# Simcenter STAR-CCM+ Understanding the Force Report result computed on a unclosed boundary.

2022-04-28T10:06:58.000-0400
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## Summary

This article will explain the output provided by a force report when it is defined on an unclosed boundary

## Details

In order to understand the output provided by a Force Report, we can consider a very simple scenario:
where a cubic body is interacting with a water current travelling at a constant velocity, Vx, and includes gravity, g

Under these conditions, there will be two main forces acting on the body: the fluid force due to the relative motion between the cube and the fluid, and the hydrostatic force due to the gravity.

If in Simcenter STAR-CCM+ we define a Velocity Vector [5,0,0} m/s at the inlet, and, at the end of the run, we set a Force Report (where, for simplicity we do not include the viscous contribution) to compute the X component of the Force acting on the faces -X and +X of the body, we get:

01_ForceReport(X_component) = 9610.9 N

If we define a new Force Report that, this time, computes the X component of the Force acting only on the face -X, we get:

02_ForceReport(X_componet)@-X = 31713.5 N.

What this the reason of this difference?

The report 01_ForceReport(X_component) computes the sum of all the forces acting on the faces -X and +X.

The hydrostatic contributions to the force on these two faces are balanced, and the total hydrostatic force results null.

The report 02_ForceReport(X_componet)@-X, instead, takes into account only the forces acting on face -X on which the hydrostatic contribution is no more balanced by the presence of the opposite face, +X:

What is the physical meaning of this report?

If the hydrostatic pressure is properly defined, this report returns the real force acting on the -X face of the body.

Looking at the definition of the hydrostatic pressure:

it is evident that the hydrostatic force (and so the Force Report computed on a not-closed surface) strongly depends on the reference altitude, h0.

This highlights the important conclusions below:

- A force report computed on a closed surface always returns the total force that a fluid in movement exerts on a body;
- A force report computed on an unclosed surface returns the total force that a fluid exerts, only if the reference altitude is correctly set.

Working Pressure in case of Viscous Flow simulation

Simcenter STAR-CCM+ Documentation Sections:
Post-Processing > Reporting Results > Specific Reports > Force

SummaryDetails

### Associated Components

Design Manager Electronics Cooling In-Cylinder (STAR-ICE) Job Manager Simcenter STAR-CCM+