The paper concerns an experimental study of the fully developed turbulent pipe flow of several different aqueous polymer solutions: 0.25%, 0.3% and 0.4% carboxymethylcellulose (CMC), 0.2% xanthan gum (XG), a 0.09%/0.09% CMC/XG blend, 0.125% and 0.2% polyacrylamide (PAA). The flow data include friction factor vs. Reynolds number, mean velocity and near-wall shear rate distributions, and axial velocity fluctuation intensity u′ at a fixed radial location as a laminar/turbulent transition indicator. For each fluid we also include measurements of shear viscosity, first normal-stress difference and extensional viscosity. At high shear rates we find that the degree of viscoelasticity increases with concentration (0.3% CMC is an exception) for a given polymer, and in the sequence XG, CMC/XG, CMC, PAA, whilst at low shear rates the ranking changes to CMC, CMC/XG, XG, PAA. The extensional viscosity ranking is XG/CMC, XG, CMC, PAA at high strain rates and the same as that for the viscoelasticity at low shear rates. We find that the observed drag-reduction behaviour is consistent for most part with the viscoelastic and extensional-viscosity behaviour at the low shear and strain rates typical of those occurring in the outer zone of the buffer region. Although laminar/turbulent transition is practically indiscernible from the friction factor vs. Reynolds number plots, particularly for PAA and XG, the u′ level provides a very clear indicator and it is found that the transition delay follows much the same trend with elasticity/extensional viscosity as the drag reduction.

Drag reduction in the turbulent pipe flow of polymers

In this review, we report recent developments on the shear-induced transitions and instabilities found in surfactant wormlike micelles. The survey focuses on the nonlinear shear rheology and covers a broad range of surfactant concentrations, from the dilute to the liquid-crystalline states and including the semidilute and concentrated regimes. Based on a systematic analysis of many surfactant systems, the present approach aims to identify the essential features of the transitions. It is suggested that these features define classes of behaviors. The review describes three types of transitions and/or instabilities: the shear-thickening found in the dilute regime, the shear-banding which is linked in some systems to the isotropic-to-nematic transition, and the flow-aligning and tumbling instabilities characteristic of nematic structures. In these three classes of behaviors, the shear-induced transitions are the result of a coupling between the internal structure of the fluid and the flow, resulting in a new mesoscopic organization under shear. This survey finally highlights the potential use of wormlike micelles as model systems for complex fluids and for applications.

https://www.springer.com/cda/content/document/cda_downloaddocument/9783642135316-c1.pdf?SGWID=0-0-45-967041-p174010264

Shear-Induced Transitions and Instabilities in Surfactant Wormlike Micelles

The viscosity as a function of concentration for xanthan gum in both salt-free solution and in 50 mM NaCl is measured and compared with a scaling theory for polyelectrolytes. In general, the zero shear rate viscosity and the degree of shear thinning increase with polymer concentration. In addition, shear thinning was observed in the dilute regime in both solvents. In salt-free solution, four concentration regimes of viscosity scaling and three associated critical concentrations were observed (c* ≈ 70 ppm, c e ≈ 400 ppm, and c D ≈ 2000 ppm). In salt solution, only three concentration regimes and two critical concentrations were observed (c* ≈ 200 ppm and c e ≈ 800 ppm). In the presence of salt, the polymer chain structure collapses and occupies much less space resulting in higher values of the critical concentrations. The observed viscosity-concentration scaling is in very good agreement with theory in the semidilute unentangled and semidilute entangled regimes in both salt-free and 50 mM NaCl solution.

Rheology and viscosity scaling of the polyelectrolyte xanthan gum

We review the current literature pertaining to the characterization of soft matter subject to flow utilizing small-angle neutron scattering, flow-SANS, with an emphasis on the simultaneous measurement of the rheology, Rheo-SANS. Experimental results are discussed in terms of the flow induced structure and direct connection to the bulk rheology in which we highlight the use of the contrast match method as a unique advantage to neutron scattering techniques. Finally, we discuss specific areas in each field that could benefit from focused flow-SANS experiments, and the projected evolution of specialized flow-SANS sample environments.

Flow-SANS and Rheo-SANS applied to soft matter

New power spectra computed from LDA measurements of the fluctuating u- and v-velocity signals in a turbulent channel flow with and without drag-reducing polymer (polyethylene oxide) injection are presented. LDA data rates were sufficiently high to reconstruct the simultaneous time-dependent u- and v-velocity signals along with the time-dependent Reynolds stress signal. Time-averaged statistics of the turbulent flow are presented in conjunction with the power spectral measurements which show a dramatic reduction in both the v-velocity fluctuations and Reynolds stress fluctuations throughout the channel over all frequencies. There is also a redistribution of energy in the u-velocity fluctuations from high frequencies to low frequencies throughout the channel. Different injection conditions were examined: different polymer concentrations were injected at different flow rates such that the total amount of polymer in the channel remained constant. For certain polymer concentrations, ‘large’ negative Reynolds stress, -〈uv〉/uτ2 ≈ − 0.2, was measured in the near-wall region. In addition, there is a marked difference in the u-velocity spectra and the Reynolds stress spectra close to the wall for the different injection conditions.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112092000600

Modifying turbulent structure with drag-reducing polymer additives in turbulent channel flows

The rheological and turbulent drag reducing characteristics of commercial and purified xanthan gum solutions of concentrations 50–500 ppm have been studied with and without addition of 100 ppm NaCl. The purification by soxlet extraction of xanthan gum using 95% ethanol is effective in removing low-molecular-weight impurities from xanthan. The increased content of higher molecular-weight xanthan in purified xanthan is evident from rheological and drag reduction behavior. The addition of 100 ppm salt to dilute solutions introduces semi-flexibility in xanthan gum solution without occurrence of self-association. The change in molecular behavior in the presence of salt is evident from rheological normal-stress and turbulent drag reduction behaviors.

Rheological and drag reduction characteristics of xanthan gum solutions

Es wird uber Experimente berichtet, bei denen eine viskoelastische Flussigkeit in die Achse einer ausgebildeten turbulenten Rohrstromung eines newtonschen Fluids injiziert wurde. Bei der Injektion von hoher konzentrierter Polymerlosung tritt eine deutliche Widerstandsverminderung auch dann auf, wenn die Losung als elastischer Faden im Kernbereich mitgefuhrt wird, d. h. das Polymer noch nicht merklich in die wandnahe Schicht hineindiffundiert ist. Es werden Ergebnisse uber die Veranderung des Druckabfalls langs des Rohres in Abhangigkeit von der Gesamtkonzentration des Polymers im Rohr, der Konzentration im injizierten Faden und der Reynoldszahl mitgeteilt und diskutiert. Weiterhin werden die Profile der Hauptstromung, der Turbulenzintensitaten in radialer und axialer Richtung sowie der Reynoldsschen Schubspannungen bei solchen Injektionsexperimenten bestimmt und mit denjenigen des reinen Losungsmittels sowie einer homogenen Polymerlosung von gleicher, uber den Rohrquerschnitt gemittelter Polymerkonzentration verglichen. Der Verlauf des dimensionslosen Geschwindigkeitsprofils und der Reynoldschen Schubspannungen zeigt, das der Ubergangsbereich zwischen viskoser Unterschicht und turbulentem Kern bei der Injektion wesentlich anders strukturiert ist als beim Vorliegen von homogener Widerstandsverminderung. Er ist sowohl im Vergleich zu einer newtonschen Flussigkeit als auch zu einer homogenen Polymerlosung deutlich vergrosert, und es findet in ihm eine drastische Reduzierung der Reynoldsschen Schubspannungen statt. Die beobachtete Struktur der turbulenten Stromung deutet in Verbindung mit den gemessenen viskoelastischen Eigenschaften der injizierten Polymerlosung darauf hin, das die grosen Turbulenzelemente durch den Polymerfaden in ihrer Bewegung behindert werden. Infolgedessen ubertragen sie weniger Energie auf die die Dissipation bewirkenden kleinen Wirbel, wodurch sich eine Widerstandsverminderung ergibt.

Heterogene Widerstandsverminderung bei turbulenten Rohrströmungen

Small-angle-neutron-scattering measurements (SANS) were done with a dilute solution of the surfactant n-tetradecyltrimethylammoniumbromide (TTAB) with sodium salicylate (Na Sal) in pipe and channel flow. The solvent used was heavy water. The anisotropy observed in the SANS-curves can be related to the drag reduction behaviour of the surfactant solution. The breakdown of the drag reducing properties above the critical wall shear stress is accompanied by a strong decrease in the scattering anisotropy. However, in this flow region the scattering curves reveal a very similar micellar structure to that found in the fluid at rest. This shows that the micelles exist above the critical wall shear stress.

The conformation of drag reducing micelles from small-angle-neutron-scattering experiments

Surfactant solutions containing rodlike micelles have been investigated by small angle neutron scattering and by light scattering at rest and under laminar and turbulent flow conditions. At rest and at room temperature long rods of average length 249 nm are found with static light and small angle neutron scattering. To interprete the orientation of these rods in a laminar shear gradient the a ssumption of an increased average rod length is necessary. In turbulent pipe flow the rod-like micelles are initially oriented in the bulk flow direction with more pronounced orientation in the near wall regions compared to the core regions of the pipe. Eventually a destruction of the initially rod-like micelles takes place leading to a rearrangement of the surfactant molecules into smaller aggregates part of which, in turn, build up loose but lasting (over several months at room temperature) superstructures.

Drag-reducing surfactant solutions in laminar and turbulent flow investigated by small-angle neutron scattering and light scattering

SANS-measurements of dilute surfactant solutions containing rodlike micelles were performed under laminar and turbulent (drag-reduced) flow conditions in a pipe geometry. Experiments indicate that the increased local stresses in turbulent flow partially break up the micellar structure, leading to a broader length-distribution of the particles and a decrease of the average rod length.

H. W. Bewersdorff

Papers

Rheological and drag reduction characteristics of xanthan gum solutions

Heterogene Widerstandsverminderung bei turbulenten Rohrströmungen

The conformation of drag reducing micelles from small-angle-neutron-scattering experiments

Drag-reducing surfactant solutions in laminar and turbulent flow investigated by small-angle neutron scattering and light scattering

Sans- and LS-studies on drag-reducing surfactant solutions