Mechanical Engineering - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemExperimental investigation of velocity and vorticity in turbulent wall flows( 2019)This thesis details the results of a research effort both to acquire sufficiently-resolved velocity and vorticity vector time-series in turbulent wall-bounded flows, as well as to use the acquired data to juxtapose two canonical wall-bounded flows: the zero-pressure-gradient boundary layer and the fully-developed pipe. Towards these ends, a novel configuration of measurement sensors has been designed, evaluated, and deployed in three of the largest canonical wall-flow facilities in existence: the Flow Physics Facility (FPF) at the University of New Hampshire, the High Reynolds Number Boundary Layer Wind Tunnel (HRNBLWT) at the University of Melbourne, and the Centre for International Cooperation in Long Pipe Experiments (CICLoPE) at the University of Bologna. The datasets presented herein are the first to contain simultaneously-acquired velocity and vorticity statistics in both pipe and boundary layer flows under matched conditions. The capacity of the measurement probe to resolve the velocity and vorticity vectors under idealised conditions is evaluated via `synthetic experiments', whereby the response of the probe to a simulated turbulent flow is modeled, and the resulting aggregate statistics compared to those of the known input. The synthetic experimental results are then compared to statistics obtained from physical experiments, and show close agreement for most quantities despite differences in Reynolds number. Disagreement between the physical and synthetic experimental results in several quantities is used to diagnose a limitation of the present sensor system. An awareness of the measurement capabilities (and limitations) afforded by the comparison between the synthetic and physical experiments pervades the ensuing analysis and discussion of additional physical experimental results. Normalised statistical moments (up to the kurtosis) of velocity and vorticity are presented for both pipe and boundary layer cases. The two flows are shown to exhibit virtually no differences from one another wallward of the wake region aside from the transverse velocity component variances. The boundary layer wake is characterised by higher turbulence enstrophy and turbulence kinetic energy (TKE) than the pipe, despite containing both turbulent and non-turbulent states. Although there is no `free-stream' in the fully-developed pipe, a significant time-fraction of the flow can be described as `quasi-irrotational' near the centreline. This results in a departure of the velocity and vorticity kurtosis (and, when not identically zero, skewness) from Gaussian behaviour in the outer region of the pipe, as is known to occur in the boundary layer (owing to turbulent/non-turbulent intermittency). Spectral properties of the velocity and vorticity signals acquired in both flows are examined, both for their own content as well as to compare the two flows. Despite very little difference in the observed streamwise turbulence kinetic energy, the contributions to the total differ considerably by scale between the two flows. The pipe is observed to contain more streamwise TKE than the boundary layer in scales longer than 10 times the outer length scale $\delta$, while the opposite is true for scales shorter than $10\delta$. This `crossover' scale also applies to the spectra of the transverse velocity components and Reynolds shear stress. The measured enstrophy spectrum is shown to be approximately invariant under Kolmogorov normalisation for wall-distances greater than about 200 viscous-lengths. Finally, local isotropy and axisymmetry in the velocity components are evaluated. Local isotropy is observed to be satisfied over a range of scales for which the characteristic timescale of inertial energy transfer is expected to be small relative to the timescale of the mean strain rate. The relationship between the transverse velocities and the streamwise velocity, however, does not appear to approach isotropy along the pipe centreline, suggesting that the bounding wall also plays a role in imposing a sense of direction on the turbulence. Indeed, the aforementioned timescale criterion is shown to identify a cutoff scale that increases proportionally with wall-distance, confounding a conclusive statement regarding the primary source of anisotropy away from the centreline.