P. C. Valente

Universal Dissipation Scaling for Nonequilibrium Turbulence

It is experimentally shown that the nonclassical high Reynolds number energy dissipation behavior, C(ε)≡εL/u(3)=f(Re(M))/Re(L), observed during the decay of fractal square grid-generated turbulence (where Re(M) is a global inlet Reynolds number and Re(L) is a local turbulence Reynolds number) is also manifested in decaying turbulence originating from various regular grids. For sufficiently high values of the global Reynolds numbers Re(M), f(Re(M))~Re(M).

The non-equilibrium region of grid-generated decaying turbulence

The previously reported non-equilibrium dissipation law is investigated in turbulent flows generated by various regular and fractal square grids. The flows are documented in terms of various turbulent profiles which reveal their differences. In spite of significant inhomogeneity and anisotropy differences, the new non-equilibrium dissipation law is observed in all of these flows. Various transverse and longitudinal integral scales are measured and used to define the dissipation coefficient

Dependence of decaying homogeneous isotropic turbulence on inflow conditions

C_{\varepsilon }

The energy cascade in grid-generated non-equilibrium decaying turbulence

. It is found that the new non-equilibrium dissipation law is not an artefact of a particular choice of the integral scale and that the usual equilibrium dissipation law can actually coexist with the non-equilibrium law in different regions of the same flow.

Energy spectra in elasto-inertial turbulence

Abstract A careful data analysis of far downstream turbulent flows generated by conventional and multiscale grids shows that these decaying flows are very clearly different from both Saffman and Loitsyansky turbulence. The analysis also shows that there are marked differences between the far downstream turbulence behaviours generated by different types of grid. There is an inflow condition dependence on both the normalised energy dissipation and the conserved large-scale invariant.

Comment on “Dissipation and decay of fractal-generated turbulence” [Phys. Fluids 19, 105108 (2007)]

We investigate non-equilibrium turbulence where the non-dimensionalised dissipation coefficient Ce scales as Ce∼ReMm/Reln with m ≈ 1 ≈ n (ReM and Rel are global/inlet and local Reynolds numbers, respectively) by measuring the downstream evolution of the scale-by-scale energy transfer, dissipation, advection, production, and transport in the lee of a square-mesh grid, and compare with a region where Ce ≈ constant. These are the main terms of the inhomogeneous, anisotropic version of the von Karman-Howarth-Monin equation. It is shown in the grid-generated turbulence studied here that, even in the presence of non-negligible turbulence production and transport, production and transport are large-scale phenomena that do not contribute to the scale-by-scale balance for scales smaller than about a third of the integral-length scale, l, and therefore do not affect the energy transfer to the small-scales. In both the non-equilibrium region where Ce∼ReMm/Reln and further downstream where Ce ≈ constant, the peak of ...

The Imbalance Between Enstrophy Production and Destruction in Homogeneous Isotropic Unsteady Turbulence

Direct numerical simulations of statistically steady homogeneous isotropic turbulence in viscoelastic fluids described by the FENE-P model are presented. Emphasis is given to large polymer relaxation times compared to the eddy turnover time, which is a regime recently termed elasto-inertial turbulence. In this regime the polymers are ineffective in dissipating kinetic energy but they play a lead role in transferring kinetic energy to the small solvent scales which turns out to be concomitant with the depletion of the usual non-linear energy cascade. However, we show that the non-linear interactions are still highly active, but they lead to no net downscale energy transfer because the forward and reversed energy cascades are nearly balanced. Finally, we show that the tendency for a steeper elasto-inertial power-law spectra is reversed for large polymer relaxation times and the spectra tend towards the usual k−5/3 functional form.

The decay of turbulence generated by a class of multiscale grids

We comment on the experimental results of Seoud and Vassilicos “Dissipation and decay of fractal-generated turbulence” [Phys. Fluids 19, 105108 (2007)] and show that, although their main observation that Ce~Reλ-1 for the fractal square grid-generated turbulence is essentially correct, their quantitative estimations of turbulent quantities depending on small scales (e.g., ɛ, λ, Reλ, and Cɛ) are biased by their anemometry system.

Denoising of time-resolved PIV for accurate measurement of turbulence spectra and reduced error in derivatives

We show in direct numerical simulations of homogeneous isotropic non-stationary turbulence that there is a systematic and significant imbalance between enstrophy production and its destruction which is concomitant with the previously observed imbalance between the non-linear energy cascade to fine scales and its dissipation (Valente, Onishi, da Silva, Phys Rev E 90(023003), 2014, [12]). However, contrary to the former, the imbalance between enstrophy production and destruction is affected by the ‘cascade time-lag’, i.e. the time it takes for the energy injected on the large-scales to reach the fine-scales.