Summary
Objective
In recent years imaging based measurement techniques have progressed rapidly to allow detailed flow velocity measurements and structure motion monitoring in ocean engineering studies. Such information is especially important when greenwater occurs – local flow velocities on structure deck and wall are essential in order to accurately estimate induced momentum and forces. The most widely used method for velocity measurement in single-phase flows is particle image velocimetry (PIV). PIV obtains fluid velocity using images with laser as an illumination source and tracking tiny artificial seeding particles in the images. Based on the principle of PIV, bubble image velocimetry (BIV) has been recently developed for velocity measurement in bubbly flows. Unlike PIV, BIV correlates images of gas-liquid interfaces, such as bubbles and droplets, in gas-liquid flows to determine velocity of the bubble-liquid mixture. Although BIV is considered as a derivative of PIV, BIV applies the shadowgraphy method for illumination so no laser is needed – making the method safe and practical to implement in various facilities. BIV was developed by the PI with support from OTRC. It has been successfully employed for velocity measurements in greenwater, wave breaking, and open channel flows. Its implementation in large facilities such as the OTRC wave basin has not been completed. A proof-of-concept test of greenwater over a fixed structure using BIV in the OTRC wave basin was successfully conducted. Further refinement on the BIV apparatus and its applications on floating structures are needed if BIV is to be used as a reliable measurement tool in the OTRC basin. In addition to BIV, two other laboratory techniques for pressure and void fraction measurements that have been successfully employed in the small scale Ocean Engineering Laboratory on fixed and floating structures have not been implemented in the OTRC wave basin. Void fraction data is especially important in greenwater flow since, as waves break and overtop, fluid density is no longer constant. Flow momentum and force distributions will be overestimated if a constant water density is assumed.
The objective of this study is to implement the BIV technique on a floating model in the OTRC wave basin. New capability for flow pressure and void fraction measurements in the wave basin will also be assessed and, if feasible, implemented. Currently greenwater velocity measurements using BIV and pressure and void fraction measurements have been successfully performed on fixed and floating structures in small-scale two-dimensional wave tank. Recently promising results were also obtained for applying the BIV technique on a fixed structure in the OTRC wave basin. Implementing BIV as well as pressure and void fraction measurement techniques in the OTRC wave basin on floating structures becomes a logical move. BIV will augment the existing OTRC capability on free surface elevation, wave loading, and structure response measurements to allow for the determination of flow kinematics in the vicinity of a model structure when wave breaking, wave runup, or greenwater overtopping occurs. Adding pressure and void fraction measurements will further extend the existing measurement capability to allow for the determination of local loading on a specific part of the structure.
Approach
Interactions between large waves and a floating structure typically result in complex flows around and over the structure, and impact pressure and loads that can cause structure damage. Efforts based on numerical and experimental studies have greatly improved our understanding of the multi-phased, turbulent flow. The flow can be simulated with some degree of success, based on advanced numerical models with free surface tracking and turbulent closure. Measurements of flow velocities and void fraction in such flow have become amenable in laboratory only in the recent years, with the help of newly developed imaging and fiber-optic techniques. Nevertheless, laboratory measurements have been conducted only in small scale flumes with simplified wave conditions and mostly fixed structures, whereas numerical simulations have rarely been validated due to the availability of laboratory data from the complex flow.
The recently developed BIV technique has been successfully tested in the OTRC wave basin on a fixed structure. The biggest issue in the implementation was the lighting – the technique requires uniform back-lit so the “shadow” of bubbles can be captured by an imaging system. To obtain velocity maps on the structure, a large LED panel was placed as the structure deck, and a high-speed digital camera was mounted above the model for down-looking image recording. The apparatus has been successful applied to obtain horizontal velocity maps. This proposal is to continue the successful but preliminary model test in the wave basin using BIV, and to extend the measurement to side looking (for vertical runup velocity) and, more importantly, on a floating structure. To obtain velocity maps on a vertical plane, illumination using LED panels needs to come from the side wall of the test model, and a high-speed digital camera needs to be mounted at the same level or higher (to prevent water intrusion) to the model deck for side-looking image recording. A down-looking camera needs to be synchronized with the side-looking camera so the relative position of the moving structure can be identified. Pressure transducers and void fraction sensors will also be mounted on the floating model in the OTRC basin to test their feasibility. Setup and integration of the illumination system and image recording system for a floating model will be investigated, in addition to the installation of the pressure transducers and void fraction sensors and their wiring, signal control, and synchronization on floating structures in the OTRC wave basin.
The research is to be carried out in the following order: (1) Design and construct a TLP model and mooring for model tests in the OTRC basin. (2) Implement and test the BIV system on the floating model, including measurements on vertical and horizontal planes. (3) Add pressure transducers and fiber optic reflectometer (FOR) on the floating model for pressure and void fraction (density) measurements. (4) Refine the floating model and the BIV system and the pressure and void fraction measurement systems. (5) Test the refined model and measurement systems in the OTRC wave basin.
Deployment of Results
Implementing BIV, pressure, and void fraction measurement systems in the OTRC wave basin will greatly enhance its data acquisition capability. With the added capability, OTRC will be able to provide its clients more choices of detailed flow data for their model tests. This would eventually provide the industry and designers a better understanding of certain complex phenomenon, and in turn minimize uncertainty in their design – a step not only increasing safety but also reducing the occurrence of over design.
Project Organization & Timing
The project will last for six years. The PI and one graduate student will be responsible for implementing and refining the BIV technique in OTRC wave basin so the technique can be used for industrial tests to provide accurate greenwater prediction and flow kinematics. Flow velocities, void fraction, pressure distributions, and forces on the model structure will be obtained, adding to the existing capability of measuring free surface and structure response at the center. The PI will coordinate closely with Dr. Richard Mercier, who will guild the design and construction of the TLP model and bridge the proposed research with the need of OTRC and industry and provide inputs. The PI and his graduate student currently have offices at OTRC so they can better interact with the center for optimizing the implementation and scheduling of the proposed measurement systems.
Project Plan for Phase 1 (2010 -2011)
Scope of Work
The main tasks are as follows:
- Design and set up illumination and camera apparatus in OTRC wave basin for the BIV system
- Measure velocity using BIV on a horizontal plane on a fixed structure.
- Measure velocity using BIV on a vertical plane on a fixed structure
For velocity measurements using BIV, the PI and graduate student will set up the imaging system, including camera, illumination, data acquisition, signal control, and image processing systems at OTRC using the current systems in the Ocean Engineering Laboratory. A fixed structure will be tested in the basin to examine the feasibility and to refine the setup.
Anticipated Results & Deliverable
We expect to finish modifying the apparatus of the existing BIV system and set it up in the OTRC wave basin for velocity measurements. Test of a fixed structure in the basin is expected using the apparatus. The apparatus will be refined based on the test results. It is expected the BIV system will work well for fixed structures in the basin.
Project Plan for Phase 2 (2011 -2014)
Scope of Work
The main tasks are as follows:
- Measure velocities on both horizontal and vertical planes on a floating structure
- Set up pressure measurement system in OTRC wave basin
- Set up void fraction measurement system in OTRC wave basin
- Simultaneously measure velocity, pressure, void fraction, loading, and wave characteristics of green water on a floating structure
For velocity measurements, the setup and tests of BIV system will be continued using a floating structure in the basin. The setup will be refined, and the accuracy, capability, and limitation of the system will be examined and documented. For pressure and void fraction measurements, the PI and graduate student will move the existing systems from the Ocean Engineering Laboratory to OTRC and modify their wiring to accommodate the size of the basin.
Project Plan for Phase 3 (2015 -2016)
Scope of Work
The main tasks are as follows:
- Design and construct a TLP model and mooring for OTRC wave basin tests
- Implement BIV system (illumination and camera apparatus) for flow kinematic measurements on a horizontal plane (deck) and a vertical plane (wall/column)
- Set up pressure transducers and fiber optic reflectometer (FOR) on the model for pressure, and void fraction (density) measurements, in addition to force, free surface, and motion measurements
- Conduct tests in OTRC wave basin
- Analyze measurement data and report finding
- Refine the model design in step 1
- Refine the setup and implementation in steps 2-3
- Repeat steps 4-5 for refined model tests in OTRC Basin, perform data analysis and report finding
Dr. Mercier will help design and construct the TLP model and its mooring system for the OTRC wave basin tests. the PI and the graduate student will design and setup of the imaging system, including multiple cameras, illumination, data acquisition, signal control, and image processing for velocity measurements using BIV. They will also integrate the system with pressure and void fraction probes and synchronize them with the wave probes and motion sensing system at OTRC. One camera will be mounted above the deck to capture down-looking images for greenwater velocity determination, while a second camera will be mounted at the deck level to capture side-looking images for runup velocity determination. Focusing waves will be first tested so greenwater events are predetermined. Spectral waves will then be tested; all the measurement systems will be at stand-by mode and triggered by greenwater events for data recording.
Lesson learned from the floating model tests in the OTRC basin in the first year will be applied to the preparation of the OTRC wave basin tests in the second year. The test condition will be mainly spectral waves to simulate realistic ocean environment. Pressure sensors and void fraction probes will be mounted on the model so greenwater velocity, pressure, void fraction, and model motion measurements as well as free surface elevations will all be obtained.
Anticipated Results & Deliverable
An integrated apparatus for velocity maps, pressure, void fraction, and motion measurements in the OTRC wave basin under normal test conditions is expected to be completed. Recommendation on installing a permanent BIV system at the center will be assessed. A joint JIP on greenwater study will be proposed once the working system is validated.
Related Publications
Chuang, W.-L., Chang, K.-A. & Mercier, R. (2017) “Impact pressure and void fraction due to plunging breaking wave impact on a 2D TLP structure.” Experiments in Fluids, to appear, DOI: 10.1007/s00348-017-2356-4.
Chuang, W.-L., Chang, K.-A. & Mercier, R. (2015) “Green water velocity due to breaking wave impingement on a tension leg platform.” Experiments in Fluids, 56(7), 139, Springer.
Song, Y.K., Chang, K.-A., Ariyarathne, K. & Mercier, R. (2015) “Surface velocity and impact pressure of green water flow on a fixed model structure in a large wave basin.” Ocean Engineering, 104, 40-51, Elsevier.
Ariyarathne, K., Chang, K.-A. & Mercier, R. (2012) “Green water impact pressure on a three-dimensional model structure.” Experiments in Fluids, 53, 1879-1894, Springer.
Chang, K.-A., Ariyarathne, K. & Mercier, R. (2011) “Three-dimensional green water velocity on a model structure.” Experiments in Fluids, 51, 327-345, Springer.
Ryu, Y. and Chang, K.-A. (2008) “Green water void fraction due to breaking wave impinging and overtopping” Experiments in Fluids, 45, 883-898, Springer.
Ryu, Y., Chang, K.-A. & Mercier, R. (2007) “Application of dam-break flow to green water prediction.” Applied Ocean Research, 29, 128-136, Elsevier.
Ryu, Y., Chang, K.-A. & Mercier, R. (2007) “Runup and green water velocities due to breaking wave impinging and overtopping” Experiments in Fluids, 43, 555-567, Springer.
Ryu, Y., Chang, K.-A. & Lim, H.-J. (2005) “Use of bubble image velocimetry for measurement of plunging wave impinging on structure and associated greenwater.” Measurement Science and Technology, 16, 1945-1953, Institute of Physics, U.K.
