scholarly journals Linear Hydrodynamic Stability of Fluid Flow in Curved Rectangular Ducts: Semi-Analytical Study

2019 ◽  
Vol 35 (5) ◽  
pp. 747-765 ◽  
Author(s):  
H. Nowruzi ◽  
H. Ghassemi ◽  
S. S. Nourazar
Keyword(s):  
Fluid Flow ◽  
Aspect Ratio ◽  
Eigenvalue Problem ◽  
Flow Instability ◽  
Wave Number ◽  
Dean Number ◽  
Analytical Technique ◽  
Dean Flow ◽  
Aspect Ratios ◽  
Analytical Results

ABSTRACTIn the current study, for the first time, a semi-analytical technique is used for solving eigenvalue problem arising from linear hydrodynamics stability of fluid flow through the curved rectangular ducts at different curvature ratios and aspect ratios. To this accomplishment, symmetric disturbances are assumed and the Homotopy perturbation method (HPM) is applied to solve our eigenvalue problem for curvature ratios ranging from 0.01 to 0.8 and aspect ratios ranging from 0.05 to 20. Our semi-analytical results are validated through the existing numerical and experimental data, showing good agreement. The semi-analytical results indicate that, as the curvature ratio increases the critical Dean number (Dnc) is increased and the flow becomes more stable, especially for aspect ratios lower than 1.Moreover, for all intended curvature ratios, irregular behavior in variation of Dnc is detected by an increase in the aspect ratio. So that, the Dnc is decreased when the aspect ratio increases from 0.05 up to 1 and the fluid flow becomes unstable. When the aspect ratio is increased from 1 to 5, it causes to increase the Dnc and fluid flow becomes stable. Furthermore, when the aspect ratio increases from 5 to 20, the Dnc is decreased again. In addition, Dnc and eigenvalues of critical complex wave number corresponding to Dnc for the onset of Dean flow instability is reported under different curvature ratios and aspect ratios.

Fluid-Induced Forces in Pump Liquid Seals With Large Aspect Ratio

2011 ◽  
Author(s):  
Alexandrina Untaroiu ◽  
Vahe Hayrapetian ◽  
Costin D. Untaroiu ◽  
Paul E. Allaire ◽  
Houston G. Wood ◽  
...  
Keyword(s):  
Shear Stress ◽  
Fluid Flow ◽  
Aspect Ratio ◽  
Stokes Equations ◽  
Complex Geometry ◽  
Dynamic Properties ◽  
Fluid Velocity ◽  
Bulk Flow ◽  
Large Aspect Ratio ◽  
Aspect Ratios

The instability due to fluid flow in seals is a known phenomenon that can occur in pumps and compressors as well as in steam turbines. Traditional annular seal models are based on bulk flow theory. While these methods are computationally efficient and can predict dynamic properties fairly well for short seals, they lack accuracy in cases of seals with complex geometry or with large aspect ratios (above 1.0). Unlike the bulk flow models, computational fluid dynamics (CFD) makes no simplifying assumption on the seal geometry, shear stress at the wall, relationship between wall shear stress and mean fluid velocity, or characterization of interfaces between control volumes through empirical friction factors. This paper presents a method to calculate the linearized rotor-dynamic coefficients for a liquid seal with large aspect ratio (balance drum) subjected to incompressible turbulent flow by means of a three dimensional CFD analysis to calculate the fluid-induced forces acting on the rotor. The Reynolds-averaged Navier-Stokes equations for fluid flow are solved by dividing the volume of fluid into a discrete number of points at which unknown variables are computed. As a result, all the details of the flow field, including the fluid forces with potential destabilizing effects, are calculated. A 2nd order curve fit is then used to express the fluid-induced forces in terms of equivalent linearized stiffness, damping, and fluid inertia coefficients.

scholarly journals Research for a Non-Standard Kenics Static Mixer with an Eccentricity Factor

Processes ◽  
2021 ◽  
Vol 9 (8) ◽  
pp. 1353
Author(s):  
Chenfeng Wang ◽  
Hanyang Liu ◽  
Xiaoxia Yang ◽  
Rijie Wang
Keyword(s):  
Fluid Flow ◽  
Aspect Ratio ◽  
Twist Angle ◽  
Univariate Analysis ◽  
Flow Simulation ◽  
Asymmetric Structure ◽  
Static Mixer ◽  
Static Mixers ◽  
Aspect Ratios ◽  
Symmetric Structure

The Kenics static mixer is one of the most widely studied static mixers, whose structure–function relationship has been studied by varying its aspect ratio and modifying the surface. However, the effect of the symmetric structure of the Kenics static mixer itself on twisting the fluid has been neglected. In order to study how the symmetrical structure of the Kenics static mixer impacts the fluid flow, we changed the center position of elements at twist angle 90° and introduced the eccentricity factor γ. We applied LHS-PLS to study this non-standard Kenics static mixer and obtained the statistical correlations of the aspect ratio, Reynolds number, and eccentricity factor on relative Nusselt number and relative friction factor. We analyzed the results by comparing the PLS model with the univariate analysis, and it was found that the underlying logic of the Kenics static mixer with an asymmetric structure became different. In addition, a non-standard Kenics static mixer with an asymmetric structure was investigated using vortex generation and dissipation through fluid flow simulation. The results demonstrated that the classical symmetric structure has a minor pressure drop, but the backward eccentric one has a higher thermal-hydraulic performance factor. It was found that the nature of the eccentric structure is that two elements with different aspect ratios are being combined at θ=90°, and this articulation leads to non-standard Kenics static mixers with different underlying logic, which finally result in the differences between the PLS model and the univariate analysis.

Heat Transfer Analysis Of Nano-fluid Flow In A Converging Nozzle With Different Aspect Ratios

2017 ◽  
Vol 20 (4) ◽  
pp. 161-167 ◽  
Author(s):  
T. Sathish
Keyword(s):  
Heat Transfer ◽  
Fluid Flow ◽  
Aspect Ratio ◽  
Flow Rate ◽  
Flow Characteristics ◽  
Heat Transfer Analysis ◽  
Element Analysis ◽  
Frictional Loss ◽  
Nano Fluid ◽  
Aspect Ratios

The study evaluates the nanofluid using finite element analysis with base fluid (water) and seeding particles (Aluminum oxide). This is placed over a convergence channel consisting of varying aspect ratio that are evaluated quantitatively to enhance the heat transfer properties of the nanofluid.We have considered frictional loss characteristics that increases the flow of the fluid with Reynolds numbers varying from 100-2000 is compared.A baseline modeling is established using the methodology analysis for the fluid flow over a rectangular chamber that is designed in the form of a square duct of ratio 1:1. The analysis is carried out over the heat transfer and flow rate characteristics of the nanofluid that converges into the square ducts with different aspect ratio, is analyzed.The concentration of the nano fluid is maintained at the constant rate, which is used for studying the flow rate influence over different aspect ratios. The thermal and flow characteristics is analyzed in such situation and validated against other literatures to check the efficiency in the converging rectangular oxygen free copper channel.The simulation results shows an increase in temperature on the duct out and drop in temperature on the inlet walls of the tube.The pressure changes and shear stress along the walls of the chamber is not much noticed and it is constant throughout the entire chamber.

scholarly journals Secondary flows due to finite aspect ratio in inertialess viscoelastic Taylor–Couette flow

2018 ◽  
Vol 857 ◽  
pp. 823-850 ◽  
Author(s):  
M. Davoodi ◽  
S. Lerouge ◽  
M. Norouzi ◽  
R. J. Poole
Keyword(s):  
Aspect Ratio ◽  
Perturbation Technique ◽  
Viscoelastic Fluids ◽  
Secondary Flows ◽  
Creeping Flow ◽  
End Effects ◽  
Dean Flow ◽  
Aspect Ratios ◽  
Secondary Motion ◽  
Taylor Couette

Both in rheometry and in fundamental fluid mechanics studies, the Taylor–Couette geometry is used frequently to investigate viscoelastic fluids. In order to ensure a constant shear rate in the gap between the inner and outer cylinders, such studies are usually restricted to the small-gap limit where the assumption of a linear velocity distribution is well justified. In conjunction with a sufficiently large aspect ratio$\unicode[STIX]{x1D6EC}$(i.e. ratio of cylinder height to gap), the flow is then assumed to be viscometric. Here we demonstrate, using a perturbation technique with the curvature ratio (i.e. ratio of the half-gap to the mid-radius of the cylinders) as the perturbation parameter, full nonlinear simulations using a finite-volume technique, and supporting experiments, that, even in the creeping-flow (inertialess) narrow-gap limit, for viscoelastic fluids end effects due to finite aspect ratio always give rise to a secondary motion. Using the constant-viscosity Oldroyd-B model we are able to show that this secondary motion, as has been observed in related pressure-driven flows with curvature, such as the viscoelastic Dean flow, is solely a consequence of the combination of gradients of the first normal-stress difference and curvature. Our results show that end effects can significantly change the flow characteristics, especially for small aspect ratios, and this may have important consequences in some situations such as the onset criteria for purely elastic instabilities.

On Hydrodynamic Stability of Dean Flow by Using Energy Gradient Methods

2018 ◽  
Vol 35 (3) ◽  
pp. 405-418 ◽  
Author(s):  
H. Nowruzi ◽  
H. Ghassemi ◽  
S. S. Nourazar
Keyword(s):  
Flow Instability ◽  
Hydrodynamic Instability ◽  
Gradient Methods ◽  
Critical Value ◽  
Dean Number ◽  
Dean Flow ◽  
Energy Gradient ◽  
Entire Flow ◽  
Gradient Function ◽  
Curved Walls

ABSTRACTIn the present paper, we investigate the hydrodynamic instability of Dean flow under different Dean numbers ranging from 1 to 2500, curvature ratios from 0.0001 up to 1000 and temperatures ranging from 273.15 K to 373.15 K. To study of fluid flow instability, analytical velocity profiles under intended conditions and energy gradient function K in the energy gradient method are evaluated. The results of present study show that, as the curvature ratio increases the flow becomes more stable. Moreover, no regular and significant effects on the energy gradient function K were achieved by increasing of temperatures. We found that, the origin of instability in the entire flow field is located on the inner wall of the parallel curved walls, especially for larger curvature ratios. We also reported the critical value of the energy gradient function K for the onset of instability corresponding to the critical Dean number.

scholarly journals Effects of Steel Fiber Percentage and Aspect Ratios on Fresh and Harden Properties of Ultra-High Performance Fiber

2021 ◽  
Vol 2 (3) ◽  
pp. 501-515
Author(s):  
Rajib Kumar Biswas ◽  
Farabi Bin Ahmed ◽  
Md. Ehsanul Haque ◽  
Afra Anam Provasha ◽  
Zahid Hasan ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Aspect Ratio ◽  
High Performance ◽  
Steel Fiber ◽  
Setting Time ◽  
Fiber Reinforced Concrete ◽  
Future Research ◽  
Manufacturing Cost ◽  
Aspect Ratios ◽  
High Performance Fiber

Steel fibers and their aspect ratios are important parameters that have significant influence on the mechanical properties of ultrahigh-performance fiber-reinforced concrete (UHPFRC). Steel fiber dosage also significantly contributes to the initial manufacturing cost of UHPFRC. This study presents a comprehensive literature review of the effects of steel fiber percentages and aspect ratios on the setting time, workability, and mechanical properties of UHPFRC. It was evident that (1) an increase in steel fiber dosage and aspect ratio negatively impacted workability, owing to the interlocking between fibers; (2) compressive strength was positively influenced by the steel fiber dosage and aspect ratio; and (3) a faster loading rate significantly improved the mechanical properties. There were also some shortcomings in the measurement method for setting time. Lastly, this research highlights current issues for future research. The findings of the study are useful for practicing engineers to understand the distinctive characteristics of UHPFRC.

scholarly journals Surface Texturing of Si with Periodically Arrayed Oblique Nanopillars to Achieve Antireflection

Materials ◽  
2021 ◽  
Vol 14 (2) ◽  
pp. 380
Author(s):  
Jun-Hyun Kim ◽  
Sanghyun You ◽  
Chang-Koo Kim
Keyword(s):  
Aspect Ratio ◽  
Plasma Etching ◽  
Surface Texturing ◽  
Light Reflection ◽  
Si Substrate ◽  
Weighted Mean ◽  
Optical Reflectance ◽  
Si Substrates ◽  
Aspect Ratios ◽  
Shadowing Effect

Si surfaces were texturized with periodically arrayed oblique nanopillars using slanted plasma etching, and their optical reflectance was measured. The weighted mean reflectance (Rw) of the nanopillar-arrayed Si substrate decreased monotonically with increasing angles of the nanopillars. This may have resulted from the increase in the aspect ratio of the trenches between the nanopillars at oblique angles due to the shadowing effect. When the aspect ratios of the trenches between the nanopillars at 0° (vertical) and 40° (oblique) were equal, the Rw of the Si substrates arrayed with nanopillars at 40° was lower than that at 0°. This study suggests that surface texturing of Si with oblique nanopillars reduces light reflection compared to using a conventional array of vertical nanopillars.

scholarly journals Strength Analysis of Alternative Airframe Layouts of Regional Aircraft on the Basis of Automated Parametrical Models

Aerospace ◽  
2021 ◽  
Vol 8 (3) ◽  
pp. 80
Author(s):  
Dmitry V. Vedernikov ◽  
Alexander N. Shanygin ◽  
Yury S. Mirgorodsky ◽  
Mikhail D. Levchenkov
Keyword(s):  
Aspect Ratio ◽  
High Aspect Ratio ◽  
High Performance ◽  
Strength Analysis ◽  
Labor Input ◽  
Aerodynamic Loading ◽  
Aspect Ratios ◽  
Wide Range ◽  
Parametrical Design

This publication presents the results of complex parametrical strength investigations of typical wings for regional aircrafts obtained by means of the new version of the four-level algorithm (FLA) with the modified module responsible for the analysis of aerodynamic loading. This version of FLA, as well as a base one, is focused on significant decreasing time and labor input of a complex strength analysis of airframes by using simultaneously different principles of decomposition. The base version includes four-level decomposition of airframe and decomposition of strength tasks. The new one realizes additional decomposition of alternative variants of load cases during the process of determination of critical load cases. Such an algorithm is very suitable for strength analysis and designing airframes of regional aircrafts having a wide range of aerodynamic concepts. Results of validation of the new version of FLA for a high-aspect-ratio wing obtained in this work confirmed high performance of the algorithm in decreasing time and labor input of strength analysis of airframes at the preliminary stages of designing. During parametrical design investigation, some interesting results for strut-braced wings having high aspect ratios were obtained.

Influence of nozzle exit geometrical parameters on supersonic jet decay

2021 ◽  
Vol 0 (0) ◽  
Author(s):  
Prasanta Kumar Mohanta ◽  
B. T. N. Sridhar ◽  
R. K. Mishra
Keyword(s):  
Aspect Ratio ◽  
Nozzle Exit ◽  
Ambient Air ◽  
Schlieren Image ◽  
Geometrical Parameters ◽  
Image Study ◽  
Aspect Ratios ◽  
Core Length ◽  
Supersonic Core Length ◽  
The Impact

Abstract Experiments and simulations were carried on C-D nozzles with four different exit geometry aspect ratios to investigate the impact of supersonic decay characteristics. Rectangular and elliptical exit geometries were considered for the study with various aspect ratios. Numerical simulations and Schlieren image study were studied and found the agreeable logical physics of decay and spread characteristics. The supersonic core decay was found to be of different length for different exit geometry aspect ratio, though the throat to exit area ratio was kept constant to maintain the same exit Mach number. The impact of nozzle exit aspect ratio geometry was responsible to enhance the mixing of primary flow with ambient air, without requiring a secondary method to increase the mixing characteristics. The higher aspect ratio resulted in better mixing when compared to lower aspect ratio exit geometry, which led to reduction in supersonic core length. The behavior of core length reduction gives the identical signature for both under-expanded and over-expanded cases. The results revealed that higher aspect ratio of the exit geometry produced smaller supersonic core length. The aspect ratio of cross section in divergent section of the nozzle was maintained constant from throat to exit to reduce flow losses.

scholarly journals Aspect Ratio Driven Relationship between Nozzle Internal Flow and Supersonic Jet Mixing

Aerospace ◽  
2021 ◽  
Vol 8 (3) ◽  
pp. 78
Author(s):  
Kalyani Bhide ◽  
Kiran Siddappaji ◽  
Shaaban Abdallah
Keyword(s):  
Boundary Layer ◽  
Aspect Ratio ◽  
Nozzle Exit ◽  
Supersonic Jet ◽  
Internal Flow ◽  
Shear Stresses ◽  
Loss Mechanism ◽  
Potential Core ◽  
Jet Mixing ◽  
Aspect Ratios

This work attempts to connect internal flow to the exit flow and supersonic jet mixing in rectangular nozzles with low to high aspect ratios (AR). A series of low and high aspect ratio rectangular nozzles (design Mach number = 1.5) with sharp throats are numerically investigated using steady state Reynolds-averaged Navier−Stokes (RANS) computational fluid dynamics (CFD) with k-omega shear stress transport (SST) turbulence model. The numerical shadowgraph reveals stronger shocks at low ARs which become weaker with increasing AR due to less flow turning at the throat. Stronger shocks cause more aggressive gradients in the boundary layer resulting in higher wall shear stresses at the throat for low ARs. The boundary layer becomes thick at low ARs creating more aerodynamic blockage. The boundary layer exiting the nozzle transforms into a shear layer and grows thicker in the high AR nozzle with a smaller potential core length. The variation in the boundary layer growth on the minor and major axis is explained and its growth downstream the throat has a significant role in nozzle exit flow characteristics. The loss mechanism throughout the flow is shown as the entropy generated due to viscous dissipation and accounts for supersonic jet mixing. Axis switching phenomenon is also addressed by analyzing the streamwise vorticity fields at various locations downstream from the nozzle exit.

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