Session G25: Flow Instability: Kelvin-Helmholtz, Wakes & Pulsatile Flow

On the critical spacing of stationary tandem circular cylinders

For steady flow past two stationary circular cylinders that are aligned in tandem with their axes perpendicular to the flow direction, the wake pattern, vortex shedding frequency, and forces on the cylinders typically show an abrupt change as a function of the dimensionless center-to-center cylinder spacing over a range of critical spacing values.  This fundamental problem has been studied for over a century, but knowledge gaps remain and published results are inconsistent.  For Re<190, the flow is expected to be laminar and two dimensional, and hence well behaved, yet prior experimental and computational results do not agree regarding the value of the critical spacing.  Using flow visualization techniques in a flowing soap film system with Re=100, we have obtained, apparently for the first time, experimental results that are consistent with published computational results.  We will discuss the physical characteristics of this flow, particularly the hysteresis that occurs in the value of the critical spacing as a function of the initial conditions.     

Effect of Reynolds number on the separated-reattached flow upstream of a fence

The flow development upstream of a rigid fence is investigated experimentally for three Reynolds numbers 18 000, 36 000, and 54 000. Time-resolved, planar particle image velocimetry is used to capture the instantaneous velocity fields with high spatio-temporal resolution in key areas, while smoke wire flow visualization is used to obtain a global description of the flow. The results reveal a complex separating-reattaching flow region two to three fence heights upstream of the fence. In addition to a local flow separation in the immediate vicinity of the fence, a laminar separation bubble is formed upstream and its mean extent decreases with increasing Reynolds number. Coherent structures are shed from the upstream separation region significantly affecting flow development downstream and exhibiting a strong dependence on the Reynolds number.   

Transition in Pulsatile Flows: The Role of Pressure Field

Pulsatile flows are of interest because for certain range of characteristic non-dimensional parameters, they display laminar and turbulent behavior at different times of the pulsating cycle. Addressing how turbulence appears, decays and is suppressed is challenging due to the flow unsteadiness and flow-wall interactions. An experiment was setup to replicate pulsatile motion of water in a clear, rigid pipe. The flow is driven by a piston-motor assembly controlled by a computer to induce cyclic motion of the mean flow. Time-Resolved Particle Image Velocimetry (TR-PIV) techniques are used to acquire velocity data on the plane of a CW laser illumination sheet. Pressure sensors were installed along the length of the pipe to monitor the flow and its variation during the transition process. Depending on the Reynolds number the hydrostatic pressure difference along the vertical diameter of the pipe causes the velocity profile to become asymmetric. Time-dependent activities related to transition and turbulence in the near wall are highly asymmetric.    

Transition mechanisms of pulsatile flow in a constricted channel

Physical mechanisms based on the energy variation of two-dimensional modes are proposed for a pulsatile flow in a constricted channel with 50% occlusion. Floquet stability analysis and direct numerical simulations are carried out to explain the transition mechanisms observed in this system. This system is characterized by two distinct states; the first state lies in a two-dimensional space and the second in a three-dimensional space. When a three-dimensional nonlinear simulation is seeded with a two-dimensional base flow and an infinitesimal perturbation, there is energy transfer from the three-dimensional modes to the two-dimensional mode, such that the energy of the latter slightly increases and the three-dimensional modes go to zero, leading the system to its unconstrained state. In addition, the base flow behaviour follows the energy variation of the two-dimensional modes with Reynolds number. Thus, base flow changes occur when the Reynolds number is increased, even after the primary instability. Besides that, the two-dimensional modes go through an energy minimum in this process which lead the system to a different dynamic.