Investigation of the Plunging Pressure Pulsation in a Swirling Flow with Precessing Vortex Rope in a Straight Diffuser

The paper investigates an unexpected feature of the unsteady pressure field resulting from the self-induced instability of the decelerated swirling flow in a straight diffuser. Firstly, the self-induced instability is experimentally investigated on the swirl generator test rig. As a result, the asynchronous (rotating) pressure pulsation associated with the rotating vortex rope of 15 Hz and it second harmonic are discriminated. Also, a low frequency synchronous (plunging) pulsation around of 2.5 Hz is identified based on unsteady pressure field measured at the wall and LDV measurement of the velocity components in the flow. The low frequency plunging pressure fluctuations is superimposed on the rotating pressure pulsations associated with the vortex rope. The numerical simulations are performed to explore the vortex rope dynamics. The numerical results are compared against experimental data to assess the accuracy of the models. Next, the pressure pulsation dynamics is correlated with the time evolution of the vortex rope. The main conclusion emerging from the analysis of the vortex rope evolution in time is that the cycle with low frequency is responsible for the plunging (synchronous) pressure fluctuations superimposed over the rotating (asynchronous) pressure field associated with the precession of the vortex rope.