An efficient multi-dimensional implementation of VSIAM3 and its applications to free surface flows

We propose an efficient multidimensional implementation of VSIAM3 (volume/surface integrated average-based multi-moment method). Although VSIAM3 is a highly capable fluid solver based on a multi-moment concept and has been used for a wide variety of fluid problems, VSIAM3 could not simulate some simple benchmark problems well (for instance, lid-driven cavity flows) due to relatively high numerical viscosity. In this paper, we resolve the issue by using the efficient multidimensional approach. The proposed VSIAM3 is shown to capture lid-driven cavity flows of the Reynolds number up to Re = 7500 with a Cartesian grid of 128 × 128, which was not capable for the original VSIAM3. We also tested the proposed framework in free surface flow problems (droplet collision and separation of We = 40 and droplet splashing on a superhydrophobic substrate). The numerical results by the proposed VSIAM3 showed reasonable agreements with these experiments. The proposed VSIAM3 could capture droplet collision and separation of We = 40 with a low numerical resolution (8 meshes for the initial diameter of droplets). We also simulated free surface flows including particles toward non-Newtonian flow applications. These numerical results have showed that the proposed VSIAM3 can robustly simulate interactions among air, particles (solid), and liquid

A conservative semi-Lagrangian method for oscillation-free computation of advection processes

The semi-Lagrangian method using the hybrid-cubic-rational interpolation function [M. Ida, Comput. Fluid Dyn. J. 10 (2001) 159] is modified to a conservative method by incorporating the concept discussed in [R. Tanaka et al., Comput. Phys. Commun. 126 (2000) 232]. In the method due to Tanaka et al., not only a physical quantity but also its integrated quantity within a computational cell are used as dependent variables, and the mass conservation is completely achieved by giving a constraint to a forth-order polynomial used as an interpolation function. In the present method, a hybrid-cubic-rational function whose optimal mixing ratio was determined theoretically is employed for the interpolation, and its derivative is used for updating the physical quantity. The numerical oscillation appearing in results by the method due to Tanaka et al. is sufficiently eliminated by the use of the hybrid function.Comment: 17 pages, 8 figures, accepted for publication in Comput. Phys. Commun., Some misprint correcte

Efficient implementation of volume/surface integrated average based multi-moment method

We investigated discretization strategies of the conservation equation in VSIAM3 (volume/surface integrated average based multi-moment method) which is a numerical framework for incompressible and compressible flows based on a multi-moment concept. We investigated these strategies through the lid-driven cavity flow problem, shock tube problems, 2D explosion test and droplet splashing on a superhydrophobic substrate. We found that the use of the CIP-CSLR (constrained interpolation profile-conservative semi-Lagrangian with rational function) method as the conservation equation solver is critically important for the robustness of incompressible flow simulations using VSIAM3 and that numerical results are sensitive to discretization techniques of the divergence term in the conservation equation. Based on these results, we proposed efficient implementation techniques of VSIAM3

Transient removal of contaminant from a channel with differentially heated wall of cavity

Cleaning accumulated deposits inside pipe cavity are by disassembling and cleaning it part by part. Hydrodynamic cleaning of the cavity is an alternative method to clean accumulated deposits or contaminants inside the pipe cavity instead of dissembling them part by part is a tedious process or using a solvent which is not suitable in the food processing industry. This study aims to investigate the contaminants removal process from a cavity by resorting to natural flow to clean the deposits in different cavity sizes and includes different heating locations with different flow configurations. An experimental method is used to visualize the flow behaviour inside the cavity of a channel at a large aspect ratio in isothermal conditions. These results are used to validate numerical results obtained in isothermal flow conditions. For numerical study, Constrained Interpolated Profile (CIP) method is used for the advection phase of momentum and energy equation, and central difference is used to solve the non-advection phase of momentum and energy equations. The numerical studies include different aspect ratios (AR), 1 to 4, various Reynolds numbers (Re), 50 to 1000, and different locations of the heated wall inside the cavity (left wall, bottom wall, & right wall) for three different Grashof numbers (Gr), 1000, 10 000, and 100 000. The particles removal percentage at the transient and steady states are then compared and discussed. A larger aspect ratio and a more significant Reynolds number for isothermal conditions will give a higher percentage of contaminants removal except for AR = 4 and Re = 50. This particular flow shows a higher percentage of contaminant removal than AR = 4; Re = 100, 200, and 400. For mixed convection flow, one typical result can be concluded: at small Gr, the contaminant removal percentage is not changing significantly for all different heated wall positions. It is also shown that a more significant aspect ratio will produce a better contaminant removal process, and a higher Grashof number will improve the contaminant removal process. It is also found that when Gr equals 1000 and 10000, there is no significant change in the contaminant removal process and constant heat flux from the bottom wall for Gr = 100,000 gives the highest contaminant removal percentage for every aspect ratio. The highest percentage removal of contaminant is 98.94% for Gr =100 000, AR=4

The Impacts Of Airborne Cloud Microphysical Instrumentation Mounting Location On Measurements Made During The Observations Of Aerosols And Clouds And Their Interactions (ORACLES) Project

ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) was a five-year NASA investigation into the climate impacts of Southern Africa’s biomass burning aerosols. The University of North Dakota, in coordination with the Cooperative Institute for Severe and High-Impact Weather Research and Operations, the University of Oklahoma and University of Illinois at Urbana-Champaign integrated and operated a suite of in-situ cloud microphysical instrumentation into the NASA P-3 Orion aircraft to study aerosol-cloud interactions within this region. However, during the course of the individual ORACLES campaigns, the accuracy of the cloud microphysical observations were uncertain due to the mounting location of instruments with respect to the aircraft wing. To address these concerns, an additional wing-mounted pylon design was created and was installed moving the instruments ahead of the leading edge of the aircraft wing in order to sample freestream conditions for ORACLES-2017 and ORACLES-2018. To study the impact of mounting location on cloud microphysical observations taken during ORACLES, a computational fluid dynamical analysis of the NASA P-3 Orion with both pylon designs is performed. Utilizing the OpenFOAM software package, a Eulerian-Lagrangian framework is utilized to simulate compressible flow with particle tracking around the aircraft, mounting locations, and instrumentation. Simulations of the predominant ORACLES vertical cloud sampling profiles, known as sawtooths, and multiple environmental factors are considered. Within the simulated Cloud Droplet Probe sample volume, the departure of the velocity field from freestream conditions was found to vary by up to twelve percent during sawtooth maneuvers for the NASA P-3 original pylon design. While the new pylon design did not achieve freestream conditions, it did minimize this distortion in flow caused by the sawtooth maneuvers, with a five percent difference in the departure of the velocity field from freestream between ascent and descent sawtooth profiles. Overall, the original NASA P-3 pylon design observed the closest velocities to freestream conditions across all simulations

A method for enhancing the stability and robustness of explicit schemes in astrophysical fluid dynamics

A method for enhancing the stability and robustness of explicit schemes in computational fluid dynamics is presented. The method is based in reformulating explicit schemes in matrix form, which cane modified gradually into semi or strongly-implicit schemes. From the point of view of matrix-algebra, explicit numerical methods are special cases in which the global matrix of coefficients is reduced to the identity matrix $I$. This extreme simplification leads to severer stability range, hence of their robustness. In this paper it is shown that a condition, which is similar to the Courant-Friedrich-Levy (CFL) condition can be obtained from the stability requirement of inversion of the coefficient matrix. This condition is shown to be relax-able, and that a class of methods that range from explicit to strongly implicit methods can be constructed, whose degree of implicitness depends on the number of coefficients used in constructing the corresponding coefficient-matrices. Special attention is given to a simple and tractable semi-explicit method, which is obtained by modifying the coefficient matrix from the identity matrix $I$ into a diagonal-matrix $D$. This method is shown to be stable, robust and it can be applied to search for stationary solutions using large CFL-numbers, though it converges slower than its implicit counterpart. Moreover, the method can be applied to follow the evolution of strongly time-dependent flows, though it is not as efficient as normal explicit methods. In addition, we find that the residual smoothing method accelerates convergene toward steady state solutions considerably and improves the efficiency of the solution procedure.Comment: 33 pages, 15 figure

Simulation of flows with violent free surface motion and moving objects using unstructured grids

This is the peer reviewed version of the following article: [Löhner, R. , Yang, C. and Oñate, E. (2007), Simulation of flows with violent free surface motion and moving objects using unstructured grids. Int. J. Numer. Meth. Fluids, 53: 1315-1338. doi:10.1002/fld.1244], which has been published in final form at https://doi.org/10.1002/fld.1244. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.A volume of fluid (VOF) technique has been developed and coupled with an incompressible Euler/Navier–Stokes solver operating on adaptive, unstructured grids to simulate the interactions of extreme waves and three-dimensional structures. The present implementation follows the classic VOF implementation for the liquid–gas system, considering only the liquid phase. Extrapolation algorithms are used to obtain velocities and pressure in the gas region near the free surface. The VOF technique is validated against the classic dam-break problem, as well as series of 2D sloshing experiments and results from SPH calculations. These and a series of other examples demonstrate that the ability of the present approach to simulate violent free surface flows with strong nonlinear behaviour.Peer ReviewedPostprint (author's final draft