Drag and Turbulent Characteristics of Mobile Bed Channel With Mixed Vegetation Densities Under Downward Seepage

2016 ◽  
Vol 138 (7) ◽  
Author(s):  
Thokchom Bebina Devi ◽  
Rishabh Daga ◽  
Sumit Kumar Mahto ◽  
Bimlesh Kumar
Keyword(s):  
Drag Coefficient ◽  
Reynolds Stress ◽  
Flow Resistance ◽  
Flow Characteristics ◽  
Vegetation Density ◽  
Streamwise Direction ◽  
Mobile Bed ◽  
Downward Seepage ◽  
Turbulent Characteristics ◽  
Sediment Particles

The present study addresses the drag owing to the presence of vegetation and turbulent characteristics in a mobile bed channel, characterized by fully submerged vegetation formed by nonuniform vegetation densities. The influence of seepage on the velocity profiles, Reynolds stress, and turbulence intensities is discussed. Experimental results show that vegetation density is one of the important parameters that affect the flow resistance. It is found that higher vegetation density when placed at the downstream side leads to a reduction in velocity, Reynolds stress, and turbulent intensities. Downward seepage increases the near bed velocity, Reynolds stress, and turbulent intensities. Moment analysis shows that there is an increase in the inrush of flow, and sediment particles are transported more toward the streamwise direction with the application of seepage. The dominance of sweep events over ejection events increases more sediment transport. However, high vegetation density when placed at the downstream portion slightly decreases the dominance of sweep event. Drag coefficient decreases near the vegetation top and increases near the bed. Downward seepage decreases the effect of drag offered by the vegetation stems. The reduction in flow characteristics, viz., velocity, Reynolds stress, turbulent intensities, in the downstream portion of lesser spacing vegetation stems is attributed an increased drag coefficient.

Turbulence Characteristics of Vegetated Channel With Downward Seepage

2016 ◽  
Vol 138 (12) ◽  
Author(s):  
Thokchom Bebina Devi ◽  
Anurag Sharma ◽  
Bimlesh Kumar
Keyword(s):  
Reynolds Stress ◽  
Reynolds Shear Stress ◽  
Experimental Studies ◽  
Flow Characteristics ◽  
Distribution Functions ◽  
Quadrant Analysis ◽  
Reynolds Stresses ◽  
Vegetation Zone ◽  
Increase With Increase ◽  
Downward Seepage

Experimental studies were carried out for investigating changes in flow characteristics with the presence of flexible vegetation in a channel. The study focuses on the effect of introducing downward seepage on velocity profiles, Reynolds shear stress (RSS), and different turbulent length scales in a vegetative channel. The presence of vegetation provides drag and reduces the flow velocity. The turbulence generation mainly comes from the oscillations occurring near the top of the vegetation as is evident from the achievement of maximum Reynolds stress near the top of the vegetation. Application of downward seepage results in a higher velocity zone in the lower vegetation zone and a higher Reynolds stress. Quadrant analysis shows that sweep and ejection contribute most to Reynolds stress. The dominance of sweep event over ejection event is more with the application of downward seepage which means more bed transport. Different turbulent length and time scales increase with increase in downward seepage percentage due to reduction in energy dissipation. The increase in the length scale and time scale with downward seepage infers that higher level of turbulence prevail near the bed with an increased eddy size resulting in higher Reynolds stresses with downward seepage. The universal probability distribution functions (PDFs) of velocity fluctuations, RSS, and conditional RSS of vegetative channel follow Gram Charlier (GC) series based on exponential distribution except that a slight departure of inward and outward interactions of conditional RSS is observed which may be due to weaker events.

scholarly journals Estimation of Vegetation-Induced Flow Resistance for Hydraulic Computations Using Airborne Laser Scanning Data

Water ◽  
2021 ◽  
Vol 13 (13) ◽  
pp. 1864
Author(s):  
Peter Mewis
Keyword(s):  
Laser Scanning ◽  
Flow Resistance ◽  
Laser Scanner ◽  
Airborne Laser Scanning ◽  
Density Parameter ◽  
Vegetation Density ◽  
Airborne Laser Scanner ◽  
Airborne Laser ◽  
Bed Roughness ◽  
Available Information

The effect of vegetation in hydraulic computations can be significant. This effect is important for flood computations. Today, the necessary terrain information for flood computations is obtained by airborne laser scanning techniques. The quality and density of the airborne laser scanning information allows for more extensive use of these data in flow computations. In this paper, known methods are improved and combined into a new simple and objective procedure to estimate the hydraulic resistance of vegetation on the flow in the field. State-of-the-art airborne laser scanner information is explored to estimate the vegetation density. The laser scanning information provides the base for the calculation of the vegetation density parameter ωp using the Beer–Lambert law. In a second step, the vegetation density is employed in a flow model to appropriately account for vegetation resistance. The use of this vegetation parameter is superior to the common method of accounting for the vegetation resistance in the bed resistance parameter for bed roughness. The proposed procedure utilizes newly available information and is demonstrated in an example. The obtained values fit very well with the values obtained in the literature. Moreover, the obtained information is very detailed. In the results, the effect of vegetation is estimated objectively without the assignment of typical values. Moreover, a more structured flow field is computed with the flood around denser vegetation, such as groups of bushes. A further thorough study based on observed flow resistance is needed.

scholarly journals Attenuation of Wave Energy Due to Mangrove Vegetation off Mumbai, India

Energies ◽  
2019 ◽  
Vol 12 (22) ◽  
pp. 4286 ◽  
Author(s):  
Samiksha S. V. ◽  
P. Vethamony ◽  
Prasad K. Bhaskaran ◽  
P. Pednekar ◽  
M. Jishad ◽  
...  
Keyword(s):  
Climate Change ◽  
Drag Coefficient ◽  
Wave Energy ◽  
Measured Data ◽  
Stem Diameter ◽  
Coastal Hazards ◽  
Sensitivity Analyses ◽  
Vegetation Density ◽  
Mangrove Vegetation ◽  
Set Up

Coastal regions of India are prone to sea level rise, cyclones, storm surges, and human-induced activities, resulting in flood, erosion, and inundation, and some of these impacts could be attributed to climate change. Mangroves play a very protective role against some of these coastal hazards. The primary aim of the study was to estimate wave energy attenuation by mangrove vegetation using modeling, and to validate the model results with measurements conducted off Mumbai coast, where a mangrove forest is present. Wave measurements were carried out from 5–8 August 2015 at three locations in a transect normal to the coast using surface-mounted pressure-level sensors in spring tide conditions. The measured data presented wave height attenuation of the order of 52%. Model set-up and sensitivity analyses were conducted to understand the model performance with respect to vegetation parameters. It was observed that wave attenuation increases with an increase in drag coefficient, vegetation density, and stem diameter. For a typical set-up in the Mumbai coastal region having a vegetation density of 0.175 per m2, stem diameter of 0.3 m, and drag coefficient varying from 0.4 to 1.5, the model reproduced attenuation ranging from 49% to 55%, which matches reasonably well with the measured data. Spectral analysis performed for the cases with and without vegetation very clearly portrays energy dissipation in the vegetation area. This study also highlights the importance of climate change and mangrove vegetation.

scholarly journals Numerical Study of Heat Transfer Intensification in a Circular Tube Using a Thin, Radiation-Absorbing Insert. Part 1: Thermo-Hydraulic Characteristics

Energies ◽  
2021 ◽  
Vol 14 (15) ◽  
pp. 4596
Author(s):  
Piotr Bogusław Jasiński
Keyword(s):  
Heat Transfer ◽  
Flow Resistance ◽  
Numerical Study ◽  
Flow Characteristics ◽  
Cylindrical Tube ◽  
Radiation Absorption ◽  
Fluid Temperature ◽  
Gas Temperature ◽  
Insert Size ◽  
Channel Diameter

The presented paper, which is the first of two parts, shows the results of numerical investigations of a heat exchanger channel in the form of a cylindrical tube with a thin insert. The insert, placed concentrically in the pipe, uses the phenomenon of thermal radiation absorption to intensify the heat transfer between the pipe wall and the gas. Eight geometric configurations of the insert size were numerically investigated using CFD software, varying its diameter from 20% to 90% of the pipe diameter and obtaining the thermal-flow characteristics for each case. The tests were conducted for a range of numbers Re = 5000–100,000 and a constant temperature difference between the channel wall and the average gas temperature of ∆T = 100 °C. The results show that the highest increase in the Nu number was observed for the inserts with diameters of 0.3 and 0.4 of the channel diameter, while the highest flow resistance was noted for the inserts with diameters of 0.6–0.7 of the channel diameter. The f/fs(Re) and Nu/Nus(Re) ratios are shown on graphs indicating how much the flow resistance and heat transfer increased compared to the pipe without an insert. Two methods of calculating the Nu number are also presented and analysed. In the first one, the average fluid temperature of the entire pipe volume was used to calculate the Nu number, and in the second, only the average fluid temperature of the annular portion formed by the insert was used. The second one gives much larger Nu/Nus ratio values, reaching up to 8–9 for small Re numbers.

scholarly journals Wave energy dissipation in the mangrove vegetation off Mumbai, India

2017 ◽  
Author(s):  
Samiksha S. Volvaiker ◽  
Ponnumony Vethamony ◽  
Prasad K. Bhaskaran ◽  
Premanand Pednekar ◽  
MHamsa Jishad ◽  
...  
Keyword(s):  
Energy Dissipation ◽  
Drag Coefficient ◽  
Wave Height ◽  
Wave Attenuation ◽  
Coastal Region ◽  
Measured Data ◽  
Vegetation Density ◽  
Mangrove Vegetation ◽  
Wave Height Attenuation ◽  
Set Up

Abstract. Coastal regions of India are prone to sea level rise, cyclones, storm surges and human induced activities, resulting in flood, erosion, and inundation. The primary aim of the study is to estimate wave attenuation by mangrove vegetation using SWAN model in standalone mode, as well as SWAN nested with WW3 model for the Mumbai coastal region. To substantiate the model results, wave measurements were carried out during 5–8 August 2015 at 3 locations in a transect normal to the coast using surface mounted pressure level sensors under spring tide conditions. The measured data presents wave height attenuation of the order of 52 %. The study shows a linear relationship between wave height attenuation and gradual changes in water level in the nearshore region, in phase with the tides. Model set-up and sensitivity analyses were conducted to understand the model performance to vegetation parameters. It was observed that wave attenuation increased with an increase in drag coefficient (Cd), vegetation density, and stem diameter. For a typical set-up for Mumbai coastal region having vegetation density of 0.175 per m2, stem diameter of 0.3 m and drag coefficient varying from 0.4 to 1.5, the model reproduced attenuation, ranging from 49 to 55 %, which matches well with the measured data. Spectral analysis performed for the cases with and without vegetation very clearly portrays energy dissipation in the vegetation area as well as spectral changes. This study has the potential of improving the quality of wave prediction in vegetation areas, especially during monsoon season and extreme weather events.

Analysis of Flow Resistance Inside Microchannels With Different Inlet Configurations Using Micro-PIV System

2003 ◽  
Author(s):  
Sang-Joon Lee ◽  
Guk-Bae Kim
Keyword(s):  
Flow Resistance ◽  
Reduction Rate ◽  
Flow Characteristics ◽  
Entrance Region ◽  
Curvature Radius ◽  
Microfluidic Chips ◽  
Mean Velocity ◽  
Dimethyl Siloxane ◽  
Micro Piv ◽  
Macro Scale

Most microfluidic chips consist of several microchannels inside. In order to design microfluidic chips efficiently, it is important to predict the flow passage and to understand the flow characteristics on the chip. In this study, the flow structure inside microchannels has been investigated using a micro-PIV system. We focused on the flow resistance with respect to the inlet configuration of microchannels. The microchannels made of poly-dimethyl-siloxane (PDMS) material were fabricated by a micro-molding technique using SU-8 (photoresist) master. The width (w) and depth of the microchannels were fixed as 100 μm and 58 μm, respectively. Six different inlet configurations with curvature radii in the ranges from r = 0.2w to 1.5w were tested in this study. As a result, with increasing the curvature radius of the inlet corner, the streamwise mean velocity develops slowly in the entrance region, but the fully developed velocity at further downstream is increased. When the curvature radius is larger than r = 0.6w, the reduction rate of flow resistance is not so significant. For the microchannels with r = 0.6w, 0.8w and 1.0w the downstream mean velocity at channel center has nearly the same value of about 276 mm/sec, 10.5% larger than that of r = 0.2w. The simple rounding of microchannel inlet corner reduces flow resistance effectively by smoothing the incoming flow. The length of entrance region is much smaller than that of macro-scale channel.

Flow Resistance Characteristics of a Specific Fuel RP-3 in Helical Tubes at Supercritical Pressure With Uniform Heat Flux

2017 ◽  
Author(s):  
Nan Zhang ◽  
Yanchen Fu ◽  
Haoran Huang ◽  
Jie Wen ◽  
Nigeer Te
Keyword(s):  
Heat Flux ◽  
Friction Factor ◽  
Mass Flux ◽  
Flow Resistance ◽  
Flow Characteristics ◽  
Supercritical Pressure ◽  
Uniform Heat Flux ◽  
Inlet Temperature ◽  
Uniform Heat ◽  
Helical Tubes

The flow resistance characteristics of aviation kerosene RP-3 in horizontal helical tubes at the supercritical pressure under heating condition are investigated. Both pressure drop and friction factor were examined under uniform heat flux of 50kW/m2−300kW/m2, mass flux from 786kg/m2s to 1375kg/m2s, and helical diameter from 20mm to 40mm. The influence of viscous factors on the resistance is analyzed to explore flow characteristics in a helical tube and provide a reference for the design of heat exchangers. Friction factor decreases with the increase of heat flux at low inlet temperatures 323K and 423K. It is explained that the viscosity changes more dramatically than the density. When the fluid inlet temperature is 523K and the fluid mean temperature Tb is close to pseudo-critical temperature, frictional flow resistance becomes significantly larger Tpc due to huge variations in thermal properties in the radical direction. The effect of centrifugal force makes the friction factor decline slowly. The friction factor goes up with the enlargement of mass flux when Tb>0.81Tpc. This phenomenon is caused by the larger radial velocity gradient under the large mass flux. Different helical diameters play the leading roles for the bending flow in the tubes.

scholarly journals Study the effect of angle of attack on flow characteristics at racing bike helmet using CFD

2018 ◽  
Vol 204 ◽  
pp. 06001
Author(s):  
Syamsuri ◽  
M Hasan Syafik ◽  
Yudho Putro Iswanto
Keyword(s):  
Numerical Simulations ◽  
Drag Coefficient ◽  
Aerodynamic Force ◽  
Angle Of Attack ◽  
Flow Characteristics ◽  
Reynold Number ◽  
Attack Angle ◽  
Large Area ◽  
Drag Racing ◽  
Simulation Results

At a cyclist drag racing champions greatly affect the speed of the bike, especially on the use of racing bike helmets. If the aerodynamic force from the racing bike helmet is getting smaller than the use of helmets on the bike racing will be more optimal and will affect the rate of the racer. In this study, numerical simulations were used to investigate the magnitude of the drag force that occurs around the surface of the helmet. With CFD software, 4 variations of attack angle 0°, 10°, 20° and 30° and variations of Reynold number 7.14x104, 1.00x105, and 1.16x105 are simulated to determine the flow characteristics of each state. The simulation results show that large area vortex is formed at the bottom of the helmet curve and dominates at the attack angle 30°. The result of the drag coefficient generated at the angle of attack 0° to 20° tends to decrease but at the attack angle 200 and 30° the drag coefficient increases.

FLOOD FLOW CHARACTERISTICS AND BED LOAD TRANSPORT IN NON-VEGETATED COMPOUND STRAIGHT CHANNELS

2016 ◽  
Vol 78 (9-4) ◽  
Author(s):  
Mazlin Jumain ◽  
Zulkiflee Ibrahim ◽  
Zulhilmi Ismail ◽  
Mohd Fuad Samsudin ◽  
Muhd Zubair Tajol Anuar ◽  
...  
Keyword(s):  
Flow Characteristics ◽  
Bed Load ◽  
Mobile Bed ◽  
Load Transport ◽  
Bed Load Transport ◽  
Flow And Sediment Transport ◽  
Depth Increase ◽  
Bed Slope ◽  
Overbank Flows ◽  
Flood Flow

Floods are the most common natural disasters in Malaysia and have damaged structures, infrastructures, crops and even causes fatalities. It may also lead to erosion and sedimentation in rivers and this will result to complex river behaviour.  A hydraulic laboratory experimental study was carried out. Also, flood flow and sediment transport in straight compound channels involving flow resistance, distribution of depth-averaged velocity, stream-wise vorticity patterns, channel bed morphology and bed load transport rate in non-vegetated compound straight mobile bed channels were investigated. The finding showed that the Darcy Weisbach friction factor  f  increased by 40% and 54% for floodplain and main channel, respectively when relative flood flow depth increase from 0.30 to 0.50. The small bed load transport rates of 0.09 g/s and 0.03 g/s for shallow and deep overbank flows, respectively were measured due to effect of very gentle or mild channel bed slope which was fixed at a gradient of 0.1%. 

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