Side Wall Of Duct Research Articles

Low frequency sound insulation of membrane-type acoustic metamaterials with negative pressure cavity

The membrane-type acoustic metamaterial with negative pressure cavity (MAM-NPC) is designed to achieve low frequency sound insulation and tunable frequency on the duct side wall. A one-dimensional analytical method was used to discuss the sound insulation effect of MAM-NPC on the sidewall of the duct. As the negative pressure increases, the first valley of sound insulation moves to high frequency. High sound insulation can be achieved in the low frequency range. The theoretical and finite element method results are compared. The one-dimensional analysis method basically agrees with the finite element results in the low frequency band. Because only plane waves are considered in the one-dimensional analytical method.

Interfacial Dynamics of Miscible Displacement of Shear-Thinning Fluid in a Vertical Channel

The displacement of a shear-thinning fluid by a denser and less viscous Newtonian fluid in a vertical duct is investigated using experiments and numerical simulations. We study how shear-thinning and increased viscosity contrast between the fluids affect the displacement. Our results show that the degree of shear-thinning significantly influences the development of interfacial patterns and the growth of perturbations. In the weakly shear-thinning regime, the displacement progresses as a stable displacement with no visible instabilities. Increasing the viscosity of the displaced fluids result in a Saffman–Taylor type instability with several finger-shaped channels carved across the width of the duct. In the strongly shear-thinning regime, a unique viscous finger with an uneven interface is formed in the middle of the displaced fluid. This finger eventually breaks through at the outlet, leaving behind considerably stagnant wall layers at the duct side walls. We link the onset of viscous fingering instability to the viscosity contrast between the fluids, and the stabilizing density difference, as expressed through a modified, unperturbed pressure gradient for the two fluids. Numerical simulations are performed with both an initial flat interface, and with a perturbed interface, and we find good qualitative agreement between experimental observations and computations.

Multi-modal thermoacoustic instability suppression via locally resonant and Bragg bandgaps.

Thermoacoustic instability is a common occurrence in combustors, yielding self-sustained oscillations and causing potential risk, such as severe structural damage. In this paper, modal instability suppression inside a duct is studied using periodically arranged membranes within the framework of a linear heat release n-τ model embedded into a fully coupled energy-based model. The periodic arrangement of the membranes along the duct sidewall enables locally resonant and Bragg scattering bandgaps, shown to be conducive for the stabilization of unstable thermoacoustic modes. Eigen-modes are classified into different groups, which call for specific control actions in relation with the bandgap frequencies. While multi-modal instability control of low-order modes can be achieved through the tuning of the resonant bandgaps, the densely packed modal cluster, regrouping modes featuring similar mode shapes, requires proper adjustment of the flame position for avoiding modal instability. Compared with the Bragg bandgaps, locally resonant bandgaps, which should be formed near the unstable modes even without stringent periodicity, are shown to play a decisive role in the control process. Meanwhile, strict periodicity is not necessary for the proposed control strategy, showing the practicability of the proposed control strategy. The study shows a promising route to achieve simultaneous suppression of multi-modal instability.

S2326 Grasping for Straws: A Rare and Challenging Complication of Biliary Stenting

INTRODUCTION: Proximal (cephalad) migration of a biliary stent (PMBS) may be associated with severe complications. Prompt recognition and retrieval is important, but may be quite difficult. We present a challenging case of proximal migration of a plastic CBD stent, highlighting techniques used for successful retrieval. CASE DESCRIPTION/METHODS: A 73-year-old F with an indeterminate proximal CBD and common hepatic duct (CHD) stricture undergoing biliary endotherapy presented for a scheduled ERCP. Due to inadequate drainage via retrograde approach, a percutaneous biliary drain (PBD) was placed 9 months ago and patient had undergone multiple biliary drain exchanges and one liver biopsy. Surgical resection was not pursued due to underlying comorbidities. Two weeks prior to presentation, ERCP for internalization of PBD revealed that a previously placed 7Fr x 12cm straight plastic stent had migrated proximally within the anterior RHD (Figure 1). Attempts to retrieve the migrated stent with balloon and snare were unsuccessful, so a 10Fr x 10cm stent was placed and PBD was pulled. On repeat ERCP, direct visualization with Spyglass cholangioscopy revealed the stent to be embedded within the bile duct sidewall at the stricture level (Figure 2). Multiple tools, including balloon catheter, snare and pediatric biopsy forceps failed to remove the stent. Even the cholangioscope biopsy forceps could not grasp stent sufficiently to enable traction and removal. After multiple failed attempts, the stent was successfully retrieved using a small rat tooth forceps via direct grasping of its distal end within the CHD under fluoroscopy. Repeat cholangiogram showed stable findings and no perforation and a 10Fr x 12cm straight plastic stent was replaced. DISCUSSION: PMBS is rare and endoscopic retrieval can be challenging. Risk factors of PMBS include malignant stricture, large stent diameter and shorter stent length. Retrieval of PMBS is critical to avoid complications such as hepatic/ductal perforation and fistula. Endoscopic retrieval methods include Soehendra retriever, basket, lasso technique, balloon and forceps. Cholangioscopy can also be used but was not helpful in this case. Fluoroscopic positioning of rat tooth forceps enabled stent retrieval in our case. We postulate that multiple biliary drain exchanges may have caused proximal migration of the biliary stent. Knowledge of the various tools and techniques to retrieve proximally migrated biliary stents is essential to successfully manage these technically challenging cases.Figure 1.: Fluoroscopic visualization of proximally migrated straight biliary stent appearing as an upside down “V”.Figure 2.: Direct visualization of the embedded distal end of the proximally migrated straight biliary stent using Spyglass cholangioscopy.

Direct numerical simulation of turbulent flow through a ribbed square duct

Abstract

Flow dynamics in longitudinally grooved duct

Flow in a finite-width rectangular duct with a corrugated top-bottom wall has been studied. The primary goal is to establish geometries that allow early flow destabilization at a possibly low drag increase. The flow is assumed periodic in the streamwise direction and bounded by the duct sidewalls in the spanwise direction; the top and bottom wall corrugations have a form of sinusoidal waves oriented transversely to the flow and form longitudinal grooves; i.e., the lines of constant elevation (or phase) are parallel to the direction of the flow. The analysis is performed up to the Reynolds numbers resulting in the formation of secondary states. The first part of the analysis is focused on the properties of the two-dimensional base flow. Mainly, the dependence of hydraulic losses and drag reducing properties on duct’s geometry is characterized. The second part of the analysis discusses the onset of the three-dimensional travelling wave instability over a wide spectrum of geometric configurations. Linear stability is investigated by means of the direct numerical simulation of the Navier-Stokes equations. Critical conditions for the onset of instabilities at a range of geometric parameters are determined. Finally, the nonlinear saturation of unstable modes and the resulting secondary flows are examined. We have shown that in the state resulting from the nonlinear saturation of the disturbance, the flow becomes more complex while the drag reducing properties of the base flow can be maintained.

Heat transfer enhancement using rectangular vortex promoters in confined quasi-two-dimensional magnetohydrodynamic flows

Heat transfer efficiency from a duct side-wall in which an electrically conducting fluid flows under the influence of a transverse magnetic field is investigated. A quasi-two-dimensional magnetohydrodynamic model is employed to model the flow using high-resolution numerical simulation. The gap height and angle of attack of a rectangular cylinder, with aspect ratio α=1/2 and blockage ratio β=1/4, are independently varied to establish relationships between obstacle configuration and heat transfer efficiency. The heat transfer efficiency is measured through an efficiency index given by the ratio of heat transfer enhancement to pressure drop penalty in comparison to an empty duct case. At gap height ratios 1.15⩽G/Lc<2 for an upright cylinder above a heated lower wall, thermal enhancement and efficiency can be improved; with a peak thermal efficiency of η=1.6 occurring at G/Lc=1.5. Additional increases in thermal efficiency for an obstacle at the duct centre-line (G/Lc=2) can be achieved through inclining the cylinder at γ=-7.5°,γ=-37.5° and 0°<γ⩽22.5°. However, these configurations offered no improvement over simply offsetting an upright cylinder at a gap height ratio of G/Lc=1.5. For a cylinder offset at G/Lc=1.5, varying the incidence angle through -37.5°<γ⩽22.5°,-7.5°⩽γ<0° and 0°<γ⩽15° can lead to additional thermal efficiency benefits; with a global peak efficiency of η=1.7 occurring at γ=-37.5°. The streamwise distribution of the local time-averaged Nusselt number and the effect of Hartmann dampening for 100⩽Ha⩽2000 on heat transfer and flow dynamics are also investigated. A net power balance analysis reveals that in fusion applications the heat transfer enhancement dominates over the pumping power cost to produce net benefits for even modest heat transfer enhancement.

Localized turbulence structures in transitional rectangular-duct flow

Direct numerical simulations of transitional flow in a rectangular duct of cross-sectional aspect ratio $A\equiv s/h=1$–9 ($s$ and $h$ being the duct half-span and half-height, respectively) have been performed in the Reynolds number range $\mathit{Re}\equiv u_{b}h/{\it\nu}=650$–1500 ($u_{b}$ and ${\it\nu}$ being the bulk velocity and the kinematic viscosity, respectively) in order to investigate the dependence on the aspect ratio of spatially localized turbulence structures. It was observed that the lowest Reynolds number $\mathit{Re}_{T}$, estimated in a specific way, for localized (transiently sustaining) turbulence decreases monotonically from $\mathit{Re}_{T}=730$ for $A=1$ (square duct) with increasing aspect ratio, and for $A=5$ it nearly attains a minimal value $\mathit{Re}_{T}\approx 670$ that is consistent with the onset Reynolds number of turbulent spots in a plane channel ($A=\infty$). Turbulent states consist of localized structures that undergo a fundamental change around $A=4$. At $\mathit{Re}=\mathit{Re}_{T}$ turbulence for $A=1$–$3$ is streamwise-localized similar to turbulent puffs in pipe flow, while for $A=5$–9 turbulence at $\mathit{Re}=\mathit{Re}_{T}$ is also localized in the spanwise direction, similar to turbulent spots in plane channel flow. This structural change in turbulent states at $\mathit{Re}=\mathit{Re}_{T}$ is attributed to the exclusion of turbulence from the vicinity of the duct sidewalls in the case of a wide duct with $A\gtrsim 4$: here the friction length on the sidewalls is so long that the size (around 100 times the friction length) of a self-sustaining minimal flow unit of streamwise vortices and streaks is larger than the duct height and, therefore, it cannot be accommodated.

Effect of bend on premixed flame dynamics in a closed duct

The premixed flame dynamics and pressure build-up in a closed duct with a 90° bend are experimentally and numerically investigated, and compared to a previous analytical theory. Emphasis is placed on the effect of the bend on the flame propagation, especially on the detailed flame front evolution. The results show that the finger flame is curved along the bend. A noticeable tulip flame is produced in the horizontal straight section as the upper parts of the indented flame nearly catches up with the lower parts. The lower parts tend to dominate the flame propagation after the complete formation of the tulip flame. The tulip flame disappears with the vanishing of the upper parts near the end of combustion. The flame dynamics in the experiment is reasonably reproduced by the numerical simulations, especially with isothermal wall. It is found that the full formation of the tulip flame is accompanied by a second drastic flame deceleration arising from the rapid reduction of the surface area of the outer flame front near the outer sidewall. It is revealed that the takes on the appearance of a “trough” shape after the flame touches the outer walls. The flame is still concaved from the center toward the burnt gas after the disappearance of the tulip upper parts. The pressure rise is closely related to the flame behavior. The heat transfer through the duct walls has a significant influence on the combustion dynamics. Both the flame tip location and pressure rise in the later stages are greater in the case of adiabatic duct walls than in the experiment. The theoretical analysis demonstrates that the flame propagation mechanism in the early stages in the curved duct coincides with that in a straight duct despite the presence of the bend. However, the flame behavior after its contact with the duct sidewall and tulip formation differs from those in a straight duct under the effects of the bend.

MHD flow and heat transfer behind a square cylinder in a duct under strong axial magnetic field

We capture through numerical simulation the MHD (magnetohydrodynamic) vortex dynamics around a square object in a square duct subjected to a strong externally imposed axial magnetic field. A quasi two-dimensional conditionality allows us to follow a two-dimensional modeling approach. The pertinent MHD control parameters such as the Reynolds and Hartmann numbers are kept in the range 0<Re⩽6000 and 0<Ha⩽2160, respectively. The various regimes of the MHD wake are found in-line with those obtained by Dousset and Pothérat under similar conditions using a circular object (Dousset and Pothérat, 2008). Four different regimes are identified out of which the first three regimes are the classical non-MHD 2D cylinder wakes. The transition from one to another regime is controlled by the friction parameter Re/Ha. The fourth regime is characterized by the vortices evolved from the duct side walls due to the boundary layer separation which strongly disturbs the Kármán vortex street. In order to explore the thermal transport phenomena under the action of the axially imposed magnetic field, the channel bottom wall is considered heated while the top wall is maintained at the free stream temperature keeping the cylinder adiabatic. The heat transfer rate from the heated channel wall strongly depends on the imposed magnetic field strength as well as Reynolds number. Additionally, an enhancement in heat transfer is experienced by placing the square cylinder in the channel over the bare channel, however, simultaneously a degradation in heat transfer would occur if the square object is replaced by a same size circular object.

Faisabilité d’un observatoire de cancers ciblés sur le département des Bouches-du-Rhône

An experimental laboratory study was performed to measure the effect of acoustic noise on the magnitude of tube vibrations in a staggered tube array and to measure the effectiveness of baffles on suppressing transverse duct modes. The acoustically induced tube vibration was experimentally determined for a flexible tube in three different transverse locations in the tube array. It was found that a tube located in the center of the duct, where a nodal point of the acoustic pressure resided, was most influenced by the transverse mode. On the other hand, a tube located near the duct side wall, where the acoustic pressure oscillation was highest, was not affected by the acoustic standing wave since it produced almost imperceptible vibration. It is shown that the acoustic pressure gradient across a tube is the cause of the tube vibration. Experiments were also performed to determine the effect of baffles for reducing the noise of acoustic resonance. The results showed that a single baffle substantially reduced the first mode resonance, as long as it was located close to the center of the bundle. The insertion of two baffles, in various locations, effectively eliminated acoustic resonance in the 1st and 2nd transverse duct modes. It is seen that the third acoustic mode is not significantly affected by either one or two baffles.

NUMERICAL INVESTIGATIONS OF PSEUDO-SHOCK WAVES IN VARIABLE CROSS-SECTION DUCTS

The Pseudo-Shock Wave (PSW), which appears when supersonic flow in duct decelerates to subsonic, is a complicated process due to the interaction between boundary layer and shock wave. It significantly affects the performance and efficiency of flow devices. In this paper, PSW in two kinds of variable cross-section ducts, edge-varied and corner-varied, was investigated through CFD numerical simulation. Compared to the rectangular duct, a shorter and wider separation region is appeared in the corner of the edge-varied duct while the strongest separation is laterally propagated across the entire plane of the corner-varied duct's side wall. This makes the performances of varied ducts different from traditional constant cross-section ducts.

Numerical simulations of a cylinder wake under a strong axial magnetic field

We study the flow of a liquid metal in a square duct past a circular cylinder in a strong externally imposed magnetic field. In these conditions, the flow is quasi-two-dimensional, which allows us to model it using a two-dimensional (2D) model. We perform a parametric study by varying the two control parameters Re and Ha (Ha2 is the ratio of Lorentz to viscous forces) in the ranges [0…6000] and [0…2160], respectively. The flow is found to exhibit a sequence of four regimes. The first three regimes are similar to those of the non-magnetohydrodynamic (non-MHD) 2D circular wake, with transitions controlled by the friction parameter Re∕Ha. The fourth one is characterized by vortices raising from boundary layer separations at the duct side walls, which strongly disturbs the Kármán vortex street. This provides the first explanation for the breakup of the 2D Kármán vortex street first observed experimentally by Frank, Barleon, and Müller [Phys. Fluids 13, 2287 (2001)]. We also show that, for high values of Ha (Ha⩾1120), the transition to the fourth regime occurs for Re∝0.56Ha, and that it is accompanied by a sudden drop in the Strouhal number. In the first three regimes, we show that the drag coefficient and the length of the steady recirculation regions located behind the cylinder are controlled by the parameter Re∕Ha4∕5. Also, the free shear layer that separates the recirculation region from the free stream is similar to a free MHD parallel layer, with a thickness of the order of Ha−1∕2 that is quite different to that of the non-MHD case, and therefore strongly influences the dynamics of this region. We also present one case at Re=3×104 and Ha=1120, where this layer undergoes an instability of the Kelvin–Helmholtz-type.

Acoustic resonance in a staggered tube array: Tube response and the effect of baffles

An experimental laboratory study was performed to measure the effect of acoustic noise on the magnitude of tube vibrations in a staggered tube array and to measure the effectiveness of baffles on suppressing transverse duct modes. The acoustically induced tube vibration was experimentally determined for a flexible tube in three different transverse locations in the tube array. It was found that a tube located in the center of the duct, where a nodal point of the acoustic pressure resided, was most influenced by the transverse mode. On the other hand, a tube located near the duct side wall, where the acoustic pressure oscillation was highest, was not affected by the acoustic standing wave since it produced almost imperceptible vibration. It is shown that the acoustic pressure gradient across a tube is the cause of the tube vibration. Experiments were also performed to determine the effect of baffles for reducing the noise of acoustic resonance. The results showed that a single baffle substantially reduced the first mode resonance, as long as it was located close to the center of the bundle. The insertion of two baffles, in various locations, effectively eliminated acoustic resonance in the 1st and 2nd transverse duct modes. It is seen that the third acoustic mode is not significantly affected by either one or two baffles.

Experimental study on flow and local heat/mass transfer characteristics inside corrugated duct

The present study investigates the flow and heat/mass transfer characteristics of wavy duct for the primary surface heat exchanger application. Local heat/mass transfer coefficients on the corrugated duct side walls are measured using a naphthalene sublimation technique. The flow visualization technique and a numerical analysis using a commercial code, FLUENT, are used to understand the overall flow structures inside the duct. The corrugation angle of the wavy duct is 145° and the duct aspect ratio is 7.3. The Reynolds numbers, based on the duct hydraulic diameter, vary from 100 to 5000. The results show that complex secondary flows and transfer processes exist inside the wavy duct resulting in non-uniform distributions of the heat/mass transfer coefficients on the duct side walls. At low Reynolds numbers ( Re ⩽ 1000), relatively high heat/mass transfer regions like cell shapes appear on both pressure- and suction-side walls due to the secondary vortex flows called Taylor–Görtler vortices perpendicular to the main flow direction. However, at high Reynolds numbers ( Re > 1000), these secondary flow cells disappear and boundary layer type flow develop on pressure-side wall. On the suction-side wall, high heat/mass transfer region appears by the flow reattachment. The average heat/mass transfer coefficients are higher than those of the smooth circular duct and pressure drop increases due to the secondary flows inside wavy duct.

煙突からの低周波音の放射とその対策(Part1,ワークショップ「低周波音に関する最新技術」,振動・騒音改善技術)

This paper introduces a new concept which can easily reduce sound reflection coefficient at a duct open end. When we make a duct side wall next to open end of appropriate perforated plate or porous material, sound reflection at that open end is remarkably reduced and the radiated sound is also reduced. Here this phenomenon is analyzed theoretically and proved experimentally. This technique is useful and applicable to reduce acoustic resonant intensity in the duct such as stacks of plants, mufflers of engines and so on. As the result, it can reduce the radiating sound from the duct open end.

Numerical Prediction of Three-Dimensional Turbulent Structure through a Circular-to-Rectangular Transition Duct.

Numerical analysis is used to investigate three-dimensional turbulent structure and fluid flow behavior in a circular-to-rectangular transition duct. Turbulent flow through a circular-to-rectangular duct is characterized by streamwise curvature and streamwise vorticity embedded in the boundary layer. In the calculation, an algebraic Reynolds stress model together with a boundary-fitted coordinate system is applied to to a circular-to rectangular duct in order to solve anisotropic turbulent flow precisely. The calculated results are compared with the experimental results to examine the validity of the present method. As a result of this calculation, it is found that the present method predicts well a secondary flow pattern which develops into a discrete vortex pair along the duct sidewalls. This secondary flow arises as a result of lateral skewing of the near-wall flow in the vicinity of the sidewall induced by transverse pressure gradients associated with wall curvature. Moreover, the calculated contours of six Reynolds stress components are shown for comparison with the experiment. The distortion of Reynolds stresses by the vortex pair is clearly observed as well as the primary flow. Although agreement is not perfect, the main features are reproduced by the present method using the algebraic Reynolds stress model.

Driving of axial acoustic waves by side-wall stabilized premixed flames

The mechanisms responsible for the driving of axial acoustic waves in a duct by a premixed, flat flame, stabilized on a side wall of the duct have been investigated both theoretically and experimentally. Measurements included acoustic pressures, burner surface admittances, and distributions of the temperature and heat release rates within the flame region. These revealed that the reaction rate oscillates with the frequency of the acoustic wave and that it possesses a spatial distribution which exhibits a large and a small peak upstream and downstream of the flat flame location, respectively. It is shown in the paper that the investigated flame driving is acoustically equivalent to that provided by a combination of a monopole and a dipole acoustic sources. These findings are in agreement with the predictions of the developed flame model which show that the flame driving is caused by the oscillation of the flame front relative to the duct side wall, caused by its interaction with the acoustic field. These newly uncovered flame driving mechanisms are believed to also play a role in solid propellant rocket motor instabilities where flame oscillations relative to the propellant surface will produce an oscillatory heat transfer to the propellant. This, in turn, will result in an oscillatory surface regression rate capable of driving the instability inside the rocket motor.

Measurements of Wall Shear Stress in Axial Flow in a Square Lattice Rectangular Rod Bundle

Hot film probe measurements of the distribution of the wall shear stress were made for axial flow along a rectangular 3 × 6 array of rods with a pitch to diameter ratio, P/D = 4/3, and a wall to diameter ratio, W/D = 2/3. Measurements were performed on rods at several locations and on two duct side walls at a position 62 hydraulic diameters from the entrance. Local shear stress maxima occur near the largest subchannel flow areas with the lowest maximum local shear stress on rods nearest the sidewalls. Maximum to the minimum shear stress ratio on an individual rod is largest for the corner rod. Side wall maximum local shear stress occurs in the first wall subchannel. Overall friction factors calculated from the wall shear stress measurements agree with those calculated from pressure drop data.