Intermittent Region Research Articles

Characteristics of the turbulent/nonturbulent interface in boundary layers, jets and shear-free turbulence

The characteristics of turbulent/nonturbulent interfaces (TNTI) from boundary layers, jets and shear-free turbulence are compared using direct numerical simulations. The TNTI location is detected by assessing the volume of turbulent flow as function of the vorticity magnitude and is shown to be equivalent to other procedures using a scalar field. Vorticity maps show that the boundary layer contains a larger range of scales at the interface than in jets and shear-free turbulence where the change in vorticity characteristics across the TNTI is much more dramatic. The intermittency parameter shows that the extent of the intermittency region for jets and boundary layers is similar and is much bigger than in shear-free turbulence, and can be used to compute the vorticity threshold defining the TNTI location. The statistics of the vorticity jump across the TNTI exhibit the imprint of a large range of scales, from the Kolmogorov micro-scale to scales much bigger than the Taylor scale. Finally, it is shown that contrary to the classical view, the low-vorticity spots inside the jet are statistically similar to isotropic turbulence, suggesting that engulfing pockets simply do not exist in jets.

Non-dimensional correlations on flame height and axial temperature profile of a buoyant turbulent line-source jet fire plume

This article investigates flame height and axial temperature profile of a buoyant turbulent line-source jet fire plume. Previous correlations have been mainly for axi-symmetrical fire sources or linear pool-type (no initial momentum) fire sources. Experiments were carried out for this study using a 3 mm (width) × 95 mm (length) line-source nozzle with propane as the fuel. Flame heights and axial temperature profiles were measured for different heat release rates. It was found that the flame heights can be well correlated by flame Froude number with a 2/3 power function based on scaling analysis. A global non-dimensional four-regions correlation (continuous flame region, intermittent flame region, line-plume region, and axi-symmetric-plume region) is proposed to characterize the axial temperature profile of a line-source jet fire plume.

The ballistic resistance of thin aluminium plates with varying degrees of fixity along the circumference

The ballistic performance of thin aluminium targets and influence thereon of different circumferential fixity conditions were studied both experimentally and by finite element simulations. A pressure gun was employed to carry out the experiments while the numerical simulations were performed on ABAQUS/Explicit finite element code using Johnson–Cook elasto-viscoplastic material model. 1 mm thick 1100-H12 aluminium plates of free span diameter 255 mm were normally impacted by 19 mm diameter ogive and blunt nosed projectiles. The boundary conditions of the plate were varied by varying the region of fixity along its circumference as 100%, 75%, 50% and 25% in experiments and the numerical simulations. Further, simulations were carried out to compare the response of the plates with 50% and 75% continuous fixity with those with two and three symmetrical intermittent regions of 25% fixity respectively.The variation in the boundary condition has been found to have insignificant influence on the failure mode of the target however; it significantly affected the mechanics of target deformation and its energy absorption capacity. The ballistic limit increased with decrease in the region of fixity. It decreased for intermittent fixity in comparison with equivalent continuous fixity. And, it has been found to be higher for the impact with projectile having blunt nose in comparison with the one having ogive nose.

J0570103 乱流拡散火炎のエントレインメント特性に及ぼす火炎構造の影響([J057-01]乱流における運動量,熱,物質の輸送現象(1))

Large-eddy simulations of a fire plume with flames and a buoyant plume without flames were performed. The effects of flame structure on entrainment characteristics in the flame region of a plume were then examined through comparison between the two numerical results. The effects of flame structures on plume characteristics were remarkable in the flame and intermittent regions in a plume. The existence of a flame increased the mean and variance of the entrainment coefficient, which corresponds to entrainment efficiency, and the frequency of high entrainment coefficient. Moreover, a characteristic of a fire plume which rarely forms wide high-temperature regions in the vicinity of the fire source was suggested to increase the frequency of highly entraining motions.

Experimental study on Jet-A pool fire at high altitude

For the further assessment on cargo fire hazard, full-scale Jet-A pool fire tests under the specifications of FAA Minimum performance standard (MPS) were conducted in sea-level Hefei and high-altitude Lhasa. A square insulation board (1.2m×1.2m) was place upon the fuel pan to simulate the effect of ceiling on fire plume. The experimental results indicates that the mass burning rate is proportional to the 2/3 power of the ambient pressure, in accordance with the theoretical prediction for convection-dominant fires. The analysis shows that the flame height increases slightly at high altitude. At higher altitude, the centerline temperature decreases in the flame region but increases in the intermittent region. The addition of ceiling increases the mass burning rate and the centerline temperature, although the effect on centerline temperature is observed to be weaker in Lhasa.

Simple Method to Predict Downward Heat Flux from Flame to Floor

This work presents a simple model to predict the radiation heat flux from a flame to a floor surrounding it. This heat flux was measured both in an unconfined space (open air) and under a ceiling. Flame lengths, flame temperatures, and ceiling surface temperatures, all of which are necessary to predict radiation heat flux, were also measured. Flame shapes were modeled by two cylinders and two disks representing the impinging flame's continuous and intermittent flame regions. The emissivity of the cylinders was calculated from the heat balance at the flame surface, and the radiation heat flux to the floor was predicted well by the model.

Experimental Investigation of Unsteady Transition on a Pitching Rotor Blade Airfoil

The unsteady flow around a pitching helicopter main rotor blade airfoil, the EDI-M109, was experimentally investigated at conditions similar to those existing on a retreating rotor blade in forward flight. High-speed pressure measurements and hot film anemometry were used to investigate the unsteady transition characteristics of the airfoil. Results are presented for dynamic test points with attached flow, light dynamic stall, and deep dynamic stall at M = 0.3, Re = 1.8 × 106, and f = 6.6 Hz. The results include the discussion of the periodicity of the hot film signals for different flow states. The transition process of the pitching airfoil is analyzed, and the significance of the intermittent region is described. A time delay between the transition and the model motion is discussed, and a linear relationship between the transition position and time is observed. The influences of the pitching amplitude on the transition characteristics are discussed, and the flow separation initiating dynamic stall is analyzed.

Hysteresis, Switching and Anomalous Behaviour of a Quartz Tuning Fork in Superfluid 4He

We have been studying the behaviour of commercial quartz tuning forks immersed in superfluid 4He and driven at resonance. For one of the forks we have observed hysteresis and switching between linear and non-linear damping regimes at temperatures below 10 mK. We associate linear damping with pure potential flow around the prongs of the fork, and non-linear damping with the production of vortex lines in a turbulent regime. At appropriate prong velocities, we have observed metastability of both the linear and the turbulent flow states, and a region of intermittency where the flow switched back and forth between each state. For the same fork, we have also observed anomalous behaviour in the linear regime, with large excursions in both damping, resonant frequency, and the tip velocity as a function of driving force.

First Study of Intersubband Absorption in Electrons on Helium under Quantizing Magnetic Fields

We present the first measurements of inter-subband absorption of microwaves in surface electrons on liquid helium subjected to perpendicular magnetic field \(B\). In quantizing \(B\), the power absorption shows intermittent regions of enhanced and suppressed absorption. This behavior is caused by strong variation of the excited-electron decay rate with \(B\). Particularly, fast decay due to elastic scattering provides condition for strong absorption and overheating of the electron system, while slow decay due to inelastic scattering limits absorption and causes its saturation. An unexpected feature is the strong suppression of absorption at magnetic fields where the inter-subband energy splitting is a multiple number of the cyclotron energy.

Low-dimensional dynamical system for Rayleigh-Bénard convection subjected to magnetic field

We have numerically investigated the dynamical behavior of Rayleigh-Bénard (RB) convection in an incompressible conducting fluid subjected to a magnetic field by solving a low-dimensional dynamical system. Its dynamical properties are quantified by nonlinear time series analysis based on chaos theory. The stretching and folding in the phase space for the chaos region (normalized Rayleigh number r = 28) and the intermittent chaos region (r = 166.1) of RB convection at a high magnetic Prandtl number of Pm = 10 become complex with increasing applied magnetic field, and the degeneration of chaos is induced by the limit of the strong magnetic field owing to the overwhelming Lorentz force compared with the buoyancy. The results obtained in this study show the importance of the magnetic Prandtl number to the dynamical behavior of RB convection subjected to a magnetic field.

High-Reynolds-number turbulence in complex fluids

We here examine the structure of turbulence in the case of a complex fluid made up of water and surfactants. This fluid has the particular property of shear thickening when driven at shear rates above a certain threshold. Through a study of the spectral properties and the structure function scalings, important differences arise with respect to the reference case, i.e., water. The surfactant solution shows strong intermittency at small scales. The large scales are, on the other hand, free of intermittency. While this transition is observed in the structure function scalings, no sign of this transition is seen in the power spectrum of velocity fluctuations which shows a single scaling range. The strongly intermittent small-scale region, despite the scaling of the power spectrum, exhibits properties reminiscent of the near-dissipative range.

Shockwave/Boundary-Layer Interaction Control on a Compression Ramp Using Steady Micro Jets

An experimental investigation was conducted to control the amplitude of shock unsteadiness associated with a 24 deg compression-ramp-induced interaction in a Mach 2 flow.Two control configurations in the form of an array of 1) 16 90-deg-pitched steady micro-air-jet vortex-generating devices (AJVG1), and 2) eight pairs of 45-deg-pitched steady micro-air-jet vortex-generating devices (AJVG2) were studied. EachAJVGdevice was placed upstream of the interaction region at 12:5� from the compression corner. Both micro-AJVG configurations show a reduction in separation shock strength and help considerably reduce the height of the lambda-wave triple point with increase in Poj . Pitching the microjets at 45 deg, as in the AJVG2 configuration, prevents a stronger control-generated bow shock to form ahead of the injectors and, hence, reduces the obstruction component of the interaction significantly. A well defined separation line for no control is seen to get replaced by a highly corrugated separation line with control. Significant reduction (up to 67%) in the peak rms value is observed in the intermittent region of separation with AJVG1 for Poj � 208:5 kPa while the same is achieved with AJVG2 at a much higher Poj�>500 kPa�. The spectral content of the pressure fluctuations also indicate that, relative to the AJVG2 configuration, AJVG1 is successful in reducing the amplitude of fluctuations in the range of unsteadiness by an order of magnitude as soon as Poj exceeds 208.5 kPa. The amplitude of these fluctuations is seen to further decrease with increase in Poj.

Thermal performance improvement with scale imprints over boiling surface of two-phase loop thermosyphon at sub-atmospheric conditions

Thermal performances of two-phase loop thermosyphons (TPLT) with smooth and scale-roughened boiling surfaces are compared by examining their boiling flow structures, boiling heat transfer properties with the associated instability phenomena, thermodynamic cycles, boiling flow maps and networks of thermal resistances along the flow pathway. With cooling applications to electronic chipsets which limit the range of operating temperatures for these TPLTs, the working fluid of treated water has to circulate at sub-atmospheric pressures. With each tested TPLT, the flow visualization and thermal performance tests are individually performed at the identical test conditions controlled by boiling heater power (Q) and condenser thermal resistance (Rth,con) to image the boiling flow structures and to measure the boiling heat transfer rates, phase-change pressures in evaporator and condenser and the loop-wise temperature distributions, respectively. The differential Q and Rth,con impacts on boiling flow structures and thermal performances between the TPLTs with smooth and scaled boiling surfaces are illustrated using a set of comparative results collected from these TPLTs. With the scaled boiling surface over evaporator at the filling ratio (FR) of 50%, the Q-driven transition of boiling flow structures still follows the smooth-walled TPLT route; but the increase of nucleation sites and the improved bubble departure from the scaled surface elevate the boiling heat transfer rates. Particularly, the segmentation of Taylor bubbles by the scale imprints considerably suppresses the oscillatory amplitudes of liquid level and temperatures in evaporator; and thereof enhances thermal stabilities to reduce the constituent thermal resistance triggered by boiling instabilities. Acting together by the suppression of boiling instabilities and the improved boiling heat transfer properties, the cooling powers required for the scaled TPLT are noticeably reduced from the smooth-walled TPLT counterparts at the conditions with similar overall thermal resistances. Heat transfer correlations determining the boiling heat transfer coefficients over pool-boiling, intermittent and vapor regions for the scale-roughened evaporator along with the empirical correlations that permit the evaluation of individual and interdependent Q and Rth,con effects on the overall thermal resistances and evaporator pressures of the scaled TPLT are generated to assist design activities.

Simultaneous measurement of velocity and pressure in a plane jet

A new technique was developed for the simultaneous measurement of velocity and pressure in turbulent flows. To accomplish this objective, a new probe (hereafter called the combined probe) that consists of an X-type hot-wire probe and a newly devised pressure probe was developed. The pressure probe was miniaturized by the MEMS fabrication process and by using a 0.1-in. microphone as a pressure sensor for improving the spatial resolution. This pressure probe was placed between two hot-wire sensors of which the X-type hot-wire probe was composed. The pressure probe was given a hemispherical tip, like that of a pitot tube, because an earlier pressure probe with a conical tip suffered from a reduction in spatial resolution. The spatial arrangement of the pressure probe and the hot-wire probe for the combined probe was carefully determined, because there was a risk that the measurement accuracy of one probe will be influenced by disturbances caused by the other probe when the two probes were placed very close to each other. Therefore, the combined probe was arranged to engender no noticeable interference between the velocity data and the pressure data measured by their respective probes. As one application of this combined probe, simultaneous measurements of pressure and two components of instantaneous velocity were performed in a plane jet. The turbulent energy budget and the cross-correlation coefficient of velocity and pressure in the intermittent region of the plane jet were estimated. The results show that the mean streamwise velocity, velocity fluctuation, and pressure fluctuation profiles were consistent with those measured individually using the X-type hot-wire probe or pressure probe. Moreover, it was shown that the integral value of the diffusion term (which should theoretically be equal to zero) in the turbulent energy transport equation was closer to zero than previous reports (Bradbury in J Fluid Mech 23(Part 1):31–64, 1965). In addition, the time variation of the cross-correlation coefficient in the intermittent region supports the vortex structure model predicted in previous studies (Browne et al. in J Fluid Mech 149:355–373, 1984; Tanaka et al. JSME Int J Ser B 49(4):899–905, 2006; Sakai et al. J Fluid Sci Technol 2(3):611–622, 2007).

Control of unsteadiness of a shock wave/turbulent boundary layer interaction by using a pulsed-plasma-jet actuator

A pulsed-plasma jet actuator is used to control the unsteady motion of the separation shock of a shock wave/boundary layer interaction formed by a compression ramp in a Mach 3 flow. The actuator is based on a plasma-generated synthetic jet and is configured as an array of three jets that can be injected normal to the cross-flow, pitched, or pitched and skewed. The typical peak jet exit velocity of the actuators is about 300 m/s and the pulsing frequencies are a few kilohertz. A study of the interaction between the pulsed-plasma jets and the shock/boundary layer interaction was performed in a time-resolved manner using 10 kHz schlieren imaging. When the actuator, pulsed at StL ≈ 0.04 (f = 2 kHz), was injected into the upstream boundary layer, the separation shock responded to the plasma jet by executing a rapid upstream motion followed by a gradual downstream recovery motion. Schlieren movies of the interaction showed that the separation shock unsteadiness was locked to the pulsing frequency of the actuator, with amplitude of about one boundary layer thickness. Wall-pressure measurements made under the intermittent region showed about a 30% decrease in the overall magnitude of the pressure fluctuations in the low-frequency band associated with unsteady large-scale motion of the separated flow. Furthermore, by increasing the pulsing frequency to 3.3 kHz, the amplitude of the separation shock oscillation was reduced to less than half the boundary layer thickness. Investigation into the effect of the actuator location on the shock wave/boundary layer interaction (SWBLI) showed qualitatively and quantitatively that the actuator placed upstream of the separation shock caused significant modification to the SWBLI unsteadiness, whereas injection from inside the separation bubble did not cause a noticeable effect.

Control of shock unsteadiness in shock boundary-layer interaction on a compression corner using mechanical vortex generators

An experimental study was conducted to control the unsteadiness of separation shock in a Mach 2 24° compression ramp-induced interaction using mechanical vortex generators (VG). Control devices in the form of an array of single-row delta-ramps were placed upstream of the interaction region and tested for two streamwise locations with respect to the boundary layer thickness (δ) at the interaction location and height ‘h’ of the delta-ramps, i.e., at 27.5δ or h/δ = 0.65 and at 12.5δ or h/δ = 0.26, respectively. Surface oil study revealed traces of streamwise counter-rotating vortex pairs generated downstream of these devices. Measurements using pressure-sensitive paint also showed a spanwise sinusoidal pattern of wall pressure variation indicating generation of streamwise vortices from these control devices. These vortices, on interaction with the reverse flow in the separation bubble, replaced a well-defined separation line (for no control) by a highly corrugated separation line. In the region of separation, the mean pressure distribution gets modified while the peak rms value in the intermittent region of separation showed significant changes. Additionally, the spanwise spacing ‘s’ of the vertex of the delta ramps seemed to be an important parameter in controlling the peak rms value. A decrease in this spacing, i.e., VG1 with s = 0, significantly reduced the peak rms value (by 50 and 35 %) while an increase in the spacing, i.e., VG2 with s = 1 mm, consistently showed an increase (by 12 and 30 %) in the separation shock unsteadiness relative to no control, irrespective of their placement location (of h/δ = 0.65 and 0.26, respectively).

Characterization of gas–solid flow in conical spouted beds operating with heavy particles

This study presents the results of an experimental investigation of the hydrodynamics of conical spouted beds operating with high density particles. This type of spouted beds is frequently encountered in chemical vapor deposition coating of nuclear fuel elements. Measurements were performed in three 15cm ID full circular (γ=30°, 45°, 60°) and one half circular conical spouted (γ=30°) bed with yttria-stabilized zirconia particles (dp=0.5, 1mm; ρp=6050kg/m3). For the complete characterization of the hydrodynamic regimes, simultaneous high speed camera and bed pressure drop measurements were carried out in a half circular conical spouted bed to visualize the gas–solid flow patterns and match them with the corresponding bed pressure drop values and its spectral characteristics. The results show that the minimum spouting velocity increases with cone angle, particle diameter and static bed height. The average bed pressure drop decreases with cone angle. Minimum spouting velocity values obtained from full and half bed experiments resulted in a maximum 15% difference. The spectral analyses of the bed pressure drop indicated a dominant frequency of 12Hz in the stable spouting region. After the initiation of external spouting, an unstable intermittent spouting region which extends up to approximately 1.2Ums has been identified. The results of this work can be successfully used in the hydrodynamic design of spouted bed nuclear fuel coaters.

Shock-Wave Boundary-Layer Interaction Study on a Compression Corner Using Pressure-Sensitive Paint

Experiments have been performed to study a compression corner induced turbulent boundary layer interactions in a freestream Mach number of 2.05 without and with control. Two configurations of vortex generators (VG), in the form of an array of deltaramps placed upstream of the interaction region at 27.5δ or h/δ = 0.65, have been tested to control the interaction. Pressure sensitive paint (PSP) measurements were made using a binary paint developed in-house. Reasonably good agreement of PSP results with mean static pressure measurements has been observed. The spanwise wall pressure distribution immediately downstream of the control devices show a sinusoidal pattern indicating the generation of streamwise vortices from VG devices. The interaction of these vortices with the reverse flow in the separated region replaced a well-defined separation line for no control by a highly corrugated separation line. Relative to the no-control case, the peak rms value in the intermittent region of separation with control sh...

Sub-atmospheric boiling heat transfer and thermal performance of two-phase loop thermosyphon

This experimental study investigates the thermal performances of a two phase loop thermosyphon (TPLT) by examining the boiling heat transfer and instabilities, the thermodynamic cycles and the constituent and overall thermal resistances with the aid of boiling flow structures collected from visualization tests. With water as the working fluid, the phase-change pressures in this TPLT at the temperature range typical for electronic cooling applications are sub-atmospheric. Followed by increasing the boiling heat flux at sub-atmospheric pressures, the transition of boiling structures from intermittent Taylor bubble to continuous bubbly flows reverses the transition route at positive pressures and triggers two different types of boiling instabilities. A set of selected results illustrates the interdependent impacts of boiling heater power (Q) and condenser thermal resistance (Rth,con) on each thermal property investigated. Three sets of heat transfer correlations for determining the boiling heat transfer coefficients over pool-boiling, intermittent and vapor regions of the evaporator along with two sets of empirical correlations that permit the evaluation of individual and interdependent Q and Rth,con effects on overall thermal resistances and evaporator pressures of the tested TPLT are generated. The applicable conditions for these empirical correlations considerably extend the lower end of pressure range available in the open literature, which add design capabilities for two-phase heat transfer devices operating at sub-atmospheric pressures.

Simultaneous Measurement of the Velocity and Pressure in a Plane Jet (Estimation of Conditional Averaged Value and Diffusion Term in the Intermittent Region)

The simultaneous measurement of the velocity and pressure in a plane jet has been performed by using a combined probe that consists of an X-type hot-wire probe and a static pressure probe. Firstly, the conditional ensemble averaged statics are investigated on the basis of the intermittency function obtained from the velocity signal by the hot-wire sensor set in the intermittent region of a plane jet. Secondly, the validity of the models for a turbulent and pressure diffusion term is investigated. The measurement results show that the cross-streamwise profile of the diffusion term in the turbulent energy transport equation differs significantly, according to the state of the flow (turbulent or non-turbulent flow) in the intermittent region. By decomposing the diffusion term into two terms, i.e., the turbulent diffusion term and the pressure diffusion term and estimating each term separately, it is found that the cross-streamwise profile of the pressure diffusion term is virtually the same, irrespective of the state of the flow in the intermittent region. This indicates that the difference in the cross-steramwise profile of the diffusion term is cause by the difference in the turbulent diffusion term. In addition, the evaluation of the two kinds of gradient diffusion models of the Reynolds stress transport equation shows that the model constants become constant and the validity of these models can be confirmed except for the region near the jet centerline and intermittent region. Finally, it is shown that both slow-part and rapid-part of the velocity-pressure correlation term must be modeled appropriately to predict the velocity-pressure correlation correctly.