Abstract

The unsteady wake dynamics of two finite wall-mounted cylinders of unequal height and arranged in tandem are investigated using time-resolved particle image velocimetry (TR-PIV). The cylinders were fully submerged in a turbulent boundary layer with a Reynolds number based on the cylinder diameter (d) of 5540 and boundary layer thickness of $\delta /d = 8.7$ . The centre-to-centre spacing between the cylinders was fixed at 4d. The degree of sheltering was studied by varying the height of the upstream cylinder (UC) $(h/d \in [0.7\unicode{x2013} 7.0])$ while keeping the height of the downstream cylinder (DC) constant at $H/d\; = \; 7.0$ . The resulting height ratios were $h/H = 0.10,0.25,0.50,0.75$ and $1.00$ denoted as HR10, HR25, HR50, HR75 and HR100, respectively. The wake dynamics of the DC were also compared to those of an isolated single cylinder (SC) with similar Reynolds number, aspect ratio and submergence ratio. TR-PIV measurements were performed in the symmetry plane for all test cases and five spanwise planes along the height of HR75. The results showed that as the height ratio increases, the downwash from the free end of the UC impinges directly on the frontal surface of the DC and induces a strong upwash on the opposite rear side of the DC. The induced upwash impedes the downwash from the free end of the DC much earlier than observed behind the SC, resulting in a reduced reverse flow area and high velocity deficit in the sheltered portion of the DC. Nonetheless, the reverse flow area behind the UC and DC undergoes a quasi-periodic pumping motion with frequencies that are synchronized for HR25 to HR100. Spectral analysis and proper orthogonal decomposition also reveal that the attachment of the shear layers of the UC on the DC also promotes a lock-in anti-symmetric vortex shedding behaviour.

No abstract

No abstract

This paper presents experimental investigation of upstream roughness and Reynolds number effects on the recirculation region over a smooth forward facing step. The upstream rough wall was produced from 1.5 mm sand grains and the Reynolds number based on step height, Re h , was varied from 2040 to 9130 for both the upstream smooth and rough walls. For the smooth wall, the reattachment length increased monotonically with Re h to an asymptotic value of 2.2 step heights for Re h ≥ 6380. Upstream roughness reduced the reattachment length by 44% because of larger momentum deficit and higher turbulence level in the rough wall boundary layer. The mean velocities and Reynolds stresses were also reduced by roughness. The Reynolds shear stress and production of turbulent kinetic energy showed high negative values at the leading edge of the step indicating counter-gradient diffusion. The implications of these results for standard eddy viscosity models are discussed.