Characteristics of Flow Past a Slender, Emergent Cylinder in Shallow Open Channels

The complex wake created by an emergent circular cylinder in a shallow open channel flow is studied experimentally using Laser Doppler Velocimetry (LDV) Technique. While previous wake studies in shallow condition are focused on the small aspect ratio cylinder, here the wake features of shallow flows past a bed mounted cylinder with large aspect ratio (L/D) of 13.7; is investigated where L is the wetted length of the cylinder and D is the cylinder diameter. The complexity of the wake is investigated by conducting velocity measurements on horizontal planes at near-bed, mid-depth, and near the free surface as well as on the vertical mid-plane. Measurements were taken in both upstream and downstream of the cylinder. The wake region is clearly demarcated from series of measurements in a very fine grid made of X/D- 0.25-0.5 intervals where X is the distance from the centre. The unique characteristics of the bed-mounted slender cylinder wake are critically analyzed. Velocity fields, turbulence parameters, and wake development in shallow open channel flow are studied at a cylinder Reynolds number(Re_D) of 3313 in water and at a subcritical Froude number of 0.58. It is found that stagnant re-circulation region behind the cylinder in all horizontal planes, where the size of the mean re-circulation region is found to be different. Consequently, it is found that wake length (lc) (distance to the wake closure point from the centre) tends to decrease as the distance from channel bed increases; i.e near the channel bed lc is highest and lowest lc is obtained near free surface. This suggests that the presence of the free surface has only a weak effect on the re-circulation region when the cylinder is in emergent condition. On the other hand, the wake is wider in the near free surface in the transverse direction and more narrow in near bed region close to the cylinder confirming the suppression effect. Further, in the near-bed plane, higher turbulence intensities and Reynolds shear stresses were observed. Combined with stronger vertical velocity and turbulence intensities noted near the bed in the vertical mid-plane, this suggests increased activity of the vortex structures at the low Reynolds numbers. In addition, after comparing the similar studies in the literature it is also noted that wake characteristics of slender cylinders are significantly different to wake generated by small aspect ratio cylinders.