Article
Mechanics
Henrique Raposo, Shahid Mughal, Antoine Bensalah, Richard Ashworth
Summary: The study investigates the generation of viscous-inviscid instability through scattering of an acoustic wave by localized and distributed roughness on an aerofoil, using a time-harmonic compressible adjoint linearized Navier-Stokes approach. The research models the inviscid acoustic field external to the aerofoil boundary layer and shows that the externally imposed acoustic pressure field drives the acoustic boundary layer, affecting the amplitudes of acoustic-roughness receptivity. Parametric studies reveal significant non-parallel effects and higher receptivity amplitudes for distributed roughness at lower angles of attack.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Youcheng Xi, Jie Ren, Liang Wang, Song Fu
Summary: This study obtained the characteristics of the leading attachment-line mode with sweep angle variations and successfully applied a bi-orthogonal eigenfunction system to address the receptivity problem of hypersonic flows. The analyses showed that global modes are most responsive to external forces and surface perturbations near the attachment line.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Omar Kamal, Matthew T. Lakebrink, Tim Colonius
Summary: Linear input-output analysis is used to determine worst-case disturbances in a laminar base flow based on a generic forcing term. However, physically realizable worst-case disturbance problems lack a generic framework. In natural receptivity analysis, disturbances are forced by matching local solutions within the boundary layer to outer solutions. A scattering formalism is proposed to restrict the input forcing to realizable disturbances associated with plane-wave solutions of the outer problem. The method is validated through numerical simulations and provides insights into transition mechanisms.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Joan Gomez, Huidan Yu, Yiannis Andreopoulos
Summary: The primary focus of this experimental work is the instability and transition to turbulence in pulsatile flows, which involve reverse flows and unsteady flow separations. By conducting experiments with different Reynolds and Womersley numbers, it was found that transition to turbulence is a spontaneous event covering the whole near-wall region.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Engineering, Aerospace
Yuhan Lu, Jianhan Liang, Zaijie Liu, Hongkang Liu, Chao Yan
Summary: Here, a numerical approach for the three-dimensional global instability analysis of high-speed boundary layer is developed and validated in two cases. The results demonstrate that this approach can accurately predict the wavelength, growth rate, and disturbance amplitude of unstable modes.
AEROSPACE SCIENCE AND TECHNOLOGY
(2023)
Article
Mechanics
Y. Zhang, S. Goertz, M. Oberlack
Summary: In this study, the reflection and over-reflection of acoustic waves in two-dimensional inviscid compressible boundary layer flows were analyzed. It was found that energy is always transferred from the base flow to the reflected wave in supersonic conditions, and this phenomenon is closely linked to the presence of the critical layer. Additionally, resonant over-reflection was observed, which is related to the resonant frequencies of unstable modes of the temporal stability of the base flow.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
K. Jain, A. I. Ruban, S. Braun
Summary: This paper investigates the receptivity of the boundary layer to suction/blowing in marginally separated flows, showing that two-dimensional perturbations initially grow inside the separation region but decay downstream, while three-dimensional perturbations form a wave packet downstream of the perturbation source in the boundary layer.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
M. Vernet, M. Pereira, S. Fauve, C. Gissinger
Summary: An experiment was conducted to study the flow of an electrically conducting fluid in a thin disc under the influence of an azimuthal Lorentz force. Different flow patterns were observed, including quasi-Keplerian velocity profiles, turbulent flow with large fluctuations and a Keplerian mean rotation profile, and a quasi-bidimensional turbulent flow with the formation of large scale condensates in the horizontal plane. These results are attributed to the instability of the Bodewadt-Hartmann layers at large Reynolds numbers and are discussed within the context of similar astrophysical turbulent flows.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Jie Ren, Markus Kloker
Summary: This study investigates the linear instability of three-dimensional boundary layers in thermodynamically non-ideal conditions. Using carbon dioxide at supercritical pressure as a representative fluid, the researchers found that different thermodynamic states and wall heating/cooling have significant effects on flow stability.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Qizhen Hong, Jiaao Hao, Ken Chun Kit Uy, Chih-Yung Wen, Quanhua Sun
Summary: This study investigates hypersonic laminar flow over double wedges using computational fluid dynamics and global stability analysis. The research reveals that thermal and chemical nonequilibrium have minimal effects on the onset of separation in high-enthalpy conditions. Furthermore, the study extends the criterion of global stability to hypersonic flow under high-enthalpy conditions.
Article
Mechanics
G. Nastro, J. -C. Robinet, J. -C. Loiseau, P. -Y. Passaggia, N. Mazellier
Summary: In this study, the stability and sensitivity of two-and three-dimensional global modes developing on steady spanwise-homogeneous laminar separated flows around NACA 4412 swept wings were numerically investigated. It was found that the introduction of a sweep angle attenuates the growth of all unstable modes and entails a Doppler effect in the leading modes' dynamics. The sensitivity analysis revealed that a streamwise-oriented force on the suction side of the wing has a stabilizing effect, and a spanwise-homogeneous force can suppress the Hopf bifurcation and stabilize the entire branch of von Karman modes.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Engineering, Aerospace
Andrey Boiko, Kirill Demyanko, Stanislav Kirilovskiy, Yuri M. Nechepurenko, Tatiana Poplavskaya
Summary: This paper presents a technology for computing three-dimensional transonic laminar-turbulent flows using ANSYS Fluent and a special module created based on LOTRAN 3. Results on the position of the laminar-turbulent transition in boundary layers in transonic flow regimes and the dominating transition mechanism are obtained and shown to agree with experimental data.
Article
Mechanics
Shuyi Liu, Bingbing Wan, Xianxu Yuan, Ligeng Zhang, Jianqiang Chen, Xi Chen
Summary: A systematic parametric study was conducted on the modal global linear instability of Mach 6 boundary-layer flow over an inclined cone at a 6° angle of attack under typical wind tunnel conditions using spatial BiGlobal theory. Two classes of global instabilities were observed in the outboard region away from the leeward plane: high-frequency second mode instabilities and low-frequency crossflow instabilities. The global stability characteristics of the crossflow instability over a conical configuration were uncovered for the first time, revealing many unstable modes with a complex pattern. The most amplified frequency of these modes was around 30 kHz, and they were mainly distributed on the leeward side of the cone.
Article
Mechanics
Alexander J. Smits, Marcus Hultmark, Myoungkyu Lee, Sergio Pirozzoli, Xiaohua Wu
Summary: The new scaling shows a Reynolds-number-independent profile for all components of the Reynolds stress in the near-wall region of wall-bounded flows, highlighting the significance of wall shear stress fluctuations and the role of large eddies in determining the Reynolds number dependence of near-wall turbulence behavior.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Parshwanath S. Doshi, Rajesh Ranjan, Datta V. Gaitonde
Summary: Flows past cavities at high speeds play a significant role in flame-holding and propulsion unstart control. The characteristics of cavity dynamics, including oscillation modes and the influence of shear layers, change significantly as the flow speed increases.
Article
Mechanics
Mathias Lemke, Vincenzo Citro, Flavio Giannetti
Summary: This paper discusses the active control of compressible flow past a bluff-body by means of external acoustic sources, successfully suppressing the transition leading to the von-Karman vortex street. The derived adjoint-based framework allows determination of acoustic sources for various bluff-body shapes, with optimal spatial position and associated temporal signal for control. The proposed methodology and results can be used to design a control strategy coupled with a continuation procedure to obtain unstable solutions beyond the critical threshold.
FLUID DYNAMICS RESEARCH
(2021)
Article
Computer Science, Interdisciplinary Applications
A. Chiarini, M. Quadrio, F. Auteri
Summary: The new finite-difference solver for the incompressible Navier-Stokes equations utilizes the direction-splitting method and a co-located grid approach, achieving high performance levels on thousands of processors through parallelization with the Schur-complement method. Several test cases were proposed to assess the accuracy and efficiency of the method.
JOURNAL OF COMPUTATIONAL PHYSICS
(2021)
Article
Mechanics
A. Chiarini, M. Quadrio, F. Auteri
Summary: The proposed scaling law predicts the onset of the primary Hopf instability in steady flow past two-dimensional symmetric bluff bodies by using the spatial extent of the separation bubble and the largest reverse-flow speed as scales. The Reynolds number collapses nicely for bodies of different shapes and aspect ratios, even with a small angle of attack perturbing the symmetry, and provides insight into the flow mechanism producing instability.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
A. Chiarini, M. Quadrio, F. Auteri
Summary: This study investigates the primary instability of flow past rectangular cylinders, analyzing the influence of aspect ratio and rounded corners on stability. It is found that the critical Reynolds number increases with aspect ratio, and the presence of small circular cylinders in the flow can modify instability significantly. Rounded corners affect the primary instability in a manner dependent on both aspect ratio and curvature radius, with differing effects on stability based on the location and curvature of the corners.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
A. Chiarini, M. Quadrio, F. Auteri
Summary: This study investigates the shedding phenomenon and interaction between vortices in elongated rectangular cylinders at moderate Reynolds numbers. It is found that with increasing aspect ratio, two different flow behaviors are observed, and there is hysteresis in the Strouhal number near certain aspect ratios.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Engineering, Aerospace
Franco Auteri, Alberto Savino, Alex Zanotti, Giuseppe Gibertini, Daniele Zagaglia, Yannick Bmegaptche Tekap, Marianna Braza
Summary: The Smart Morphing & Sensing (SMS) project investigated the use of intelligent electro-active actuators to modify the lifting surface of an aircraft. Wind-tunnel tests were performed to evaluate the morphed aerodynamic shapes of a high-lift morphing flap. The results showed that morphing the trailing-edge flap can optimize the aerodynamic performance of the wing during take-off and landing.
AEROSPACE SCIENCE AND TECHNOLOGY
(2022)
Article
Engineering, Multidisciplinary
Javier Sierra-Ausin, Vincenzo Citro, Flavio Giannetti, David Fabre
Summary: A systematic approach is proposed to analyze compressible time-periodic flows using a Fourier-Galerkin strategy. The method is free of numerical constraints and aliasing effects, and efficient algorithms are provided to solve the nonlinear problem.
COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING
(2022)
Article
Mechanics
J. Sierra-Ausin, M. Lorite-Diez, J. I. Jimenez-Gonzalez, V. Citro, D. Fabre
Summary: This study examines the spatio-temporal pattern formation of wake flow past a streamwise rotating sphere from a dynamical system perspective. It reveals the connection between rotating waves and helical patterns within the wake. The research finds that, for low rotation rates, the flow field undergoes a transition similar to that of flow past a static sphere, but with a rapid variation in flow frequencies due to rotation. The transition from a single helix pattern to a double helix structure within the wake displays hysteric behavior in several regions. Eventually, the interaction between single and double helix structures leads to temporal chaos.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
J. Sierra-Ausin, D. Fabre, V. Citro, F. Giannetti
Summary: The mechanisms leading to acoustic whistling for a jet passing through a circular hole in a thick plate connecting two domains are investigated. Two generic situations are considered: one with a closed upstream cavity and an open downstream domain, and the other with both upstream and downstream regions considered open. A low Mach asymptotic model is used to derive a global impedance of the system, and the knowledge of this impedance provides an instability criterion and eigenvalue predictions. The method is validated against linearized fully compressible solutions and used to characterize the range of instabilities.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mathematics, Applied
I. Bochicchio, F. Giannetti, A. Sellitto
Summary: This study considers a model of nonlocal heat transfer at nanoscale in rigid bodies and analyzes three different strategies for setting up boundary conditions. The results suggest the influence of interactions on unknown fields and demonstrate the well-posedness of the problem.
ZEITSCHRIFT FUR ANGEWANDTE MATHEMATIK UND PHYSIK
(2022)
Article
Mechanics
Alessandro Chiarini, Maurizio Quadrio, Franco Auteri
Summary: The three-dimensional instability of the flow past a 5:1 rectangular cylinder is investigated using Floquet analysis and direct numerical simulations. A quasi-subharmonic unstable mode is found to become unstable at Reynolds number Re≈480, with a spanwise wavelength approximately three times the cylinder thickness. The structural sensitivity analysis reveals that the instability is triggered by the mutual inviscid interaction of vortices generated by the leading edge shear layer.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Engineering, Mechanical
Antonia Tirri, Alessandro Nitti, Javier Sierra-Ausin, Flavio Giannetti, Marco D. de Tullio
Summary: In this work, a novel approach for linear stability analysis of fluid-structure interaction problems is presented. The method combines a validated immersed boundary solver with Krylov-based techniques to obtain a robust and accurate global stability solver. The proposed algorithm avoids complex analytical linearization of equations while retaining all relevant aspects of the fully-coupled fluid-structure system. Testing on various cases demonstrates good quantitative agreement with literature results.
JOURNAL OF FLUIDS AND STRUCTURES
(2023)
Article
Mechanics
Alessandro Chiarini, Franco Auteri
Summary: The primary instability of steady two-dimensional flow past rectangular cylinders moving parallel to a solid wall is investigated, considering the cylinder length-to-thickness aspect ratio AR and the dimensionless distance from the wall g. Two types of primary instability are identified: a Hopf bifurcation leading to unsteady two-dimensional flow for g >= 0.5, and a regular bifurcation leading to steady three-dimensional flow for g < 0.5. The critical Reynolds numbers for both bifurcations depend on the gap height and aspect ratio, and exhibit different trends for different conditions.
JOURNAL OF FLUID MECHANICS
(2023)