Mechanical Engineering - Research Publications

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    On the universality of inertial energy in the log layer of turbulent boundary layer and pipe flows
    Chung, D ; Marusic, I ; Monty, JP ; Vallikivi, M ; Smits, AJ (SPRINGER, 2015-07)
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    Structure Inclination Angles in the Convective Atmospheric Surface Layer
    Chauhan, K ; Hutchins, N ; Monty, J ; Marusic, I (SPRINGER, 2013-04)
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    Towards Reconciling the Large-Scale Structure of Turbulent Boundary Layers in the Atmosphere and Laboratory
    Hutchins, N ; Chauhan, K ; Marusic, I ; Monty, J ; Klewicki, J (SPRINGER, 2012-11)
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    Spatial averaging effects on the streamwise and wall-normal velocity measurements in a wall-bounded turbulence using a cross-wire probe
    Baidya, R ; Philip, J ; Hutchins, N ; Monty, JP ; Marusic, I (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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    Sensitivity of turbulent stresses in boundary layers to cross-wire probe uncertainties in the geometry and calibration procedure
    Baidya, R ; Philip, J ; Hutchins, N ; Monty, JP ; Marusic, I (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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    Simultaneous skin friction and velocity measurements in high Reynolds number pipe and boundary layer flows
    Baidya, R ; Baars, WJ ; Zimmerman, S ; Samie, M ; Hearst, RJ ; Dogan, E ; Mascotelli, L ; Zheng, X ; Bellani, G ; Talamelli, A ; Ganapathisubramani, B ; Hutchins, N ; Marusic, I ; Klewickil, J ; Monty, JP (Cambridge University Press (CUP), 2019-07-25)
    Streamwise velocity and wall-shear stress are acquired simultaneously with a hot-wire and an array of azimuthal/spanwise-spaced skin friction sensors in large-scale pipe and boundary layer flow facilities at high Reynolds numbers. These allow for a correlation analysis on a per-scale basis between the velocity and reference skin friction signals to reveal which velocity-based turbulent motions are stochastically coherent with turbulent skin friction. In the logarithmic region, the wall-attached structures in both the pipe and boundary layers show evidence of self-similarity, and the range of scales over which the self-similarity is observed decreases with an increasing azimuthal/spanwise offset between the velocity and the reference skin friction signals. The present empirical observations support the existence of a self-similar range of wall-attached turbulence, which in turn are used to extend the model of Baars et al. (J. Fluid Mech., vol. 823, p. R2) to include the azimuthal/spanwise trends. Furthermore, the region where the self-similarity is observed correspond with the wall height where the mean momentum equation formally admits a self-similar invariant form, and simultaneously where the mean and variance profiles of the streamwise velocity exhibit logarithmic dependence. The experimental observations suggest that the self-similar wall-attached structures follow an aspect ratio of in the streamwise, spanwise and wall-normal directions, respectively.
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    Spatial averaging of velocity measurements in wall-bounded turbulence: single hot-wires
    Philip, J ; Hutchins, N ; Monty, JP ; Marusic, I (IOP Publishing Ltd, 2013-11)
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    Spatial averaging of streamwise and spanwise velocity measurements in wall-bounded turbulence using ν- and x-probes
    Philip, J ; Baidya, R ; Hutchins, N ; Monty, JP ; Marusic, I (IOP PUBLISHING LTD, 2013-11)
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    Distance-from-the-wall scaling of turbulent motions in wall-bounded flows
    Baidya, R ; Philip, J ; Hutchins, N ; Monty, JP ; Marusic, I (AIP Publishing, 2017-02)
    An assessment of self-similarity in the inertial sublayer is presented by considering the wall-normal velocity, in addition to the streamwise velocity component. The novelty of the current work lies in the inclusion of the second velocity component, made possible by carefully conducted subminiature ×-probe experiments to minimise the errors in measuring the wall-normal velocity. We show that not all turbulent stress quantities approach the self-similar asymptotic state at an equal rate as the Reynolds number is increased, with the Reynolds shear stress approaching faster than the streamwise normal stress. These trends are explained by the contributions from attached eddies. Furthermore, the Reynolds shear stress cospectra, through its scaling with the distance from the wall, are used to assess the wall-normal limits where self-similarity applies within the wall-bounded flow. The results are found to be consistent with the recent prediction from the work of Wei et al. [“Properties of the mean momentum balance in turbulent boundary layer, pipe and channel flows,” J. Fluid Mech. 522, 303–327 (2005)], Klewicki [“Reynolds number dependence, scaling, and dynamics of turbulent boundary layers,” J. Fluids Eng. 132, 094001 (2010)], and others that the self-similar region starts and ends at z+∼O(δ+) and O(δ+), respectively. Below the self-similar region, empirical evidence suggests that eddies responsible for turbulent stresses begin to exhibit distance-from-the-wall scaling at a fixed z+ location; however, they are distorted by viscous forces, which remain a leading order contribution in the mean momentum balance in the region z+≲O(δ+), and thus result in a departure from self-similarity.
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    Numerical investigation of the behaviour of wall shear stress in three-dimensional pulsatile stenotic flows
    Li, S ; Chin, C ; Barlis, P ; MARUSIC, I ; Ooi, A (Australasian Fluid Mechanics Society (AFMS), 2014)