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On the tip sharpness of riblets for turbulent drag reduction

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Acta Mechanica Sinica 2022年第4期38卷 39-55,I0001页

作者： Yiqian Wang Yi Huang Song Fu School of Mathematical Sciences Soochow UniversitySuzhou 215006China School of Aerospace Engineering Tsinghua UniversityBeijing 100084China

It is commonly known that riblets with sharper tip generally have better turbulent drag reduction capacity,which,however,poses great challenges for manufacturing and makes the riblets vulnerable to tip *** this study,we show that a scalloped riblet which is not as sharp in the tip as corresponding triangular riblet with same height-width ratio,nevertheless has a larger protrusion height,a quantity solely depending on the riblet shape and calculated through a boundary element algorithm in this study,and thus a higher projected drag reduction *** addition,it is found that,when subjected to tip rounding,this scalloped riblet performs better in terms of protrusion height than corresponding parabolic riblet,which indicates stronger resilience to riblet tip *** the class of scalloped riblets,designed by smoothly connecting two third-order polynomials and thus the tip sharpness and valley curvature can be well defined,it is revealed that two mechanisms,one for the valley curvature at the viscous limit and one for the tip sharpness at infinite deep limit,determine the protrusion height,and thus the projected drag reduction *** numerical simulations are then carried out to investigate controlled boundary layer transition with the scalloped riblet of width s+=20 and 5+=60.A 7.8%drag reduction in the turbulent region is found for the smaller riblet with a preferable transition delay,while for the larger riblet transition is promoted and drag is increased in the turbulent *** is also found that the area fraction of high drag region around the riblet tips is basically the same for the two ***,even higher drag is found around the tip region for the smaller drag-reducing *** the other hand,a much smaller drag coefficient is found in the valley of the smaller riblet,which results in the reduction of turbulent *** is thus inferred that the issue of sharp riblet tip,that hard to manufacture and deteriorate substantially

关键词： Riblet turbulent drag reduction Tip sharpness Protrusion height Direct numerical simulation

drag reduction of wall bounded incompressible turbulent flow based on active dimples/pimples

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Journal of Hydrodynamics 2017年第2期29卷 261-271页

作者：葛铭纬方乐刘永前 State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources North China Electric Power University Sino-French Engineering School Beihang University

The control of turbulence by dimples/pimples has drawn more and more attention. The objective of this paper is to investigate the effectiveness of the active dimples/pimples for the drag reduction in the incompressible turbulent flow. Firstly, the drag reduction by the opposition control based on active dimples/pimples at the lower wall is studied via the direct numerical simulation of the turbulent channel flow. It is found that large active dimples/pimples can not suppress the streamwise vortices significantly and thus almost no drag reduction is achieved. Small active dimples and pimples with the diameter of one fourth of the streak width can both reduce the friction drag, but pimples will induce a larger pressure drag than dimples. Then the suboptimal control scheme is examined based on small active dimples using the spanwise wall shear information only, It is shown that the friction drag decreases by about 4.5% but the total drag is only reduced by about 2.7% abated by the pressure drag. Compared with the actuation of the all-point blowing/suction or the all-point wall movement, the effectiveness of the turbulent drag reduction based on active shallow dimples is much smaller.

关键词： turbulent drag reduction active dimple opposition control suboptimal control

drag reduction EFFECT OF COUPLING FLEXIBLE TUBES WITH turbulent FLOW

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Journal of Hydrodynamics 2008年第1期20卷 96-100页

作者： CAI Shu-peng JIN Guo-yu LI Da-mei Yang Lin Institute of High Pressure Water Jet Hunan University of Technology Zhuzhou 421008 China State Key Laboratory of Water Resources and Hydropower Engineering Science Wuhan University Wuhan430072 China College of Mechanical and Electrical Engineering Chongqing Jiaotong University Chongqing 400074 China

To analyze the mechanism of drag reducing effect by coupling flexible tubes with turbulent flow, based on experimental examination of more obvious turbulent drag reduction effect in flexible tubes than in rigid tubes, experimental investigation was performed on the effect of turbulent drag reduction, fluctuating vibration characteristics of flexible tube and the correlations by using a double-tube system and laser displacement sensor. The results are as follows： with the decrease of the thickness of the flexible tubes, the root mean square of fluctuating amplitude of the outer wall of the tubes increases, and the non-dimensional burst period increases, resulting in the increase of the reduction rate of drag coefficient by coupling flexible tubes with turbulent flow. At applied pressure-balanced air on the outer wall and the Reynolds number of about 1.75×10^4, the non-dimensional burst periods of the flexible tubes with the thickness of 2 mm, 3 mm, 4 mm are 141, 126, 105, respectively.

关键词： flexible tubes fluctuating displacement turbulent drag reduction burst period

Active control of turbulence for drag reduction based on the detection of near-wall streamwise vortices by wall information

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Acta Mechanica Sinica 2015年第4期31卷 512-522页

作者： Mingwei Ge Chunxiao Xu Gui-Xiang Cui School of Renewable Energy North China Electric Power University School of Aerospace Tsinghua University

The spatial relations between the measurable wall quantities （streamwise shear stress τwx, spanwise shear stress τwz, and pressure fluctuations Pw） and the near-wall streamwise vortices （NWSV） are investigated via direct numerical simulation （DNS） databases of fully developed turbulent channel flow at a low Reynolds number. In the stan- dard turbulent channel flow, the results show that all the wall measurable variables are closely associated with the NWSV. But after applying a stochastic interference, the relation based on τwx breaks down while the correlations based on Pw and τwz are still robust. Hence, two wall flow quantities based on Pw and τwz are proposed to detect the NWSV. As an appli- cation, two new control schemes are developed to suppress the near-wall vortical structures using the actuation of wall blowing/suction and obtain 16 % and 11% drag reduction, respectively.

关键词： Active control Measurable wall information Near-wall streamwise vortices turbulent drag reduction

Analysis of heat transfer performance for turbulent viscoelastic fluid-based nanofluid using field synergy principle

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Science China(Technological Sciences) 2015年第7期58卷 1137-1145页

作者： YANG JuanCheng LI FengChen NI MingJiu YU Bo School of Physical Sciences University of Chinese Academy of Sciences School of Energy Science and Engineering Harbin Institute of Technology Beijing Key Laboratory of Urban Oil and Gas Distribution Technology China University of Petroleum

In this paper,the field synergy principle is firstly performed on the viscoelastic fluid-based nanofluid and other relevant fluid in channel at turbulent flow state to scrutinize their heat transfer performance based on our direct numerical simulation *** cosine values of intersection angle between velocity vector and temperature gradient vector are calculated for different simulated cases with varying nanoparticle volume fraction,nanoparticle diameter,Reynolds number and Weissenberg *** is found that the filed synergy effect is enhanced when the nanoparticle volume fraction is increased,nanoparticle diameter is decreased and Weissenberg number is decreased,*** heat transfer is also ***,the filed synergy effect is weakened with the increase of Reynolds number which may be the possible reason for the power function relationship in empirical correlation of heat transfer between heat transfer performance and Reynolds number with the constant power exponent lower than ***,it is also observed that the field synergy principle can be used to analyze the heat transfer process of viscoelastic fluid-based nanofluid at the turbulent flow state even if some negative cosine values of intersection angle exist in the flow field.

关键词： viscoelastic fluid-based nanofluid filed synergy principle intersection angle turbulent drag reduction heat transferenhancement

A new mixed subgrid-scale model for large eddy simulation of turbulent drag-reducing flows of viscoelastic fluids

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Chinese Physics B 2015年第7期24卷 293-309页

作者：李凤臣王璐蔡伟华 School of Energy Science and Engineering Harbin Institute of Technology

A mixed subgrid-scale（SGS） model based on coherent structures and temporal approximate deconvolution（MCT） is proposed for turbulent drag-reducing flows of viscoelastic fluids. The main idea of the MCT SGS model is to perform spatial filtering for the momentum equation and temporal filtering for the conformation tensor transport equation of turbulent flow of viscoelastic fluid, respectively. The MCT model is suitable for large eddy simulation（LES） of turbulent dragreducing flows of viscoelastic fluids in engineering applications since the model parameters can be easily obtained. The LES of forced homogeneous isotropic turbulence（FHIT） with polymer additives and turbulent channel flow with surfactant additives based on MCT SGS model shows excellent agreements with direct numerical simulation（DNS） results. Compared with the LES results using the temporal approximate deconvolution model（TADM） for FHIT with polymer additives, this mixed SGS model MCT behaves better, regarding the enhancement of calculating parameters such as the Reynolds *** scientific and engineering research, turbulent flows at high Reynolds numbers are expected, so the MCT model can be a more suitable model for the LES of turbulent drag-reducing flows of viscoelastic fluid with polymer or surfactant additives.

关键词： turbulent drag reduction large eddy simulation viscoelastic fluid subgrid-scale model

Numerical analysis of non-Newtonian rheology effect on hydrocyclone flow field

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Petroleum 2015年第1期1卷 68-74页

作者： Lin Yang Jia-Lin Tian Zhi Yang You Li Chuan-Hong Fu Yong-Hao Zhu Xiao-Lin Pang School of Mechanical Engineering Southwest Petroleum UniversityChengdu 610500China School of Mechanical Engineering Southwest Jiaotong UniversityChengdu 610031China

In view of the limitations of the existing Newton fluid effects on the vortex flow mechanism study,numerical analysis of non Newton fluid effects was *** Reynolds stress turbulence model(RSM)and mixed multiphase flow model(Mixture)of FLUENT(fluid calculation software)and combined with the constitutive equation of apparent viscosity of non-Newtonian fluid,the typical non-Newtonian fluid(drilling fluid,polymer flooding sewage and crude oil as medium)and Newton flow field(water as medium)were compared by quantitative *** on the research results of water,the effects of non-Newtonian rheology on the key parameters including the combined vortex motion index n and tangential velocity were *** study shows that:non-Newtonian rheology has a great effect on tangential velocity and n value,and tangential velocity decreases with non-Newtonian *** three kinds of n values(constant segment)are:0.564(water),0.769(polymer flooding sewage),0.708(drilling fluid)and their variation amplitudes are larger than Newtonian *** same time,non-Newtonian rheology will lead to the phenomenon of turbulent drag reduction in the vortex flow *** with the existing formula calculation results shown,the calculation result of non-Newtonian rheology is most consistent with the simulation result,and the original theory has large *** study provides reference for theory research of non-Newtonian cyclone separation flow field.

关键词： Non-Newtonian rheology Cyclone separation Key parameters turbulent drag reduction FLUENT

Direct numerical simulation of viscoelastic-fluid-based nanofluid turbulent channel flow with heat transfer

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Chinese Physics B 2015年第8期24卷 404-420页

作者：阳倦成李凤臣蔡伟华张红娜宇波 School of Energy Science and Engineering Harbin Institute of Technology School of Physics Sciences University of Chinese Academy of Sciences Beijing Key Laboratory of Urban Oil and Gas Distribution Technology China University of Petroleum

Our previous experimental studies have confirmed that viscoelastic-fluid-based nanofluid(VFBN) prepared by suspending nanoparticles in a viscoelastic base fluid(VBF, behaves drag reduction at turbulent flow state) can reduce turbulent flow resistance as compared with water and enhance heat transfer as compared with VBF. Direct numerical simulation(DNS) is performed in this study to explore the mechanisms of heat transfer enhancement(HTE) and flow drag reduction(DR) for the VFBN turbulent flow. The Giesekus model is used as the constitutive equation for VFBN. Our previously proposed thermal dispersion model is adopted to take into account the thermal dispersion effects of nanoparticles in the VFBN turbulent flow. The DNS results show similar behaviors for flow resistance and heat transfer to those obtained in our previous experiments. Detailed analyses are conducted for the turbulent velocity, temperature, and conformation fields obtained by DNSs for different fluid cases, and for the friction factor with viscous, turbulent, and elastic contributions and heat transfer rate with conductive, turbulent and thermal dispersion contributions of nanoparticles, respectively. The mechanisms of HTE and DR of VFBN turbulent flows are then discussed. Based on analogy theory, the ratios of Chilton–Colburn factor to friction factor for different fluid flow cases are investigated, which from another aspect show the significant enhancement in heat transfer performance for some cases of water-based nanofluid and VFBN turbulent flows.

关键词： viscoelastic-fluid-based nanofluid direct numerical simulation thermal dispersion model turbulent drag reduction heat transfer e