Numerical issues in Lagrangian tracking and topological evolution of fluid particles in wallbounded turbulent flows

The determination of the local Lagrangian evolution of the flow topology in wall-bounded turbulence, and of the Lagrangian evolution associated with entrainment across the turbulent / non-turbulent interface into a turbulent boundary layer, require accurate tracking of a fluid particle and its local velocity gradients. This paper addresses the implementation of fluid-particle tracking in both a turbulent boundary layer direct numerical simulation and in a fully developed channel flow simulation. Determination of the sub-grid particle velocity is performed using both cubic B-spline, four-point Hermite spline and higher-order Hermite spline interpolation. Both wall-bounded flows show similar oscillations in the Lagrangian tracers of both velocity and velocity gradients, corresponding to the movement of particles across the boundaries of computational cells. While these oscillation in the particle velocity are relatively small and have negligible effect on the particle trajectories for time-steps of the order of CFL = 0.1, they appear to be the cause of significant oscillations in the evolution of the invariants of the velocity gradient tensor

Distortion correction of two-component - two-dimensional PIV using a large imaging sensor with application to measurements of a turbulent boundary layer flow at Reτ=2386Re_{\tau} = 2386

In the past decade, advances in electronics technology have made larger imaging sensors available to the experimental fluid mechanics community. These advancements have enabled the measurement of 2-component 2-dimensional (2C-2D) velocity fields using particle image velocimetry (PIV) with much higher spatial resolution than previously possible. However, due to the large size of the sensor, the lens distortion needs to be taken into account as it will now have a more significant effect on the measurement quality that must be corrected to ensure accurate high-fidelity 2C-2D velocity field measurements. In this paper, two dewarping models, a second-order rational function (R2) and a bicubic polynomial (P3) are investigated with regards to 2C-2D PIV measurements of a turbulent boundary layer (TBL) using a large imaging sensor. Two approaches are considered and compared: (i) dewarping the images prior to the PIV cross-correlation analysis and (ii) undertaking the PIV cross-correlation analysis using the original recorded distorted images then followed by using the mapping functions derived for image dewarping to provide the correct spatial location of the velocity measurement point. The results demonstrate that the use of P3 dewarping model to correct lens distortion yields better results than the R2 dewarping model. Furthermore, both approaches for the P3 dewarping model yield results which are statistically indistinguishable

High Cyclic Fatigue Behaviour of Varied Pitch Compression Springs for Automotive Applications

In this paper, fatigue life of coil springs was investigated as failure due to high cyclic loading during their service life is more common. For this purpose, two exclusive rear coil compression spring geometries were investigated using material type - siliconchromium steel SAE-AISI 9254 for this application. For these design models, the effect of changing component geometry using varying pitch arrangements of coils were examined to understand their structural durability and ability to withstand cyclic loading conditions. A finite element model based on industry standard designs was created to simulate real-life spring performance. In this study, the varied pitch designs were subjected to lower stresses in static analysis and have longer life estimates in the fatigue analysis. This confirms that a varied pitch can improve the performance of a coil spring. The result shows that design-1 incorporated a pitch design that reduced pitch at the centre coils of the spring performed better

mNCEA policy brief - Mind the Gap – The need to continue long-term plankton monitoring

This policy brief argues that while it is beneficial to explore novel plankton survey technology, it is essential that we also continue to maintain traditional long-term monitoring programmes to generate the necessary information to inform policy. Changes in plankton have important implications for the continued provision of ecosystem services, including supporting commercial fish stocks, carbon sequestration, and oxygen production. Such changes can only be detected by studying long-term, consistent plankton datasets which are needed to understand the pressures driving these changes and how we can manage them. Traditional long-term plankton monitoring relies on light microscopy to identify and count plankton taxa, with methods fully supported by national / international QA/QC standards and providing high quality trusted data. Novel technologies, including imaging and molecular methods, offer more efficient means of collecting some types of plankton data, filling targeted knowledge gaps left by traditional monitoring. However, these data are often semi-quantitative, lacking in QA/QC standards, and/or in taxonomic resolution. While these technologies are developed it remains critical to maintain the continuity of traditional plankton monitoring to inform policy assessments of important changes in biodiversity. Losing these time-series, many of which span multiple decades, would impair our ability to detect important change in pelagic habitats, as most changes cannot be detected from short-term data. This would also accelerate the loss of taxonomic expertise, already under threat globally, diminishing our UK skill-base. Novel technologies should be explored in parallel to traditional monitoring, as they can provide complementary data to support policy assessments and research, however, it is important that we do not attempt to replace traditional monitoring with new technology before it has been thoroughly integrated into long-term monitoring programmes. This project was funded by the Department for Environment, Food and Rural Affairs (Defra) as part of the marine arm of the Natural Capital and Ecosystem Assessment (NCEA) programme. The marine NCEA programme is leading the way in supporting Government ambition to integrate natural capital approaches into decision making for the marine environment. Find out more at https://www.gov.uk/government/publications/natural-capital-and-ecosystem-assessment-programme

High spatial resolution 2C-2D PIV measurements using a 47 MPx sensor of high Reynolds number turbulent boundary layer flow

In the past decade, advances in electronics technology have made larger imaging sensors available to the experimental fluid mechanics community. These advancements have enabled the measurement of 2-component 2-dimensional (2C-2D) velocity fields using particle image velocimetry (PIV) with much higher spatial resolution than previously possible using a single camera. Although previously reported experiments have incorporated multiple-camera array to acquire high spatial resolution PIV, using a single large camera can greatly reduce the complexity of the experimental setup as well as the error introduced by the calibration between the cameras. In this paper, the ability of a single large sensor for high spatial resolution PIV is demonstrated by performing the measurement of a zero-pressure-gradient turbulent boundary layer (ZPG-TBL). In post-processing the PIV images, the lens distortion error is of particular importance, as the lens distortion error increases with the size of the imaging sensor. The third-order polynomial functions are used to model the lens distortion in this study, and the correction is performed on the PIV vectors to save computational cost. The first- and second-order statistics are calculated and compared with the profiles captured by small camera arrays, and the result shows that the corrected profiles agree well with the previously acquired data, therefore, the lens distortion error can be corrected

Mimicking superinfection exclusion disrupts alphavirus infection and transmission in the yellow fever mosquito Aedes aegypti

Multiple viruses, including pathogenic viruses, bacteriophages, and even plant viruses, cause a phenomenon termed superinfection exclusion whereby a currently infected cell is resistant to secondary infection by the same or a closely related virus. In alphaviruses, this process is thought to be mediated, at least in part, by the viral protease (nsP2) which is responsible for processing the nonstructural polyproteins (P123 and P1234) into individual proteins (nsP1–nsP4), forming the viral replication complex. Taking a synthetic biology approach, we mimicked this naturally occurring phenomenon by generating a superinfection exclusion-like state in Aedes aegypti mosquitoes, rendering them refractory to alphavirus infection. By artificially expressing Sindbis virus (SINV) and chikungunya virus (CHIKV) nsP2 in mosquito cells and transgenic mosquitoes, we demonstrated a reduction in both SINV and CHIKV viral replication rates in cells following viral infection as well as reduced infection prevalence, viral titers, and transmission potential in mosquitoes.</p

Effect of streamwise domain size on the POD mode characteristics in an adverse pressure gradient turbulent boundary layer

Proper orthogonal decomposition (POD) is used to study coherent structures on measurements of wall-bounded turbulent flows. In order to apply POD in the study of turbulent boundary layers, it is important to determine the appropriate size of the flow domain to be used in POD analysis. This study uses the 2C-2D PIV measurements of an adverse pressure gradient (APG) turbulent boundary layer (TBL) with beta ~ 0 - 3.74 and Re_delta2 ~ 1720 - 23430 where Re_delta2 is the momentum thickness based Reynolds number and beta is the Clauser's pressure gradient parameter. The measurements were obtained in the Laboratoire de Mecanique des Fluides de Lille (LMFL) High-Reynolds-Number (HRN) Boundary Layer Wind Tunnel, Lille, France. Spanning over 20 delta along the streamwise direction (where delta is the boundary layer thickness in the middle of the field of view), these are appropriate to be used in the study of the extent of the large-scale motions. POD analysis on the variable domain size along the streamwise direction (delta x) suggests that the POD mode shape changes with delta x. In this particular TBL, the shape of the first mode which represents the largest scales in the fluid flow, changes until delta x is equal to or greater than 8 delta. It is also observed the smaller scales of higher-order POD modes take longer delta x to converge in the mode shape. The streamwise integral-length-scale of 6.68 delta also confirms the size of the largest scales to be around 8 delta. Therefore, for this particular TBL, a minimum streamwise domain size of 8 delta is appropriate for the analysis of large-scale motions using POD

Investigation of Large Scale Motions in Zero and Adverse Pressure Gradient Turbulent Boundary Layers Using High-Spatial-Resolution PIV

Particle image velocimetry (PIV) has been used to capture the high-spatial-resolution (HSR) two-component, two-dimensional (2C-2D) velocity fields of a zero-pressure-gradient (ZPG) turbulent boundary layer (TBL) and of an adverse-pressure-gradient (APG) TBL. Proper Orthogonal Decomposition (POD) is performed on the measured velocity fields to characterize the velocity fields as large or small scale motions (LSMs or SSMs), with further characterisation of the LSMs into high and low momentum events. This paper reports the findings of the PIV experiment and the subsequent analysis of the high Reynolds number ZPG and APG TBLs

Intense large-scale motions in zero and adverse pressure gradient turbulent boundary layers

Proper orthogonal decomposition (POD) is used to study coherent structures in wall-bounded turbulent flows. The present study uses POD in turbulent boundary layers to determine the contributions of the intense large-scale motions (LSMs) to the Reynolds stresses. This study uses the 2C-2D PIV measurements of zero pressure gradient turbulent boundary layers (ZPG-TBL) at Re_delta2 = 7750, and adverse pressure gradient turbulent boundary layer (APG-TBL) at beta=2.27 and Re_delta2=16240, where Re_delta2 is the momentum thickness based Reynolds number and beta is the Clauser's pressure gradient parameter. The measurements were obtained in the Laboratoire de Mecanique des Fluides de Lille (LMFL) High-Reynolds-Number (HRN) Boundary Layer Wind Tunnel, Lille, France. The snapshots of the flow field are segregated into those dominated by the intense and mild LSMs based on the intensity of the temporal coefficients of the first POD mode. The intense LSMs are further decomposed into high-momentum (HM) and low-momentum (LM) motions. The relative contributions of the HM motions to the Reynolds stresses are larger near the wall as compared to the LM motions. At the wall-normal distance of the displacement thickness (delta_1), HM and LM motions have similar contributions. Beyond delta_1, the LM motions have larger contributions with their peaks located closer to the displacement thickness height. This shows that in the presence of an APG, the turbulence activity is shifted closer to the displacement thickness height