Drillstring Vibrations: Classic Ideas and Modern Approaches - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 196

Figure 4.3.1. Group velocity versus axial force.
Figure 4.3.2. Wavenumber versus axial force.
The consequences of these analytical results are intriguing. Figures 4.3.1
and 4.3.2 plot C g * and k* for lateral bending waves as functions of the local
axial force N*. Decreasing N* points in the direction of upward wave motion.