Oscillatory Flow Structures Research Articles

Oscillating onset of the Rayleigh–Bénard convection with viscoelastic fluids in a slightly tilted cavity

The oscillating onset of the Rayleigh–Bénard convection with viscoelastic fluids in a slightly tilted two-dimensional rectangular cavity with an aspect ratio of Γ = 2 was investigated for the first time via direct numerical simulation. A series of simulations were conducted in the plane of the Rayleigh number (Ra) and the tilt angle (α∈[0°, 5°]) with three Weissenberg numbers [Wi=(0.1,0.15,0.2)] at a fixed Prandtl number Pr = 7.0. The evolutionary path of the oscillating convection onset in the (Wi,α)-plane was determined, and the corresponding complex flow structures were observed. The inclination of the box delays the onset of the oscillations and the corresponding Rayleigh number Rac as compared to the horizontal configuration. Oscillating flow structures acquire the attributes of a traveling wave. A specific feature of the oscillating convection in the case of the horizontal cavity is that the periodicity in space and time exists in the inclined box case as well. However, the evolution of the oscillatory flow structure is very different from the horizontal case in that the counterclockwise cell assimilates the clockwise cell [Zheng et al., “Pattern selection in Rayleigh–Bénard convection with nonlinear viscoelastic fluids,” Phys. Rev. Fluids 7, 023301 (2022)].

Oscillations in Rectangular Supersonic Inlets with Large Internal Contraction Ratio

Increasing the internal contraction ratio (defined as the area ratio of the duct entrance and throat, denoted as ICR) can improve the operating performance of a variable geometry supersonic inlet, however, excessive ICR typically will bring the inlet into an unstarted state with oscillation. To uncover the underlying mechanism of a large ICR-induced oscillatory-unstarted phenomenon, we employed a high-speed schlieren and dynamic pressure acquisition system, and experimentally examined the flowfields of a rectangular supersonic inlet with different ICRs of 1.5, 2.0, 2.5, and 3.0 in the wind tunnel at a freestream Mach number of 2.9. The inlet stayed unstarted while the ICR ranged from 2.0 to 3.0, and the flowfield had three features. First, the inlet flowfields showed high- and broadband-frequency oscillations, whose dominant frequency and oscillatory amplitude increased with ICR. Second, the inlet flowfields revealed similar oscillatory flow structures, which contained the periodic variation of the ramp-side boundary layer separation, cowl-side flow spillage, and shock train in the internal contraction part. Third, the unstarted flow structure and oscillatory phenomenon mainly occurred upstream of the inlet geometric throat, downstream of which the main flowfield was supersonic. To characterize these three features, we proposed a self-excited oscillatory model consisting of a disturbing source, a disturbing feedback terminal, and a signal feedback loop. Practically, the disturbing source is a scale variation of the entrance separation, the disturbing feedback terminal is an aerodynamic throat near the geometric throat, and the signal feedback loop is composed of communication pathways of shock train motion, acoustic waves, and convection waves. With this model, we explained the underlying mechanism of the high- and broadband-frequency characteristics of the oscillation caused by large ICR, and the ICR’s influence on the flow oscillation.

Fluid dynamics of oscillatory flow in three-dimensional branching networks

The present study is aimed at understanding and thoroughly documenting the complex unsteady fluid dynamics in six generations of a model human bronchial tree, comprising 63 straight sections and 31 bifurcation modules, during a complete breathing cycle. The computational task is challenging since the complexity of an elaborate network is augmented with adopted stringent criteria for spatial and temporal accuracy and convergence at each time step (10−8 for each scaled residual). The physical understanding of the fluid dynamics of steady expiratory flow is taken to a similar level of fine details that have been previously established for steady inspiratory flow in earlier publications of the authors. The effects of three-dimensional arrangement of the same branches on the oscillatory flow structure are determined. It is found that the quasisteady assumption is approximately valid in the neighborhood of the peak flow rate, both during inspiration and expiration. Unsteady effects are at their maximum during the changeover from expiration to inspiration and inspiration to expiration. At these time instants, regions of bidirectional flow are observed in all branches with significant secondary motion at various cross sections (none of these features can be predicted by steady state simulations). It is described how the symmetry of the solution with respect to both space and time—found in the oscillating, fully developed flow in a pipe—are destroyed in the unsteady effects that occur in the oscillating flow in a branching network. As the Womersley number is increased, the unsteady effects at all branches increase, and bidirectional flow exists over a greater portion of a cycle. The flow division at a bifurcation module during inspiratory flow generates large asymmetry in the flow field with nonuniform mass flow distribution among the branches of a generation (even in a geometrically symmetric network), whereas flow combination at the same bifurcation module during expiratory flow tends to produce more symmetry in the flow field, displaying essential irreversibility of fluid dynamics.

Numerical investigation of oscillatory thermocapillary flows under zero gravity in a circular liquid film with concave free surfaces

NASA astronaut Pettit has conducted thermocapillary flow experiments in water films suspended in a solid ring onboard the International Space Station (ISS) in 2003 and 2011. In one of these experiments, an oscillatory thermocapillary flow was observed. The developed flow broke its symmetry along the centerline of the film. To the best of our knowledge, there are no studies on such oscillatory thermocapillary flows in thin films, and the flow-mechanism giving rise to such oscillatory flows is also not well understood. In order to shed light on the subject, we have carried out a numerical simulation study. The simulation results have shown that the water film geometry (film surface shape; being concave) is an important parameter and give rise to three oscillatory flow structures in the film, namely, a hydrothermal wave developing near the heated section, a symmetric oscillatory flow due to temperature variations, and a symmetry breaking flow due to the hydrodynamic instability along the free boundary layer (mixing layer) and the development of the hydrothermal waves. Simulation results show that the symmetry-breaking phenomenon observed in the thin film experiment on the ISS can be explained by the hydrodynamic instability and the development of hydrothermal waves.

Cyclically-arranged, storm-controlled, prograding lithosomes in Messinian terrigenous shelves (Bajo Segura Basin, western Mediterranean)

This work focuses on a Messinian shallow-marine terrigenous unit, termed the La Virgen Formation, which forms part of the sedimentary infill of the Bajo Segura Basin (Betic margin of the western Mediterranean). This formation was deposited during a high sea level phase prior to the onset of the Messinian Salinity Crisis. Stratigraphically, it comprises a prograding stack of sandstone lithosomes alternating with marly intervals (1st-order cyclicity). These lithosomes are characterized by a homoclinal geometry that tapers distally, and interfinger with pelagic sediments rich in planktonic and benthic microfauna (Torremendo Formation). An analysis of sedimentary facies of each lithosome reveals a repetitive succession of sandy storm beds (tempestites), which are separated by thin marly layers (2nd-order cyclicity). Each storm bed contains internal erosional surfaces (3rd-order cyclicity) that delimit sets of laminae. Two categories of storm beds have been differentiated. The first one includes layers formed below storm wave base (SWB), characterized by traction structures associated to unidirectional flows. The second category consists of layers deposited above the SWB, which display typical high regime oscillatory flow structures. The 1st-order cyclicity recorded in the La Virgen Formation corresponds to sapropel/homogeneous marl precessional cycles formed in a pelagic basin context (Torremendo Formation).

Benchmark Simulation of Turbulent Flow through a Staggered Tube Bundle to Support CFD as a Reactor Design Tool. Part II: URANS CFD Simulation

In Part II, we described the unsteady flow simulation and proposed a modification of a traditional turbulence flow model. Computational fluid dynamics (CFD) simulations of an isothermal, fully periodic flow across a tube bundle using unsteady Reynolds averaged Navier-Stokes (URANS) equations, with turbulence models such as the Reynolds stress model (RSM) were investigated at a Reynolds number of 1.8 × 104, based on the tube diameter and inlet velocity. As noted in Part I, CFD simulation and experimental results were compared at five positions along (x; y) coordinates. The steady RANS simulation showed that four diverse turbulence models were efficient for predicting the Reynolds stresses, and generally, SRANS results were marginal to poor, using a consistent evaluation terminology. In the URANS simulation, we modeled the turbulent flow field in a manner similar to the approach used for large eddy simulation (LES). The time-dependent URANS results showed that the simulation reproduces the dynamic stability as characterized by transverse oscillatory flow structures in the near-wake region. In particular, the inclusion of terms accounting for the time scales associated with the production range and dissipation rate of turbulence generates unsteady statistics of the mean and fluctuation flow. In spite of this, the model implemented produces better agreement with a benchmark data set and is thus recommended.

Numerical Simulation of Oscillatory Flow in Realistic Model Human Airways

Computational fluid dynamics (CFD) simulations were carried out for an oscillatory flow in a 3D realistic model of the human central airways to reveal the effect of airway geometry on the oscillatory flow structure. This 3D realistic airway model is a computational model of multi-branching airway constructed based on X-ray CT images of actual human airways in which the airway diameter ranged from approx. 2 to 14 mm, and the flow in this airway model was simulated using CFD software (Fluent). The resultant inspiratory flow patterns in this 3D realistic airway model were relatively similar to patterns observed in a simplified planar multi-branching airway model, whereas the expiratory flow patterns strongly depended on the realistic airway geometry and showed more complicated secondary flow patterns. Secondary flow velocities were higher in the realistic airway model than in the simplified airway model in both the inspiratory and expiratory flows. Langrangian fluid particle tracking clarified the convective dispersion due to asymmetric inspiratory and expiratory velocity profiles.

An experimental study of the oscillatory flow structure of tone-producing supersonic impinging jets

An experimental investigation into the structure of a supersonic jet impinging on a large plate is presented. Digital particle image velocimetry (DPIV), shadowgraph photography and acoustic measurements are used to understand the relationship between the unsteady jet structure and the production of tones for nozzle-to-plate spacings between 1 and 5 nozzle exit diameters at a nozzle–pressure ratio equal to 4. Results indicate that the instability of the jet depends on the location of the plate in the shock cell structure of the corresponding free jet and the strength of the standoff shock wave, rather than on the occurrence of recirculation zones in the impingement region. Phase-locked studies show streamwise displacements of the stand-off shock wave, a moving recirculation zone in the subsonic flow in front of the plate, and significant oscillations of both the compression and expansion regions in the peripheral supersonic flow when tones are produced. Sound is shown to be generated by periodic pulsing of the wall jet boundary resulting from periodic motion of the flow in the impingement and near-wall regions of the flow.

Oscillatory Flow Structure and Unidirectional Flow in Model Avian Bifurcation

In the circular channel of avian lungs, there exists unidirectional flow, which is thought to be induced by the mechanism of 'aerodynamic valves'. To clarify flow phenomena in the avian lung, oscillatory flow in a right angle branched tube was experimentally studied. Periodical variation of flow rate was measured with Particle-Imaging velocimetry (PIV). Velocity fields near the bifurcation were visualized with illuminated tracers and measured by PIV technique. The main concern was focused on the separation vortex near the bifurcation and convective inertia, which were found to cause unidirectional net flow from a side-daughter tube to a daughter tube. Based on the experimental observation, a flow model was proposed. It was suggested that the periodical variation of pressure drop induced by the separation vortex near the bifurcation in the inspiratory phase should work as the 'aerodynamic valves', and cause the unidirectional flow. Due to convective inertia, the unidirectional flow persisted even in the expiratory phase, in which no vortex existed near the bifurcation.

Spatial and temporal variation of secondary flow during oscillatory flow in model human central airways.

Axial and secondary velocity profiles were measured in a model human central airway to clarify the oscillatory flow structure during high-frequency oscillation. We used a rigid model of human airways consisting of asymmetrical bifurcations up to third generation. Velocities in each branch of the bifurcations were measured by two-color laser-Doppler velocimeter. The secondary velocity magnitudes and the deflection of axial velocity were dependent not only on the branching angle and curvature ratio of each bifurcation, but also strongly depended on the shape of the path generated by the cascade of branches. Secondary flow velocities were higher in the left bronchus than in the right bronchus. This spatial variation of secondary flow was well correlated with differing gas transport rates between the left and right main bronchus.

Oscillatory Natural Convection of a Liquid Metal in Circular Cylinders

An experimental study is made of natural convection oscillations in gallium melts enclosed by right circular cylinders with differentially heated end walls. Cases heated from below are examined for angles of inclination (φ) ranging from 0 deg (vertical) to 75 deg with aspect ratios Ar (height/diameter) of 2, 3, and 4. Temperature measurements are made along the circumference of the cylinder to detect the oscillations, from which the oscillatory flow structures are inferred. The critical Rayleigh numbers and oscillation frequencies are determined. For Ar=3 and φ = 0 deg, 30 deg the supercritical flow structures are discussed in detail.