Internal degrees of freedom of an actuator disk model

Actuator disk models can have internal degrees of freedom as, for example, in the case for models with lagged losses, governed by additional differential equations. Generally, being a system with distributed parameters, flow in the interblade passage has an infinite number of internal degrees of freedom. An attempt is made to estimate how many of them can be distinguished as the most important. The response of a blade row to time-periodic excitations is modeled by an actuator disk with internal degrees of freedom and by linearized Navier-Stokes equations, and the results are compared. It is found that in the case of subsonic flow one internal degree of freedom can be considered as the most important, both for design and off-design regimes. In the case of transonic flow in off-design regime, two internal degrees of freedom are more important than the rest. However, for the transonic design regime, no internal degrees of freedom could be distinguished as especially significant. The physical mechanisms associated with distinguished internal degrees of freedom are investigated.