Drag-reducing Polymer Additives Research Articles

Experimental Investigation of a Cavitating Water Flow With the Addition of Drag- Reducing Agents

Local reduction of the pressure in a flow field to levels lower than the saturation pressure triggers the production of tiny vaporous bubbles and causes the cavitation phenomenon. Higher pressure reductions significantly increase the cavitation bubbles’ population, which can coalesce and generate large-scale cavitation structures such as cloud cavitation that shed downstream of the flow channel. The chaotic collapse of cavities produces strong shockwaves in regions with a recovered pressure which causes serious erosion on solid surfaces and high levels of noise. Hence, there is a growing interest in cavitation control methods. In this study, drag-reducing polymer additives are utilized as cavitation reducing (CR) agents in a converging-diverging mesoscale nozzle to verify the applicability of these agents in the control of the cavitation process. Analysis of high-speed images of the cavitating flow fields reveals that the viscoelastic flow of a 400 ppm polymer solution reduced the cavitation intensity by nearly 60 % relative to the pure water flow at a similar Reynolds number. Ultra-high-speed imaging of single cavitating bubbles at the inception showed that in a viscoelastic flow, the collapse period of cavities is longer, and their sizes are shrunk at a lower rate relative to their counterparts in the puer water. Particle image velocimetry (PIV) was used to study the near-wall turbulent flow fields at the flow conditions close to the cavitation inception, at different flow locations with non-zero pressure gradients present on the curved surfaces. Preliminary analysis of the results reveals that viscoelasticity alters the near-wall turbulence and distribution of the pressure gradient fluctuations, which might link to the significant reduction of cavitation intensity in polymeric flows.

Turbophoresis attenuation in a turbulent channel flow with polymer additives

AbstractTurbophoresis occurs in wall-bounded turbulent flows where it induces a preferential accumulation of inertial particles towards the wall and is related to the spatial gradients of the turbulent velocity fluctuations. In this work, we address the effects of drag-reducing polymer additives on turbophoresis in a channel flow. The analysis is based on data from a direct numerical simulation of the turbulent flow of a viscoelastic fluid modelled with the FENE-P closure and laden with particles of different inertia. We show that polymer additives decrease the particle preferential wall accumulation and demonstrate with an analytical model that the turbophoretic drift is reduced because the wall-normal variation of the wall-normal fluid velocity fluctuations decreases. As this is a typical feature of drag reduction in turbulent flows, an attenuation of turbophoresis and a corresponding increase in the particle streamwise flux are expected to be observed in all of these flows, e.g. fibre or bubble suspensions and magnetohydrodynamics.

Influence of polymer additives on turbulent energy cascading in forced homogeneous isotropic turbulence studied by direct numerical simulations

Direct numerical simulations (DNS) were performed for the forced homogeneous isotropic turbulence (FHIT) with/without polymer additives in order to elaborate the characteristics of the turbulent energy cascading influenced by drag-reducing effects. The finite elastic non-linear extensibility-Peterlin model (FENE-P) was used as the conformation tensor equation for the viscoelastic polymer solution. Detailed analyses of DNS data were carried out in this paper for the turbulence scaling law and the topological dynamics of FHIT as well as the important turbulent parameters, including turbulent kinetic energy spectra, enstrophy and strain, velocity structure function, small-scale intermittency, etc. A natural and straightforward definition for the drag reduction rate was also proposed for the drag-reducing FHIT based on the decrease degree of the turbulent kinetic energy. It was found that the turbulent energy cascading in the FHIT was greatly modified by the drag-reducing polymer additives. The enstrophy and the strain fields in the FHIT of the polymer solution were remarkably weakened as compared with their Newtonian counterparts. The small-scale vortices and the small-scale intermittency were all inhibited by the viscoelastic effects in the FHIT of the polymer solution. However, the scaling law in a fashion of extended self-similarity for the FHIT of the polymer solution, within the presently simulated range of Weissenberg numbers, had no distinct differences compared with that of the Newtonian fluid case.

Depletion layer formation in suspensions of elastic capsules in Newtonian and viscoelastic fluids

Motivated by observations of the effects of drag-reducing polymer additives on various aspects of blood flow, suspensions of fluid-filled elastic capsules in Newtonian fluids and dilute solutions of high molecular weight (drag-reducing) polymers are investigated during plane Couette flow in a slit geometry. A simple model is presented to describe the cross-stream distribution of capsules as a balance of shear-induced diffusion and wall-induced migration due to capsule deformability. The model provides a theoretical prediction of the dependence of capsule-depleted layer thickness on the capillary number. A computational approach is then used to directly study the motion of elastic capsules in a Newtonian fluid and in polymer solutions. Capsule membranes are modeled using a neo-Hookean constitutive model and polymer molecules are modeled as bead-spring chains with finitely extensible nonlinearly elastic springs, with parameters chosen to loosely approximate 4000 kDa poly(ethylene oxide). Simulations are performed with a Stokes flow formulation of the immersed boundary method for the capsules, combined with Brownian dynamics for the polymer molecules. Results for an isolated capsule near a wall indicate that the wall-induced migration depends on the capillary number and is strongly reduced by addition of polymer. Numerical simulations of suspensions of capsules in Newtonian fluid illustrate the formation of a capsule-depleted layer near the walls. The thickness of this layer is found to be strongly dependent on the capillary number. The shear-induced diffusivity of the capsules, on the other hand, shows only a weak dependence on capillary number. These results thus indicate that the mechanism of wall-induced migration is the primary source for determining the capillary number dependence of the depletion layer thickness. Both the wall-induced migration and the shear-induced diffusive motion of the capsules are attenuated under the influence of polymer; reduction of migration dominates, however, so the net effect of polymers on the capsule suspension is to reduce the thickness of the capsule-depleted layer. This prediction is in qualitative agreement with experimental observations.

An experimental-based model for prediction of flow noise with drag reducing polymers

The drag reduction characteristics of certain high molecular weight polymers have been studied by various investigators. Because of the polymer’s ability to reduce turbulent shear stress and dependence of the boundary layer wall pressure spectral amplitude on the shear stress, polymer has the potential to suppress noise and vibration caused by the boundary layer unsteady pressures. Compared to its effect on drag reduction, polymer additive effects on turbulent boundary layer (TBL) wall pressure fluctuations have received little attention. Kadykov and Lyamshev [Sov. Phys. Acoust. 16 (1970) 59], Greshilor et al. [Sov. Phys. Acoust. 21 (1975) 247] showed that drag reducing polymer additives do indeed reduce wall pressure fluctuations, but they have not established any scaling relationship which effectively collapse data. Some effort has been made by Timothy et al. [JASA 108 (1) (2000) 71] at Penn State University to develop a scaling relationship for TBL wall pressure fluctuations that are modified by adding drag reducing polymer to pure water flow. This paper presents a theoretical model based on the work of the Timothy et al. team at ARL, Penn State University. Through this model one can estimate, reduction in TBL flow induced noise and vibration for rigid smooth surfaces due to release of drag reducing polymers in boundary layer region. Using this theoretical model, flow noise as experienced by a typical flush mounted hydrophone has been estimated for a smooth wall plate as a function of polymer additive concentration. Effect of non-dimensionalisation of the wall pressure fluctuations frequency spectra with traditional outer, inner and mixed flow variables will also be addressed in the paper. The paper also covers a model based on molecular relaxation time in polymer additives which not only reduce drag but also flow induced noise up to certain polymer concentration.

The effect of drag-reducing polymer additives on wall-pressure fluctuations in turbulent channel flows

The present paper is concerned with the experimental study of the influence of Toms' phenomenon on the convective part of the hydrodynamic turbulent wall-pressure field. The experiments were carried out in a water tunnel, at flow velocities of 2.3, 3.3 and 4.3 m/s, where amounts of drag reduction up to 40% were obtained by injecting dilute polymer solutions. An investigation of the spectral feature of the wall-pressure frequency spectrum and the wall-pressure frequency cross-spectrum was achieved from data taken using pinhole transducers. The scaling laws for the mid- and high-frequency regions of the spectrum were examined both for the water flow and the drag-reducing flow. The use of a noise cancellation procedure allowed observation of the maximum of the spectrum in the mid-frequency range at an approximately reduced frequency ωδ u τ of 80 for the water flow. This maximum is shifted towards a higher value of the reduced frequency in the presence of Toms' effect. From cross-spectral data obtained over separation distances ranging from 0.4 δ to 2.8 δ, the effect of drag reduction was estimated on the coherence functions as well as the convection velocity. These data show a correlated shift of the threshold which delineates the low wave number group identified as the large-scale turbulence contributor in the outer region of the flow and the high wave number group associated with turbulent sources in the log-region. The effect of drag reduction on the convection velocity data supports the fact that the contributions to the different wave number groups should be attributable to activity in the outer and inner regions, respectively. In addition, a computation of the root mean square pressure was achieved from the wall-pressure frequency spectrum. When expressed as functions of the percentage of drag reduction, the rms pressures related to the wall shear stress are found to increase monotonicaly. The equivalent ratio obtained by scaling the rms pressures using the dynamic pressure is found to decrease in the drag-reducing flow. Finally, an estimation of the probability density distribution showed that the skin friction reduction affects predominantly the occurrence of higher pressure amplitudes as well as very low pressure amplitudes. This confirms the important contribution of the large pressure peak events to the overall features of the wall-pressure.

Effect of polymer additives on Görtler vortices in Taylor—Couette flow

Taylor—Couette flow is ideal for studying drag-reducing polymer additives because, unlike turbulent boundary layers, the instabilities are better understood. Video records of laser-induced fluorescence experiments with and without polymers will be presented. Polyethylene-oxide (PEO) ‘oceans’ were used in concentrations of 20 and 100 p.p.m. In the Taylor number range, 3 × 104 ≤ Ta ≤ 108, Newtonian flow consisted of Taylor vortices which span the gap between cylinders and much smaller Görtler vortices at the inner cylinder wall. Measurements of core-to-core separation between counter-rotating vortices were made to estimate the Görtler instability wavenumber. These measurements show that PEO addition increases the Görtler wavenumber for a given Taylor number. At the lower Taylor numbers, Görtler vortex formation was suppressed by PEO. This implies that polymers directly affect the evolution of centrifugal instabilities.

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

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.

EFFECT OF DRAG REDUCING POLYMER ADDITIVES ON FLOW THROUGH COMMERCIAL BENDS AND TEES.

Experimental studies of the total loss coefficient and pressure distribution, for turbulent Newtonian and drag reduction flows in bends and tees are presented. Dilute solutions of polyethylene glycol in water were used as the drag reducing fluids. Drag reduction was deduced from plots of experimental pipe flow data, on Prandtl-Karman coordinates. Two main test sections were studied. The first test section was a commercial 90° bend,while the second was a commercial tee section. The later test section was studied in different situations such as a distributer , a collector, a by-pass and a bend. Estimation of the total loss coefficient required an evaluation of a kinetic energy correction factor. Based on previous theoretical and experimental verifica-tions, this factor was determined from the friction factor Reynolds number pipe flow data, for the different flowing fluids. Thus, a reasonally accurate evaluation of the total loss coeffi-cient could be obtained. The results showed that drag reducing polymer additives cause a reduction in the total loss coefficient for all flow situations studied. Pressure distributions along the different sections were presented both with physical inter-pretations and discussion of some flow separation aspects. The results showed that the pressure distributions along the studied sections had the same trend for both Newtonian and drag reduction flows.

Laser anemometer measurements of Reynolds stress in a turbulent channel flow with drag reducing polymer additives

Turbulence measurements with a high resolution laser Doppler anemometer in a channel flow with injection of a concentrated polyethylene oxide solution are reported. With polymer injection sufficient to yield a well mixed concentration of 10 ppm the total drag is reduced by 38%, but the sum of the Reynolds stress and the conventional molecular shear stress is only (2)/(3) of the force produced by the streamwise pressure gradient. Thus, the injected polymer solution has a significant effect on the turbulence and causes an additional retarding force equal to approximately (1)/(3) of the total drag.

An Experimental Study for The Flow of Dilute Polymer Solutions in Nozzles and Diffusers.(Dept.M)

This work presents an experimental study for the effect of drag reducing polymer additives upon a water flow in nozzles and diffusers. Five circular nozzles and diffusers of constant area ratio of 4 and half angle α = 4o ,6o , 10o, and 12o respectively are used. Two polymer types; Polyox coagulant and polyacrylamide are used at different concentrations. The results show that both polyacrylamide and polyox have no effect on the flow through nozzles at any flow rate and polyner concentrations. However, for diffuser flow, polymer additives results in a substantial increase in the pressure recovery of the diffuser. This effect increases with the flow rate and polymer concentration up to 10 wppm. For higher concentrations as 50 wppm, the effect is complicated and more interesting especially with polyacrylamide additives. The pressure recovery of the diffuser begins to decrease at some value of the flow rate. The decrease becomes more pronounced as the diffuser half angle increases. This is thought to be due to the increase of the viscoelastic properties with con centration and the elongation strains with the flow rate and diffuser half angle. The results also show that the pressure recovery increases with the diffuser half angle. They also show that polymer effect starts at a certain flow rate similar to the onset of drag reduction in pipe flow.

DRAG REDUCTION IN SEWERS: FIRST RESULTS FROM A PERMANENT INSTALLATION

Tests are described in which drag-reducing polymer additives are used to increase the flow velocity in a municipal sewer. The selected sewer has a diameter of 300 mm and carries the domestic sewage for a population of 10,000 but floods during heavy rainfall due to rainwater infiltration. A permanent polymer dosing station has been designed and constructed on this sewer and flow and depth measurement instruments have been included. Although the system has been designed to operate automatically during peak discharges tests were made under dry weather flow conditions which showed increases in sewer velocities of up to 70% using polymer concentrations ranging up to 60 parts per million of Polyox WSR-301.

Effect of Polymer Additives on the Process of Transition

The process of transition from laminar to turbulent flow on a flat plate in the presence of drag-reducing polymer additives has been investigated. In particular, the effect of polymer concentration and plate inclination on the formation of turbulent spots and their subsequent growth has been studied. It is observed that polymer addition causes a delay in both onset of turbulence and complete transition. The turbulent spots are seen to move at a lower velocity in the presence of the polymer additives. Further, the pressure gradient affects the transition in polymer solutions in the same way that it does in pure water.

Calculations of turbulent boundary layer flows with drag reducing polymer additives

Results of investigations into numerical predictions of turbulent flows in plane two‐dimensional channels and circular pipes with and without polymer additives are presented. These predictions are based on an available computer program for two‐dimensional boundary layer flows and on the k‐ε turbulence model. The model takes into account the low Reynolds number effects in the immediate vicinity of the wall and is modified to take into account the influence of the polymer additives in smooth as well as rough conduits in accordance with experimental results available in the literature. Results of the predictions are presented and compared with experiments. Based on the reliability of the present predictions and the uncertainty in the available measurements, a practical approach is outlined to study turbulent flows with drag reducing polymer additives. Degradation of polymer molecules is predictable with the described theoretical model.

Optimizing loss-compensated long delay lines : Coldren, L. A. 20 (January 1973) 17

The drag reduction characteristics of certain high molecular weight polymers have been studied by various investigators. Because of the polymer’s ability to reduce turbulent shear stress and dependence of the boundary layer wall pressure spectral amplitude on the shear stress, polymer has the potential to suppress noise and vibration caused by the boundary layer unsteady pressures. Compared to its effect on drag reduction, polymer additive effects on turbulent boundary layer (TBL) wall pressure fluctuations have received little attention. Kadykov and Lyamshev [Sov. Phys. Acoust. 16 (1970) 59], Greshilor et al. [Sov. Phys. Acoust. 21 (1975) 247] showed that drag reducing polymer additives do indeed reduce wall pressure fluctuations, but they have not established any scaling relationship which effectively collapse data. Some effort has been made by Timothy et al. [JASA 108 (1) (2000) 71] at Penn State University to develop a scaling relationship for TBL wall pressure fluctuations that are modified by adding drag reducing polymer to pure water flow. This paper presents a theoretical model based on the work of the Timothy et al. team at ARL, Penn State University. Through this model one can estimate, reduction in TBL flow induced noise and vibration for rigid smooth surfaces due to release of drag reducing polymers in boundary layer region. Using this theoretical model, flow noise as experienced by a typical flush mounted hydrophone has been estimated for a smooth wall plate as a function of polymer additive concentration. Effect of non-dimensionalisation of the wall pressure fluctuations frequency spectra with traditional outer, inner and mixed flow variables will also be addressed in the paper. The paper also covers a model based on molecular relaxation time in polymer additives which not only reduce drag but also flow induced noise up to certain polymer concentration.

Ferroelectric non-linearities in transudcer ceramics : Woollet, R. S. LeBlanc, C. L. 20 (January 1973) 24

The drag reduction characteristics of certain high molecular weight polymers have been studied by various investigators. Because of the polymer’s ability to reduce turbulent shear stress and dependence of the boundary layer wall pressure spectral amplitude on the shear stress, polymer has the potential to suppress noise and vibration caused by the boundary layer unsteady pressures. Compared to its effect on drag reduction, polymer additive effects on turbulent boundary layer (TBL) wall pressure fluctuations have received little attention. Kadykov and Lyamshev [Sov. Phys. Acoust. 16 (1970) 59], Greshilor et al. [Sov. Phys. Acoust. 21 (1975) 247] showed that drag reducing polymer additives do indeed reduce wall pressure fluctuations, but they have not established any scaling relationship which effectively collapse data. Some effort has been made by Timothy et al. [JASA 108 (1) (2000) 71] at Penn State University to develop a scaling relationship for TBL wall pressure fluctuations that are modified by adding drag reducing polymer to pure water flow. This paper presents a theoretical model based on the work of the Timothy et al. team at ARL, Penn State University. Through this model one can estimate, reduction in TBL flow induced noise and vibration for rigid smooth surfaces due to release of drag reducing polymers in boundary layer region. Using this theoretical model, flow noise as experienced by a typical flush mounted hydrophone has been estimated for a smooth wall plate as a function of polymer additive concentration. Effect of non-dimensionalisation of the wall pressure fluctuations frequency spectra with traditional outer, inner and mixed flow variables will also be addressed in the paper. The paper also covers a model based on molecular relaxation time in polymer additives which not only reduce drag but also flow induced noise up to certain polymer concentration.