Modeling coherent structures in the atmosphere and assessing their impact on aircraft

The paper introduces an engineering method for assessing the aerodynamic effect of disturbed atmosphere on an aircraft. As a source of vortex structures, we can consider vortex wind wakes that arise when the atmospheric wind flows around the landscape, large structures, moving or stationary aircraft-carrying platforms, vortex wakes behind aircraft, etc. In this study, we consider the situation when a light transport aircraft and an aircraft of the MC-21 type get into the vortex wake behind the super-heavy aircraft A-380 when flying along the glide path. A coherent vortex structure behind the A-380 is formed by the grid method within the framework of the boundary value problem for the Reynolds-averaged Navier —Stokes equations. The evolution and stochastics of the far wake are carried out using the author’s computer code written in the MATLAB system, within the framework of discrete vortices with a Rankine core. The assessment of the increment of forces and moments from the effect of the vortex system on the aircraft was carried out using the panel method.

A Study on the Evolution Characteristics of the Large Structure of Turbulence in a Meander Channel

In this paper, we adopt theoretical method to study the evolution characteristic of the two dimensional turbulent vortex structures in a meander channel. The disturbance growth rates under different bank curvatures are simulated. The result showed that the change of growth rate of smaller vortices is more intensive than bigger vortices. Future more, we consider the coherent vortex structure as a kind of disturbance to study the evolution characteristics of multi-scale turbulent structures in a meander channel, make basis for finding “meander channel-forming vortices” which is controlling the river shape and adapt to the meander river in theoretic.

New Findings by High-Order DNS for Late Flow Transition in a Boundary Layer

This paper serves as a summary of new discoveries by DNS for late stages of flow transition in a boundary layer. The widely spread concept “vortex breakdown” is found theoretically impossible and never happened in practice. The ring-like vortex is found the only form existing inside the flow field. The ring-like vortex formation is the result of the interaction between two pairs of counter-rotating primary and secondary streamwise vortices. Following the first Helmholtz vortex conservation law, the primary vortex tube rolls up and is stretched due to the velocity gradient. In order to maintain vorticity conservation, a bridge must be formed to link two Λ-vortex legs. The bridge finally develops as a new ring. This process keeps going on to form a multiple ring structure. The U-shaped vortices are not new but existing coherent vortex structure. Actually, the U-shaped vortex, which is a third level vortex, serves as a second neck to supply vorticity to the multiple rings. The small vortices can be found on the bottom of the boundary layer near the wall surface. It is believed that the small vortices, and thus turbulence, are generated by the interaction of positive spikes and other higher level vortices with the solid wall. The mechanism of formation of secondary vortex, second sweep, positive spike, high shear distribution, downdraft and updraft motion, and multiple ring-circle overlapping is also investigated.

Experimental investigations of quasi-two-dimensional vortices in a stratified fluid with source–sink forcing

This paper describes laboratory experiments on the self-organizing character of geophysical turbulence. The experiments were carried out in a linearly stratified fluid, forced horizontally with sources and sinks around a horizontal ring. The flow was visualized with small particles illuminated by a horizontal light sheet, recorded with a camera and analysed with an advanced particle tracking system. Qualitative and quantitative data, such as flow patterns, velocity and vorticity fields, were obtained. In the experiments, the inverse energy cascade was clearly observed: the flow organized into a single quasi-steady, coherent vortex structure of the largest available scale. This vortex is maintained against diffusion of momentum by entrainment of vorticity from its exterior. In this process, patches of vorticity of the same sign as the core of this large structure intermittently cross the vorticity barrier around the vortex. Patches of opposite vorticity were observed to be effectively blocked by the barrier. The direction of rotation of the vortex was set by a slight bias in the experimental apparatus and could be changed by imposing a small initial circulation in the opposite sense, the magnitude of which suggested a measure for the bias. A detailed study of the effects of changing forcing parameters was carried out. The number of sinks (which play only a passive role) does not affect the flow, whereas the number of sources sets the lengthscale of the forcing and thereby determines the size of the vortices that are created close to the ring, as well as that of the large central vortex that emerges. However, after longer times of forcing, the vortex size also depends on the strength of the forcing. The velocities in the large vortex structure scale with the mean velocity from the sources, and with the square root of their number. Measurements were also taken of the decay of the vortex. After switching off the forcing it quickly becomes axisymmetric and a linear functional relationship is established between the vorticity and streamfunction. The spin-down time was observed to be much shorter than can be accounted for by vertical viscous diffusion alone: initially the short horizontal scale of the vorticity barrier causes a relatively fast decay, whereas at later times the size of the vortex as a whole is important.

Mixing Process Induced by the Vorticity Associated with the Penetration of a Jet into a Cross Flow

The turbulent process associated with the penetration of a jet into a subsonic cross flow has been investigated in regard to various motivations: cooling of combustion products, discharge of effluents into a waterway, aerodynamic phenomena related to VTOL aircraft. Starting from a theoretical model of the penetration and mixing processes for jets issued from a circular orifice which agrees with experimental data within a wide range of injection patterns, a simple expression was found for the mass flow engulfed by the contrarotating vortices. The coherent vortex structure, which has been identified by several investigators, is superimposed on the turbulent diffusion mixing process, and can be further analyzed in connection with the penetration model. Closed form equations are presented for the vortex strength and spacing, assuming the bidimensional straight vortex filament approximation first proposed by Fearn and Weston. The resulting predictions are found in fair agreement with data from velocity surveys by various techniques.