Paper detail

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 -<uv> ~ 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 -<uv> ~ u_τ^2 is taken into account.