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Many river engineers have hitherto suggested that there might exist cellular secondarycurrents in a straight river which had a pair of counter-rotating streamwise vortices with avortex diameter of about the flow depth. These cellular secondary currents create variousbed configurations such as sand ribbons which are alternating ridges and broad hollows andaligned parallel to the mean flow. Also, they may cause the three-dimensional flow patternsand sediment transportation in an actual river. However, the origin of the cellular secondarycurrent and its physical mechanism are quite unknown at present.In order to reveal the characteristics of the secondary currents and also their interrelationto the bursting phenomena occurring originally in the wall region, the present study hasexamined the turbulent structures in air duct flows with and without the cellular secondarycurrents. Considering that it was very difficult to measure precisely the secondary currentsin water flow because they were only a few percentage of the main flow, they could bemeasured by hot-wire anemomaters in two-dimensional air duct flow which correspondedsimilarly to open-channel flow. By systematically varying the bed configurations in whichthe longitudinal and alternate stripes of smooth and rough beds were set spanwisely, thegeneration and maintenance mechanism of the secondary currents were investigated.As the result, the strongest secondary currents were created when the width of the stripeswas equal to half of the width of the duct which corresponded to the flow deth in open-channel.The effects of the secondary currents on the mean velocity, turbulence intensities andReynolds stress were examined in detail, and then they could be explained well theoreticallyby means of (1) equation of motion, (2) equation of vorticity and (3) equation of turbulentenergy.Next, the effect of the secondary current on the bursting motion was examined by usingthe pattern-recognition method. Although the relative intensity of sweep to ejection motionswas affected by the secondary current, the bursting period was not varied. So, it issuggested that the bursting and cellular secondary current are phenomena different fromeach other and both of them coexist self-consistently.

An investigation of three-dimensional turbulent structure, I.e.cellular secondary currentsor longitudinal vortices, is very important in hydraulic engineering because these currentscause three-dimensional bed shear stress, sediment distributions and bed configurations suchas sand ridges in straight rivers.The present study has carried out highly accurate measurements of secondary currentsin fully-developed open channel flows by making use of a powerful two-color Laser DopplerAnemometer (LDA) system with a direct digital signal processing. The primary and secon-dary velocity distributions, the maximum-velocity-dip phenomena, the bed and side-wallshear stress distributions and also the turbulence characteristics were made clear by varyingthe aspect ratio of channel. The effects of free surface on three-dimensional structurewere then examined by comparison with the data of closed duct flow. A strong free-surfacevortex is produced due to high an-isotropy of turbulence which is caused by the existenceof free surface. This vortex produces a velocity dip and also restrains the development ofthe bottom vortex. Although the bottom vortex may be divided into smaller ones with anincrease of the aspect ratio, multi-cellular secondary currents are not produced any longerin the central zone. The variation of bed shear stress, however, suggests strongly that multi-cellular secondary currents will be produced in all the cross section by a mutual interactionof the flow and sand ridges.

Numerical calculation techniques of turbulent shear flows are classified into two categories:one is K-e turbulence model and the other is large eddy simulation (LES). The standard K-emodel has been established at present to predict turbulent structure in closed duct flows, whileLES is being developed to predict coherent eddy structure in more simple duct flows. Thestandard K-e model cannot be, however, applied to open channel surface flows, because theturbulence near the free surface is more depressed than the closed duct flows.In the present study a new modified K-e model is proposed to predict reasonably the turbulentstructure in open channel flows with both of low and high Reynolds numbers. The numericalcalculations indicate a splendid agreement with the experimental data which were obtained bymaking use of hot-film and Laser Doppler anemometers.

For establishment of a reasonable hydraulic design of multi-diffuser by which various wastewaters such as sewage or warmed water can be efficiently diffused in river or sea, an experimen-tal investigation on the turbulent structure of two-dimensional dual-jet whose two nozzles wereset in parallel has been performed. By systematically varying the ratio of flow quantity of eachjet and the space between both nozzles, the mean velocity distribution and the turbulence charac-teristics of the dual jet have been investigated through hot-film measurements and visual obser-vations.Since the flow axes of dual jet were attracted and then merged each other, the flow fieldcould be divided into three subregions: (1) two-single-jet region (before mergence), (2) one-single-jetregion (after mergence) and (3) transition region between (1) and (2). The turbulent structure ofeach subregion could be described fairly well by a simple model in which two Gaussian profileswere combined under the conservation law of momentum flux. A noticeable fact has been alsoobtained from the characteristics of intermittency factor, macro-scale of turbulence and spectraldistribution that a coherent motion, so-called 'puff phenomenon', appeared near the edge of potentialcore of both single- and dual-jets.