J. Christos Vassilicos

Direct Numerical Simulation of Fractal-Generated Turbulence

The flow obtained behind a fractal square grid is studied by means of direct numerical simulation. An innovative approach which combines high order schemes, Immersed boundary method and a dual domain decomposition method is used to take into account the multiscale nature of the grid and the resulting flow.

Wall shear stress fluctuations: Mixed scaling and their effects on velocity fluctuations in a turbulent boundary layer

The present work investigates numerically the statistics of the wall shear stress fluctuations in a turbulent boundary layer (TBL) and their relation to the velocity fluctuations outside of the near-wall region. The flow data are obtained from a Direct Numerical Simulation (DNS) of a zero pressure-gradient TBL using the high-order flow solver Incompact3D [S. Laizet and E. Lamballais, “High-order compact schemes for incompressible flows: A simple and efficient method with quasi-spectral accuracy,” J. Comput. Phys. 228(16), 5989 (2009)]. The maximum Reynolds number of the simulation is Re𝜃≈2000, based on the free-stream velocity and the momentum thickness of the boundary layer. The simulation data suggest that the root-mean-squared fluctuations of the streamwise and spanwise wall shear-stress components τx and τz follow a logarithmic dependence on the Reynolds number, consistent with the empirical correlation of Orlu and Schlatter [R. Orlu and P. Schlatter, “On the fluctuating wall-shear stress in zero pres...

Effect of Fractal Endplates on the Wingtip Vortex

© 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. An experimental investigation was performed on the effect of fractal endplates on the wingtip vortex of a NACA 0012 semi span wing at a Reynolds number of 2 × 105. The endplates were obtained by introducing three different fractal patterns. Constant temperature anemometry and stereoscopic particle image velocimetry were employed to assess both the local flow properties as well as the spatial organization of the wingtip vortex. The results show that the introduction of a fractal endplate strongly affects both the geometry and the turbulence features of the vortex. In particular, it is found that the fractal geometry weakens the vortex by spreading the turbulent kinetic energy over a broader range of frequencies. We relate this loss of coherence to a faster dissipation of the vortex, thus paving the way to the employment of fractal endplates to reduce the hazard associated to such flow features.

The Stagnation Point Structure of Wall-Turbulence and the Law of the Wall in Turbulent Channel Flow

DNS of turbulent channel flows propose the following picture. (a) The Taylor microscale λ(y) is proportional to l s (y), the average distance between stagnation points of the fluctuating velocity field, i.e. λ(y)=B 1 l s (y) where B 1 is constant, for δ ν≪y<δ. (b) The number density of stagnation points n s varies with height as \(n_{s}=C_{s}y_{+}^{-1}/\delta_{\nu}^{3}\) with C s constant in the range δ ν≪y<δ. (c) In that same range, the kinetic energy dissipation rate per unit mass, \(\epsilon=\frac{2}{3}E_{+}u_{\tau}^{3}/(\kappa_{s}y)\) where \(E_{+}=E/u_{\tau}^{2}\) is the normalised total kinetic energy per unit mass and \(\kappa_{s}=B_{1}^{2}/C_{s}\) is the stagnation point von Karman coefficient. (d) For Re τ ≫1, large enough for the production to balance dissipation locally and for \(-\langle{uv}\rangle \sim u_{\tau}^{2}\) in the range δ ν≪y≪δ, \(d\langle{u}\rangle /dy\simeq\frac{2}{3}E_{+}u_{\tau}/(\kappa_{s}y)\) in that same range. (e) The von Karman coefficient κ is a meaningful and well-defined coefficient and the log-law holds only if E + is independent of y + and Re τ in that range, in which case κ∼κ s . The universality of \(\kappa_{s}=B_{1}^{2}/C_{s}\) depends on the universality of the stagnation point structure of the turbulence via B 1 and C s , which are conceivably not universal.

The Mean Flow Profile of Wall-Bounded Turbulence and Its Relation to Turbulent Flow Topology

The mean velocity profile’s scaling in different turbulent wall-bounded flows and the so called von Karman constant, k in the case of a logarithmic velocity profile, have been the source of controversy for the last decade. The classical log-law theory of von Karman [11] and Prandtl [7] is questioned by some who propose power laws to describe the mean velocity profile [1,3]. Moreover, the most widely accepted value of k used to be roughly 0.41 but recent estimates place this value as low as 1/e for channel flows [6] and as high as 0.43 for pipe flows [6]. It is also argued that for zero-pressure-gradient boundary layer flows e ≈ 0.38 [6].