Turbulence Structures Generated By An Oscillating Grid In A Stably-stratified Two-layer Fluid In The Early Period

Laboratory experiments,together with the Particle Image Velocimetry（PIV）technique,were used to study the turbulence structures produced by an oscillating grid in a stably-stratified mean shear-free two-layer fluid（fresh water on salt water）.The instantaneous velocity and vorticity of the two-dimensional flow fields（perpendicular to the plane of the grid）were obtained from PIV analyses of recorded video.These data were used to calculate the following physical quantities of the flow fields:1)the time-averaged velocity and time-averaged vorticity;2)the root-mean-square velocity;3)the degree of homogeneity and isotropy;4)the strength of mean flow;5)the time-averaged Taylor’s Eulerian integral length scale;6)the Eulerian frequency spectrum and 7)the time-averaged turbulent kinetic energy and time-averaged vertical flux of turbulence kinetic energy.Results show that:i)the time-averaged velocity above the grid is upward,while the time-averaged vorticity is alternately positive and negative on both sides of the bars,suggesting that the jet-like structures are the dominant feature near the grid and counter-rotating vortices pairs are present;ii)the root mean square velocity around the grid decreases rapidly with the increasing height above the grid（horizontal plane）,satisfying a power law:’-1.425（ca.-3/2）th’power of height;iii)the degree of homogeneity and isotropy close to the two sidewalls of the mixing box are larger than 1,suggesting anisotropic regions exit near the two sidewalls;iv)the magnitude of the inverse turbulence intensities is not 0 but relatively small,suggesting that the present results can be compared with previous results of zero-mean turbulence due to the lower strength of mean flow;v)the time-averaged turbulent kinetic energy and time-averaged vertical flux of turbulence kinetic energy decrease with the increasing height above the midplane of the grid（horizontal plane）and the latter one is positive,suggesting that the time-averaged turbulent kinetic energy decays far away from the grid but always transfers upwards;vi)the Taylor’s Eulerian integral length scale is approximately linearly related to the height above the midplane of the grid（horizontal plane）,which increases with the height,suggesting that the average size of an eddy increases with the upward development of turbulence;vii)the Eulerian frequency spectrum for the horizontal and vertical velocities decrease with the increasing height above the midplane of the grid（horizontal plane）and the power laws are approximately ω^-1（ω refers to the frequency）,suggesting that the horizontal and vertical turbulent kinetic energy decrease far away from the grid and they may be subjected to the finite Reynolds number effect of turbulence.By using ANSYS Fluent,based on the large eddy simulation,together with the multiphase flow model and the moving mesh technique,this thesis simulated the vertically-oscillating-grid mixing box which has the same size as the one used in the laboratory above to further study the physics within the turbulent layer.Results show:i)the vorticities around the square bars are significantly higher before the grid reaches at both the top and bottom of the stroke,suggesting that the jet generated by the grid is more intense between the highest and lowest positions of the grid;ii)the distribution shape of vorticity in the horizontal plane is similar to the geometry of the grid,and then becomes irregular,suggesting that turbulence is fully developed;iii)the root mean square velocity together with the turbulent kinetic energy and vertical flux of turbulence kinetic energy within the turbulent layer decrease with the increasing height above the grid（horizontal plane）.They are in good agreement with those experimental ones above.