Turbulent Structure in Steep Open-Channel Flows

Velocity measurement with a fiber-optic laser-Doppler anemometer were conducted in steep open-channel flows over smooth and incompletely rough beds. The channel slopes were changed between 1/500 and 1/25 by steps, and as a result, the Froude number changed from 0.5 to 3. The law of the wall and the distributions of turbulence intensity were examined in more detail. The von Kármán constant κ in the log law was verified to be unversally equal to 0.41. On the other hand, the integral constant A in the log law coincided with the usual value of 5.29 in subcritical flows, whereas it decreased with an increase of the bed slope in supercritical flows. This decrease of A was explained by a decrease of the damping factor B in the van Driest's mixing-length formula. The decrease of B was well correlated with an increase of the friction velocity and the roughness. The streamwise turbulence intensity became smaller in the near-wall region than the semi-empirical universal formula, as the friction velocity became larger

Numerical Calculation of Turbulent Open-Channel Flows in Consideration of Free-Surface Effect

Numerical calculation techniques of turbulent shear flows are classified into two categories : one is the k-ε turbulence model, and the other is the large eddy simulation (LES). The standard k-ε model has been established at present to predict a turbulent structure in jets, boundary layers and closed channel flows, while LES is being developed to predict a coherent eddy structure in simpler channel flows. The standard k-ε model cannot be, however, easily applied to open channel surface flows, because the turbulence near the free surface is more depressed than the closed channel flows. In the present study, a new modified k-ε model is proposed to predict reasonably a turbulent structure in open channel flows with both the low and high Reynolds numbers. The numerical calculations indicate a good agreement with the experimental data which were obtained by making use of hot-film and Laser Doppler anemometers

Flow resistance law in open-channel flows with rigid and flexible vegetation

River hydrodynamicsBed roughness and flow resistanc

Coherent turbulence structure generated by wind-induced water waves

River engineeringInnovative field and laboratory instrumentatio

Bursting Phenomenon near the Wall in Open-channel Flows and its Simple Mathematical Model

In this paper we propose a new evaluation method for the bursting period, on the basis of the phenomenological consideration that the number of the occurrences of interaction-like motions should be removed from those of the ejection or the sweep events in the sorted Reynolds-stress fluctuating signals. Then, it is confirmed by this method that the mean bursting period in open-channel flows may be universally expressed by outer rather than inner parameters, and that its probability distribution becomes log-normal, irrespective of the Reynolds and the Froude numbers, as well as the wall roughness. Next, in order to explain even quantitatively the bursting process or the turbulent structure in the wall region, we propose a simple mathematical model on the basis of the Einstein-Li model and also the knowledge of the bursting-period characteristics obtained above. Though the present model is inherently quasi-two-dimensional and quasi-linear, this model can describe fairly well some distributions of mean-velocity, turbulence intensities and Reynolds stress. In particular, it can satisfactorily explain a sequence of the bursting process

Structure of Instantaneous Reynolds Stress over a Permeable Open-channel with Suction or Injection

In the present study, we investigate experimentally the structure of the instantaneous Reynolds stress in open-channel flow over a permeable bed with suction or injection. Then, the fluctuating signals obtained from the X-type hot-films are conditionally analyzed in order to examine the effect of suction or injection on the turbulence production mechanism or the bursting phenomenon. We can then obtain the following results. The absolute magnitudes of the turbulence intensities and the Reynolds stress near the wall increase with an enlargement of the injection, while they decrease with an enlargement of the suction. However, the fraction of time occupied by each bursting event and the contributions of its event to the Reynolds stress against any hole size are almost the same, irrespective of suction or injection. The promotion of turbulence by injection or its suppression by suction may be caused by similar variations of three parameters of the bursting intensity, the bursting period and the bursting duration time. To sum up, the internal structure of the turbulence or the bursting mechanism is not essentially influenced by suction or injection, as long as the flow is still turbulent

Turbulence Measurements in Compound Open Channels

An investigation on a three-dimensional ( 3 -D) turbulent structure including turbulence-driven secondary currents in compound open-channel flows is a very important topic in hydraulic and river engineerings, as well as in fluid mechanics. In this study, accurate measurements in fully-developed compound open-channel flows were conducted by making use of a filder-optic laser Doppler anemometer (FLDA). Secondary velocities could be measured very accurately with the present 3-D measurement system. The characteristics peculiar to compound open channel flows are recognized in a junction region between the main channel and the flood plain, whereas the characteristics in rectangular open channel flows are observed in a region near the side wall of the main channel. Strong inclined secondary currents, which are associated with a pair of longitudinal vortices, are generated in a junction region between the main channel and the flood plain. The primary mean velocity filed is directly influenced by these secondary currents. Turbulence intensisies and the Reynolds stresses are also reveald in detail. Moreover, the effects of channel gemetry and bed roughness on turbulent structures are examined

The effect of coherent waving motion on turbulence structure in flexible vegetated open channel flows

River hydrodynamicsOverbank flows and vegetatio

Simultaneous measurements of concentration and velocity with combined PIV and planar LIF in vegetated open-channel flows

River hydrodynamicsOverbank flows and vegetatio

Structure of Space-time Correlations of Bursting Phenomenon in an Open-channel Flow

The present study is to investigate the structure of space-time correlations of bursting motions, such as ejections and sweeps in an open-channel flow, by a new conditional sampling method in which simultaneous measurements of the instantaneous Reynolds stress have been performed by two dual-hot-film probes. One probe was fixed near the edge of buffer layer and used as a detecting probe of the bursting motions, while the other probe was moved in both the streamwise and flow-depth-wise directions. The main conclusions obtained from the above are as follows : (1) The spatial and time scales of the streamwise turbulent velocity component are larger than those of the vertical velocity component. (2) The spatial and time scales of the sweep motion are also larger than those of the ejection motion. (3) The spatial scale of the ejection motion extends more widely downstream than upstream, and vice versa for the sweep motion. (4) The bursting motion is a kind of a large-scale eddy structure, and its coherent structure is fairly inclined downstream toward the wall. It is convected downstream with a longer life-time than the bursting passing-period, namely in the frozen-turbulence-like manner. Next, a qualitative model is proposed which attempts to explain the space-time structures of the bursting phenomenon, on the basis of the above anemometry information and other visual information