Drag reduction and turbulent structure in two-dimensional channel flows

1991 ◽  
Vol 336 (1640) ◽  
pp. 19-34 ◽  
Keyword(s):  
Shear Stress ◽  
Drag Reduction ◽  
Strain Rate ◽  
Reynolds Stress ◽  
Polymer Concentration ◽  
Turbulent Structure ◽  
Channel Flows ◽  
Channel Height ◽  
Normal Velocity ◽  
Two Dimensional

A two-component laser velocimeter has been used to determine the effect of wall strain rate, polymer concentration and channel height upon the drag reduction and turbulent structure in fully developed, low concentration, two-dimensional channel flows. Water flows at equal wall shear stress and with Reynolds numbers from 14430 to 34640 were measured for comparison. Drag reduction levels clearly depended upon wall strain rate, polymer concentration and channel height independently.However, most of the turbulent structure depended only upon the level of drag reduction. The slope of the logarithmic law of the wall increased as drag reduction increased. Similarly, the root-mean-square of the fluctuations in the streamwise velocity increased while the r.m.s. of the fluctuations in the wall-normal velocity decreased with drag reduction. The production of the streamwise normal Reynolds stress and the Reynolds shear stress decreased in the drag-reduced flows. Therefore it appears that the polymer solutions inhibit the transfer of energy from the streamwise to the wall-normal velocity fluctuations. This could occur through inhibiting the newtonian transfer mechanism provided by the pressure-strain correlation. In six drag-reducing flows, the sum of the Reynolds stress and the mean viscous stress was equal to the total shear stress. However, for the combination of highest concentration (5 p.p.m.), smallest channel height (25 mm) and highest wall strain rate (4000 s - 1 ), the sum of the Reynolds and viscous stresses was substantially lower than the total stress indicating the presence of a strong non-newtonian effect. In all drag-reducing flows the correlation coefficient for uv decreased as the axes of principal stress for the Reynolds stress rotated toward the streamwise and wall-normal directions.

Turbulence characteristics of the boundary layer on a rotating disk

1994 ◽  
Vol 266 ◽  
pp. 175-207 ◽  
Author(s):  
Howard S. Littell ◽  
John K. Eaton
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Reynolds Stress ◽  
Structural Model ◽  
Rotating Disk ◽  
Two Dimensional ◽  
Mean Flow ◽  
Velocity Correlation ◽  
Dimensional Boundary ◽  
The Mean

Measurements of the boundary layer on an effectively infinite rotating disk in a quiescent environment are described for Reynolds numbers up to Reδ2 = 6000. The mean flow properties were found to resemble a ‘typical’ three-dimensional crossflow, while some aspects of the turbulence measurements were significantly different from two-dimensional boundary layers that are turned. Notably, the ratio of the shear stress vector magnitude to the turbulent kinetic energy was found to be at a maximum near the wall, instead of being locally depressed as in a turned two-dimensional boundary layer. Also, the shear stress and the mean strain rate vectors were found to be more closely aligned than would be expected in a flow with this degree of crossflow. Two-point velocity correlation measurements exhibited strong asymmetries which are impossible in a two-dimensional boundary layer. Using conditional sampling, the velocity field surrounding strong Reynolds stress events was partially mapped. These data were studied in the light of the structural model of Robinson (1991), and a hypothesis describing the effect of cross-stream shear on Reynolds stress events is developed.

Properties of the mean recirculation region in the wakes of two-dimensional bluff bodies

1997 ◽  
Vol 351 ◽  
pp. 167-199 ◽  
Author(s):  
S. BALACHANDAR ◽  
R. MITTAL ◽  
F. M. NAJJAR
Keyword(s):  
Reynolds Number ◽  
Shear Stress ◽  
Reynolds Stress ◽  
Vortex Shedding ◽  
Reynolds Numbers ◽  
Recirculation Region ◽  
Bluff Bodies ◽  
Two Dimensional ◽  
Stress Components ◽  
The Mean

The properties of the time- and span-averaged mean wake recirculation region are investigated in separated flows over several different two-dimensional bluff bodies. Ten different cases are considered and they divide into two groups: cylindrical geometries of circular, elliptic and square cross-sections and the normal plate. A wide Reynolds number range from 250 to 140000 is considered, but in all the cases the attached portion of the boundary layer remains laminar until separation. The lower Reynolds number data are from direct numerical simulations, while the data at the higher Reynolds number are obtained from large-eddy simulation and the experimental work of Cantwell & Coles (1983), Krothapalli (1996, personal communication), Leder (1991) and Lyn et al. (1995). Unlike supersonic and subsonic separations with a splitter plate in the wake, in all the cases considered here there is strong interaction between the shear layers resulting in Kármán vortex shedding. The impact of this fundamental difference on the distribution of Reynolds stress components and pressure in relation to the mean wake recirculation region (wake bubble) is considered. It is observed that in all cases the contribution from Reynolds normal stress to the force balance of the wake bubble is significant. In fact, in the cylinder geometries this contribution can outweigh the net force from the shear stress, so that the net pressure force tends to push the bubble away from the body. In contrast, in the case of normal plate, owing to the longer wake, the net contribution from shear stress outweighs that from the normal stress. At higher Reynolds numbers, separation of the Reynolds stress components into incoherent contributions provides more insight. The behaviour of the coherent contribution, arising from the dominant vortex shedding, is similar to that at lower Reynolds numbers. The incoherent contribution to Reynolds stress, arising from small-scale activity, is compared with that of a canonical free shear layer. Based on these observations a simple extension of the wake model (Sychev 1982; Roshko 1993a, b) is proposed.

Reynolds stress development in pressure-driven three-dimensional turbulent boundary layers

1989 ◽  
Vol 202 ◽  
pp. 263-294 ◽  
Author(s):  
Shawn D. Anderson ◽  
John K. Eaton
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Reynolds Stress ◽  
Free Stream ◽  
Three Dimensional ◽  
Radius Of Curvature ◽  
Two Dimensional ◽  
Stress Vector ◽  
Dimensional Boundary ◽  
Cross Wire

The development of the Reynolds stress field was studied for flows in which an initially two-dimensional boundary layer was skewed sideways by a spanwise pressure gradient ahead of an upstream-facing wedge. Two different wedges were used, providing a variation in the boundary-layer skewing. Measurements of all components of the Reynolds stress tensor and all ten triple products were measured using a rotatable cross-wire anemometer. The results show the expected lag of the shear stress vector behind the strain rate. Comparison of the two present experiments with previous data suggests that the lag can be estimated if the radius of curvature of the free-stream streamline is known. The magnitude of the shear stress vector in the plane of the wall is seen to decrease rapidly as the boundary-layer skewing increases. The amount of decrease is apparently related to the skewing angle between the wall and the free stream. The triple products evolve rapidly and profiles in the three-dimensional boundary layer are considerably different than two-dimensional profiles, leaving little hope for gradient transport models for the Reynolds stresses. The simplified model presented by Rotta (1979) performs reasonably well providing that an appropriate value of the T-parameter is chosen.

Drag Reduction Effect of Dimples Arranged in Two-Dimensional Quasicrystal Structure

2010 ◽  
Vol 146-147 ◽  
pp. 331-335
Author(s):  
Wen Hui Xue ◽  
Xing Guo Geng ◽  
Feng Li ◽  
Jie Li ◽  
Yao Zhang ◽  
...  
Keyword(s):  
Shear Stress ◽  
Drag Reduction ◽  
Two Dimensional ◽  
Rotating Fluids ◽  
Periodic Arrays ◽  
Visualization Experiment ◽  
Reduction Efficiency ◽  
Reduction Effect ◽  
Coherent Effect ◽  
Quasicrystal Structure

Rotating fluids experiments were carried out by CAP2000+ cone viscometer, to examine the drag reduction properties of dimples arranged in quasicrystal structure. The dimples were fabricated on the surface of duralunmin (LY12) plates. Compared with the periodic arrays, the dimples arranged in quasicrystal structure, especially the 12-fold quasicrystal structure, could significantly reduce the wall shear stress. And the relative drag reduction efficiency changes periodically with the depth of dimple. Flow-visualization experiment verified that the coherent effect of dimples arranged in quasicrystal structure and the fluids could efficiently inhibit the extending intensity of radial secondary flow, which strengthens the drag reduction effect.

Drag Reduction Due to Streamwise Grooves in Turbulent Channel Flow

2016 ◽  
Vol 138 (12) ◽  
Author(s):  
C. T. DeGroot ◽  
C. Wang ◽  
J. M. Floryan
Keyword(s):  
Shear Stress ◽  
Drag Reduction ◽  
Turbulent Channel Flow ◽  
Surface Modifications ◽  
Channel Flows ◽  
Wave Number ◽  
Stress Profile ◽  
Bulk Fluid ◽  
Geometry Model ◽  
Turbulent Channel Flows

Drag reduction in turbulent channel flows has significant practical relevance for energy savings. Various methods have been proposed to reduce turbulent skin friction, including microscale surface modifications such as riblets or superhydrophobic surfaces. More recently, macroscale surface modifications in the form of longitudinal grooves have been shown to reduce drag in laminar channel flows. The purpose of this study is to show that these grooves also reduce drag in turbulent channel flows and to quantify the drag reduction as a function of the groove parameters. Results are obtained using computational fluid dynamics (CFD) simulations with turbulence modeled by the k–ω shear-stress transport (SST) model, which is first validated with direct numerical simulations (DNS). Based on the CFD results, a reduced geometry model is proposed which shows that the approximate drag reduction can be quantified by evaluating the drag reduction of the geometry given by the first Fourier mode of an arbitrary groove geometry. Results are presented to show the drag reducing potential of grooves as a function of Reynolds number as well as groove wave number, amplitude, and shape. The mechanism of drag reduction is discussed, which is found to be due to a rearrangement of the bulk fluid motion into high-velocity streamtubes in the widest portion of the channel opening, resulting in a change in the wall shear stress profile.

High-fidelity measurements in channel flow with polymer wall injection

2018 ◽  
Vol 859 ◽  
pp. 851-886 ◽  
Author(s):  
John R. Elsnab ◽  
Jason P. Monty ◽  
Christopher M. White ◽  
Manoochehr M. Koochesfahani ◽  
Joseph C. Klewicki
Keyword(s):  
Shear Stress ◽  
Drag Reduction ◽  
Channel Flow ◽  
Polymer Concentration ◽  
Streamwise Velocity ◽  
Velocity Profiles ◽  
Order Structure ◽  
Dynamical Structure ◽  
Molecular Tagging Velocimetry ◽  
The Mean

Streamwise velocity profiles and their wall-normal derivatives were used to investigate the properties of turbulent channel flow in the low polymer drag reduction$(DR)$regime ($DR=6.5\,\%$to$26\,\%$), as realized via polymer injection at the channel surface. Streamwise velocity data were obtained over a friction Reynolds number ranging from$650$to$1800$using the single-velocity-component version of molecular tagging velocimetry (1c-MTV). This adaptation of the MTV technique has the ability to accurately capture instantaneous profiles at very high spatial resolution (${\gtrsim}850$data points per wall-normal profile), and thus generate well-resolved derivative information as well. Owing to this ability, the present study is able to build upon and extend the recent numerical simulation analysis of Whiteet al. (J. Fluid Mech., vol. 834, 2018, pp. 409–433) that examined the mean dynamical structure of polymer drag-reduced channel flow at friction Reynolds numbers up to$1000$. Consistently, the present mean velocity profiles indicate that the extent of the logarithmic region diminishes with increasing polymer concentration, while statistically significant increases in the logarithmic profile slope begin to occur for drag reductions less than$15\,\%$. Profiles of the r.m.s. streamwise velocity indicate that the maximum moves farther from the wall and increases in magnitude with reductions in drag. Similarly, with increasing drag reduction, the profile of the combined Reynolds and polymer shear stress exhibits a decrease in its maximum value that also moves farther from the wall. Correlations are presented that estimate the location and value of the maximum r.m.s. streamwise velocity and combined Reynolds and polymer shear stress. Over the range of$DR$investigated, these effects consistently exhibit approximately linear trends as a function of$DR$. The present measurements allow reconstruction of the mean momentum balance (MMB) for channel flow, which provides further insights regarding the physics described in the study by Whiteet al. In particular, the present findings support a physical scenario in which the self-similar properties on the inertial domain identified from the leading-order structure of the MMB begin to detectably and continuously vary for drag reductions less than$10\,\%$.

Experimental Study on Turbulent Drag Reduction and Polymer Concentration Distribution With Blowing Polymer Solution From the Channel Wall

2010 ◽  
Author(s):  
Masaaki Motozawa ◽  
Taiki Kurosawa ◽  
Hening Xu ◽  
Kaoru Iwamoto ◽  
Hirotomo Ando ◽  
...  
Keyword(s):  
Experimental Study ◽  
Drag Reduction ◽  
Polymer Solution ◽  
Concentration Distribution ◽  
Channel Wall ◽  
Polymer Concentration ◽  
Channel Height ◽  
Wall Region ◽  
Turbulent Drag Reduction ◽  
Turbulent Drag

Experimental study on turbulent drag reduction (DR) and polymer concentration distribution with blowing polymer solution from whole surface of the channel wall was carried out. A set of measurements for drag reduction were performed with blowing rate for the sintered porous metal plate (0.45m × 0.45m × 3) adjusted from 0.5 L/min to 4.0 L/min, and concentration of polymer solution varied from 10 ppm to 200 ppm. Reynolds number based on the channel height was chosen for 20000 and 40000 in this experiment. The polymer concentration distribution in the near-wall region (0.5 mm < y < 20 mm) at three locations of the downstream from the leading edge of the blower wall was also measured. Polymer concentration can be analyzed via Total Organic Carbon (TOC) analyzer. Through the analysis of mass transfer by polymer concentration distribution, we found that polymer which exists in buffer layer (10 < y+ < 70) has important influence on drag reduction.

The Impact of a Vortex Induced by a Baffle on the Turbulent Structure

2008 ◽  
Author(s):  
Shivani T. Gajusingh ◽  
Nasiruddin Shaikh ◽  
Kamran Siddiqui
Keyword(s):  
Experimental Study ◽  
Reynolds Stress ◽  
Reynolds Numbers ◽  
Velocity Fields ◽  
Turbulent Structure ◽  
Turbulent Regime ◽  
Two Dimensional ◽  
Square Channel ◽  
Order Of Magnitude ◽  
The Impact

An experimental study was conducted to investigate the impact of a rectangular baffle inside a square channel. PIV was used to measure the two-dimensional velocity fields. The measurements were conducted for two Reynolds numbers (Reτ) equal to 134 and 152 in the turbulent regime. Results show that the turbulent intensities and the Reynolds Stress are enhanced by more than an order of magnitude in the presence of a baffle. Significant enhancement was observed in a region up to two times the baffle height.

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