Unsteady dynamics in a subsonic duct flow with a bluff body

TL;DR

This study uses reduced-order simulations to analyze how geometrical variations of a bluff body in subsonic flow affect wake unsteadiness, vortex shedding, and pressure fluctuations, revealing stabilization effects of slits and flow frequency characteristics.

Contribution

It introduces a detailed numerical analysis of how v-gutter geometry influences unsteady wake dynamics and vortex shedding in subsonic duct flows, highlighting stabilization mechanisms.

Findings

Higher velocity and pressure fluctuations for larger bluff body angles.

Slits on streamlined bodies stabilize wake and reduce fluctuations.

Maximum shedding frequency observed at largest slit size.

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.