Analysis of Unsteady Confined Viscous Flows With Variable Inflow Velocity and Oscillating Walls

2010 ◽  
Vol 132 (4) ◽  
Author(s):  
Dan Mateescu ◽  
Manuel Muñoz ◽  
Olivier Scholz
Keyword(s):  
Reynolds Number ◽  
Flow Separation ◽  
Oscillation Frequency ◽  
Stokes Equations ◽  
Fluid Dynamic ◽  
Viscous Flows ◽  
Staggered Grid ◽  
Engineering Systems ◽  
Flow Induced Vibration ◽  
Inflow Velocity

The inflow velocities in various components of many engineering systems often display variations in time (fluctuations) during the operation cycle, which may substantially affect the flow-induced vibrations and instabilities of these systems. For this reason, the aeroelasticity study of these systems should include the effect of the inflow velocity variations, which until now has not been taken into account. This paper presents a fluid-dynamic analysis of the unsteady confined viscous flows generated by the variations in time of the inflow velocities and by oscillating walls, which is required for the study of flow-induced vibration and instability of various engineering systems. The time-accurate solutions of the Navier–Stokes equations for these unsteady flows are obtained with a finite-difference method using artificial compressibility on a stretched staggered grid, which is a second-order method in space and time. A special decoupling procedure, based on the utilization of the continuity equation, is used in conjunction with a factored alternate direction scheme to substantially enhance the computational efficiency of the method by reducing the problem to the solution of scalar tridiagonal systems of equations. This method is applied to obtain solutions for the benchmark unsteady confined flow past a downstream-facing step, generated by harmonic variations in time of the inflow velocity and by an oscillating wall, which display multiple flow separation regions on the upper and lower walls. The influence of the Reynolds number and of the oscillation frequency and the amplitudes of the inflow velocity and oscillating wall on the formation of the flow separation regions are thoroughly analyzed in this paper. It was found that for certain values of the Reynolds number and oscillation frequency and amplitudes, the flow separation at the upper wall is present only during a portion of the oscillatory cycle and disappears for the rest of the cycle, and that for other values of these parameters secondary flow separations may also be formed.

Steady and Unsteady Three-Dimensional Flows in Confined Configurations With Constant and Variable Inflow Velocity

2012 ◽  
Author(s):  
Araz Panahi ◽  
Dan Mateescu
Keyword(s):  
Stokes Equations ◽  
Three Dimensional ◽  
Staggered Grid ◽  
Navier Stokes ◽  
Engineering Systems ◽  
Flow Induced Vibration ◽  
Navier Stokes Equations ◽  
Inflow Velocity ◽  
Operation Cycle ◽  
Tridiagonal Systems

This paper presents a three-dimensional analysis of the unsteady confined viscous flows generated by the variations in time of the inflow velocities (fluctuations) which often are present during the operation cycle of various engineering systems, and have to be taken into account in the study of flow-induced vibration and instability of these systems. Time-accurate solutions of the Navier-Stokes equations for these unsteady flows are obtained with a numerical method developed by the authors, which is second-order accurate in space and time and is based on a finite difference formulation on a stretched staggered grid and uses artificial compressibility. A factored alternate direction implicit (ADI) scheme and a special decoupling procedure, based on the utilization of the continuity equation, are used to substantially enhance the computational efficiency of the method by reducing the problem to the solution of scalar tridiagonal systems of equations. This method is applied to obtain solutions for the benchmark unsteady confined flow past a downstream-facing step, generated by the harmonic variations in time of the inflow velocity. The formation of the flow separation regions is thoroughly analyzed in the paper, including the influence on the flow separations of the Reynolds number, and of the oscillation frequency and amplitude of the inflow velocity variations.

Numerical Simulation of Flow Field Inside Reactor Upper Plenum for PWR With Code_Saturne

2017 ◽  
Author(s):  
Jiawei Liu ◽  
Puzhen Gao ◽  
Tingting Xu ◽  
Jiesheng Min ◽  
Guofei Chen
Keyword(s):  
Stokes Equations ◽  
Fluid Dynamic ◽  
Structure Design ◽  
Navier Stokes ◽  
Guide Tube ◽  
Flow Induced Vibration ◽  
Local Pressure ◽  
Pressurized Water ◽  
Local Flow ◽  
Control Rod

Flow characteristic in upper plenum has a strong influence on reactor functional margin and rod cluster control assembly (RCCA) guide tube wear. Upper plenum flow governs loops flow rate measurement via hot leg temperature which has also an influence on the reactor protection system. For RCCA guide tube wear, it appears in operation with RCCA flow-induced vibration, leading to its replacement. It is important to know the flow condition in the upper plenum, and in particular the outlet. Existing Generation III reactors have their own specialties on the design. Comparison between current technologies is a good way for better understanding on the key structure design for the upper plenum. In this paper, simplified models based on upper plenum structure of Korean advanced pressurized water reactor (PWR) and Westinghouse design AP1000 are constructed and meshed with a volume around 6 million cells to obtain a 3-dimensional global and local flow distributions inside the upper plenum and to characterize the vital flow features for reactor safety. The Navier-Stokes equations are solved with standard k-ε turbulence model by using EDF in-house open source computational fluid dynamic (CFD) software: Code_Saturne. Through calculations, pressure and velocity distributions are obtained, axial and lateral variations have been analyzed. Compared with APR1400, it can be observed that for the design of AP1000, the rotational flow entrained in the edge of upper plenum and high velocity area due to the hot leg suction effect contribute to the relatively lower local pressure, and may have an impact on the drop velocity of control rod.

Vortex Shedding From a Square Cylinder With Ground Effect

2003 ◽  
Author(s):  
S. Bhattacharyya ◽  
D. K. Maiti
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Stokes Equations ◽  
Numerical Study ◽  
Ground Effect ◽  
Staggered Grid ◽  
Navier Stokes ◽  
Square Cylinder ◽  
Navier Stokes Equations ◽  
Cylinder Length

Numerical study on the wake behind a square cylinder placed parallel to a wall has been made. Flow has been investigated in the laminar Reynolds number (based on the cylinder length) range. We have studied the flow field for different values of the non-dimensional gap length between cylinder and the wall. The case when the cylinder is placed on the wall has also been considered. The governing unsteady Navier-Stokes equations are discretised through the finite volume method on staggered grid system. A SIMPLER type of algorithm has been used to compute the discretised equations iteratively. Vortex shedding has been found to be influenced by the wall. Vortex shedding suppression occurs beyond a critical value of the gap length. Due to the shear, the drag experienced by the cylinder is found to increase with the reduction of gap length. The flow is found to be steady when the cylinder is placed on the wall at a range of Reynolds number.

Numerical study on flow over a confined oscillating cylinder with a splitter plate

2019 ◽  
Vol 29 (5) ◽  
pp. 1629-1646 ◽  
Author(s):  
Arya Ghiasi ◽  
Seyed Esmaeil Razavi ◽  
Abel Rouboa ◽  
Omid Mahian
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Drag Reduction ◽  
Oscillation Frequency ◽  
Stokes Equations ◽  
Numerical Study ◽  
Splitter Plate ◽  
Content Type ◽  
Plate Length ◽  
Parametric Studies

Purpose This study aims to investigate the effect of the simultaneous usage of active and passive methods (which in this case are rotational oscillation and attached splitter plate, respectively) on the flow and temperature fields to find an optimum situation which this combination results in heat transfer increment and drag reduction. Design/methodology/approach The method of the solution was based on finite volume discretization of Navier–Stokes equations. A dynamic grid is coupled with the solver by the arbitrary Lagrangian–Eulerian (ALE) formulation for modeling cylinder oscillation. Parametric studies were performed by altering oscillation frequency, splitter plate length and Reynolds number. Findings Oscillation in different frequencies was found to be complicated. Higher frequencies provide more heat transfer, but in the lock-on region, they bring remarkable increment to the drag coefficient. It was observed that simultaneous usage of oscillation and splitter plate may have both positive and negative effects on drag reduction and heat transfer increment. Finally F = 2 and L = 0.5 were chosen as an optimum combination. Originality/value In this study, the laminar incompressible flow and heat transfer from a confined rotationally oscillating circular cylinder with an attached splitter plate are investigated. Parametric studies are performed by changing oscillation frequency, splitter plate length and Reynolds number.

Flow Separation and Heat Transfer in Shear Flow Around a Wall-Mounted Obstacle

2002 ◽  
Author(s):  
S. Mahapatra ◽  
S. Bhattacharyya
Keyword(s):  
Heat Transfer ◽  
Flow Separation ◽  
Buoyancy Force ◽  
Stokes Equations ◽  
Plane Surface ◽  
Surface Heat ◽  
Staggered Grid ◽  
Grashof Number ◽  
Surface Heat Transfer ◽  
Uniform Shear

Flow structure and surface heat transfer around a single small obstacle of rectangular cross-section mounted on a smooth plane surface are investigated. The obstacle is considered to be submerged in the viscous layer so that the far field flow may be viewed as uniform shear. The obstacle is considered to be at a temperature higher than that of the surrounding fluid and the flat surface is insulated. The unsteady Navier-Stokes equations and heat transport equation are solved numerically through a finite volume method on a staggered grid system. Solutions are obtained over a range of Reynolds number Re, which is based on the obstacle height and the incident uniform shear and Grashof number Gr. An investigation of the influence of buoyancy on the upstream and downstream flow separation from the obstacle and the interaction of the separation with the thermal field is also made. Numerical results reveal that in absence of the buoyancy force, the recirculating eddy upstream of the obstacle elongates with increasing Re. It is found that the buoyancy effect reduces the size of the upstream eddy when Re ≤ 200 with Gr (which is a measure of buoyancy) equal to 100. At an increased value of buoyancy force, Gr = 104, the upstream separation zone shifts further close to the obstacle. It is also found that the downstream separation length (which increases with increasing Re) further increases with increasing Gr as long as Re ≤ 200. Buoyancy effects on the flow are not prominent when Re is above 200. The surface heat transfer is quite high at the protruding corners and it increases with increase in Re. Increase of Grashof number produces an increment on surface heat transfer.

Analysis of the Performance of Plasma Actuators Under Low-Pressure Turbine Conditions Based on Experiments and URANS Simulations

2017 ◽  
Author(s):  
D. S. Martínez ◽  
E. Pescini ◽  
F. Marra ◽  
M. G. De Giorgi ◽  
A. Ficarella
Keyword(s):  
Experimental Data ◽  
Boundary Layer ◽  
Reynolds Number ◽  
Numerical Model ◽  
Flow Separation ◽  
Stokes Equations ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Suction Surface ◽  
Flow Separation Control

The present work is focused on the investigation of an alternate current driven single dielectric barrier discharge plasma actuator (AC-SDBDPA) for the control of separated flow at Reynolds numbers up to 2·104. Laminar boundary layer separation typically occurs on the suction surface of the low pressure turbines (LPT) blades when operating at high altitude cruise conditions, as the Reynolds number can drop below 2.5·104. In this context, the implementation of an active boundary layer control system able to operate in suppressing separation — only at the critical Reynolds numbers — is of great interest. The SDBDPA was manufactured by means of the photolithographic technique, which ensured a thin metal deposition with high manufacturing reliability control. Actuator operation under sinusoidal voltage at 8 kV amplitude and 2 kHz frequency was considered. Investigations were performed in a closed loop wind tunnel. A curved plate with a shape designed to reproduce the suction surface of a LPT was mounted directly over the bottom wall of the test section. The SDBDPA was inserted in a groove made at the middle of the curved plate, located at the front side of the adverse pressure gradient region. The flow pattern and velocities in absence of actuation were experimentally measured by a two-dimensional (2-D) particle image velocimetry (PIV) system and a laser Doppler velocimetry (LDV) system. PIV measurements were performed in presence of actuation. Simultaneously to the velocity measurements, the voltage applied to the AC-SDBDPA and the discharge current flowing through the circuit were acquired in order to determine the power dissipated by the device. The experimental data were supported by computational fluid dynamics (CFD) simulations based on the finite volume method. In order to deeply investigate the effect of flow separation control by the AC-SDBDPA on the LPT blade performances, the viscous and unsteady Reynolds-averaged Navier-Stokes equations were solved to predict the characteristics of the flow with and without actuation. The actuation effect was modelled as a time-constant body force calculated prior to the fluid flow simulations by using the dual potential algebraic model. The experimental data were used to calibrate and successfully validate the numerical model. An unsteady RANS (URANS) approach, using the k-ω Lam and Bremhorst Low-Reynolds turbulence model was employed, accounting with the main transient flow structures. Results showed that the mixing action of the streamwise fluid with higher momentum and the boundary layer fluid with the lower momentum -due to the AC-SDBDPA-led, depending on the tested Reynolds number, to the alleviation or suppression of the boundary layer flow separation which occurred on the suction surface of the LPT blade. The validated numerical model will allow expanding the study of the actuation effect including different locations and multiple devices, saving considerably experimental efforts.

scholarly journals Mixing Enhancement of Non-Newtonian Shear-Thinning Fluid for a Kenics Micromixer

Micromachines ◽  
2021 ◽  
Vol 12 (12) ◽  
pp. 1494
Author(s):  
Abdelkader Mahammedi ◽  
Naas Toufik Tayeb ◽  
Kwang-Yong Kim ◽  
Shakhawat Hossain
Keyword(s):  
Reynolds Number ◽  
Stokes Equations ◽  
Fluid Dynamic ◽  
Reynolds Numbers ◽  
Shear Thinning ◽  
Navier Stokes ◽  
Mixing Enhancement ◽  
Pressure Losses ◽  
Shear Thinning Fluids ◽  
Wide Range

In this work, a numerical investigation was analyzed to exhibit the mixing behaviors of non-Newtonian shear-thinning fluids in Kenics micromixers. The numerical analysis was performed using the computational fluid dynamic (CFD) tool to solve 3D Navier-Stokes equations with the species transport equations. The efficiency of mixing is estimated by the calculation of the mixing index for different cases of Reynolds number. The geometry of micro Kenics collected with a series of six helical elements twisted 180° and arranged alternately to achieve the higher level of chaotic mixing, inside a pipe with a Y-inlet. Under a wide range of Reynolds numbers between 0.1 to 500 and the carboxymethyl cellulose (CMC) solutions with power-law indices among 1 to 0.49, the micro-Kenics proves high mixing Performances at low and high Reynolds number. Moreover the pressure losses of the shear-thinning fluids for different Reynolds numbers was validated and represented.

Viscous Flows

2018 ◽  
Author(s):  
S. G. Rajeev
Keyword(s):  
Exact Solution ◽  
Reynolds Number ◽  
Stokes Equations ◽  
Viscous Flows ◽  
Navier Stokes ◽  
Small Reynolds Number ◽  
Navier Stokes Equation ◽  
Navier Stokes Equations ◽  
The Core ◽  
Flow Past A Sphere

Here some solutions of Navier–Stokes equations are found.The flow of a fluid along a pipe (Poisseuille flow) and that between two rotating cylinders (Couette flow) are the simplest. In the limit of large viscosity (small Reynolds number) the equations become linear: Stokes equations. Flow past a sphere is solved in detail. It is used to calculate the drag on a sphere, a classic formula of Stokes. An exact solution of the Navier–Stokes equation describing a dissipating vortex is also found. It is seen that viscosity cannot be ignored at the boundary or at the core of vortices.

scholarly journals Numerical analysis of high-lift configurations with oscillating flaps

2021 ◽  
Author(s):  
Johannes Ruhland ◽  
Christian Breitsamter
Keyword(s):  
Reynolds Number ◽  
Flow Separation ◽  
Separated Flow ◽  
Navier Stokes ◽  
Rotational Axis ◽  
High Lift ◽  
Hinge Point ◽  
Reduced Frequency ◽  
Flap Deflection ◽  
The Impact

AbstractThis study presents two-dimensional aerodynamic investigations of various high-lift configuration settings concerning the deflection angles of droop nose, spoiler and flap in the context of enhancing the high-lift performance by dynamic flap movement. The investigations highlight the impact of a periodically oscillating trailing edge flap on lift, drag and flow separation of the high-lift configuration by numerical simulations. The computations are conducted with regard to the variation of the parameters reduced frequency and the position of the rotational axis. The numerical flow simulations are conducted on a block-structured grid using Reynolds Averaged Navier Stokes simulations employing the shear stress transport $$k-\omega $$ k - ω turbulence model. The feature Dynamic Mesh Motion implements the motion of the oscillating flap. Regarding low-speed wind tunnel testing for a Reynolds number of $$0.5 \times 10^{6}$$ 0.5 × 10 6 the flap movement around a dropped hinge point, which is located outside the flap, offers benefits with regard to additional lift and delayed flow separation at the flap compared to a flap movement around a hinge point, which is located at 15 % of the flap chord length. Flow separation can be suppressed beyond the maximum static flap deflection angle. By means of an oscillating flap around the dropped hinge point, it is possible to reattach a separated flow at the flap and to keep it attached further on. For a Reynolds number of $$20 \times 10^6$$ 20 × 10 6 , reflecting full scale flight conditions, additional lift is generated for both rotational axis positions.

The inlet flow structure of a conceptual open-nose supersonic drone

2021 ◽  
pp. 095441002098304
Author(s):  
Eiman B Saheby ◽  
Xing Shen ◽  
Anthony P Hays ◽  
Zhang Jun
Keyword(s):  
Flow Structure ◽  
Stokes Equations ◽  
Fluid Dynamic ◽  
Three Dimensional ◽  
Computational Fluid Dynamic Simulation ◽  
Navier Stokes ◽  
Ansys Fluent ◽  
Navier Stokes Equations ◽  
Aerodynamic Efficiency ◽  
Performance Effects

This study describes the aerodynamic efficiency of a forebody–inlet configuration and computational investigation of a drone system, capable of sustainable supersonic cruising at Mach 1.60. Because the whole drone configuration is formed around the induction system and the design is highly interrelated to the flow structure of forebody and inlet efficiency, analysis of this section and understanding its flow pattern is necessary before any progress in design phases. The compression surface is designed analytically using oblique shock patterns, which results in a low drag forebody. To study the concept, two inlet–forebody geometries are considered for Computational Fluid Dynamic simulation using ANSYS Fluent code. The supersonic and subsonic performance, effects of angle of attack, sideslip, and duct geometries on the propulsive efficiency of the concept are studied by solving the three-dimensional Navier–Stokes equations in structured cell domains. Comparing the results with the available data from other sources indicates that the aerodynamic efficiency of the concept is acceptable at supersonic and transonic regimes.

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