ORCID Profile
0000-0003-1149-4440
Current Organisation
Beijing Institute of Technology
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Publisher: Cambridge University Press (CUP)
Date: 11-04-2022
DOI: 10.1017/JFM.2022.266
Abstract: The effects of wall-normal vibration on bypass transition in a boundary-layer flow over a flat plate with roughness elements in the form of circular cylinders are investigated using direct numerical simulations (DNS), linear Floquet analyses and dynamic mode decompositions (DMD). The vibration of the plate strengthens the streamwise vorticity, consequently enhancing the velocity streaks and reducing the critical Reynolds number for transition. A map is constructed to identify the coupling effect of the vibration litude and Reynolds number on transition. Among all investigated combinations of height and diameter of roughness, the critical Reynolds numbers at different vibration litude $A$ can be unified by a scaling function of $(1-10A)$ . Two instability modes are identified in the vibration-induced transition process: a wake mode immediately downstream of the roughness, related to the inviscid Kelvin–Helmholtz instability of the wall-normal shear in the wake and a streak mode, in response to the spanwise shear of the streaky flow, occurring further downstream. The first one results in the generation of central hairpin vortices which then feed the second one. Further development of the streak instability leads to two arrays of hairpin vortices. Results from linear Floquet analyses and DMD further confirm the two modes of instability observed in DNS. A quantitative study suggests that the lification of vibration-induced disturbance by the base shear dominates the production of streamwise vorticity and subsequently the hairpin vortices.
Publisher: AIP Publishing
Date: 09-2022
DOI: 10.1063/5.0105820
Abstract: We propose a spatial-temporal multi-fidelity Gaussian process regression framework for the fusion of flow field data with various availabilities and fidelities but not sufficiently large to train neural networks commonly encountered in fluid mechanics studies. For ex le, fluid experiments lead to data with high fidelity but sparse in time and space, while most of the numerical data are generally regarded as less accurate but are spatially temporally continuous. The proposed framework aims at generating a new set of fused data by combining the merits of those in the spatial-temporal space. Numerical simulations [e.g., direct numerical simulation (DNS), large eddy simulation, Reynolds-averaged Navier–Stokes] of flow around a National Advisory Committee for Aeronautics 0012 airfoil are performed to collect the original raw data with various fidelities, and a fraction of the DNS result is used to mimic the high-fidelity but sparse experimental data. It is found that the accuracy of the fused data increases with the density of high-fidelity points until reaching a threshold, above which the fusion accuracy becomes insensitive. This limit can be overcome by introducing extra dimensions, such as the gradients of the low-fidelity data field. By examining the error fields, it is found that the high-fidelity points can tune low-fidelity fields but only within a limited local region. The accuracy can be firmly improved by introducing more high-fidelity points or higher levels of spatial gradients if the data set captures the temporal development.
Publisher: Cambridge University Press (CUP)
Date: 19-10-2021
DOI: 10.1017/JFM.2021.835
Abstract: Bypass transition in flow over a flat plate triggered by a pair of dielectric-barrier-discharge plasma actuators mounted on the plate surface and aligned in the streamwise direction is investigated. A four-species plasma–fluid model is used to model the electrohydrodynamic force generated by the plasma actuation. A pair of counter-rotating streamwise vortices is created downstream of the actuators, leading to the formation of a high-speed streak in the centre and two low-speed streaks on each side. As the length of actuators increases, more momentum is added to the boundary layer and eventually a turbulent wedge is generated at an almost fixed location. With large spanwise distance between the actuators (wide layout), direct numerical simulations indicate that the low-speed streaks on both sides lose secondary stability via an inclined varicose-like mode simultaneously, leaving a symmetric perturbation pattern with respect to the centre of the actuators. Further downstream, the perturbations are tilted by the mean shear of the high- and low-speed streaks and consequently a ‘W’-shaped pattern is observed. When the pair of plasma actuators is placed closer (narrow layout) in the spanwise direction, the mean shear in the centre becomes stronger and secondary instability first occurs on the high-speed streak with an asymmetric pattern. Inclined varicose-like and sinuous-like instabilities coexist in the following breakdown of the negative streaks on the side and the perturbations remain asymmetric with respect to the centre. Here the tilting of disturbances is dominated by the mean shear in the centre and the perturbations display a ‘V’ shape. Linear analysis techniques, including biglobal stability and transient growth, are performed to further examine the fluid physics the aforementioned phenomena at narrow and wide layouts, such as the secondary instabilities, the ‘V’ and ‘W’ shapes, and the symmetric and asymmetric breakdown, are all observed.
Publisher: Cambridge University Press (CUP)
Date: 29-12-2020
DOI: 10.1017/JFM.2020.923
Publisher: AIP Publishing
Date: 05-2022
DOI: 10.1063/5.0089997
Abstract: The aim of this paper is to investigate the linear and weakly nonlinear dynamics in flow over a flat-plate with leading edge. Linear optimal and suboptimal inflow perturbations are obtained using a Lagrangian multiplier technique. In particular, the suboptimal inflow conditions and the corresponding downstream responses are investigated in detail for the first time. Unlike the suboptimal dynamics reported in other canonical cases such as the backward-facing step flow, the growth rate of the suboptimal perturbation is in the same order as the optimal one, and both of them depend on the lift-up mechanism even though they are orthogonal. The suboptimal mode has an additional layer of vorticity that penetrates into the boundary layer farther downstream, generating a second patch of high- and low-speed streaks. The farther suboptimal ones spread to the free-stream without entering the boundary layer. The weakly nonlinear dynamics are examined by decomposing the flow field into multiple orders of perturbations using the Volterra series. Small structures in the higher order perturbations mainly concentrate in the region farther away from wall, suggesting a mechanism of outward perturbation developments, which is opposite with the well reported inward development of perturbations, i.e., from free-stream to boundary layer. The significance of these modes is then demonstrated through a prediction of flow field from the inflow condition by exploiting the orthogonality of the modes.
No related grants have been discovered for Dandan Xiao.