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?...

Part 1:  General Approach

1.0 Introduction

Just fresh out of college, and the boss assigned your first project for computational fluid dynamics (CFD).  Giddy with excitement, you can’t wait to begin the challenge.  You sit down at your computer, stretch your fingers, start up the CFD software . . . and freeze like a deer in headlights.  How to begin?  What to do first?  What is your plan?  Today we discuss the general workflow for a CFD project and highlight some general modeling advice. 

2.0 General Approach

Table 2‑1 shows the general workflow for a typical CFD project.  Notice that the initial setup and diagnostic runs form a major portion of the project costs.  Don’t undervalue this initial phase.  An unproven CFD simulation has no value.  Invest the time and budget to perform mesh independence studies and ensure you can trust your simulation.

This entire process can be organized and documented into a CFD setup spreadsheet.  These spreadsheets capture critical inputs for the CFD simulation like:

• Reynolds number
• Input velocity
• Starting cell size
• Cell size at boundary layer
• Time step

Capturing this information into a single spreadsheet helps keep you organized and documents the CFD process.  It is well worth the development effort.

3.0 Domain Size

The next step requires you to select the domain size.  Begin with the size of your body and position your domain boundaries some distance away from the body.  But how far away?

3.1 Boundaries Far Enough Away for Predictable Flow

Remember that you dictate flow at boundaries.  You force the computer to match the velocity magnitude, direction, pressure, and other boundary conditions that you set at the boundaries.  Make sure you pick a position that you can reliably justify as free of local flow conditions around your body.

3.2 MUCH Farther than Your Flow Patterns Show

When sizing the domain, try a few different positions for your boundaries.  Remember that the computer will always match the conditions at the boundaries to whatever boundary conditions you dictated.  Don’t look at variation in flow patterns near the boundary.  Those are governed by your boundary conditions.  Instead, examine changes in pressure on your body.  As you move the boundary positions, the pressure patterns on your boundary will adjust to reflect reduced influence on your body.  You seek a boundary position with further movement no longer changes the pressure pattern on your body.

3.3 Sizing for Ship Free Surface Flow

Use the following general guidelines to place your boundary positions for free surface flow.  All boundary positions are relative to the nearest point on your body.  For example, upstream boundary is measured from the forward end of the body.  Downstream boundary is measured from the downstream end of the body.

• Upstream = 2.0 – 3.0 L
  • Downstream = 6.0 – 8.0 L
  • Sides = 2.0 – 3.0 L
  • Top = 2.0 – 3.0 D
  • Bottom = 3.0 – 4.0 D (look at your wave numbers for this one)

L = ship length

D = ship depth

Bottom should be deep enough to ensure any waves generated do not suffer from shallow water effects.  This is typically achieved by setting the bottom depth as a minimum of half of the longest wave length.

4.0 Boundary Conditions

There are four major types of boundary conditions:

• Wall
• Velocity
• Pressure
• Symmetry

Apply wall boundary conditions at your body.  Most wall conditions also include skin friction effects.

Always include at least one velocity and one pressure boundary condition in each simulation.  Velocity and pressure and both variables included in the momentum equation.  And you need to have at least one boundary condition specified for every variable in your simulation.  This prevents irregular solutions to the underlying differential equations.  Remember, the computer just solves the math.  It doesn’t care if the math creates realistic solutions.  That is why you need the boundary conditions for every variable in your simulation.

5.0 Conclusion

In summary, the CFD workflow divides into three major phases:  geometry preparation, diagnostic runs, and production work.  Basic modeling advice included some simple suggestions for positioning boundary positions and setting boundary conditions.  Be sure to always set one pressure and one velocity boundary condition in each simulation.  In general, remember that CFD does not begin with the software.  It begins with you, the CFD engineer.

6.0 References