 Mechanical Engineering  Research Publications
Mechanical Engineering  Research Publications
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ItemSimultaneous microPIV measurements and realtime control trapping in a crossslot channelAkbaridoust, F ; Philip, J ; Hill, DRA ; Marusic, I (Springer, 20181201)Here we report novel microPIV measurements around micronsized objects that are trapped at the centre of a stagnation point flow generated in a crossslow microchannel using realtime control. The method enables one to obtain accurate velocity and strain rate fields around the trapped objects under straining flows. In previous works, it has been assumed that the flow field measured in the absence of the object is the one experienced by the object in the stagnation point flow. However, the results reveal that this need not be the case and typically the strain rates experienced by the objects are higher. Therefore, simultaneously measuring the flow field around a trapped object is needed to accurately estimate the undisturbed strain rate (away from the trapped object). By combining the microPIV measurements with an analytical solution by Jeffery (Proc R Soc Lond A 102(715):161–179, 1922), we are able to estimate the velocity and strain rate around the trapped object, thus providing a potential fluidic method for characterising mechanical properties of micronsized materials, which are important in biological and other applications.

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.

ItemRevisiting the law of the wake in wall turbulenceKrug, D ; Philip, J ; Marusic, I (CAMBRIDGE UNIV PRESS, 20170125)The streamwise mean velocity profile in a turbulent boundary layer is classically described as the sum of a log law extending all the way to the edge of the boundary layer and a wake function. While there is theoretical support for the log law, the wake function, defined as the deviation of the measured velocity profile from the log law, is essentially an empirical fit and has no real physical underpinning. Here, we present a new physically motivated formulation of the velocity profile in the outer region, and hence for the wake function. In our approach, the entire flow is represented by a twostate model consisting of an inertial selfsimilar region designated as ‘pure wall flow state’ (featuring a loglaw velocity distribution) and a free stream state, which results in a jump in velocity at the interface separating the two. We show that the model provides excellent agreement with the available high Reynolds number mean velocity profiles if this interface is assumed to fluctuate randomly about a mean position with a Gaussian distribution. The new concept can also be extended to internal geometries in the same form, again confirmed by the data. Furthermore, adopting the same interface distribution in a twostate model for the streamwise turbulent intensities, with unchanged parameters, also yields a reliable and consistent prediction for the decline in the outer region of these profiles in all geometries considered. Finally, we discuss differences between our model interface and the turbulent/nonturbulent interface (TNTI) in turbulent boundary layers. We physically interpret the twostate model as lumping the effects of internal shear layers and the TNTI into a single discontinuity at the interface.

ItemGlobal and local aspects of entrainment in temporal plumesKrug, D ; Chung, D ; Philip, J ; Marusic, I (CAMBRIDGE UNIV PRESS, 20170210)To date, the understanding of the role buoyancy plays in the entrainment process in unstable configurations such as turbulent plumes remains incomplete. Towards addressing this question, we set up a flow in which a plume evolves in time instead of space. We demonstrate that the temporal problem is equivalent to a spatial plume in a strong coflow and address in detail how the temporal plume can be realized via direct numerical simulation. Using numerical data of plume simulations up to $Re_{\unicode[STIX]{x1D706}}\approx 100$, we show that the entrainment coefficient can be determined consistently using a global entrainment analysis in an integral framework as well as via a local approach. The latter is based on a study of the local propagation of the turbulent/nonturbulent interface relative to the fluid. Locally, this process is dominated by smallscale diffusion which is amplified by interface convolutions such that the total entrained flux is independent of viscosity. Further, we identify a direct buoyancy contribution to entrainment by baroclinic torque, which accounts for 8 %–12 % of the entrained flux locally, comparable to the 15 % buoyancy contribution at the integral level. It appears that the baroclinic torque is a mechanism that might explain higher values of the entrainment coefficient in spatial plumes compared with jets.

ItemMinimization of divergence error in volumetric velocity measurements and implications for turbulence statisticsde Silva, CM ; Philip, J ; Marusic, I (SPRINGER, 20130701)

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.

ItemThe effects of lasersheet misalignment on StereoPIV measurements in wallbounded turbulenceRama Reddy, GV ; Philip, J ; MARUSIC, I (Australasian Fluid Mechanics Society, 2016)

ItemCharacteristics of the entrainment velocity in a developing wakePhilip, J ; BermejoMoreno, I ; Chung, D ; MARUSIC, I (International Symposium on Turbulence and Shear Flow Phenomena, 2015)

ItemThe influence of the turbulent/nonturbulent interface geometry on local entrainmentMistry, D ; Dawson, JR ; Philip, J ; Marusic, I ( 20170101)
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