Degradation effects of dilute polymer solutions on turbulent drag reduction in pipe flows

An important practical problem in the application and study of drag reduction by polymer additives is the degradation of the polymer, for instance due to intense shearing, especially in recirculatory flow systems. Such degradation leads to a marked loss of the drag-reducing capability of the polymer.Three different polymer types were tested on degradation effects in a closed pipe flow system. The polymers used were Polyox WSR-301, Separan AP-273 and Superfloc A-110, dissolved in water in concentrations of 20 wppm each. The flow system consisted of a 16.3 mm pipe of 4.25 m length. Two different pumps were used: a centrifugal pump and a disc pump. Different solution-preparation procedures were tried and the experiments were performed at different flow rates.Superfloc A-110 proved to be both the most effective drag reducer and most resistant to degradation. Because of very fast degradation, Polyox WSR-301 was found to be unsuitable for being used as a drag reducer in re-circulatory systems. The disc pump proved to be much better suited for pumping the polymer solutions than the centrifugal pump. The degradation curve of the combination Superfloc/disc pump showed a plateau-like region with reasonable drag reduction, which makes it possible to perform (laser Doppler) measurements under nearly constant circumstances during a sufficient time.     

Reynolds-averaged modeling of polymer drag reduction in turbulent flows

A novel eddy viscosity model for predicting friction drag reduction induced by polymers in turbulent wall-bounded flows is presented. The approach is based on the elliptic relaxation model modified to account for the modified Reynolds-stress equilibrium established by the presence of elastic polymer chains in the fluid. The increased wall damping of the turbulent fluctuations is obtained by modifying the pressure–strain redistribution term. Polymer solutions are represented using the Finite Extensibility Non-linear elastic FENE-P dumbbell model; only one transport equation for the elongation of the polymer chains is considered. The model reproduces the level of drag reduction observed over a wide range of rheological parameters. In addition, both the mean velocity and the turbulent fluctuations are predicted with good accuracy. The approach is computationally attractive because of its limited increase in computational cost in comparison with its Newtonian counterpart.     

Heterogeneous drag reduction in turbulent pipe flows using various injection techniques

The results of an experimental study of the injection of concentrated polymer solutions into the near-wall region of a turbulent pipe flow are reported. The injection experiments described here show drag reduction that was significantly larger than that obtained for homogeneous polymer solutions of the same average concentration. Local drag reduction and friction behavior was obtained by measuring pressure differences over a test section of 13 m in length. Furthermore the flow behaviour of the injected polymer solution was investigated by flow visualization experiments. Velocity profile measurements elucidate in case of near-wall injection that the turbulent structure could be altered in the near-wall and also in the core region of the pipe flow, indicating that the polymer lumps and threads created by the near-wall injection are able to influence a much wider spectrum of turbulent eddies in comparison to centreline injection or, all the more, to homogeneous drag reduction.     

Time-dependent drag reduction and ageing in aqueous solutions of a cationic surfactant

The turbulent pipe flow of a highly dilute aqueous cationic surfactant solution is investigated by means of a pulsed ultrasound Doppler method with special emphasis on the wall boundary layer. The velocity profiles are recorded for several Reynolds numbers at varying ages of the solution. The wall shear stress velocities u _τ used for the normalization of the velocity profiles are determined by fitting the measured profiles to the universal linear velocity profile in the viscous sublayer. The theoretical pressure loss is then calculated from the numerical values of u _τ and compared to the experimental values. Two different scaling methods are discussed for the velocity fluctuations concerning the correlation of the root-mean square values with the effect and the amount of drag reduction. It is shown that outer scaling with the mean velocity is appropriate for the detection of drag reduction in surfactant solutions, rather than inner scaling with the wall shear stress velocity, which is common practice in investigations of 'usual' turbulent flows.     

Turbulent drag reduction in dam-break flows

The role of turbulence is investigated in dam-break flows, where a finite volume of fluid is released from a compartment into a long, rectangular channel. After a sudden removal of the lock gate, a gravity current, undular bore, or solitary wave develops, depending on the ambient fluid height in the channel. The temporal evolution of the moving front has been measured and evaluated. It was observed that the dilution using a very small amount (a few weight ppm) of a long chain polymer (polyethylene-oxide) in the fluid strongly affected flow properties. Pronounced drag reduction has been found in dry bed flows (whereas the polymer increased the viscosity of the fluid). The presence of a few mm-thick ambient fluid layer in the channel effectively destroyed drag reduction, in spite of the fact that strong turbulence was obvious and the propagation velocity of the front was almost unchanged.     

Energy-based decomposition of friction drag in turbulent square-duct flows

The generation of friction drag in turbulent duct flows has direct connection with statistical quantities and corresponding turbulence dynamics in the duct cross-section. In this study, we generalize the RD identity (Renard and Deck, 2016) to a ‘two-dimensional’ form which we exploit to decompose the mean friction drag in turbulent square-duct flows into contributions associated with viscosity, turbulence and cross-stream convection. The friction Reynolds number of the duct flows ranges from 220 to 2000. The scaling, spatial distribution and local normalization of the contributions to friction are investigated and compared with those in pipe and channel flows. As in other canonical flows, we find logarithmic growth of the turbulent contribution in contrast to the viscous one, the former thus becoming dominant at high enough Reynolds numbers. Whereas cross-stream convection has no net effect on friction, its contribution may be locally comparable to the other two, hence may be responsible for redistribution of friction along the duct perimeter.     

The effect of solvent solubility parameter on turbulent flow drag reduction in polyisobutylene solutions

Drag reduction is the effective reduction of the fluid flow friction brought about by the addition of small amounts of dissolved polymer, suspended particles, or emulsions. This study has focused on the turbulent-flow drag reduction effected by small amounts (10 ^-6–10 ^-3 g/ml) of polyisobutylene dissolved in organic solvents of varying solubility parameters. The data show that a maximum drag reduction (up to 70% for Reynolds numbers of 20,000) occurs in solvents with a solubility parameter near that of the polymer.     

Microbubble drag reduction in rough walled turbulent boundary layers with comparison against polymer drag reduction

Experiments were conducted in the 12-inch diameter tunnel at the Applied Research Laboratory, Pennsylvania State University using the tunnel wall boundary layer to determine the influence of surface roughness on microbubble drag reduction. To accomplish this, carbon dioxide was injected through a slot at rates of 0.001 m³/s to 0.011 m³/s, and the resulting skin friction drag measured on a 317.5-mm long by 152.4-mm span balance. In addition to the hydrodynamically smooth balance plate, additional plates were covered with roughly 75, 150, and 300 micron grit. Over the speed range tested of 7.6, 10.7, and 13.7 m/s, the roughness ranged from smooth to fully rough. Not only was microbubble drag reduction achieved over the rough surfaces, but the % drag reduction at a given gas flow rate was larger for larger roughness. Scaling of the data is discussed. Comparison against results of a polymer drag reduction experiment, using the same facility, is made. Finally, a measure of the expected persistence of the phenomenon is given.     

Effect of drag force on backward-facing step gas-particle turbulent flows

An improved drag force coefficient of gas-particle interaction based on the traditional Wen’s 1966 model is proposed. In this model, a two-stage continuous function is used to correct the discontinuous switch when porosity less than 0.2. Using this proposed correlation and the Wen’s 1966 model, a gas-particle kinetic energy and particle temperature model is developed to predict the hydrodynamic characteristics in backward-facing step gas-particle two-phase turbulent flows. Numerically results showed that they are in good agreement with experiment measurements and presented model are better due to a improvement of momentum transport between gas and particle phases. Particle dispersions take on the distinctively anisotropic behaviors at every directions and gas phase fluctuation velocity are about twice larger than particle phases. Particle phase has a unique transportation mechanism and completely different from the gas phase due to different density. Furthermore, the correlation values of axial–axial gas-particle are always greater than the radial–radial values at fully flow regions. The gas-particle two-phase interactions will make influence on two-phase turbulent flow behaviors.     

Surfactant turbulent drag reduction in an enclosed rotating disk apparatus

An enclosed rotating disk apparatus (RDA) with rotational speed up to 5,500 rpm and with temperature control from −5 to 55°C was designed to screen the turbulent drag reducing effectiveness of small samples of newly synthesized drag reducing additives. First, the rotating disk was calibrated with water using both logarithmic and power law models. Then experiments were carried out to measure the frictional torque reduction for a drag reducing aqueous cationic surfactant system (5 mM Ethoquad O12 with 12.5 mM sodium salicylate) over a range of Re. The maximum drag reduction at 30°C was about 47% at Re = 1.90 × 10⁶. For the first time, results with the RDA were compared with those in a circular pipe flow system. They showed similar trends indicating it is a useful screening device for small samples, giving conservative estimates of surfactant effectiveness compared with pipe flow.

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Effect of insoluble surfactant on turbulent bubbly flows in vertical channels

The effect of an insoluble surfactant on the structure of turbulent bubbly upflow in a vertical channel is examined by direct numerical simulations (DNS). For nearly spherical bubbles the presence of a surfactant reduces the lateral lift on the bubbles and changes the structure of the flow in major ways. Clean bubbles are driven to the walls by the lift force and the void fraction distribution has a well defined peak near the walls, resulting in significant reduction in flow rate. Bubbles with strong enough surfactants do not experience significant lateral lift and remain in the bulk flow. Indeed, when surfactant is present the addition of bubbles to a turbulent flow has relatively little effect on the flow, once the pressure gradient is adjusted to account for the reduced weight of the mixture.     

The role of small scale coherent structures in turbulent drag reduction

With a new visualization technique, a strong suppression of small scale coherent structures (filaments) in turbulence is observed when a drag reducing polymer is added. Simultaneously, the rate of formation of large scale structures (eddies) out of these filaments decreases, which may be an important observation in the explanation of turbulent drag reduction.     

The effect of a zwitterionic and cationic surfactant in turbulent flows

A zwitterionic and a cationic surfactant were compared concerning their effectiveness in drag reduction. Both reach even lower friction factors than can be determined by the known maximum drag reduction asymptote in polymers. It is shown that both the maximum effectiveness and the range of surfactant activity depend not only on the temperature and concentration of the solution, but also on the length of the direct entering section and on the quality of water. The influence of the particular locality of the flow rate regulation is also notified. Received: 1 December 1999/Accepted: 30 April 2000     

Polymer drag reduction with surface roughness in flat-plate turbulent boundary layer flow

Experimental results from a study of surface roughness effects on polymer drag reduction in a zero-pressure gradient flat-plate turbulent boundary layer are presented. Both slot-injected polymer and homogeneous polymer ocean cases were considered over a range of flow conditions and surface roughness. Balance measurements of skin friction drag reduction are presented. Drag reductions over 60% were measured for both the injected and homogeneous polymer cases even with fully rough surfaces. As the roughness increased, higher polymer concentration was required to achieve a given level of drag reduction for the homogeneous case. With polymer injection, increasing surface roughness caused the drag reduction to decrease to low levels more quickly when the polymer expenditure was decreased or the freestream velocity was increased. However, the percent drag reductions on the rough surfaces with polymer injection were often substantially larger than on the smooth surface. Remarkably, in some cases, the skin friction drag force on a rough surface with polymer injection was less than the drag force observed on a smooth surface at comparable conditions. An erratum to this article can be found at     

Skin-friction drag reduction in turbulent channel flow based on streamwise shear control

It is known that stretching and intensification of a hairpin vortex by mean shear play an important role to create a hairpin vortex packet, which generates the large Reynolds shear stress associated with skin-friction drag in wall-bounded turbulent flows. In order to suppress the mean shear at the wall for high efficient drag reduction (DR), in the present study, we explore an active flow control concept using streamwise shear control (SSC) at the wall. The longitudinal control surface is periodically spanwise-arranged with no-control surface while varying the structural spacing, and an amplitude parameter for imposing the strength of the actuating streamwise velocity at the wall is introduced to further enhance the skin-friction DR. Significant DR is observed with an increase in the two parameters with an accompanying reduction of the Reynolds stresses and vorticity fluctuations, although a further increase in the parameters amplifies the turbulence activity in the near-wall region. In order to study the direct relationship between turbulent vortical structures and DR under the SSC, temporal evolution with initial eddies extracted by conditional averages for Reynolds-stress-maximizing Q2 events are examined. It is shown that the generation of new vortices is dramatically inhibited with an increase in the parameters throughout the flow, causing fewer vortices to be generated under the control. However, when the structural spacing is sufficiently large, the generation of new vortex is not suppressed over the no-control surface in the near-wall region, resulting in an increase of the second- and fourth-quadrant Reynolds shear stresses. Although strong actuating velocity intensifies the near-wall turbulence, the increase in the turbulence activity is attributed to the generation of counter-clockwise near-wall vortices by the increased vortex transport.