arXiv subjects

Chihyung Wen

Publications and source records attributed to Chihyung Wen.

3 recordsLinked to original sources

arXiv2026

A Compression-Directional Entropic Stress Method for Shock-Regularized Compressible Flow

We introduce the Compression-Directional Entropic Stress (CoDeS) method inspired by information geometric regularization. CoDeS replaces scalar multidimensional entropic pressure with a tensor stress aligned with the principal directions of compression. The stress has the form $\boldsymbolΠ_Σ=σ\boldsymbol{M}$, where $σ$ is obtained from a modified-Helmholtz equation and $\boldsymbol{M}$ is constructed from the compressive eigenspace of the symmetric velocity-gradient tensor. The source is gated by volumetric and principal-strain compression, so the regularization vanishes in smooth expansion, rigid-body rotation, and ideal contacts, while recovering the compressive one-dimensional IGR mechanism at planar shocks. The same tensor stress is used in the conservative momentum flux and the stress-work energy flux. CoDeS is tested on one-, two-, and three-dimensional problems including smooth expansion, double rarefaction, the Sod shock tube, multidimensional Riemann flow, a viscous shock tube, a two-fluid triple point, a Mach-3 slot jet, and a supersonic Taylor--Green vortex. The results show that CoDeS remains inactive in expansive and contact regions, supplies localized stress at shocks, and concentrates regularization along compressive wave structures while remaining weak in shear- and vorticity-dominated regions. At matched resolutions, the three-dimensional Taylor--Green results are comparable to or more energetic than seventh-order WENO/TENO references. These results indicate that CoDeS provides a compression-selective shock regularization compatible with high-order finite-volume resolution of contacts, interfaces, shear layers, and vortical structures. All the code, case settings, and code for plotting figures of this paper are available at https://github.com/xubonan/code\_for\_CoDeS.

physics.flu-dyn↗

arXiv2026

Nonlinear dynamics involving multiple modes in high-speed transitional boundary layer

Extensive studies have investigated the transition mechanism of boundary layers initiated by a single primary instability. In a real-world scenario, however, multiple primary instabilities of different physical nature would coexist and generate more complicated stages of mode--mode interactions. For this scenario, conventional secondary stability analysis may not be applicable. In this work, a general framework is established to decompose the input--output system and to quantify the transfer of energy involving various modes. The linearized governing equation with nonlinear forcings is applied in a Mach 6 boundary layer, where two different types of primary instabilities are added simultaneously. As the primary-wave amplitudes increase to certain threshold, the nonlinear effect causes the saturation of the second mode and secondary growth of the first mode. In the generation stage of each higher-order mode, a specific leading triadic forcing term can be identified. These higher-order waves manifest solely in response to the identified dominant forcing during their generation. At the moderate and late transitional stages, the forcings are, however, not equally transferred to the response via the resolvent operator. In other words, the base-flow-associated resolvent operator exerts different levels of `leverage' to transfer different forcings to responses. The nonlinear energy transfer via triadic forcings also drives the higher-order instability to inherit physical signature from the associated lower-order instability. Finally, the interplay between secondary/tertiary waves and primary waves occurs notably earlier then one may expect, namely before transition onset or in the early transitional region. This differs from the traditional secondary instability analysis that a large-amplitude primary wave is developed first to perform the bi-global analysis in the distorted base flow.

physics.flu-dyn↗

arXiv2025

Role of acoustic metasurface in the nonlinear mode-mode interaction and breakdown of hypersonic boundary layer

Boundary-layer instability and transition control have drawn extensive attention from the hypersonic community. The acoustic metasurface has become a promising passive control method. Currently, the effects of the acoustic metasurface on the early and late transitional stages remain evidently less understood than the linear instability stage. In this study, the transitional stage of a flat-plate boundary layer at Mach 6 is investigated, with a particular emphasis on the nonlinear mode-mode interaction. The acoustic metasurface is modelled by the well-validated time domain impedance boundary condition (TDIBC). First, the resolvent analysis is performed to obtain the optimal disturbances, which reports two peaks corresponding to the oblique first mode and the planar Mack second mode. Subsequently, both optimal forcings are introduced upstream in the direct numerical simulation, which leads to pronounced detuned modes before breakdown. The takeaway is that the location of the acoustic metasurface is significant in minimizing skin friction and delaying transition onset simultaneously. The placement of the metasurface in the linearly unstable region of the second mode delays the transition, which is due to the suppressed streak in the oblique breakdown scenario. However, in the late stage of the transition, the acoustic metasurface induces an undesirable increment of skin friction overshoot due to the augmented shear-induced dissipation work, which mainly arises from reinforced detuned modes related to the combination resonance. Meanwhile, by restricting the location of the metasurface upstream of the overshoot region, this undesirable augment of skin friction can be eliminated. As a result, the reasonable placement of metasurface is crucial to damping the early instability while causing less negative impacts on the late transitional stage.

physics.flu-dyn↗