 Mechanical Engineering  Research Publications
Mechanical Engineering  Research Publications
Permanent URI for this collection
35 results
Filters
Reset filtersSettings
Statistics
Citations
Search Results
Now showing
1  10 of 35

ItemEvolution of the turbulent/nonturbulent interface of an axisymmetric turbulent jetKhashehchi, M ; Ooi, A ; Soria, J ; Marusic, I (SPRINGER, 20130101)

ItemOn the universality of inertial energy in the log layer of turbulent boundary layer and pipe flowsChung, D ; Marusic, I ; Monty, JP ; Vallikivi, M ; Smits, AJ (SPRINGER, 20150701)

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.

ItemStructure Inclination Angles in the Convective Atmospheric Surface LayerChauhan, K ; Hutchins, N ; Monty, J ; Marusic, I (SPRINGER, 20130401)

ItemTowards fullyresolved PIV measurements in high Reynolds number turbulent boundary layers with DSLR camerasde Silva, CM ; Grayson, K ; Scharnowski, S ; Kaehler, CJ ; Hutchins, N ; Marusic, I (SPRINGER, 20180601)

ItemTowards Reconciling the LargeScale Structure of Turbulent Boundary Layers in the Atmosphere and LaboratoryHutchins, N ; Chauhan, K ; Marusic, I ; Monty, J ; Klewicki, J (SPRINGER, 20121101)

ItemWalldrag measurements of smooth and roughwall turbulent boundary layers using a floating elementBaars, WJ ; Squire, DT ; Talluru, KM ; Abbassi, MR ; Hutchins, N ; Marusic, I (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 signaltonoise ratios. In this paper, we present new direct measurements of $$øverlineτ _w$$ τ ¯ w where high signaltonoise ratios are achieved using a new design of a largescale 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 largescale turbulent boundary layer facility. Walldrag induced by both smooth and roughwall zeropressuregradient flows are considered. Results for the smoothwall 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 thicknessbased 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 smoothwall benchmark verification of the experimental apparatus is critical before attempting any roughwall studies. For a roughwall 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 walldrag indicates the anticipated trend from the transitionally to the fully rough regime.

ItemWavelet analysis of wall turbulence to study largescale modulation of small scalesBaars, WJ ; Talluru, KM ; Hutchins, N ; Marusic, I (SPRINGER, 20151001)

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.