Effect of pulsatile flow to drag of a two-dimensional bluff body

Abstract

Drag and velocity measurement on a two-dimensional bluff body has been performed in a pulsatile flow. Circular and rectangular cylinders were selected in the present study. The projected width and length of the cylinders were d =30 mm and L =300 mm, respectively. The cycle-averaged velocity was 〈Vθ〉 ≅15 m/s and the amplitude was in the range of V′θm =0.45 to 3.6 m/s. The pulsation period of the flow was set in the range of T =1.5 to 10.0 s. The Reynolds number based on the phaseaveraged mean velocity and the projected width was in the range of Reθ =20000 to 40000. The cycle-averaged drag coefficient increases with the root mean value of the deviation from the cycle-averaged velocity of approaching flow to the cylinder. The cycle-averaged drag coefficient for T <6.0 s ( T〈Vθ〉/d <3.0× 103) increases compared with that estimated from the drag coefficient in the steady flow. For T ≥6.0 s ( T〈Vθ〉/d ≥3.0× 103), the drag coefficient of the steady flow can be used to calculate the cycle-averaged drag. The effect of the pulsation period on the phase-averaged drag occurs during temporal deceleration of the pulsatile flow and can be related with the increase of the velocity deficit and width in the wake behind the cylinder. The phase-averaged drag will be represented as the sum of the momentum deficit due to the phase-averaged flow, which contributes to the cycle-averaged drag, and the pressure gradient by the pulsatile flow. This can be formulated semi-empirically with the cycle-averaged value of the phase-averaged drag coefficient, phase-averaged velocity and the derivative of the pulsatile flow with respect to time.

Keywords

• unsteady flow
• pulsatile flow
• two-dimensonal bluff body
• drag
• wake