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How To Assess a Pressure Drop in the Post-Processor?

This article shows how to determine a pressure drop value with SimScale using two workflows: result controls and the online post-processor.

Solution

Before we can calculate the pressure drop in the domain, first we have to obtain the pressure levels in the regions of interest, such as inlets and outlets. In Figure 1, we have a representation of a fluid moving through a pipe to help us explain this process.

pipe model with a representation of the internal flow
Figure 1: Pipe model flow situation. The fluid enters through the horizontal and vertical inlet and leaves through the outlet.

There are two main ways to obtain the pressure drop from a simulation. Find below the workflows available.

Result Controls

One option to obtain the pressure drop between the inlet and outlet is to use the Surface data item under Result control. After the simulation finishes running, this allows the user to directly obtain the pressure values on the inlet and outlet faces.

surface data setup simscale
Figure 2: Setting up an area average control. The area average result controls can be accessed after the simulation run finishes.

Once the simulation finishes, you can access these results and extract the pressure values from the plots available under Run -> Area averages, as the image above shows.

Figure 3 shows what the area average results look like, for pressure:

area average results for pressure differences
Figure 3: Pressure value on the inlet and outlet face (averaged over face area) for every iteration. The values are fairly converged and the difference between inlet and outlet can be considered as the pressure drop.

By calculating the difference between the two pressure values of the last iteration, we get the pressure drop of the model.

Cutting Plane

Another option to determine the pressure drop between the inlet and outlet of the pipe is within SimScale’s post-processor. The workflow consists of creating cutting planes next to the inlet and outlet, and use the Bulk calculator function to plot the average pressure levels. Find the steps below:

bulk calculator to determine pressure drop of a simulation
Figure 4: By using the bulk calculator, you will receive information about the selected field on the cutting plane.
  1. Click on the Add Filter button and select a Cutting plane.
  2. Adjust the Position, Orientation, and Coloring of the cutting plane. The intention is to place the cutting plane just next to the inlets and outlets, plotting pressure.
  3. Activate the Bulk calculator.

For the bulk calculator, please note that the recommendation is to use cell data, instead of node data. Figure 5 shows how to switch to cell data in the post-processor:

switching to cell data post-processor
Figure 5: Cell data contains the raw data from the solver, without interpolations, and is more accurate for the bulk calculator.

To display cell data, simply right-click on the legend bar and then on Switch to cell data. The average pressure at the inlet is obtained from the information box. At this point, we can follow the same approach, creating a plane next to the outlet:

cutting plane downstream of the pipe pressure drop bulk calculator
Figure 6: Adjust the orientation and position, so that the cutting plane is next to the outlet.

Applying the averaged values available above, we can calculate an approximation of the pressure drop between the inlet and outlet of the pipe:

$$\Delta\ P=P_1-P_2=428.7\ – 15.2 =413.5\ Pa \tag{1} $$

Try It Yourself

If the above article has sparked interest in you, then make sure to check this Fluid Flow Simulation tutorial. In this tutorial, you can learn to simulate the pipe junction flow and also try the post-processor filters.

Note

If none of the above suggestions solved your problem, then please post the issue on our forum or contact us.

Last updated: April 29th, 2021

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