Experimental Study on Interference Effect of Three Risers Arranged in Tandem with Variable Spacing Ratios

Taking the three-riser group arranged in tandem as the research subject,an experimental study was carried out on the risers arranged in tandem.The purpose is to explore the sensitivity of the dynamic response of each riser to spacing ratio and reveal the physical mechanism of riser groups under the interference effect.The spacing ratios of the adj acent risers are 4.0,5.0,6.0,and 8.0.At the spacing between the risers of 4.0D,the strong feedback effect increases the cross-flow(CF) displacement amplitude of the upstream riser.The shielding effect is the key factor affecting the interference effect on the midstream and downstream risers.At low reduced velocities,the shielding area initially appears,the displacement amplitude of the midstream and downstream risers varies greatly,the vibration of the two risers is still dominated by the first-order mode,and the transition between adjacent vibration modes is restrained.The multi-frequency superposition phenomenon is very significant at high reduced velocities.The most sensitive interference spacing under the test conditions is obtained.Due to the separation of the incoming flow and the double shielding effect of the upstream and midstream risers,the regular vortex-induced vibration in the wake area of the downstream riser is broken,and the vibration in the two directions is weakened.In general,the interference effect is more significant for the CF vibration of the three-riser groups than the in-line(IL) vibration.

NUMERICAL COMPUTATION OF THE FLOW AROUND TWO SQUARE CYLINDERS ARRANGED SIDE-BY-SIDE

The numerical method is used to calculate the flow around two square cylinders arranged side-by-side and the mean and fluctuating aerodynamic forces, and Strouhal numbers and power spectrum of lift force and drag force are obtained. An improved MAC method proposed by Chen Suqin et al.,which uses three order upwind scheme to discretize the convection term and uses multigrid method to solve the Poisson equation for pressure is applied to simulate the flow around two square cylinders arranged side-by-side. Results show that the interference characteristic of two square cylinders arranged side-by-side is completely different with the different spacing ratio. When the spacing ratio is smaller than a certain critical value, the gap flow between two cylinders is biased to one side in a stable or unstable manner.

Flow and passive scalar transfer characteristics around a row of interacting tandem cylinders

Transfer and transport of a passive scalar as well as flow structures of unbounded flow over a row of 2,3,4 and 5 equispaced identical cylinders at Re=90 and pitch ratios 1.05D≤S≤7D are investigated.Studies are performed through high fidelity simulations using a novel sharp-interface immersed boundary method.Five flow regimes are identified based on the nature of flow in the gap between the first two cylinders:dormant in the gap where fluid in the gap is motionless;symmetric in the gap where fluid in the gap behaves like a cavity flow;alternating in the gap where fluid in the gap is trapped but flaps up and down;wake in the gap where periodic vortices are shed in the gap;and bi-stable flow regime where vortices are shed in the gap for some time and then quasi-periodic twin vortices prevail for other times.A jump in values of hydrodynamic forces,pressure coefficient,Strouhal number,and Sherwood number is observed as pitch ratio increases due to change in flow structure.At high pitch ratio,a convectively unstable two-row vortical train is formed by the second cylinder.Member cylinders immersed in the two-row vortical structures have no effect on flow structure downstream or upstream and portray poor scalar transfer characteristics.The effect of configuration size on flow structures is found not to be as prominent as that of pitch ratio.