Energy balance in the flow around a square cylinder was examined with the data of numerical simulations at the Reynolds number 100. The mechanical energy, the sum of kinetic energy and pressure, decays due to the viscous dissipation resulted from a drastic change near the edge of the front surface and the viscous friction in the boundary layer on the both sides of the cylinder. Small amount of dissipation was also found in the formation region of the Karman vortex street. The kinetic energy shows an excess over the pressure energy drop at the edge of the front side and the pressure energy, on the contrary, shows a deficit in the formation region of the Karman vortex street. The difference of the input and the output energies, however, is within one percent of the input kinetic energy.
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