Wave Inception Research Articles

A two-scale approach to widen a predictive blast propagation model around a hemicylindrical obstacle

The aim of the present paper was to report on an experimental study of the characterization of a blast wave initiated by a solid explosive and its interaction with a rigid obstacle in the form of a hemicylinder. Pressure transducers located along the path of the blast wave and high-speed imaging allow (1) the measurement of the overpressure at different locations and (2) the characterization of the blast wave inception, propagation, and reflection off the hemicylinder. The scaling effect has been investigated by performing experiments in two different facilities, where one is at twice the scale of the other.

A stall diagnosis method based on entropy feature identification in axial compressors

AbstractA stall diagnosis method based on the entropy feature extraction algorithm is developed in axial compressors. The reliability of the proposed method is determined and a parametric sensitivity analysis is experimentally conducted for two different types of compressor stall diagnoses. A collection of time‐resolved pressure sensors is mounted circumferentially and along the chord direction to measure the dynamic pressure on the casing. Results show that the stall and prestall precursor embedded in the dynamic pressures are identified through nonlinear feature perturbation extraction using the entropy feature extraction algorithm. Further analysis demonstrates that the prestall precursor with the peak entropy value is related to the unsteady tip leakage flow for the spike‐type stall diagnosis. The modal wave inception with increasing amplitude is identified by the considerable increase of the entropy value. The flow field in the tip region indicates that the modal wave corresponds to the flow separation in the suction side of the rotor blade. The warning time is 100–300 rotor revolutions for both types of stall diagnoses, which is beneficial for stall control in different axial compressors. Moreover, a parametric study of the embedding dimension m, similar tolerance n, similar radius r, and data length N in the fuzzy entropy method is conducted to determine the optimal parameter setting for stall diagnosis. The stall warning based on the entropy feature extraction algorithm provides a new stall diagnosis approach in the axial compressor with different stall types. This stall warning can also be adopted as an online stability monitoring index when using the concept of active stall control.

Spike type of stall inception in the tandem rotor

There are two known mechanisms of inception of rotating stall in the compressor, modal wave inception and spike inception. However, the stall inception mechanisms for a tandem-rotor have not been explored so far in detail. The paper explores the unsteady behaviour of the tandem rotor. Unsteady casing pressure measurements are carried out with the help of seven equispaced unsteady pressure sensors. The casing pressure traces are analysed using wavelet transforms, fast Fourier transform, discrete spatial Fourier series, travelling wave energy method and correlation techniques to identify the nature of the stall cells. The changes observed in the stall cells during the transition from the stable mode of operation to the fully stalled mode and back from the stall regime to the stable operating condition are investigated in detail. It is observed that the stall incepts in the form of a low-intensity spike, which later evolves as a long-length scale disturbance in the fully developed stall region. The initial stall disturbance is found to be rotating with 75% of the rotor speed. The speed of the stall cells is reduced to 61% of the rotor speed in a fully developed region. The number and size of stall cells increase as the mass flow is throttled. The number and size of the stall cells are strongly coupled with the leading-edge incidence pattern. During the transition from the stall regime to the stable operating condition, the mass flow of the compressor increases, which reduces the flow incidence angle of the tandem rotor. Thus, the separation region shrinks during this transition leading to a reduction in the size of the stall cells. The stall in the tandem-rotor is primarily associated with the tip leakage flow. The rotating disturbances in the tandem-rotor originate due to the leading-edge flow separation of the forward rotor blade. Leading-edge separation of the aft rotor, which arises due to interaction of the forward rotor tip leakage vortex, is effectively suppressed with the help of the gap-nozzle flow. For a comprehensive unsteady analysis, the different techniques need to be used in combination.

Configurational complexity of nonautonomous discrete one-soliton and rogue waves in Ablowitz-Ladik-Hirota waveguide

We compute the configurational complexity (CC) for discrete soliton and rogue waves traveling along an Ablowitz-Ladik-Hirota (ALH) waveguide and modeled by a discrete nonlinear Schrödinger equation. We show that for a specific range of the soliton transverse direction κ propagating along the parametric time ζ(t), CC reaches an evolving series of global minima. These minima represent maximum compressibility of information in the momentum modes along the Ablowitz-Ladik-Hirota waveguide. Computing the CC for rogue waves as a function of background amplitude modulation μ, we show that it displays two essential features: a maximum representing the optimal value for the rogue wave inception (the “gradient catastrophe”) and saturation representing the rogue wave dispersion into constituent wave modes. We show that saturation is achieved earlier for higher values of modulation amplitude as the discrete rogue wave evolves along time ζ(t).

Periodic jetting and monodisperse jet drops from oblique gas injection

When air is blown in a straw or tube near an air-liquid interface, typically one of two behaviors is observed: a dimple in the liquid's surface, or a frenzy of sputtering bubbles, waves, and spray. Here we report and characterize an intermediate regime that can develop when a confined air jet enters the interface at an angle. This regime is oscillatory with a distinct characteristic frequency and can develop periodic angled jets that can break up into monodisperse aerosols. The underlying mechanisms responsible for this highly periodic regime are not well understood. Here we flow a continuous stream of gas through a tube near a liquid surface, observing both optically and acoustically the deformation of the liquid-air interface as various parameters are systematically adjusted. We show that the Kelvin-Helmholtz instability is responsible for the inception of waves within a cavity formed by the gas. Inertia, gravity, and capillary forces both shape the cavity and govern the frequency and amplitude of these gas-induced cavity waves. The flapping cavity focuses the waves into a series of periodic jets that can break up into droplets following the Rayleigh-Plateau instability. We present scaling arguments to rationalize the fundamental frequencies driving this system, as well as the conditions that bound the periodic regime. These frequencies and conditions compare well with our experimental results.

Numerical study of wave disturbance in liquid cooling film

Transient numerical simulations are carried out to investigate the liquid-gas interface characteristics associated with liquid film cooling flows. A two-dimensional axisymmetric multi-phase numerical model using finite volume formulation is developed. The model has been validated against available experimental data for liquid-film cooling flows inside tubes. The model has been used to predict the interface characteristics for a variety of imposed parameters and momentum flux ratios under cold flow conditions wherein both the coolant and mainstream are maintained at the same temperature. Disturbance waves are observed at the liquid-gas interface for coolant flows above a critical value and after a finite distance from the inlet. The distance toward the wave inception point increased with the increase of momentum flux ratio. However, at higher momentum flux ratios, the properties of the disturbance waves did not vary significantly. The parameters related to the liquid-gas interface waves, namely, wave velocity, frequency, amplitude and wave length have been analyzed in detail. Analysis indicates that the liquid entrainment is due to the shearing of the disturbance wave crest.

Investigation of Secondary Waves Dynamics in Annular Gas–Liquid Flow

Annular gas–liquid flow was investigated using high-speed modification of LIF technique. The evolution of liquid film surface was studied with high spatial and temporal resolution. It was found that all waves may be classified into primary and secondary waves; all the secondary waves arise due to instability of the back fronts of primary waves. The areas of inception and velocity of secondary waves were compared for annular flow with and without entrainment. The similarities and differences of wavy structure in both cases were demonstrated.

Effects of liquid viscosity on inception of disturbance waves and droplets in gas–liquid annular two‐phase flow

AbstractThe structure of gas–liquid two‐phase flow is investigated in order to establish a reliable criterion for the development of disturbance waves and droplets considering the effects of liquid viscosity. The structure of the gas–liquid interface and the flow rate of droplets entrained in gas are measured simultaneously at five kinematic viscosities (1.0, 3.2, 9.9, 30, 70 mm2/s). The time‐series traces of liquid film thickness measured by five holdup probes reveal that the inception of disturbance waves occurs at a liquid Reynolds number of 200 or a non‐dimensional liquid film thickness of 6.5. It is also shown that droplets are generated before the inception of disturbance waves with increasing liquid kinematic viscosity at a liquid velocity of 0.02 to 0.03 m/s. As previously published criteria for the inception of droplets are found to be unsatisfactory, a new critical condition for droplet generation balancing the interfacial shear stress $τi$ with the wave height h and surface tension σ is proposed: $τih/σ=0.025$. This relation describes the action of shear force and surface tension on wave crests, and is notably independent of liquid viscosity. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(8): 529–541, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20176

Effects of Liquid Viscosity on Inception of Disturbance Waves and Droplets in Gas-Liquid Annular Two-Phase Flow

In order to clarify the effects of liquid viscosity on inception of disturbance waves and droplets, structure of gas-liquid interface and flow rates of droplets entrained with gas were measured simultaneously at five kinds of kinematic viscosities, 1.0, 3.2, 9.9, 30 and 70 mm2/s. Time-spatial traces of liquid film thickness measured by five holdup probes were presented and it was found that the inception of disturbance waves occurred at Rel= 200 or at the non-dimensional liquid film thickness of tfm+=6.5. Furthermore, entrainment ratios were measured and it was clarified that droplets occurred before the inception of disturbance waves with increasing liquid kinematic viscosity and the critical condition of the droplet inception was jl=0.02-0.03 m/s. The obtained results were compared with the published criteria on the inception of droplets and the previous correlations were found unsatisfactory. Considering the balance between shear force and surface tention acting on the wave crests, a new criterion of droplet inception was also proposed.

Characteristics of developing free falling films at intermediate Reynolds and high Kapitza numbers

Experiments on developing free falling films have been carried out in a vertical rectangular channel, using smooth inlet conditions, with three liquids characterized by high Kapitza numbers, Ka (i.e. water, 1.5% butanol and 2.5% butanol solutions with surface tension 75, 50 and 40 mN/m, respectively) at intermediate Reynolds numbers ( Re L<400). Film thickness measurements supported by visual observations suggest that reduced surface tension significantly affects the inception of waves at the entrance region of the film. However, no significant qualitative difference is observed concerning developing wave patterns in the longitudinal direction, as large “tear-drop” type waves appear downstream in all cases, with the smaller Ka liquids being more unstable as expected. The new data show that above Re L∼200, the RMS values of film thickness fluctuations, beyond the wave inception region, tend to be nearly independent of Re L. Results concerning other typical falling film characteristics also show that above Re L∼200 (for the Re L range of the present experiments) the wave structure is weakly dependent on Re L beyond the wave inception region. The dominant disturbance frequencies on the wavy film surface as well as the wave celerity are in good agreement with earlier findings. Finally, results from previous linear stability analyses are compared with the new data showing in general satisfactory agreement especially with measurements corresponding to developing nearly two-dimensional waves.

On the modeling of sand wave migration

We describe a simple mathematical model capable of reproducing the main features of sand wave inception and growth. In particular we focus on the prediction of the migration rates that sand waves undergo because of tidal and residual currents. The model adequately predicts migration rates even for the cases of upstream‐propagating sand waves, i.e., for sand waves which migrate in the direction opposite to that of the residual current. We show that upstream/downstream propagation is mainly controlled by the relative strength of the residual current with respect to the amplitude of the quarter‐diurnal tide constituent and by the phase shift between the semi‐diurnal and quarter‐diurnal tide constituents. Therefore, to accurately predict field cases, a detailed knowledge of the direction, strength, and phase of the different tide constituents is required.

The onset of the flexural mode in fluid loaded elastic shells revisited, and its value in object identification in both the frequency and time domain

The onset of the excitation of flexural resonances for fluid loaded evacuated elastic shells produces a striking event. This issue has been interpreted theoretically in 1988 by means of a partial wave decomposition which showed a very narrow-peaked subsonic wave and a broad-peaked weak wave that begins at the speed of sound of the entraining fluid and increases. The narrow peaks are identified with subsonic water-borne waves that resonate in the fluid along the surface of an elastic object, and the broad peaks correspond to the inception of flexural waves. They exist over a small interval in wave number at the point when the flexural wave begins to couple with the fluid. Here we examine the pulse solution. With increasing frequency the partial waves change phase (a 90 degree phase change) and the overlapping flexural waves transition from a partially coherent constructive signal to one that is partially destructive. This leads to an envelope or hump effect, also called mid-frequency enhancement; it is a function of shell thickness as well as material property. We demonstrate how this effect may be employed to identify a submerged elastic shell in either the pulse or frequency solution.

Evaporation waves in flashing processes

An experimental study on the inception and propagation of evaporation waves in adiabatic flashing of Refrigerant 11 has been performed in glass tubes with diameters 32 mm≤d≤252 mm . The formation of evaporation waves was observed at superheats 35 K≤ Δϑ max ≤50 K and at depressurization rates as low as Δ p/Δ t≈1 bar/s. Additional experiments with metallic inserts as starters in the test tubes indicate, that the presence of metal/liquid contacts decreases the necessary superheat for the formation of evaporation waves and increases the propagation velocity of the wave along the metallic surface into the superheated liquid.

419 気液環状二相流における波と液滴の発生に及ぼす液粘性の影響(G.S. 二相流)

419 気液環状二相流における波と液滴の発生に及ぼす液粘性の影響(G.S. 二相流)

Numerical Analysis for Flow Dynamics and Heart Transfer of Wavy Condensate Film.

Wavy condensate films flowing on a vertical wall have been simulated with a finite difference method in which the algorithm is based on the HSMAC method and free boundary conditions are treated with newly proposed method. A random perturbation given at the leading edge quickly diminishes once, and propagates downstream as small-amplitude long waves. Then the amplitude of the wave increases rapidly at a certain position. The wave shape changes from a sinusoidal wave to a solitary wave which is composed of a big wave and capilary waves. A large circulation flow occurs in the big wave and temperature contour lines are distorted by the circulation flow. The heat transfer is enhanced by space-time variation of the film thickness and convection effects. The higher enhancement rate is obtained for higher Prandtl number. In the present simulation condition, the wave inception occurs at lower film Reynolds number region for higher Prandtl number.

Simulation of noise‐driven wave dynamics on a falling film

AbstractA numerical model developed simulates inlet noise‐driven wave dynamics on a falling film at relatively high Reynolds numbers. Two parameters, a normalized Reynolds number and a noise index, are sufficient to specify the wave statistics on most channels. Observed phenomena, like wave inception, downstream wave texture coarsening, initial deceleration and subsequent acceleration of wave speeds, are quantitatively reproduced and explained. Statistical analysis from our simulations suggests that beyond a critical Reynolds number this complex noise‐driven spatio‐temporal dynamics can be modeled by a deterministic interaction theory based on stable solitary waves that resemble onehump pulses and a statistical theory with a random‐phase description of noise. The “chaotic” wave dynamics at higher Reynolds number is hence due to both noise amplification/filtering and intrinsic dynamics.

Suppression of wind‐generated ripples by natural films: A laboratory study

The suppression of wind‐generated ripples by natural films was investigated in an oval‐shaped wind‐wave tank filled with seawater. Experiments were performed over the water surface with and without natural films; the former were produced through aging and the latter was assured through overflowing. Time series of the water surface slope were obtained with a computer‐interfaced matrix camera system to determine the slope spectra. The inception of waves by wind was much delayed over the slick surface, while at higher winds there were insignificant differences between the mean square slopes over clean and slick surfaces. At low winds, suppression of ripples by natural films was observed from the intrinsic wave frequency of about 7 Hz and up, with the maximum suppression at about 19 Hz. No obvious spectral dip was observed to account for the Marangoni effect observed by others over artificial slick surfaces.

Methods of deterministic chaos applied to the flow of thin wavy films

AbstractThe structure of thin, wavy falling films was studied to evaluate whether the random‐appearing wave structure is a result of deterministic chaos or a purely stochastic process. The time‐varying film thickness was obtained at different spatial locations near the point of wave inception for flow rates in the range of Re=3–10. Under all conditions the wave structure was aperiodic in nature and displayed none of the known transitions to chaos. However, the power spectra followed an exponential decay law at high frequencies that is characteristic of chaotic systems. The estimated attractor dimension, used to characterize the complexity of a chaotic system, was much higher than those of known model chaotic systems. It is demonstrated that these high values could be explained due to small levels of noise present in experimental situations. Since experimental data are seldom noise free, a basic limitation in applying these methods to experimental measurements is demonstrated.

Condensation of refrigerants on horizontal tubes-characteristics of vapor shear controlled regime.

Heat transfer measurements and flow visualization studies are reported for condensation of R 113 at near atmospheric pressure on single horizontal tubes of 8.0 mm and 19.0 mm in outer diameter with vertical vapor downflow. The vapor ranged from 0.34 m/s to 15.6m/s, and the condensation temperature difference from 3.1 K to 32.3 K. The slow of condensate was visualized by injecting a dye tracer into the condensate film. The film showed the inception of waves and the change of wave pattern with the increase of vapor velocity. In the three dimensional wave regime the film showed an abrupt change of thickness at an angular position somewhat lower than the tube axis. The heat transfer results are compared with available experimental data and with theory. Empirical equations for the average heat transfer coefficient based on the equivalent Reynolds number concept are proposed for the vapor shear controlled regime.

Stability of the flow during sedimentation in inclined channels

The conditions for wave formation and growth during sedimentation in inclined channels is investigated using a linear stability analysis in the limit of large sedimentation Grashof number with the Reynolds number remaining O(1). An asymptotic solution for long waves reveals that the critical Reynolds number is proportional to the tangent of the angle of inclination of the channel times the ratio of the thickness of the thin pure-fluid layer, which forms beneath the downward-facing surface of the channel, to the width of the channel. Hence, the critical Reynolds number is very small unless the channel is extremely long and narrow. Numerical calculations indicate, however, that the maximum amplification rates are often so small that the disturbances may be convected out of the channel before they reach an observable size. This conclusion is consistent with experiments performed in our laboratory, and the observed wave inception point and the calculated maximum amplification rates exhibit remarkably similar dependence on the angle of inclination and the suspension concentration.