Inlet Regions

Inlet regions mimic the behavior of inlets with a superposed impose region and allow spawning particles anywhere within the computational domain.


Particle arrangement that is put into the domain at inlet region position.
The act of introducing particle stencils into the domain.
Spawn interval
The interval at which the stencil is spawned.
An extended region in the direction of inlet region normal where a constant particle velocity equal to inlet velocity is enforced.

Inlet Region Types

Currently there are five types of inlet regions:

Figure 1 shows schematics of inlet regions and example stencils. Figure 2 and Figure 3 show the relationship between .cfg parameters and inlet regions. It is worth noting that the inlet region type does not define the shape of the stencil but rather the shape of the extension.

Figure 1. . Stencils and extensions for different inlet region types. The top row shows the inlet regions at spawn time while bottom row is at a later time before the next spawn time. (a) Rectangular inlet region with complete stencil. (b) Circular inlet region with partial stencil. (c) Custom inlet region. Particles are color coded as brown for spawned particles, magenta for particles affected by the extension and cyan for free particles. Continuous lines show extension borders.

Figure 2. . Rectangular and Rectangular_Arbor inlet region definitions, color coded as Figure 1. Note that extension is half dx larger on each side compared to cfg definition. Since there is not a particle on the center of the stencil, the case shown here either had its inletregion_meshmatch set to true or was provided with an inletregion_stf_file. (left) front view, flowing into page; (right) side view, flowing left to right.

Figure 3. . Circular and Circular_Arbor inlet region definitions, color coded as Figure 1. Note that extension is half dx larger on each side compared to cfg definition. Since the stencil does not fill the region, the case shown here was provided with an inletregion_stf_file. (left) front view, flowing into page; (right) side view, flowing left to right.

There are two methods for defining the inlet region stencil for Rectangular and Circular types, including Arbor (arbitrary orientation) variants. The first method is to ask the inlet region to create the stencil and the second method is to provide a stencil file. While the former method leads to a stencil fitting the inlet region type (Figure 1-a), the stencil shape in the latter method is not restricted by the inlet region type (Figure 1-b).

In contrast to Rectangular and Circular types, Custom type inlet regions always require a stencil file and create a matching extension by creating a cuboid extension per stencil particle (Figure 1-c). As a result, Custom inlet regions are computationally expensive and should be used sparingly.
Note: Only Arbor type inlet regions are able to follow MOVINGWALL motions.
All inlet region types support varying velocity with a tvs file, otherwise constant velocity with ramp from zero and back to zero.
Table 1. Main Features of Inlet Region Types
INREG Internal Stencil Stencil File Extension Shape Orientation Movement
CIRCULAR Circle Any shape Cylinder Global x, y, z Static
RECTANGULAR Rectangle Any shape Cuboid Global x, y, z Static
CUSTOM Not available Any shape Matches stencil Global x, y, z Static
CIRCULAR ARBOR Circle Any shape Cylinder Arbitrary Static/moving
RECTANGULAR ARBOR Rectangle Any shape Cuboid Arbitrary Static/moving

Auxiliary Files

Inlet regions have two auxiliary files:
  • Stencil file, inletregion_stf_file
  • Time-velocity series file, inletregion_tvs_file

Stencil file includes the particle stencil to be spawned by the inlet region. The particle stencil should be planar. All particles in the stencil file should be inside the domain box defined by min_domain and max_domain to spawn. Stencils may have three (x-position, y-position and z-position) or four columns (x-position, y-position, z-position, and phase). The four-column stencil is only used for multiphase stencils.

Time-velocity series (tvs) files are used for inlets with time dependent velocity. The expected velocity magnitude of inlet region at times of interest are given in the tvs file and the solver linearly interpolates the velocity magnitude values between the given points. The tvs file has two columns: (1) time and (2) velocity magnitude.

Basic Operation

Inlet region is a velocity inlet region without backflow, and only accepts velocity as input.
Note: Pressure automatically rises in streamwise direction, right after the end of the extension, to the level needed to sustain the flow.
An inlet region consists of two parts:
  • Stencil spawn surface
  • Extension (can be turned off)
One inlet region cycle consists of the following operations:
  1. Spawn a stencil with the velocity in sync with fluid.
  2. Integrate the velocity in time to estimate the position of the spawned stencil.
  3. Once the current spawn has moved by dx, spawn the next layer.
  4. The extension imposes the inlet velocity and ensures the distance between different spawn layers remain at dx.

All particles spawned by the inlet region belong to the FLUID phase the moment they appear in the simulation. Inlet regions have an internal storage of the complete stencil on each rank and away from the simulation. This may result in more memory consumption for very large stencils. For Arbor type inlet regions, all transformations are applied to this internal stencil storage.

Inlet region stencil should be indistinguishable from the rest of the discretized geometry. For non-Arbor inlet regions, this means that the stencil particles should align at dx with the particles of the solid phases at the beginning of the simulation. For Arbor inlet regions, this condition means the stencil particles should match their conduit. Turning on the mesh match option will align non-arbor stencils of Circular and Rectangular inlet regions with the rest of the particles when the inlet region is asked to create the stencil.
Note: When an external stencil is supplied to the inlet region, the solver is not responsible for stencil alignment.
Here are two examples of what may happen when inlet region stencils don't match their surrounding particles:
Misaligned stencil and conduit:
As the fluid particles leave the extension, the distance between fluid-fluid particles and fluid-wall particles are not similar. This results in a pressure rise on the side where fluid particles are closer to the wall. Depending on the severity of the misalignment, this may result in undesired secondary flows.
An angled conduit discretized in Cartesian with an arbor type inlet region:
As well as experiencing the same problem as the above item, this case will face an additional issue. Since the conduit cross section changes in the direction of inlet region normal, the extension will push the particles through the conduit wall. It is not guaranteed that the particles will be out of the wall at the end of the extension.
It is important that the spawn area is not disturbed by other fluid particles. As a result, extension is an important component of the inlet region. The fluid particles within the extension have the same velocity as the inlet region.
Note: Transport velocity and artificial particle displacement are deactivated in the extension. Additionally, the extension is strict, i.e. the velocity of the particles in the extension is completely defined by the extension in x, y and z.
The extension may be active before and/or after an inlet region's active interval. The extension only applies to the fluid phase specified for that specific inlet region.

The extension covers up to half dx, further in each direction than the definitions given in the cfg to ensure all spawned inlet region particles are covered. If extension definitions are missing from the cfg and are detected automatically based on the given stencil, the extension covers up to half dx, further from the outermost stencil particle.

While possible, not using an extension is generally not recommended unless it is guaranteed that the spawn area will remain undisturbed during the active interval of the inlet region. For example, it is acceptable to disable the extension when the inlet region is stationary and sprays into void. Moving inlet regions always require an extension to function properly.

For inlet regions that are not discharging into void, it is recommended that all sides except for the front of the inlet region (where fluid particles exit the extension) be encapsulated. It is possible to leave the back side of the inlet region uncovered as well, provided that no other particles disturb the stencil spawn surface.

It is possible to have overlapping inlet regions, provided that their stencils do not overlap. This mechanism is the basis of having multiple fluid phases within the same conduit, which we may somewhat erroneously call multiphase inlet regions. There are two methods to achieve this:
  • Creating multiple superposed inlet regions with separate stencils.
  • Creating multiple superposed inlet regions with a multiphase stencil.
Note: Please note that while the fluid particles will have an orderly distribution within the extension, the behavior after the extension is strictly dependent on the particle arrangement of the stencil and inlet region velocity. While it is possible to have different velocities for each of the superposed inlet regions, this may lead to undesired behavior after the fluid particles exit the extension. As a result, it is recommended to achieve the desired flowrate by changing the number of particles within the stencil.

It is important to pay attention to (cnt, circle_rad) and (cnt, rectangle_len) parameter pairs when creating multiphase Circular and Rectangular type inlet regions, respectively. Ideally, the extension should not cover outside of the conduit walls.

Here are two examples on creating multiphase inlet regions with separate stencils:
  • Assume two Circular inlet regions, where one phase has a circular stencil and the other has an annular stencil encompassing the first. These inlet regions will be called INREG1 and INREG2. The cnt} of both inlet regions should be place at the center of stencils. The circle_rad of INREG1 may only cover up to the circle radius, or the outer radius of INREG2. The cnt of INREG2 should cover the outer radius of the annular stencil.
  • Assume two Circular inlet regions, INREG1 and INREG2, where each phase has one half of a circular stencil. Combining these two stencils results in a complete circle. In this case, cnt should be at the center of combined stencils of INREG1 and INREG2 and circle_rad should be equal to the distance from cnt to the outermost particle of either stencil.
If you were to use a multiphase stencil and omit cnt and circle_rad from the cfg, you would get the same behavior as described above. Of course, it is not possible to anticipate all the combinations and as a result, it is always a good idea to guide the solver in the right direction.
Activating the indexsim option will generate unique indices for spawned fluid particles. Using this option will generate an additional message in the log, printing the particle count that each inlet region would spawn into the domain during the simulation time. This count is not consistent in a restart/continue situation.
Note: In general, indexsim is more of a diagnostic feature and it is not recommended to activate indexsim when output_prtlID is turned off.


Below are some known limitations of inlet regions.
  • Inlet regions are not checked for collision with each other to allow for multiphase inlet regions.
  • Arbor and Custom inlet regions do not check for collision with domain box.
  • Arbor inlet regions do not support meshmatch option.
  • An inlet region does not impose a force on its base.
  • Inlet regions do not support backflow.
  • Inlet regions do not support velocity profiles. It is recommended to use an extended conduit if fully developed profile is needed.