Use Of Lagrangian Methods Research Articles

Computations of the shear stresses distribution experienced by passive particles as they circulate in turbulent flow: A case study for vWF protein molecules.

The stress distribution along the trajectories of passive particles released in turbulent flow were computed with the use of Lagrangian methods and direct numerical simulations. The flow fields selected were transitional Poiseuille-Couette flow situations found in ventricular assist devices and turbulent flows at conditions found in blood pumps. The passive particle properties were selected to represent molecules of the von Willebrand factor (vWF) protein. Damage to the vWF molecule can cause disease, most often related to hemostasis. The hydrodynamic shear stresses along the trajectories of the particles were calculated and the changes in the distribution of stresses were determined for proteins released in different locations in the flow field and as a function of exposure time. The stress distributions indicated that even when the average applied stress was within a safe operating regime, the proteins spent part of their trajectories in flow areas of damaging stress. Further examination showed that the history of the distribution of stresses applied on the vWF molecules, rather than the average, should be used to evaluate hydrodynamically-induced damage.

Investigating transport in a tidally driven coral atoll flow using Lagrangian coherent structures

AbstractA field experiment study of flow transport around a coral reef was conducted at Scott Reef, an offshore atoll in the Timor Sea. A drifter deployment was designed based on the insight derived from two Lagrangian data analysis approaches, the finite‐time Lyapunov exponent method and the optimized‐parameter spectral clustering method, which were used to analyze the predictions of a numerical model. This analysis predicted the formation of a key transport barrier during a critical time of the tidal cycle that separated two bodies of water, one remaining trapped within the lagoon, and one advected offshore; this transport structure had no clear signature upon inspection of the velocity fields and thus the use of Lagrangian methods was crucial. The observed drifter trajectories confirmed the predictions, with the drifters separating into two clusters, one on each side of the transport barrier. The results demonstrate how Lagrangian approaches elucidate the processes governing connectivity and water exchanges between atolls and the surrounding ocean.

Water Pathways for the Hindu-Kush-Himalaya and an Analysis of Three Flood Events

The climatology of major sources and pathways of moisture for three locales along the Hindu-Kush-Himalayan region are examined, by use of Lagrangian methods applied to the ERA-Interim dataset, over the period from 1980 to 2016 for both summer (JJA) and winter (NDJ) periods. We also investigate the major flooding events of 2010, 2013, and 2017 in Pakistan, Uttarakhand, and Kathmandu, respectively, and analyse a subset of the climatology associated with the 20 most significant rainfall events over each region of interest. A comparison is made between the climatology and extreme events, in the three regions of interest, during the summer monsoon period. For Northern Pakistan and Uttarakhand, the Indus basin plays the largest role in moisture uptake. Moisture is also gathered from Eastern Europe and Russia. Extreme events display an increased influence of sub-tropical weather systems, which manifest themselves through low-level moisture transport; predominantly from the Arabian sea and along the Gangetic plain. In the Kathmandu region, it is found that the major moisture sources come from the Gangetic plain, Arabian Sea, Red Sea, Bay of Bengal, and the Indus basin. In this case, extreme event pathways largely match those of the climatology, although an increased number of parcels originate from the western end of the Gangetic plain. These results provide insights into the rather significant influence of mid-latitudinal weather systems, even during the monsoon season, in defining the climatology of the Hindu-Kush-Himalaya region, as well as how extreme precipitation events in this region represent atypical moisture pathways. We propose a detailed investigation of how such water pathways are represented in climate models for the present climate conditions and in future climate scenarios, as this may be extremely relevant for understanding the impacts of climate change on the cryosphere and hydrosphere of the region.

On temperature prediction at low Re turbulent flows using the Churchill turbulent heat flux correlation

The present work investigates the prediction of mean temperature profiles in turbulent channel flow using the fraction of the heat flux due to turbulence. According to this new model, suggested by Churchill and co-workers, fully developed flow and convection can be expressed as local fractions of the shear stress and the heat flux density due to turbulent fluctuations, respectively. The fully developed temperature profile can be predicted if the velocity field and the turbulent Prandtl number are known. Temperature profiles for Pr between 0.01 and 50,000 have been obtained theoretically and with simulations through the use of Lagrangian methods for both plane Poiseuille flow and plane Couette flow. The half channel height for all simulations was h = 150 in wall units. The theoretical predictions have been found to agree with the data quite well for a range of Pr, but there are deviations at very high Pr.

The Use of Lagrangian Methods To Describe Turbulent Transport of Heat from a Wall

Tracking of heat markers released instantaneously from a point source on a wall boundary in a numerically simulated turbulent channel flow is used to describe the transfer of heat. The trajectories of 16 129 heat markers are monitored in time and space. These results are used to calculate temperature fields generated by continuous line or planar sources and the temperature profile in a channel with a heated bottom wall and a cooled top wall. The good agreement between the calculated fully-developed profile and the profile generated by a direct numerical simulation illustrates the equivalence of the Lagrangian and Eulerian approaches. The main point to be made is that dispersion from an instantaneous point source is a basic element. Its understanding could provide a better foundation for establishing the physics of wall heat transfer than is supplied by Eulerian analyses.

Use of lagrangian methods to describe drop deposition and distribution in horizontal gas—liquid annular flows

Drop distribution and deposition in horizontal gas—liquid annular flow is described by a diffusion model, which views the concentration field as the result of dispersion from a distribution of sources. Drops originating from a wall source are considered to diffuse in a field of homogeneous turbulence, while simultaneously being swept downward by the gravitational field. Deposition is assumed to be controlled by two mechanisms operating in parallel, and boundary conditions are derived which correctly satisfy conservation of mass. This analysis for an instantaneous source is shown to be equivalent to considering diffusion in a coordinate system moving with the settling velocity of the particles. The results are found to be useful for understanding droplet distribution and deposition.

A STUDY OF CONSTANT ABSOLUTE VORTICITY TRAJECTORIES ON ISENTROPIC SURFACES

Abstract It is shown that constant absolute vorticity (CAV) trajectories are more useful on isentropic surfaces than on constant pressure surfaces. A form of the vorticity equation is derived by use of Lagrangian methods. This form is similar to the Rossby form, and ordinary methods of computing CAV trajectories can be used with isentropic charts. A series of isentropic streamline charts was prepared and CAV trajectories were constructed at certain points on these charts. CAV trajectories were also constructed on 500-mb charts for the same time. 24- and 48-hr forecasts of wind direction and speed were made from CAV trajectories on both sets of charts, and verified. The results of these forecasts are treated statistically, and forecasts from CAV trajectories on isentropic charts are shown to be significantly better than similar forecasts from 500-mb charts. The effects of the divergence term of the vorticity equation are discussed qualitatively.