The effect of local surface heating on friction in a turbulent boundary layer on a plate

The paper describes the results of numerical. investigations of the effect of local surface heating of a flat plate flown about by air on the flow in a developed turbulent boundary layer. Downstream of the heating area, the surface was assumed to be heat-insulated. It is demonstrated that the friction increases at the beginning of the heating area, and then the friction coefficient decreases, with its minimum values attained in the heat-insulated area of the surface immediately following the heating area. It is found that the decrease of the turbulent friction coefficient, as compared with its value on an isothermal adiabatic surface, extends to cover a considerable distance downstream of the heating area. It is shown that, owing to the observed effect of ''long memory'' of turbulent boundary layer, an appreciable reduction of friction is attained in the case of local heat input as compared with uniform heating of the entire streamlined surface. In the presence of several successive heating areas, this ''long memory'' effect increases, and a more substantial reduction of turbulent friction may be accomplished.