Theoretical and Computational Fluid Dynamics最新文献

A methodology to numerically assess wall-curvature effects in boundary layers is introduced. Wall curvature, which directly induces streamline curvature, is associated with several changes in boundary-layer flow. By necessity, a local radial pressure gradient emerges to balance mean flow turning. Moreover, a streamwise (wall-tangential) pressure gradient can appear for configurations with non-constant wall curvature or a particular freestream condition; zero pressure gradient is a special case. In laminar concave flow, the Görtler instability and the associated Taylor-Görtler vortices destabilize the flow and promote laminar-turbulent transition, whereas in the fully turbulent regime, unsteady coherent structures formed by the centrifugal instability mechanism dramatically redistribute turbulent shear stress. One difficulty of assessing centrifugal effects on boundary layers is that they often appear simultaneously with other phenomena, such as a streamwise pressure gradient, making their individual evaluation often ambiguous. For numerical studies of transitional and turbulent boundary layers, it is therefore beneficial to understand the interactive nature of such coupled effects for generic configurations. A methodology to do so is presented, and is verified using the case of a subsonic, compressible turbulent boundary layer. Four direct numerical simulations have been computed, forming a (2{times }2) matrix of turbulent boundary-layer states; namely with and without concave wall curvature, each having a zero and a non-zero streamwise-pressure-gradient realization. The setup and accompanying procedures to determine appropriate boundary conditions are discussed, and the methodology is evaluated through analysis of the mean flow fields. Differences in mean flow properties such as wall shear stress and boundary-layer thickness due to either streamwise pressure gradient or wall curvature are shown to be remarkably independent of one another.

This paper reports a parametric investigation of the effect of the nozzle exhaust initial conditions on the wavelet separated acoustic pressure generated by a single stream compressible jet in its near field from a database obtained by Large-Eddy Simulations of jet flows at M = 0.9 and Re = (10^5). The nozzle–exit boundary–layer conditions consist of different turbulence intensities for fixed thickness and several thicknesses in laminar conditions. Pressure time series are extracted from virtual probes distributed in the near field of the jets and the acoustic components of the near field pressure are extracted using a wavelet-based procedure able to decontaminate the signals from the hydrodynamic contribution. The reconstructed acoustic time series are analyzed in the frequency domain and in terms of Overall Sound Pressure Level (OASPL). The results show that both the boundary-layer thickness and the turbulence level significantly affect the acoustic pressure in terms of both intensity and directivity. In the laminar case, strong sideline components are observed and strongly depend on the boundary layer thickness. These components clearly appearing in the energy spectra are associated with the Kelvin–Helmholtz instability waves. For large nozzle-exit turbulence intensities, the acoustic field is more uniform and less intense in the sideline direction. On the other hand, the streamwise directivity of the acoustic pressure appears to be strictly correlated to the length of the jet potential core which strongly varies with the initial conditions.

This paper presents an overlay-based one-way coupled Eulerian–Lagrangian computational approach designed to investigate the dynamics of particulate phases in extreme high-speed, high-altitude flight conditions characterized by very low particulate mass loading. Utilizing the Direct Simulation Monte Carlo method to generate accurate gas flow fields, this study explores two canonical hypersonic flow systems. First we focus on the hypersonic flow over a sphere-cone, revealing the formation of dust-free zones for small particulate diameters and describing the particulate interaction with gas shocks. As particulate diameter and flight speed increase, the characteristics of the particulate phase evolve, leading to the emergence of distinctive features such as high particulate concentration bands or regions void of particulates. Subsequently, the investigation considers flow over a double-cone, emphasizing the behavior of particulate phases in separated vortex-dominated systems where particulate-inertia-driven interactions with vortices result in unique particulate-free zones in the vicinity of the primary and secondary vortices. Additionally, the paper addresses the importance of using realistic fractal-like particulate shapes and demonstrates that the shape effect tends to decelerate the fractal aggregates and trap them along the boundaries of the primary vortex. This research contributes to a deeper understanding of particulate phase dynamics in extreme flight conditions, offering insights relevant to aerospace and aerodynamic applications.

This work delves into the peristaltic rheology of two-wave sinusoidal cilia beating within a tubular pipe. Cilia movement drives the dynamic phenomenon of peristaltic fluid flow. Employing the traditional long-wavelength lubrication assumption, the flow equations are transformed into similarity form. The main objective is to take into account the true peristaltic-ciliary motion effects. We then derive analytical solutions for the radial and axial velocities of fluid particles within the tube. Notably, at this leading approximation level, the impacts of cilia beating are negligible, suggesting the motion is solely driven by peristaltic surface waves. However, analyzing the correction to the long-wavelength limit reveals the emergence of ciliated boundary effects through their largely eccentric elliptic paths. This correction enables us to extract expressions for the pressure gradient, stream function, axial and radial velocities, resultant pressure rise, and drag force, all based on the time-averaged mean flow rate across the pipe. Finally, we present a general discussion of fluid rheology due to cilia-assisted peristaltic motion, illustrated with informative graphical displays. It is shown that the drag force on the tube walls owing to the cilia beating waves in biology or biomedical applications necessitates addition of correction terms to the traditional long-wavelength adoption.

This work aims to provide a more complete understanding of the resonance mechanisms that occur in turbulent jets at high subsonic Mach number, as shown by Towne et al. (J. Fluid Mech., vol. 825, 2017, pp. 1113-1152). Resonance was suggested by that study to exist between upstream- and downstream-travelling guided waves. Five possible resonance mechanisms were postulated, each involving different families of guided waves that reflect in the nozzle exit plane and at a number of downstream turning points. However, that study did not identify which of the five resonance mechanisms underpin the observed spectral peaks. In this work, the waves underpinning resonance are identified via a biorthogonal projection of Large Eddy Simulation data on eigenbases provided by a locally parallel linear stability analysis. Two of the five scenarios postulated by Towne et al. are thus confirmed to exist in the turbulent jet. The reflection-coefficients in the nozzle exit and turning-point planes are, furthermore, identified. Such information is required as input for simplified resonance-modelling strategies such as developed in Jordan et al. (J. Fluid Mech., vol. 853, 2018, pp. 333-358) for jet-edge resonance, and in Mancinelli et al. (Exp. Fluids, vol. 60, 2019, pp. 1-9) for supersonic screech.

This study introduces the Extended Cluster-based Network Modeling (eCNM), a methodology to analyze complex fluid flows. The eCNM focuses on characterizing dynamics within specific subspaces or subsets of variables, providing valuable insights into complex flow phenomena. The effectiveness of the eCNM is demonstrated on a swirl flame in unforced conditions, characterized by a precessing vortex core (PVC), using synchronized data from PIV measurements, UV-images filtered around the OH* chemiluminescence wavelength, featuring the heat release rate distribution, and pressure signals from jet inlet probes. The analysis starts with choosing the distance metric for the coarse-graining process and the number of clusters of the model. This has been pursued by designing a filtered distance metric based on the filtered correlation matrix and minimizing the Bayesian information criterion (BIC) score, balancing the goodness of the fit of a model with its complexity. The standard cluster-based network model on the velocity fluctuations allowed for determining the characteristic frequency of the PVC. The construction of extended cluster centroids of the heat release rate reveals a rotating flame pattern, predominantly localized within regions influenced by PVC’s vortices roll-up. Spatial subdomain analysis is carried out, demonstrating the benefits of focusing on specific regions of interest within the fluid system and providing significant computational savings. Furthermore, eCNM allows for the handling of different sampling frequencies among datasets. Leveraging high-resolution pressure measurements as a reference dataset and velocity components as undersampled data, extended cluster centroids for velocity are successfully estimated, even when the velocity sampling frequency is artificially reduced. This study showcases the adaptability and robustness of eCNM as a valuable tool for comprehending and analyzing coherent structures in complex fluid flows.