A novel experimental facility to impose unsteady pressure gradients on turbulent boundary layers

Abstract: The design and characterization of a new removable wind tunnel installation to impose unsteady pressure gradients (PGs) on flat plate turbulent boundary layers (TBLs) are presented. An electropneumatic actuation mechanism was used to rapidly deform a flat ceiling section into an inverted convex bump, producing a temporally strengthening favorable and adverse PG in spatial sequence. The design allowed the vertical extent of deformation and the speed of deformation of the ceiling to be independently varied in a controlled manner to access a series of spatial and temporal strengths of PGs. High-frequency pressure measurements were carried out to characterize the spatio-temporal pressure distributions in the test area for 18 test cases. The resulting range of PGs is presented in terms of non-dimensional parameters relevant to PG TBLs: the acceleration parameter (K), which varied in the range [3 , - 2.5] × 10 ^{- 6}, the Clauser PG parameter (β), in the range ± 7 , and the non-dimensional gradient of pressure coefficient (dCpd(x/L)), in the range ± 2.6. The temporal rates of change of PGs are presented in terms of the reduced frequency (k) and are in the range [0.19, 2.75]. The current and future potential for using this facility to impose a wide range of steady and unsteady PGs in a wind tunnel to enable fundamental studies of various engineering flows of interest are discussed. Graphical Abstract: [Figure not available: see fulltext.].