A discrete vortex method analysis tool for control of flows with trapped vortices
Passive and active control studies have been conducted to investigate the feasibility of stabilising a trapped vortex. Passive stabilisation is achieved using distributed steady wall suction. An enhanced aerodynamic performance with lower drag and higher lift has been achieved as a result of passive stabilisation, but only up to moderate suction rates. When the trapped vortex is stabilised, vortex shedding is completely suppressed and flow unsteadiness is significantly reduced. Compared with steady suction, pulsed suction reduces the required suction rate for stabilisation by over 50% at an optimum pulsing frequency. Using slow suction reduction from a stable state, savings in mean suction are also possible provided the rate of suction reduction is small enough. It appears, however, that such reduction is dependent on the specific shape of the aerofoil’s cavity. A bifurcation analysis has shown that the loss of stability of a trapped vortex, from a stable state to unstable cortex shedding, is also dependent on the cavity configuration. A linear feedback controller with an optimum gain G extends the slow suction reduction results by achieving stabilisation at a reduced mean suction compared with passive stabilisation. Using DVM’s ability to simulate unsteady suction and blowing, active stabilisation using vorticity flux control achieves 16% reduction in mean flow rate compared with linear control and 19% compared with passive stabilisation. The pulsed suction technique remains the most efficient method of stabilisation. Simplified power calculations have shown suction control to be effective but only within a small range of suction rates. Using DVM’s capability of providing controlled input-output data, a linear System Identification study of the trapped vortex has resulted in a dynamical model description of a stable trapped vortex which, despite being a crude estimate, is useful in future stabilisation studies.