Boundary Lyer Theory

a Intninn.r l)out~rlary I:~.ycr, but; rlccrcasrs to about 10 0 for n turh~llent hountlary
Iayor; t,l~r! syn~l)ol 8 tlcne>lcs hrrc t,l~c: I)o~tntlary-layer t,lliolrness in i,hc si~oolr region.
The liigliar tlcgrcc of tliffusior~ wl~icli is cli:tmcte:rislic of laminar bouticI:~ry layers
can IN untltrsi,ootl if it, is not,ctl that tllc subsonic mgion of flow extm~ds furfhr
away from the wall in a laminar than in a trirbulcnt boundary layer.
Irrrspcclivc of wl~ntllcr sepxmt,ior~ elocs or clocs not occur, the bountlary-layer
t.l~ic:kncss incrtascs alicatl of the poinl, of :~rrival of the shock wave. The pressure
increme at Lhc out,cr ctlgc: ol the Iiountl:iry I:~yer, ant1 hcncc also insitlc the I~ountlnry
layer, corrcspondn to the c~lrvrtl st~rcan~linc wllic:h is convex in the direction of
t.11~ wall ant1 which scp:~,mtc~s the: exl,t:r~~:il from the I)oirntlary-hycr flow. lhwn in
the clomain of influcncc or i,l~c cxpnnsion waves which appear in the rcflcxion of
an ol)liquc shock wave, the sligl~t, tlccrc:asc in pressure in the bountlary hycr, l'ig 13.22,
corresponds to the fact that the curvature of the dividing streamline is concave
lowards tho wall. A laminar boundary layer which has not sepsratetl can support
only very small pressure rises because tlw external flow imprcsscs on it the prcssurc
cxclasively through viscous forces. A non-sepnmt,ecl tnrl)rllcr~t ho~tndnry
layer can take up much larger pressure padicnts because now the turbulent mixing
motion aids the process. Both laminar and tirrbulent boundary layers nre in a position
to snpport the large pressure increasaq of strong shocks if they separate. Tn particular.
Fig. 13.23. Ilrllrsion oi n sl~ock wnvc from a f.nrldoril bonndnry lnyor on a flat wall, after
S. M. Ih)grlot~oll' nntl C. IF. I