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Using CFD Boundary Conditions Analysis to Optimize System Design

Key Takeaways

  • The CFD analysis of boundary conditions.

  • Properly setting the boundary conditions is key to accurate simulation results. 

  • CFD analysis of external aerodynamics vs. hydrodynamic boundary conditions.

Model of a ship for CFD boundary conditions analysis

A CFD boundary conditions model for hydrodynamics analysis

The existence of boundaries is a truism that adequately describes the physical world. And, studying how the physical states of matter interact at these boundaries is critical for understanding the world and everything in it. CFD boundary conditions (BCs) are no exception; CFD boundary conditions analysis enables us to design and build systems to optimally operate in the physical world. In this article, we will explore what CFD boundary conditions analysis is, why it is important, and discuss the analysis of external aerodynamics vs. hydrodynamic BCs.

What Is CFD Boundary Conditions Analysis?

The term CFD refers to the use of numerical methods or computation to analyze fluid flow into, out of, through, and around systems. For system design, understanding flows occurring at or near internal and/or external surfaces or boundaries provides invaluable insight that can be used to improve the design and operation of the system. This analysis can be performed using any of the conditions listed below:

  1. Dirichlet BC
    The Dirichlet BC is referred to as the first-type BC in mathematics. This condition typically specifies a value that the solution variable must assume at certain points along the boundary. The solution format may be an ordinary differential equation (ODE), partial differential equation (PDE), or other solvable mathematical equation. Many essential fluid dynamics equations are PDEs.

  2. Neuman BC
    Also known as the second-type BC,  the Neuman BC specifies the solution to the derivative of the variable as opposed to the variable itself.

  3. Robin BC
    The Robin BC is a combination of Neuman and Dirichlet conditions.

Additional BC problem types—for example, mixed and Cauchy—also exist. However, for CFD, the ones above (primary Dirichlet and Neuman) are most often used. For fluid dynamics, analysis typically involves initial condition constraints and constraints on the variable(s) being evaluated.

Common CFD boundary conditions are:

  • Physical BCs
  • Pressure conditions
  • Cyclic conditions
  • Symmetry conditions
  • Intake conditions
  • Output or exit conditions

Thus, we can define CFD boundary conditions analysis as the study of the fluid flow for these BCs using a BC type. The obvious next questions are:

  1. Why is this type of analysis important for aerodynamics or fluid flow systems? For example, when defining the laminar and turbulent flow regions across a surface.
  2. How does a design benefit from its utilization?

The Benefits of Boundary Conditions Analysis 

The Navier-Stokes equation, shown below, is one of the most implemented equations in fluid dynamics.

 The Navier-Stokes equation

Navier-Stokes equation

This PDE is used to solve boundary value problems related to fluid flow, which is applicable to aerodynamics and hydrodynamic BCs. In both cases, the objective is to understand how systems will be affected by variations in the flow of fluid (this includes air) at system boundaries, such as across external surfaces. There are several benefits to this type of analysis:

  • Aids in the selection of the best materials for your design.
  • Helps to define operational capabilities and limitations.
  • Provides data for risk analysis and contingency plan protocols. 
  • Allows for safe operational testing via simulation prior to physical build.  
  • Is significantly more cost-effective than trial and error evaluation.  

These benefits, if leveraged appropriately, can improve development time, lower costs, and perhaps even prevent catastrophe once the system is deployed. 

CFD Analysis of External Aerodynamics vs. Hydrodynamic Boundary Conditions

The benefits of CFD boundary conditions analysis are applicable to any system where fluid-fluid, fluid-gas, fluid-solid, and gas-solid regions exist. Examples include aerodynamics and hydrodynamic boundaries, as shown in the figures below.  

CFD analysis of boundary conditions for aerodynamics

Aerodynamics CFD boundary conditions analysis with Omnis

CFD Analysis for Hydrodynamic Boundaries

Omnis CFD analysis of hydrodynamic boundary conditions

Performing CFD boundary conditions analysis, although significantly beneficial, does involve complex analysis that necessitates the utilization of an advanced CFD solver like Omnis. This tool employs proven mathematical solution methods, such as FEA, with advanced system and border model generation to help you perform high-level evaluations of aerodynamic and hydrodynamic systems.

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