Turbulence structure and mass transport in a channel flow with polymer injection

David Thomas Walker, Purdue University

Abstract

This study examined a fully developed turbulent channel flow with polymer injection at the wall and had two complementary objectives. The first objective was to gain more understanding of the way in which long-chain, high-molecular-weight, water soluble polymers effect viscous drag reduction in turbulent wall flows. The second was to acquire information useful in the modeling of the transport processes in flows with polymer injection at the wall, particularly the high Schmidt number mixing process which occurs. A fluorescence technique was developed for time-resolved measurements of species concentration profiles. This technique was combined with laser velocimetry to obtain measurements of turbulent mass transport in a flow with polymer injection and, for comparison purposes, a flow with water injection. Two-component laser velocimeter measurements were made in both flows to examine the effect of both the injection process and the evolving polymer concentration field on the structure of turbulence. In the flows with injection, the major transport mechanism was identified as long (in the streamwise direction) filaments of high concentration fluid periodically lifting away from the near-wall region. When polymer was injected, mixing of the injected fluid with the outer flow required a longer streamwise distance than for water injection. The high extensional viscosity of the polymer solution resulted in a strong correlation between large instantaneous polymer concentrations and small wall-normal velocity fluctuations causing lower levels of turbulent mass transport. After the polymer solution began to mix with the flow, the turbulent mass transport was smaller than in the flow with water injection but the degree of correlation between velocity and concentration fluctuations was comparable. This indicates that the mass transport processes were similar for these two cases. In both cases, the high Schmidt number resulted in high concentration fluid mixing from the near-wall region to the outer flow and then back near the wall yielding low levels of net turbulent mass transport. Measurements of turbulence structure show that while the root-mean-square (RMS) fluctuation level of the streamwise velocity was increased by the presence of the polymer solution, the RMS of the wall-normal velocity and the Reynolds shear stress were reduced as the polymer solution mixed with the flow. Production of the streamwise Reynolds normal stress was decreased but the production of the Reynolds shear stress was unchanged. This shows that the pressure-strain correlation terms in the Reynolds stress transport equations may be directly affected by the polymers.

Degree

Ph.D.

Advisors

Tiederman, Purdue University.

Subject Area

Mechanical engineering

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