Flow induced oscillations (FIO) of two rough, rigid, tandem-cylinders on springs are investigated for hydrokinetic power conversion at Reynolds number 30,000 ≤ Re ≤ 120,000. Passive turbulence control (PTC) in the form of roughness strips is applied to enhance FIO and increase the power harness efficiency of the VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) converter. Numerical simulations are performed using two-dimensional, Unsteady Reynolds-Averaged Navier-Stokes equations with the Spalart-Allmaras turbulence model. The center-to-center spacing ratio d/D of the two cylinders is set as 2.0 or 2.57 with mass ratio m* = 1.343, damping ratio ζ = 0.26, and stiffness K = 1,200N/m. Amplitude response, frequency response, interaction, energy harvesting, and conversion efficiency are presented and discussed. Results are compared to experimental data for validation and reveal how the interaction of two tandem cylinders may enhance the harnessed power. CFD provides complementary valuable information on visualization of wake and vortex structures. The main conclusions are: (1) In the VIV region at Re = 60,000, the amplitude response, frequency response, harnessed power, and power conversion efficiency of the upstream cylinder is the same for the two spacing ratios tested. Due to the shedding effect, the motion of the downstream cylinder for spacing ratio d/D = 2.0 is more severely suppressed than spacing ratio d/D = 2.57, which reduces the harnessed power and conversion efficiency for the downstream cylinder. (2) In the galloping region at Re = 110,000, due to the different timing of impingement of the shed vortices on the downstream cylinder, the upstream cylinder harnesses more power and has higher energy conversion efficiency for spacing ratio d/D = 2.0 than d/D = 2.57.