scholarly journals Flexible conformable hydrophobized surfaces for turbulent flow drag reduction

2015 ◽  
Vol 5 (1) ◽  
Author(s):  
Joseph C Brennan ◽  
Nicasio R Geraldi ◽  
Robert H Morris ◽  
David J Fairhurst ◽  
Glen McHale ◽  
...  
Keyword(s):  
Turbulent Flow ◽  
Drag Reduction ◽  
Flow Drag

Turbulent flow drag reduction and degradation with dilute polymer solutions

1970 ◽  
Vol 43 (4) ◽  
pp. 689-710 ◽  
Author(s):  
R. W. Paterson ◽  
F. H. Abernathy
Keyword(s):  
Molecular Weight ◽  
Turbulent Flow ◽  
Drag Reduction ◽  
Average Molecular Weight ◽  
Experimental Studies ◽  
Polymer Degradation ◽  
High Viscosity ◽  
Turbulent Pipe Flow ◽  
Shear Stresses ◽  
Flow Drag

Experimental studies of drag reduction and polymer degradation in turbulent pipe flow with dilute water solutions of unfractionated polyethylene oxide are described. Drag reduction results indicate that the magnitude of the reduction cannot be correlated on the basis of weight average molecular weight, rather the phenomenon depends strongly on the concentration of the highest molecular weight species present in the molecular weight distribution. Polymer degradation in turbulent flow is found to be severe for high molecular weight polymers causing appreciable changes in drag reduction and molecular weight with the duration of flow. Data indicates that drag reduction exists in the limit of infinite dilution suggesting that the phenomenon is due to the interaction of individual polymer molecules with the surrounding solvent and that the extent of reduction is relatively independent of pipe diameter when a comparison is carried out at equal solvent wall shear stresses. Consideration of the high viscosity obtained with solutions in an irrotational laminar flow field suggests this is due to polymer molecule deformation and that this phenomenon is central to the mechanism of turbulent flow drag reduction.

scholarly journals Study on the Law of Pseudo-Cavitation on Superhydrophobic Surface in Turbulent Flow Field of Backward-Facing Step

Fluids ◽  
2021 ◽  
Vol 6 (6) ◽  
pp. 200
Author(s):  
Xuecheng Lv ◽  
Wei-Tao Wu ◽  
Jizu Lv ◽  
Ke Mao ◽  
Linsong Gao ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Flow Field ◽  
Turbulent Flow ◽  
Drag Reduction ◽  
Superhydrophobic Surface ◽  
Large Bubble ◽  
Backward Facing Step ◽  
Bubble Generation ◽  
Turbulent Flow Field ◽  
Flow Drag

Superhydrophobic surface is regarded as important topic in the field of thermal fluids today due to its unique features on flow drag reduction and heat transfer enhancement. In this study, the pseudo-cavitation phenomenon on the superhydrophobic surface in the backward-facing step turbulent flow field is observed through experiments. The underlying reason for this phenomenon is studied with experimental observation and analysis, and the time variant mechanisms of this phenomenon with various Reynolds number is summarized. The research results indicate that the superhydrophobic surface and the backward-facing step provide the material basis and dynamic condition for the generation of pseudo-cavitation. The pseudo-cavitation induces a large bubble on the superhydrophobic surface below the backward-facing step. The size, position, shape, oscillation amplitude, detachment, and splitting of the large bubble show regularity with the changes of Reynolds number. Meanwhile, the bubble growth, oscillation, detachment, split, and regeneration over time also show regularity. The study of bubble generation and development laws can be used to better control the perturbation of the flow field. Importantly, the present study has meaning in better understanding the flow mechanisms and gas coverage of superhydrophobic surface under condition of backward-facing step, paving the way for studying the flow drag reduction effect of superhydrophobic surface.

scholarly journals A note to further explain the mechanism of turbulent flow drag reduction by polymer from chemistry view

2021 ◽  
Author(s):  
Xin Zhang ◽  
Xiaodong Dai ◽  
Jishi Zhao ◽  
Dengwei Jing ◽  
Fei Liu ◽  
...  
Keyword(s):  
Fourier Series ◽  
Turbulent Flow ◽  
Drag Reduction ◽  
Phase Angle ◽  
Linear Polymers ◽  
Flexible Polymers ◽  
Viscoelastic Theory ◽  
Maximum Drag Reduction ◽  
Flow Drag ◽  
Drag Reducing Polymer

In our previous work regarding the mechanism of drag reduction and degradation by flexible linear polymers, we proposed a correlation based on the Fourier series to predict the drag reduction and its degradation, where a phase angle was involved, but the physical meaning for the correlation especially of the employed phase angle was not clear, which is however important for reasonable explanation of the drag reduction mechanism over flexible linear polymers. This letter aims to clarify this issue. We use several steps of deduction from the viscoelastic theory, and conclude that the Fourier series employed to predict the drag reduction and its degradation is due to viscoelastic property of drag-reducing polymer solution, and the phase angle represents the hysteresis of polymer in turbulent flow. Besides, our new view of drag reduction by flexible polymers can also explain why a maximum drag reduction in rotational flow appears before degradation happens.

Transition to turbulence in pipe flow for water and dilute solutions of polyethylene oxide

1972 ◽  
Vol 51 (1) ◽  
pp. 177-185 ◽  
Author(s):  
R. W. Paterson ◽  
F. H. Abernathy
Keyword(s):  
Turbulent Flow ◽  
Drag Reduction ◽  
Polyethylene Oxide ◽  
Transition To Turbulence ◽  
Small Scale ◽  
Outer Flow ◽  
Pure Solvent ◽  
Inlet Conditions ◽  
Scale Turbulence ◽  
Flow Drag

An experimental study of the transition from laminar to turbulent flow in a long 0·248in. I.D. pipe is reported for both water and dilute water solutions of polyethylene oxide which exhibit turbulent flow drag reduction (the Toms phenomenon). The drag-reducing solutions, ranging in effectiveness from near zero to the maximum attainable, are observed to undergo transition in a similar way to the Newtonian solvent in that the solutions exhibit intermittency and the growth rates of the turbulent patches are essentially equal to those of the pure solvent. The growth rate of turbulent patches indicates that drag reduction is associated with the small-scale structure of the turbulence near the pipe wall while patch growth is associated with the larger-scale turbulence in the outer flow. For low-disturbance pipe inlet conditions the strong drag-reducing solutions are observed to undergo transition at lower Reynolds numbers than the pure solvent.

Drag reduction for turbulent flow over a projectile. Part II

1994 ◽  
Vol 31 (1) ◽  
pp. 93-98 ◽  
Author(s):  
Shen-Min Liang ◽  
Jan-Kuang Fu
Keyword(s):  
Turbulent Flow ◽  
Drag Reduction

Durability of a Drag Reducing Solution

2008 ◽  
Vol 18 (1) ◽  
pp. 12421-1-12421-5
Author(s):  
V. Mik ◽  
J. Myska ◽  
Z. Chara ◽  
P. Stern
Keyword(s):  
Turbulent Flow ◽  
Drag Reduction ◽  
Turbulence Intensity ◽  
Surfactant Concentration ◽  
Surfactant Solution ◽  
Small Addition ◽  
Rheological Parameters ◽  
Aging Effects ◽  
Flow Disturbance ◽  
Solution Change

AbstractEffectiveness of drag reduction by small addition of a surfactant in the turbulent flow of water depends on the structure and concentration of the additive, temperature of the solution and turbulence intensity, possible flow disturbance by a mechanical obstacle and the content of ions in water, but also on the age of the surfactant solution. We show how important aging effects are in connection with total surfactant concentration, in particular how rheological parameters of the drag reducing solution change with time.

Turbulent flow over superhydrophobic surfaces with streamwise grooves

2014 ◽  
Vol 747 ◽  
pp. 186-217 ◽  
Author(s):  
S. Türk ◽  
G. Daschiel ◽  
A. Stroh ◽  
Y. Hasegawa ◽  
B. Frohnapfel
Keyword(s):  
Boundary Conditions ◽  
Turbulent Flow ◽  
Drag Reduction ◽  
Secondary Flow ◽  
Slip Length ◽  
Laminar Flows ◽  
Superhydrophobic Surfaces ◽  
Mean Velocity ◽  
Effective Slip Length ◽  
Effective Slip

AbstractWe investigate the effects of superhydrophobic surfaces (SHS) carrying streamwise grooves on the flow dynamics and the resultant drag reduction in a fully developed turbulent channel flow. The SHS is modelled as a flat boundary with alternating no-slip and free-slip conditions, and a series of direct numerical simulations is performed with systematically changing the spanwise periodicity of the streamwise grooves. In all computations, a constant pressure gradient condition is employed, so that the drag reduction effect is manifested by an increase of the bulk mean velocity. To capture the flow properties that are induced by the non-homogeneous boundary conditions the instantaneous turbulent flow is decomposed into the spatial-mean, coherent and random components. It is observed that the alternating no-slip and free-slip boundary conditions lead to the generation of Prandtl’s second kind of secondary flow characterized by coherent streamwise vortices. A mathematical relationship between the bulk mean velocity and different dynamical contributions, i.e. the effective slip length and additional turbulent losses over slip surfaces, reveals that the increase of the bulk mean velocity is mainly governed by the effective slip length. For a small spanwise periodicity of the streamwise grooves, the effective slip length in a turbulent flow agrees well with the analytical solution for laminar flows. Once the spanwise width of the free-slip area becomes larger than approximately 20 wall units, however, the effective slip length is significantly reduced from the laminar value due to the mixing caused by the underlying turbulence and secondary flow. Based on these results, we develop a simple model that allows estimating the gain due to a SHS in turbulent flows at practically high Reynolds numbers.

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