Jet-Spike Bifurcation in High-Speed Flows

Abstract

In recent plasma injection experiments, counter ow jet interaction has been identi ed as one of the mechanisms that signi cantly reduces the drag. There is a sudden change of the dynamic state from an oscillatory to nearly steady motion of this jet-shock interaction which depends on the relative magnitude of injection and the stagnation pressure. The shock-wavebifurcation associated with a counter ow jet from a hemispherical cylinder is investigated by a side-by-side experimental and computationaleffort. Shock-wave bifurcation has been discovered by the present experiments over the entire range of tested conditionsin the Mach 6 wind tunnel. This oscillatory motionis sustained by the upstream propagationof selectively ampli ed frequencies from the free-shear layer to the Mach disk through the embedded subsonic domain. The breakdown of this feedback loop occurs when a higher injecting jet pressure creates a supersonic zone separating the interconnecting embedded subsonic domains. Nomenclature D = total drag p = air pressure Q = conservative variables q = primitive variable r = radius of hemispheric nose T = air temperature u = velocity vector, u, v, w D = standoff distance of bow shock q = air density

Numerical investigation of a jet from a blunt body opposing a supersonic flow

“…The evolution of the jet structures is closely associated with the unsteady off-axis motion, consistent with the experimental observations (e.g. Karashima & Sato 1975;Shang et al 2001). These 3D flow phenomena cannot be predicted by an axisymmetric simulation (e.g.…”

“…As the conical shear layer reattaches to the body surface, the flow realigns with the body contour and also causes a pressure rise; a reattachment shock is then generated which intersects with the bow shock to form a triple point. Because of the unsteady motion of the shock system shown below, the shocks are blurred based on the mean pattern in figure 2(a), consistent with the experimental photographs (Karashima & Sato 1975;Shang et al 2001). Moreover, the phenomena described above can be confirmed based on the temporal evolution of the flow field and will be further discussed later.…”

“…Moreover, the jet layer becomes unstable quickly and the vortical coherent structures convect downstream on the outside of the jet column. Since the subsonic region occurs as typically shown in figure 3(a), there exists a feedback resonance mechanism between the conical shear layer and the Mach disk of this flow field (Shang et al 2001). The jet structures undergo a corresponding change subsequently.…”

“…Figure 4(a,b) shows the flow structures using the iso-contours of ∇ρ in the meridian planes for P = 0.816. The deviation of the jet back to the body results in a complicated conical shear layer (Finley 1966;Shang et al 2001), as marked in figure 4(a). The conical shear layer eventually reattaches to the body surface, triggering a series of compression waves in the near region of body surface and causing a pressure rise, which is also confirmed by the mean pressure distribution on the body surface in figure 1(a).…”

“…Moreover, the disturbances possibly propagate in the subsonic region to influence the jet structure. A feedback mechanism of the free shear layer and the Mach disk was proposed by Shang et al (2001). The relevant feedback mechanisms for the two flow states will be discussed in detail in § 4.3.2.…”