Adaptive Grid Refinement for Two-Phase Offshore Applications

2018 ◽  
Author(s):  
Peter van der Plas ◽  
Arthur E. P. Veldman ◽  
Henk Seubers ◽  
Joop Helder ◽  
Ka-Wing Lam
Keyword(s):  
Cfd Simulation ◽  
Simulation Method ◽  
Breaking Waves ◽  
Absorbing Boundary Conditions ◽  
Grid Refinement ◽  
Two Phase ◽  
Absorbing Boundary ◽  
Irregular Wave ◽  
Area Of Interest ◽  
Wide Range

In the past, the CFD simulation method ComFLOW has been successfully applied in a wide range of offshore applications, involving wave simulations and impact calculations. In many of these calculations the area of interest comprises a small part of the domain and remains fixed in time, which allows for efficient grid refinement by means of grid stretching or static local refinement. However, when trying to accurately resolve the surface dynamics and kinematics of irregular and breaking waves, the resolution requirements are strongly time-dependent and difficult to predict in advance. Efficient grids can only be obtained by means of time-adaptive refinement. A Cartesian block-based refinement approach is followed which allows for efficient grid adaptation, with moderate overhead. An array-based data structure is employed which exploits the semi-structured nature of the Cartesian block grid. Currently we are testing the method with the simulation of lifeboat drops in regular and irregular wave conditions. This poses several challenges such as accurately imposing the incoming waves and modifying the absorbing boundary conditions to support two-phase flow. To reduce the wall-clock time, the simulation method has been parallelized.

CFD Simulations of a Semi-Submersible With Absorbing Boundary Conditions

2009 ◽  
Author(s):  
Peter R. Wellens ◽  
Roel Luppes ◽  
Arthur E. P. Veldman ◽  
Mart J. A. Borsboom
Keyword(s):  
Boundary Conditions ◽  
Stokes Equations ◽  
Computing Time ◽  
Height Function ◽  
Absorbing Boundary Conditions ◽  
Navier Stokes ◽  
Two Phase ◽  
Absorbing Boundary ◽  
Wide Range ◽  
Grid Points

The CFD tool ComFLOW is suitable for simulations of two-phase flows in offshore applications. ComFLOW solves the Navier-Stokes equations in both water and (compressible) air. The water surface is advected through a Volume-of-Fluid method, with a height-function approach for improved accuracy. By employing Absorbing Boundary Conditions (ABC), boundaries can be located relatively close to an object, without influencing outgoing waves or generating numerical reflections that affect the waves inside the flow domain. Traditionally, boundaries are located far from the obstacle to avoid reflections; even when numerical damping zones are used. Hence, with the ABC approach less grid points are required for the same accuracy, which reduces the computing time considerably. Simulations of a semi-submersible model are compared to measurements. The overall agreement is reasonably good, for a wide range of wave conditions. The ABC performs well; numerical reflections are almost absent. Moreover, computing times reduce with a factor four compared to damping zone techniques.

Stability of wide‐angle absorbing boundary conditions for the wave equation

Geophysics ◽  
1989 ◽  
Vol 54 (9) ◽  
pp. 1153-1163 ◽  
Author(s):  
R. A. Renaut ◽  
J. Petersen
Keyword(s):  
Wave Equation ◽  
Boundary Conditions ◽  
Least Squares ◽  
Absorbing Boundary Conditions ◽  
Computational Domain ◽  
Wide Angle ◽  
Courant Number ◽  
Absorbing Boundary ◽  
Wide Range ◽  
Artificial Boundaries

Numerical solution of the two‐dimensional wave equation requires mapping from a physical domain without boundaries to a computational domain with artificial boundaries. For realistic solutions, the artificial boundaries should cause waves to pass directly through and thus mimic total absorption of energy. An artificial boundary which propagates waves in one direction only is derived from approximations to the one‐way wave equation and is commonly called an absorbing boundary. Here we investigate order 2 absorbing boundary conditions which include the standard paraxial approximation. Absorption properties are compared analytically and numerically. Our numerical results confirm that the [Formula: see text] or Chebychev‐Padé approximations are best for wide‐angle absorption and that the Chebychev or least‐squares approximations are best for uniform absorption over a wide range of incident angles. Our results also demonstrate, however, that the boundary conditions are stable for varying ranges of Courant number (ratio of time step to grid size). We prove that there is a stability barrier on the Courant number specified by the coefficients of the boundary conditions. Thus, proving stability of the interior scheme is not sufficient. Furthermore, waves may radiate spontaneously from the boundary, causing instability, even if the stability bound on the Courant number is satisfied. Consequently, the Chebychev and least‐squares conditions may be preferred for wide‐angle absorption also.

A Direct Numerical Simulation of the Unsteady Development of a Deformable Rising Bubble in a Quiescent Liquid

2008 ◽  
Author(s):  
Alpana Agarwal ◽  
C. F. Tai ◽  
J. N. Chung
Keyword(s):  
Weber Number ◽  
Bubble Dynamics ◽  
Stokes Equations ◽  
Control Volume ◽  
Density Ratio ◽  
Simulation Method ◽  
Liquid Pool ◽  
Accurate Information ◽  
Two Phase ◽  
Wide Range

An accurate finite-volume based numerical method for the simulation of an isothermal two-phase flow, consisting of a deformable bubble rising in a quiescent, unbounded liquid, is presented. This direct simulation method is built on a sharp interface concept and developed on an Eulerian, Cartesian fixed grid with a cut-cell scheme and marker points to track the moving interface. The unsteady Navier-Stokes equations in both liquid and gas phases are solved separately. The mass continuity and momentum flux conditions are explicitly matched at the true phase boundary to determine the interface shape and movement of the bubble. The highlights of this method are that it utilizes a combined Eulerian-Lagrangian approach, and is capable of treating the interface as a sharp discontinuity. A fixed underlying grid is used to represent the control volume. The interface, however, is denoted by a separate set of marker particles which move along with the interface. A quadratic curve fitting algorithm with marker points is used to yield smooth and accurate information of the interface curvatures. This numerical scheme can handle a wide range of density and viscosity ratios. The bubble is assumed to be spherical and at rest initially, but deforms as it rises through the liquid pool due to buoyancy. Additionally, the flow is assumed to be axisymmetric and incompressible. The bubble deformation and dynamic motion are characterized by the Reynolds number, the Weber number, the density ratio and the viscosity ratio. The effects of these parameters on the translational bubble dynamics and shape are given and the physical mechanisms are explained and discussed. Results for the shape, velocity profile and various forces acting on the bubble are presented here as a function of time until the bubble reaches terminal velocity. The range of Reynolds numbers investigated is 1 < Re < 100, and that of Weber number is 1 < We < 10.

scholarly journals Absorbing boundary conditions for acoustic and elastic wave equations

1977 ◽  
Vol 67 (6) ◽  
pp. 1529-1540 ◽  
Author(s):  
Robert Clayton ◽  
Björn Engquist
Keyword(s):  
Boundary Conditions ◽  
Elastic Wave ◽  
Wave Equations ◽  
Absorbing Boundary Conditions ◽  
Limited Area ◽  
Absorbing Boundary ◽  
Physical Behavior ◽  
Wide Range ◽  
Elastic Wave Equations ◽  
Numerical Wave

abstract Boundary conditions are derived for numerical wave simulation that minimize artificial reflections from the edges of the domain of computation. In this way acoustic and elastic wave propagation in a limited area can be efficiently used to describe physical behavior in an unbounded domain. The boundary conditions are based on paraxial approximations of the scalar and elastic wave equations. They are computationally inexpensive and simple to apply, and they reduce reflections over a wide range of incident angles.

scholarly journals Simulation and Experimental Research on a Gas Liquid Separator with Rotary Impeller

2020 ◽  
Vol 316 ◽  
pp. 03001
Author(s):  
Baihui Wang ◽  
Yingbin Li ◽  
Mingjun Deng ◽  
Baolu Shi ◽  
Wenjin Shang ◽  
...  
Keyword(s):  
Life Support ◽  
Environmental Control ◽  
Simulation Method ◽  
Two Phase ◽  
Design And Optimization ◽  
Manned Spacecraft ◽  
Vof Model ◽  
Wide Range ◽  
Two Parameters ◽  
Liquid Separator

Gas-liquid separation technology under microgravity is the basis for various gas and liquid treatments on a manned spacecraft, which has a wide range of applications in Environmental Control and Life Support System. Dynamic gas-liquid separator is commonly used for the separation of gas-liquid two-phase flow, which has two essential performance parameters called liquid outlet pressure and separating efficiency. Predicting these two parameters accurately under a specific structure has guiding significance for design and application of the dynamic gas-liquid separator. In this study, CFD simulations were conducted using the Volume of Fluid (VOF) model at steady state conditions. In addition, experiments were designed to verify the accuracy of numerical results. Finally, the performance of the separator under microgravity was predicted. It is showed that the simulation method can be utilized to determine the transport performance of dynamic gas-liquid separator, which has significant value in design and optimization.

Calibration of the CFD code based on testing of a standard AGARD-B model for determination of aerodynamic characteristics

2020 ◽  
pp. 095441002096685
Author(s):  
Čedomir Kostić ◽  
Aleksandar Bengin ◽  
Boško Rašuo ◽  
Dijana Damljanović
Keyword(s):  
Cfd Simulation ◽  
Simulation Method ◽  
Aerodynamic Characteristics ◽  
Advisory Group ◽  
Test Model ◽  
Computational Domain ◽  
Technical Institute ◽  
Strong Point ◽  
Wide Range

The goal of this work is to build a unique numerical method to obtain the basic aerodynamic characteristics of the aircraft and to enable a wide application of the method in the analysis of some aerodynamic characteristics of the aircraft, without use of empirical methods. The Computational fluid dynamics (CFD) simulation method was being calibrated based on test results of the standard AGARD-B (Advisory Group for Aerospace Research and Development) test model, which were obtained in the T-38 trisonic wind tunnel facility of the Military Technical Institute (VTI) in Belgrade, Serbia.The paper presents the CFD simulation through a description of the conditions of flow, geometry of the computer domain, grid density and mesh strategy, boundary conditions, initial strategy and turbulence model. The CFD simulation was carried out for flow cases with similarity parameters M = 0.6, M = 0.85 and M = 1.6 and Re = from 7.7(x106) to 9.9(x106) . The results of calculations were compared with the appropriate experimental ones and presented in the form of comparative diagrams for the drag, lift and pitching moment coefficients. The results of investigation presented in divergence diagrams show very good agreement between numerical and experimental ones. Simulated flows are illustrated by the distribution of pressure and velocity components on the surface of the tested model and the computational domain. This CFD simulation will be applied to other similar aerodynamic designs for a wide range angles of attack and Mach numbers and can be a strong point for the development of different aerodynamic designs.The ultimate aim of the work is to use the previous calibrated CFD simulation method as the basis for future determination of the aerodynamic characteristics of aircraft in non-stationary flight modes, caused by motion of the aircraft and/or by changing the free-velocity vector.

Generating and Absorbing Boundary Conditions for Free-Surface Flow Simulations in Offshore Applications

2015 ◽  
Author(s):  
Bülent Düz ◽  
René H. M. Huijsmans ◽  
Peter R. Wellens ◽  
Mart J. A. Borsboom ◽  
Arthur E. P. Veldman ◽  
...  
Keyword(s):  
Boundary Conditions ◽  
Cfd Simulation ◽  
Free Surface Flow ◽  
Surface Flow ◽  
Absorbing Boundary Conditions ◽  
Computational Domain ◽  
Wave Impact ◽  
Absorbing Boundary ◽  
Floating Structures ◽  
Hydrodynamic Wave

Numerical simulations of wave phenomena necessarily have to be carried out in a limited computational domain. This implies that incoming waves should be prescribed properly, and the outgoing waves should leave the domain without causing reflections. In this paper we will present an enhanced type of such generating and absorbing boundary conditions (GABC). The new approach is applied in studies of extreme hydrodynamic wave impact on rigid and floating structures in offshore and coastal engineering, for which the VOF-based CFD simulation tool ComFLOW has been developed.

Adaptive Grid Refinement for Free-Surface Flow Simulations in Offshore Applications

2015 ◽  
Author(s):  
Peter van der Plas ◽  
Arthur E. P. Veldman ◽  
Henri J. L. van der Heiden ◽  
Roel Luppes
Keyword(s):  
Free Surface ◽  
Moving Objects ◽  
Cfd Simulation ◽  
Free Surface Flow ◽  
Surface Flow ◽  
Adaptive Refinement ◽  
Efficiency Gain ◽  
Grid Refinement ◽  
Wave Impact ◽  
Area Of Interest

In many (wave) impact problems the area of interest does not change in time and is readily pointed out by hand, allowing for a one-time design of an efficient computational grid. However, for a large number of other applications, e.g. involving violent free-surface motion or moving objects, a reasonable efficiency gain can only be obtained by means of time-adaptive refinement of the grid. In previous studies a fixed, block-based Cartesian local grid refinement method was developed and implemented in the CFD simulation tool ComFLOW [1], a VOF-based Navier-Stokes solver on Cartesian grids with cut-cell discretization of the geometry. Special attention was paid to the interface discretization in cut-cells as well as the fluid displacement algorithm across refinement boundaries. The method was successfully applied to a range of offshore applications, including for example wave-impact on a semi-submersible (figure 1)and sloshing in a moonpool. In the present paper we present the first results of our attempts to extend the method to support adaptive refinement.

CFD Simulation of a Low NOx Oil Fired Boiler

2005 ◽  
Author(s):  
C. Hochenauer ◽  
G. Brandstetter
Keyword(s):  
Cfd Simulation ◽  
Nox Reduction ◽  
Fuel Oil ◽  
Detailed Chemical Kinetics ◽  
Heavy Fuel Oil ◽  
Two Phase ◽  
Boiler Load ◽  
Excess Air ◽  
Wide Range ◽  
The Impact

This paper compares the results of an advanced CFD calculation with measurements of a heavy fuel oil fired low NOx boiler. First, a state of the art boiler was investigated and the impact of boiler load and excess air on the NOx emissions was measured. In a second test run a staged combustion technology was integrated using the over fire air concept. The over fire technology is well known and well tested in coal fired boilers. In this pilot boiler it was shown that the over fire air technology could be used for oil fired boilers, too — leading to an enormous NOx reduction without any increase in CO and soot emissions. It was shown that the influence of boiler load, excess air and over fire air on the NOx and CO emissions can be predicted very well in the CFD calculation. Detailed numerical investigations showed that two-phase effects, a good turbulence model, gas and soot radiation and a detailed chemical kinetics mechanism are a must when modeling (staged) heavy oil combustion. The results of the CFD calculation showed an excellent agreement with the measurements over a very wide range of boiler settings and load factors although NOx is extremely difficult to predict.

OPTIMAL CONVERGENCE OF NON-OVERLAPPING SCHWARZ METHODS FOR THE HELMHOLTZ EQUATION

2005 ◽  
Vol 13 (03) ◽  
pp. 525-545 ◽  
Author(s):  
FRÉDÉRIC MAGOULÈS ◽  
ROMAN PUTANOWICZ
Keyword(s):  
Boundary Conditions ◽  
Helmholtz Equation ◽  
Absorbing Boundary Conditions ◽  
Dirichlet Boundary Conditions ◽  
Absorbing Boundary ◽  
Comparative Performance ◽  
Schwarz Method ◽  
Wide Range ◽  
Schwarz Algorithm ◽  
Overlapping Schwarz

The non-overlapping Schwarz method with absorbing boundary conditions instead of the Dirichlet boundary conditions is an efficient variant of the overlapping Schwarz method for the Helmholtz equation. These absorbing boundary conditions defined on the interface between the subdomains are the key ingredients to obtain a fast convergence of the iterative Schwarz algorithm. In a one-way subdomains splitting, non-local optimal absorbing boundary conditions can be obtained and leads to the convergence of the Schwarz algorithm in a number of iterations equal to the number of subdomains minus one. This paper investigates different local approximations of these optimal absorbing boundary conditions for finite element computations in acoustics. Different approaches are presented both in the continuous and in the discrete analysis, including high-order optimized continuous absorbing boundary conditions, and discrete absorbing boundary conditions based on algebraic approximation. A wide range of new numerical experiments performed on unbounded acoustics problems demonstrate the comparative performance and the robustness of the proposed methods on general unstructured mesh partitioning.

Sign in / Sign up

Export Citation Format

Share Document