OTRC Project Summary
Project Title:
Implementation of Bubble Image Velocimetry in OTRC Wave Basin Prinicipal Investigators:
Kuang-An Chang Sponsor:
OTRC Industry Consortium Completion Date:
August 2012 Status Report:
Due August 2011 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. 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) is been recently developed for velocity measurement in bubbly flows. Unlike PIV, BIV correlates images of liquid-gas interfaces, such as bubbles, in bubbly 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 easier to implement in various facilities. BIV was developed by the PI with support from OTRC. It has been successfully employed in velocity measurements in green water, wave breaking, and open channel flows. Nevertheless, BIV has only been tested in small scale laboratories. Its applications in large facilities such as the OTRC wave basin have not been tested. Moreover, two other laboratory techniques, for pressure and void fraction measurements, are also not been implemented in the OTRC wave basin. All these techniques that are currently been used in the small scale Ocean Engineering Laboratory have never been implemented and tested in OTRC.
The objective of this research is to implement the BIV technique in the OTRC wave basin. Pressure and void fraction measurement capability in the wave basin will also be assessed and, if feasible, implemented. Currently wave breaking and green water measurements using BIV are being performed in small two-dimensional wave flumes with fixed structures. Implementing BIV in the OTRC wave basin is therefore a logical move. BIV will augment the existing OTRC capability on free surface elevation, wave loading, and structure response measurements, to allow the determination of flow kinematics when wave breaking, wave runup, or green water occurs. Adding pressure and void fraction measurements will also extend the existing data acquisition capability in OTRC to allow the determination of local loading on a specific part of testing structure.
APPROACH:
Interactions between large waves and a floating structure typically result in complex flows around and over the structure, and the resulting impact loads and pressure that can cause damage. Efforts based on numerical and experimental studies have greatly improved the understanding of the multi-phased, turbulent flow. Measurements of flow field and void fraction in such flow have became amenable in laboratory only in the recent years, with the help of newly developed imaging and optics based techniques. The flow can be simulated with some degree of success, based on advanced numerical models with free surface tracking and turbulent closure. However, laboratory measurements have been conducted in small scale flumes with simplified wave conditions and structure geometry, whereas numerical simulations have rarely been validated due to the availability of laboratory data from the complex flow.
The recently developed and tested BIV technique will be implemented in the OTRC wave basin. The biggest issue in the implementation is likely to be lighting – the technique requires uniform back-lit so the “shadow” of bubbles can be captured by an imaging system. To obtain velocity maps on a vertical plane, a large LED panel or equivalent is required to be placed behind the test model, and a high-speed digital camera is needed to be placed in front of the model for side-looking image recording. Similarly, to obtain velocity maps on a horizontal plane, illumination needs to come underneath the test model or below the water surface, and a high-speed digital camera needs to be mounted above the model for down-looking image recording. Design and setup of the illumination system and image recording system are necessary, in addition to the need on wiring, signal control, and synchronization.
The research is to be carried out by the following plan: (1) Design of illumination system and setup of imaging and control systems in OTRC for BIV measurement. (2) Preliminary test of BIV measurement on a fixed model structure in OTRC basin. (3) Repeat the design and test on another measurement plane (from vertical to horizontal). (4) Test of BIV measurement on a floating model in OTRC basin. (5) Set up the pressure and void fraction measurement systems in OTRC along with the BIV system. (6) Test of BIV, pressure, and void fraction measurements on a floating model in OTRC 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 two years. The PI with one graduate student will be responsible for transferring the BIV technique from the small scale Ocean Engineering Laboratory to OTRC, including implementing the technique in the wave basin and testing it on a model structure in the basin. Velocity maps on both horizontal plane and vertical plane will be obtained. Pressure and void fraction measurements will also be implemented in the OTRC wave basin. The PI will coordinate closely with Dr. Richard Mercier, who will bridge the proposed research with the need of OTRC and industry and provide inputs. The PI will also have an office at OTRC to better interact and understand the Center and its working environment 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 -2012):
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.
Anticipated Results & Deliverable: We expect the velocity, pressure, and void fraction measurement systems to be functioning in the OTRC wave basin. Test of green water flow on a floating structure will be conducted with measurements taken using the new systems and the current OTRC measurement system. Recommendation on installing a permanent BIV system in the Center will be assessed.