China clay and kerosene is an easy mixture which can be applied to the surfaces of wind tunnel models. We recommend a mixture containing a weight ratio of 1:10 for clay to kerosene. The flow is visualised by running the tunnel until the kerosene has fully evaporated, leaving the clay pattern behind.

The mixture should be applied liberally using a paintbrush to the surface of the model, without much care being taken unless there are some precious electronics (e.g. a loadcell) nearby. The mixture will evaporate quite quickly, so it is paramount that the tunnel be started as quickly as possible.

Note that if you are applying the clay to the underside of the model, gravity will cause the mixture to drip off and the results may not be clear. Surface flow visualisation works best on the top surfaces of models at moderate AoA.

The top picture shows the results on the top surface of a swept wing, while the second photo shows the bottom surface. It is clear that the patterns on the suction side are clearer. The interference from the mounting points, as well as the spanwise flow and wingtip vortices are seen clearly. Similar results are seen on the straight wing in image 3, with perhaps a boundary layer transition point seen at approximately the mid chord point. Finally, the last image shows a model Concorde tested at moderate AoA. The leading edge flow separation and subsequent formation of a vortex is clearly seen. Note that in this test, the suction side was upside down in the tunnel (i.e. fighting gravity). Better results can be achieved when the model is oriented differently.

When using oil for flow visualisation, it is key to have some fluorescent dye to aid in seeing the flow pattern. A good mixture is 75ml of parraffin oil with 3 drops of dye. The dye does not have to be a specific type, but can be similar to the ones used for identifying cracks in engine casings. Along with this, a UV light is required to illuminate the dye in the dark. As the oil does not evaporate, turning on the tunnel is not "time critical". The oil mixture can be applied with a brush to the surface of the model. The higher the viscosity of the oil, the longer it will take before a result is achieved. The oil weight can also be used to the testers advantage, e.g. If you have a vertical 2D wind section, the oil will be fighting gravity and you may get drops forming. A more viscous oil can be used in this case to prevent this from happening.