In automobile engines,vapor can be condensed when exhaust gas including water vapor is cooled at such as an intake manifold,and condensed water becomes liquid film.When the liquid film is scattered to sensors in a pipe,their performances will be deteriorated.Therefore,it is important to predict and control liquid film formation and behavior.When gas flows on liquid film,it is confirmed that two or three-dimensional steady waves appear,or their breakup and scattering occur depends on gas and liquid conditions.There have been some studies to classify steady wave or transition to breakup and scattering by cross-flow.However,clear visualization measurements of interaction between liquid film and cross-flow are also needed because transition and scattering are indefinite to be predicted.In our experiments,air from wind tunnel flows on the water film in a square duct,and the film is photographed using the high speed camera in various conditions of air and water velocities.It is found that the liquid film behavior can be definitely classified in four,that is,smooth region,wave region,transition region and scattering region by dimensionless parameters of Reynolds number,Weber number and Capillary number defined by liquid phase(water)flow quantity and gas phase(air)velocity.All flow region is varied velocity differences of gas-liquid phase and liquid film thickness and Capillary number is approximately constant on the border of each region.In future,our research will progress numerical analysis of two phase flow.