Duct Curvature Research Articles

Non-isothermal magnetohydrodynamic fluid flow along the centerline in a rotating curved square duct with Hall and ion-slip current

This study is considered a fully developed non-isothermal, viscous, incompressible fluid flow that passes through the centerline of a rotating square curved duct in the presence of the magnetic field with Hall and ion-slip current. This non-isothermal duct is generated by keeping the heat on the outer wall and cool on the inner wall; the upper and lower walls are taken as adiabatic. The aspect ratio of the duct is taken l = 1 so that the duct cross section is viewed as square. The gravitational force acts on the flow as an external force, and a pressure gradient force (namely, Dean number effects) has been applied along the centerline of the duct. Due to the rotation of the system and the duct curvature, the flow is influenced by the centrifugal and Coriolis forces. The calculations are carried out by the spectral method as the primary tool, while the Chebyshev polynomial, collocation method, Newton–Raphson method, and arc-length method are used as secondary tools. The focus is to show the effects of the magnetic parameter (M), Hall parameter (m), and ion-slip parameter (α) on the flow characteristics of this rotationally curved square duct. The new findings have been shown in various values of M, m, and α for the three cases of Dean numbers, such as case I: Dn = 1000, case II: Dn = 3000, and case III: Dn = 5000, with different curvatures, while the rotational parameter is fixed at Tr = 20. Here, it is considered that Dn, Gr, Tr, M, m, and α are varied, while the curvature of the duct δ ranges from 0.01 to 0.5.

Experimental simulation methodology and spatial transition of complex distortion fields in a S-shaped inlet

The S-shaped inlet of the modern military aircraft propulsion system suffers from severe inlet distortion during flight maneuvering. The complex distorted flow field is generally decomposed into total pressure and swirl distortion, both of which could substantially degrade the performance and stability of an aero-engine. In the ground test of fans and low-pressure compressors, how to simulate the complex inlet distortion occurring in flight is non-trivial. In this paper, a subsonic S-shaped inlet test rig was constructed, and eight configurations of curved-edge plate-type distortion generators were installed in the intake channel to create a complex distorted flow field downstream. Results show that the distorted flow field is dependent on the circumferential position of the blocking plate due to the duct curvature. Prominent vortex structures caused by the pressure gradient and the cross secondary flow exist between the main flow and separated flow. Conventional distortion indices DC, CDI/RDI, and the SAE AIR1419/SAE AIR 5686 standard system are implemented to represent the total pressure distortion and swirl distortion and the effects of the size and location of the curved-edge plate on distortion intensity and distribution are investigated. Additionally, this paper also studied the spatial variation of the total pressure and swirl distortion between the duct circular exit section and the annular section, termed as spatial transition characteristics. This spatial transition process can be used in the future to specify a real inlet flow condition for stability assessment of a fan or low-pressure compressor. According to the experimental results, the circumferential total pressure distortion index is approximately unaltered along the contraction duct, while the radial index reduces by 62% on average. The spatial transition of the swirl distortion differs for bilaterally symmetrical and asymmetric swirling fields. The overall swirl intensity index decays by 37%∼70% only for bilaterally symmetrical modes, while being the same for bilaterally asymmetric approximately. At last, the generation and evolution mechanism of the above complex distorted flow field is analyzed theoretically.

A computational modeling on transient heat and fluid flow through a curved duct of large aspect ratio with centrifugal instability

Due to remarkable applications of the curved ducts in engineering fields, scientists have paid much attention to invent new characteristics of curved-duct flow in mechanical systems. In the ongoing study, a computational modeling of fluid flow and energy distribution through a curved rectangular duct of large aspect ratio is presented. Governing equations are enumerated by using a spectral-based numerical technique together with the function expansion and collocation method. The main purpose of the paper is to analyze the effect of centrifugal force in the flow transition as well as heat transfer in the fluid. The investigations are performed for the aspect ratio, Ar = 4; the curvature ratio, delta = 0.5; the Grashof number, {text{Gr}} = 1000; and varying the Dean number, 0 < {text{Dn}} le 1000. It is found that various types of flow regimes including steady-state and irregular oscillations occur as Dn is increased. To well understand the characteristics of the flow phase spaces and power spectrum of the solutions are performed. Next, pattern variations of axial and secondary flow velocity with isotherms are illustrated for different Dn’s. It is revealed that the flow velocity and the isotherms are significantly influenced by the duct curvature and the aspect ratio. Convective heat transfer and temperature gradients are calculated which explores that the fluids are diversified due to centrifugal instability, and as a consequence the overall heat transfer is enhanced significantly in the curved duct.

Full Three-Dimensional Inverse Design Method for S-Ducts Using a New Dimensionless Flow Parameter

In this study, a new inverse design method is proposed for the full 3-D inverse design of S-ducts using curvature-based dimensionless pressure distribution as a target function. The wall pressure distribution in a 3-D curved duct is a function of the centerline curvature and the cross-sectional profile and area. A dimensionless pressure parameter was obtained as a function of the duct curvature and height of the cross-sections based on the normal pressure gradient equation. The dimensionless pressure parameter was used to eliminate the effect of the cross-sectional area on the wall pressure distribution. Full 3-D inverse design of an S-shaped duct was carried out by substituting the 3-D duct with a large number of 2-D planar ducts. The ball-spine inverse design method with vertical spins was coupled with the dimensionless pressure parameter as a target function for the design of the planar ducts. The inverse design process was performed in two steps. First, the height of each cross-section was considered constant, and only the duct centerline was allowed to be deformed by applying the difference between the dimensionless pressure on the upper and lower lines of symmetry plane. Then, a constant curvature was considered for each centerline in the equation, and the difference between the current and the target dimensionless pressure was applied to each upper and lower line of the planar sections to correct the heights of the 2-D planar sections, separately. The method was validated by choosing a straight duct as an initial guess, which converges to the target S-shaped duct. The results showed that the method is an efficient physical-based residual-correction method with low computational cost and good convergence rate. The 3-D wall pressure distribution of a high-deflected 3-D S-shaped diffuser was modified to eliminate the separation, secondary flow, and outlet distortion. Finally, the geometry corresponding to the modified pressure was obtained by the proposed 3-D inverse design method, which revealed higher pressure recovery, lower total pressure loss, and lower outlet flow distortion and swirl angle.

Effects of Rayleigh-Taylor instabilities on turbulent premixed flames in a curved rectangular duct

Large-eddy simulations are used to demonstrate the effects of Rayleigh-Taylor instabilities (RTIs) on constant-pressure, turbulent premixed flames stabilized in a curved rectangular duct. A splitter plate of constant radius separates a reactant and pilot stream such that the flame stabilizes in the mixing layer between the two streams. Centrifugal acceleration due to the duct curvature induces the RTI, increasing the mixing of the higher-density reactant stream with the lower-density pilot stream. In both non-reacting and reacting flows, the resulting mixing layer thickness grows at rates comparable to those in unconfined RTIs until the flame occupies approximately half of the duct, at which point the duct walls limit the growth rate. The conservative equations are modified with artificial body forces to negate the centrifugal effect (and the RTI) to isolate the impact of the curved geometry. A comparison between the flames with and without the RTI shows that the RTI increases the turbulent flame speed primarily through increased flame surface area. The RTI also increases the range of local flame stretches and curvatures. The increased turbulent flame speed and growth rates due to RTI suggest a viable mechanism to increase turbulent flame speed in gas turbine engines though the application of flow path curvature.

Hydrodynamic stability study in a curved square duct by using the energy gradient method

Curved non-circular ducts have remarkable applications in the macro and micro scales engineering. Stability of flow through the curved ducts is a challenging topic in the context of fluid mechanics. In the current paper, hydrodynamic stability of fully developed 3D steady flow of incompressible fluid through the 180° curved square duct is investigated via energy gradient method. To this accomplishment, different Dean numbers (De) ranging from 19.60 to 1181.25 at curvature ratio 6.45 are considered. To investigate of flow stability, we analyzed the distributions of velocity, total pressure and control parameter of energy gradient function K. Obtained results indicated an appropriate agreement via both experimental and CFD data. Results of our investigation show that the maximum of energy gradient function Kmax is decreased by a reduction in the Dean number. In addition, we found that the origin of Dean hydrodynamic instability (i.e., position of the Kmax) is placed in the radial position between the center of the duct and outer curvature wall at azimuthally angular position θ < 72°. Results of Kmax and its cylindrical coordinates corresponding to the onset of Dean flow instability under various Dean numbers are reported.

Fluid flow in a spiral microfluidic duct

We consider the steady, pressure driven flow of a viscous fluid through a microfluidic device having the geometry of a planar spiral duct with a slowly varying curvature and height smaller than width. For this problem, it is convenient to express the Navier–Stokes equations in terms of a non-orthogonal coordinate system. Then, after applying appropriate scalings, the leading order equations admit a relatively simple solution in the central region of the duct cross section. First-order corrections with respect to the duct curvature and aspect ratio parameters are also obtained for this region. Additional correction terms are needed to ensure that no slip and no penetration conditions are satisfied on the side walls. Our solutions allow for a top wall shape that varies with respect to the radial coordinate which allows us to study the flow in a variety of cross-sectional shapes, including trapezoidal-shaped ducts that have been studied experimentally. At leading order, the flow is found to depend on the local height and slope of the top wall within the central region. The solutions are compared with numerical approximations of a classical Dean flow and are found to be in good agreement for a small duct aspect ratio and a slowly varying and small curvature. We conclude that the slowly varying curvature typical of spiral microfluidic devices has a negligible impact on the flow in the sense that locally the flow does not differ significantly from the classical Dean flow through a duct having the same curvature.

An audit of lymph node retrieval and histopathology reporting of pancreaticoduodenctomy specimens undertaken at a tertiary care referral center

Pancreaticoduodenectomy (PD) is currently the main surgical option for malignancies in the ampullary region, which includes ampulla of Vater tumours (AVT), distal bile duct tumours (DBDT), periampullary duodenal tumours (DT) and tumours of the head of the pancreas (PT). Nodal status and many other important pathological features have a significant impact on tumour prognosis and therapy. The aim of this study was to determine the total number of lymph nodes (LNs) retrieved from PD specimens, whether grouping of LNs improves the total yield and to assess the level completeness of histopathology reporting of PD specimens. Forty two PD requests and histopathology reports were assessed to determine the total number of LNs retrieved and whether the LN were grouped (G)or non-grouped (NG). The significance of difference in the number of LNs in the two groups were assessed using the Wilcoxon signed rank test. The tumours were subcategorized as AVT, DBDT, DT and PT and the reports were audited against the respective minimum data sets of the Royal College of Pathologists of United Kingdom to determine the overall completeness and the parameters poorly reported in the reports. The overall median LN yield was 14.5 and the median LN yield was 15 and 10 in G and NG respectively which was statistically significant. The completeness of the histopathology reporting was 63.6%- 77.3% in AVT (n-18), 73.9% - 95.6% in DBDT (n-5), 68.1% - 90.1% in DT (n-8), 70.8% - 83.3% in PT (n-11). The lengths of the bile duct, lesser and greater curvature of the stomach, tumour differentiation, involvement of resection margins and named blood vessels were poorly reported. In conclusion, the total LN retrieval improved by grouping according to the Union of International Cancer Control (UICC) protocol. Histopathology reporting of some of the data items requires improvement. Hence adoption of a pro forma for synoptic reporting and establishment of national guidelines on reporting and handling of specimens is recommended.

Behavior of Shock Train in Curved Isolators with Complex Background Waves

To study the duct curvature effects on a shock train in rectangular hypersonic inlet/isolator models, four curved isolators are designed and tested at a freestream Mach number of 4.92. Strong interactions between the shock train and background waves are encountered. In small curvature isolators, the shock train intersects with the left-running or right-running shock stably, with the massive separation situated near the top or bottom wall, respectively; whereas the large separation always locates near the bottom wall in large curvature isolators. When the shock train switches from intersecting with one background shock to another, rapid forward movement occurs in large curvature isolators; whereas violent flow oscillations may appear in small curvature isolators. The radial pressure gradient direction in the shock train region alternatively changes from being centripetal to being centrifugal in small curvature isolators, whereas the direction remains centripetal in large curvature isolators due to the enhanced left-running expansion waves and right-running compression waves. In addition, the amplitude of the radial pressure gradient increases with the duct curvature, which induces significant secondary flow and accumulates a large amount of low-energy fluid near the bottom surface. On the other hand, the streamwise adverse pressure gradient in the shock train decreases with the duct curvature, resulting in a longer and weaker shock train, and the maximum sustainable backpressure ratio decreases with a percentage as high as 17% of that of straight isolator. Besides, the root-mean-square values of static pressure on the exit plane can be reduced to some extent by increasing the duct curvature. In general, large duct curvature has detrimental effects on the shock train characteristics, but adequate duct curvature can help to avoid violent flow oscillation and decrease pressure fluctuating level.

Unthrottled Flows with Complex Background Waves in Curved Isolators

The unthrottled flows in curved isolators are studied experimentally and numerically. Wind-tunnel tests are conducted at a freestream Mach number of 4.92. The internal flowfield is visualized and measured. Large-scale separation bubbles in the inlet are observed, and the downstream duct curvature is found to be able to decrease the separation bubble size via lowering the local pressure downstream of the separation bubble. Complex background waves appear in these isolators, and the increase of duct curvature tends to intensify the reflected shocks. Particularly, the strength distributions along the shock direction of the background shocks behave differently under the influence of duct curvature. For the right-running shocks, the strength distribution changes from a descending trend to an ascending trend with the increase of duct curvature, whereas the strength of left-running shocks always descends with an increasing speed. The strength of background shocks is particularly related to the left-running expansion waves, which get intensified and become the dominant expansion waves in the isolators as the duct curvature increases. The schlieren images indicate a significant transverse density gradient and pressure gradient in the curved isolators. The wall pressure distributions further show that the radial pressure gradient in small curvature isolators switches between centripetal and centrifugal repeatedly; whereas it remains centripetal consistently in large curvature isolators, which is also determined by the left-running expansion waves. As a result, the secondary flow gets stronger as the duct curvature increases, resulting in a larger low-momentum flow region near the bottom wall. In general, the compression capability of the isolator gets attenuated with the increase of the duct curvature.

An audit of histopathology reporting in Kausch-Whipple pancreaticoduodenectomy specimens

Introduction Since histopathology reports of the resection specimens of malignant tumours should provide information of therapeutic and prognostic significance, national guidelines have been formulated on reporting common malignancies. However, no such guidelines are available locally for Kausch-Whipple pancreaticoduodenectomy (KWPD) specimens. The aim of this study was to audit the completeness of reporting of KWPD specimens against the minimum data sets formulated by the Royal College of Pathologists (RCP) UK. Methodology The histopathology reports of KWPD specimens of malignant tumours which were received at the Department of Pathology, Faculty of medicine, Ragama over a four year period were analyzed to determine whether the core data identified in the RCP minimum data set (BT-23, PT-24, DT-22 and AVT-22) was included. The percentage of items reported in each of the histopathology reports was calculated to ascertain the overall level of adequacy in each of the report. Results There were a total of 42 KWPD specimen reports including 18 tumours of the Ampulla of Vater (AVT), 11 pancreatic tumours (PT), 8 duodenal tumours (DT) and 5 biliary tumours (BT). The minimum number of core data items included was, BT-17/23, PT-17/24, DT-15/22 and AVT-14/22. Data that was poorly reported included the degree of differentiation, lengths of bile duct and lesser curvature and greater curvature of stomach, maximum tumour diameter (most poorly reported in AVT 16.67%, 3/18) and tumour involvement of margins in PT(0/11) and AVT(1/18, 5.56%). Conclusion Though overall reporting was found to be satisfactory in many of the reports, further improvements are required in reporting of certain data items. It is recommended that national guidelines and a proforma are introduced and followed.

Analysis of bi-directional ducted tidal turbine performance

Several commercial tidal turbine designs feature axial flow rotors within bi-directional ducts. Such devices are typically intended to increase power extraction through a flow-concentrating effect, operating on flood and ebb tides without a yawing mechanism. Research focused on such devices has been limited so far, with available results indicating poor performance relative to bare rotors. This study further investigates the relative performance of bi-directional ducted tidal turbines in confined flow.Several duct profiles are evaluated relative to unducted rotors using the Reynolds-averaged Navier–Stokes solver ANSYS Fluent. The rotor is represented as an actuator disc, which mimics the streamwise thrust of a real device but does not reproduce its swirl or additional turbulence generation. This idealised model achieves optimal energy extraction and enables fair comparison of duct geometries. Device power is reported relative to total frontal area, reflecting the fact that the overall dimension of the device will be limited by water depth. Comparisons based on rotor area show how the absolute power is increased by a duct, but that this is attributable to an increase in blockage.The fundamental effect of a duct on a rotor, as well as the secondary effects of duct camber and thickness, are identified by analysing streamwise distributions of velocity, pressure and cross-sectional area along the rotor streamtube. Ducts are found to limit the expansion of the downstream flow, in turn restricting the pressure reduction immediately behind the rotor. This effect, in combination with the reduced volumetric flux through a ducted rotor relative to a bare rotor, results in reduced power extraction.The effects of duct curvature and thickness on turbine performance are also examined. Where a ducted rotor is desirable, e.g. for the protection of rotor blades, a thick profile with slight curvature performs best.

Investigation on Secondary Flow Characteristics in a Curved Annular Duct with Struts

Under the influence of duct curvature, cross-sectional area variation and internal struts, the internal flow field within a curved annular duct becomes rather complicated and contains strong secondary flow. In this paper, the secondary flow characteristics in an annular duct with struts are experimentally and numerically investigated. The results show that large pressure gradients exist on the bends of hub and shroud. Meanwhile, two counter-rotating vortex pairs appear both along the hub-side and shroud-side surfaces. The hub-side vortex pair of which the vortex cores travel downstream parallelly evolves from the horseshoe vortex which is induced by the leading edge of the upstream strut, whereas the shroud-side vortex pair originates from the strut trailing edge and the corresponding vortex cores develop in a divergent way. Additionally, the effects of the duct exit Mach number on the secondary flow characteristics are also studied. As the exit Mach number increases, the streamwise pressure gradients increase and lead to more intense vortices, higher total pressure loss and larger flow distortion.

Evaluation of simplified evaporation duct refractivity models for inversion problems

AbstractTo assess a radar system's instantaneous performance on any given day, detailed knowledge of the meteorological conditions is required due to the dependency of atmospheric refractivity on thermodynamic properties such as temperature, water vapor, and pressure. Because of the significant challenges involved in obtaining these data, recent efforts have focused on development of methods to obtain the refractivity structure inversely using radar measurements and radar wave propagation models. Such inversion techniques generally use simplified refractivity models in order to reduce the parameter space of the solution. Here the accuracy of three simple refractivity models is examined for the case of an evaporation duct. The models utilize the basic log linear shape classically associated with evaporation ducts, but each model depends on various parameters that affect different aspects of the profile, such as its shape and duct height. The model parameters are optimized using radiosonde data, and their performance is compared to these atmospheric measurements. The optimized models and data are also used to predict propagation using a parabolic equation code with the refractivity prescribed by the models and measured data, and the resulting propagation patterns are compared. The results of this study suggest that the best log linear model formulation for an inversion problem would be a two‐layer model that contains at least three parameters: duct height, duct curvature, and mixed layer slope. This functional form permits a reasonably accurate fit to atmospheric measurements as well as embodies key features of the profile required for correct propagation prediction with as few parameters as possible.

Secondary flow vortex structures and forced convection heat transfer in fluid flow through curved elliptical ducts

Fluid flow through curved ducts is essentially characterised by the secondary flow effects due to duct curvature and cross-sectional flow geometry. Such flows produce vortex structures making the fluid behaviour vastly different than those in straight ducts while intrinsically promoting forced convection through fluid mixing. Examining the unique features of secondary flow and wall heat transfer, this paper presents a numerical simulation on the fluid flow through curved elliptical ducts, including circular geometry. The study develops and validates a novel numerical model based on three-dimensional vortex structures (helicity) and a curvilinear mesh system to overcome previous modelling limitations. Considering several duct aspect ratios, flow rates and wall heat fluxes, computations are performed to obtain the flow patterns and thermal characteristics. Parametric influences on flow features and forced convection are described through physical interpretation. The onset of vortices due to secondary flow instability is carefully examined in relation to the duct aspect ratio and flow rate. Appraising their merits, two techniques are developed for accurate detection of secondary flow instability and integrated into the computational process, which was not previously feasible. An approach based on the Second Law irreversibility is evaluated for thermal optimisation of fluid flow through curved elliptical ducts.

S0530105 モータリングエンジンにおける曲がり管内排気脈動流の実験的および数値解析的研究([S053]内部流れ)

This paper presents experimental and numerical investigations of pulsating flow from an exhaust manifold of an automobile engine. Pulsating exhaust with 1500 rpm flows in S-shaped duct with a square cross section. Thus, velocity fields at first and second bends in a horizontal plane are measured using PIV. It shows that fast flow and slowdown appear around inner and outer walls in both bends, respectively. Numerical simulations using RNG k-e model are in good agreement with measurements. Simulations have also displayed secondary flows grow after passing through the first bend like steady flows, because of pressure gradient and centrifugal force by the duct curvature. However, it is also found that flow direction of the secondary flow at the second bend outlet is opposite to centrifugal force; this result is different from researches of the past. Influences of the intensity of the secondary flows between two bend inlets on flow field are discussed in the present paper.

An analytical solution for Dean flow in curved ducts with rectangular cross section

In this paper, a full analytical solution for incompressible flow inside the curved ducts with rectangular cross-section is presented for the first time. The perturbation method is applied to solve the governing equations and curvature ratio is considered as the perturbation parameter. The previous perturbation solutions are usually restricted to the flow in curved circular or annular pipes related to the overly complex form of solutions or singularity situation for flow in curved ducts with non-circular shapes of cross section. This issue specifies the importance of analytical studies in the field of Dean flow inside the non-circular ducts. In this study, the main flow velocity, stream function of lateral velocities (secondary flows), and flow resistance ratio in rectangular curved ducts are obtained analytically. The effect of duct curvature and aspect ratio on flow field is investigated as well. Moreover, it is important to mention that the current analytical solution is able to simulate the Taylor-Görtler and Dean vortices (vortices in stable and unstable situations) in curved channels.

Velocity Field in 90°Curved Duct of Cycle Section

Flow in curved duct is different from that happens in straight pipeline. Affected by the curved duct curvature, the flow field in a curved duct is uneven. The flow in the curved duct suffers serious loss. Considering the flow field characteristic and engineering applications, the characteristic of the flow field in curved duct without guide plate and the curved duct with different guide plates have been studied based on the numerical results. The velocity distribution and unevenness on the outlet of the curved duct have been analyzed too. Then the best conditions for guide plates installed in curved duct are achieved.

Effects of geometrical parameters and internal wall velocity on turbulent flow and heat transfer in helical square duct with moving wall

In this study the effect of geometrical dimensions and the internal wall velocity on turbulent flow and heat transfer in a helical square duct with movable wall are numerically analyzed. RNG k-e turbulent model and finite volume method are used in the numerical solutions. The validity of the numerical solutions are tested by previous experimental studies with different boundary conditions and it has been determined that the helical duct curvature ratio and the internal wall velocity have significant effects but the dimensionless helical pitch has not on the internal wall Nusselt number. Key words: Helical square duct, RNG k-e turbulent model, finite volume method, curvature ratio, internal wall velocity, dimensionless pitch.

Effects of a curved vocal tract with grid-generated tongue profile on low-order formants

A hyperbolic grid-generation algorithm allows investigation of the effect of vocal-tract curvature on low-order formants. A smooth two-dimensional (2D) curve represents the combined lower lip, tongue, and anterior pharyngeal wall profile as displacements from the combined upper lip, palate, and posterior pharyngeal wall outline. The algorithm is able to generate tongue displacements beyond the local radius of strongly curved sections of the palate. The 2D grid, along with transverse profiles of the lip, oral-pharyngeal, and epilarynx regions, specifies a vocal conduit from which an effective area function may be determined using corrections to acoustic parameters resulting from duct curvature; the effective area function in turn determines formant frequencies through an acoustic transmission-line calculation. Results of the corrected transmission line are compared with a three-dimensional finite element model. The observed effects of the curved vocal tract on formants F1 and F2 are in order of importance, as follows: (1) reduction in midline distances owing to curvature of the palate and the bend joining the palate to the pharynx, (2) the curvature correction to areas and section lengths, and (3) adjustments to the palate-tongue distance required to produce smooth tongue shapes at large displacements from the palate.