1,720,966 research outputs found
Surface tension-induced instability in spatially developing subcritical liquid curtains
No full textAn energy budget approach based on numerical simulations of a linear low-order model, combined with linear global stability analysis, is used to investigate the unsteady dynamics of subcritical (We < 1) gravitational liquid sheet flows. It is found that surface tension is the physical mechanism responsible for the modal flow instability as the Weber number is progressively decreased down to a critical threshold Weth for which the sheet is entirely subcritical. A transient algebraic growth of the perturbation characterized by the power law t 1 3 is found in both asymptotically stable (W e t h < W e < 1) and unstable (W e < W e t h) conditions. This finding agrees with a previous result of the literature obtained by employing a local spatiotemporal stability technique (for an infinite domain) for which in the subcritical regime an absolute instability occurs. However, in the present study, the temporal evolution of disturbances in the unstable case eventually follows an asymptotic modal growth, which is also recovered in the eigenvalue spectra evaluated using linear stability analysis. Asymptotic stability of the flow detected in the range W e t h < W e < 1 is not caused by the damping effect of viscosity, but by the energy exchanges through the domain boundaries. Surface tension-induced instability is further studied by means of parametric analysis involving the Froude number Fr and the slenderness ratio parameter ε. It is found that decreasing ε and increasing Fr have the same destabilizing effect. The present work represents a further step toward a deeper understanding of liquid sheet dynamics in the subcritical regime, with the aim of providing a theoretical background to establish connections between results of two-dimensional modeling and three-dimensional observations of real occurrence
Robust spectral proper orthogonal decomposition
Full text linkExperimental measurements often present corrupted data and outliers that can strongly affect the main coherent structures extracted with the classical modal analysis techniques. This effect is amplified at high frequencies, whose corresponding modes are more susceptible to contamination from measurement noise and uncertainties. Such limitations are overcome by a novel approach proposed here, the robust spectral proper orthogonal decomposition (robust SPOD), which implements the robust principal component analysis within the SPOD technique. The new technique is firstly presented with details on its algorithm, and its effectiveness is tested on two different fluid dynamics problems: the subsonic jet flow field numerically simulated, and the flow within an open cavity experimentally analyzed in [48]. The analysis of the turbulent jet data, corrupted both with salt and pepper and Gaussian noise, shows how the robust SPOD produces more converged and physically interpretable modes than the classical SPOD; moreover, the use of the robust SPOD as a tool for de-noising data, based on the signal reconstruction from de-noised modes, is also presented. Applying robust SPOD to the open cavity flow has revealed that it yields smoother spatial distributions of modes, particularly at high frequencies and when considering higher-order modes, compared to standard SPOD
Reduced-Order Model Approaches for Predicting Airfoil Performance
Full text linkThis study delves into the construction of reduced-order models (ROMs) of a flow field over a NACA 0012 airfoil at a moderate Reynolds number and an angle of attack of (Formula presented.). Numerical simulations were computed through the finite-volume solver OpenFOAM. The analysis considers two different reduction techniques: the standard Galerkin projection method, which involves projecting the governing equations onto proper orthogonal decomposition modes (POD−ROMs), and the cluster-based network model (CNM), a fully data-driven nonlinear approach. An analysis of the topology of the dominant POD modes was conducted, uncovering a traveling wave pattern in the wake dynamics. We compared the performances of both ROM techniques regarding their prediction of flow field behavior and integral quantities. The ROM framework facilitates the practical actuation of control strategies with significantly reduced computational demands compared to the full-order approach
Cluster-based network reduced order modeling for flow fields around airfoil profiles
No full textA Reduced Order Model investigation of the flow field over a NACA 0012 airfoil at moderate Reynolds number and angle of attack α = 8° is presented in this work. Numerical simulations have been carried out by means of the finite-volume solver OpenFOAM. Two main reduction techniques have been employed for this analysis, the standard Galerkin projection of the governing equations onto proper orthogonal decomposition modes (POD-ROM) and an original application of the cluster-based network model (CNM), a fully data-driven nonlinear methodology. The topology of the leading POD modes is analyzed, revealing the travelling wave pattern of the wake dynamics. Performances of both ROMs techniques are compared in terms of flow fields behaviours and integral quantities
Energy insights into the unsteady dynamics of a viscous gravitational liquid sheet
No full textThe impulse response of planar liquid sheet flows, subjected to gravity, and interacting with unconfined gaseous environments located on both sides of the liquid phase, is numerically investigated from an energy perspective by means of a combined approach of linear stability analysis and direct numerical simulations, carried out with the volume-of-fluid technique. The computation of global eigenmodes and eigenvalues is based on a simplified one-dimensional model also accounting for viscous effects. Physical insights are gained by means of an original energy balance equation for sinuous perturbations, identifying the energy budgets associated with the different terms governing the flow dynamics. Two distinctive features of the sheet flow, the flow instability in the supercritical regime at relatively high gas-to-liquid density ratio and the discontinuity in frequency at the supercritical-to-subcritical transition, have been recovered and discussed. The pressure work is responsible for the instability of supercritical regimes at relatively high density ratio. This finding is confirmed by the direct numerical simulations, showing a convective amplification of the perturbation as it travels downstream: for high density ratios, the large convective amplification cannot be expelled from the domain and the flow suffers from a global instability. The frequency discontinuity occurring at the supercritical-to-subcritical transition is basically due to the left-going wave expulsion; therefore, the subcritical sheet stabilizes more rapidly than the supercritical one, and the slow branch of the spectrum disappears. The high frequency oscillations observed in subcritical regime are attributed to the removal of constraint on the meanline slope when We < 1, which produces an increase in the oscillation frequency of the sheet analogous to that occurring for elastic solid beams when the clamped constraint is substituted by pinned constraint
Computation of liquid jets BiGlobal spectrum via randomly perturbed numerical data
No full textA data-driven approach to estimate the global spectrum of gravitational planar liquid jets is presented in this work. Numerical data have been obtained by means of the BASILISK code, based on the one-fluid formulation and the volume-of-fluid approach. The Dynamic Mode Decomposition (DMD) technique is applied to extract the underlying linear operator, considering random perturbations of the base flow. This methodology is applied to a two-dimensional configuration obtaining the BiGlobal spectrum. The analysis highlights that in supercritical regime (We > 1) the spectrum presents three branches: the upper and lower ones exhibit a purely sinuous behavior; the middle branch presents a predominant varicose component, increasing with the frequency. Experimental evidence shows that the varicose behavior is related to the rupture of the curtain
VOLUME-OF-FLUID SIMULATION OF THREE-DIMENSIONAL GRAVITATIONAL LIQUID SHEET FLOWS
No full textThe volume-of-fluid (VOF) method is employed to simulate the dynamics of gravitational liquid sheets (curtains) issuing into an initially quiescent gaseous environment. The flow behaviour is investigated by varying two governing parameters, namely the sheet aspect ratio, (formula presented), and the Weber number, (formula presented), where (formula presented) and (formula presented) are the curtain inlet thickness and width, respectively, (formula presented) the inlet mean liquid velocity, l and σ the liquid density and surface tension coefficient. For a reference Weber number greater than one, three different aspect ratio values are considered, namely AR = 40, 25 and 10, and it is found that a varicose mode progressively arises as AR decreases, dramatically affecting the flow at AR = 10. The analysis performed by varying We reveals that a stable liquid sheet can be obtained in both supercritical (We > 1) and subcritical (We < 1) regimes, down to We = 0:8, where rupture mechanisms (holes formation and their amplification) start to occur. Linear stability analysis predictions of the sheet oscillation frequency based on a simplified linear low-dimensional model of the flow system are found to be in good agreement with corresponding values arising from the three-dimensional simulations
Modal decomposition analysis of unsteady viscous liquid sheet flows
No full textThe unsteady dynamics of a gravitational liquid sheet, driven by a continuous harmonic perturbation in the lateral velocity component applied at the inlet section, is analyzed. The topology and the dynamics of the relevant flow structures are characterized by applying POD (Proper Orthogonal Decomposition) and spectral POD (SPOD) modal decompositions on two-dimensional two-phase numerical simulation data obtained with the volume-of-fluid approach. The investigation is carried out by varying the Weber number, the forcing frequency (Strouhal number), and the Reynolds number. The supercritical regime (We > 1) features a traveling perturbation, exhibiting a spatial structure with leading sinuous modes. SPOD spectra confirm the occurrence of a discontinuity in frequency response between the supercritical and subcritical regimes. In the subcritical regime (We < 1), the investigation highlights the excitation of a combined sinuous-varicose motion when the system is driven at resonance frequency for a relatively high Reynolds number (approaching the inviscid limit). The emergence of varicose modes is favored by low Weber numbers. The excitation of these modes occurs when the Weber number is decreased from We = 0.90 down to 0.75, with a progressive shift of the varicose mode from higher harmonics toward the main frequency; it can be considered as a possible mechanism of breakup observed in experiments when the inlet flow rate is progressively reduced. The flow reconstruction based on both POD and SPOD confirms the good capability of SPOD modes to capture dynamically relevant features of the fluid motion in subcritical conditions
MODAL ANALYSIS OF A 3D GRAVITATIONAL LIQUID SHEET
No full textModal analysis of three-dimensional gravitational thin liquid sheet flows, interacting with unconfined gaseous environments located on both sides of the liquid phase, is performed in the present work. Numerical data of this relevant two-phase flow configuration are obtained through the single-phase formulation and the Volume-of-Fluid (VOF) technique implemented in the flow solver Basilisk. This class of flows exhibits a variety of spatial and temporal relevant structures, both in free and forced configurations, that are investigated through the Spectral Proper Orthogonal Decomposition (SPOD). By means of such methodology, we explore the effect of two main governing parameters on the flow dynamics, namely the liquid sheet aspect ratio, AR = W/H, where H and W are the sheet inlet thickness and width, and the Weber number, We = piU2H/(2a), in which U is the inlet liquid velocity, pi the liquid density, and a the surface tension coefficient. Finally, for the highest aspect ratio value considered (AR = 40), we investigate the forced dynamics of the system excited by a harmonic perturbation in transverse velocity component applied at the inlet section, comparing results with ones arising from a purely two-dimensional analysis of the flow. The obtained results highlight the low rank behavior exhibited by the flow, suggesting that reduced order modeling could be particularly appealing to reduce complexity and computational effort in numerical simulation of this class of flows
Numerical investigation of hole propagation in a three-dimensional liquid curtain
No full textThe propagation of a hole in a gravitational liquid sheet (curtain) issuing into a quiescent gaseous environment is numerically investigated in supercritical flow conditions, namely for acurta in Weber number We > 1. The analysis is based on three-dimensional direct numerical simulations carried out by means of the open-source code BASILISK, which implements the volume-of-fluid method to track the gas-liquid interface. For a selected reference configuration, the steady flow solution is first examined. The investigation reveals a triangular shape of the steady curtain, due to the surface tension-induced sheet borders retraction towards its center plane. In addition, capillary waves forming a striped pattern arise at the curtain interface. These waves result from the competition between viscous and surface tension forces, and accordingly vanish when the former become increasingly dominant, namely when the Oh nesorge number (Oh) is relatively high. The unsteady dynamics is then analyzed as the curtain response to a hole perturbation artificially superposed to the steady flow. Several streamwise (xh) and spanwise (zh) hole initial locations are examined, as well as different Weber number values. As major results, it is found that the hole evolution is governed by the interplay between gravity and capillary forces acting on its rims, and by the hole-curtain rims interaction. The hole area undergoes an initial transient growth, it reaches a peak value, and then it decreases, leading to the hole closure or to the hole expulsion at the downstream outflow, respectively for low and high values of xh. Moreover, the effect on the hole amplification of increasing the Weber number at fixed xh is the same of introducing the hole perturbation more upstream (i.e., reducing xh) at fixed,We. The main result found here is that the holeperturbation does not influence the supercritical curtain flow dynamics in the long time limit
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