arXiv subjects

Christos Vassilicos

Publications and source records attributed to Christos Vassilicos.

5 recordsLinked to original sources

arXiv2026

Passive scalar cascade in the intermediate layer of turbulent channel flow for $Pr\leq 1$

Similarities and differences between Kolmogorov scale-by-scale equilibria/non-equilibria for velocity and scalar fields are investigated in the intermediate layer of a fully developed turbulent channel flow with a passive scalar/temperature field driven by a uniform heat source. The analysis is based on intermediate asymptotics and direct numerical simulations at different Prandtl numbers lower than unity. Similarly to what happens to the velocity fluctuations, for the fluctuating scalar field Kolmogorov scale-by-scale equilibrium is achieved asymptotically around a length scale $r_{min}$, which is located below the inertial range. The lengthscale $r_{min}$ and the ratio between the inter-scale transfer and dissipation rates at $r_{min}$ vary following power laws of the Prandtl number, with exponents determined by matched asymptotics based on the hypothesis of homogeneous two-point physics in non-homogeneous turbulence. The interscale transfer rates of turbulent kinetic energy and passive scalar variance are globally similar but show evident differences when their aligned/anti-aligned contributions are considered.

physics.flu-dyn↗

arXiv2011

The decay of turbulence generated by a class of multi-scale grids

A new experimental investigation of decaying turbulence generated by a low-blockage space-filling fractal square grid is presented. We find agreement with previous works by Seoud & Vassilicos (2007) and Mazellier & Vassilicos (2010) but also extend the length of the assessed decay region and consolidate the results by repeating the experiments with different probes of increased spatial resolution. It is confirmed that this moderately high Reynolds number Reλ turbulence (up to Reλ {\simeq}350 here) does not follow the classical high Reynolds number scaling of the dissipation rate ε ~ u'^3/L and does not obey the equivalent proportionality between the Taylor-based Reynolds number Reλ and the ratio of integral scale L to Taylor micro-scale λ. Instead we observe an approximate proportionality between L and λ during decay. This non-classical behaviour is investigated by studying how the energy spectra evolve during decay and examining how well they can be described by self-preserving single-length scale forms. A detailed study of homogeneity and isotropy is also presented which reveals the presence of transverse energy transport and pressure transport in the part of the turbulence decay region where we take data (even though previous studies found mean flow and turbulence intensity profiles to be approximately homogeneous in much of the decay region). The exceptionally fast turbulence decay observed in the part of the decay region where we take data is consistent with the non-classical behaviour of the dissipation rate. Measurements with a regular square mesh grid as well as comparisons with active grid experiments by Mydlarski & Warhaft (1996) and Kang, Chester & Meveneau (2003) are also presented to highlight the similarities and differences between these turbulent flows and the turbulence generated by our fractal square grid.

physics.flu-dyn↗

arXiv2011

An infinity of possible invariants for decaying homogeneous turbulence

The von Karman-Howarth equation implies an infinity of invariants corresponding to an infinity of different asymptotic behaviours of the double and triple velocity correlation functions at infinite separations. Given an asymptotic behaviour at infinity for which the Birkhoff-Saffman invariant is not infinite, there are either none, or only one or only two finite invariants. If there are two, one of them is the Loitsyansky invariant and the decay of large eddies cannot be self-similar. We examine the consequences of this infinity of invariants on a particular family of exact solutions of the von Karman-Howarth equation.

physics.flu-dyn↗

arXiv2010

Turbulence without Richardson-Kolmogorov cascade

We present an experimental investigation of intense turbulence generated by a class of low-blockage space-filling fractal square grids. We confirm the existence of a protacted production region followed by a decaying region, as first reported by Hurst & Vassilicos (Physics of Fluids, 2007). We show that the centerline streamwise variation of most of the statistical properties of the turbulent flow can be scaled by a wake interaction length-scale $x_*$. We also confirm the finding of Seoud and Vassilicos (Physics of Fluids, 2007) that the ratio of the integral length-scale $L_u$ to the Taylor micro-scale $λ$ remains constant in the decaying region whereas the Reynolds number $Re_λ$ strongly decreases. As a result the scaling $L_{u}/λ\sim Re_λ$ which follows from the $u'^{3}/L_u$ scaling of the dissipation rate in boundary-free shear flows and in usual grid-generated turbulence does not hold here. However, we show that the ratio $L_u/λ$ is an increasing function of the inlet Reynolds number $Re_0$. This extraordinary decoupling is consistent with a self-preserving single length-scale decaying homogeneous turbulence proposed by George & Wang (Physics of Fluids, 2009) with which our results are compared.

physics.flu-dyn↗

arXiv2002

Richardson's pair diffusion and the stagnation point structure of turbulence

DNS and laboratory experiments show that the spatial distribution of straining stagnation points in homogeneous isotropic 3D turbulence has a fractal structure with dimension D_s = 2. In Kinematic Simulations the time exponent gamma in Richardson's law and the fractal dimension D_s are related by gamma = 6/D_s. The Richardson constant is found to be an increasing function of the number of straining stagnation points in agreement with pair duffusion occuring in bursts when pairs meet such points in the flow.

physics.flu-dyn↗