An Experimental Study On Hydrodynamic Characteristics Of Jet Flow Under Wave Condition

The engineering practice at home and abroad shows that the organic combination of underwater multi-hole discharge and land sewage treatment is the best choice for sewage treatment in riverside and coastal cities.Underwater multi-hole discharge is a typical form of jet flow in environmental hydraulics.The hydrodynamic characteristics of jet flow is closely correlated with the receiving water environment,influenced by open channel flow in river environment and by wave and tidal current in coastal zone.The effect of open channel flow and tidal current on jet flow can be regarded as the effect of steady flow on jet flow,while the reciprocating motion of water induced by wave results in significant difference between the characteristics of jet flow under wave and steady flow conditions.Therefore,it is of great theoretical significance and practical application value to study the hydrodynamic characteristics of jet flow under wave condition.The analysis of single-jet and dual-jets phenomena under wave condition was combined to measure the pressure and velocity distribution within the single-jet and dual-jets zones under wave condition,and analyze the effect of different wave heights,cycles,initial velocities of jet flow and hole spacings on the hydrodynamic characteristics of jet flow from the perspectives of velocity and pressure.The main conclusions are described as follows:1.Compared with the jet flow in static water environment,the jet flow under wave condition has a more complex motion mechanism,and it is not suitable to directly use the static water jet pressure and the wave pressure alone to represent the effect of wave on jet flow.Wave action enhances the attenuation of axial velocity and pressure of jet flow,and the attenuation amplitude is correlated with the incident wave height and the wave period.At the same wave height,the greater the wave period is,the faster the attenuation of jet flow will be;at the same wave period,the bigger the wave height is,the faster the attenuation will be.2.From analysis of the rule of distribution and attenuation of velocity and pressure,the zonal structure of jet flow can be divided into the jet dominant zone,the transition zone,and the wave dominant zone.In the jet dominant zone,the velocity,pressure,jet flow axis position change insignificantly compared with the jet flow under static water condition.In the transition zone,the amplitudes of velocity and pressure attenuation exhibit major difference under different wave conditions,and offset of the jet flow axis occurs.In the wave dominant zone,the effect of jet flow is weakened;the wave action is further enhanced;and the velocity and pressure attenuation greatly slows down.The ranges of jet flow zones vary with the condition of incident wave.Generally,z/d<6~8 is the jet dominant zone;6~8<z/d<21~25 is the transition zone;z/d>21~25 is the wave dominant zone.Bigger wave height and period result in smaller values.3.Under wave condition,the main axis of jet flow swings left and right obviously,and its swing period is basically consistent with the wave period.From the mean state,the wave makes the peak pressure on the cross-section of jet flow offset along the wave propagation direction,and the offset section and position are correlated with the incident wave elements.At the same height,as the period increases,the section where the axis starts offsetting moves downward,i.e.,z/d is decreased,and the range of the jet dominant zone is reduced and the range of the transition zone increased.At the same wave period,as the wave height increases,the position where the axis starts offsetting lowers down,i.e.,z/d is decreased,and the range of the jet dominant zone is reduced and the range of the wave dominant zone increased.4.The degree of mixing between the holes of dual jets is closely correlated with the jet spacing Do and the initial velocity of jet flow wo.With the increase of hole spacing,the effect of mixing between the holes reduces slowly.When Do=3d~6d,the effect of mixing is obvious;when the spacing increases to Do=9d,the jet flows begin to show the single-jet trend until they are not mixed with each other and maintain the flow characteristics of single jet flow.5.Due to the mutual effect between wave and jet flow and the mutual entrainment effect between two jet flows,the hydrodynamic characteristics of dual jets under wave condition are more complex than those of the dual-jets under static water condition and the single-jet under wave condition.In the jet dominant zone,the velocity,pressure and jet flow axis of dual jets change insignificantly compared with the dual jets under static water condition.In the transition zone,the first hole is similar to the single jet;due to block by the first hole,the velocity and pressure attenuation of the second hole is lower than that of the first hole;the distance of the axis of the second hole from the main axis is reduced in comparison with the first hole,and there is certain lagging.In the wave dominant zone,the dual jets are combined to one jet flow,the hydrodynamic characteristics of which are similar to the single jet.Under wave condition,the blocking effect of dual jets is associated with the hole spacing Do and the initial velocity of jet flow wo.With the increase of jet spacing and the decrease of initial velocity of jet flow,the blocking effect of the first hole on the second hole is obviously reduced.6.Under certain wave condition,the existence of wave promotes the adsorption and mixing between dual jets,resulting in earlier occurrence of section merging of the two jet flows,i.e.,z/d is decreased,and such promoting effect becomes more obvious with the increase of wave height and period.