A hot-wire anemometer is used to determine the boundary layer velocity profile in the air flow over a scale model of an automobile. The hot-wire is held in a traversing mechanism that moves the wire in a direction normal to the surface of the model. The hot-wire is operated at constant temperature. The boundary layer thickness is to be defined as the distance from the model urface at which the velocity is 90% of the free stream. If the probe current is low when the hot-wire is held in the free stream velocity, U∞, What current will indicate the edge of the boundary layer? Neglect radiation heat transfer from the hot-wire and conduction from the ends of the wire.
GIVEN
Thin, electrically-heated constant-temperature wire in air flow near an automobile model
Boundary layer thickness ° point when velocity (Uy) = 90% free stream velocity Vo
Probe current at U∞ = Io
ASSUMPTIONS
Radiation is negligible
Conduction from the ends of the hot-wire is negligible
Reynolds number is small