Mechanical Engineering - Theses
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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.
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ItemMechanisms of momentum transport associated with the interaction of concentrated and distributed regions of vorticity( 2018)Turbulent flows are of ubiquitous technological importance due to their wide variety of applications. The action of vortical motions plays a vital role in turbulence production, dissipation, and time-averaged turbulence statistics. Therefore, it is essential to understand the flow features responsible for the inertial mechanisms of turbulence and ultimately the mean distribution of momentum. The flow field associated with a vortex ring advecting towards a stationary/moving wall is investigated using planar PIV. This study aims to clarify the mechanisms of turbulent inertia associated with the interaction of advecting regions of concentrated vorticity and distributed vorticity. These physical simulations represent aspects of the instantaneous flow field interactions known to exist in turbulent wall-bounded flows. To allow for an explicit study of these interactions and avoid background turbulence, unsteady, laminar, vortex ring experiments are conducted under reproducible initial conditions. The experiments are conducted in a large water tank. The bottom wall of the tank is fitted with a conveyor belt driven by a servo motor to generate a time evolving shear layer. Laminar vortex rings are produced using a piston cylinder apparatus that is driven by a stepper motor and controlled using a computer. For opposite sign vorticity interactions between the vortex ring and shear layer vorticity, the passage of the vortex ring above the wall results in a lifting of the near wall fluid. This gives rise to the formation of a primary hairpin vortex with the same sign vorticity as the top core of the vortex ring. Results indicate that the generation of new coherent vortex motion introduce geometric and kinematic asymmetries that generates a contribution to turbulent inertia. This action creates local imbalances in the stress field leading to momentum inhomogeneities. To gain an in-depth knowledge of the parameters governing the formation of a primary hairpin vortex, a parametric study is conducted using four factors: the initial wall-normal location of the vortex ring, the circulation ratio between the vortex ring and shear layer, the displacement thickness, and the incidence angle of the vortex ring. This led to a precise and detailed characterisation of the primary hairpin. The development of a unique framework based upon the mean momentum equation to analyse momentum transport and exchange between the ring and the shear-layer is discussed in detail. New observations on the vortex ring/moving wall simulations are presented. In general, the time rate of change of momentum trends for the vortex ring and hairpin indicates a flux of momentum from the ring to hairpin vortex during the initial stages of interaction. Some of the momentum from the vortex ring is transported to the hairpin that is contributing towards wall-layer vorticity roll-up. There is compelling evidence that vortex rings are stable, coherent and long-lived features of the flow, and capable of transporting momentum to near-wall vorticity field without coming too close to the wall. Once instabilities are initiated in the shear-layer, the hairpin development and formation is inevitable. The resulting hairpin convection velocity is within the range of 0.6-0.8 of the wall velocity. The evolution angle of the hairpin is invariant under variations in shear-layer Reynolds number. The results answer fundamental `kernel' questions to establish an understanding of hypothetically significant events for regeneration and self-sustenance mechanisms in wall-turbulence. This information is ultimately required from flow control perspectives.