Numerical simulations are performed for mixing and ignition of a round ethylene jet injected into a cavity adjacent to a M = 3 oxidizer flow. For mixing, injector-placement effects are quantified by simulating two injection configurations: vertical from the floor near the cavity’s leading edge, and horizontal from the 45◦ ramp. Vertical injection produces a leaner cavity, which reduces ignition probability. Ignition is then seeded in the horizontal-injection configuration by modifying the local temperature field with a Gaussian hotspot as a reduced-cost approximation to laser-induced breakdown. Three seed locations are compared. Kernel dynamics are quantified through line-of-sight-integrated hydroxyl radical, and these are shown to be sensitive to seed placement.