Some Notes on Drag Reduction in the Near-Wall Region

2007 ◽  
pp. 155-198 ◽  
Author(s):  
Ron F. Blackwelder
Keyword(s):  
Drag Reduction ◽  
Wall Region ◽  
Near Wall

Effects of Spatial Resolution and Box Size on Numerical Solutions of Turbulent Flows

2005 ◽  
Author(s):  
Dongmei Zhou ◽  
Kenneth S. Ball
Keyword(s):  
Drag Reduction ◽  
Channel Flow ◽  
Spatial Resolution ◽  
Numerical Solutions ◽  
Velocity Profiles ◽  
Wall Region ◽  
Computational Domain ◽  
Mean Velocity ◽  
Significant Difference ◽  
Near Wall

This paper has two objectives, (1) to examine the effects of spatial resolution, (2) to examine the effects of computational box size, upon turbulence statistics and the amount of drag reduction with and without the control scheme of wall oscillation. Direct numerical simulation (DNS) of the fully developed turbulent channel flow was performed at Reynolds number of 200 based on the wall-shear velocity and the channel half-width by using spectral methods. For the first objective, four different grids were applied to the same computational domain and the biggest impact was observed on the logarithmic law of mean velocity profiles and on the amount of drag reduction with 28.3% for the coarsest mesh and 35.4% for the finest mesh. Other turbulence features such as RMS velocity fluctuations, RMS vorticity fluctuations, and bursting events were either overpredicted or underpredicted through coarse grids. For the second objective, two different minimal channels and one natural full channel were studied and 3% drag reduction difference was observed between the smallest minimal channel of 39.1% and the natural full channel of 36.2%. In the near-wall region, however, the minimal channel flow did not exhibit significant difference in the mean velocity profiles and other lower-order statistics. Finally, from this systematical study, it showed that the accuracy of DNS depends more on the spanwise resolution, and it also confirmed that a minimal channel model is able to catch key structures of turbulence in the near-wall region but is much less expensive.

Turbulent drag reduction in plane Couette flow with polymer additives: a direct numerical simulation study

2018 ◽  
Vol 846 ◽  
pp. 482-507 ◽  
Author(s):  
Hao Teng ◽  
Nansheng Liu ◽  
Xiyun Lu ◽  
Bamin Khomami
Keyword(s):  
Numerical Simulation ◽  
Drag Reduction ◽  
Large Scale ◽  
Core Region ◽  
Wall Region ◽  
Plane Couette Flow ◽  
The Core ◽  
Elastic Potential Energy ◽  
Conformation Tensor ◽  
Near Wall

Drag reduction (DR) in plane Couette flow (PCF) induced by the addition of flexible polymers has been studied via direct numerical simulation (DNS). The similarities and differences in the drag reduction features of PCF and plane Poiseuille flow (PPF) have been examined in detail, particularly in regard to the polymer-induced modification of large-scale structures (LSSs) in the near-wall turbulence. Specifically, it has been demonstrated that in the near-wall region, drag-reduced PCF has features similar to those of drag-reduced PPF; however, in the core region, intriguing differences are found between these two drag-reduced shear flows. Chief among these differences is the significant polymer stretch that arises from the enhanced exchanges between elastic potential energy and turbulent kinetic energy and the commensurate observation of peak values of the conformation tensor components $\unicode[STIX]{x1D60A}_{yy}$ and $\unicode[STIX]{x1D60A}_{zz}$ in this region. This finding is in stark contrast to that of drag-reduced PPF where the polymer stretch and the exchanges between elastic potential energy and turbulent kinetic energy in the core region are insignificant; to this end, in drag-reduced PPF, peak values of the conformation tensor components appear in the near-wall region. Therefore, this study paves the way for understanding the underlying flow physics in drag-reduced PCF, particularly in the context of elastic theory. Moreover, the longitudinal large-scale streaks at the channel centre of drag-reduced PCF are greatly strengthened due to the increased production/dissipation ratio; the LSS imprint effects on the near-wall flow of drag-reduced PCF monotonically increase as the Weissenberg number is enhanced.

Experimental and Numerical Study of Bubble Drag Reduction on a Flat Plate

2017 ◽  
Author(s):  
ShiJie Qin ◽  
DaZhuan Wu
Keyword(s):  
Drag Reduction ◽  
Flat Plate ◽  
Experimental Results ◽  
Air Injection ◽  
Wall Region ◽  
Buffer Region ◽  
Wide Range ◽  
Flow Speeds ◽  
Near Wall ◽  
Injection Rates

The presence of near wall bubbles may reduce the skin friction drag. This phenomenon has been studied by well designed experiments and combined computational fluid dynamics (CFD) and population balance model (PBM) simulations in this paper. Drag reductions and bubble distributions over a flat plate have been implemented in cavitation tunnel experiments at various flow speeds and air injection rates. CFD-PBM modeling for bubble drag reduction (BDR) has been modified and validated by the flat plate experiments. Drag and lift forces are fully modeled, and bubble breakup and coalescence are calculated. A wide range of bubble sizes are well captured base on the aforementioned numerical consideration. And this modeling work can be further used to design full-scale BDR ships and to discover detailed BDR mechanisms. The predicted drag reductions and bubble distributions are in reasonable accordance with the experimental results. Approximately 30% of BDR is achieved both in the numerical and experimental results. The influence of flow speeds and air injection rates on drag reductions and bubble distributions is discussed. In particular, the mechanism of BDR is analyzed based on the detailed flow filed profiles from numerical simulations. Higher air injection rates generally lead to thicker bubble layer thickness from the rear-part of buffer region (20 < y+ < 30) to turbulent region (y+ > 30). And noticeable increases of air volume fraction in the laminar region (y+ < 5) and forepart of buffer region (5 < y+ < 20). The change of the velocity gradient in the near wall region is considered to be directly related to drag reduction.

scholarly journals Large eddy simulation of near-wall turbulent flow over streamlined riblet-structured surface for drag reduction in a rectangular channel

2020 ◽  
Vol 24 (5 Part A) ◽  
pp. 2793-2808
Author(s):  
Hussain Al-Kayiem ◽  
Desmond Lim ◽  
Jundika Kurnia
Keyword(s):  
Large Eddy Simulation ◽  
Turbulent Flow ◽  
Drag Reduction ◽  
Rectangular Channel ◽  
Wall Region ◽  
Mean Velocity ◽  
Structured Surface ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Near Wall

Sharkskin-inspired riblets are widely adopted as a passive method for drag reduc?tion of flow over surfaces. In this research, large eddy simulation of turbulent flow over riblet-structured surface in a rectangular channel domain were performed at various Reynolds numbers, ranging from 4200-10000, to probe the resultant drag change, compared to smooth surface. The changes of mean streamwise velocity gradient in wall-normal direction at varied locations around riblet structures were also investigated to reduce mechanisms of streamlined riblet in reducing drag. The computational model is validated by comparing the simulation results against analytical and experimental data, for both smooth and riblet surfaces. Results in?dicating that the performance of the proposed streamlined riblet shows 7% drag reduction, as maximum, which is higher than the performance of L-shaped riblet with higher wetted surface area. The mean velocity profile analysis indicates that the streamlined riblet structures help to reduce longitudinal averaged velocity component rate in the normal to surface direction of near-wall region which leads to laminarization process as fluid-flows over riblet structures.

scholarly journals Effects of a new drag reduction agent on natural gas pipeline transportation

2019 ◽  
Vol 11 (10) ◽  
pp. 168781401988192
Author(s):  
Yachao Ma ◽  
Zhiqiang Huang ◽  
Zhanghua Lian ◽  
Weichun Chang ◽  
Huan Tan
Keyword(s):  
Natural Gas ◽  
Drag Reduction ◽  
Gas Flow ◽  
Reduction Rate ◽  
Field Tests ◽  
Wall Region ◽  
Pipeline Transportation ◽  
Frictional Drag ◽  
Inner Surface ◽  
Near Wall

Pipeline transportation is the major way to transport natural gas. How to reduce energy dissipation and retain the gas delivery capacity are the main problems of pipeline transportation. In this article, a new drag reduction agent named CPA is synthesized. An experimental investigation on the roughness-reducing effect of CPA on the inner surface of the pipeline is carried out. The effect of CPA on natural gas flow regime in the near-wall region of the pipeline is researched with Fluent software. Field tests for calculating the drag reduction rate of CPA are performed. The results show that CPA can reduce the roughness of the inner surface effectively, and the maximum roughness-reducing percentage is 38.74%. Meanwhile, CPA can reduce the frictional drag and thereby improve transportation capacity of pipelines. After injecting CPA, the streamline of the natural gas in the near-wall region is more consistent. The velocity fluctuation decreases by 93.2%. The mean turbulence intensity decreases by 53.01%. The pipeline pressure further decreases the roughness of the inner surface of the pipeline. The field test shows that the maximum drag reduction rate of CPA is 25%, and it is suitable for application in gathering and transportation pipelines of high flow velocity and turbulent rough region.

Direct numerical simulations of Rayleigh–Bénard convection in water with non-Oberbeck–Boussinesq effects

2019 ◽  
Vol 881 ◽  
pp. 1073-1096 ◽  
Author(s):  
Andreas D. Demou ◽  
Dimokratis G. E. Grigoriadis
Keyword(s):  
Numerical Simulations ◽  
Two Dimensions ◽  
Direct Numerical Simulations ◽  
Wall Region ◽  
Number Range ◽  
Bénard Convection ◽  
Benard Convection ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard ◽  
Near Wall

Rayleigh–Bénard convection in water is studied by means of direct numerical simulations, taking into account the variation of properties. The simulations considered a three-dimensional (3-D) cavity with a square cross-section and its two-dimensional (2-D) equivalent, covering a Rayleigh number range of $10^{6}\leqslant Ra\leqslant 10^{9}$ and using temperature differences up to 60 K. The main objectives of this study are (i) to investigate and report differences obtained by 2-D and 3-D simulations and (ii) to provide a first appreciation of the non-Oberbeck–Boussinesq (NOB) effects on the near-wall time-averaged and root-mean-squared (r.m.s.) temperature fields. The Nusselt number and the thermal boundary layer thickness exhibit the most pronounced differences when calculated in two dimensions and three dimensions, even though the $Ra$ scaling exponents are similar. These differences are closely related to the modification of the large-scale circulation pattern and become less pronounced when the NOB values are normalised with the respective Oberbeck–Boussinesq (OB) values. It is also demonstrated that NOB effects modify the near-wall temperature statistics, promoting the breaking of the top–bottom symmetry which characterises the OB approximation. The most prominent NOB effect in the near-wall region is the modification of the maximum r.m.s. values of temperature, which are found to increase at the top and decrease at the bottom of the cavity.

The Effects of Adverse Pressure Gradients on Momentum and Thermal Structures in Transitional Boundary Layers: Part 2—Fluctuation Quantities

1996 ◽  
Vol 118 (4) ◽  
pp. 728-736 ◽  
Author(s):  
S. P. Mislevy ◽  
T. Wang
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Transition Region ◽  
Boundary Layers ◽  
Reynolds Shear Stress ◽  
Turbulent Shear ◽  
Wall Region ◽  
Pressure Gradients ◽  
Baseline Case ◽  
Near Wall

The effects of adverse pressure gradients on the thermal and momentum characteristics of a heated transitional boundary layer were investigated with free-stream turbulence ranging from 0.3 to 0.6 percent. Boundary layer measurements were conducted for two constant-K cases, K1 = −0.51 × 10−6 and K2 = −1.05 × 10−6. The fluctuation quantities, u′, ν′, t′, the Reynolds shear stress (uν), and the Reynolds heat fluxes (νt and ut) were measured. In general, u′/U∞, ν′/U∞, and νt have higher values across the boundary layer for the adverse pressure-gradient cases than they do for the baseline case (K = 0). The development of ν′ for the adverse pressure gradients was more actively involved than that of the baseline. In the early transition region, the Reynolds shear stress distribution for the K2 case showed a near-wall region of high-turbulent shear generated at Y+ = 7. At stations farther downstream, this near-wall shear reduced in magnitude, while a second region of high-turbulent shear developed at Y+ = 70. For the baseline case, however, the maximum turbulent shear in the transition region was generated at Y+ = 70, and no near-wall high-shear region was seen. Stronger adverse pressure gradients appear to produce more uniform and higher t′ in the near-wall region (Y+ < 20) in both transitional and turbulent boundary layers. The instantaneous velocity signals did not show any clear turbulent/nonturbulent demarcations in the transition region. Increasingly stronger adverse pressure gradients seemed to produce large non turbulent unsteadiness (or instability waves) at a similar magnitude as the turbulent fluctuations such that the production of turbulent spots was obscured. The turbulent spots could not be identified visually or through conventional conditional-sampling schemes. In addition, the streamwise evolution of eddy viscosity, turbulent thermal diffusivity, and Prt, are also presented.

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