogeneities on small scale turbulence. Four different flows were extensively investigated: the homogeneous and isotropic turbulence (HIT), the turbulent shearless mixing layer (TSLML), and the turbulent shear mixing layer (TSML), all three flows generated by the three different classical and two different composite grids. The fourth flow was a turbulent boundary layer over a rough wall. Classical grid turbulence measurements were made with the aim of ascertaining whether the decay of q² follows a power law. It was found that the conventional turbulence power law decay (q² ∼ xn, with n is constant) does not describe the entire decay period (the initial to final period of decay, including the transition period). A family of the power law of the form xni, where ni is a different constant over a restrained portion of decay is more appropriate. It was also found that the Kolmogorov scaling holds down to Rλ ≃ 25; below this value, it breaks down. The effects of Rλ on the energy distribution across the different scales of motion and the validity of the local isotropy, were investigated. It also was found that the assumption of self-preservation (SP) in the classical grid turbulence was not valid. The grid generated turbulent shearless mixing layer (TSLML) was investigated with the aim of studying the turbulence decay with non-homogeneous turbulence,and in particular whether or not SP could be satisfied. The SP analysis was carried out using the transport equation of the second-order velocity structure function or the scale-by-scale (SBS) energy budget equation. It was observed that the SBS energy budget on the centreline of the grid is similar to that found in the decaying HIT. Moreover, the measurements on the centreline showed that the complete SP was achieved at all scales of motion. This mean that the Taylor microscale Reynolds number (Rλ = u′λ/ν) is constant and decay power law exponent n is equal to - 1. A grid generated turbulent with a mean shear (TSML) was used to study the effect of a mean shear on the turbulence decay. The one-point statistics showed that the distribution of the mean velocity and the Reynolds stresses u′2, v′2, w′2 and uv across the shear layer collapsed when x/ML ≥ 50, suggesting SP is approximated for one-point statistics. The power law decay exponent n was about zero on the centreline of the grid, but differed from zero outside the shear layer region. The SBS energy budget equation was shown that the TSML flow did not conform to SP conditions (e.g., Rλ is not constant). For the turbulent boundary layer over a rough wall,… Advisors/Committee Members: University of Newcastle. Faculty of Engineering & Built Environment, School of Engineering.