巴格诺尔德数（Bagnold number，Ba）是在间质性牛顿流体中的颗粒材料，其碰撞应力和黏滞应力之间的比值，最早是由巴格诺尔德找到。

巴格诺尔德数定义如下：

其中

其中

流体的巴格诺尔德数较小（Ba < 40）时，流体的黏滞力大于颗粒的碰撞应力，流体在「巨观黏性」（macro-viscous）状态。若巴格诺尔德数较大（Ba > 450）时，流体在「颗粒应力」（grain-inertia）状态。在这两个状态之间有临界状态。

The Bagnold number (Ba) is the ratio of grain collision stresses to viscous fluid stresses in a granular flow with interstitial Newtonian fluid, first identified by Ralph Alger Bagnold.

The Bagnold number is defined by

where is the particle density, is the grain diameter, is the shear rate and is the dynamic viscosity of the interstitial fluid. The parameter is known as the linear concentration, and is given by