These are Bar and Fin Oil coolers commonly known as D Cup Coolers. Typically these have bars with internal fins or tabulators & external fins. The bars act as heat sinks for the heat from the oil or fluid flowing through the heat exchanger. External fins are attached to plates or bars. This increases the surface area for better cooling effectiveness. The heat is transferred from the oil to the bars. The heat in turn is transferred from the bars to the external fins. From the external fins the heat is dissipated by the air flow generated by the fan.