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(a) Vortex without head in viscous region (b) Vortex must have bridge (head) when rising
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Abstract In this paper a new study for the mechanism,of the ring-like vortex formation is presented. There were two existing theories, one believes the ring-like vortex is produced by the shear layer and,the other thinks the ring-like vortex generation follows,the Crow,theory. According to our recent DNS results, a new hypothesis is presented that...
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... A nonslip condition was applied at the wall, periodic boundary was used for the symmetric side walls, and a nonreflection condition was used for the upper boundary. The inflow condition was obtained from a direct numerical simulation (DNS) of a turbulent boundary layer [20] so that a turbulent inflow is ascertained. Analysis of the accuracy of the present code has been provided in91011. ...
The flow past a microramp immersed in a supersonic turbulent boundary layer is studied by means of numerical simulations with the implicit large-eddy simulation technique and experiments conducted with tomographic particle image velocimetry. The experimental data are mostly used to verify the validity of the numerical results by ample comparisons on the time-averaged velocity, turbulent statistics, and vortex intensity. Although some discrepancies are observed on the intensity of the upwash motion generated by the streamwise vortex pair, the rates of the recovery of momentum deficit and the decay of streamwise vortex pair intensity are found in good agreement. The instantaneous flow organization is inspected, making use of the flow realizations available from implicit large-eddy simulation. The flow behind the microramp exhibits significant large-scale unsteady fluctuations. Notably, the quasi-conical shear layer enclosing the wake is strongly undulated under the action of Kelvin–Helmholtz (K–H) vortices. The resulted vortices induce localized high-speed arches in the outer region and a deceleration within the wake associated with ejection of low-momentum fluid. Because of the presence of the K–H vortex, the streamwise vortex filaments exhibit downward and outward motions. The further evolution of vortical structures within the wake features the development of K–H vortices from arch shape to full ring in the far wake, under the effects of the streamwise vortices, which induce an inward motion of the vortex legs and eventually connect the vortex at the bottom.
Read More: http://arc.aiaa.org/doi/abs/10.2514/1.J052649
This study presents DNS results of the laminar-turbulent spatial transition in a plane channel flow. The transition is achieved imposing at the inlet the most unstable modes of the associated Orr–Sommerfeld and Squire eigenvalue problems. First, a study of the dependence of the transition on the intensity of the perturbations is presented. For Re=5000, eleven simulations employing different amplitudes of the Tollmien-Schlichting and oblique waves were analyzed to find that the variation of the friction Reynolds number and shape factor downstream the departure of the transition is roughly independent on the amplitude of the perturbations and that the location of the peak in the friction Reynolds number is strongly dependent on the amplitude of each wave. This implies that, for the type of perturbations simulated here, the transitional phenomenon is essentially delayed or accelerated by the amplitude of the perturbations. Second, two cases with well different amplitude of perturbations are compared in detail. Results show that in both cases the following stages can be identified: quasi-linear stage, late stage, spike stage, peak transitional zone, post-transitional zone and fully turbulent zone. Moreover, downstream the first state of the spike stage, both cases are essentially equal despite the fact that both transitions are separated by 50 channel half-height diameters in the streamwise coordinate. Finally, the physical phenomenon of the peak zone in the friction Reynolds number is explained considering the coherent vortices packet found across the height of the channel in the super-late stage of the transition.
In this study, we investigate the interaction between vortex rings and the oblique shocks by the MVG controlled ramp flow at M=2.5 and Re=5760. A kind of large eddy simulation method is used by solving the unfiltered form of the Navier-Stokes equations with the 5th order Bandwidth-optimized WENO scheme. A series of turbulent profiles are obtained from previous DNS simulation result. It shows that the ring structure does not break down and keeps it topology after penetrating the strong shock wave and the oblique shocks is influenced a lot by the induced flow field from rings. The bump of the 3D shock wave surface is discovered and its mechanism is explained.
The interaction of vortex rings and oblique shocks, generated by the micro vortex generator (MVG) controlled ramp flow at M = 2.5 and Re θ =5760, is studied by implicit large eddy simulation (LES) with the 5th order Bandwidth-optimized WENO scheme. A turbulent inflow was generated by a separate DNS for boundary layer transition. It shows that the vortex ring structure generated by MVG is very stable, does not break down and keeps it original topology after penetrating the strong shock wave. However, the oblique shocks are broken when they interact with the vortex rings. The bump on the 3D shock wave surface is observed. The separation zone, which is originally generated by the shock-boundary layer interaction, is significantly reduced due to the vortex ring-shock interaction.
Implicitly implemented large eddy simulation (LES) with a fifth-order WENO scheme was conducted in this study. Based on Navier-Stokes equations, this LES was carried out to explore the origin of momentum deficit caused by a supersonic micro-ramp at flow conditions of M
a
= 2.5 and Re
θ = 5760. The numerical results were validated through qualitative and quantitative comparisons with existing experimental data. After describing the aerodynamic properties of the supersonic wake, such as the deficit and the streamwise vortices, the momentum deficit was later detected to originate from the lower portion of the upstream boundary layer, while the high momentum fluid originated from close to the wall at the upper portion. Position alternation trigged by the micro-ramp was finally proposed as a revised mechanism.
In this paper, we studied the influence of different inflow conditions on the MVG controlled supersonic ramp flow. In front of the MVG, the inflow of three turbulent inflow profiles with different boundary layer thickness are generated. It is found that the generation of ring-like vortical structure is not dominated by the inflow conditions. However, the inflow conditions give significant influences to the ring-like vortex shapes, vorticity origins, momentum deficit zones, streamwise velocity profiles after MVG and vortex organizations. Further, it is seen that the interaction between ring-like vortices and shock waves at the ramp corner is also affected.
This paper serves as a review of our recent new DNS study on physics of late
boundary layer transition. This includes mechanism of the large coherent vortex
structure formation, small length scale generation and flow randomization. The
widely spread concept vortex breakdown to turbulence,which was considered as
the last stage of flow transition, is not observed and is found theoretically
incorrect. The classical theory on boundary layer transition is challenged and
we proposed a new theory with five steps, i.e. receptivity, linear instability,
large vortex formation, small length scale generation, loss of symmetry and
randomization to turbulence. We have also proposed a new theory about
turbulence generation. The new theory shows that all small length scales
(turbulence) are generated by shear layer instability which is produced by
large vortex structure with multiple level vortex rings, multiple level sweeps
and ejections, and multiple level negative and positive spikes near the laminar
sub-layers.Therefore, turbulence is not generated by vortex breakdown but
rather positive and negative spikes and consequent high shear layers. Shear
layer instability is considered as the mother of turbulence. This new theory
may give a universal mechanism for turbulence generation and sustenance which
shows that the energy is brought by large vortex structure through multiple
level sweeps.
The mechanism of randomization in late boundary layer transition is a key
issue of late boundary layer transition and turbulence theory. We studied the
mechanism carefully by high order DNS. The randomization was originally
considered as a result of large background noise and non-periodic spanwise
boundary conditions. It was addressed that the large ring structure is affected
by background noises first and then the change of large ring structure affects
the small length scale quickly, which directly leads to randomization and
formation of turbulence. However, what we observed is that the loss of symmetry
starts from the middle level rings while the top and bottom rings are still
symmetric. The nonsymmetric structure of second level rings will influence the
small length scales at the boundary layer bottom quickly. The symmetry loss at
the bottom of the boundary layer is quickly spread to up level through
ejections. This will lead to randomization of the whole flow field. Therefore,
the internal instability of multiple level ring structure, especially the
middle ring cycles, is the main reason for flow randomization, but not the
background noise. A hypothesis is given that the loss of symmetry may be caused
by the shift from C-type transition to K-type transition or reverses.
In this paper, the initial evolution of ring-like vortices which are found behind the MVG-controlled supersonic flow is studied. It is verified that the ring-like vortices can come from the upstream vortex structure and be generated by shear layer instability at the boundary of the momentum deficit as well. However, because of the inflow’s fully developed turbulent flow, the lower boundary layer underneath the ring-like vortex structure will influence the formation of the vortex rings a lot. Furthermore, it is also found that certain vortex structures in the lower boundary layer can be lifted up and become the ring-like vortex. The formation and development of the ring-like vortices is more complicated under the consideration of the influence of the fully developed turbulent inflow.
Micro vortex generators are a new kind of passive flow control instruments for shock-boundary layer interaction problems. In contrary to the conventional vortex generator, they have heights approximately 20-40% (more or less) of the boundary layer. Among them, Mircoramp vortex generators (MVG) are given special interest by engineers because of their structural robustness. The mechanism of the flow control was thought that a pair of streamwise vortex is generated by MVG and remains in the boundary layer for relatively long distance; the down-wash effect by the streamwise vortices will bring about momentum exchange, which makes the boundary layer less liable to separation. During such process, a specific phenomenon called as momentum deficit will happen [1], i.e., a cylindrical region consisted of low speed flows will be formed after the MVG. It was pointed out by Li and Liu [2] that the origin of deficit comes from the shedding of boundary layer over MVG.
Numerical simulations have been made on MVG for comparative study and further design purposes. Ghosh, Choi and Edwards [3] made detailed computations under the experimental conditions given by Babinsky by using RANS, hybrid RANS/LES and immersed boundary (IB) techniques. Lee et al [4, 5] also made computations on the micro VGs problems by using Monotone Integrated Large Eddy Simulations (MILES). Basic flow structures like momentum deficit and streamwise vortices were reproduced in the computation. Further studies were also conducted on the improvement of the control effect.
