Unsteady dynamics in a subsonic duct flow with a bluff body

Abstract

A series of reduced-order numerical simulations on a specific bluff body type (v-gutters) in a subsonic duct flow is performed to assess the unsteady wake dynamics. Two of the v-gutter's geometrical parameters are varied: the v-gutter's base angle (θ) and the size of a slit (ξ) at the leading-edge of the v-gutter. Turbulent flow kinematics and pressure field are analyzed to evaluate the unsteadiness at a freestream Mach number of M∞=0.25 and a freestream Reynolds number based on bluff body's transverse length (L = 17.8 mm) of ReL=0.1×106. Five v-gutter angles are considered (θ, rad=π/6,π/4,π/3,5π/12,π/2) and three slit sizes (ξ, mm =0,0.25,0.5) are considered only for a particular θ=[π/6]. In general, high fluctuations in velocity and pressure are seen for the bluffest body in consideration (θ=π/2) with higher drag (cd) and total pressure loss (Δp0). However, bluffer bodies produce periodic shedding structures that promote flow mixing. On the other hand, the presence of a slit on a streamlined body (θ=π/6) tends to efficiently stabilize the wake and thus producing almost a periodic shedding structure with lower cd and Δp0. For θ=[π/6], broadened spectra in vortex shedding are seen with a peak at [fL/u∞]∼0.08. For θ≥[π/4], a dominant discrete shedding frequency is seen with a gradual spectral decay. Similarly, the effects of ξ on the θ=[π/6] case produce a discrete shedding frequency instead of a broadened one, as seen in the cases of θ≥[π/4]. The shedding frequency increases to a maximum of [fL/u∞]∼0.26 for the maximum slit size of ξ=0.5 mm. From the analysis of the x–t diagram and the modal analysis of vorticity and velocity magnitude in the wake, the peaks are indeed found to agree with the spectral analysis. More insights on the shedding vortices, momentum deficit in the wake, varying energy contents in the flow field, and the dominant spatiotemporal structures are also provided.