1,720,980 research outputs found
Natural frequency discontinuity of vertical liquid sheet flows at transcritical threshold
Full text linkThe natural and forced dynamic response of a gravitational plane liquid sheet (curtain) of finite length interacting with an unconfined gaseous ambient is numerically and experimentally investigated. The global eigenvalue spectrum obtained by means of a linear inviscid one-dimensional model, accounting for the coupling between the curtain motion and the ambient pressure disturbances, clearly shows an abrupt increase (jump) in the characteristic natural frequency of the flow when the supercritical (We>1) to subcritical (We<1) transition occurs, with the Weber number defined as the ratio between inertia and capillary forces. On the other hand, the numerical simulation of the forced sheet response does not show any discontinuity between supercritical and subcritical conditions, as recently found by Torsey et al. (J. Fluid Mech., vol. 910, 2021, pp. 1-14) in the case of an infinite liquid sheet subjected to imposed ambient pressure disturbances not coupled with the curtain motion. It is argued that the forced liquid sheet behaviour varies continuously in shape and amplitude between the two regimes, not depending on the specific liquid-gas interaction model considered, whilst the natural frequency of the finite flow system does undergo a discontinuity, which can be theoretically predicted by the model of sheet-ambient interaction employed here. As a major result, the experimental evidence of the natural frequency jump is for the first time provided as well
Numerical and experimental frequency response of plasma synthetic jet actuators
No full textPlasma synthetic jet (PSJ), or Sparkjet, actuators seem to be a promising technology to improve the aircraft performances due to their short response time, high jet velocities and absence of moving parts. This paper aims at presenting a combined numerical and experimental investigation, to obtain information about the frequency response of the device. From the numerical point of view, an innovative lumped-element model (LEM), able to predict the temporal evolution of the main fluid-dynamic variables of the device, is presented. It is fully based on the gasdynamics equations, it includes viscous losses as well as radiative and convective heat transfer mechanisms at walls, and it considers the air as a real gas. OpenFOAM numerical computations have been carried out to perform a first calibration of the lumped model through the determination of key fitting parameters. Results for both single pulse mode and repetitive working regimes are reported, providing insights on the major actuation characteristics. To validate the model, a home-designed actuator has been manufactured, together with the control electric circuit. Experimental measurements of the jet velocity complete the actuator characterization and the model validation
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
Analysis of the wake flow behind concave curved cylinders with velocity measurements by particle image velocimetry and modal decomposition
Full text linkThe properties of the wake flow behind concave curved cylinders have been investigated analyzing simultaneously the mean velocity profiles extracted along concentric arcs parallel to the cylinder axis and the dynamically most relevant coherent structures obtained by spectral proper orthogonal decomposition. These analyzes have allowed us to determine the position, the extension, and the evolution of the different wake regimes. The velocity measurements have been obtained through the stereo particle image velocimetry (stereo-PIV) technique. The study considers various Reynolds numbers, based on cylinder diameter ( 240 < R e < 840 ), and curvatures. The quarter-of-ring cylinder is first analyzed. Near the cylinder root, the flow exhibits a topology dominated by an oblique vortex shedding, contrary to what is observed in the literature for similar geometric configurations. We attribute this disagreement to differences in the treatment of the free-end conditions that play a role in triggering the shedding regime. We find that the local shedding inclination is driven by the axial velocity, whereas its extension and wavelength depend mainly on the Reynolds number. The near free-end region presents, instead, two counter-rotating standing vortices that induce a cross-wise velocity directed toward the cylinder root and a tip vortex that expands evolving downstream. As the Reynolds number increases, the wake presents irregularities and the shedding spreads becoming nearly normal to the incoming flow. At smaller curvatures, the cylinder free-end becoming more inclined with respect to the incoming flow, and the free-end effects are enhanced. The interaction with the vortex sheets of the standing vortices that develop at the leeward side weakens. As a consequence, these vortices stretch in the stream-wise direction giving rise to the trailing vortices that stem from the cylinder surface and affect the cross-wise velocity distributions
Varicose dynamics of liquid curtain: Linear analysis and volume-of-fluid simulations
No full textThe varicose dynamics of a forced gravitational liquid sheet (curtain) issuing into a quiescent gaseous ambient is numerically investigated in this work. The study is relevant for technological applications such as coating deposition, where varicose perturbations of the curtain interfaces can arise due to axial velocity fluctuations coming from the delivering pump placed upstream of the coating die. The investigation is performed in supercritical regime, namely, for Weber number We>1. Two methodologies are employed: a simplified one-dimensional (1D) linear model and two-dimensional (2D) volume-of-fluid simulations. Using harmonic forcing perturbations of the streamwise velocity applied at the inlet section, the curtain varicose dynamics is excited by varying the forcing frequency f and amplitude Au of the perturbations for different values of We. As a significant result, the 1D analysis reveals that the curtain oscillations amplitude reaches a maximum value for a certain forcing frequency f=fmax. In other terms, it is found that the flow manifests a resonance behavior, with the oscillation frequency fmax and corresponding amplitude Ah,max both scaling as We1/3, while the average wavelength λ ̄max scales as We-1/3. These scaling laws are confirmed both by theoretical insights and 2D simulations. Moreover, it is found that the 2D curtain breaks up numerically by increasing the forcing amplitude Au. The numerical rupture is determined by a progressive curtain thinning induced by the varicose deformation, which moves upstream by increasing We, i.e., downstream by increasing the surface tension coefficient. In this respect, surface tension is found to play a stabilizing role on the varicose oscillations of the curtain
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
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