Experimental study on the characteristics of the near wake of a blade-like body with an axisymmetric cross-section rotating about one end. (2nd report. Influences of the tip clearance flow).
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Abstract:
The characteristics of the wake of a blunt body near its tip were investigated experimentally. When the tip clearance is large, the downwash flow is so large that the radial component of the velocity in the wake promotes mixing and diffusion of the wake. Therefore the velocity recovery is rapid and the width of the wake increases quickly in the wake. The velocity fluctuation in the wake becomes irregular, and vortex shedding disappears near the tip of the body. On the other hand when the tip clearance is small, the downwash flow is weak and near the tip of the body the dead air region becomes large as a result of gathering the low energy fluid. So the mixing and diffusion in the wake are inactive, and the regularity of the velocity fluctuation in the wake is kept up to the tip of the body.Keywords:
Downwash
Vortex shedding
Obstructed flow around single cylinder and two paratactic cylinders in nanoscale were investigated in the view of discrete particles. Transient and temporal-averaged flow and density fields were obtained to analyse the wake flow. For single cylinder case, Stokes flow, steady vortex flow, periodic vortex-shedding flow with the Karman vortex street and supersonic flow were distinguished based on Re. For two paratactic cylinders case, periodic vortex-shedding flow, periodic vortex-shedding flow with gap-flow, bistable flow and synchronised vortex-shedding flow were observed with different centre-to-centre pitch ratios. Despite of some special characteristics, the results indicate most macroscopic flow patterns still exist in nanoscale.
Vortex shedding
Kármán vortex street
Hele-Shaw flow
Transient (computer programming)
Starting vortex
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In order to better understand the von Kármán vortex shedding on the basis of the concept of absolute instability, the vortex shedding from a blunt-ended flat-plate is examined experimentally through controlling the wake development. The stability analysis is also made for the velocity distributions in the near wake region to obtain their local instability characteristics. When the vortex shedding is controlled by another flat-plate in the wake, the shedding frequency is remarkedly changed. Such change in frequency is found to be closely related to the change in the absolute instability characteristics of the wake velocity distributions immediately behind the plate base.
Vortex shedding
Kármán vortex street
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The characteristics of the wake of a blunt body near its tip were investigated experimentally. When the tip clearance is large, the downwash flow is so large that the radial component of the velocity in the wake promotes mixing and diffusion of the wake. Therefore the velocity recovery is rapid and the width of the wake increases quickly in the wake. The velocity fluctuation in the wake becomes irregular, and vortex shedding disappears near the tip of the body. On the other hand when the tip clearance is small, the downwash flow is weak and near the tip of the body the dead air region becomes large as a result of gathering the low energy fluid. So the mixing and diffusion in the wake are inactive, and the regularity of the velocity fluctuation in the wake is kept up to the tip of the body.
Downwash
Vortex shedding
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Vortex shedding
Wake turbulence
Kármán vortex street
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Bluff
Vortex shedding
Kármán vortex street
Wake turbulence
Flow Control
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Vortex shedding
Kármán vortex street
Lock (firearm)
Load Shedding
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Vortex shedding
Kármán vortex street
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A passive control is numerically investigated in this paper to
effectively suppress the vortex shedding from bluff bodies. To
implement the control, thr
ee sinusoidal leading edge
configurations are considered and compared to a straight square
cylinder case. Large Eddy Simulati
on (LES) is used to model the
wall effects, as well as the near wake.
Numerical velocity fluctuations in the near wake compare well
with experimental data. The LES observations provide an accurate
prediction of wake instability and near wake topology, otherwise
not provided experimentally. This advantage can be used for
further investigation of
a thorough understanding and
enhancement of control.
Numerical case studies are presented using the software package
FLUENT
and the observations are pres
ented in the form of design
charts, as well as velocity spectra and near wake flow details.
Vortex shedding
Bluff
Flow Control
Kármán vortex street
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Abstract The phenomenon of Karman-type vortex shedding from finite square cylinders of the aspect ratios 2, 4 and 6 at Reynolds number of 1.9 × 105 were examined with the employment of a three-component force balance to measure the aerodynamic forces. The signals of the lateral force measured were first analyzed by the HHT (Hilbert-Huang Transformation) together with a conditional sampling technique for identifying the time periods during which the vortex shedding frequency component was prominent. Meanwhile, the force measured in the vertical direction was analyzed by the same procedure to identify the events of pronounced unsteady downwash motion induced by the flow over the finite end of the model. Therefore, the unsteady flow motions around a finite cylinder model could be categorized into four patterns. Namely, the patterns 11 and 10 denote the situations of pronounced Karman-type vortex shedding with and without strong downwash motion, respectively; and the patterns 01 and 00 denote the situations of no pronounced vortex shedding with and without strong downwash motion, respectively. The results obtained show that the pattern 00 occupied more than 60% of the time sampled, apparently dominant over the other three patterns, whereas the second popular pattern 10 (Karman type vortex shedding) occupied no more than 30% of the time sampled. Further experiments were made for the square cylinder of the aspect ratio 6, with a hot-wire situated near either side of the cylinder. By analyzing the unsteady lateral forces experienced by the cylinder and the hot-wire velocity data with the data reduction scheme employed, it is unveiled that the Karman-type vortex shedding induced an anti-symmetric, unsteady flow motion around the cylinder, although this component was not the dominant one. On the other hand, it is found that a large portion of the fluctuating energy was resided in the low-frequency component featuring a symmetric unsteady flow motion around the cylinder. This finding further supports the earlier observation that the flow pattern 00 is most commonly seen in the unsteady flow motions around a finite square cylinder.
Vortex shedding
Downwash
Kármán vortex street
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Vortex shedding
Bluff
Wake turbulence
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Citations (95)