Effects of unsteadiness on turbulent boundary layers

Experimental analyses of oscillating turbulent flat plate boundary layers are discussed. Effects of adverse pressure gradient on oscillating turbulent boundary layers are described. Calculation of unsteady boundary layers is considered. It is shown that time mean flow is not affected by forced oscillation of the external flow. Average velocity profiles cannot be deduced from the time mean velocity profile by a quasisteady analysis, but can be represented by steady profiles. Unsteady effects are stronger with an adverse pressure gradient. Hysteresis in relationships between boundary layers characteristics was observed. Turbulence characteristics are modulated by the flow oscillation even at low frequency. At high frequency, turbulence characteristics are frozen in the outer region of the boundary layer. For low reduced frequencies integral methods give acceptable results. As the reduced frequency increases, use of a quasisteady skin friction law becomes wrong. Even at low reduced frequency, classic mixing length models give poor results (except for the time mean flow). Better results are obtained from transport equations models.