Coherent Structures in a Jet in the Vicinity of the Free Surface

In this study, the coherent structures formed in a round turbulent jet in the vicinity of a free surface are investigated by using proper orthogonal decomposition (POD). The jet issued from a nozzle located at a depth five times the nozzle diameter (d = 10 mm) below and parallel to the free surface. The jet exit velocity was 2.8 m/s and the resulting Reynolds number was 28,000. Instantaneous two-dimensional PIV measurements were obtained in horizontal (x–z) plane at the center of the nozzle at y/d = 0. The velocity fields containing only 50% of the kinetic energy was reconstructed using POD and the difference between the original fluctuating velocity field and POD reconstructed velocity field that represent the smaller-scale (residual) velocity field were further analyzed and compared with the velocity field of a free jet. INTRODUCTION Jets are used in various manufacturing processes and waste disposal operations. While turbulence characteristics of unconfined (free) jets are well studied and documented (Wygnanski and Fiedler, 1969; Rajaratnam, 1976; Hussein et al., 1994), less is known about confined jets. When turbulent jets are discharged into a shallow environment, they can be vertically confined by the free surface. In the present study, the modification of the jet characteristics due to the free surface interaction is studied experimentally. The velocity field is measured using a two-dimensional Particle Image Velocimeter (PIV), which provides instantaneous global velocity measurements with good spatial resolution. PIV measurements are obtained in planes parallel to the free surface at various distances from the axis of the nozzle. This paper examines the effect of the coherent structures caused by the jet interaction with the free surface. EXPERIMENTAL The experiments were conducted in a jet tank facility 2 m long, 1 m wide and 0.7 m deep (Tandalam et al., 2010). A circular nozzle was machined and mounted on the end wall of the tank, which was made of 0.75-inch thick aluminum plate. The centre of the nozzle was located 0.3 m above the bottom of the tank, and 0.5 m away from both side walls of the tank. The nozzle exit opening was 10 mm in diameter and was flushed with the inside wall of the tank. The jet discharge was provided by an overhead reservoir with a constant head supply of 2.0 m. The flow from the overhead reservoir was controlled by a valve to deliver a constant velocity of 2.8 m/s at the nozzle exit. The jet facility was operated at two different conditions depending on the jet submergence, h/d. Here, h denotes the vertical height of the water measured from the free surface to the center of the nozzle. While at h/d = 30 the jet was expected to behave as a free jet (FJ), at a lower submergence h/d = 5, the jet is constrained vertically by the free surface and it is expected to behave as a surface jet (SJ). The Reynolds number based on the jet diameter (d) and exit velocity (Ue) for both jet conditions was kept constant 000 , 28 / Re d Ue . The Froude number was 4 / gh U Fr e for the case of surface jet. Velocity measurements were carried out using a planar Particle Image Velocimetry (PIV) system. PIV velocity fields in several horizontal (x-z) planes passing through the centerline of the jet were measured in the far field of the jet at 30 < x/d < 60 where the jet interacts significantly with the free surface at h/d = 5. The light source was Nd:YAG laser set ( = 523nm ) with 50 mJ/pulse energy and a pulse duration of 10 ns. The laser sheet with thickness of about 1mm was created by a combination of spherical and cylindrical lenses with focal length of 1000 mm and -15 mm, respectively. The field of view was 12 x 12 mm. To avoid reflection of the laser sheet by the waves on the free surface, measurements were obtained only up to 4.5d above the central plane for the surface jet. A high resolution Powerview Plus 4MP CCD camera with a resolution of 2048 x 2048 pixels was employed to record pairs of time delayed images of the particles. 2000 images at a sampling rate of 1.04 Hz were obtained for each field of view (FOV). The image acquisition was performed with the software Insight 3G [TSI Inc]. The pair