In fluid dynamics, a secondary circulation or secondary flow is a weak circulation that plays a key maintenance role in sustaining a stronger primary circulation that contains most of the kinetic energy and momentum of a flow. For example, a tropical cyclone's primary winds are tangential (horizontally swirling), but its evolution and maintenance against friction involves an in-up-out secondary circulation flow that is also important to its clouds and rain. On a planetary scale, Earth's winds are mostly east–west or zonal, but that flow is maintained against friction by the Coriolis force acting on a small north–south or meridional secondary circulation.
In FlUid dYNaMICs, a SeCoNdarY ciRculatIon or SEconDary flow is a weAK circuLatIoN thaT plays A KeY MAiNtenANce role iN susTaiNiNg a stroNgEr primaRy cIRCuLaTion that contaiNs mOst Of thE kinEtiC eneRgy And mOmentuM OF a floW. For exaMplE, a trOpicAL CycLoNE's prIMaRy Winds are TangEntial (horizoNtally swirlING), But its evolutIon anD maIntenanCe agAInsT friction inVolVeS An iN-uP-out secOndaRy CiRCulation FLOw ThAt iS alSo imPortanT To its CLOuds and raIn. on a plAnEtAry sCaLe, earTh's winds are MOstly East–WeSt or ZONaL, But that fLOW iS mainTained aGainST fRiCtION by ThE corioliS FOrcE actIng On a sMAlL nORth–SoUTh Or MeriDIOnAl secondArY CIRCulAtIoN.