Research on data assimilation strategy of turbulent separated flow over airfoil

In order to increase the accuracy of turbulence field reconstruction, this paper combines experimental observation and numerical simulation to develop and establish a data assimilation framework, and apply it to the study of S809 low-speed and high-angle airfoil flow. The method is based on the ensemble transform Kalman filter(ETKF) algorithm, which improves the disturbance strategy of the ensemble members and enhances the richness of the initial members by screening high flow field sensitivity ...     

Energy Transfer in Spectra of the d-Dimensional Past Grid Turbulence

Free decay theory of the homogeneous and isotropic developed turbulence isconsidered in the d-dimensional case. The basic quantities under our consideration are the kinetic energy spectrum E(k,t) and energy transfer spectrum T(k,t) as functions of wave number k and decay time t. Starting point for studying E and T represents their adaptation from the stationary model which predicts the Kolmogorov spectrum which is multiplicatively dependent on an unknown scaling function F. In order to study the spectra of decaying turbulence both parameters l and εɛ are supposed to be dependent on t. Formerly derived basic integro–differential equation for F (by Adzhemyan, et al., 1998) has been here solved numerically in the dimension interval d∈(2, 3) for two cases of the Saffman invariant and the Loitsyansky integral fixing an arbitrary theor parameter α (α ⩵ 2 and 4, correspondingly). The energy transfer spectrum T(k) has been analyzed for several dimensions d≤3 showing the presence of integration regions in the wavenumber space where an inverse energy cascade can occur. PACS numbers: 47.27.ef, 47.11.-j, 47.27.er     

An explicit algebraic Reynolds stress model in turbulence

A new algebraic Reynold stress model is constructed with recourse to the realizability constraints. Model coefficients are made a function of strain and vorticity invariants through calibration by reference to homogeneous shear flow data. The anisotropic production in near‐wall regions is accounted for substantially by modifying the model constants C_{ε(1, 2)} and adding a secondary source term in the ε equation. Hence, it reduces the kinetic energy and length scale magnitudes to improve prediction of adverse pressure gradient flows, involving flow separation and reattachment. To facilitate the evaluation of the turbulence model, some extensively used benchmark cases in the turbulence modelling are convoked. The comparisons demonstrate that the new model maintains qualitatively good agreement with the direct numerical simulation (DNS) and experimental data. Copyright © 2006 John Wiley & Sons, Ltd.     

Intermittency in stochastically perturbed turbulent models

Random dynamical models obtained as a perturbation of the GOY (Gledzer-Ohkitani-Yamada) shell model for three-dimensional turbulence are defined. Both static (time-independent) and dynamical scaling properties of the randomly perturbed model are studied. The random static-inviscid manifold, in contrast to the dynamical evolution, does not show intermittent scaling laws. This behavior is linked to the absence of large deviation in the random-map connecting fluctuations of velocities at different scales. The importance of inviscid conserved quantities on the intermittent statistics is discussed. Different random dynamical perturbations such that only energy is conserved in the inviscid and unforced limit are investigated. Intermittency is weakly affected by random perturbations.     

Self-similar solutions of the second-order model of the far turbulent wake

A second-order semi-empirical two-dimensional model of turbulence in the approximation of the far turbulent wake is considered. The sought quantities are the velocity defect, kinetic turbulent energy, energy dissipation, and Reynolds stress. The full group of transformations admitted by this model is found. Self-similar solutions satisfying natural boundary conditions are constructed. The solutions obtained agree with experimental data. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 2, pp. 74–78, March–April, 2008.     

Completeness of general solutions for three-dimensional transversely isotropic piezoelectricity

In this paper, a general solution for three-dimensional transversely isotropic piezoelectricity in terms of four quasi-quadri-harmonic functions is established first. Owing to complexity of the higher-order equation, it is difficult to obtain rigorous analytic solutions and in most cases this general solution is not applicable. By virtue of the generalized Almansi’s Theorem, the simplified generalized LHN solution and E–L solution expressed by lower order functions are achieved, respectively, by taking a decomposition and superposition technique. In the absence of piezoelectric coupling, these two simplified general solutions can be degenerated into those for transversely isotropic elasticity, i.e. LHN and E–L solutions. More importantly, the completeness of these two generalized solutions is proved if the domain is z-convex, no matter whether the characteristic roots are distinct or possibly equal to each other.     

Breakdown of the Reynolds analogy over drag-reducing riblets surface

An experiment was carried out in a low-speed wind tunnel to study the near-wall turbulence structure over a heated riblets surface. The results confirmed the main conclusion of previous study that riblets can enhance the convective heat-transfer rate by as much as 35 percent even within the drag- reducing region. The logarithmic profile of the mean temperature was shifted downwards, indicating that viscous-sublayer thickness of the thermal boundary layer was reduced over a riblets surface. The reason for the apparent breakdown of the Reynolds analogy seems to be related to the difference in the turbulence length scale between the thermal and momentum boundary layers in the near-wall region.     

Orthogonal interaction of bernstein mode with ion-acoustic wave through plasma maser effect

Interaction of Bernstein mode wave with ion-acoustic turbulence is treated as the plasma maser effect. Ion-acoustic turbulence is considered as resonant mode, propagating orthogonal to the test Bernstein mode. The external magnetic field, in the direction of ion-acoustic turbulence, is playing key role in transferring energy in upconversion process from resonant low frequency wave to the high frequency wave through modulated electric field. It is shown that the direct coupling term and the associated reverse absorbtion effect do not contribute individually to the growth or damping to the Bernstein mode. Only the polarization coupling term is found to be effective in the energy upconversion process as the external magnetic field is providing momentum in the direction of propagation of Bernstein mode. The polarization coupling term is identified to be the dominant upconversion factor in the plasma maser effect.