Mikio Hino

Experiments on transition to turbulence in an oscillatory pipe flow

Experiments on transition to turbulence in a purely oscillatory pipe flow were performed for values of the Reynolds number R δ , defined using the Stokes-layer thickness δ = (2ν/ω) ½ and the cross-sectional mean velocity amplitude U, from 19 to 1530 (or for values of the Reynolds number R e , defined using the pipe diameter d and U , from 105 to 5830) and for values of the Stokes parameter λ = ½ d (ω/ 2ν ) ½ ( ν = kinematic viscosity and ω = angular frequency) from 1·35 to 6·19. Three types of turbulent flow regime have been detected: weakly turbulent flow, conditionally turbulent flow and fully turbulent flow. Demarcation of the flow regimes is possible on R λ , λ or R e , λ diagrams. The critical Reynolds number of the first transition decreases as the Stokes parameter increases. In the conditionally turbulent flow, turbulence is generated suddenly in the decelerating phase and the profile of the velocity distribution changes drastically. In the accelerating phase, the flow recovers to laminar. This type of partially turbulent flow persists even at Reynolds numbers as high as R e = 5830 if the value of the Stokes parameter is high.

STRUCTURE OF THE TURBULENT FLOW FIELD UNDER BREAKING WAVES IN THE SURF ZONE

The structure of turbulence and its role in the breaking wave dynamics within the surf zone have been investigated through laboratory experiments using several flow visualization techniques and a fibre-optic LDV system. The results indicate that there exists a characteristic structure of large-scale eddies referred to here as ‘horizontal eddies’ and ‘obliquely descending eddies’, which has a significant role in the generation of Reynolds stress and thus affects the deformation of the mean flow field. The experiments also reveal that these eddies caused by the wave breaking bring a large amount of vorticity (with non-zero average) into otherwise almost irrotational velocity fields, resulting in the generation of vorticity-related mean flow fields as well as turbulence (vorticity-containing velocity fluctuation). This means that the breaking waves in the surf zone can be regarded as pseudowaves which consist of irrotational velocity components as ‘wave motion’ and appreciable amounts of rotational mean velocity components as ‘eddying motion’ (with non-zero mean vorticity) together with turbulence. It is found that the generation of the mean rotational velocity component due to wave breaking causes considerable increase in mass and momentum transport, as compared with ordinary non-breaking waves, and thus a decrease in wave height.

Organized structures in developing turbulent flow within and above a plant canopy, using a Large Eddy Simulation

A Large Eddy Simulation (LES) model representing the air flow within and above a plant canopy layer has been completed. Using this model, the organized structures of turbulent flow in the early developmental stages of a crop are simulated and discussed in detail.The effect of the drag due to vegetation is expressed by a term added to the three-dimensional Navier-Stokes equation averaged over the grid scale. For the formulation of sub-grid turbulence processes, the equations for the time-dependent SGS (Sub-Grid-Scale) turbulence energy equation is used, which includes the effects of dissipation (both by viscosity and leaf drag), shear production and diffusion.The organized structure of turbulent flow at the air-plant interface, obtained numerically by the model, yields its contribution to momentum transfer. The three-dimensional large eddy structures, which are composed of spanwise vortices (‘rolls’) and streamwise vortices (‘ribs’), are simulated near the air-plant interface. They are induced by the shear instability at inflection points of the velocity profile. The structure clearly has a life cycle. The instantaneous image of the structure is similar to those observed in the field observations of Gaoet al. (1989) and in the laboratory flume experiments of Ikeda and Ota (1992). These organized structures also account for the well known fact that the sweep motion of turbulence dominates momentum transport within and just above a plant canopy, and the motion of ejection prevails in the higher regions.

THEORY ON FORMATION OF RIP-CURRENT AND CUSPIDAL COAST

The steady state profile of the longshore current induced by regular, obliquely incident, breaking waves, over a bottom with arbitrary parallel bottom contours, is predicted. A momentum approach is adopted. The wave parameters must be given at a depth outside the surf zone, where the current velocity is very small. The variation of the bottom roughness along the given bottom profile must be prescribed in advance. Depth refraction is included also in the calculation of wave set-down and set-up. Current refraction and rip-currents are excluded. The model includes two new expressions, one for the calculation of the turbulent lateral mixing, and one for the turbulent bottom friction. The term for the bottom friction is non-linear. Rapid convergent numerical algorithms are described for the solution of the governing equations. The predicted current profiles are compared with laboratory experiments and field measurements. For a plane sloping bottom, the influence of different eddy viscosities and constant values of bottom roughness is examined.The calculation of turbulent flow using Naviers equations assumes the introduction of a turbulent viscosity coefficient the value of which is normally constant, conforming with Boussinesqs hypothesis. It was shown that setting aside this hypothesis, a velocity profile quite different to that resulting from the classic theory is obtained in the case of flow induced by wind. This result appears to be confirmed by the tests carried out in the Mediterranean. The advantage of this method is that it gives the vertical turbulent diffusion which is of particular interest to pollution studies.In the numerical method of prediction of wind waves in deep water, Hasselmanns nonlinear interaction theory is applied. This method assumes the energy balance of individual component waves. However, the total energy balance must exist in the transformation of irregular waves in shoaling water. In this investigation, experiments were carried out on the transformations in shoaling water of composite waves having two components and random waves having one or two main peaks. It was found that the elementary component wave height of the composite waves and the elementary peak power of the random waves decrease with decrease in the water depth. This reason can be explained qualitatively by the theory of the elementary component wave height change of finite amplitude waves in shoaling water. The secondary component wave height of the composite waves and the secondary peak power of the random waves increase with decrease in the water depth. This can be explained qualitatively by Hamadas theory of nonlinear interaction in uniform depth.Experiments have been carried out by using non-breaking waves and breaking waves to investigate the wave forces on a vertical circular cell located in the shallow water. Based on the experimental data, the drag coefficient and the inertia coefficient of a circular cylinder and the curling factor of breaking waves are estimated, and the computation methods of wave forces are examined. As a result, it is shown that the phase lag of inertia forces behind the accelerations of water particles should be considered for the estimation of the drag coefficient as well as the inertia coefficient. In addition the previous formula of the maximum breaking wave forces acting on a cell or a pile is revised by introducing the effects of the above-mentioned phase lag and another phase difference, both of which are functions of the ratio of the cell diameter to the wave length. • It is confirmed that the proposed formula is applicable even to the large cell with the diameter comparable to the wave length. INTRODUCTION Many studies have been done on the impulsive pressures acting on a vertical wall, but there has been very little investigation of breaking wave forces on a cell-type structure. The breaking wave forces should be taken into consideration all the same in the design of pile-type or cell-type structures in nearshore area, because breaking waves cause extreme shock pressures on a cell structure asThe air bubble plume induced by the steady release of air into water has been analyzed with an integral technique based on the equations for conservation of mass, momentum and buoyancy. This approach has been widely used to study the behavior of submerged turbulent jets and plumes. The case of air-bubble induced flow, however, includes additional features. In this study the compressibility of the air and the differential velocity between the rising air bubbles ,and the water are introduced as basic propertie s of the air bubble plume in addition to a fundamental coefficient of entrainment and a turbulent Schmidt number characterizing the lateral spreading of the air bubbles. Theoretical solutions for twoand three-dimensional air-bubble systems in homogeneous, stagnant water are presented in both dimensional and normalized form and compared to existing experimental data. The further complication of a stratified environment is briefly discussed since this case is of great practical interest. This paper is to be considered as a progress report, as future experimental verification of various hypotheses is needed.

Analysis Of Hydrologic Characteristics From Runoff Data - A Hydrologic Inverse Problem

Abstract In this paper, the characteristics of hydrologic basins as well as rainfall are derived only from daily runoff data. This is a sort of inverse hydrologic problem. First, time series of daily runoffs are separated into components of surface flow, inter-flow and groundwater flow by numerical filters whose cut-off infrequencies are deter-mined from the order of magnitude comparison ofAR (autoregressive) coefficients of runoff data and confirmed by the coherence gap in rainfall-runoff data. By assuming the daily rainfall to be of white noise, the time series of each component thus separated are fitted to the AR model from which the hydrologic impulse response characteristics (unit hydrographs for each subsystem) are determined. Time series of the daily rainfall are inversely generated from the original time series of the daily runoff and the coefficients of AR model determined as above. The estimated time series of rainfalls agree relatively well with the real, but screened, rainfall data. Finally, a nonlinear separation law of effective rainfall into several rainfall components is estimated.

A laboratory experiment on the role of grass for infiltration and runoff processes

Abstract Two lysimeters with the same dimensions were provided, and filled with the same loam clay. On the soil surface of one lysimeter, grass was planted to compare the hydrologic response of the grassed lysimeter with that of the other bare soil lysimeter. About half of the runoff from the bare soil lysimeter occurred as overland flow, the rest being groundwater flow. Overland flow scarcely occurred from the grassed lysimeter. Grass roots that developed deep into the soil layer play an important role in increasing the infiltration rate as well as in drying the soil uniformly throughout the soil layer by evapotranspiration, preparing for high infiltration and large rainwater storage for the subsequent rainfall event. Accordingly, the total loss by evapotranspiration from the grassed soil amounts to almost twice that from the bare soil. For an evaporation- and evapotranspiration-prohibited experiment, the recession characteristics from a saturation state showed similar features for the bare and grassed soils, indicating the same microstructure of high moisture reservability for both soils. The well-developed grass root system reformed the soil structure considerably to produce the seemingly contradicting characteristics of high moisture conductivity and high moisture reservability; i.e. a high infiltration rate and prolonged groundwater discharge. Finally, the importance of the initial soil moisture in the rainfall-runoff process, rainfall loss and runoff ratio is stressed.

Effect of initial soil moisture content on the vertical infiltration process: a guide to the problem of runoff-ratio and loss

Initial soil moisture content plays an important role in hydrologic processes. In this paper, the relationships among the initial soil suction (an index of the soil-moisture content), total amount of rainfall, rainfall intensity, runoff loss, runoff ratio and lag between rainfall and runoff occurrence are examined with a simplified one-dimensional analog of the infiltration process based on a cylindrical lysimeter with artificial rainfalls. It is conclusively shown that the loss of infiltrated rainfall is uniquely correlated with the initial soil-moisture content measured by tensiometers and is independent of the total amount of rainfall as well as of the rainfall intensity, if it does not exceed the infiltration rate of the soil. Consequently, the runoff ratio is a function of the initial soil-moisture content and total rainfall, but not a function of the rainfall intensity. The nondimensional propagation velocity of the wetting front or inversely the nondimensional lag of runoff occurrence defined by T★1 = T1rH (T1 = lag between rainfall and runoff; r = infiltration rate, in this case, rainfall intensity; and H = depth of the lysimeter or distance from the surface to the less pervious layer) is also well correlated with the initial soil moisture. These results are further proven and extended through the use of numerical experiments, solving the Richards equation of unsaturated infiltration. An approximate analytical solution has been derived for the runoff ratio and rainfall loss.

Identification and prediction of nonlinear hydrologic systems by the filter-separation autoregressive (AR) method: Extension to hourly hydrologic data

Abstract In this paper, our method — from the hydrologic inverse detection method originally proposed for daily runoff data analysis — is extended to hourly hydrologic data analysis. A few modifications of the original inverse method (filter separation and autoregressive (AR) method) are necessary in order to apply it to hourly data. 1. (1) The cut-off frequency to separate the total runoff time series into component runoffs was determined by the slope of the semi-logarithmic plot of the recession curve. 2. (2) Coefficients of the autoregressive moving average (ARMA) model applied to each of the subsystems were determined by the least-squares criterion from the recession period data when rainfall stopped; thus the ARMA model is reduced to the AR model with a white-noise error. A remaining coefficient to be multiplied by the rainfall (input) is obtained by the continuity condition of effective rainfall and runoff. The conclusions of this analysis are as follows: 1. (1) Nonlinear hourly hydrologic systems are easily and precisely identified and predicted by the present method. 2. (2) Each subsystem of surface runoff, interflow and groundwater runoff is linear; the nonlinearity of the rainfall-runoff system is caused mainly by the nonlinearity of the separation of rainfall into component rainfalls. The nonlinear separation rule of rainfall into rainfall components is derived from inversely estimated rainfalls. 3. (3) Time series of hourly rainfalls can be inversely estimated from hourly runoff by this method and it compares well with the observed effective precipitation time series regardless of the size of watershed.

Computer experiment on smoke diffusion over a complicated topography

Abstract A technique of numerical experiments on smoke diffusion over a complicated topography by means of a high-speed digital computer is developed. The technique consists of the computation of field of wind velocity from the dynamic equations of fluid motion, and then solving the Fickian equation of diffusion to obtain the distribution of smoke concentration. Comparison of the computed distribution with some model results suggests that the method of computer experiments is as reliable as, or more reliable than, the conventional wind-tunnel experiments for the estimation of the distributions of wind velocity and smoke concentration over the area with complex topographical features.

Separation of a storm hydrograph into runoff components by both filter-separation AR method and environmental isotope tracers

Abstract The purpose of this paper is to separate the total storm runoff into runoff components of overland flow, interflow and groundwater flow. The hourly storm hydrograph is separated into runoff components by the following two methods and the results are compared with each other: 1. (1) The first method is to separate the time series of the total flow into overland flow, interflow, and groundwater flow by using numerical filters determined by the slope of the semi-logarithmic plot of the recession curve. 2. (2) The second one is to separate the total storm runoff, using the geochemical data, into each component by the simultaneous solution of the mass balance equations describing the fluxes of water and the tracer isotope in the stream. 3. (3) Both results compare relatively well with each other.