Yonghua Yan

Numerical and Experimental Studies on the Separation Topology of the MVG Controlled Flow at M=2.5

In this paper, the implicitly implemented LES method and fifth order bandwidth optimized WENO scheme are used to make comprehensive studies on the separation topology of the MVG controlled flow at M=2.5 and Re{\theta}=5760. Experiments are also made to verify the prediction of the computation. Analyses are conducted on three categories of the topology: the surface separation, cross-section separation and the three dimensional structure of the vortices. A complete description about the separation topology and a series of new findings are obtained. Among them, a pair of spiral point is first predicted by the computation and verified by the experiment. A corresponding new vortex model with 7 vortex tubes is presented also.

DNS study on Λ-vortex and vortex ring formation in flow transition at Mach number 0.5

Large vortex structure in late boundary layer transition with an inflow Mach number of 0.5 is studied by DNS (Direct Numerical Simulation) in this paper. First, we found that there are no Λ-vortex tubes, contradicting to what the existing literatures and textbooks addressed. The so-called Λ-vortex is always open on head, which has a different shape from Λ. Λ-vortex is really a pair of open rotation cores with a lower half of the Λ shape. It is also found that the Λ-vortex and ring-like vortex are formed separately and independently. There is no such a process that the Λ-vortex self-deforms to a hairpin vortex at the tip as many literatures indicated. Λ-vortex and ring-like vortex can be visualised by the iso-surface of λ2. However, the iso-surfaces of λ2 only represent rotation cores but not necessarily vortex tubes. In fact, many spanwise vortex filaments can easily penetrate the so-called Λ-vortex (iso-surface of λ2), change the direction toward the streamwise direction, and then leave the iso-surface of λ2. The vortex ring is not part of the original Λ-vortex but is formed separately. The Λ-vortex cores were originated from the 2D and 3D T-S waves, amplified and became strong by attracting neighbouring spanwise vortex filaments from the boundary shear layer. As the Λ-vortex becomes strong, a strong shear layer is formed above the Λ-vortex roots, which is caused by ejection of the Λ-vortex rotation to bring low speed flow from the bottom of the boundary layer to form an olive-like low speed zone. As a result, the instability of the shear layer leads to the formation of new ring-like vortex tubes one by one.

Numerical and Experimental Investigations of the Supersonic Microramp Wake

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....

Numerical, Experimental and Theoretical Studies on Mechanism of K-H Instability and Ring Generation behind Supersonic MVG

This paper illustrates the dominant mechanism of the ring-like vortex formation which was found generated by micro vortex generated (MVG) for shock-boundary layer interaction control by our implicit large eddy simulation (ILES) with high order accuracy and our experiment. An averaged velocity profile after MVG is taken as the base flow. The disturbed viscous incompressible and inviscit compressible stability equations have been derived in a 3D coordinates, axially symmetric in particular. After we introduced the effect of simplified spanwise counter-rotating vortices, we theoretically conclude that the structure of vortex rings behind the MVG is a consequence of the momentum deficit according to the axisymmetic instability theorem.

The Interaction between Vortex Rings and Oblique Shocks by the MVG Controlled Ramp Flow at M=2.5

In this paper, the implicitly implemented LES method and fifth order bandwidthoptimized WENO scheme are used to make comprehensive studies on the interaction of vortex rings and shock waves of the MVG controlled flow at M=2.5 and Reθ θ θ θ=5760. The kinetic information of the vortex rings is carefully studied and provided. The interaction of the streamwise vortices and vortex rings are investigated using vorticity and three stages of evolution are found. The study between the oblique shock and vortex rings is made on 2-D section analyses, 3-D shock surface investigation and the relation between vortex ring structure and the interaction. The multilayer shock structure, different shock reflection type, etc are investigated, and characteristics are obtained about the 3-D hump shape of the distorting shock caused by the vortex ring.

Study on Shock wave-Vortex Ring Interaction by the MVG Controlled Ramp Flow with Turbulence inlet from DNS

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 Reθ=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.

Shear Layer Stability Analysis in Later Boundary Layer Transition and MVG controlled flow

A 2D flow linear stability analysis based on standard Orr-Sammerfeld equation and the corresponding equation for axisymmetric flow is investigated in this paper. A typical velocity profile in a shear layer with infection point is studied first. The fluid motion is decomposed to two parts: the rotational part and shear part. Linear stability analysis shows the rotational part (like the motion of a rigid body) is stable but the shear part is unstable. Following this analysis, a velocity profile taken from the later boundary layer transition and another one from flow field generated by the micro vortex generator are considered as the base flow. A global figure has been obtained that is the ring-like vortices found in both boundary layer transition and MVG controlled flow can be generated by the shear layer instability. The shear layers are located at the boundary of the olive-shaped momentum deficit zones.

LES Study on the Mechanism of Vortex Rings behind Supersonic MVG with Turbulent Inflow

In this study, we investigate the interaction between vortex rings behind MVG and the oblique shocks in the MVG controlled ramp flow at M=2.5 and Re θ=5760. Implicit large eddy simulation (ILES) method is used by solving the unfiltered form of the Navier-Stokes equations with the 5th order Bandwidth-optimized WENO scheme. The fully developed inflow is given by a series of turbulent profiles obtained from previous DNS simulation. It shows that the ring structure does not break down and keeps its 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.

Numerical Study on Mechanism of Multiple Rings Formation

In this paper, the flow around each ring-like vortex is investigated by high order DNS including first sweep, first ejection, second sweep, second ejection, positive spike, momentum deficit, vortex shape, vortex location, strength of sweeps, etc. Meanwhile, the mechanism about formation of momentum deficit is deeply studied. A new mechanism on how the multiple rings are formed one by one found both by experiment and by DNS in late boundary layer transition is presented. It also reveals that the relation between streamwise vortex and spanvise vertex rings, and how the vorticity is transferred between them.

DNS study on role of linearly unstable modes in flow transition

Flow transition from laminar to turbulent flow is widely considered as caused by linear unstable modes through absolute or convective instability. However, our DNS study shows that is not the case. In our previous AIAA paper, the nature of flow transition is described as an inherent property of fluid flow that fluid cannot tolerate shear layer and shear must transfer to rotation when the Reynolds number is large, which will lead to flow transition. In the current DNS study, two inflow disturbances, 2D T-S waves and 2D+3D T-S waves, are tested separately and carefully compared. The development of disturbances and late flow transition are investigated by DNS. It is found that the late flow transition are all caused by shear layer instability including the vortex ring formation, multilevel shear layers, multiple level sweeps and ejections. The role of all unstable modes are same which is to trigger the vorticity rollup and change the base flow profile to have inflection points. Since all disturbances, like dust, gust, noise, mosquito, fly, sands, etc., can trigger the vorticity rollup, the idea to use control of linear modes to control flow transition is hard to get success and the key issue is to avoid the vorticity rollup, like use of suctions.