Numerical studies on the effect of surface roughness on flow field and flow-induced vibration in the in-line tube bundle

Surface roughness effects on flow field and flow-induced vibration (FIV) in the in-line tube bundle are studied by three-dimensional refined fully coupled numerical simulation. The equivalent sand grain approach is used to simulate rough wall. The research focuses on a smooth tube bundle and rough tube bundles with four different degrees of surface roughness, K-s/d = 0, 1 x 10(-4), 5 x 10(-3), 1 x 10(-2), 2 x 10(-2). The uncoupled flow field without FIV is analyzed first. Then, the flow-induced vibration responses are analyzed for typical cases in the three continuous FIV mechanisms, namely turbulence-induced vibration, vortex-induced resonance, fluidelastic instability. The results show that the increasing roughness causes the delayed boundary layer separation, which causes the lower mean drag coefficient (C-D), root-mean-square (RMS) C-D, RMS lift coefficient (C-L) and the lower strength of vortex in uncoupled flow field. For the lower flow velocity of vortex-induced resonance, 0.15-0.27 m/s, compared with the behaviour of smooth tubes, the vibration of rough tube bundle is more difficult to synchronize the deflections of flow patterns, which causes the amplitude of the downstream two row tubes is lower than that of smooth tubes. However, for the higher flow velocity of vortex-induced resonance and fluidelastic instability, 0.27-0.41 m/s, compared with smooth tube bundle, the amplitude of the total rough tube bundle is larger and increases faster, because the smaller drag causes the smaller total damping of the quasi-steady streamwise motion of tube. The critical velocity of instability for the rough tube bundle at K-s/d = 0.02 is earlier about 6.7 % than that for smooth tube bundle.