Mechanical Engineering - Research Publications
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ItemHeat Transfer Coefficient Estimation for Turbulent Boundary Layers(The University of Queensland, 2020-12-11)Convective heat transfer in rough wall-bounded turbulent flows is prevalent in many engineering applications, such as in gas turbines and heat exchangers. At present, engineers lack the design tools to accurately predict the convective heat transfer in the presence of non-smooth boundaries. Accordingly, a new turbulent boundary layer facility has been commissioned, where the temperature of an interchangeable test surface can be precisely controlled, and conductive heat losses are minimized. Using this facility, we can estimate the heat transfer coefficient (Stanton number, St), through measurement of the power supplied to the electrical heaters and also from measurements of the thermal and momentum boundary layers evolving over this surface. These methods have been initially investigated over a shorter smooth prototype heated surface and compared with existing St prediction models. Preliminary results suggest that we can accurately estimate St in this facility.
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ItemAn investigation of cold-wire spatial resolution using a DNS database(The University of Queensland, 2020-12-11)The effect of spatial resolution of cold-wire anemometry on both the variance and energy spectrum of temperature fluctuations is analyzed through the use of a numerical database. Temperature fluctuation snapshots from a direct numerical simulation (DNS) of a heated smooth-wall turbulent channel flow are spatially averaged in the spanwise direction to simulate the wire filtering. The results show that the wire length does not affect the mean temperature while it significantly attenuates the variance of temperature fluctuations, particularly in the vicinity of the wall. As the filter length grows, the peaks of the one- and two-dimensional energy spectrograms are further attenuated. Limited attenuation is seen when the filter length is smaller than 30 wall units in the vicinity of the wall, whereas a complete suppression of the near-wall energetic peak is observed when the filter length exceeds 100 wall units.
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ItemStructure Inclination Angles in the Convective Atmospheric Surface Layer(SPRINGER, 2013-04-01)
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ItemWall-drag measurements of smooth- and rough-wall turbulent boundary layers using a floating element(SPRINGER, 2016)The mean wall shear stress, $$øverlineτ _w$$ τ ¯ w , is a fundamental variable for characterizing turbulent boundary layers. Ideally, $$øverlineτ _w$$ τ ¯ w is measured by a direct means and the use of floating elements has long been proposed. However, previous such devices have proven to be problematic due to low signal-to-noise ratios. In this paper, we present new direct measurements of $$øverlineτ _w$$ τ ¯ w where high signal-to-noise ratios are achieved using a new design of a large-scale floating element with a surface area of 3 m (streamwise) × 1 m (spanwise). These dimensions ensure a strong measurement signal, while any error associated with an integral measurement of $$øverlineτ _w$$ τ ¯ w is negligible in Melbourne’s large-scale turbulent boundary layer facility. Wall-drag induced by both smooth- and rough-wall zero-pressure-gradient flows are considered. Results for the smooth-wall friction coefficient, $$C_f \equiv øverlineτ _w/q_\infty $$ C f ≡ τ ¯ w / q ∞ , follow a Coles–Fernholz relation $$C_f = \left[ 1/κ \ln \left( Re_θ \right) + C\right] ^-2$$ C f = 1 / κ ln R e θ + C - 2 to within 3 % ( $$κ = 0.38$$ κ = 0.38 and $$C = 3.7$$ C = 3.7 ) for a momentum thickness-based Reynolds number, $$Re_θ > 15,000$$ R e θ > 15 , 000 . The agreement improves for higher Reynolds numbers to <1 % deviation for $$Re_θ > 38,000$$ R e θ > 38 , 000 . This smooth-wall benchmark verification of the experimental apparatus is critical before attempting any rough-wall studies. For a rough-wall configuration with P36 grit sandpaper, measurements were performed for $$10,500< Re_θ < 88,500$$ 10 , 500 < R e θ < 88 , 500 , for which the wall-drag indicates the anticipated trend from the transitionally to the fully rough regime.
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ItemWavelet analysis of wall turbulence to study large-scale modulation of small scales(SPRINGER, 2015-10-01)
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ItemThe effect of spanwise wavelength of surface heterogeneity on turbulent secondary flows(Cambridge University Press (CUP), 2020-07-10)We examine the behaviour of turbulent boundary layers over surfaces composed of spanwise-alternating smooth and rough strips, where the width of the strips varies such that, where is the boundary-layer thickness averaged over one spanwise wavelength of the heterogeneity. The experiments are configured to examine the influences of spanwise variation in wall shear stress over a large range. Hot-wire anemometry and particle image velocimetry (PIV) reveal that the half-wavelength governs the diameter and strength of the resulting mean secondary flows and hence the observed isovels of the mean streamwise velocity. Three possible cases are observed: limiting cases (either or), where the secondary flows are confined near the wall or near the roughness change, and intermediate cases (), where the secondary flows are space filling and at their strongest. These secondary flows, however, exhibit a time-dependent behaviour which might be masked by time averaging. Further analysis of the energy spectrogram and fluctuating flow fields obtained from PIV show that the secondary flows meander in a similar manner to that of large-scale structures occurring naturally in turbulence over smooth walls. The meandering of the secondary flows is a function of and is most prominent when.
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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.