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.

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.

scholarly journals Turbulent boundary layers over permeable walls: scaling and near-wall structure

2011 ◽  
Vol 687 ◽  
pp. 141-170 ◽  
Author(s):  
C. Manes ◽  
D. Poggi ◽  
L. Ridolfi
Keyword(s):  
Turbulent Flows ◽  
Laser Doppler Anemometry ◽  
Wall Turbulence ◽  
Shear Instability ◽  
Wall Structure ◽  
Wall Region ◽  
Mean Velocity ◽  
Wide Range ◽  
Permeable Walls ◽  
Near Wall

AbstractThis paper presents an experimental study devoted to investigating the effects of permeability on wall turbulence. Velocity measurements were performed by means of laser Doppler anemometry in open channel flows over walls characterized by a wide range of permeability. Previous studies proposed that the von Kármán coefficient associated with mean velocity profiles over permeable walls is significantly lower than the standard values reported for flows over smooth and rough walls. Furthermore, it was observed that turbulent flows over permeable walls do not fully respect the widely accepted paradigm of outer-layer similarity. Our data suggest that both anomalies can be explained as an effect of poor inner–outer scale separation if the depth of shear penetration within the permeable wall is considered as the representative length scale of the inner layer. We observed that with increasing permeability, the near-wall structure progressively evolves towards a more organized state until it reaches the condition of a perturbed mixing layer where the shear instability of the inflectional mean velocity profile dictates the scale of the dominant eddies. In our experiments such shear instability eddies were detected only over the wall with the highest permeability. In contrast attached eddies were present over all the other wall conditions. On the basis of these findings, we argue that the near-wall structure of turbulent flows over permeable walls is regulated by a competing mechanism between attached and shear instability eddies. We also argue that the ratio between the shear penetration depth and the boundary layer thickness quantifies the ratio between such eddy scales and, therefore, can be used as a diagnostic parameter to assess which eddy structure dominates the near-wall region for different wall permeability and flow conditions.

The Effect of Air Lubrication on a Flat Plate at Varying Angles of Trim With Regard to Drag Reduction

2013 ◽  
Author(s):  
Sean P. Murphy ◽  
Colin T. Spillane
Keyword(s):  
Drag Reduction ◽  
Flat Plate ◽  
Fuel Consumption ◽  
Technological Development ◽  
Frictional Resistance ◽  
Flow Volume ◽  
Resistance Reduction ◽  
Flow Speeds ◽  
Air Lubrication ◽  
Air Layer Drag Reduction

One of the driving factors of technological development in ship design is the reduction of fuel consumption. One way to reduce fuel consumption is to reduce the total resistance experienced by a vessel. The methods of resistance reduction covered in this document are Air-layer-drag-reduction (ALDR) and Bubble Drag Reduction (BDR). This research, conducted in Webb Institute’s circulating flow channel, investigates the applications of ALDR and BDR to a flat plate. These tests measured frictional resistance at varying air flow volume and angles of trim over a range of flow speeds. Results from these tests offer compelling evidence that ALDR is an effective method of reducing frictional resistance.

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.

Numerical Investigation of the Compressible Flat-Plate Turbulent Boundary Layer with Extended GAO-YONG Turbulence Model

2013 ◽  
Vol 444-445 ◽  
pp. 416-422
Author(s):  
Yang Yang Tang ◽  
Zhi Qiang Li ◽  
Yong Wang ◽  
Ya Chao Di ◽  
Huan Xu ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flat Plate ◽  
Turbulence Model ◽  
Turbulence Models ◽  
Wall Region ◽  
Flow And Heat Transfer ◽  
Empirical Coefficients ◽  
Near Wall

The extended GAO-YONG turbulence model is used to simulate the flow and heat transfer of flat-plate turbulent boundary layer, and the results indicate that GAO-YONG turbulence model may well describe boundary layer flow and heat transfer from near-wall region to far outer area, without using any empirical coefficients and near-wall treatments, such as wall-function or modified low Reynolds number model, which are used widely in all RANS turbulence models.

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.

Turbulent Drag Reduction by Hydrogen Microbubbles Made by Electrolysis

2007 ◽  
Author(s):  
Takahisa Endo ◽  
Hiroharu Kato ◽  
Xinlin Lu
Keyword(s):  
Drag Reduction ◽  
Void Fraction ◽  
Water Electrolysis ◽  
Experimental Results ◽  
Air Injection ◽  
Air Bubbles ◽  
Turbulent Drag Reduction ◽  
Turbulent Drag

Turbulent drag reduction realized by hydrogen microbubbles was investigated experimentally. A method of generating microbubbles of 10–60μm diameter by water electrolysis was established. Experiments were performed using a water circulating tunnel. Microbubbles generated by electrolysis can achieve the same drag reduction as the injected air bubbles at a much lower void fraction. The present experimental results suggest that microbubbles produced by electrolysis are 10∼100 times more effective in terms of drag reduction than large bubbles generated by air injection. Thus, it is considered that the diameters of microbubbles play an important role in drag reduction.

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.

Sign in / Sign up

Export Citation Format

Share Document