Annular velocity
Annular velocity is the speed of a fluid's movement in a column called an annulus in wells being drilled with circulating drilling fluid. It is commonly measured in feet per minute (ft/min) or meters per minute (m/min). Annular velocity is often abbreviated as AV, though this is not exclusively so, as AV also refers to apparent viscosity which is calculated from rheometer readings from tests that the mud engineer performs. Scope. For this article, annular velocity is described, as used in drilling fluid applications in the oil exploration industry. There may be other applications in  other fields of study such as fluid mechanics (the study of the movement of fluid) or fluid dynamics (the study of the flow of fluid). Determination. The annular velocity can be calculated using one of the following formulas. Or Where: Application. The annular velocity is one of two major variables in the process of cleaning solids (drill cuttings) from the wellbore. By maintaining the annular velocity at certain rates (speeds) in conjunction with the rheological properties of the drilling fluid, the wellbore is kept clean of the drill cuttings to prevent them from settling back down to the bottom and  causing drilling problems. The other major variable is the rheology of the drilling fluid. Rheology is sometimes thought of as viscosity to the uninitiated, though improperly. Viscosity (sometimes thought of as its thickness) is a very basic measurement of the fluids resistance to change in movement or flow. The viscosity of a fluid can be measured with a Marsh Funnel. Rheology is the study of viscosity and requires more precise and complicated procedures and equipment for its determination. For drilling fluid applications a rheometer is used.