Unsteadiness in the turbulent separated flow over a three-dimensional Gaussian bump

The unsteady separated flow over the three-dimensional Boeing Gaussian bump is investigated at a Reynolds number based on bump height italic Re Subscript upper H Baseline equals 2.26 times 10 Superscript 5 Re H = 2.26 × 10 5 ReH = 2.26× 10⁵ using unsteady wall-pressure measurements and planar particle image velocimetry (PIV). Four major unsteady broadband phenomena spanning more than two decades in frequency are identified: (i) a very-low-frequency (VLF) spanwise motion centred at a Strouhal number of upper S t Subscript upper H Baseline tilde 10 Superscript negative 3 S t H ∼ 10 − 3 StH~ 10⁻³ (1 Hz) based on bump height, (ii) a low-frequency (LF) breathing motion of the separation zone centred at upper S t Subscript upper L Sub Subscript italic sep Baseline equals 0.068 S t L sep = 0.068 St_Lₛₑₚ=0.068 (13.5 Hz) where upper L Subscript italic sep L sep Lₛₑₚ is the mean separation length, (iii) a localised 20 Hz LF signature near the lateral shear-layer/surface-vortex region and (iv) a centreline shear-layer vortex shedding at upper S t Subscript upper L Sub Subscript italic sep Baseline equals 0.68 minus 1.01 S t L sep = 0.68 − 1.01 St_Lₛₑₚ=0.68-1.01 (135–200 Hz). Interestingly, while the VLF mode has a characteristic frequency of the same order as that often reported for other rectilinear bodies and hills that exhibit bistable asymmetric wake switching, it is found that the VLF mode for this geometry exhibits a continuous spanwise meandering motion. Joint symmetric–antisymmetric proper orthogonal decomposition modal statistics from top-down PIV data further show that the spanwise meandering and streamwise stretching of the wake – likely associated with the breathing motion – are dynamically coupled, with the separation zone reaching its greatest streamwise extent when in a symmetric state. In this paper, the observed hierarchy of spectral features is comparable to those observed for a wide range of geometries, suggesting connections between geometric length scales and the LF dynamics.