Frame Suite

General Frame Definitions

Road Top Profile Stencil: A set of Y and Z coordinates which defines the road top section profile.
Road Path Stencil: A set of X and Z coordinates which defines the road path in the direction of the moving vehicle.
Spawning: The act of introducing particle stencils into the domain as the frame is moving along the specified vehicle path.

Sliding Frame

Sliding frame is a part of the frame suite feature targeting water wading applications in nanoFluidX. To facilitate the simulation of experiment-like motion of descending into a water filled channel at a reasonable computational cost, sliding frame follows a phase of interest during simulation by moving the computational domain and spawning fluid and road particles as necessary.

There are two modes in which the sliding frame follows the phase of interest. In line lock mode, the sliding frame translates in the X direction only, and in point lock mode, the sliding frame follows both X and Z translational movement of the phase of interest. While the phase of interest may move arbitrarily, the trajectory of the sliding frame is confined to the XZ plane.

Temporal (Static) Frame

The temporal frame is intended for use cases where the car is moving inside a channel or a part of it with a flat road path and constant water depth, which is a quasi steady state problem. Therefore, the case setup can be further streamlined and optimized in such a way, that the reference frame does not have to follow a moving car body as reference object, but the motion can be imposed through a velocity boundary condition on the road and an inlet-like boundary condition for fluid particles while keeping the car body stationary (similar to belt systems in wind tunnels).

Contrary to sliding frame, which translates the domain based on the motion of an obj_phase of type MOVINGWALL, the relative motion of the car and the road is imposed by defining a motion for the road_phase of type WALL. Note that the only supported motion is IMPOSE_VEL, with a velocity vector parallel to the X coordinate, where a negative value corresponds to the car moving forward relative to the road. Due to the path being flat and therefore predefined, the road definition is simplified and only the top profile, which defines the top side of the road in the YZ plane in absolute Y and Z coordinates, is required. A separate definition of a road path is not necessary and therefore is removed.

Note: The TEMPORAL frame type requires only the road top profile to be defined, as in such scenario the road is flat. Therefore, in that context, when using the TEMPORAL frame, definitions and descriptions relating to the road path can be ignored.

Road/Channel Definition

The road definition is split into two parts: the road path, which defines the elevation of the road in the XZ plane, and the road top profile, which defines the top side of the road in the YZ plane.

It is recommended that both road top profile and road path point coordinates (which define road path and profile lines) are provided in absolute coordinates. Also, you should align the road top profile with the car at the start of the simulation, meaning the road should be close to or touching the tires.

Road top profile notes:

Is a two column file of the Y Z form.
Consists of a minimum of two Y Z pairs (two points defining one line segment).
Has a strictly increasing Y coordinate. Every value in the first column must be bigger than that of the previous line.

Road path profile notes:

Is a two column file with X coordinate in the first and Z coordinate in the second column.
Consists of a minimum of two X Z pairs (two points defining one line segment).
Has a strictly increasing X coordinate.

Figure 1. Schematic View of the Road Generated for the Case of SLIDING frame. Road Path (green line) and the Road Top Profile (blue line). Respective circular markers on the lines represent points in the stencil definition files (pairs of coordinates) which define the lines.

Note: It is critical that in the case of the Road Path, the X coordinate must be strictly increasing; and in the case of Road Top Profile the Y coordinate must be strictly increasing. Furthermore, it is important to note that it is assumed that the Z coordinate axis is always pointing upwards.

Figure 2. Schematic View of the Road Generated for the Case of TEMPORAL Frame. Note that for the TEMPORAL frame type only the Road Top Profile (blue line) needs to be provided. Respective circular markers on the lines represent points in the stencil definition files (pairs of coordinates) which define the lines.

The road stencil is created from the initial road top profile by first creating a road top stencil. Road top stencil is a set of particles with unique Y coordinates and a distance equal to the particle spacing dx that approximates the profile and is aligned with the bounding box of the case in Y and Z directions to ensure alignment with the vehicle particles. The road top stencil is then filled to create the road stencil, assuring that there are approximately three particles in wall normal direction to impose the wall boundary condition. This stencil is then copied and translated along the X direction and following the road path coordinates to create the full length channel. Fluid particles are spawned accordingly, in conjunction with the frame_fluid_height command input.

Note: It is critical that in order for Frame Suite to work, the positive Z direction needs to be directed upwards (gravity acting in the negative Z direction). Make sure your geometry is oriented in the correct manner during your pre-processing phase.

Freeze and sponge layers

Freeze and sponge layers are used to avoid fluid spill through X and Y (and potentially Z) bounding planes in the middle of the channel. The following image shows a schematic of the available configurations of these layers. A freeze layer sets the velocity of all particles within the layer to zero. A sponge layer multiplies a hyperbolic tangent profile with the calculated particle velocity. The two layers aim to satisfy the requirement of quiescent fluid near the frame boundaries.

Figure 3. Schematic of Inner Layers of Domain Box . Depending on frame_min_bc_overlay and frame_max_bc_overlay. Blue and purple bars represent freeze and sponge layers, respectively. (a) Option 0; (b) Option 1; (c) Option 2; (d) Option 3