▁time , ▁calculation ▁of ▁the ▁Reyn olds ▁number ▁can ▁be ▁complicated . ▁ ▁It ▁is ▁one ▁of ▁the ▁most ▁important ▁dimension less ▁numbers ▁in ▁fluid ▁dynamics ▁and ▁is ▁used , ▁usually ▁along ▁with ▁other ▁dimension less ▁numbers , ▁to ▁provide ▁a ▁criter ion ▁for ▁determ ining ▁dynamic ▁sim il itude . ▁When ▁two ▁geomet r ically ▁similar ▁flow ▁patterns , ▁in ▁perhaps ▁different ▁flu ids ▁with ▁possibly ▁different ▁flow ▁rates , ▁have ▁the ▁same ▁values ▁for ▁the ▁relevant ▁dimension less ▁numbers , ▁they ▁are ▁said ▁to ▁be ▁dynamically ▁similar . ▁ ▁Typ ically ▁it ▁is ▁given ▁as ▁follows : ▁ ▁where : ▁ ▁us ▁– ▁mean ▁flow ▁velocity , ▁[ m ▁s − 1 ] ▁ ▁L ▁– ▁characteristic ▁length , ▁[ m ] ▁▁ μ ▁– ▁( absolute ) ▁dynamic ▁fluid ▁vis cos ity , ▁[ N ▁s ▁m − 2 ] ▁or ▁[ Pa ▁s ] ▁▁ ν ▁– ▁kin ematic ▁fluid ▁vis cos ity : ▁, ▁[ m 2 ▁s − 1 ] ▁▁ ρ ▁– ▁fluid ▁density , ▁[ kg ▁m − 3 ]. ▁ ▁Me asure ment ▁ ▁R he om eters ▁are ▁instruments ▁used ▁to ▁character ize ▁the ▁r he ological ▁properties ▁of ▁materials , ▁typically ▁flu ids ▁that ▁are ▁mel ts ▁or ▁solution . ▁These ▁instruments ▁imp ose ▁a ▁specific ▁stress ▁field ▁or ▁de formation ▁to ▁the ▁fluid , ▁and ▁monitor ▁the ▁result ant ▁de formation ▁or ▁stress . ▁Inst ru ments ▁can ▁be ▁run ▁in ▁steady ▁flow ▁or ▁oscill atory ▁flow , ▁in ▁both ▁she