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The spatiotemporal dynamics of a turbulent boundary layer subjected to an unsteady pressure gradient are studied. A dynamic sequence of favourable to adverse pressure gradients (FAPGs) is imposed by deforming a section of the wind tunnel ceiling, transitioning the pressure gradient from zero to a strong FAPG within 0.07 s. At the end of the transient, the acceleration parameter is K=6x10^-6 in the favourable pressure gradient (FPG) region and K=-4.8x10^-6 in the adverse pressure gradient (APG) region. The resulting unsteady response of the boundary layer is compared with equivalent steady pressure gradient cases in terms of turbulent statistics and coherent structures. While the steady FAPG effects, as shown by Parthasarathy & Saxton-Fox (2023), caused upstream stabilisation in the FPG, a milder APG response downstream, and the formation of an internal layer, the unsteady case presented in this paper shows a reduced stabilisation in the FPG region, a stronger APG response and a weaker internal layer. This altered response is hypothesised to stem from the different spatiotemporal pressure gradient histories experienced by turbulent structures when the pressure gradient changes at a time scale comparable to their convection. 

A statically deformable ceiling was used at the Turbulence Dynamic Research facility at the University of Illinois at Urbana-Champaign. This facility was used to impose static spatially varying pressure gradients in a favorable-adverse (FAPG) arrangement. This study focuses on the FPG region of this flowfield which experiences a rapid spatial variation of its pressure gradient over 6.2𝛿. Non-time-resolved particle imaging velocimetry data were captured for 6 pressure gradients and 6 Reynolds numbers. This manuscript focuses on 𝑅𝑒𝜏 = 953 and four pressure gradient cases that have maximum acceleration parameters of 𝐾𝑚𝑎𝑥 × 106 = 0, 2.43, 4.77, and 5.97. The Reynolds stresses and turbulent kinetic energy of the boundary layer were investigated. The results were analyzed for the occurrence of relaminarization and it was concluded that no relaminarization occurred in any of the cases. The Reynolds stresses showed behavior suggestive of an internal layer, including knee points in 𝑢′𝑢′. However, no conclusive evidence was found to support this hypothesis.