Calculating Forces on Solid Structures in FLOW-3D
Engineers are often faced with a challenge of designing systems that have liquids and/or gases interacting with solid structures. These structures can be stationary or moving and are completely or partially submerged in the fluid flow. It is often important to know the loading (hydraulic force) that the fluid flow exerts on these solid structures. FLOW-3D offers different ways of calculating the force and moments on structures.
Conventions in Version 9.3
Hydraulic Torque = Moment about the reference point. (Only computed for general moving objects.) If the reference point is not specified by the user, the hydraulic torque is calculated about the mass center provided that the mass properties are specified, or else the hydraulic torque is calculated about the geometric center, where
A. Computing Forces on Stationary Structures
For stationary structures like spillways, bridge piers, and offshore oil platforms, there are two options available to compute the forces:
Figure 1: Force window encompassing multiple solid structures:
The output will be hydraulic forces and moments in all
three directions on all three sturctures: A, B and C combined.
Figure 2: Force window partially encompassing a solid structure:
Hydraulic forces and moments will be computed only on the
part of the object inside the force window.
Option 1: Force Window
Force window is a diagnostic tool in FLOW-3D that is useful for computing hydraulic forces and moments on solid structures. A force window is basically a “3D rectangular show box” defined by user-specified minimum and maximum limits in each direction. The solver will then output combined hydraulic forces and moments on all the solid structures and wall boundaries inside the 3D domain defined by the force window. Hydraulic moments are computed relative to the origin of the coordinate system.
Force window limitation:
Force window may not be useful in certain cases. For example, if the geometry has two structures A and B, as seen in Fig. 4, it is not possible to define a single force window around just the structure A, since a force window can only have a 3D box shape aligned with coordinate axes.
Figure 3: Version 9.3 will allow users to create a force window from the
output tab in the graphical user interface.
Figure 4: Multiple force windows
One option is to use multiple force windows as shown in Fig. 4 and then the net hydraulic force will be sum of the force output from the multiple force windows. The second option is to create structures A and B as separate components. Then request “Pressure Force Output” for component A (see next section for “Pressure Force Output”).
Option 2: Component Pressure Force
If just the fluid pressure force for a component in the geometry is desired, then set the option “Pressure force output” to “Yes” for that component in the geometry tree. The pressure force output, as the name suggests, only includes fluid pressure effects. The viscous effects are not included. With the “Pressure force output” option the solver also computes the “Pressure moments,” which are about the origin of the coordinate system.
The output from both the force window and pressure force output option is available in the results file as history data under Analyze tab > Probe > General history. The frequency of output of the force window data can be controlled by changing the output frequency of the history data on the output tab.
To determine just the viscous force on a solid object, both the force window and pressure force output can be used together and the difference between these two outputs will be the viscous force.
B. Computing Forces on Moving Structures
Moving solid structures, such as the piston in a shot sleeve assembly, spillway gates, etc. can be modeled in FLOW-3D by using the general moving object model and defining the moving structures as either coupled motion or prescribed motion components. For such components, hydraulic force (sum of pressure and viscous force) and hydraulic torques are always available as history data under Analyze > Probe> History data. For a coupled motion component the hydraulic torques are calculated about the center of mass of the object. For a prescribed motion object the hydraulic torques are calculated about the center of mass of the object if the mass properties are specified, otherwise hydraulic torques are calculated about the origin of the coordinate system.
Note that a force window can be used to compute forces on moving components but forces and moments computed by the force window on such components will be correct only if the components stay within the limits of the force window.
C. Forces on Baffles
Structures like porous plates or porous screens can be modeled in FLOW-3D by creating baffles of the desired shape and desired porosity properties. For such porous baffles FLOW-3D calculates “Baffle force” if variable IFRCBF is set to one in the namelist “BF” in the text input (prepin) file. Baffle force calculation includes only the effect of fluid pressure, viscous force is not included in Baffle force. Baffle force is available as history data in the results file and can be accessed under Analyze tab> Probe > General history.