Understanding sketch animation for hydraulic lines

Modeling of fluids lines in Simcenter Amesim: an overview

Simcenter Amesim has a variety of ways of modeling flow phenomena between components where a fluid connection exists. A simplified list of these is presented below:

• Direct lines: these can be viewed as simple data connections, transferring information from the output port of one component to the input port of another (usually the exchange is bi-directional).

• Lines with lumped-parameter representations of pipes: in this case, lines are no longer seen as data funnels. When lumped submodels are used, the intent is to signify the presence of a physical pipe or line in which fluid flow occurs. Lumped submodels make the assumption that the physical properties of the fluid vary negligibly as position evolves down a line. In cases where this is no longer considered true from a modeling perspective, you can also use distributed lumped submodels, which split lines into several regions where the physical properties are assumed to be constant.

• CFD-1D lines: for these types of lines, the length of the pipe is discretized (more or less finely depending on the frequency of the phenomena one wants to observe), and the full 1D Navier-Stokes equations are solved, accounting for the variation of density, pressure and flow rate as a function of the position in the pipe.

Please refer to the Hydraulic library user manual for detailed information on the underlying hypotheses and equations. Sketch Animation is a post-processing and analysis tool dedicated to the description of the macro- level (i.e. global system-level) operation of a model. As such, it has been introduced with a feature set that targets this specific use-case.

How are lines handled by sketch animation?

Given the definitions stated above, you might expect to be able to perform a certain number of analyses on lumped or CFD-1D submodels. For example, you might expect to see a pressure and flowrate animation between each node individually. This would make it possible to observe dynamic pressure and flowrate evolution along the length of a line, which is key to analyzing certain types of phenomena. This has been reproduced below in an equivalent circuit:

Figure 8: Discretized/distributed line analysis

This figure illustrates how analysis would be possible with animation for discretized/distributed lines, here performed on an equivalent circuit split into a series of elementary inertial and compressibility hydraulic lines separated by nodes. In the above example, the resulting overall line, composed of a series of 6 sets of inertial and compressibility lines connected via hydraulic nodes, is equivalent (in terms of the simulation results) to a 5-node distributed inertia line with C-IR *** C-IR effects.

However, the current iteration of Sketch Animation does not provide the functionality for this type of analysis. Indeed, the manner in which data is collected and displayed by Sketch Animation can be summarized as shown below.

For color animation of effort variables:

• Sketch Animation only uses external variables when attempting to collect results to display in an animation.

• Specifically, it looks for the port through which the variable being "searched for" is output. In the case of color animation for hydraulic lines, this is usually pressure, so Sketch Animation will try to find variables which carry pressure units.

• Sketch Animation then performs unit conversions (when necessary), and infers the color to be displayed, based on a color map and upper/lower bounds set for the animation.

• Finally, Sketch Animation displays the line with the color in question.

The method for displaying symbols for flux variables is equivalent to above, except the last two steps, where the outcome of the process is the number of symbols that are displayed in order indicate the magnitude and direction of the flux variable (for hydraulic lines, usually L/min or equivalent units).

As a result, when attempting to perform the same kind of animation on a distributed HL0040 line, this is the animation you would get:

Figure 9: Animation on a distributed HL0040 line

This is of course the expected result. Pressure is imposed by the tank, which is a constant 10 bar:

Figure 10: Tank pressure source - TK000

Similarly, the flow rate is imposed by the zero-flow hydraulic source, which needless to say outputs a constant 0 L/min:

Figure 11: Zero hydraulic flow force - Q000

Given that animation for distributed hydraulic lines is not currently a supported feature, there is no display of what is happening between nodes. Of course, there are other ways of observing the detailed flow dynamics within this kind of line, specifically by plotting the flow characteristics at each node. Below is a plot showing pressure evolutions at each node for both the equivalent circuits shown previously, and the single distributive line discussed above.

Figure 12: Pressure evolutions

For CFD-1D line submodels, you can go further using 1D plots, which allow you to check the spatial evolution of state variables, for example, making it possible to observe pressure waves along the pipe's length