This experimental protocol is to visualize the detail flow fields and the near boundary share in normal stresses within an equilibrium scour hole induced by a forced vibrating pipeline. The key advantage of this measurement technique is its capacity to simultaneously obtain pipeline dynamics, flow fields and near boundary flow stresses in high resolution. By using this technique, more in-depth studies of the two dimensional flow field in a complex environment can be conducted to better understand the scour mechanism.
The experiment takes place in a flume 11 meters long. The cross section is square with side length of 0.6 meters. This schematic view of the flume provides additional details including the location of an erodible seabed model.
The water level is 0.4 meters above the seabed. In the seabed model, use uniformly distributed medium sand that has been compacted and leveled. Have the structure for the vibration system in place over the flume.
This consists of a fixed frame that is locked on the top rails of the flume. The fixed frame has a moveable pole that supports an aluminum frame. The aluminum supporting the frame holds the pipeline model above the seabed model in the flume.
This schematic provides an overview of the setup. Note that there are four bearings that ensure the aluminum supporting frame can only vibrate vertically. A connecting rod between the movable pole and a servo motor drives the motion of the aluminum frame.
The setup depends on the pipe geometry. This duplicate of the acrylic pipeline model has a diameter of 35 millimeters. Adjust the supporting frame and pole so the bottom of the pipeline is one diameter above the initial seabed surface.
Respect all laser safety protocols and begin working with the laser. Place the 532 nanometer laser and optics for velocimetry on top of the flume. The optics include elements to form a sheet of illumination.
With the laser on, adjust the optics so that a flat sheet of illumination is formed in the field of interest in the flume. The sheet should be along the flume center and parallel to its side walls. These schematic front and side views indicate the position of the laser and optics and the created laser sheet in the setup.
Next, set up the camera of the particle image velocimetry apparatus. Use a high speed camera with the appropriate focal length directed perpendicularly to the laser sheet. Connect the camera to a computer with the correct control software.
With the camera on, adjust the field of view to ensure the pipeline fluid seabed region is visible and the image is clear. To calibrate the setup, begin with the seeding particles. This aluminum powder provides particles with a diameter of 10 microns.
Add about 20 grams of seeding particles to the test section of the flume. Verify that the camera brings the seeding particles into sharp focus. Then, place a calibration ruler inside the field of view on the laser sheet plane and capture a calibration image.
After choosing a sampling rate for data collection, turn off the laser and camera. For the experiment, obtain a transparent acrylic plate. Support it over the test bed below the laser source and on the water surface to suppress surface fluctuations.
This diagram provides details of the use of strings attached to the flume rails to support the plate in this setup. Next, turn on the servo motor on the frame. This will begin to induce forced vibrations on the pipeline model.
Keep the vibration system running for 24 hours. After 24 hours, turn on the laser to create the light sheet. Start the camera and its control software using the calibrated settings.
Then, turn off the lights and begin data collection. Once the data is collected, check that the seeding particle density for 32 by 32 pixel interrogation window is greater than eight before collecting additional datasets. Once all of the datasets have been collected, begin data processing.
Work with the particle image velocimetry software with the calibration image opened. Next, go to the toolbar and click the scale setup button. Move the cross hairs to a mark on the ruler's image and tag it.
Next, tag a second mark on the ruler's image. In the dialogue box that opens, enter the distance between the marks according to the ruler. Note the scale that is computed.
Return to the toolbar and click the origin button. From there, use the mouse to set the origin of the coordinates for all of the data images. Click yes when done.
Then, click on the file menu and load the first of the raw images that were collected as data. Check that the other files are accessible but return to the first file. Next, click on the parameter menu.
In the dialogue box, enter the number of data files and the sample rate to load all the images. Save the values and close the box. Now, go to the image filter menu.
There, apply the low pass filter. In the toolbar, click the PTV module. Follow this by clicking tracing point.
Then in the image, find the center point at the right half of the pipeline circumference and select it. Okay the selection before clicking on PTV tools in the toolbar. In the dialogue box that opens, adjust the gamma, light gate and median filter settings to single out the pipeline outline in the image.
After approving the changes, click the object tracking button. Use the mouse to select an identifiable portion of the pipeline on the processed image. Once this is done, the software tracks the displacement in the images and records the time series.
After the data is saved, go to and click on PTV tools. In the dialogue box, click the default button and OK to recover the raw image for subsequent analysis. Click on PTV module to deactivate the module.
Remain in the toolbar and open the parameter panel. Verify the velocity vector calculation parameter and others before closing the dialogue box. Then, go to the image filter menu.
Apply a laplacian filter function to the raw images to highlight the seeding particles and filter out undesired scattered light. Now, go back to the toolbar and click on boundary. Use the mouse to set the geometric mask on the images to exclude the seabed region.
Confirm that the boundary has been set. When done, click boundary save to save the boundary data. Finally, go to the toolbar and click the run button to calculate the instantaneous velocity fields using the cross correlation method.
Export and save the instantaneous velocity field's data for further analysis. This an image of a quasi-equilibrium scour profile and vibrating pipeline taken after 24 hours of pipeline vibration. The origin for analysis is set at the intersection point at the original seabed surface and the pipeline vertical center line.
Seeding particles are visible but very few sediment particles are suspended in the flow, suggesting the system is in a quasi-equilibrium stage. Data collected with the protocol allows visualization of the phase averaged velocity field and vorticity dynamics. This video consists of 72 frames of flow fields from one pipeline vibration cycle.
This method can also be applied to investigate vortex induced vibration processes such as pipeline vibration induced by asymmetry vortex shedding.
