排序方式: 共有3条查询结果,搜索用时 0 毫秒
1
1.
Madan Mohan Guin Hiroharu Kato Hajime Yamaguchi Masatsugu Maeda Masaru Miyanaga 《Journal of Marine Science and Technology》1996,1(5):241-254
Determination of the flow structure near the wall is essential for a clear insight into the phenomenon of skin friction reduction
by microbubbles in a turbulent boundary layer. An important parameter, is the bubble concentration or void fraction in the
wall region in drag-reducing conditions. The purpose of this paper is to show drag-reducing effects due to microbubbles in
a water channel and, more importantly, to show the dependence of the drag-reduction values on the near-wall void fraction.
A two-dimensional channel with an aspect ratio of 10 was specially built for this purpose with provisions for air injection
through porous plates. Skin friction was directly measured by a miniature floating element transducer with a 5-mm circular
sensing disk mounted flush on the top wall 67 channel-heights downstream of the injector. The wall friction in the presence
of air bubbles was found to be reduced under the same bulk velocity when compared with the value without air. Detailed void
fraction profiles across the channel were obtained by a sampling probe and a fiber-optic probe. Better collapse of the drag
reduction data, independent of different profile shapes, was found when plotted against the near-wall void fraction than against
a cross-sectional mean void fraction. While this dependence reconfirms that the phenomena are essentially inner-region dependent,
the lack of influence of the bubble distribution patterns away from the wall implies lack of outer region influence. 相似文献
2.
Hyun Woong Cho Bhargava R. Chilukuri Jorge A. Laval Angshuman Guin Wonho Suh 《运输规划与技术》2020,43(5):475-487
ABSTRACT This paper presents a case study of the optimal ALINEA ramp metering system model of a corridor of the metro Atlanta freeway. Based on real-world traffic data, this study estimates the origin-destination matrix for the corridor. Using a stochastic simulation-based optimization framework that combines a micro-simulation model and a genetic algorithm-based optimization module, we determine the optimal parameter values of a combined ALINEA ramp metering system with a queue flush system that minimizes total vehicle travel time. We found that the performance of ramp metering with optimized parameters, which is very sensitive possibly because bottlenecks are correlated, outperforms the no control model with its optimized parameters in terms of reducing total travel time. 相似文献
3.
Yuki Yoshida Yoshiaki Takahashi Hiroharu Kato Madan Mohan Guin 《Journal of Marine Science and Technology》1998,3(1):30-36
A method of enveloping the hull with a sheet of microbubbles is discussed. It forms part of a study on means of reducing the skin friction acting on a ship's hull. In this report, a bubble traveling through a horizontal channel is regarded as a diffusive particle. Based on this assumption, an equation based on flow flux balance is derived for determining the void fraction in approximation. The equation thus derived is used for calculation, and the calculation results are compared with reported experimental data. The equation is further manipulated to make it compatible with a mixing length model that takes into account the presence of bubbles in the liquid stream. Among the factors contained in the equation thus derived, those affected by the presence of bubbles are the change of mixing length and the difference in the ratio of skin friction between cases with and without bubbles. These factors can be calculated using the mean void fraction in the boundary layer determined by the rate of air supply into the flow field. It is suggested that the ratio between boundary layer thickness and bubble diameter could constitute a significant parameter to replace the scale effect in estimating values applicable to actual ships from corresponding data obtained in model experiments.List of symbols
a
1
proportionality constant indicating directionality of turbulence
-
B
law-of-the-wall constant
-
C
f
local skin-friction coefficient in the presence of bubbles
-
C
f0
local skin-friction coefficient in the absence of bubbles
-
d
b
bubble diameter (m)
-
g
acceleration of gravity (m/s2)
-
j
g
flow flux of gas phase accountable to buoyancy (m/s)
-
j
t
flow flux of gas phase accountable to turbulence (m/s)
-
k
4
constant relating reduction of liquid shear stress by bubble presence to decrease of force imparted to bubble by its displacement due to turbulence
-
l
b
mixing length of gas phase (m)
-
l
m
mixing length of liquid phase (m)
-
l
mb
diminution of liquid phase mixing length by bubble presence (m)
-
Q
G
rate of air supply to liquid stream (l/min)
-
q
/g
velocity of bubble rise (m/s)
- 2R
height of horizontal channel (m)
-
T
*
integral time scale (s)
-
U
m
mean stream velocity in channel (m/s)
-
U
friction velocity in channel (m/s)
-
V
volume of a bubble (m3)
-
u, ¯ v
time-averaged stream velocities inx- andy-directions, respectively (m/s)
-
u, v
turbulent velocity components inx- andy-directions, respectively (m/s)
- v
root mean square of turbulence component in they-direction (m/s)
-
root mean square of bubble displacement iny-direction with reference to turbulent liquid phase velocity (m)
-
y
displacement from ceiling (m)
-
local void fraction
- m
mean void fraction in boundary layer
- m
constant relating local void fraction to law-of-the-wall constant
- t
reduction of turbulent stress (N/m2)
-
law-of-the-wall constant in turbulent liquid region in absence of bubbles
- 1
law-of-the-wall constant in turbulent liquid region in presence of bubbles
- 2
law-of-the-wall constant in gas phase
- m
constant indicating representative turbulence scale (m)
-
viscosity (Pa × s)
-
v
kinematic viscosity (m2/s)
-
density (kg/m3)
Suffixes
G
gas
-
L
liquid
- 0
absence of bubbles 相似文献
1