arXiv subjects

Eman Bagheri

Publications and source records attributed to Eman Bagheri.

3 recordsLinked to original sources

arXiv2026

Generalised Perturbed Convective Wave Theory

The theory of the perturbed convective wave equation for compressible flows (cPCWE) is generalised to spatially varying mean-density fields. The resulting equation is an exact scalar reformulation of the acoustic perturbation equations and describes sound generation and propagation in moving inhomogeneous media using a single unknown. The intermediate variables of the associated workflow, in which a Helmholtz decomposition problem, a Poisson equation and the cPCWE are solved successively, are related to the vortical, entropy and acoustic modes of Kovasznay, providing a physical interpretation of each processing step. The quantitative accuracy is assessed against fully compressible direct numerical simulations (DNS) of two-dimensional isothermal mixing layer and Lighthill's analogy computed in the same framework at Mach numbers between M=0.2 and M=0.4, based on the velocity difference across the layer and the ambient speed of sound. Over this range of Mach numbers, the radiated power spans several orders of magnitude. For M>=0.25, the sound power levels obtained using the three methods agree within 0.9dB, and within 0.5dB for M>=0.3. At M=0.2, where the acoustic fluctuations are weakest relative to the hydrodynamic ones, Lighthill's analogy over-predicts the radiated power by 2.8dB. In contrast, the cPCWE deviates from the DNS reference by only -1.2dB. This closer agreement is because the cPCWE source term is confined to the vortex-pairing region, while convection and refraction are represented by its convective wave operator. Beyond reproducing the far-field sound, the cPCWE resolves the acoustic field within the shear zone itself, where the DNS' fields are masked by vortical fluctuations.

physics.flu-dyn↗

arXiv2026

Disentangling coherent structures and the origin of swirl-switching

The physical origin of swirl-switching in turbulent bent-pipe flow remains the subject of ongoing research. We perform three direct numerical simulations (DNS) of flow through a $180^{\circ}$ bent pipe for curvature $\gamma=0.2$ at $Re_D=5,300$ and $Re_D=10,000$, and for curvature $\gamma=0.4$ at $Re_D=5,300$. Here, $Re_D$ is the Reynolds number based on the bulk velocity $U_b$ and pipe diameter $D$. The DNS data are subsequently used to conduct modal decompositions and local stability analysis (LSA). We discuss the limitations of common modal decomposition methods and introduce a new decomposition method, filtered Hilbert proper orthogonal decomposition (FHPOD), enabling us to isolate different instabilities as distinct FHPOD modes. FHPOD reveals seven modes belonging to four distinct families: a low-frequency axial mode, two swirl-switching modes, two swirl-breathing modes and a pair of downstream shear-layer modes. The swirl-switching and swirl-breathing modes are localised to the curved section of the bent-pipe flow and represent the sinuous and varicose instabilities of the Dean vortices, respectively. The DNS data are used to perform local stability analysis of the mean flow using the frozen eddy-viscosity method at the two streamwise wavenumbers associated with the two swirl-switching and swirl-breathing modes. The linear stability analysis reveals six unstable branches, with the swirl-switching mode being the most unstable branch. Another unstable branch represents the swirl-breathing mode. The modes obtained from the local stability analysis are in close agreement with their FHPOD counterparts in both spatial features and the range of Strouhal numbers. The results support the interpretation that swirl-switching arises as an intrinsic instability of the bent-pipe mean flow, which can be excited by incoming turbulent structures.

physics.flu-dyn↗

arXiv2025

Collapse of turbulence in optimised curved pipe flow

The increased friction caused by turbulence is a significant contributor to energy consumption in the fluid-transport and piping industries. Here we describe a passive approach to reduce friction: we show that a local increase in streamwise flow curvature, combined with changing the circular cross-section to an oval, relaminarizes turbulent flow in curved pipes. We exemplify this effect in a $180^\circ$ bend at $Re_D = 10\,000$ and $20\,000$, well above the linear-stability limit. Curvature inhibits streamwise Reynolds stresses, and cross-sectional modifications weaken the secondary flow, together disrupting the near-wall regeneration cycle and collapsing turbulence. Simulations and experiments confirm that these geometric modifications suppress turbulence and reduce pressure loss by 53% and 36% compared with the baseline $180^\circ$ bend and an equal-length fully developed straight pipe, respectively. The results establish a passive, mechanism-based route to relaminarization in curved pipes with implications for energy-efficient control in other wall-bounded flows with curvature.

physics.flu-dyn↗