def HB(x, ystress=1.0, K=1.0, n=0.5):
    """Hershel-Bulkley Model

    Note:

    .. math::
       \sigma= \sigma_y + K \cdot \dot\gamma^n

    Args:
        ystress: yield stress [Pa]

        K : Consistency index [Pa s^n]

        n : Shear thinning index []

    Returns:
        stress : Shear Stress, [Pa]
    """
    return ystress + K * x ** n