scholarly journals Strengthen resilience to rivers flooding by the drag reduction technique

2022 ◽  
Vol 964 (1) ◽  
pp. 012021
Author(s):  
Walid Bouchenafa ◽  
Trong Dang-Vu ◽  
Huyen Xuan Dang-Vu
Keyword(s):  
Drag Reduction ◽  
Water Depth ◽  
Large Scale ◽  
Reduction Technique ◽  
Measuring Instruments ◽  
Protection Measure ◽  
River Flooding ◽  
Climate Change Effects ◽  
Flooding Risk ◽  
Reduction Effect

Abstract Urban agglomerations face the risk of overflowing rivers due to intense urbanization in flood-prone areas and the climate change effects. Despite the important protective measures deployed to reduce the fluvial flooding risk, additional efforts are still needed. This work aims to propose a new complementary non-structural protection measure, used to reduce the river flooding risk. The study is part of the NABRAPOL (NEBRASKA POLYMER) project, which aims to improve knowledge of the drag reduction effect by adding polymers in open-channel flows. The addition of polymers, even in limited concentrations, allows high friction to decrease with the typical Manning coefficient reduced up to 45%. An application case on a real watercourse is presented in this article. Two measurement campaigns are carried out on a river along 30 km. Experimental devices are deployed, and non-intrusive hydraulic measuring instruments are installed at the study field. Surface velocities are evaluated by the Large-Scale Particle Image Velocimetry (LSPIV) technique, and water depth is measured using ultrasonic radar sensors over the river. Measurement results show that the addition of 20 ppm of polymers in the flows results in a marked drag reduction by decreasing the water depth to 18% of its initial depth. The drag reduction technique by addition of small concentrations of polymers can be considered as a new and effective method to reinforce the measures already deployed in the flood risk management strategy since it allows the water depth to be decreased thus avoid overflowing rivers in the extreme flooding event.

scholarly journals Boundary Layer Mixture Model for a Microbubble Drag Reduction Technique

2011 ◽  
Vol 2011 ◽  
pp. 1-9 ◽  
Author(s):  
Jing-Fa Tsai ◽  
Chi-Chuan Chen
Keyword(s):  
Boundary Layer ◽  
Drag Reduction ◽  
Flat Plate ◽  
Mixture Model ◽  
Frictional Resistance ◽  
Reduction Technique ◽  
Measuring System ◽  
Water Tunnel ◽  
Towing Tank ◽  
Reduction Effect

The boundary mixture model is derived to predict the performance of the microbubble drag reduction technique for a flat plate. The flat plate with a porous material microbubble injecting system and resistance-measuring system are set up to measure the frictional resistance of the flat plate without and with injected microbubbles. The tests are conducted in a water tunnel and a towing tank. The test results show that the boundary mixture model predicts the drag reduction well for the flat plate when testing with injected microbubbles in the water tunnel. However, the boundary mixture model overestimates the drag reduction effect for the flat plate tested in the towing tank. The possible mechanism for the overestimation of drag reduction effect in the towing tank may be due to the different behaviors of microbubbles in the velocity gradient of boundary layer.

scholarly journals Study on turbulence drag reduction of riblet plate in hypersonic turbulent flows

2020 ◽  
Vol 31 (03) ◽  
pp. 2050046
Author(s):  
Hao Zhou ◽  
Xinliang Li ◽  
Changping Yu
Keyword(s):  
Drag Reduction ◽  
Turbulent Flows ◽  
Large Scale ◽  
Three Dimensional ◽  
Turbulence Control ◽  
Eddy Simulation ◽  
Number Formula ◽  
Large Scale Vortex ◽  
Reduction Effect ◽  
Parameter Values

This paper focuses on turbulence drag reduction of riblet plate in hypersonic turbulent flows. We use direct numerical simulation (DNS) and large eddy simulation (LES) to simulate three-dimensional spatially-developing boundary layer over the flat plate and riblet plate with a free-stream Mach number [Formula: see text]. The results reveal the influence of different riblet heights [Formula: see text] and riblet distances [Formula: see text] on drag reduction effect. The drag reduction effect increases with the increase of riblet height and the decrease of riblet distance within suitable range of parameter values. Through analysis, it can be seen that the riblet plate affects the turbulent contribution of the skin friction by suppressing or destroying the large-scale vortex structure. Combined with the actual engineering design requirements, we can use the riblet plate with appropriate parameters to achieve the purpose of turbulence control.

scholarly journals Molecular Insight into the Possible Mechanism of Drag Reduction of Surfactant Aqueous Solution in Pipe Flow

2021 ◽  
Vol 22 (14) ◽  
pp. 7573
Author(s):  
Yusei Kobayashi ◽  
Hirotaka Gomyo ◽  
Noriyoshi Arai
Keyword(s):  
Aqueous Solution ◽  
Drag Reduction ◽  
Molecular Simulation ◽  
Pipe Flow ◽  
Large Scale ◽  
Coarse Grained ◽  
Turbulence Suppression ◽  
Nonequilibrium States ◽  
Reduction Effect ◽  
Surfactant Aqueous Solution

The phenomenon of drag reduction (known as the “Toms effect”) has many industrial and engineering applications, but a definitive molecular-level theory has not yet been constructed. This is due both to the multiscale nature of complex fluids and to the difficulty of directly observing self-assembled structures in nonequilibrium states. On the basis of a large-scale coarse-grained molecular simulation that we conducted, we propose a possible mechanism of turbulence suppression in surfactant aqueous solution. We demonstrate that maintaining sufficiently large micellar structures and a homogeneous radial distribution of surfactant molecules is necessary to obtain the drag-reduction effect. This is the first molecular-simulation evidence that a micellar structure is responsible for drag reduction in pipe flow, and should help in understanding the mechanisms underlying drag reduction by surfactant molecules under nonequilibrium conditions.

scholarly journals Inundation Analysis of the Dike Breach of the Chikuma River Taking Drainage Process and House Damage into Consideration

2021 ◽  
Vol 16 (3) ◽  
pp. 343-350
Author(s):  
Makoto Takeda ◽  
Daisuke Sato ◽  
Kenji Kawaike ◽  
Masashi Toyota ◽  
◽  
...  
Keyword(s):  
Water Depth ◽  
Large Scale ◽  
Heavy Rain ◽  
Left Bank ◽  
Analysis Model ◽  
River Flooding ◽  
Inundation Area ◽  
Water Behavior ◽  
Model Improvement ◽  
Good Agreement

Heavy rain and river flooding due to Typhoon No. 19 in October 2019 led to overflow and a dike breach on the left bank of the Chikuma River that caused large-scale inundation damage in Nagano City, Japan. To devise countermeasures, an inundation analysis model is an important tool. In this study, an inundation analysis model was developed to examine the inundation water behavior. The calculated inundation water depth and inundation area showed good agreement with the observed inundation water depth and the inundated area, confirming the validity of the analysis model. In addition, temporal changes of the inundation state were calculated considering the drainage process. However, the sewerage system, waterway, and drainage pump car were not taken into consideration in this analysis, and future issues for model improvement were also revealed. In addition, an analysis model with a 2 m grid was developed in the dike breach site, and the inundation water flow on roads and the fluid force around houses were obtained after taking into consideration the effect of houses. In paticular, the calculated value of the specific force exerted on damaged houses was very high. Moreover, it was proposed that house hazard should be evaluated while taking into consideration the loss of houses around the dike.

scholarly journals Noise Reduction Effect of Superhydrophobic Surfaces with Streamwise Strip of Channel Flow

2021 ◽  
Vol 11 (9) ◽  
pp. 3869
Author(s):  
Chen Niu ◽  
Yongwei Liu ◽  
Dejiang Shang ◽  
Chao Zhang
Keyword(s):  
Reynolds Number ◽  
Drag Reduction ◽  
Noise Reduction ◽  
Channel Flow ◽  
Superhydrophobic Surface ◽  
Sound Field ◽  
Flow Noise ◽  
Slip Boundary ◽  
Eddy Simulation ◽  
Reduction Effect

Superhydrophobic surface is a promising technology, but the effect of superhydrophobic surface on flow noise is still unclear. Therefore, we used alternating free-slip and no-slip boundary conditions to study the flow noise of superhydrophobic channel flows with streamwise strips. The numerical calculations of the flow and the sound field have been carried out by the methods of large eddy simulation (LES) and Lighthill analogy, respectively. Under a constant pressure gradient (CPG) condition, the average Reynolds number and the friction Reynolds number are approximately set to 4200 and 180, respectively. The influence on noise of different gas fractions (GF) and strip number in a spanwise period on channel flow have been studied. Our results show that the superhydrophobic surface has noise reduction effect in some cases. Under CPG conditions, the increase in GF increases the bulk velocity and weakens the noise reduction effect. Otherwise, the increase in strip number enhances the lateral energy exchange of the superhydrophobic surface, and results in more transverse vortices and attenuates the noise reduction effect. In our results, the best noise reduction effect is obtained as 10.7 dB under the scenario of the strip number is 4 and GF is 0.5. The best drag reduction effect is 32%, and the result is obtained under the scenario of GF is 0.8 and strip number is 1. In summary, the choice of GF and the number of strips is comprehensively considered to guarantee the performance of drag reduction and noise reduction in this work.

scholarly journals Why does the spread of COVID-19 vary greatly in different countries? Revealing the efficacy of face masks in epidemic prevention

2021 ◽  
Vol 149 ◽  
Author(s):  
Jincheng Wei ◽  
Shurui Guo ◽  
Enshen Long ◽  
Li Zhang ◽  
Bizhen Shu ◽  
...  
Keyword(s):  
Social Isolation ◽  
Large Scale ◽  
Experimental Studies ◽  
Weighted Average ◽  
Filtration Efficiency ◽  
Volume Distribution ◽  
Particle Sizes ◽  
Economic Activities ◽  
Protection Measure ◽  
Source Of Infection

Abstract The severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) is highly contagious, and the coronavirus disease 2019 (COVID-19) pandemic caused by it has forced many countries to adopt ‘lockdown’ measures to prevent the spread of the epidemic through social isolation of citizens. Some countries proposed universal mask wearing as a protection measure of public health to strengthen national prevention efforts and to limit the wider spread of the epidemic. In order to reveal the epidemic prevention efficacy of masks, this paper systematically evaluates the experimental studies of various masks and filter materials, summarises the general characteristics of the filtration efficiency of isolation masks with particle size, and reveals the actual efficacy of masks by combining the volume distribution characteristics of human exhaled droplets with different particle sizes and the SARS-CoV-2 virus load of nasopharynx and throat swabs from patients. The existing measured data show that the filtration efficiency of all kinds of masks for large particles and extra-large droplets is close to 100%. From the perspective of filtering the total number of pathogens discharged in the environment and protecting vulnerable individuals from breathing live viruses, the mask has a higher protective effect. If considering the weighted average filtration efficiency with different particle sizes, the filtration efficiencies of the N95 mask and the ordinary mask are 99.4% and 98.5%, respectively. The mask can avoid releasing active viruses to the environment from the source of infection, thus maximising the protection of vulnerable individuals by reducing the probability of inhaling a virus. Therefore, if the whole society strictly implements the policy of publicly wearing masks, the risk of large-scale spread of the epidemic can be greatly reduced. Compared with the overall cost of social isolation, limited personal freedoms and forced suspension of economic activities, the inconvenience for citizens caused by wearing masks is perfectly acceptable.

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