Grass waves Presented by Bulanchuk Pavel

Build a theory of grass waves excitation

Structure Features Models: А) Independent cones B) Collisions C) Thick grass Details of “Thick grass” Demonstration

Features 2 types of waves: “quick small waves” and “slow big waves” Weak wind→ only quick waves

Independent cones (quick waves) Waves - a consequence of heterogeneity of air flow (turbulence) Disturbances propagate at the speed of wind

Collisions Cones collide→disturbances propagate Problem: quick dying-out

Thick grass The higher the blade above the rest → → The stronger it is bend→ → The grass starts to sway → → effect of self-oscillations

Thick grass: theory 1)1D model 2)Stable wind 3)Grass surface is described by

Thick grass: theory Windelasticityfading Linear approximation

Thick grass: theory Separate for sine wavesSolve individuallySum solutions

Thick grass: theory Dispersion law:

Thick grass: theory Disturbance will exponentially rise if

Dispersion law No waves

Conclusion Slow and fast waves Self-oscillations in constant wind Critical wind velocity

Thank you for attention!

Experiment ArtificialReal

Thick grass: analysis Phase speed: Group speed:

Dispersion law