Transition of a Steady to a Periodically Unsteady Flow for Various Jet Widths of a Combined Wall Jet and Offset Jet

2016 ◽  
Vol 138 (7) ◽  
Author(s):  
Tanmoy Mondal ◽  
Manab Kumar Das ◽  
Abhijit Guha
Keyword(s):  
Flow Field ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Progressive Increase ◽  
Separation Distance ◽  
Wall Jet ◽  
Two Dimensional ◽  
Two Jets ◽  
Offset Jet

In the present paper, a dual jet consisting of a wall jet and an offset jet has been numerically simulated using two-dimensional unsteady Reynolds-Averaged Navier–Stokes (RANS) equations to examine the effects of jet width (w) variation on the near flow field region. The Reynolds number based on the separation distance between the two jets (d) has been considered to be Re = 10,000. According to the computational results, three distinct flow regimes have been identified as a function of w/d. For w/d ≤ 0.5, the flow field remains to be always steady with two counter-rotating stable vortices in between the two jets. On the contrary, within the range of 0.6 ≤ w/d < 1.6, the flow field reveals a periodic vortex shedding phenomenon similar to what would be observed in the wake of a two-dimensional bluff body. In this flow regime, the Strouhal number of vortex shedding frequency decreases monotonically with the progressive increase in the jet width. For w/d ≥ 1.6, the periodic vortex shedding is still evident, but the Strouhal number becomes insensitive to the variation of jet width.

Numerical study on mean flow field of turbulent dual offset jet

2021 ◽  
pp. 095440622110007
Author(s):  
Tanmoy Mondal ◽  
Shantanu Pramanik
Keyword(s):  
Flow Field ◽  
Numerical Study ◽  
Jet Flow ◽  
Separation Distance ◽  
Bottom Wall ◽  
Mean Flow ◽  
Reattachment Point ◽  
The Mean ◽  
Two Jets ◽  
Offset Jet

A numerical investigation on the mean flow and turbulence characteristics of dual offset jet for various separation distances between the two jets with a fixed offset height of the lower jet from the bottom wall is reported in this study. The numerical simulations have been performed by solving the Reynolds-averaged Navier-Stokes equations (RANS) with two-equation standard [Formula: see text] turbulence model. The Reynolds number based on the jet width and the inlet turbulence intensity are considered as 15,000 and 5%, respectively. The computational results for the mean flow reveal that after issuing from the nozzles, the adjacent shear layers of the offset jets meet together at the merging point and then the merged jets reattaches on the bottom wall at the reattachment point before they combine together at the combined point forming a single jet flow. In the far downstream, the flow field behaves like a classical single wall jet flow. The self-similarity of mean flow field is achieved at far down stream of combined point. An increase in separation distance between the two jets [Formula: see text] results in a decrease in magnitude of the streamwise maximum velocity of the combined jet but with same rate of decay. The converging region of the jets has depicted considerable growth of turbulence as the jet centrelines bend towards the merging point. According to the mean flow results, the distances of the reattachment point and the combined point from the nozzle exit gradually increase with the progressive increase in separation distance between the two jets within the range d/ w = 3–8.

Blockage Effects on Two-dimensional Bluff Body Flow

1975 ◽  
Vol 26 (4) ◽  
pp. 243-253 ◽  
Author(s):  
J E Fackrell
Keyword(s):  
Flow Field ◽  
Strouhal Number ◽  
Bluff Body ◽  
The Body ◽  
Base Pressure ◽  
Two Dimensional ◽  
Mean Flow ◽  
Free Air ◽  
Bluff Body Flow ◽  
Good Agreement

SummaryThe time mean flow past a two-dimensional bluff body in a wind-tunnel is modelled by an adaptation of the numerical free-streamline method of Bearman and Fackrell. Provided the base pressure and separation positions are specified in advance, the method allows the calculation of the flow field outside the wake and in particular of the pressures on the wetted surface of the body. Good agreement of the predicted pressures with experimental results is obtained for a flat plate, wedge and circular cylinder at various blockage ratios. In addition to predicting the confined flow, it is shown that a free-air base pressure can be simply deduced from the confined base pressure. With an assumption that the separation positions are unaffected by blockage, this allows the free-air flow field to be calculated. By using an analogy to Roshko’s Strouhal number, which has been found to be invariant under constraint, a free-air Strouhal number can also be deduced.

Aerodynamics of a two-dimensional bluff body with the cross-section of a train

2020 ◽  
Vol 23 (12) ◽  
pp. 2679-2693 ◽  
Author(s):  
Huan Li ◽  
Xuhui He ◽  
Hanfeng Wang ◽  
Si Peng ◽  
Shuwei Zhou ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Large Amplitude ◽  
High Speed ◽  
Bluff Body ◽  
Mean Amplitude ◽  
Two Dimensional ◽  
Aerodynamic Forces ◽  
Moderate Reynolds Number ◽  
Amplitude Mode

Experiments on the aerodynamics of a two-dimensional bluff body simplified from a China high-speed train in crosswinds were carried out in a wind tunnel. Effects of wind angle of attack α varying in [−20°, 20°] were investigated at a moderate Reynolds number Re = 9.35 × 104 (based on the height of the model). Four typical behaviors of aerodynamics were identified. These behaviors are attributed to the flow structure around the upper and lower halves of the model changing from full to intermittent reattachment, and to full separation with a variation in α. An alternate transition phenomenon, characterized by an alteration between large- and small-amplitude aerodynamic fluctuations, was detected. The frequency of this alteration is about 1/10 of the predominant vortex shedding. In the intervals of the large-amplitude behavior, aerodynamic forces fluctuate periodically with a strong span-wise coherence, which are caused by the anti-symmetric vortex shedding along the stream-wise direction. On the contrary, the aerodynamic forces fluctuating at small amplitudes correspond to a weak span-wise coherence, which are ascribed to the symmetric vortex shedding from the upper and lower halves of the model. Generally, the mean amplitude of the large-amplitude mode is 3 times larger than that of the small one. Finally, the effects of Reynolds number were examined within Re = [9.35 × 104, 2.49 × 105]. Strong Reynolds number dependence was observed on the model with two rounded upper corners.

Simulation of Two-Dimensional Turbulent Recirculating Flow Field Behind a V-Shaped Bluff Body

1994 ◽  
Author(s):  
Z. Gu ◽  
M. A. R. Sharif
Keyword(s):  
Flow Field ◽  
Bluff Body ◽  
Flame Stabilization ◽  
Bluff Bodies ◽  
Two Dimensional ◽  
Combustion Chambers ◽  
Recirculating Flow ◽  
Conservative Form ◽  
Curvilinear Grid ◽  
Predictor Corrector

Abstract The two-dimensional turbulent recirculating flow fields behind a V-shaped bluff body have been investigated numerically. Similar bluff bodies are used in combustion chambers for flame stabilization. The governing transport equations in conservative form are solved by a pressure based predictor-corrector method. The standard k-ϵ turbulence closure model and a boundary fitted multi-block curvilinear grid system are used in the computation. The code is validated against turbulent flow over a backward facing step problem. The predicted flow field behind the bluff body is also compared with experiment. It is found that while the qualitative features of the flow are well predicted, there is quantitative disagreement between the measurement and prediction. This disagreement can be partially attributed to the k-ϵ turbulence model which is known to be inadequate for recirculating flows. Parametric investigation of the flow field by varying the shape and size of the bluff body is also performed and the results are reported.

Numerical analysis of bluff body wakes under periodic open-loop control

2013 ◽  
Vol 739 ◽  
pp. 94-123 ◽  
Author(s):  
Derwin J. Parkin ◽  
M. C. Thompson ◽  
J. Sheridan
Keyword(s):  
Strouhal Number ◽  
Cross Section ◽  
Vortex Shedding ◽  
Open Loop ◽  
Wake Flow ◽  
Dynamic Mode ◽  
Two Dimensional ◽  
Near Wake ◽  
Recirculation Bubble ◽  
Number Range

AbstractLarge eddy simulations at$Re= 23\hspace{0.167em} 000$are used to investigate the drag on a two-dimensional elongated cylinder caused by rear-edge periodic actuation, with particular focus on an optimum open-loop configuration. The 3.64 (length/thickness) aspect-ratio cylinder has a rectangular cross-section with rounded leading corners, representing the two-dimensional cross-section of the now genericAhmed-body geometry. The simulations show that the optimum drag reduction occurs in the forcing Strouhal number range of$0. 09\leq S{t}_{act} \leq 0. 135$, which is approximately half of the Strouhal number corresponding to shedding of von Kármán vortices into the wake for the natural case. This result agrees well with recent experiments of Henninget al. (Active Flow Control, vol. 95, 2007, pp. 369–390). A thorough transient wake analysis employing dynamic mode decomposition is conducted for all cases, with special attention paid to the Koopman modes of the wake flow and vortex progression downstream. Two modes are found to coexist in all cases, the superimposition of which recovers the majority of features observed in the flow. Symmetric vortex shedding in the near wake, which effectively extends the mean recirculation bubble, is shown to be the major mechanism in lowering the drag. This is associated with opposite-signed vortices reducing the influence of natural vortex shedding, resulting in an increase in the pressure in the near wake, while the characteristic wake antisymmetry returns further downstream. Lower-frequency actuation is shown to create larger near-wake symmetric vortices, which improves the effectiveness of this process.

Vortex Shedding From Two Circular Cylinders in a Staggered Arrangement

2003 ◽  
Author(s):  
D. Sumner ◽  
M. D. Richards
Keyword(s):  
Strouhal Number ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Incidence Angle ◽  
Power Spectra ◽  
Circular Cylinders ◽  
Aerodynamic Forces ◽  
Staggered Arrangement ◽  
Pitch Ratio ◽  
Small Increments

Vortex shedding from two circular cylinders of equal diameter in a staggered configuration was studied experimentally in the subcritical Reynolds number regime, for Re = 3.2×104–7.4×104. The dimensionless centre-to-centre pitch ratio of the staggered cylinders was ranged from P/D = 1.125–4.0, and the incidence angle was varied in small increments from α = 0°–90°. The behaviour of the Strouhal number measurements was broadly classified according to whether the cylinders were closely, moderately, or widely spaced, corresponding to P/D < 1.5, 1.5 ≤ P/D ≤ 2.5, and P/D > 2.5, respectively. For closely spaced staggered configurations, the flow around the cylinders is similar to a single bluff body, and only a single Strouhal number is measured. For moderately spaced cylinders, two distinct Strouhal numbers are measured when α > 30°, but there is considerable scatter in the Strouhal data when α < 30°. For widely spaced cylinders, the Strouhal numbers remain close to that of a single circular cylinder, in contrast to the behaviour of the aerodynamic forces. Evidence of the outer lift peak is seen in the power spectra for the downstream cylinder.

scholarly journals The onset of unsteadiness of two-dimensional bodies falling or rising freely in a viscous fluid: a linear study

2011 ◽  
Vol 690 ◽  
pp. 173-202 ◽  
Author(s):  
Pauline Assemat ◽  
David Fabre ◽  
Jacques Magnaudet
Keyword(s):  
Reynolds Number ◽  
Viscous Fluid ◽  
Strouhal Number ◽  
Cross Section ◽  
Vortex Shedding ◽  
The Body ◽  
Mode Switching ◽  
Two Dimensional ◽  
Body Dynamics ◽  
Mass Ratios

AbstractWe consider the transition between the steady vertical path and the oscillatory path of two-dimensional bodies moving under the effect of buoyancy in a viscous fluid. Linearization of the Navier–Stokes equations governing the flow past the body and of Newton’s equations governing the body dynamics leads to an eigenvalue problem, which is solved numerically. Three different body geometries are then examined in detail, namely a quasi-infinitely thin plate, a plate of rectangular cross-section with an aspect ratio of 8, and a rod with a square cross-section. Two kinds of eigenmodes are observed in the limit of large body-to-fluid mass ratios, namely ‘fluid’ modes identical to those found in the wake of a fixed body, which are responsible for the onset of vortex shedding, and four additional ‘aerodynamic’ modes associated with much longer time scales, which are also predicted using a quasi-static model introduced in a companion paper. The stability thresholds are computed and the nature of the corresponding eigenmodes is investigated throughout the whole possible range of mass ratios. For thin bodies such as a flat plate, the Reynolds number characterizing the threshold of the first instability and the associated Strouhal number are observed to be comparable with those of the corresponding fixed body. Other modes are found to become unstable at larger Reynolds numbers, and complicated branch crossings leading to mode switching are observed. On the other hand, for bluff bodies such as a square rod, two unstable modes are detected in the range of Reynolds number corresponding to wake destabilization. For large enough mass ratios, the leading mode is similar to the vortex shedding mode past a fixed body, while for smaller mass ratios it is of a different nature, with a Strouhal number about half that of the vortex shedding mode and a stronger coupling with the body dynamics.

Flow Over an Inclined Plate

1962 ◽  
Vol 84 (3) ◽  
pp. 380-388 ◽  
Author(s):  
F. H. Abernathy
Keyword(s):  
Flow Field ◽  
Flat Plate ◽  
Strouhal Number ◽  
Angle Of Attack ◽  
Separation Distance ◽  
Lateral Flow ◽  
Vortex Street ◽  
Arbitrary Angle ◽  
Inclined Plate ◽  
Free Vortex

A free-streamline theory for an inclined flat plate in an infinite flow field, at an arbitrary angle of attack, is presented. Measurements of the location of the free-vortex layers, of the vortex-street frequency, and of the pressure behind an inclined sharp-edge plate are reported as a function of both the angle of attack of the plate and the lateral constriction of the flow. The separation between free-vortex layers is found experimentally to be essentially independent of lateral flow constriction. A form of the Strouhal number, using this separation distance as the characteristic flow dimension, is shown to be independent of lateral constriction of the flow and of the inclination of the plate.

Part I: Chordwise Distribution of Fluctuating Pressure

1981 ◽  
Vol 32 (2) ◽  
pp. 97-110 ◽  
Author(s):  
R.H. Wilkinson
Keyword(s):  
Vortex Shedding ◽  
Bluff Body ◽  
Lift Coefficient ◽  
Relative Magnitude ◽  
The Body ◽  
Front Face ◽  
Two Dimensional ◽  
Fluctuating Pressure ◽  
Pressure Distributions ◽  
Dimensional Section

SummaryThe fluctuating loading on a cylindrical bluff body due to vortex shedding increases if the body is capable of vibration. This is a result of amplification of the fluctuating pressures around a two-dimensional section of the body together with an improvement of the spanwise correlation of the vortex shedding. Measurement of the fluctuating forces on the cylinder during this process gives no guide as to the relative magnitude of these effects. In this paper, root mean square fluctuating pressure distributions and pressure correlations across a chord are presented for a square cylinder with front face normal to the approach flow whilst stationary and during forced vibration. The fluctuating lift coefficient for a two-dimensional section of the cylinder and its maximum amplification during vibration are calculated.

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