Introduction to Fluid Flow and CFD

Fluid Scenario Creation App

Fluid Scenario Creation allows the analysis of both internal and external flows; for example, a fluid flowing inside a pipe or air drag over the body of a moving car. Internal flow occurs when the fluid passes through confined solid boundaries. External flow occurs when the fluid or gas passes over the external surface of an object. Complex models can capture and visualize transient and steady-state behavior of incompressible flow.

The Fluid Model Creation app allows automatic generation of the fluid domain, starting from the CAD geometry of the structure (for example, a car's exhaust manifold). You can create fluid domains with very little user intervention and geometry preparation. A fluid domain represents the physical fluid to be simulated in a flow simulation. Think of the fluid domain as a special kind of part in a fluid-structure simulation model. Similar to a solid structural part, you assign material (section) properties to the fluid domain, which requires meshing before executing the simulation.

Hex-dominant meshing with boundary layers provides easier meshing of the fluid. The Fluid Model Creation app also supports orphan meshes for modeling a fluid.

In a pure flow simulation, for example fluid in a rigid pipe, the fluid boundaries do not deform. You can use a pure flow simulation to see interesting results such as flow separation and reattachment. Net forces on the pipe due to flow are measurable. In a fluid-structure interaction (FSI) simulation, you can analyze the deformation of the structure and its effect on the fluid flow.

Material Properties for Fluids

Simulating a fluid requires defining material properties. These properties enable the solvers to determine how the fluid or gas responds to flow boundary conditions and physical loads. A fluid material must have density and viscosity specified; for example, the density of water is 998.2 kg/m³ and the viscosity is 0.001003 kg/ms. If you use the fluid material in a thermal simulation, for example a conjugate heat transfer co-simulation, you must also define its specific heat capacity.

You use the Material Definition app to create a new material and then use the Material Palette to apply the fluid material to the fluid domain object (in the Fluid Model Creation app). The fluid domain requires an assigned fluid section; the fluid section includes the material definition. See Setup Section for more details.

CFD Basics

In a CFD simulation, the liquid or gas deforms continuously under the application of shear stresses. Fluid viscosity is the key material property that relates shear stress to the rate of deformation. For a Newtonian fluid such as air or water, the shearing stress varies linearly with the rate of shearing strain. A non-Newtonian fluid such as blood or alcohol is distinguishable by how the viscosity changes with the shearing rate. A viscous fluid can exhibit either laminar or turbulent flow. Modeling viscosity is especially important in flows close to a solid boundary, as in an FSI simulation.

For inviscid flows, the effect of viscosity is negligible. Inviscid flow can slip at solid boundaries, where no-penetration conditions apply.

The Reynolds number (Re) is a dimensionless value that gives a measure of the ratio of inertial forces to viscous forces. The Reynolds number quantifies the relative importance of inertial vs. viscous forces for given flow conditions.

Re = \frac{Inertial
		  forces}{Viscous
		  forces} = \frac{ρ V L}{μ}

where

L is the characteristic length scale of the flow, and

V is characteristic velocity.

You can also use Reynolds numbers to characterize different flow regimes, such as laminar or turbulent flow:

Laminar flow occurs at low Reynolds numbers, where viscous forces dominate. This type of flow is smooth, constant fluid motion.
Turbulent flow occurs at high Reynolds numbers (> 200), where inertial forces dominate. This type of flow tends to produce chaotic eddies, vortices, and other flow instabilities.

Laminar flow has the following characteristics:

Smooth motion in layers (laminae).
Time-dependent flows characterized by a single frequency.
Limited or localized mixing and transport.

Turbulent flows have the following characteristics:

Highly random three-dimensional motion.
Exhibit a broad range of length and time scales.
Enhanced mixing and transport processes.

The Fluid Scenario Creation app can simulate incompressible flows in which the velocity field is divergence free. The energy contained in acoustic waves is small relative to the energy transported by advection. For compressible flow, in contrast, the density variations within the flow are not negligible and shock/rarefaction waves might be present.

Mach number is a dimensionless quantity representing the ratio of the speed of an object moving through a fluid and the local speed of sound:

Ma = \frac{Flow
		  speed}{Local speed of
		  sound} = \frac{V}{c}

Mach number varies by the composition of the surrounding fluid and by local conditions, especially temperature and pressure. You can use the Mach number to determine if a flow models an incompressible flow. If the Mach number is less than approximately 0.3 and the flow is (quasi-) steady and isothermal, compressibility effects will be small and the simplified incompressible flow model applies.

Boundary Conditions

Computational fluid dynamics simulations typically require the prescription of multiple variables such as pressure, temperature, and velocity for flow boundary conditions. In practice, the boundary conditions tend to appear together to collectively define a physical behavior; for example, no-slip/no-penetration conditions at a wall. Grouping combinations of boundary conditions that represent an inflow, outflow, or wall behavior eases use. The flow conditions group the momentum (velocity, pressure), thermal (temperature), and turbulence variables together.

The primary flow boundary conditions—velocity inlet and pressure outlet—vary only in their momentum condition. They reflect the turbulence model and thermal parameters defined in the physics action.