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Published online by Cambridge University Press: 28 April 2025
History effects play a significant role in determining the velocity in boundary layers with pressure gradients, complicating the identification of a velocity scaling. This work pivots away from traditional velocity analysis to focus on fluid acceleration in boundary layers with strong adverse pressure gradients. We draw parallels between the transport equation of the velocity in an equilibrium spatially evolving boundary layer and the transport equation of the fluid acceleration in temporally evolving boundary layers with pressure gradients, establishing an analogy between the two. To validate our analogy, we show that the laminar Stokes solution, which describes the flow immediately after the application of a pressure gradient force, is consistent with the present analogy. Furthermore, fluid acceleration exhibits a linear scaling in the wall layer and transitions to logarithmic scaling away from the wall after the initial period, mirroring the velocity in an equilibrium boundary layer, lending further support to the analogy. Finally, by integrating fluid acceleration, a velocity scaling is derived, which compares favourably with data as well.