Mechanical Engineering - Research Publications
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ItemSpatial averaging effects on the streamwise and wall-normal velocity measurements in a wall-bounded turbulence using a cross-wire probe(IOP Publishing, 2019-08-01)The spatial averaging effects due to a cross-wire probe on the measured turbulence statistics in a wall-bounded flow are investigated using a combined approach of direct numerical simulation data, theoretical methods and experiments. In particular, the wire length (l), spacing ( ) and angle ( ) of a cross-wire probe configured to measure the streamwise and wall-normal velocities are systematically varied to isolate effects of each parameter. The measured streamwise velocity from a cross-wire probe is found to be an average of the filtered velocities sensed by the two wires. Thus, in general, an increase in the sensor dimensions when normalised by viscous units leads to an attenuated variance for the streamwise velocity ( ), resulting from a larger contribution to the spatial averaging process from poorly correlated velocities. In contrast, the variance for the wall-normal velocity ( ) can be amplified, and this is shown to be the result of an additional contributing term (compared to ) due to differences in the filtered wire-normal velocity between the two wires. This additional term leads to a spurious wall-normal velocity signal, resulting in an amplified variance recorded by the cross-wire probe. Compared to the streamwise and wall-normal velocity variances, the Reynolds shear stress ( ) perhaps surprisingly shows less variation when l, and are varied. The robustness of Reynolds shear stress to the finite sensor size is due to two effects: (i) Reynolds shear stress is devoid of energetic contributions from the near-isotropic fine scales unlike the and statistics, hence cross-wire probe dimensions are typically sufficiently small in terms of viscous unit to adequately capture the statistics for a range of l and investigated; (ii) the dependency arises due to cross terms between the filtered velocities from two wires, however, it turns out that these terms cancel one another in the case of Reynolds shear stress, but not for the and statistics. We note that this does not, however, suggest that is easier to measure accurately than the normal stresses; on the contrary, in a companion paper (Baidya et al 2019 Meas. Sci. Technol. 30 085301) we show that measurements are more prone to errors due to uncertainty in probe geometry and calibration procedure.
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ItemSensitivity of turbulent stresses in boundary layers to cross-wire probe uncertainties in the geometry and calibration procedure(IOP Publishing, 2019-08-01)The sensitivity of measured turbulent stresses to uncertainties in the probe geometry and calibration procedure is investigated for a cross-wire probe in a turbulent boundary layer using direct numerical simulation data. The errors investigated are guided by experiments, and to replicate the full experimental procedure, the cross-wire calibration procedure is simulated to generate a voltage-to-velocity mapping function, which is then utilised to calculate the measured velocity from simulated cross-wire voltages. We show that wire misalignment can lead to an incorrect mean wall-normal velocity and Reynolds shear stress in the near-wall region due to the presence of shear. Furthermore, we find that misalignment in the wire orientation cannot be fully accounted for through the calibration procedure, presumably due to increased sensitivity to an out-of-plane velocity component. This has strong implications if using a generic commercial cross-wire probe, since inclining these probes to gain access to the near-wall region can lead to a large error (up to 10%) in turbulent stresses and these errors can manifest in the log region and beyond to half the boundary layer thickness. For uncertainties introduced during the calibration procedure, the Reynolds shear stress is observed to exhibit an elevated sensitivity compared with other turbulent stresses. This is consistent with empirical observations where the repeatability in the Reynolds shear stress is found to be the poorest.
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ItemCFD simulations of vertical surface piercing circular cylinders and comparison against experiments(Australian fluid mechanics society, 2018)When predicting the susceptibility of a submarine to above water detection, it is important to consider the impact of the wake generated by the periscope(s). Computational Fluid Dynamics (CFD) tools can be used to predict the physical size and shape of the wake, which can be combined with periscope models for input into detectability prediction models. For this application, it is important that CFD predictions of the wake are accurate not only in the mean calculations, but that the physical characteristics of the wake are captured at instantaneous snapshots in time. In a previous experimental study, Keough et al. [10] presented time resolved measurements of the wake from vertical surface piercing cylinders, utilising an automated method of extracting these measurements as a function of time from video recordings of the experiment. In the present work, CFD simulations have been performed to model this experimental data set. The open source CFD software Caelus was used, with the improved Defence Science and Technology Group version of vofSolver—the multiphase volume of fluid solver. A numerical wave gauge is implemented in order to measure the free surface elevation during the simulation and this data is compared to bow wave data obtained from animations of the CFD results, using the same automated visual tracking technique utilised for the experimental measurements. Analysis of these time-resolved measurements is performed, comparing transient statistics and spectral characteristics of the CFD predictions against the experimental data.
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ItemTurbulent flow above wind-generated waves: conditional statistics and POD structures(Australian Fluid Mechanics Society, 2018)Large field-of-view particle image velocimetry (PIV) measurement is performed to characterise the turbulent boundary layer above evolving wind waves, which are developed over 3.5 m fetch at U∞ = 8.2 m/s. This multi-camera experiment captures a streamwise domain of 0.4 m, slightly longer than two dominant wavelength of these wind waves. Instantaneous velocity observations reveal strong flow separations on the leeward side of most dominant waves, and these events are also marked by strong vertical velocity fluctuations. The spatially-averaged velocity profile further indicates a large velocity gradient below the wave crest, which occupies a significant proportion of the boundary layer. The conditionally-averaged flow fields around larger dominant waves show that turbulence stresses are high downwind the wave crest, indicating the highly varying form of the separation events. These events are further elucidated using proper orthogonal decomposition (POD) analysis, where the first few stronger modes reveal several common attributes around the separation events.
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ItemEvolution of the Turbulent Far Wake of a Sphere(Australasian Fluid Mechanics Society, 2018)The classic turbulent axisymmetric wake derivation for the spreading of wake half-width, δ, and maximum mean velocity, ūmax decay comes from arguments of a high local Reynolds number, Re, and thus negligible viscosity. If instead one assumes the local Reynolds number is small, then at some distance sufficiently far downstream the turbulent production term in the Reynolds shear stress equation will decay and a new similarity solution will arise: as shown by [2, 4]. This solution features the scaling of δ ∼ (x/d)1/2 and ūmax ∼ (x/d)−1. In other words, the turbulent wake is scaling itself at rates that match the theoretical laminar wake, yet with a local Reynolds number high enough for the turbulent fluctuations to be non-negligible. Whilst the derivation of a low Reynolds number solution is a mathematical exercise, obtaining data to confirm or deny its existence has proved difficult. No experiment has been conducted at a combination of high enough initial Reynolds number and far enough downstream to capture this transition behaviour. Furthermore, only the DNS study of Gourlay [3] has been able to achieve this behaviour; leading some researchers to question whether this decay state would occur or if the wake instead would relaminarise [7]. This paper presents results for a towed a sphere through water at a Reynolds number, based on sphere diameter, of 13000. Our experiments have been able to capture the wake transitioning from the high local Reynolds number solution to the low local Reynolds number solution via high-speed time-resolved PIV. The value of local Reynolds number that exhibits itself in the extreme far wake during the low local Reynolds number solution suggests the wake is still turbulent, supporting the claim of [2, 4].
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ItemTime Resolved Measurements of Wake Characteristics from Vertical Surface-Piercing Circular Cylinders(Australasian Fluid Mechanics Society, 2016)
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ItemSpatial averaging of velocity measurements in wall-bounded turbulence: single hot-wires(IOP Publishing Ltd, 2013-11-01)
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ItemSpatial averaging of streamwise and spanwise velocity measurements in wall-bounded turbulence using nu- and x-probes(IOP PUBLISHING LTD, 2013-11-01)
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ItemDistance-from-the-wall scaling of turbulent motions in wall-bounded flows(AIP Publishing, 2017-02-01)