A pitot tube method for measuring the first normal stress difference and its influence on laminar velocity profile determinations

A scheme is outlined which enables the calculation of the normal stress difference P11‐P22 in fluids undergoing steady Poiseuille flow from measurements with a pitot tube of local velocity distributions. This new technique should enable measurements in a region of shear rate comparable to that encountered in the torsional and circular flow methods. A particularly interesting consequence of the method is it leads one to the inescapable conclusion that the presence of finite normal stresses should produce significant aberrations on the measurement of the velocity profile in the laminar flow of viscoelastic fluids. The presence of a P11‐P22 induced aberration on the velocity profile measurement suggests that caution be exercised in the practice of deducing estimates of the thickness of a boundary layer in the nonlaminar flow of these fluids.