There have been a few reports in the literature of counter-Evershed flows observed in well developed sunspot penumbrae , i.e .
flows directed towards the umbra along penumbral filaments .
Here we investigate the driving forces of such counter-Evershed flows in a radiative magnetohydrodynamic simulation of a sunspot and compare them with the forces acting on the normal Evershed flow .
The simulation covers a timespan of 100 solar hours and generates an Evershed outflow exceeding 8 km s ^ { -1 } in the penumbra along radially aligned filaments where the magnetic field is almost horizontal .
Additionally , the simulation produces a fast counter-Evershed flow ( i.e. , an inflow near \tau = 1 ) in some regions within the penumbra , reaching peak flow speeds of \sim 12 km s ^ { -1 } .
The counter-Evershed flows are transient and typically last a few hours before they turn into outflows again .
By using the kinetic energy equation and evaluating its various terms in the simulation box , we found that the Evershed flow occurs due to overturning convection in a strongly inclined magnetic field while the counter-Evershed flows can be well described as siphon flows .