ALBERT

Description: In this paper, we describe improvements to the in situ cosmogenic radiocarbon extraction system at SUERC made since 2004, highlighting the factors that potentially control the reduction of analytical variability. We also present new results on system blanks and of measurements of in situ14C in shielded quartz and a surface quartz sample used at the University of Arizona as an in situ14C standard (PP-4). The SUERC in situ14C extraction system was built in 2001 and is based on a combustion technique following the design of the extraction system at the University of Arizona. Our preliminary results suggest that the continuous running of the extraction system and the monitoring of gas collecting time and of the temperature of the cryogenic traps used in the gas cleaning steps are key to maintaining low and stable system blanks. Our latest average system blank is 2.02 ± 0.23 x 10514C atoms. This is consistent with those recently published by the University of Arizona and ETH in situ14C labs. Measurements of in situ14C concentrations in sample PP-4 yield an average of 3.82 ± 0.23 x 105 atoms g–1 quartz, again consistent with published values.

Description: Many of the Loch Tay crannogs were built in the Early Iron Age and so calibration of the radiocarbon ages produces very broad calendar age ranges due to the well-documented Hallstatt plateau in the calibration curve. However, the large oak timbers that were used in the construction of some of the crannogs potentially provide a means of improving the precision of the dating through subdividing them into decadal or subdecadal increments, dating them to high precision and wiggle-matching the resulting data to the master 14C calibration curve. We obtained a sample from 1 oak timber from Oakbank Crannog comprising 70 rings (Sample OB06 WMS 1, T103) including sapwood that was complete to the bark edge. The timber is situated on the northeast edge of the main living area of the crannog and as a large and strong oak pile would have been a useful support in more than 1 phase of occupation and may be related to the earliest construction phase of the site. This was sectioned into 5-yr increments and dated to a precision of approximately ±8–16 14C yr (1 σ). The wiggle-match predicts that the last ring dated was formed around 500 BC (maximum range of 520–465 BC) and should be taken as indicative of the likely time of construction of Oakbank Crannog. This is a considerable improvement on the estimates based on single 14C ages made on oak samples, which typically encompassed the period from around 800–400 BC.

Description: The SUERC Radiocarbon Laboratory employs a one-step “background subtraction” method when calculating 14C ages. An interglacial wood (VIRI Sample K) is employed as the non-bone organic background standard, while a mammoth bone (LQH12) from Latton Quarry is used as the bone background standard. Results over several years demonstrate that the bone background is consistently around a factor of two higher and more variable than the wood background. As a result, the uncertainty on routine bone measurements is higher than for other sample types. This study investigates the factors that may contribute to the difference in F14C values and the higher variability. Preparations of collagen using modified Longin or ultrafiltration methods show no significant difference, nor does eliminating the collagen dissolution step. Two bone samples of known infinite age with respect to radiocarbon are compared and again no significant difference is observed. Finally, the quantity and age of the organic matter in the water used during the pretreatment is investigated and it is shown that there is insufficient organic matter in the reverse osmosis water to influence background values significantly. The attention is now on determining if incomplete demineralization could lead to contaminants being retained by the phosphate in the hydroxyapatite.

Description: The wake of a streamwise oscillating circular cylinder has been experimentally investigated over a range of oscillation amplitude and frequency ratios using laser-induced-fluorescence flow visualization, particle image velocimetry and hot-wire techniques. Five typical flow structures, referred to as S-I, S-II, A-I, A-III and A-IV, are identified. Special attention is given to the S-II mode because this flow structure is observed experimentally for the first time. It consists of two rows of binary vortices symmetrically arranged about the centreline of the wake. Each binary vortex contains two counter-rotating vortices shed from the same side of the cylinder. This flow structure corresponds to zero mean and fluctuating lift on the cylinder, which could be of engineering significance. A theoretical analysis for this flow has been conducted based on the governing equations. The solution to the two-dimensional vorticity equation suggests that the flow may be considered to be the superposition of two components, i.e. that due to a stationary cylinder in a steady uniform cross-flow and to a cylinder oscillating in fluid at rest, which are characterized by alternate and symmetric vortex shedding, respectively. The solution provides insight into the formation of the various modes of the flow structure. A semi-empirical prediction of the S-II mode structure is developed, which is in excellent agreement with experimental data as well as with previous numerical results. © 2006 Cambridge University Press.

Description: Gaseous detonation propagation in a bifurcated tube was experimentally and numerically studied for stoichiometric hydrogen and oxygen mixtures diluted with argon. Pressure detection, smoked foil recording and schlieren visualization were used in the experiments. Numerical simulation was carried out at low initial pressure (8.00 kPa), based on the reactive Navier-Stokes equations in conjunction with a detailed chemical reaction model. The results show that the detonation wave is strongly disturbed by the wall geometry of the bifurcated tube and undergoes a successive process of attenuation, failure, re-initiation and the transition from regular reflection to Mach reflection. Detonation failure is attributed to the rarefaction waves from the left-hand corner by decoupling leading shock and reaction zones. Re-initiation is induced by the inert leading shock reflection on the right-hand wall in the vertical branch. The branched wall geometry has only a local effect on the detonation propagation. In the horizontal branch, the disturbed detonation wave recovers to a self-sustaining one earlier than that in the vertical branch. A critical case was found in the experiments where the disturbed detonation wave can be recovered to be self-sustaining downstream of the horizontal branch, but fails in the vertical branch, as the initial pressure drops to 2.00 kPa. Numerical simulation also shows that complex vortex structures can be observed during detonation diffraction. The reflected shock breaks the vortices into pieces and its interaction with the unreacted recirculation region induces an embedded jet. In the vertical branch, owing to the strength difference at any point and the effect of chemical reactions, the Mach stem cannot be approximated as an arc. This is different from the case in non-reactive steady flow. Generally, numerical simulation qualitatively reproduces detonation attenuation, failure, re-initiation and the transition from regular reflection to Mach reflection observed in experiments. © 2008 Cambridge University Press.

Description: In this paper we present results from a numerical investigation of turbulent channel flow in the presence of a compliant wall. The compliant wall is modelled as a homogeneous spring-supported plate. The simulation code is validated both by comparison with an alternative code and by reproducing results of linear stability theory. Our results demonstrate that with the wall compliance we used in the simulation there is little change in the very long-time behaviour of the turbulent skin friction drag and little modification to the near-wall turbulent coherent structures. The values of pertinent statistical quantities of the turbulence near the compliant walls converge to those near a rigid wall and the statistical effect of the wall compliance on the turbulent channel flow is small.

Description: The wake of polygonal cylinders with side number N = 2 ∼ ∞ is systematically studied based on fluid force, hot-wire, particle image velocimetry and flow visualisation measurements. Each cylinder is examined for two orientations, with a flat surface or a corner leading and facing normally to the free stream. The Reynolds number Re is 1:0 × 104 × 1:0 × 105, based on the longitudinally projected cylinder width. The time-averaged drag coefficient CD and fluctuating lift coefficient on these cylinders are documented, along with the characteristic properties including the Strouhal number St, flow separation point and angle θs, wake width and critical Reynolds number Rec at which the transition from laminar to turbulent flow occurs. It is found that once N exceeds 12, Rec depends on the difference between the inner diameter (tangent to the faces) and the outer diameter (connecting corners) of a polygon, the relationship being approximately given by the dependence of Rec on the height of the roughness elements for a circular cylinder. It is further found that CD versus ξ or St versus ξ for all the tested cases collapse onto a single curve, where the angle ξ is the corrected θs associated with the laterally widest point of the polygon and the separation point. Finally, the empirical correlation between CD and St is discussed. © 2016 Cambridge University Press.

Description: Active control of a turbulent boundary layer has been experimentally investigated with a view to reducing the skin-friction drag and gaining some insight into the mechanism that leads to drag reduction. A spanwise-aligned array of piezo-ceramic actuators was employed to generate a transverse travelling wave along the wall surface, with a specified phase shift between adjacent actuators. Local skin-friction drag exhibits a strong dependence on control parameters, including the wavelength, amplitude and frequency of the oscillation. A maximum drag reduction of 50 % has been achieved at 17 wall units downstream of the actuators. The near-wall flow structure under control, measured using smoke-wire flow visualization, hot-wire and particle image velocimetry techniques, is compared with that without control. The data have been carefully analysed using techniques such as streak detection, power spectra and conditional averaging based on the variable-interval time-average detection. All the results point to a pronounced change in the organization of the perturbed boundary layer. It is proposed that the actuation-induced wave generates a layer of highly regularized streamwise vortices, which acts as a barrier between the large-scale coherent structures and the wall, thus interfering with the turbulence production cycle and contributing partially to the drag reduction. Associated with the generation of regularized vortices is a significant increase, in the near-wall region, of the mean energy dissipation rate, as inferred from a substantial decrease in the Taylor microscale. This increase also contributes to the drag reduction. The scaling of the drag reduction is also examined empirically, providing valuable insight into the active control of drag reduction. © 2014 Cambridge University Press.

Description: Free vibrations of two side-by-side cylinders with fixed support (no rotation and displacement) at both ends placed in a cross-flow were experimentally investigated. Two fibre-optic Bragg grating sensors were used to measure the dynamic strain, while a hot wire and flow visualization were employed to examine the flow field around the cylinders. Three T /d ratios, 3.00, 1.70 and 1.13, were investigated, where T is the centre-to-centre cylinder spacing and d is the diameter; they give rise to three different flow regimes. The investigation throws new light on the shed vortices and their evolution. A new interpretation is proposed for the two different dominant frequencies, which are associated with the narrow and the wide wake when the gap between the cylinders is between 1.5 and 2.0 as reported in the literature. The structural vibration behaviour is closely linked to the flow characteristics. At T /d = 3.00, the cross-flow root-mean-square strain distribution shows a very prominent peak at the reduced velocity Ur ≈ 26 when the vortex shedding frequency fs, coincides with the third-mode natural frequency of the combined fluid-cylinder system. When T /d 〈 3.00, this peak is not evident and the vibration is suppressed because of the weakening strength of the vortices. The characteristics of the system modal damping ratios, including both structural and fluid damping, and natural frequencies are also investigated. It is found that both parameters depend on T /d. Furthermore, they vary slowly with Ur, except near resonance where a sharp variation occurs. The sharp variation in the natural frequencies of the combined system is dictated by the vortex shedding frequency, in contrast with the lock-in phenomenon, where the forced vibration of a structure modifies the vortex shedding frequency. This behaviour of the system natural frequencies persists even in the case of the single cylinder and does not seem to depend on the interference between cylinders. A linear analysis of an isolated cylinder in a cross-flow has been carried out. The linear model prediction is qualitatively consistent with the experimental observation of the system damping ratios and natural frequencies, thus providing valuable insight into the physics of fluid-structure interactions.