Modeling of the HLLgd solver of each dust fluid depending on the Riemann problem at the interface: Riemann problem on the gas in lines and kinematic coupling situation in columns. In blue, the gas wave fan defined by S _g,L and S _g,R. With the dotted green lines, we show the dust normal velocities, V_d,L and V_d,R. The HLLgd flux for the dust species d is described in red by its wave fan.