PIV and DNS analyses of viscoelastic turbulent flows behind a rectangular orifice

• Viscoelastic fluid turbulence in relaxing and non-equilibrium flow is investigated.
• Numerical and experimental results of viscoelastic turbulent orifice flow are provided.
• DNS result using Giesekus model agrees well with the experimental one by surfactant solution.
• Formation of Kelvin–Helmholtz vortices behind the orifice weakens in viscoelastic flows.
• Highly viscoelastic flow reduces the form drag, but increases the skin friction due to extra stress.

We performed PIV (particle image velocimetry) measurements and DNS (direct numerical simulations) on turbulent orifice flows for the Newtonian fluid and viscoelastic fluids, and compared their results with emphasis on turbulence statistics and vortical motions just behind the orifice rib. In the experiment, a cationic surfactant solution of CTAC (cetyltrimethyl ammonium chloride) was chosen as the viscoelastic fluid that is known to provide substantial drag reduction in the case of smooth-wall turbulence. In the viscoelastic flows, the formation of the Kelvin–Helmholtz vortices emanating from the orifice edge was found to be attenuated compared to the Newtonian case, resulting in the suppression of turbulent eddies and Reynolds shear stress behind the orifice. However, the variation of the drag depended on the Reynolds number and the surfactant concentration (or the Weissenberg number): that is, the drag-reducing effect can be achieved only in limited conditions or low Reynolds-number flows. Although DNS results was found to be in qualitative agreement with the experimental data, we discussed also inconsistency between the experimental and DNS results.

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