Characterization of the wake of large depth-ratio cylinders at low Reynolds numbers

• Author / Creator

  Zargar, Arash

• This dissertation characterizes the wake dynamics of long depth-ratio wall-mounted rectangular cylinders at a range of Reynolds numbers between 250-1000 at different incidence (yaw) angles, 0° - 45° with respect to the free stream flow. The effect of large depth-ratio on flow characteristics and vortex evolution is first explored for steady and unsteady wakes of wall-mounted cylinders at zero incident angle using Large Eddy Simulations (LES). Following this fundamental wake characterization at zero incident-angle, the unsteady wake evolution by large incident (yaw) angles are investigated at similar Reynolds numbers.
  The fundamental characterization of the wake was completed through a detailed comparison of the wake features for square (small-depth ratio of 0.83) and long-rectangular (large-depth ratio of 4.15) wall-mounted cylinders at two Reynolds numbers. The wake models for the latter were developed and compared with the existing models for square cylinders. LES results showed that the simulated wake at Reynolds number of 250 is steady for both cylinders, whereas there was transition to an unsteady wake at Reynolds number of 1000. The latter wake was dominated by hairpin-like structures formed on top of the cylinder and vortex loops formed by the unsteady transition of horseshoe vortices. The long lasting effect in the wake, however, belonged to an arc-type structure in the near wake region, the frequency signature of which remained apparent farther downstream. A new skeleton model is developed and introduced for the wake of a long depth-ratio cylinder that identifies three distinct flow features. In development of this model, the Strouhal number of Sthp =0.38 and 0.64 were attributed to the hairpin-like vortices on the cylinder upper and side faces, respectively. The dominant frequency corresponding to the Strouhal number of 0.26 was due to the horseshoe vortex loops. The peak frequency corresponding to StH =0.175 was ascribed by the arc-type structure in the wake, which persisted farther downstream the wake.
  Study of the wake was then extended to characterize the unsteady wake of the large depth-ratio wall-mounted rectangular cylinder at large incident (yaw) angles even at Reynolds number of 250. LES simulations are carried out for 10 different incidence angles between i=0° and i=45° at 5° increments to understand the implications of incident angle on altering the wake topology. Numerical results showed that increasing the incidence angle changes the wake from symmetric to asymmetric, which then translates to unsteady features.
  For the incidence angles of i ≤ 35°, the separated flow from the rear and leeward leading edges reattach to the cylinder at distinct points. The onset of wake unsteadiness occurs at 35° < i ≤ 40°. The weakly unsteady flow characteristics are captured in the wake of the cylinder at i=40° with small amplitude fluctuations identified in the velocity and pressure fields. Increasing the incidence angle to 45° coincided with a stronger wake unsteadiness that incorporated low and high frequency features. The main difference in the flow due to changing incidence angle is the formation of helical structures that dominate the wake region at i=40° and 45°. Three distinct frequencies were observed in the wake of the cylinder at i=45°. The Strouhal number of St =0.13 was attributed to the regular vortex shedding process in the wake region. The low-frequency fluctuations corresponding to StH =0.0146 were due to the alterations to the shape and size of unsteadily evolving helical structures that spin out of the vortex tubes. There also existed a high-frequency fluctuation at St=0.46, which was connected to the onset of unsteadiness, or the unsteady wake transition mechanism. It was further determined that the high-frequency and low-frequency fluctuations were connected in such a manner that the former initiated the onset of unsteady wake, through the formation of helical structures, while the latter suppressed the unsteady wake features. Moreover, the low-frequency suppression mechanism appeared to start in the wake and propagate to the shear layers on the cylinder.

• Subjects / Keywords

  • Wall-mounted cylinder
  • Fluid Mechanics
  • Wake
  • Long depth-ratio rectangular cylinders
  • Wake dynamics
  • Flow coherent structures at high incidence (yaw) angles
  • DNS
  • LES

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-mh9w-p249

• License

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