 # Transient Pressure Fluctuations Against a Drill Rig Leg

Monday January 6, 2014 at 9:52am
SOLIDWORKS Flow Simulation can be used to create transient studies where the fluid boundary conditions vary with time. This means that inlet and outlet parameters such as velocity, pressure or temperature can vary over a defined time period.

An interesting application of this is the pressure fluctuations that arise when a pressure pulse from a wave passes around a static solid object - like a leg of a pier or a submerged column in seawater. One of our customers wanted to observe this phenomenon so I quickly built a simple SOLIDWORKS model to illustrate how to do it.

Here is the simple model. It is one section of a submerged leg so I was able to use the 'Periodic' option for in the Computational Domain as shown below. I set the project to be a transient one (over 30 secs) in the Flow Wizard and created a pressure pulse by varying the global velocity in the X direction using a 'Dependency'. This dependency is shown below ... The Formula I used is: 3 x sin { [ 2 * pi / 10] x t} + 2, where t = time

This creates a velocity that fluctuates over time sinusoidally with a period of 10 secs, a mean value of 2 m/s and an amplitude of +/- 3m/s. This creates a longitudinal velocity wave that represents a wave entering the domain in the X direction.

Flow allows the results to be plotted over time and animated. Below are some animations that show the transient behaviour ...

This video is a plan view showing the streamlines as they vary with time through a horizontal section. The parameter being plotted is velocity. You can see the velocity wave starting on the left hand side and passing to the right. It oscillates 3 times representing 3 waves entering the domain. It is surprising how a relatively small structure significantly affects the flow - even at a distance.

The next video shows the same result but plotted with the '3D offset' option enabled. This takes the 2D Cut Plot results and offsets them in the vertical direction. This clearly shows the wave effect. However, note that this is NOT the wave at a water/air surface but rather a graphical illustration of how the velocity changes within a given plane. Nonetheless it is an interesting insight into the dynamic behaviour of the water around the leg.

In addition to the animations, Flow can report the results in numerical and graphical form. Below are 2 examples.

The first graph is a plot of 2 points on the centreline showing the X velocity. The red curve shows the input velocity fluctuation near the edge of the domain. The blue curve shows that the X velocity close to the leg. The second graph is a plot of the X component of force acting on the front face of the leg. The force value peaks at a value between 10,000N and 12,000N. This ties in well with hand calculations for a hydraulic pressure.

by Andy Fulcher
Technical Manager
Solid Solutions Management Ltd

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