The flow field in a two-dimensional three-ramp hypersonic mixed-compression inlet in a freestream Mach number of M_∞ = 5 is numerically solved to understand the unsteady throttling dynamics. Throttling conditions are simulated by varying the exit area of the isolator in the form of plug insets. Different throttling ratios between 0 ≤ ζ ≤ 0.7 in steps of 0.1 are considered. No unsteadiness is observed for ζ ≤ 0.2, and severe unsteadiness is found for 0.3 ≤ ζ ≤ 0.7. The frequency of unsteadiness (f) increases rapidly with ζ. As ζ increases, the amount of reversed mass inside the isolator scales with the frequency and the exit mass flow rate. A general framework is attempted to scale the unsteady events based on the gathered knowledge from the numerical study. The inlet–isolator flow is modeled as an oscillating flow through a duct with known upstream design conditions such as the freestream Mach number (M_∞) and the isolator inlet Mach number (M_i). Factors such as the mass occupied by the duct volume, the characteristic unsteady frequency, the throttling ratio, and the exit mass flow rate through the duct are used to form a non-dimensional parameter β, which scales with the upstream design parameter ξ = M_i/M_∞. The scaling parameters are further exploited to formulate a semi-empirical relation using the existing experimental results at different throttling ratios from the open literature. The unsteady frequencies from the present two-dimensional numerical exercise are also shown to agree with the proposed scaling and the resulting semi-empirical relation.

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.