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TURBULENCE MODULATION AND INSTABILITY INDUCED BY SURFACTANT ADDITIVES IN BACKWARD-FACING STEP FLOW

Ryusuke II
Department of Mechanical Engineering Tokyo University of Science 2641 Yamazaki, Noda-shi, Chiba-ken, Japan

Shumpei Hara
Department of Mechanical Engineering Tokyo University of Science 2641 Yamazaki, Noda-shi, Chiba-ken, Japan

Takahiro Tsukahara
Department of Mechanical Engineering, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba, 278-8510, Japan

Yasuo Kawaguchi
Department of Mechanical Engineering, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan

Resumo

The water flow is largely changed by adding surfactant because of the fluid viscoelasticity, which is known as the Toms effect. The trubulent state can be stabilized in the straight pipe flow of surfactant solution and therefore the wall frictional drag is significantly reduced. Whereas, the enhancement of turbulence by viscoelasticity also can be observed in surfactant flow, which increases the turbulent instability along the bended the flow line. Therefore we will try to find the conditons under which the conflict between suppression and enhancement of viscoelasticity on turbulence happens.
In this study, we performed a series of experimental investigations on the viscoelastic flow through a backward-facing step (BFS) to obtain the turbulence statistics profiles along the streamwise direction in a separated shear layer using a PIV system. As a result, in the separated shear layer flow with surfactant additive, we found two flow conditions depending on the balance between surfactant concentration of viscoelastic fluid and shear rate. Firstly, the suppressed-turbulence condition was observed. Secondly, the enhanced-turbulence condition was clarified, accompanied with meandering main flow to the wall-normal direction in the instantaneous velocity field and the meandering main stream in enhanced-turbulence condition was studied in detail at some points along the streamwise direction. Finally, we evaluated the limits between suppression and enhancement of viscoelasticity on turbulence and made the meandering conditions map for the surfactant viscoelastic flow. Based on the map we found that the increase in viscoelastic force with surfactant concentration results to the expansion of the stable region.