arXiv subjects

J. Christos Vassilicos

Publications and source records attributed to J. Christos Vassilicos.

6 recordsLinked to original sources

arXiv2020

Unified description of turbulent entrainment

We present a mathematical description of turbulent entrainment that is applicable to free shear problems that evolve in space, time or both. Defining the global entrainment velocity $\overline V_g$ to be the fluid motion across an isosurface of an averaged scalar, we find that for a slender flow, $\overline V_g=\overline u_ζ- \overline{D}h_t/\overline{D}t$, where $\overline D/\overline D t$ is the material derivative of the average flowfield and $\overline u_ζ$ is the average velocity perpendicular to the flow direction across the interface located at $ζ=h_t$. The description is shown to reproduce well-known results for the axisymmetric jet, the planar wake and the temporal jet, and provides a clear link between the local (small-scale) and global (integral) descriptions of turbulent entrainment. Application to unsteady jets/plumes demonstrates that, under unsteady conditions, the entrainment coefficient $α$ no longer only captures entrainment of ambient fluid, but also time-dependency effects due to the loss of self-similarity.

physics.flu-dyn↗

arXiv2018

Length scales and eddy viscosities in turbulent free shear flows

In the present paper, we address the important point of the proportionality between the longitudinal integral lengthscale ($L$) and the characteristic mean flow width ($δ$) using experimental data of an axisymmetric wake and a turbulent planar jet. This is a fundamental hypothesis when deriving the self-similar scaling laws in free shear flows, irrespective of turbulence dissipation scaling. We show that $L/δ$ is indeed constant, at least in a range of streamwise distances between 15 and 50 times the characteristic inlet dimension ($L_{ref}$, nozzle width or wake generator size). Furthermore, we revisit turbulence closure models such as the Prandtl mixing length \cite{prandtl1925} and the constant eddy viscosity in the light of the non-equilibrium dissipation scalings. We show that the mixing length model, with $l_m\sim δ$, does not comply with the scalings stemming from the non-equilibrium version of the theory; we instead obtain $l_m\sim δ\sqrt{Re_G/Re_{0δ}}$, where $Re_G$ and $Re_{0δ}$ are a global and local Reynolds number, respectively. Similarly, the eddy viscosity model holds in the case of the non-equilibrium version of the theory provided that the eddy viscosity is constant everywhere, not only across sections orthogonal to the streamwise direction as in the equilibrium case. We conclude comparing the results of the different models with each other and with experimental data and with an improved model (following Townsend) that accounts for the intermittency of the flow and corrects for the eddy viscosity variation across the flow boundaries.

physics.flu-dyn↗

arXiv2011

Strong polymer-turbulence interactions in viscoelastic turbulent channel flow

This paper is focused on the fundamental mechanism(s) of viscoelastic turbulence that lead to polymer induced turbulent drag reduction phenomenon. A great challenge in this problem is the computation of viscoelastic turbulent flows, since the understanding of polymer physics is restricted to mechanical models. An effective state-of-the-art numerical method to solve the governing equation for polymers modelled as non-linear springs, without using any artificial assumptions as usual, was implemented here for the first time on a three-dimensional channel flow geometry. The capability of this algorithm to capture the strong polymer-turbulence dynamical interactions is depicted on the results, which are much closer qualitatively to experimental observations. This allowed a more detailed study of the polymer-turbulence interactions, which yields an enhanced picture on a mechanism resulting from the polymer-turbulence energy transfers.

physics.flu-dyn↗

arXiv2011

Rapid growth of cloud droplets by turbulence

Assuming perfect collision efficiency, we demonstrate that turbulence can initiate and sustain the rapid growth of very small water droplets in air even when these droplets are too small to cluster, and even without having to take gravity and small-scale intermittency into account. This is because the range of local Stokes numbers of identical droplets in the turbulent flow field is broad enough even when small-scale intermittency is neglected. This demonstration is given for turbulence which is one order of magnitude less intense than is typical in warm clouds but with a volume fraction which, even though small, is nevertheless large enough for an estimated a priori frequency of collisions to be ten times larger than in warm clouds. However, the time of growth in these conditions turns out to be one order of magnitude smaller than in warm clouds.

physics.flu-dyn↗

arXiv2010

A turbulent premixed flame on fractal-grid generated turbulence

A space-filling, low blockage fractal grid is used as a novel turbulence generator in a premixed turbulent combustion experiment. In contrast to the power law decay of a standard turbulence grid, the downstream turbulence intensity of the fractal grid increases until it reaches a peak at some distance from the grid before it finally decays. The effective mesh size and the solidity are the same as those of a standard square mesh grid with which it is compared. It is found that, for the same flow rate and stoichiometry, the fractal generated turbulence enhances the burning rate and causes the flame to further increase its area. Using a flame fractal model, an attempt is made to highlight differences between the flames established at the two different turbulent fields.

physics.flu-dyn↗

arXiv2010

The stagnation point von Kármán coefficient

On the basis of various DNS of turbulent channel flows the following picture is proposed. (i) At a height y from the y = 0 wall, the Taylor microscale λis proportional to the average distance l_s between stagnation points of the fluctuating velocity field, i.e. λ(y) = B_1 l_s(y) with B_1 constant, for δ_ν<< y \lesssim δ. (ii) The number density n_s of stagnation points varies with height according to n_s = C_s y_+^{-1} / δ_ν^3 where C_s is constant in the range δ_ν<< y \lesssim δ. (iii) In that same range, the kinetic energy dissipation rate per unit mass, ε= 2/3 E_+ u_τ^3 / (κ_s y) where E_+ is the total kinetic energy per unit mass normalised by u_τ^2 and κ_s = B_1^2 / C_s is the stagnation point von Kármán coefficient. (iv) In the limit of exceedingly large Re_τ, large enough for the production to balance dissipation locally and for - ~ u_τ^2 in the range δ_ν<< y << δ, dU_+/dy ~ 2/3 E_+/(κ_s y) in that same range. (v) The von Kármán coefficient κis a meaningful and well-defined coefficient and the log-law holds only if E_+ is independent of y_+ and Re_τin that range, in which case κ~ κ_s. The universality of κ_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. (vi) DNS data of turbulent channel flows which include the highest currently available values of Re_τsuggest E_+ ~ 2/3 B_4 y_+^{-2/15} and dU_+/dy_+ ~ B_4/(κ_s) y_+^{-1 - 2/15} with B_4 independent of y in δ_ν<< y << δif the significant departure from - ~ u_τ^2 is taken into account.

physics.flu-dyn↗