 Mechanical Engineering  Research Publications
Mechanical Engineering  Research Publications
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ItemSpatial averaging effects on the streamwise and wallnormal velocity measurements in a wallbounded turbulence using a crosswire probeBaidya, R ; Philip, J ; Hutchins, N ; Monty, JP ; Marusic, I (IOP Publishing, 20190801)The spatial averaging effects due to a crosswire probe on the measured turbulence statistics in a wallbounded 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 crosswire probe configured to measure the streamwise and wallnormal velocities are systematically varied to isolate effects of each parameter. The measured streamwise velocity from a crosswire 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 wallnormal 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 wirenormal velocity between the two wires. This additional term leads to a spurious wallnormal velocity signal, resulting in an amplified variance recorded by the crosswire probe. Compared to the streamwise and wallnormal 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 nearisotropic fine scales unlike the and statistics, hence crosswire 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.

ItemSensitivity of turbulent stresses in boundary layers to crosswire probe uncertainties in the geometry and calibration procedureBaidya, R ; Philip, J ; Hutchins, N ; Monty, JP ; Marusic, I (IOP Publishing, 20190801)The sensitivity of measured turbulent stresses to uncertainties in the probe geometry and calibration procedure is investigated for a crosswire 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 crosswire calibration procedure is simulated to generate a voltagetovelocity mapping function, which is then utilised to calculate the measured velocity from simulated crosswire voltages. We show that wire misalignment can lead to an incorrect mean wallnormal velocity and Reynolds shear stress in the nearwall 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 outofplane velocity component. This has strong implications if using a generic commercial crosswire probe, since inclining these probes to gain access to the nearwall 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.

ItemInterfaces of uniform momentum zones in turbulent boundary layersde Silva, CM ; Philip, J ; Hutchins, N ; Marusic, I (CAMBRIDGE UNIV PRESS, 20170610)In this paper we examine the characteristics of the interfaces that demarcate regions of relatively uniform streamwise momentum in turbulent boundary layers. The analysis utilises particle image velocimetry databases that span more than an order of magnitude of friction Reynolds number ($Re_{\unicode[STIX]{x1D70F}}=10^{3}$–$10^{4}$), enabling us to provide a detailed description of the interfacial layers as a function of Reynolds number. As reported by Adrianet al.(J. Fluid Mech., vol. 422, 2000, pp. 1–54), these interfaces appear as persistent regions of strong shear with distinct patches of vorticity consistent with a packetlike structure. Here, however, we treat these interfaces as continuous lines, thus averaging the properties of the vortical patches, and find that their geometry is highly contorted and exhibits selfsimilarity across a wide range of scales. Specifically, the lengths of the edges of uniform momentum zones exhibit a powerlaw behaviour with a fractal scaling that has a constant exponent across the boundary layer, while the topmost edge or the turbulent/nonturbulent interface shows a sudden increase in the exponent. The accompanying sharp changes in velocity that occur at these edges are found to change in magnitude as a function of wallnormal height, being larger closer to the wall. Further, a Reynolds number invariance is exhibited when the magnitude of the steplike changes in velocity is scaled by the skinfriction velocity, meanwhile, the width across which it occurs is shown to be of the order of the Taylor microscale. Based on these quantitative measures, the Reynolds number scaling observed and the persistent presence of sharp changes in momentum in turbulent boundary layers, a simple model is used to reconstruct the mean velocity profile. Insight gained from the model enhances our understanding of how instantaneous phenomena (such as a zonallike structural arrangement) manifests in the averaged flow statistics and confirms that the instantaneous momentum in a turbulent boundary layer appears to mainly consist of a steplike profile as a function of wallnormal distance.

ItemSpatial averaging of velocity measurements in wallbounded turbulence: single hotwiresPhilip, J ; Hutchins, N ; Monty, JP ; Marusic, I (IOP Publishing Ltd, 20131101)

ItemSpatial averaging of streamwise and spanwise velocity measurements in wallbounded turbulence using nu and xprobesPhilip, J ; Baidya, R ; Hutchins, N ; Monty, JP ; Marusic, I (IOP PUBLISHING LTD, 20131101)

ItemDistancefromthewall scaling of turbulent motions in wallbounded flowsBaidya, R ; Philip, J ; Hutchins, N ; Monty, JP ; Marusic, I (AIP Publishing, 20170201)

ItemThe turbulent/nonturbulent interface and entrainment in a boundary layerChauhan, K ; Philip, J ; de Silva, CM ; Hutchins, N ; Marusic, I (CAMBRIDGE UNIV PRESS, 20140301)Abstract The turbulent/nonturbulent interface in a zeropressuregradient turbulent boundary layer at high Reynolds number ($\mathit{Re}_\tau =14\, 500$) is examined using particle image velocimetry. An experimental setup is utilized that employs multiple highresolution cameras to capture a large field of view that extends $2\delta \times 1.1\delta $ in the streamwise/wallnormal plane with an unprecedented dynamic range. The interface is detected using a criteria of local turbulent kinetic energy and proves to be an effective method for boundary layers. The presence of a turbulent/nonturbulent superlayer is corroborated by the presence of a jump for the conditionally averaged streamwise velocity across the interface. The steep change in velocity is accompanied by a discontinuity in vorticity and a sharp rise in the Reynolds shear stress. The conditional statistics at the interface are in quantitative agreement with the superlayer equations outlined by Reynolds (J. Fluid Mech., vol. 54, 1972, pp. 481–488). Further analysis introduces the mass flux as a physically relevant parameter that provides a direct quantitative insight into the entrainment. Consistency of this approach is first established via the equality of mean entrainment calculations obtained using three different methods, namely, conditional, instantaneous and mean equations of motion. By means of ‘massflux spectra’ it is shown that the boundarylayer entrainment is characterized by two distinctive length scales which appear to be associated with a twostage entrainment process and have a substantial scale separation.

ItemThe quiescent core of turbulent channel flowKwon, YS ; Philip, J ; de Silva, CM ; Hutchins, N ; Monty, JP (CAMBRIDGE UNIV PRESS, 20140701)Abstract The identification of uniform momentum zones in wallturbulence, introduced by Adrian, Meinhart & Tomkins (J. Fluid Mech., vol. 422, 2000, pp. 1–54) has been applied to turbulent channel flow, revealing a large ‘core’ region having high and uniform velocity magnitude. Examination of the core reveals that it is a region of relatively weak turbulence levels. For channel flow in the range $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}Re_{\tau } = 1000\text {}4000$, it was found that the ‘core’ is identifiable by regions bounded by the continuous isocontour lines of the streamwise velocity at $0.95U_{CL}$ (95 % of the centreline velocity). A detailed investigation into the properties of the core has revealed it has a largescale oscillation which is predominantly antisymmetric with respect to the channel centreline as it moves through the channel, and there is a distinct jump in turbulence statistics as the core boundary is crossed. It is concluded that the edge of the core demarcates a shear layer of relatively intense vorticity such that the interior of the core contains weakly varying, very lowlevel turbulence (relative to the flow closer to the wall). Although channel flows are generally referred to as ‘fully turbulent’, these findings suggest there exists a relatively large and ‘quiescent’ core region with a boundary qualitatively similar to the turbulent/nonturbulent interface of boundary layers, jets and wakes.