Research Of The Detection Method For The Velocity Field Underwater Based On Particle Image Velocimetry

Image underwater is a useful technique in the detection underwater. The turbulence as an important flow phenomena becomes an important factor for the laser imaging underwater. It is necessary to find out the properties of the turbulent flow. Aimed at this purpose, the thesis implemented measurement of the turbulent velocity field underwater using particle image velocimetry(PIV).The principle of PIV is described as followed:A laser sheet illuminates a plane of the flow field, and the images of the fields are recorded in the perpendicular direction at two different times. Then velocity distribution of the flow field has been achieved using image processing. Based on the working principle, the thesis includes four parts:the selection of the tracer particles, the construction of the measured flow field, the establishment of the whole PIV system and the image processing of particle images.The tracer particles seeding in the flow fields is an important factor to accurately measure the fluid velocity. The optical scattering and following properties of seeding particles are discussed. The small bubbles as non-polluting particles are seeded into the water in these experiments. The velocity of the flow field can be achieved with the measurement of the bubbles motion. Through the simulation of two phase flow, the following properties of the bubbles with different diameters are calculated in different flow velocities.The thesis proposes a new spheroidal model to simulate the bubble's shape in the turbulent flow to research the optical properties of a bubble. With the plane wave incidence, the scattering distribution of the bubble is calculated by the geometrical optics approximation (GOA). The algorithm is verified using the Mie results for a spherical bubble, and the scattering patterns of the two methods agree well. The scattering properties of the spherical bubble with the Gaussian incidence are also calculated. The results of GOA and GLMT(Generalized Lorenz-Mie Theory) are compared, the scattering intensity distribution of different p orders rays are analyzed with Debye series in order to explain the differences between the two results.The measured turbulence flow is constructed in this thesis. Using the RKE(realized κ-ε) model, two flow fields, including the flow around cylinder and the pipe flow, are simulated. The latter one is selected in the experiment. Based on the conditions of produceing turbulence, the equipments of the measured flow are designed and chosen, and the measured flow field in experiment is achieved.The whole PIV system is established in this thesis. Based on the demands of velocity of flow and other corresponding conditions, the instruments of the laser, laser sheet optics, imaging optical path and image recordings are selected and assembled. The experiments are fulfilled and the images are achieved using the single frame-double exposed recording.The pairs of particle images are calculated using the correlation algorithm to obtain the velocity vectorgraph of flow in the field of view. Considering the background noise of CCD, the particles images are reduced noise and enhanced. With the suitable interrogation windows, the images are processing using the cross-correlation in the corresponding grids to calculating the velocity of flow in this region. Then we yield the whole by analyze the image pairs point-by-point.