Vortex statistics in two-dimensional turbulence based on IB-LBM

摘要

In this paper, the two-dimensional (2D) turbulence perturbed by arrays of cylinders placed both horizontally and vertically is investigated by Immersed Boundary Lattice Boltzmann Method (IB-LBM). The energy spectrum reveals the coexistence of the inverse and direct cascades in 2D grid turbulence. By observing at the distribution of fluxes in space, the energy and enstrophy fluxes have explained the physical mechanism of the double cascades where the two Kolmogorov laws for structure functions are simultaneously observed. The results of vortex statistics by the conditional analysis, which are based on a new and accurate vortex identification criteria called Liutex, show that the algebraic number density n(A) ～ A?2, where A is vortex area. The time-evolving vortex number density distribution constructs a theoretical framework involving a three-part: n(A,t) = A?0.5(Amin ≤ A < A?); t?1A?1(A ? < A < A+); t7A?8(A + < A ≤ Amax), which is satisfied with the prediction well. The relationship between the vortex circulation |Γ|(A) and vortex area A is |Γ|(A) ～ A and the one between the kinetic energy of vortex Ee(A) and A is Ee(A) ～ A in the range where n(A) ～ A?2. Moreover, it has been found that vortices contain about 30% of the total energy of the flow by studying the energy ratio of all vortices to the entire flow field. What is more, it is an interesting phenomenon is that there is only a range where E(k) ～ k?5/3 in the energy spectrum for the coherent structure field which is obtained by using Liutex as the extraction of vortices. The probability density function (PDF) of the fluctuations of longitudinal velocity shows that an indication of small intermittency in the direct cascade and the absence of intermittency in the inverse cascade range. On the other hand, the scaling exponents ζ(p) of the structure function for the inverse cascade are consistent with Kr67 model, which shows the absence of intermittency. While the measured intermittency parameters are J = 0.006 and H = 0.921, which explains that there is a very weak intermittent correction in the direct cascade, and ESS has verified the existence of intermittency in our 2D turbulence.