Optical methods including particle image velocimeter (PIV), digit particle image velocimeter (DPIV) and holographic particle velocimeter (HPV), are widely used to measure 2D and 3D nonstationary flow.

• 对于复杂的非定常流动，流场的测量往往要求无干扰、非接触，并且能够瞬时记录流场的二维甚至三维信息。