Uniform Fixed Value - Boundary Condition

Uniform Fixed Value - Boundary Condition   Description

Uniform Fixed Value - is a derivative of Fixed Value boundary condition. It sets the field to the uniform value (throughout the entire boundary) as a constant value or as a function of time. The Uniform Fixed Value can be applied to all variable types (scalars, vectors).

Mathematically, the Uniform Fixed Value can be described as

$\phi|_{\partial \Omega} (x,y,z,t) = f(t)$

Uniform Fixed Value - Boundary Condition   Understanding Uniform Fixed Value

When the Uniform Fixed Value is applied, it enforces a specified value at the boundary. This value is uniformly applied to all mesh faces on the boundary, ensuring that it remains constant along the boundary. In other words, the specified field, such as temperature or velocity, remains constant across the boundary. This boundary condition is similar to Fixed Value. However, the key distinction is its ability to define time-dependent values at the boundary.

Uniform Fixed Value - Graphical Representation

Uniform Fixed Value Boundary Condition Animation

The animation illustrates the temperature distribution in a long channel. The temperature at the left boundary varies over time between 300 K and 500 K, while the right boundary maintains a constant temperature of 300 K. The Uniform Fixed Value enables the definition of time-dependent profiles for various dependent variables, such as pressure, velocity, temperature, and turbulent properties.

Uniform Fixed Value - Boundary Condition   Application & Physical Interpretation

The Uniform Fixed Value is one of the most commonly used boundary conditions. It consistently fixes the value of a specific variable at the domain boundary. However, its physical meaning varies depending on the problem being considered and the solver applied. Below are a few examples demonstrating how this boundary condition can be used and how to correctly interpret its meaning.

Uniform Fixed Value in Aerodynamics

Example applications: car, aircraft aerodynamics, wind tunnel experiment

This problem can be solved by using pimpleFoam (solver). This solver has two basic independent variables: pressure and velocity. Additionally, one can define turbulence-related variables. The Uniform Fixed Value can be applied at the inlet of the domain to represent changing in time value of the incoming air (therefore the speed of the car or aircraft).

Example Boundary Conditions set for Aerodynamics applications
PhysicsVelocityPressure

Velocity Inlet

Uniform Fixed Value

Zero Gradient

Pressure Outlet

Pressure Inlet Outlet Velocity

Uniform Fixed Value

Uniform Fixed Value in Heat Transfer

Example applications: room ventilation, room heating, electronic cooling, atmospheric flows

When the flow is driven by buoyancy force, which arises from the density variation, the buoyantPimpleFoam (solver) can be used for such problem. The Uniform Fixed Value defines the temperature at a given boundary. This may represent, for example, the temperature of a hot radiator or the inlet temperature of air in a ventilation system.

Example Boundary Conditions set for Heat Transfer applications
PhysicsVelocityPressureThermal T

Water inlet

Fixed Flux Pressure

Surface Normal Fixed Value

Uniform Fixed Value

Uniform Fixed Value in Species Transport applications

Example applications: industrial furnaces, jet engines, chemical process industry (chemically reacting foams)

The reactingFoam (solver) can be applied for chemically reacting flows, that involve heat transfer and chemical reactions with the combustion. In this application, the Uniform Fixed Value boundary condition defines, for example, the gas composition at a given boundary.

Example Boundary Conditions set for Species Transport applications
PhysicsVelocityPressureSpecies

Air Inlet

Zero Gradient

Free Stream Velocity

Uniform Fixed Value

Uniform Fixed Value - Boundary Condition   How to apply Uniform Fixed Value in SimFlow

Constant value Uniform Fixed Value in SimFlow

When the constant value of variable is assigned using Uniform Fixed Value, the definition is similar to Fixed Value.

Figure 1. Constant Uniform Fixed Value Boundary Condition SimFlow

Time-dependent Uniform Fixed Value in SimFlow

The variable can be defined also as time-dependent. In SimFlow the convenient table with graphical representation is used to define the boundary condition.

Figure 2. Time-dependent Uniform Fixed Value Boundary Condition SimFlow

Uniform Fixed Value - Boundary Condition   Uniform Fixed Value - Alternatives

In this section, we propose boundary conditions that are alternative to Uniform Fixed Value. While they may fulfill similar purposes, they might be better suited for a specific application and provide a better approximation of physical world conditions.

Uniform Fixed Value - Alternatives
Boundary ConditionDescription

Fixed Value

fixed value on the patch