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Influence of surfactants on the turbulent boundary layer

By using Particle Image Velocimetry (PIV) we investigate the influence of low concentration, bio-degradable surfactant solution in a turbulent boundary layer in a flume. The flow was measured in the spanwise-streamwise plane (x-z). The results present a comparison between water and surfactant solution flow, as characterized by the root-mean-square values of the fluctuating velocity, Reynolds stress and turbulent energy production. Significant modification of the flow characteristics were observed for flow with surfactant solution, characterized mostly by decorrelation between the streamwise and the spanwise velocity components and followed by strong suppression of the turbulent kinetic energy production term.

Introduction

Drag reduction effect in turbulent flow, by low concentrations of polymers, is well known since the first publication on the phenomenon by Toms, (1948). Recently, bio-degradable surface active agents (surfactants) have become more suitable choice as the drag reducing additives, since they are environmental friendly materials, and in addition, they are known to have lower mechanical degradation Zakin et al. (1983). The influence could be spectacular with only few parts per million (ppm) of surfactant solution added to the solvent (e..g., reviews in Ohlendorf, 1986). The changes appear in a number of flow characteristics, both of large and small scales. However, the general belief is that surfactants, as polymer additives, act directly toward the small scales. In addition, the experimental results show that the drag reduction occurs only in turbulent flow when a certain wall shear stress is exceeded.

The drag reduction effect is accompanied by the modification of the turbulence structure, such as significant decrease of the Reynolds stresses Warholic et al. (1999) and the turbulence production. The suppression of Reynolds stresses does not mean that there is a significant reduction of the energy of turbulent fluctuations (i.e., r.m.s. values). The turbulent energy production in flows of drag reducing solutions, known to be sometimes smaller than in solvent flows, but also may be increased, due to the fact that both turbulent energy production and the dissipation are reduced strongly in the drag reduced flow Tsinober (1990b). One of the surfactant effects that was reported by many researches, is the increased anisotropy, characterized by the considerable suppression of
the wall-normal velocity fluctuations. Such observations have lead to the conclusion that the main reason for the reduced Reynolds stresses is the decorrelation of the streamwise (u) and wall-normal (v) components of the velocity fluctuations Tsinober (2001).

Other experimental studies of the influence of the additives on the flow were done the motion pictures of dye injection, real-time hologram interferometry and dye visualization. The researchers found an increase in the spanwise spacing between the low-speed velocity streaks and a reduction in “bursting events” in drag reduced flow when compared with Newtonian counterpart. The existing theories concerning drag eduction regard the stretching of the polymers by the turbulent flow in certain areas, and consequential local increase of the viscosity as the major mechanisms of drag reduction.

In the present research, we investigate experimentally the influence of the non-ionic, bio-degradable, non-toxic surfactants on the turbulent flow in a flume, by means of Particle Image Velocimetry (PIV). A similar experimental researches were performed by Warholic et al. 2001 and White et al. 2003, where measured planes of velocity primarily in the x-y plane and one x-z plane. White et al. (2003) measured the x-z planes very close to the wall, and both researches used polymers as a drag reducing additives. Surfactants are known to have different influence on the flow, and they are able to exceed the so-called Virk’s maximum drag reduction Virk’s limit for the polymers, for relatively low concentrations. Therefore these non-toxic and bio-degradable materials, which are also shown to be good drag reducers, have a great potential to be used in many engineering and industrial applications.

Results

Fig 1: Fluctuating velocity field in the streamwise–spanwise plane of the water flow (left) and the flow with surfactant (right). The color scale represents the magnitude of the streamwise velocity fluctuations.

Fig 2: PDF’s of the u’rms/Uq for water (circles) and surfactant (squares). PDF is representing the spatial distribution of the r.m.s. values, normalized to the flow-rate velocity Uq.

Fig 3: PDFs of the turbulent kinetic energy production term Suw

References

Gurka R., Liberzon A. and Hetsroni G. (2004) “Characterization of turbulent flow in a flume with surfactant using PIV�, Journal of Fluids Engineering, 126(6), pp. 1054-1057.