Boundary Lyer Theory

314 X11. Thermal boundary laycrs in laminar flow h. Thermnl boundnry Inyrrn in nnbitral flow 315
It iu rccognizcd that, there exiutr, a difficulty in providing nn rrnequivocal description of
nucb Iloct.uetirig ntrenma. Since turbulence involvca atochnstic fluctuations, ntrictly epcnking,
110 two turhttlctit utrcnrns can ever bc nimilnr. However, it is found by oxperinlent that certain
nvcr:rgc propcrlicu of tho osrilhtiodu arc adequate to dmtcribe them. Tlirse nre: the intensity
of titrl~uloncr, T, clcfinctl in Scc; XVI d 1, and I.ho ncnlc of b~trhulcticc, L, defined in See. XVIIltl.
It, iu found, furtltcr, that in cnaca when ttic scale of turbulenm is small comparcd with the din~elinionn
of 1.11~ I)orly. whinh occurs in most cmm in prnctice, ttic degree of turbirlence alone sr~fficca
lo ~-l~artkrtd~rizc t.hc Ilow. It in, t.hcn:fore, b bc cx]xcted t.hnL lhc NIIRRC~~ number for gconlctrically
uitnilar, inot~licrni:~l I~odicn wlticli arc placcd in fluct~unti~~g, parallel, isolhcrtnal streanis, depc~iclu
on blir trtrlntl~:ncc intenniy, T, in addition to its clc~~cntlcncc on the l'randt,l and l