Fluid Mechanics

2020-Jan-06

Guts of CFD: Multiphase Modeling

In computational fluid dynamics (CFD), we often need to model scenarios that involve more than one fluid. Volume of fluid modeling (VOF) expands the capabilities of CFD to allow limitless combinations of different fluids. The world of VOF encompasses everything from droplets of diesel spraying in cylinder all the way up to tsunami waves crashing against the city of Tokyo. How does VOF achieve this, and what are the implications for CFD modeling?
2019-Dec-09

Practical CFD Modeling: Mesh Deformation

Mesh deformation is incredibly frustrating, complicated, unstable . . . and unavoidable if you want to incorporate body motions into CFD. Modeling body motion demands mesh deformation, changing the mesh on the fly, while using it to solve transport equations. As you might expect, that brings a host of new challenges. This reviews several new strategies that the CFD engineers needs to consider.
2019-Nov-18

Practical CFD Modeling: Time Variation

When we add the time domain, simulations change from modeling steady scenarios to unsteady, where boundary conditions change over time. Beyond the physics, modeling unsteady flow requires a few changes to the CFD solver. Inner iterations, timestep, Courant Number, and data management all enter into the strategy for the CFD engineer. Today we discuss each of these.
2019-Nov-04

Practical CFD Modeling: Volume of Fluid Modeling

Computational Fluid Dynamics (CFD) can model multiple fluids with the volume of fluid method. (VOF) The volume of fluid method opens new horizons for advanced modeling, which requires additional planning from the CFD engineer. Dive into the boundary conditions, meshing strategy, stability concerns, and more. Discover the world of VOF modeling.
2019-Oct-21

Practical CFD Modeling: Turbulence

Turbulence demands modeling just like any other equation in computational fluid dynamics (CFD). As the CFD engineer, you need to describe boundary conditions for your turbulence equations. This article describes how to define boundary conditions for turbulence and provides typical values for normal simulations.
2019-Oct-07

Guts of CFD: Near Wall Effects

Turbulence does tricky things near walls. Boundary layers and laminar sublayers compact interesting flow patterns into a very small space. Small it may be, but experience proved we cannot ignore it. The boundary layer forms on the body, which is our object of interest, arguably the most critical region. Turbulence is most critical near the wall, and we need to consider near wall effects.
2019-Sep-23

Guts of CFD: Turbulence

How we address turbulence is the defining feature of modern computational fluid dynamics (CFD). No modern computer has the power to directly compute the full details of turbulence (as of 2019). Instead, we make approximations and develop empirical models. What type of approximation, and which models should you select?
2019-Sep-09

Guts of CFD: CFD Linear Solution

The heart of any CFD program is an extremely efficient linear algebra solver. But CFD equations are non-linear. How do we stretch the limits of linear algebra to accommodate non-linear CFD equations? How do we take the mathematics from one cell and apply them to millions of cells?
2019-Aug-19

Practical CFD Modeling: Judging Convergence

CFD convergence is not an exact science. The CFD engineer relies on three tools to judge when a simulation finishes: monitors, flow patterns, and residuals. But none of these tools work 100% of the time. The well-trained engineer understands how to use these tools and how to combine them into a cohesive picture and reliably judge a converged CFD simulation.
2019-Aug-05

Guts of CFD: Interpolation Equations

The core of all calculus problems require us to consider something infinitely small. Ask a computer to ponder the concept of infinity and watch its circuits fry. If we want to solve the equations of computational fluid dynamics (CFD), we need a way to fake calculus. This impacts the stability, the mesh quality, and the ultimate simulation quality. Enter interpolation equations.
2019-Jul-08

Guts of CFD: Transport Equation

What is the utility of a transport equation? What do they achieve? Transport equations form the fundamental language of computational fluid dynamics (CFD). CFD engineers use them to communicate ideas, program CFD software, and diagnose problems with their simulations. But they only work if you understand the language. Today we explain transport equations and the significance of their terms.
2019-Jun-24

Guts of CFD: Navier Stokes Equations

Navier Stokes Equation. Shrouded in mystery and intimidation. Navier Stokes is essential to CFD, and to all fluid mechanics. This equation defines the basic properties of fluid motion. But there is more to gain from understanding the meaning of the equation rather than memorizing its derivation. Today we review Navier Stokes Equation with a focus on the meaning behind the math.
2019-Jun-10

Practical CFD: General Approach

Just fresh out of college, and the boss assigned your first project for computational fluid dynamics (CFD). You are excited. You can’t wait to begin the challenge. You sit down at your computer, start up the CFD software . . . and freeze like a deer in headlights. How to begin? What to do first? Today we discuss the general workflow for a CFD project and highlight some general modeling advice.
2019-May-27

CFD Workflow

What happens behind the curtain when the CFD engineer goes to work? What goes into making a CFD simulation? As a project manager, you need to understand the workflow of a CFD project; this helps you plan the project and track budget expenses. When we understand the workflow, we know the right questions and can anticipate project delays.
2019-May-06

Which CFD?

Is there anything that CFD can’t do? Practically speaking, we can achieve the result, but you may regret paying for the answer. Several CFD projects involve combinations of different CFD methodologies. Combined together, they evolve into a major project risk. Gain some insight about the risk factors for your next CFD project. Plan a strategy to minimize project risks so that you don’t get caught by combining unknown cost increases.
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