Flow Boundary Condition types

A Flow Boundary Condition constrains the motion of the fluid through an opening in the model's geometry. You can use a Flow Boundary Condition to model fans, pumps, valves, vents, and other such types of devices.

You select different ways to model fluid flow into, out of, or within the fluid model. The type you select determines the direction and nature of the flow in relation to the fluid domain. The type does not determine mechanical characteristics of the device or the kind of fluid it moves. Different Flow Boundary Condition options are available depending on the type you select.

Inlet Flow and Outlet Flow types

Use the Inlet Flow or Outlet Flow types to model fluid entering or leaving the fluid domain at a known flow rate and location, or to set up a fluid body simulating static air through which the model moves. For example, flow through an Inlet Flow type of the Flow Boundary Condition simulation object always moves into the fluid domain from outside.

Opening type

Use the Opening type to model an external opening that allows fluid to flow into or out of the 3D flow mesh domain.

Any opening can act as either an inlet or an outlet, depending on the flow conditions at each iteration or timestep. For example, use an opening to model buoyancy driven flow around a heated body in a large room filled with still air, or to model flow through a pipe originating from a stagnant upstream reservoir.

When the fluid enters the domain via the opening, the specified pressure at the boundary is the total pressure, which is equal to the static pressure plus the dynamic pressure. When the fluid leaves the domain via the opening, the specified pressure at the boundary is only the static pressure.

The specified opening pressure value can be an absolute or a relative pressure.

Internal Fan type

Use the Internal Fan type to model fluid movement at a known location within the fluid body.

When you define the internal fan on the disjoint fluid mesh faces, select either the primary or the secondary region in theDisjoint Fluid Mesh Pairing simulation object. When the area size of the two sides of the interface differ, select the smaller side in the Internal Fan type, so that the flow is not stopped by a wall. For an interface with equal area size on both sides, but different element sizes, select the side with the finer mesh to improve accuracy.

Recirculation Loop type

Use the Recirculation Loop type to model a device or system that extracts fluid from the 3D flow domain and injects the same fluid back into the domain. With the Recirculation Loop type, you define two separate openings: a Flow Extract and a Flow Return. If the sizes of the two openings are different then the flow speeds at the two openings will differ.

• The analysis includes only upstream effects at the Flow Extract opening.

• The analysis includes only downstream effects at the Flow Return opening.

Convective Outflow type

Use the Convective Outflow type to model fluid exiting the fluid domain without specifying the pressure.

The convective outflow boundary condition lets the flow field exit and enter the flow domain on each element of the boundary as necessary, conserving the mass. A convective outflow boundary condition is recommended for transient, external flow applications. For example, this boundary condition allows you to reduce the flow domain downwind of the cylinder in the following vortex shedding example.

Warning:

You cannot specify a Convective Outflow in a fluid enclosure that already has an Opening type.

Static Pressure type

Use the Static Pressure type to model pressure-driven flows with specified static pressure.

When you apply a Static Pressure type of flow boundary condition on a region, the flow at the region acts either as an inlet or an outlet depending on the physics of the model. For example, to model pulsatile flows, such as the blood flow in an artery, you can specify the known static pressure as a function of time at one end of the artery.

When the fluid enters or leaves the fluid domain, the specified pressure at the boundary is the static pressure.

The specified static pressure value can be an absolute or a relative pressure.

Bursting Membrane and Flap types

Use the Bursting Membrane type to model an embedded flow surface that bursts when one of the following operation criteria is reached:

• The maximum static pressure difference between the two sides of the membrane reaches a specified value on the high pressure side. You specify the high pressure side to be the top side, the bottom side, or both sides. In the last case, the membrane bursts when the maximum static pressure difference between the two sides of the membrane reaches the specified value on either of the two sides.Note: The top side of a shell has the normal pointing outward.

• The solution time reaches the specified bursting time.

You specify which of these two operation criteria the parallel flow solver uses to model bursting membranes.

Static pressure results just before and after the membrane bursts

The Flap type is similar to the Bursting Membrane type. Instead of modeling the bursting membrane, it models the opening and closing of a flap. A closed flap is represented by an embedded flow surface; while, an open flap is represented by an opening where the fluid flows freely from one side to the other in the fluid domain. You must specify the initial state of the flap: open or closed in the Initial State box.

For the opening and closing of the flap, you specify one of the following operation criteria:

Pressure Difference

Opens the flap when the maximum static pressure difference between the two sides of the selected embedded surface reaches the value you specify in the Differential Pressure to Open box and closes the flap when the maximum dynamic pressure at the selected region reaches the value you specified in the Dynamic Pressure to Close box. You must also specify which side is the high pressure side: top or bottom.

Time

Opens and closes the flap when the solution time reaches the times you specify in the Opening Time and Closing Time boxes, respectively.

For both Bursting Membrane and Flap types, when closed, the flow surface is assumed to be a no slip wall with smooth friction.

Warning:

You cannot define bursting membranes and flaps on a disjoint fluid mesh surface. The matching faces on both sides must have matching non-duplicate nodes. You can ensure node matching using mesh mating conditions.

Note:

The fractional step solver scheme does not support Bursting Membrane and Flap types.

How do I

Create a Flow Boundary Condition

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Modeling fan swirl

Working with fan curves

Modeling heat loss or gain in a recirculation loop

Flow boundary condition modeling

External Conditions

Fan Speed Controller

Inputs to expressions

Mesh mating conditions

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Auto-generated expressions

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