Anisotropy in turbulent drag reduction (original) (raw)
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The effect of polymers on the dynamics of turbulence in a drag reduced flow
Thermal Science, 2005
An experimental investigation of a polymer drag reduced flow using state-of-the-art laser-Doppler anemometry in a refractive index-matched pipe flow facility is reported. The measured turbulent stresses deep in the viscous sublayer are analyzed using the tools of invariant theory. It is shown that with higher polymer concentration the anisotropy of the Reynolds stresses increases. This trend is consistent with the trends extracted from DNS data of non-Newtonian fluids yielding different amounts of drag reduction. The interaction between polymer and turbulence is studied by considering local stretching of the molecular structure of a polymer by small-scale turbulent motions in the region very close to the wall. The stretching process is assumed to re-structure turbulence at small scales by forcing these to satisfy local axisymmetry with invariance under rotation about the axis aligned with the main flow. It is shown analytically that kinematic constraints imposed by local axisymmetry...
Measured effects of polymer additives on turbulent-velocity fluctuations at various length scales
Physical Review A, 1992
Turbulent-drag reduction in a dilute polymer solution has been studied using the technique of photon-correlation homodyne spectroscopy to measure velocity differences in a concentric cylinder cell, in which the inner cylinder rotates. A large anisotropic suppression of turbulent-velocity differences is found in the bulk region of the turbulent fluid. The suppression effect occurs at various length scales up to =1 mm, which is far beyond the Kolmogorov dissipation length ld (=0.04 mm). The large-scale velocity fluctuations are suppressed, but their statistical properties over varying length scales remain unchanged. The small-scale fluctuations, on the other hand, are damped out much more strongly, resulting in a different functional form for the velocity-density function. The latter observation is consistent with the notion that the polymer-turbulence interaction causes a truncation of the turbulent-energy cascade at small scales. The measurements are also in agreement with laser Doppler velocimetry results that in turbulent polymer solutions the lifetime of large-scale vortices is increased, and the high-frequency velocity fluctuations (small-scale motions) are reduced.
Turbulent drag reduction by polymers
The reduction of turbulent energy dissipation by addition of polymers is studied experimentally. We first address the question of where the action of the polymers is taking place. Subsequently, we show that there is a direct correlation of drag reduction with the elongational viscosity of the polymers. For this, the reduction of turbulent energy dissipation by addition of the biopolymer DNA is studied. These results open the way for a direct visualization study of the polymer conformation in a turbulent boundary layer.
Saturation of turbulent drag reduction in dilute polymer solutions
Europhysics Letters (EPL), 2004
Drag reduction by polymers in turbulent wall-bounded flows exhibits universal and non-universal aspects. The universal maximal mean velocity profile was explained in a recent theory. The saturation of this profile and the crossover back to the Newtonian plug are non-universal, depending on Reynolds number Re, concentration of polymer cp and the degree of polymerization Np. We explain the mechanism of saturation stemming from the finiteness of extensibility of the polymers, predict its dependence on cp and N in the limit of small cp and large Re, and present the excellent comparison of our predictions to experiments on drag reduction by DNA.
Polymer-turbulence interactions in a complex flow and implications for the drag reduction phenomenon
Physics of Fluids
We present direct numerical simulation data for turbulent duct flow of a finite-extensibility non-linear elastic dumbbell model with the Peterlin approximation (FENE-P) fluid in the high drag reduction regime. While the secondary flow pattern is qualitatively similar to that in a Newtonian fluid, its magnitude is significantly reduced, resulting in a less uniformly distributed velocity profile and hence smaller gradients at the wall. The Reynolds stress tensor in the polymer-laden flow was found to be increasingly anisotropic with most of the turbulent kinetic energy retained in the streamwise component, [Formula: see text]. We introduce a novel approach for investigating polymer stretching using the anisotropy invariant map of the polymer stress tensor and observe the persistence of both uniaxial and biaxial extension. Analysis of the transport equation for the mean kinetic energy indicates that polymer stretching and relaxation is a highly dissipative process; hence, the introduct...
Drag reduction by polymer additives from turbulent spectra
Physical Review E, 2010
We extend the analysis of the friction factor for turbulent pipe flow reported by G. Gioia and P. Chakraborty (G. Gioia and P. Chakraborty, Phys. Rev. Lett. 96, 044502 (2006)) to the case where drag is reduced by polymer additives. * calzetta@df.uba.ar
Manifestations of Drag Reduction by Polymer Additives in Decaying, Homogeneous, Isotropic Turbulence
Physical Review Letters, 2006
The existence of drag reduction by polymer additives, well established for wall-bounded turbulent flows, is controversial in homogeneous, isotropic turbulence. To settle this controversy we carry out a high-resolution direct numerical simulation (DNS) of decaying, homogeneous, isotropic turbulence with polymer additives. Our study reveals clear manifestations of drag-reduction-type phenomena: On the addition of polymers to the turbulent fluid we obtain a reduction in the energy dissipation rate, a significant modification of the fluid energy spectrum especially in the deep-dissipation range, a suppression of small-scale intermittency, and a decrease in small-scale vorticity filaments.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2003
We address the phenomenon of drag reduction by a dilute polymeric additive to turbulent flows, using direct numerical simulations (DNS) of the FENE-P model of viscoelastic flows. It had been amply demonstrated that these model equations reproduce the phenomenon, but the results of DNS were not analyzed so far with the goal of interpreting the phenomenon. In order to construct a useful framework for the understanding of drag reduction we initiate in this paper an investigation of the most important modes that are sustained in the viscoelastic and Newtonian turbulent flows, respectively. The modes are obtained empirically using the Karhunen-Loéve decomposition, allowing us to compare the most energetic modes in the viscoelastic and Newtonian flows. The main finding of the present study is that the spatial profile of the most energetic modes is hardly changed between the two flows. What changes is the energy associated with these modes, and their relative ordering in the decreasing ord...
Turbulent viscosity profile of drag reducing rod-like polymers
The European Physical Journal E
Recent theories of drag reduction in wall turbulence assumed that the presence of the polymer leads to an effective viscosity, which increases linearly with the distance from the wall. Such a linear viscosity profile reduces the Reynolds stress (i.e., the momentum flux to the wall), which leads to drag reduction. For the usual flexible polymers employed in drag reduction, the effective viscosity is however a strongly non-linear effect that is difficult to quantify. We therefore investigate the turbulent drag reduction characteristics of a stiff rod-like polymer for which any effective viscosity changes are only due to the orientation of the polymers. The results show that close to the walls the polymers orient and the viscosity is low, whereas in the bulk the polymers are randomly oriented and the effective viscosity is high. This indeed leads to a reduction of the Reynolds stress and hence to a drag reduction.
A mechanism of polymer induced drag reduction in turbulent pipe flow
2014
Polymer induced drag reduction in turbulent pipe flow was investigated using a non-intrusive laser based diagnostic technique, namely Particle Image Velocimetry (PIV). The drag reduction was measured in a pressure-driven flow facility, in a horizontal pipe of inner diameter 25.3 mm at Reynolds numbers ranging from 35 000 to 210 000. Three highmolecular-weight polymers (polyethylene oxide 2×10 6-8×10 6 Da) at concentrations in the range of 5-250 wppm were used. The results, obtained from the PIV measurements, show that the drag reduction scales with the magnitude of the normalized streamwise and spanwise rms velocity fluctuations in the flow. This scaling seems to universal, and is independent of the Reynolds number and in some cases also independent of the distance from the wall where the velocity fluctuations are considered. Furthermore, the instantaneous PIV observations indicate that as the level of drag reduction increases, the flow in the pipe is separated into a low-momentum flow region near the pipe wall and a high-momentum flow region in the turbulent core. Based on these findings a new mechanism of polymeric drag reduction is proposed in this paper.