The geothermal heat pump market has changed quite a bit since its early commercialization in the 1980s. Back then, all geothermal earth loops operated with fixed-speed circulators. Those circulators were typically sized to ensure that the flow through the earth loop circuits remained turbulent under the harshest expected conditions. That condition was based on the Reynolds number of the flow stream. According to the IGSHPA (International Ground Source Heat Pump Association) Design and Installation Guide, Reynolds numbers need to be at least 2500 to ensure turbulent flow in earth loops.
If you’ve dealt with pipe sizing in HVAC systems chances are you’re familiar with the concept of Reynolds number. It’s essentially a ratio of the inertial forces acting on a fluid divided by the viscous forces working on that same fluid. When inertial forces dominate the flow will be turbulent. When viscous forces dominate the flow will be laminar.