Tornillos are quasi-monochromatic seismic signals with a slowly decaying coda that are observed near active volcanoes and geothermal areas worldwide. In this work a lumped parameter model describing the tornillo source process as the self-oscillations of fluid filling a cavity is investigated. A nonlinear ordinary differential equation is derived that governs the behavior of the model taking into account viscous and nonlinear damping as well as the reaction force of the fluid inside the cavity. This equation is numerically integrated both for different cavity sizes and different fluids of volcanological interest, such as gas (H2O+CO2, H2O + SO2) and gas-particle mixtures (ash-SO2, water droplets-H2O). This cavity model predicts that when the filling fluid is a mixture of ash and SO2 the signal duration will increase until the mixture becomes enriched in ash and then the duration exhibits a decrease. Additionally, the damping coefficients (=1/2Q) of the synthetic signals are estimated in the range between 0.002 and 0.014. Both results agree well with the temporal variation of tornillos duration and the estimated Q quality factors/damping coefficients observed at Galeras volcano. In the context of the cavity model, tornillo frequency variations from 4Hz to 1Hz observed prior to eruptions can be interpreted as the result of fluid composition changes as more ash particles are added. This is in agreement with the observation that gas accumulation at Galeras was a steady rather than an episodic process and that tornillos were most likely triggered after a fluid pressure threshold had been exceeded.