Enforcement of Flow-Induced Oscillations of a Circular Cylinder due to Interference Effects: Numerical Simulations and Experimental Studies

摘要

The enforcement of flow induced oscillations of a spring mounted circular cylinder (d_(cyl)) due to the presence of a fixed square cylinder (d_(s.cyl)) is investigated. An integrated approach takes place by means of 2D CFD simulations and experimental studies in the wind tunnel. The results show that a reduction of the square cylinder distance (g) leads to a higher oscillation of the upstream cylinder (until a critical distance is reached). Furthermore, there is an enlargement of the flow velocity region where lock-in takes place (factor 4 at g/d_(cyl) = 0.8). The main reason for those phenomena that can be found is the raising pressure field downstream of the cylinder due to the square cylinder interference. This increased pressure field leads to a reduction of the separation angle which controls the lift force of the upstream cylinder. The alternating vortices are now farther away from one another what increases the time of their energy absorption before they cut each other off. The reduced vortex interaction leads to a slower rise of the phase angle between lift force and cylinder movement with increasing flow velocity. Hence the vortex separation pattern remains longer in the 2S mode where higher lift forces occur than in the self-limiting 2P mode. Therefore the leaving of the lock-in region at a critical phase angle can be delayed.