Sinusoid with increasing frequency
Frequency is more likely decreasing (period is getting longer). This is a sweep sine signal or as some people used to call it - chirp tone. No point in rewriting the literature, so here are some links with very good explanation for both linear and exponential frequency change.

For linear sweep you get:

$$s(t)=\sin\left(\omega_1t+\frac{\omega_2-\omega_1}{T}\cdot \frac{t^2}{2}\right) $$

where: 

$\omega_1 $, $\omega_2$ - initial and final angular frequency respectively 

$T$  - sweep duration.


  Meng Q., et al. - Impulse Response Measurement With Sine Sweeps and
  Amplitude Modulation
  Schemes
  
  Farina A. - Advancements in impulse response measurements by sine
  sweeps