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 done to assess the unsteady wake dynamics. Two of the v-gutter's geometrical parameters are varied: the v-gutter's base angle ($\theta$) and the size of a slit ($\xi$) 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 $M_\infty = 0.25$ and a freestream Reynolds number based on bluff body's transverse length (L) of $Re_L=0.1 \times 10^6$. Five v-gutter angles are considered ($\theta$, $^\circ = \pi/6, \pi/4, \pi/3, 5\pi/12, \pi/2$) and three slit sizes ($\xi$, mm =0,0.25,0.5) are considered only for a particular $\theta = [\pi/6]$. In general, high fluctuations in velocity and pressure are seen for the bluffest body in consideration ($\theta = \pi/2$) with higher drag ($c_d$) and total pressure loss ($\Delta p_0$). On the other hand, the presence of a slit on a streamlined body ($\theta = \pi/6$) tends to efficiently stabilize the wake and thus, producing almost a periodic shedding structure with lower $c_d$ and $\Delta p_0$. For $\theta = [\pi/6]$, broadened spectra in vortex shedding is seen with a peak at $[fL/u_\infty] \sim 0.08$. For $\theta \geq [\pi/4]$, a dominant discrete shedding frequency is seen with a gradual spectral decay. Similarly, the effects of $\xi$ on the $\theta = [\pi/6]$ case produce a discrete shedding frequency instead of a broadened one, as told before. The shedding frequency increases to a maximum of $[fL/u_\infty] \sim 0.26$ for the maximum slit size of $\xi = 0.5$. 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.