Boundary Lyer Theory

380 X IV. llorlntlnry-layrr control
placed in a strcam nt right anglcs to its axis. On thc upper side, where the flow
and t,he cylinder move in tlic snmc direction, separation is completely eliminated.
I~ttrt,llcrmorc, on tho lowcr sitlc wl~cre the tlircotion of Hr~itl motion is opposite to
t,llnL of t.hc solitl wall, scpnration is clcvclopcd only incotnpletcly. 011 the whole,
t,lw flow pnt.t.ern wllich exists in this case npproxinintcs vcry closely the pnt,t,ern of
frit:l.ionlt:ss Ilow psi, x circular cylintlor with oircnla.tion. 'rhc: sl,rram exerts n consitlcmbl(:
force on t,hc cylintlcr at right :~nglcs t,o the mean flow tlircction, ;lncl t,liis
is somctimcs referred to ns t,he Magnus clTcct,. This effect can be seen, o. g., when a.
tennis 11dl is 'sliced' in 1,I;~y. Attctnpts wcrc also mntlc to ~lt~ilizc the occilrrcncc of
lift, on rotating cylintlcrs for the prol)l~lsion of ships (I'lettner's rotor 111). With
the cxcept.ion of rotating cylinders, tho itlc:l of moving thc solid wall wit,l~ the stronm
can I)(: realized only at tho cost, of vcry grcat complic:~tions as far as sl~apcs
othrr t.l~nn ryli~~tlrionl are conoornccl, ant1 conscqt~ently, this nictl~otl has not fonntl
mnalr practical application. Nevorthc:loss, A. Ihvre (261 mntle a thorougl~ experimc%nt.nl
invest.ignt.ion of the inll~~cncc of a moving bountlary on an nerofoil. A port,ion
of l.I~t~ upper sttrfacc of the norofoil was limnetl into an c:ntllcss Idt which n~ovatl
ovvr two rollcrs so t,li:~t tltc: rcLttrn tnol,ion occurred in the interior of tlic motlcl.
'J'lie arrangcrncnt, proved vcry cffcctivc for the avoidance of separakion, nntl yicltlcd
vcry high maximum lift, cocfficicnt,~ (C,,,,,, = 3.5) at high angles of incitlcnce
(a. z 55'). The Inrninar boundary laycr for a flat plat,c moving in its rear part with
t.hc shmn 11n.s I~ccn c:rlculatctl by ]I:. 'I'rncltcnhrotlt. [IOO].
2. Accclcrnhn olthc bnu~~tlnr~ layer (blowing). An nllnrna.t,ive tnc~t.ltotl of l)rcvc:nt.il~g
scp:~.rat.ion consists in supplying atltlit.ional energy to the part,iclcs of fluid wl~irh
arc bring rct.nrtlrd in the boundary laycr. This result can be at-hicvetl by tliscl~arging
fluill from t,hc itlt,c?rior of t,l~c I~ocly with the aicl of n special blower (I'ig. 14.:ln), or
by tlrriving t.hc tcquirctl energy directly from the main sham. This Iathx effect.
pa11 IIP protlricctl by connect.ing the rctmdect rcgion t.o a rcgion of l~iglier pressure
t.l~rot~gh n slot in the wing (slottcd wing, Pig. 14.3b). In citl~cr ca.sc atltlit.ionnl
crirrgy is intp:~rkd 1.0 the pnrticles of lluitl in tho boundary layer .near the wall.
When fluitl is tlischnrgcd, say in the manner shown in IGg. 14.3a. it is mnntlntory
t,o pay mrcfitl at,tcnt.ion to the sllnpc of t,hc slit in order to prevent the jet from
dimc,lving into at 8 short* distnncc behind the exit section. 1,atcr expcri-
~ncnt,s pc~rli)rmc~tl in France [04] Iinvc rnatlc it, vcry ntI.racl.ivt: t,o n.pply blowing :I.{,
t,hc (,railing ctlge of n.rl acrofoil in ortlcr to incrcnsc its mnximum lift. Att,cnipt.s
consid(.ra.l)ly to incrmsc the masimum lift of a Rnp wing tl~rongh blowing in the
slot. Imve dso mrt with success (c/. Sec. X l l b 6).
111 1.11~ c.;~sr of t .1~ slottctl wing [7], shown in Ipig. 14.311, the cfict is prot1uc:ctl
:IS follows: 'l'l~c I)o~tntl:~ry laycr formed on the forward slnL A - 13 is c:arrictl inlo the
tn:tit~ sl.rc-n.tn brfore sepnml,ion occurs, :&ntl from point (: OIIWR~~S a t~ew bot~ll(la.~.y
I:~yt-r is liwnc.tl. IJnJcr favourablc contlitions this new boundary layer will rcnclt
1.11t: t.miling rtlgc 1) without. sc?pamtion. In this way it is possiblc to relegate ~rparnt~ioti
t,o consitlem.bly larger a.nglcs nf incidnnce, nrtd to achieve much larger lifts. Fig. 14.4
shows n polar tlia.grnni (lift, cocfficicnt~ plotted against drag ~oeffi'licirnt~) for a wir~g
~(dion \vit,l~ and wil~l~ot~t, forwnr