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Flow in an impeller-stirred tank using an immersed-boundary method
No full textThe moderate Reynolds number flow developing in a cylindrical unbaffled stirred tank is studied using direct numerical simulation (DNS) and the results are compared with available experimental data and Reynolds Averaged Navier–Stokes (RANS) solutions. The geometry of the impeller is handled using an immersed-boundary procedure implemented in a solver written for cylindrical coordinates. This allows efficient simulation of the flow at a reasonable computational cost and accurate prediction of the mean and rms velocity fields. For this configuration, RANS performs poorly because of the low Reynolds and strongly unsteady and inhomogeneous nature of the flow: many different flow structures are produced, ranging from small-scale vortices generated at the blade tips to large-scale meridional recirculation. It is shown, in addition, that inaccurate results are produced by the wrong (computational) assumption that an impeller with n blades produces instantaneous fields with an n-fold symmetry. Because simple stirred-tank configurations (like the present one) have been recently used to assess the performance of several RANS closures, the main message of this study is that simple geometrical configurations and low Reynolds numbers are not benign parameters for such a task
Analysis of the sound generated by the pairing of two axisymmetric co-rotating vortex rings
No full textOne-point statistics for turbulent pipe flow up to Reτ ≈ 6000
Full text linkWe study turbulent flows in a smooth straight pipe of circular cross-section up to friction Reynolds number using direct numerical simulation (DNS) of the Navier-Stokes equations. The DNS results highlight systematic deviations from Prandtl friction law, amounting to approximately, which would extrapolate to approximately at extreme Reynolds numbers. Data fitting of the DNS friction coefficient yields an estimated von Kármán constant, which nicely fits the mean velocity profile, and which supports universality of canonical wall-bounded flows. The same constant also applies to the pipe centreline velocity, thus providing support for the claim that the asymptotic state of pipe flow at extreme Reynolds numbers should be plug flow. At the Reynolds numbers under scrutiny, no evidence for saturation of the logarithmic growth of the inner peak of the axial velocity variance is found. Although no outer peak of the velocity variance directly emerges in our DNS, we provide strong evidence that it should appear at, as a result of turbulence production exceeding dissipation over a large part of the outer wall layer, thus invalidating the classical equilibrium hypothesis
Analysis of the sound generated by the pairing of two axisymmetric co-rotating vortex rings
No full text
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