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).

Vortex Shedding From a Bluff Body Adjacent to a Plane Sliding Wall

1991 ◽  
Vol 113 (3) ◽  
pp. 384-398 ◽  
Author(s):  
M. P. Arnal ◽  
D. J. Goering ◽  
J. A. C. Humphrey
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Solid Wall ◽  
Reynolds Numbers ◽  
Careful Examination ◽  
Recirculation Region ◽  
Stream Velocity ◽  
Flow Past ◽  
Lower Reynolds Number

The characteristics of the flow around a bluff body of square cross-section in contact with a solid-wall boundary are investigated numerically using a finite difference procedure. Previous studies (Taneda, 1965; Kamemoto et al., 1984) have shown qualitatively the strong influence of solid-wall boundaries on the vortex-shedding process and the formation of the vortex street downstream. In the present study three cases are investigated which correspond to flow past a square rib in a freestream, flow past a rib on a fixed wall and flow past a rib on a sliding wall. Values of the Reynolds number studied ranged from 100 to 2000, where the Reynolds number is based on the rib height, H, and bulk stream velocity, Ub. Comparisons between the sliding-wall and fixed-wall cases show that the sliding wall has a significant destabilizing effect on the recirculation region behind the rib. Results show the onset of unsteadiness at a lower Reynolds number for the sliding-wall case (50 ≤ Recrit ≤100) than for the fixed-wall case (Recrit≥100). A careful examination of the vortex-shedding process reveals similarities between the sliding-wall case and both the freestream and fixed-wall cases. At moderate Reynolds numbers (Re≥250) the sliding-wall results show that the rib periodically sheds vortices of alternating circulation in much the same manner as the rib in a freestream; as in, for example, Davis and Moore [1982]. The vortices are distributed asymmetrically downstream of the rib and are not of equal strength as in the freestream case. However, the sliding-wall case shows no tendency to develop cycle-to-cycle variations at higher Reynolds numbers, as observed in the freestream and fixed-wall cases. Thus, while the moving wall causes the flow past the rib to become unsteady at a lower Reynolds number than in the fixed-wall case, it also acts to stabilize or “lock-in” the vortex-shedding frequency. This is attributed to the additional source of positive vorticity immediately downstream of the rib on the sliding wall.

Numerical and Experimental Evaluation of Lift Forces in Flows Around Surface-Mounted Bluff Bodies

2002 ◽  
Author(s):  
T. Stengel ◽  
F. Ebert ◽  
M. Fallen
Keyword(s):  
Velocity Field ◽  
Bluff Body ◽  
Recirculation Zone ◽  
Separation Zone ◽  
Laser Doppler Anemometry ◽  
Turbulence Models ◽  
Reynolds Numbers ◽  
The Body ◽  
Bluff Bodies ◽  
Large Eddy

The flow around a surface-mounted bluff body with cuboid shape is investigated. Therefore, the velocity field including the distribution of the turbulent kinetic energy is computed and compared with experimental Laser Doppler Anemometry data. Several different turbulence models, namely the standard k-ε model, the Wolfshtein two-layer k-ε model and a Large-Eddy approach are validated. Since the Large-Eddy model remains the only model representing the flow accurate, it is chosen for further investigations. The pressure distribution on the body and on the carrying surface around the body is analysed. The lift coefficients are computed for Reynolds numbers, ranging from 1.1 × 104 up to 4.4 × 104. The lengths of the separation zone above and the recirculation zone downstream the body are evaluated.

Unsteady drag on a sphere at finite Reynolds number with small fluctuations in the free-stream velocity

1991 ◽  
Vol 233 ◽  
pp. 613-631 ◽  
Author(s):  
Renwei Mei ◽  
Christopher J. Lawrence ◽  
Ronald J. Adrian
Keyword(s):  
Reynolds Number ◽  
Free Stream ◽  
Low Frequency ◽  
Reynolds Numbers ◽  
Free Stream Velocity ◽  
Creeping Flow ◽  
Stream Velocity ◽  
Velocity Difference ◽  
Long Time ◽  
The Time Domain

Unsteady flow over a stationary sphere with small fluctuations in the free-stream velocity is considered at finite Reynolds number using a finite-difference method. The dependence of the unsteady drag on the frequency of the fluctuations is examined at various Reynolds numbers. It is found that the classical Stokes solution of the unsteady Stokes equation does not correctly describe the behaviour of the unsteady drag at low frequency. Numerical results indicate that the force increases linearly with frequency when the frequency is very small instead of increasing linearly with the square root of the frequency as the classical Stokes solution predicts. This implies that the force has a much shorter memory in the time domain. The incorrect behaviour of the Basset force at large times may explain the unphysical results found by Reeks & Mckee (1984) wherein for a particle introduced to a turbulent flow the initial velocity difference between the particle and fluid has a finite contribution to the long-time particle diffusivity. The added mass component of the force at finite Reynolds number is found to be the same as predicted by creeping flow and potential theories. Effects of Reynolds number on the unsteady drag due to the fluctuating free-stream velocity are presented. The implications for particle motion in turbulence are discussed.

Flow due to an oscillating sphere and an expression for unsteady drag on the sphere at finite Reynolds number

1994 ◽  
Vol 270 ◽  
pp. 133-174 ◽  
Author(s):  
Renwei Mei
Keyword(s):  
Reynolds Number ◽  
Dynamic Equation ◽  
High Frequency ◽  
Free Stream ◽  
Reynolds Numbers ◽  
Stream Velocity ◽  
Particle Dynamic ◽  
History Force ◽  
The Time Domain ◽  
Oscillating Sphere

Unsteady flow due to an oscillating sphere with a velocity U0cosωt’, in which U0 and ω are the amplitude and frequency of the oscillation and t’ is time, is investigated at finite Reynolds number. The methods used are: (i) Fourier mode expansion in the frequency domain; (ii) a time-dependent finite difference technique in the time domain; and (iii) a matched asymptotic expansion for high-frequency oscillation. The flow fields of the steady streaming component, the second and third harmonic components are obtained with the fundamental component. The dependence of the unsteady drag on ω is examined at small and finite Reynolds numbers. For large Stokes number, ε = (ωa2/2v)½ [Gt ] 1, in which a is the radius of the sphere and v is the kinematic viscosity, the numerical result for the unsteady drag agrees well with the high-frequency asymptotic solution; and the Stokes (1851) solution is valid for finite Re at ε [Gt ] 1. For small Strouhal number, St = ωa/U0 [Lt ] 1, the imaginary component of the unsteady drag (Scaled by 6πU0pfva, in which Pf is the fluid density) behaves as Dml ∼ (h0Stlog St–h1St), m = 1,3,5… This is in direct contrast to an earlier result obtained for an unsteady flow over a stationary sphere with a small-amplitude oscillation in the free-stream velocity (hereinafter referred to as the SA case) in which D1∼ –h1St (Mei, Lawrence & Adrian 1991). Computations for flow over a sphere with a free-stream velocity U0(1–α1+α1cosωt’) at Re = U02a/v = 0.2 and St [Lt ] 1 show that h0 for the first mode varies from 0 (at α1 = 0) to around 0.5 (at α1 = 1) and that the SA case is a degenerated case in which the logarithmic dependence of the drag in St is suppressed by the strong mean uniform flow.The numerical results for the unsteady drag are used to examine an approximate particle dynamic equation proposed for spherical particles with finite Reynolds number. The equation includes a quasi-steady drag, an added-mass force, and a modified history force. The approximate expression for the history force in the time domain compares very well with the numerical results of the SA case for all frequencies; it compares favourably for the PO case for moderate and high frequencies; it underestimates slightly the history force for the PO case at low frequency. For a solid sphere settling in a stagnant liquid with zero initial velocity, the velocity history is computed using the proposed particle dynamic equation. The results compare very well with experimental data of Moorman (1955) over a large range of Reynolds numbers. The present particle dynamic equation at finite Re performs consistently better than that proposed by Odar & Hamilton (1964) both qualitatively and quantitatively for three different types of spatially uniform unsteady flows.

Reducing the drag on a circular cylinder by upstream installation of an I-type bluff body as passive control

2009 ◽  
Vol 223 (10) ◽  
pp. 2291-2296 ◽  
Author(s):  
Y Triyogi ◽  
D Suprayogi ◽  
E Spirda
Keyword(s):  
Circular Cylinder ◽  
Velocity Vector ◽  
Bluff Body ◽  
Passive Control ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Bluff Bodies ◽  
Subsonic Wind Tunnel ◽  
Large Cylinder

The bluff body cut from a small circular cylinder that is cut at both sides parallel to the y-axis was used as passive control to reduce the drag of a larger circular cylinder. The small bluff body cut is called an I-type bluff body, which interacts with a larger one downstream. I-type bluff bodies with different cutting angles of θs = 0°(circular), 10°, 20°, 30°, 45°, 53°, and 65° were located in front and at the line axis of the circular cylinder at a spacing S/ d = 1.375, where their cutting surfaces are perpendicular to the free stream velocity vector. The tandem arrangement was tested in a subsonic wind tunnel at a Reynolds number (based on the diameter d of the circular cylinder and free stream velocity) of Re = 5.3×104. The results show that installing the bluff bodies (circular or sliced) as a passive control in front of the large circular cylinder effectively reduces the drag of the large cylinder. The passive control with cutting angle θs = 65° gives the highest drag reduction on the large circular cylinder situated downstream. It gives about 0.52 times the drag of a single cylinder.

A proposal concerning laminar wakes behind bluff bodies at large Reynolds number

1956 ◽  
Vol 1 (4) ◽  
pp. 388-398 ◽  
Author(s):  
G. K. Batchelor
Keyword(s):  
Reynolds Number ◽  
Steady Flow ◽  
Bluff Body ◽  
The Body ◽  
Navier Stokes ◽  
Large Reynolds Number ◽  
Bluff Bodies ◽  
Proper Solution ◽  
Navier Stokes Equation ◽  
Limit Solution

This note advocates a model of the steady flow about a bluff body at large Reynolds number which is different from the classical free-streamline model of Helmholtz and Kirchhoff. It is suggested that, although the free-streamline model may be a proper solution of the Navier-Stokes equation with μ = 0, it is unlikely to be the limit, as μ → 0, of the solution describing the steady flow due to the presence of a bluff body in an otherwise uniform stream. The limit solution proposed here is one which gives a closed wake.A closed wake contains a standing eddy, or eddies, whose general features can be inferred from the results of an earlier investigation of steady flow in a closed region at large Reynolds number. In all cases, the drag (coefficient) on the body tends to zero as the Reynolds number tends to infinity. The proccedure for finding the details of the closed wake behind two-dimensional and axisymmetrical bodies is described, although no particular case has yet been worked out.

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.

Flow past a sphere with an oscillation in the free-stream velocity and unsteady drag at finite Reynolds number

1992 ◽  
Vol 237 ◽  
pp. 323-341 ◽  
Author(s):  
Renwei Mei ◽  
Ronald J. Adrian
Keyword(s):  
Reynolds Number ◽  
Finite Difference ◽  
Unsteady Flow ◽  
Large Time ◽  
Free Stream ◽  
Reynolds Numbers ◽  
Stream Velocity ◽  
Small Reynolds Number ◽  
Dependent Part ◽  
History Force

Unsteady flow over a stationary sphere with a small fluctuation in the free-stream velocity is considered at small Reynolds number, Re. A matched asymptotic solution is obtained for the frequency-dependent (or the acceleration-dependent) part of the unsteady flow at very small frequency, ω, under the restriction St [Lt ] Re [Lt ] 1, where St is the Strouhal number. The acceleration-dependent part of the unsteady drag is found to be proportional to St ∼ ω instead of the ω½ dependence predicted by Stokes’ solution. Consequently, the expression for the Basset history force is incorrect for large time even for very small Reynolds numbers. Present results compare well with the previous numerical results of Mei, Lawrence & Adrian (1991) using a finite-difference method for the same unsteady flow at small Reynolds number. Using the principle of causality, the present analytical results at small Re, the numerical results at finite Re for low frequency, and Stokes’ results for high frequency, a modified expression for the history force is proposed in the time domain. It is confirmed by comparing with the finite-difference results at arbitrary frequency through Fourier transformation. The modified history force has an integration kernel that decays as t−2, instead of t½, at large time for both small and finite Reynolds numbers.

scholarly journals Turbulence Intensity Modulation by Micropolar Fluids

Fluids ◽  
2021 ◽  
Vol 6 (6) ◽  
pp. 195
Author(s):  
George Sofiadis ◽  
Ioannis Sarris
Keyword(s):  
Reynolds Number ◽  
Turbulence Intensity ◽  
Fluid Flows ◽  
Turbulent Channel Flow ◽  
High Volume ◽  
Reynolds Numbers ◽  
Intensity Modulation ◽  
Wall Turbulence ◽  
Volume Fractions ◽  
Physical Phenomena

Fluid microstructure nature has a direct effect on turbulence enhancement or attenuation. Certain classes of fluids, such as polymers, tend to reduce turbulence intensity, while others, like dense suspensions, present the opposite results. In this article, we take into consideration the micropolar class of fluids and investigate turbulence intensity modulation for three different Reynolds numbers, as well as different volume fractions of the micropolar density, in a turbulent channel flow. Our findings support that, for low micropolar volume fractions, turbulence presents a monotonic enhancement as the Reynolds number increases. However, on the other hand, for sufficiently high volume fractions, turbulence intensity drops, along with Reynolds number increment. This result is considered to be due to the effect of the micropolar force term on the flow, suppressing near-wall turbulence and enforcing turbulence activity to move further away from the wall. This is the first time that such an observation is made for the class of micropolar fluid flows, and can further assist our understanding of physical phenomena in the more general non-Newtonian flow regime.

Model reduction and mechanism for the vortex-induced vibrations of bluff bodies

2017 ◽  
Vol 827 ◽  
pp. 357-393 ◽  
Author(s):  
W. Yao ◽  
R. K. Jaiman
Keyword(s):  
Reynolds Number ◽  
Bluff Body ◽  
Low Reynolds Number ◽  
Smooth Curve ◽  
Bluff Bodies ◽  
Sharp Corner ◽  
Mass Ratio ◽  
Square Cylinder ◽  
Low Reynolds ◽  
Lock In

We present an effective reduced-order model (ROM) technique to couple an incompressible flow with a transversely vibrating bluff body in a state-space format. The ROM of the unsteady wake flow is based on the Navier–Stokes equations and is constructed by means of an eigensystem realization algorithm (ERA). We investigate the underlying mechanism of vortex-induced vibration (VIV) of a circular cylinder at low Reynolds number via linear stability analysis. To understand the frequency lock-in mechanism and self-sustained VIV phenomenon, a systematic analysis is performed by examining the eigenvalue trajectories of the ERA-based ROM for a range of reduced oscillation frequency $(F_{s})$, while maintaining fixed values of the Reynolds number ($Re$) and mass ratio ($m^{\ast }$). The effects of the Reynolds number $Re$, the mass ratio $m^{\ast }$ and the rounding of a square cylinder are examined to generalize the proposed ERA-based ROM for the VIV lock-in analysis. The considered cylinder configurations are a basic square with sharp corners, a circle and three intermediate rounded squares, which are created by varying a single rounding parameter. The results show that the two frequency lock-in regimes, the so-called resonance and flutter, only exist when certain conditions are satisfied, and the regimes have a strong dependence on the shape of the bluff body, the Reynolds number and the mass ratio. In addition, the frequency lock-in during VIV of a square cylinder is found to be dominated by the resonance regime, without any coupled-mode flutter at low Reynolds number. To further discern the influence of geometry on the VIV lock-in mechanism, we consider the smooth curve geometry of an ellipse and two sharp corner geometries of forward triangle and diamond-shaped bluff bodies. While the ellipse and diamond geometries exhibit the flutter and mixed resonance–flutter regimes, the forward triangle undergoes only the flutter-induced lock-in for $30\leqslant Re\leqslant 100$ at $m^{\ast }=10$. In the case of the forward triangle configuration, the ERA-based ROM accurately predicts the low-frequency galloping instability. We observe a kink in the amplitude response associated with 1:3 synchronization, whereby the forward triangular body oscillates at a single dominant frequency but the lift force has a frequency component at three times the body oscillation frequency. Finally, we present a stability phase diagram to summarize the VIV lock-in regimes of the five smooth-curve- and sharp-corner-based bluff bodies. These findings attempt to generalize our understanding of the VIV lock-in mechanism for bluff bodies at low Reynolds number. The proposed ERA-based ROM is found to be accurate, efficient and easy to use for the linear stability analysis of VIV, and it can have a profound impact on the development of control strategies for nonlinear vortex shedding and VIV.

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