Boundary Lyer Theory

IGg. 2.16. Frrc stagnation flow witl~o~~tarpn. Fig. 2.17. 1)rcrlrrated 8Lag11:~tiorl flow with
ration, au pliotogmphrtl by Fotttingrr
scprntion, ns pllotogrnphed by Focttingcr
I fa1 Anaubr nose 1 I
I (b) Round nose I I
0. %?
- ( - z z 0 -
I
no separation
IFig. 2.18. I'low n.l,orrl, n ~n~(lcl of a motor vrl~inlc (Volltsw:i.gc:n tlclivrry vrm). nftrr 15. Morller
1231. n) Angulrrr noso wi1I1 mpnmtcd flow nlong tho whole of the aidc wall nnd lnrge drag codficicr~t
(C,, = 0.70); h) ltord iionc with no ~cpnrntion nntl small clrng cocflhic~lt (CD = 0.42)
b. Separation and vortcx formation 3 7
Separation is also important for the lifting properties of nn aerofoil. At small
incidence anglcs (up to about lo0) the flow does not separate on either side antl
closely approximates frictionless contlitions. The prcssurc distrihntion for slleh a cnsr
("S~IIII~" flow, Vig. 2.11)n) WILR givo11 in Vig. 1.14. Will1 inoron~ing i~tcitlo~~cn t,lrc\rc*
is tlangcr of srparnt,ioti on t h sucI,ion side of tho nerofoil, I)cer~~~so t,l~e l)rcss~~re ill.
crcnw bccomcs sleepcr. Por n given angle of incidenc~, which is nljout l!jO, ~cparation
Litinlly occurs. The scpwation point is located fairly closely behind the lcading cdge.
Thc wr-kc, Fig. 2.19b, shows a large "(lead-water" nrca. The friclionless, lift-creating
flow patter-n has Iwcornc dislurbcd, and the drag has become very largo. The ,heginning
of scpnrat.ion nwrly coincidcs with the occurrence of maximum lift of the
acrofoil.
Structural oerodynomics. Flow around land-bnsed bluff bodies, suc11 as struc-
tures antl buildings, is consiclcral~ly more complex than flow around streamlined botlies
and aircraft. The principal cause of complication is the presence of the ground ant1
the shear created in the turbulent wind as a consequence. The interaction between
the incident shcar flow and the stsruct,ure produces coexisting static and tlynamic loads
[8, 9, 101. Tlie fluctuating forces produced by vortex formation and shedding can
induce oscillat,ions in thc structures nt. their natural frcql~cncics.
The flow patterns observed on a tlctachcd rectangulnr building is shown sahrmali-
cally in Fig. 2.20. In front of the building there appears a bound vortrx whirh arises
from the interaction of the boundary layer in t,he sheared flow (d V/dz > 0) ant1 the
ground. There is, furthermore, strong vortex shedding from the sharp corllcrs of the
building and a complex wake is created behind it. So far no theoretical mcthotls have
been developed to cope with this ext,remely complicated flow pattern. It is, therefore,
necessary to rcsort to wind-tunnel studies with the aid of adequately scalctl models.