scholarly journals Fluidic Oscillators, the Effect of Some Design Modifications

2020 ◽  
Vol 10 (6) ◽  
pp. 2105
Author(s):  
Masoud Baghaei ◽  
Josep M. Bergada
Keyword(s):  
Reynolds Number ◽  
Reynolds Numbers ◽  
Bluff Bodies ◽  
Entire Study ◽  
Fluidic Oscillator ◽  
Relevant Effect ◽  
Mixing Chamber ◽  
Inclination Angles ◽  
Output Characteristics ◽  
Oscillator Output

The number of applications where fluidic oscillators are expected to be used in the future, is raising sharply, then their ability of interacting with the boundary layer to modify forces on bluff bodies, enhancing heat transfer or decreasing noise generation, are just few of the applications where fluidic oscillators can be used. For each application a particular pulsating frequency and amplitude are required to minimize/maximize the variable under study, force, Nusselt number, etc. For a given range of Reynolds numbers, fluidic oscillators present a linear relationship between the output frequency and the incoming fluid flow, yet it appears the modification of the internal fluidic oscillator geometry may affect this relation. In the present paper and for a given fluidic oscillator, several performance parameters will be numerically evaluated as a function of different internal modifications via using 3D-CFD simulations. The paper is also evaluating the relation between the momentum applied to the mixing chamber incoming jet and the oscillator output characteristics. The evaluation is based on studying the output mass flow frequency and amplitude whenever several internal geometry parameters are modified. The geometry modifications considered were: the mixing chamber inlet and outlet widths, and the mixing chamber inlet and outlet wall inclination angles. The concept behind this paper is, to evaluate how much the fluidic oscillator internal dimensions affect the device main characteristics, and to analyze which parts of the oscillator produce a higher impact on the fluidic oscillator output characteristics. For the different internal modifications evaluated, special care is taken in studying the forces required to flip the jet. The entire study is performed for three different Reynolds numbers, 8711, 16034 and 32068. Among the conclusions reached it is to be highlighted that, for a given Reynolds number, modifying the internal shape affects the oscillation frequencies and amplitudes. Any oscillator internal modification generates a much relevant effect as Reynolds number increases. Under all conditions studied, it was observed the fluidic oscillator is pressure driven.

Supercritical Reynolds number simulation for two-dimensional flow over circular cylinders

1975 ◽  
Vol 70 (3) ◽  
pp. 529-542 ◽  
Author(s):  
Edmond Szechenyi
Keyword(s):  
Surface Roughness ◽  
Reynolds Number ◽  
Reynolds Numbers ◽  
Practical Significance ◽  
Circular Cylinders ◽  
Wind Tunnels ◽  
Bluff Bodies ◽  
Wide Range ◽  
Two Dimensional Flow ◽  
Different Diameters

In wind-tunnel tests on bluff bodies the Reynolds number is often limited to values that are very much smaller than those of the flows being simulated. In such cases the experiments may have no practical significance whatsoever since both the fluctuating and the steady aerodynamic phenomena can vary considerably with Reynolds number.This difficulty was encountered in an investigation of supercritical incompressible flow over cylinders, and an attempt at artificially increasing the Reynolds number by means of surface roughness was made. In order to evaluate this simulation technique, the influence of various grades of surface roughness on the aerodynamic forces acting on cylinders of different diameters was studied over a wide range of Reynolds numbers in two very different wind tunnels. The results allow very positive conclusions to be drawn.

An approximate theory for the streaming motion past axisymmetric bodies at Reynolds numbers of from 1 to approximately 100

1977 ◽  
Vol 82 (3) ◽  
pp. 583-604 ◽  
Author(s):  
Michael S. Kolansky ◽  
Sheldon Weinbaum ◽  
Robert Pfeffer
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Reynolds Numbers ◽  
Bluff Bodies ◽  
Approximate Theory ◽  
Viscous Term ◽  
Number Range ◽  
Curvature Effects ◽  
Flow Past

In Weinbaum et al. (1976) a simple new pressure hypothesis is derived which enables one to take account of the displacement interaction, the geometrical change in streamline radius of curvature and centrifugal effects in the thick viscous layers surrounding two-dimensional bluff bodies in the intermediate Reynolds number range O(1) < Re < O(102) using conventional Prandtl boundary-layer equations. The new pressure hypothesis states that the streamwise pressure gradient as a function of distance from the forward stagnation point on the displacement body is equal to the wall pressure gradient as a function of distance along the original body. This hypothesis is shown to be equivalent to stretching the streamwise body co-ordinate in conventional first-order boundary-layer theory. The present investigation shows that the same pressure hypothesis applies for the intermediate Reynolds number flow past axisymmetric bluff bodies except that the viscous term in the conventional axisymmetric boundary-layer equation must also be modified for transverse curvature effects O(δ) in the divergence of the stress tensor. The approximate solutions presented for the location of separation and the detailed surface pressure and vorticity distribution for the flow past spheres, spheroids and paraboloids of revolution at various Reynolds numbers in the range O(1) < Re < O(102) are in good agreement with available numerical Navier–Stokes solutions.

CFD Study of the Pressure Distribution on the Back of a 3-D Bluff Body (CUBE) of Air Flow in Different Reynolds Numbers

2009 ◽  
Author(s):  
Mohammad Javad Izadi ◽  
Pegah Asghari ◽  
Malihe Kamkar Delakeh
Keyword(s):  
Reynolds Number ◽  
Bluff Body ◽  
Free Stream ◽  
Fluid Flows ◽  
Reynolds Numbers ◽  
The Body ◽  
Stream Velocity ◽  
Bluff Bodies ◽  
Unsteady Forces ◽  
Flow Round

The study of flow around bluff bodies is important, and has many applications in industry. Up to now, a few numerical studies have been done in this field. In this research a turbulent unsteady flow round a cube is simulated numerically. The LES method is used to simulate the turbulent flow around the cube since this method is more accurate to model time-depended flows than other numerical methods. When the air as an ideal fluid flows over the cube, flow separate from the back of the body and unsteady vortices appears, causing a large wake behind the cube. The Near-Wake (wake close to the body) plays an important role in determining the steady and unsteady forces on the body. In this study, to see the effect of the free stream velocity on the surface pressure behind the body, the Reynolds number is varied from one to four million and the pressure on the back of the cube is calculated numerically. From the results of this study, it can be seen that as the velocity or the Reynolds number increased, the pressure on the surface behind the cube decreased, but the rate of this decrease, increased as the free stream flow velocity increased. For high free stream velocities the base pressure did not change as much and therefore the base drag coefficient stayed constant (around 1.0).

Properties of the mean recirculation region in the wakes of two-dimensional bluff bodies

1997 ◽  
Vol 351 ◽  
pp. 167-199 ◽  
Author(s):  
S. BALACHANDAR ◽  
R. MITTAL ◽  
F. M. NAJJAR
Keyword(s):  
Reynolds Number ◽  
Shear Stress ◽  
Reynolds Stress ◽  
Vortex Shedding ◽  
Reynolds Numbers ◽  
Recirculation Region ◽  
Bluff Bodies ◽  
Two Dimensional ◽  
Stress Components ◽  
The Mean

The properties of the time- and span-averaged mean wake recirculation region are investigated in separated flows over several different two-dimensional bluff bodies. Ten different cases are considered and they divide into two groups: cylindrical geometries of circular, elliptic and square cross-sections and the normal plate. A wide Reynolds number range from 250 to 140000 is considered, but in all the cases the attached portion of the boundary layer remains laminar until separation. The lower Reynolds number data are from direct numerical simulations, while the data at the higher Reynolds number are obtained from large-eddy simulation and the experimental work of Cantwell & Coles (1983), Krothapalli (1996, personal communication), Leder (1991) and Lyn et al. (1995). Unlike supersonic and subsonic separations with a splitter plate in the wake, in all the cases considered here there is strong interaction between the shear layers resulting in Kármán vortex shedding. The impact of this fundamental difference on the distribution of Reynolds stress components and pressure in relation to the mean wake recirculation region (wake bubble) is considered. It is observed that in all cases the contribution from Reynolds normal stress to the force balance of the wake bubble is significant. In fact, in the cylinder geometries this contribution can outweigh the net force from the shear stress, so that the net pressure force tends to push the bubble away from the body. In contrast, in the case of normal plate, owing to the longer wake, the net contribution from shear stress outweighs that from the normal stress. At higher Reynolds numbers, separation of the Reynolds stress components into incoherent contributions provides more insight. The behaviour of the coherent contribution, arising from the dominant vortex shedding, is similar to that at lower Reynolds numbers. The incoherent contribution to Reynolds stress, arising from small-scale activity, is compared with that of a canonical free shear layer. Based on these observations a simple extension of the wake model (Sychev 1982; Roshko 1993a, b) is proposed.

Deep Learning Techniques for Effective Prediction of Aerodynamic Properties of Elliptical Bluff Bodies

2021 ◽  
Author(s):  
W. M. U. Weerasekara ◽  
H. M. C. D. B. Gunarathna ◽  
W. A. K. P. Wanigasooriya ◽  
T. P. Miyanawala
Keyword(s):  
Neural Network ◽  
Reynolds Number ◽  
Deep Learning ◽  
Aerodynamic Force ◽  
Flow Behavior ◽  
Reynolds Numbers ◽  
Bluff Bodies ◽  
Flow Conditions ◽  
Force Coefficients ◽  
Drag And Lift

Abstract Predicting aerodynamic forces on bluff bodies remains to be a challenging task due to the unpredictable flow behavior, specifically at higher Reynolds numbers. Experimental approaches to determine aerodynamic coefficients could be costly and time consuming. In the meantime, use of numerical techniques could also require a considerable computational cost and time depending on complexity of the flow behavior. The research focusses on developing an effective deep learning technique to predict aerodynamic force coefficients acting on elliptical bluff bodies for a given aspect ratio and given flow condition. Collecting data for drag and lift coefficients of several aspect ratios for flow conditions starting from onset of vortex shredding to verge of subcritical region is conducted by an accurate full order model. The specified region will provide a transient flow behavior and thus lift coefficient will be represented in terms of root mean square value and drag coefficient in terms of a mean value. With variations in flow behavior and vortex shredding frequencies, it requires to select an appropriate turbulence model, optimum discretization of fluid domain and time step to obtain an accurate result. Flow simulations are conducted primarily using Unsteady Reynolds Averaged Navier-Stokes Equations (URANS) model and Detached Eddy Simulations (DES) model. Effectiveness in using different turbulence models for specified flow regimes are also explored in comparison to available experimental results. At lower Reynolds numbers, aerodynamic force coefficients for a specified body will only depend on Reynolds number. But after a certain specific Reynolds number, aerodynamic forces are dependent on the Mach number in addition to Reynolds number. Therefore, for higher Reynolds numbers, aerodynamic force coefficients are recorded for multiple Mach numbers with same Reynolds number and will be fed to the neural network. With the development of the machine learning and neural network modelling, many of the fields have nourished and created effective and efficient technologies to ease complex functions and activities. Our goal is to ease the complexity in the computational fluid dynamic field with a deep neural network tool created to predict drag and lift coefficient of elliptical bluff bodies for a given aspect ratio with an acceptable accuracy level. Researchers have developed deep neural network tools to predict various flow conditions and have succeeded with sufficient accuracy and a satisfying reduction of computational cost. In our proposed deep learning neural network, we have chosen to model the network with inputs as the geometry setup and the flow conditions with validated drag and lift coefficients. The model will extract the necessary flow features into filters with the convolution operation performed on the inputs. Our main directive is to create a deep learned neural network tool to predict the target values within an acceptable range of accuracy while minimizing the computation cost.

A Numerical Investigation of Flow Around a Circular Cylinder Near a Flat Boundary

2012 ◽  
Author(s):  
Mário Caruso Neto ◽  
Juan B. V. Wanderley
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Flow Characteristics ◽  
Reynolds Numbers ◽  
Bluff Bodies ◽  
Conservative Scheme ◽  
Gap Ratio ◽  
Number Variation ◽  
High Reynolds ◽  
Number Case

Flow around a pipeline near the seabed still remains relatively unknown in spite of the efforts of many researchers to understand the complicated flow around bluff bodies. The present study contributes to this discussion numerically investigating two-dimensional fluid flow around a circular cylinder near a flat plate. The investigation contemplates Reynolds numbers of 100, 180 and 7000 and a gap ratio (G/D) of 3, 0.6, 0.3 and 0.125. The flow is simulated considering a finite difference and total variation diminishing (TVD) conservative scheme with a Chimera domain division method to solve RANS equations. The k-e turbulence model is used to simulate the turbulent flow in the high Reynolds number case. Results are obtained for force coefficients and flow visualization. The results show a significant variation of flow characteristics with gap ratio and Reynolds number variation.

Experimental and Numerical Study of the Three Dimensional Instabilities in the Wake of a Blunt Trailing Edge Profiled Body

2010 ◽  
Author(s):  
Arash Naghib Lahouti ◽  
Lakshmana Sampat Doddipatla ◽  
Horia Hangan ◽  
Kamran Siddiqui
Keyword(s):  
Reynolds Number ◽  
Numerical Study ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
The Body ◽  
Bluff Bodies ◽  
Trailing Edge ◽  
Blunt Trailing Edge ◽  
Image Velocimetry

The wake of nominally two dimensional bluff bodies is dominated by von Ka´rma´n vortices, which are accompanied by three dimensional instabilities beyond a threshold Reynolds number. These three dimensional instabilities initiate as dislocations in the von Ka´rma´n vortices near the trailing edge, which evolve into pairs of counter-rotating vortices further downstream. The wavelength of the three dimensional instabilities depends on profile geometry and Reynolds number. In the present study, the three dimensional wake instabilities for a blunt trailing edge profiled body, composed of an elliptical leading edge and a rectangular trailing edge, have been studied in Reynolds numbers ranging from 500 to 1200, based on the thickness of the body. Numerical simulations, Laser Induced Fluorescence (LIF) flow visualization, and Particle Image Velocimetry (PIV) methods have been used to identify the instabilities. Proper Orthogonal Decomposition (POD) has been used to analyze the velocity field data measured using PIV. The results confirm the existence of three dimensional instabilities with an average wavelength of 2.0 to 2.5 times thickness of the body, in the near wake. The findings are in agreements with the values reported previously for different Reynolds numbers, and extend the range of Reynolds numbers in which the three dimensional instabilities are characterized.

The Effect of Transverse Curvature on the Drag and Vortex Shedding of Elongated Bluff Bodies at Low Reynolds Number

1983 ◽  
Vol 105 (3) ◽  
pp. 308-318 ◽  
Author(s):  
D. R. Monson
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Vortex Flow ◽  
Vortex Pair ◽  
Reynolds Numbers ◽  
Finite Cylinder ◽  
Bluff Bodies ◽  
Helical Vortex ◽  
Solid Bodies ◽  
Linear Counterpart

This paper establishes the drag characteristics of finite cylinders of aspect ratio 1, 4, 10 and 100 for Reynolds numbers less than 1000 including the viscous regime. The effect of the drag and vortex shedding characteristics of curving a finite cylinder into a toroidal shape is investigated. The curvature reduces drag by as much as 13 percent over its linear counterpart in the viscous regime. Vortex shedding characteristics of tori include all the features of cylinders in addition to a solidity range that behaves like solid bodies and an intermediate range where two vortex flow patterns can exist. These patterns can occur either as alternating ring vortices or a less common but more stable counterrotating helical vortex pair.

Hybrid RANS-LES Formulations for Wake Interference Physics in Tandem Cylinders and Multi-Column Floaters

2015 ◽  
Author(s):  
Harish Gopalan ◽  
Peifeng Ma ◽  
Haihua Xu ◽  
Ankit Choudhary ◽  
Anis Hussain ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Reynolds Numbers ◽  
Hydrodynamic Forces ◽  
Hybrid Models ◽  
Bluff Bodies ◽  
Tandem Cylinders ◽  
High Reynolds Numbers ◽  
Spacing Ratio ◽  
Non Linear ◽  
High Reynolds

Accurate prediction of hydrodynamic forces on tandem bluff bodies at high Reynolds numbers is of interest in many fields of offshore engineering. The most commonly used turbulence modeling strategy for studying these flows is unsteady Reynolds-averaged Navier-Stokes methods (URANS) due to its speed. However, the accuracy of URANS results are problem dependent and usually poor for bluff bodies flow separation predictions. To overcome this deficiency, two different modeling methods have been considered: (i) large eddy simulation (LES) and (ii) non-linear URANS. LES are accurate and computationally feasible for low to moderate Reynolds number flows. However, the cost of LES makes it infeasible at high Reynolds numbers. On the other hand, non-linear URANS methods are fast like URANS, and its accuracy is comparable to LES for certain flows. It is usually not known in advance if the simulations using non-linear methods are accurate. Hybrid models have been proposed in the literature as an alternative to existing methods. They employ a URANS model in the near-body region and LES in the near and far wake regions. Simulations performed using hybrid models are computationally cheaper than LES and more accurate than URANS. Most hybrid models developed in the literature employ linear URANS models. The use of non-linear URANS models in the hybrid context has not received significant attention. In this study, we propose the use of a hybrid model based on a non-linear URANS model. Flow past tandem cylinders, with different spacing ratio, at sub-critical Reynolds number regime, is chosen as the test case. Simulations are also performed using URANS and linear hybrid models for comparison. It is shown that the non-linear hybrid models provides the best agreement to measurement data in the literature. Non-linear URANS models will be shown to provide acceptable prediction of hydrodynamic forces. The models are finally used to predict the current load on a generic multi-column floater.

Flow around six in-line square cylinders

2012 ◽  
Vol 710 ◽  
pp. 195-233 ◽  
Author(s):  
C. M. Sewatkar ◽  
Rahul Patel ◽  
Atul Sharma ◽  
Amit Agrawal
Keyword(s):  
Reynolds Number ◽  
Flow Regime ◽  
Lift Coefficient ◽  
Flow Regimes ◽  
Reynolds Numbers ◽  
Bluff Bodies ◽  
Square Cylinders ◽  
Chaotic Nature ◽  
Regime Map ◽  
Flow Regime Map

AbstractThe flow around six in-line square cylinders has been studied numerically and experimentally for $0. 5\leq s/ d\leq 10. 0$ and $80\leq \mathit{Re}\leq 320$, where $s$ is the surface-to-surface distance between two cylinders, $d$ is the size of the cylinder and $\mathit{Re}$ is the Reynolds number. The effect of spacing on the flow regimes is initially studied numerically at $\mathit{Re}= 100$ for which a synchronous flow regime is observed for $0. 5\leq s/ d\leq 1. 1$, while quasi-periodic-I, quasi-periodic-II and chaotic regimes occur between $1. 2\leq s/ d\leq 1. 3$, $1. 4\leq s/ d\leq 5. 0$ and $6. 0\leq s/ d\leq 10. 0$, respectively. These regimes have been confirmed via particle-image-velocimetry-based experiments. A flow regime map is proposed as a function of spacing and Reynolds number. The flow is predominantly quasi-periodic-II or chaotic at higher Reynolds numbers. The quasi-periodic and chaotic nature of the flow is due to the wake interference effect of the upstream cylinders which becomes more severe at higher Reynolds numbers. The appearance of flow regimes is opposite to that for a row of cylinders. The Strouhal number for vortex shedding is the same for all the cylinders, especially for synchronous and quasi-periodic-I flow regimes. The mean drag (${C}_{Dmean} $) experienced by the cylinders is less than that for an isolated cylinder, irrespective of the spacing. The first cylinder is relatively insensitive to the presence of downstream cylinders and the ${C}_{Dmean} $ is almost constant at 1.2. The ${C}_{Dmean} $ for the second and third cylinders may be negative, with the value of ${C}_{Dmean} $ increasing monotonically with spacing. The changes in root mean square lift coefficient are consistent with changes in ${C}_{Dmean} $. Interestingly, the instantaneous lift force can be larger than the instantaneous drag force on the cylinders. These results should help improve understanding of flow around multiple bluff bodies.

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