Qi
KHOA HQC - CONG NGHEcor CHE xdi cue BO TRU CAU
PGS.TS. T R A N D I N H NGHIEN Trudng Dgi hgc Giao thdng Vgn tiii
Tdm tSt: Bdi bdo trinh bdy tdm tit vi xody mdng ngua, cdch xac djnh cudng dd xody lien quan din cdc thdng sd ddng chiy vd md hinh fry trdn cung nhu qud trinh xdi eye bg trong ddy cdt vi chiiu sdu xdi eye bd ldn nhit dtfa vdo sU can bing cudng dg, ndng lugng ddng chay cua xody 6 ddy khdng xdi vd ddy cdt cua cac tdc gia khac nhau. Tdc gia cung trinh biy hai cdch xac dinh cd xody cudng bde trdn ddy khdng xdi di/a vdo thi nghigm md hinh try trdn vd trdn diu, sau dd dtfa vdo stf can bang eudng dd ddng chay eua xody d diy khdng xdi vd ddy cdt de dua ra bleu thdc xdc dinh chieu sdu xdi ci^c bg Idn nhit d tnj, biiu thdc thda man si li$u do xdi thUc ti d phgm vi thdng dyng trong thtfc ti.
Abstract: The size of the primary horseshoe vor- tex in front of a cylidrical pier correlated to the flow, pier parameters, the vortex in the scour process in sand bed are discussed in brief. The equilibrium scour depth is predicted based on the concept of flow circulation as flow intensity available in the pri- mary scour vortex or vortex power equal to the resisting flow circulation or vortex power of sedi- ment in the scour hole and combination of flow, pier parameters as well. The prediction of the maximum scour depth based on auther'experimental data and concept agrees well with field observation data in literature.
1 . Dat van de
Try cau tron, trdn d i u hay vudng dau dat trong sdng la mdt vat can lam hinh t h i n h dong c h i y bao quanh try, tao ra gradien i p s u i t ngUdc d ddng ehay den try ngay trUde chan try; khi i p suat n i y dd ldn se hinh thanh sy tach ddng c h i y ba e h i l u 6 trUdc c h i n try. Trong Idp bidn bao try cda ddng e h i y khdng deu dd'n tru, sat mat trudc try hinh t h i n h radien i p s u i t ddng hudng xudng tao ra dong thU cap dpc t h i n try d trudc va hai ben try. Sy tUdng tae gida ddng thd c i p hudng xudng nay vdi tach ldp bien 3 chieu d khu vyc g i n day lam hinh t h i n h he thdng x o i y true ngang trydc chan try vdi hai tay x o i y bao c h i n try cd dang hinh mdng ngUa trdn mdt bdng, dUde gpi la " x o i y dang mdng ngua". D d n g t h d cap cdng vdi he thdng xoay n i y tach Idp dat dd xdi d chan try, tgo ra q u i trinh xdi va hinh t h i n h vung xdi nho, sau d c h i n try dupe gpi la hd xdi eye bp tgi e h i n try, cd c h i l u s i u Idn nhat la chieu s i u xdi cue bp try c l u . H i l u b i l t v l cd che xdi, cudng dp xdi v i dy doan dUpc e h i l u s i u xdi eye bp Idn n h i t ed the khi lu t h i l t k l thdng qua dudi cau l i van de r i t quan trpng trong t h i l t k l mdng e l u an t o i n va hidu q u i v l kinh t l . T h u I n tuy v l ly thuyet, day la v i n d l khdng ddn g i i n bdi vi su phde tgp cua ddng c h i y ba ehieu tai c h i n try tUPng t i c vdi vdn t i i bdn eat v i sy thay ddi cua Idp bidn la d i y di ddng. Do vay bai bao tap trung v i o phan tich cd e h l xdi eye bd v i mdt sd bieu thdc ddn s u i t xac djnh xdi
cye bp Idn n h i t h^
2. G i d i thigu chung ve xoay mong ngya He thdng x o i y trUdc c h i n try da sdm dUpc nh?n bidt v i nghien cdu bdi n h i l u nha khoa hpc tren the gidi: Thwaites {I960) bang thi nghiem ehi ra co mot e g p x o i y trong vung t i c h ddng d ddng e h i y baochSn try trdn trdn day phang khong xdi. Baker (1979,1980 va 1985) mo t i ehi t i l t he thdng x o i y tang va xoay r6\
trong vdng tach ddng d trudc chan try trdn, chi ra kfch cd xoay va vj tri x o i y lien quan den thay ddi ap suai phfa trudc try 6 mat phang ddi xdng dpc tim try, vitri cd Ung sua't tiep tmax d d i y xoay d i u tien eung nhU da ehdng minh sU tdn tgi cda hai cdp x o i y trong vimg t i c h ddng. Eekerie va Langston (1987) chTracd mpt xoay Idn, l i x o i y ehfnh d mat ddi xdng trong vimg t i e h ddng. Melville va Raudkivi (1977) l i nhQng ngUdi dau tien do dupc dac trUng xoay mdng ngya trong ba giai doan phat trien cda xoay: 6 d i y phang khong x6i (xoay cudng bde), d giai doan p h i t t r i l n hd xdi v i l<hi hd xdi d trgng thai can bang; da xay dyng dUpc mo hinh ddng e h i y d mat eat dpc v i mdt b i n g , cUdng d6 x o i y , cUdng dd rdi va dng s u i t tidp rdi d dudi xoay d i u tien rdt ra td thf nghiem. Dargahi {1989) sd dyng ky thuat bdng khf v i dng Preston do dupe dng suit tidp day va chT ra s y xuat hien ddng thdi cda hang loat x o i y phia trUde try trong vung tdeh ddng. Deyva cac cdng sy (ccs) (1995) do dupe phan phdi tdc do trong mat phang ddi xdng qua tim try v i phat triln phUdng p h i p du toan td'c dd trong hd xdi. Ahmed va Rajaratnam (1998) va Sarker (1998) do cac dde trUng cua ddng ddng di xudng dpc t h i n try gia trudc tnJ.
Graf v i Yutistiyanto (1998) do phdn phdi tdc 6d, cudng dp rdi v i ddng nang rdi, dng suat t i l p d i y fl mat d i y eat trUde try trdn. Lhermitte va Lemmin (1994) chi ra sy tdn tai cda x o i y mdng ngya cUdng d5 mgnh trong hd xdi trudc try nhd m i y do tdc dd dang sdng am (ADA). Barbhuiya va Dey (2004) cung cho ket q u i tUdng t u nhu tren d nda hd xdi khi ddt tru dang ban nguyet d t h i n h kenh. Muzzammil v i Gangad- hariah (2003) do kfch cd x o i y ed dgng ellp trong h5 xdi, kich cd eda x o i y eung dUpe Unger v i Hager (2007) xac nhgn bang thf nghiem. N g o i i ra Unger v i Hager (2007) cdn ghi lai kieh ed x o i y d I u t i i n tang theo chieu s i u xdi trong hd xdi. Mazzammil va Gan- gadharaiah {2003) edn chT ra quan hd kfch ed xoay, tdc dd quay eda x o i y vdi chieu sau xdi trong hd xoi.
Kirkil va ccs (2008, 2009) quan sat dUpe cdp x o i y dau tidn va xoay thd c i p trong hd xdi vdi xoay dau tidn co dang elip, true dai ty Id vdi e h i l u rdng try, true ngan b i n g nda tryc dai; kich cd x o i y thd cap bang mot phan ba xoay dau tidn, trye x o i y do try tao ra co eudng dp chd yeu lien quan den dng suat t i l p day, x o i y dau tien cd gia trj Idn cung nhU quan he ciia thdng xoay mdng ngUa vdi ddng c h i y phia sau try.
Ddi vdi try tron va try trdn d i u d i t trdn mdng liln
KHOA H O C - Tap Chl GTVT 10/2012 I Q
khdi k i l u gidng chim da dupc Gangarudaiah Veerap- padevaru, Thimmaiah Gangadharaiah v i Thirumalai Ramaswamyiyenger Jagadeesh (2011) nghidn c d u bang thf nghidm de tim quan hd giOa tinh chat, dae trUng xoay gay xdi, d d la kich ed x o i y , tdc dp xoay vdi chidu s d u x d i eye b p , c h i l u sdu ddng e i i i y v i dudng kfnh cda mdng. K i t q u i thf nghigm cda eac t i c g i i rut ra tO n g u y d n ly c a n b i n g ndng lupng gdy xdi vdi nang lupng tidu hao de t i c h hgt hinh thanh h d xdi d i n d e n ehidu s d u x d i t d n g c d n g (chieu s a u ddng c h i y cdng chieu sau xdi eye bd) so vdi dUdng kinh b d m d n g quan h d v d i thdng s d eudng d d ddng c h i y t h d n g q u a luu IUdng d d n vj, c h i l u s i u d d n g e h i y , dudng kfnh xoay mdng n g y a , gia tdc trpng lye, dudng kinh trung binh hgt c i t , khdi lUpng rieng cOa hat c i t v i nude trUdc khi xdi eye bd xay ra. Quan he phd h d p v d i k i t q u i thf nghidm eda n h i l u t i c gia khac d a c d n g b d .
3. Cudng dp xoay mong ngya d chdn try va cong thdc ddn s u i t xac djnh c h i l u sau xdi
Cd c h d ed b i n eda xdi eye bd try trdn va try trdn d i u l i hd thdng xoay tryc ngang trUdc c h i n try vdi hai tay xoay bao c h i n t r y . Trdn mdt b i n g he thdng nay cd dgng hinh mdng ngya do v i y dUpc gpi l i "xoay dgng mdng ngUa", eudng dp cda hd thdng xoay n i y lien quan ehdt che va quyd't djnh q u i trinh xdi v i ehieu sdu xdi eye bp d ehdn try. Cydng dd eda xoay thudng dupe tinh nhu la luu s d dpc theo Idp bien x o i y - luu s d r , thudng eoi cUdng dd xoay la luu sd c d a loi xoay (nhu vdt t h i edng):
r = [jyds= jvgrdg = 2n:rVg (1)
^-^ l i tdc d d tiep tuydn c d a xoay.
Id tdc dp gdc cda hgt nUdc quay trong d d
r l i ban kfnh x o i y , . .
quanh Idi xody. C d ngudn gde gan l i l n vdi dmh \y Hemhdn (Helmholtz) va Kenvin (Kelvin) trong c d hpc c h i t Idng {Batchelor 1967, Panton 1984, Saffman 1992, Green 1995...) Theo dinh \y Green thi cUdng dd xoay dUdc xac djnh nhu tfnh p h i n mat c d a mat eat ngang xody, song chua p h i n i n h ddng dupc eudng dd xoay cue bd trong ddng c h i y thuc, day chfnh l i v i n d e c d n h i i u e i c h hieu, g i i i thfeh v a xac dmh cUdng dp x o i y k h i c nhau.
Do tinh c h i t phdc tgp c d a tach ddng c h i y ba chieu tgi ehan try ( H i n h l ) , t i e ddng tUdng hd eua he thdng x o i y do tach ddng vdi t i i bun eat v i s y thay ddi c u a Idp bien di ddng lai lam cho van d l trd nen phdc tap hdn. M d phdng ddng c h i y 3 chieu tgo ra xdi dUdc b i s e n va Melaaen (1993), Richardson va Pancheng (1998), Gray va Fstionto (2002), Dey v i ccs (2008), Koken va Constantinescu (2008), S a d - eque va ccs {2008)-. ^;,-,jr^ j _ ^ ^
nghidn eOu. r ' " " " ^ J t 5 > { D l x i e dmh cUdng
dd xoay Roper, Schnei- der v a Shen (1967) xet the tich k i l m tra d m a t e l t doc tim try, xac djnh
c a a n g do xoay ban dSu ^ ^ ^ ^ J , ^ . , ^ ^ ^ gSy XOI vOi gid thiet tSc ^^^g ^ ^ ^ ^ ^ a ^f,,g[, ^^^
do d p c theo day v6 mSt quan/i trv
"@
try bang khdng ( g i i thiet khdng trUpt tgi thanh r i n ) . S d dyng ly thuyet ddng the xac dinh mdc dp g i i m lUu sd AT do tru dat trong ddng e h i y :
AT = vb/2 (2) trong d d b l i chieu rpng hay dUdng kinh tru. Mdc dp
g l i m luu s d lidn quan den phan phdi iai xoay do c d try, tdc la hinh thanh he thdng xoay mdng ngya, do vdy cUdng do x o i y la ham eda dd giam lUu s d ; v i
((aA)iamx,.By = f(bv/2) (3) S d dyng he so nhdt ddng eda ddng e h i y thye thi
{(BA/v)iamxoay = f{vb/v) = t{Rei^)... {4) trong d d , co, A v i v l l n lUdt la tde dp gdc quay cda Idi
x o i y , dien tich loi xoay, v i tde dp cua dong e h i y den try. Ddt quan he xdi h^ vdi s d R e ^ c u a try (Ret, = vb/v), se dupe phUdng trinh dUdng bao kd't q u i t h i nghiem cho xdi nude trong d day e i t cd
d5o < 0,54mm
h(, = 0,00022 Retj"^^' (5) Trong cdng thUe {3) neu thay hg sd nhdt dpng cda
nudc V = 10"^m^/s vao s d Reynolds tru v i b i l n ddi se dupe quan he ddn g i i n :
hc = ^,^4{bv)°•^'^ (6) Cdng thdc nay khdng cung thU nguyen, cdng dgng
vdi cong thdc do Latyshenkov (1948 - 1 9 6 0 ) de nghj tinh xdi eye bd ldn nhat d try (tde dp ddng den trudc try vupt qua 1,3 l l n tde dp khdng xdi cda hgt da't d chan try, v > 1,3vQ)vdi pham vi xoay trudc chdn try bang 2,75b
hcmax = 0,7Ksh{vob)°.^ (7) trong dd Kgh l i hd sd hinh dang try, vo = 5h°•2d°•3^
h va d cd ddn vj la met.
Cd the ndi Shen va ccs l i nhOng ngUdi dau tien dgt quan he h^ vdi s d Rei^ try. Cdng thdc (3) dUde Knight {1975) x i c n h i n ddi vdi cat hat nhd ed Re[j=1300-5700. Song ket q u i s d dyng s d lieu t h i nghiem trong phdng (Ret, < 160000), s d lidu thf nghidm trong sdng {160000< Ret, < 4000000) v i s d lieu thyc te (Ret, > 1200000) cua Altunin {1977) cho thay hai dUdng quan he s d lieu t h i nghiem khdng trung nhau, dudng quan he sd lieu thi nghidm hau nhu vudng gdc vdi dUdng quan he s d lieu thuc te. Sau dd chfnh Shen va ccs (1969) s d dung them s d lidu thf nghidm eda Chabert va Engendinger (1956) chi ra hfJh &Frf^ -^\l4s^ {so phd rut try) vi chf c d s d Rejj l i khdng dd cho cae logi dat khdng dinh dj v i chidu rpng try k h i c nhau b; k h i c nhau, cudi edng da dUa ra hai quan he tUdng quan nhU (Hinh 2) gida xdi tUdng ddi vdi sd phd rut try Frj,:
b (8.a,8.b) b '
c _ T A1T7 "-"^ '
v i quan he -^- i,^irr (9) Vgy chi rieng s d Re^ ehUa p h i i la thdng s d b i l u thi xdi eye bd cho dat khdng dinh.
Baker (1979, 1980) nghidn edu xoay mdng ngUa e h i y t i n g v i c h i y rdi trong phong thi nghiem va chi
m
KHOA HQC - CONG NGHEHinh 2. Chiiu sdu xdi hf, Hinh 3. Ddng thU cap - Fr^ try vi xody trUdc try ra cac cap x o i y tang theo sd Re^, hg thdng dn dinh cd 3 c^p xoay {3 xoay thuan v i 3 x o i y nghjeh nhd d dudi). G i i t h i l t luu sd x o i y eUdng bUe d d i y phang khdng xdi se gay xdi, tgo ra hd xdi trong day cat
ro = k-i(2corov^) = 2n(or^ (10) trong dd to la gia tde gdc quay cda xoay.
S d dyng ddng m l u cho thay luu so xoay khdng ddi khi x o i y chim sau v i o hd xdi, ed x o i y tang l i m cho Vo d d i y g i i m ; gia thiet eUdng dp x o i y trong hd xdi nhu l i x o i y eudng bde cd b i n kinh bang cudng dd x o i y cdng mpt phan chieu sau xdi trong hd xdi:
r = ITIR Vg = 27t{r^ -\- k^h^ )Vg /.| .| <.
trong dd ro l i b i n kinh xoay d day khdng xdi, ks la h i n g sd trong qua trinh xdi v i o d i y xdi. G i i thiet cudng dd x o i y khdng ddi, can bang b i l u thdc (9) va (10) r u t r a :
Baker (1980) kien nghj td'c dd tdi han v Vo xdi dgt gia trj xdi c i n bang,
|8cos(60°-^p) r — ^
(12)
(13)
ddng thdi ndu ra bieu thdc x i e dinh xdi cue bp tUdng ddi h^/b d y a v i o kdt q u i thi nghiem tUPng ty nhu edng thdc do Breusers va ccs de nghj (1977):
j / v V trong dd k-| v i ^2 la ham cua d* = d V ^ S ' '' va h/b vdi A = (Ps-p)/p = 1,65; v la hd sd nhdt ddng eua nudc (mVs); g la gia td'c trpng lyc = 9,81 m/s^; )-\ = f{v/Vc); h v i fa l i h i m eda hinh dgng try v i hudng eda ddng c h i y d i n try.
TUdng ty nhu Shen v i ccs (1967), Nakagawa v i Suzki (1975) sd dyng the tich kiem tra v i mde dp g i i m luu sd eda x o i y mdng ngya ban d i u r-| = -vb/2.
G i i thiet he thdng x o i y trdn trong tam giac trUde c h i n try r 2 = -27noro^ C i n bang F-j vdi r 2
ri = -vb/2 = -2nmo^.= r2 (15) Ket qua thi nghiem cda Nakagawa v i Suzki eho
phgm vi xoay trudc try bang 0,6D d d i y va bang 0,65D is than tru, cd b i n kinh xoay cudng bde ro = 0,177b try hay (Dy = 0,354b), ve = 0,45umaarude try.
Sau dd xet xdi hgt dat d chdn try la n g i u nhidn de tim s y thay ddi chieu sau xoi eye bp theo thdi gian, khdng cho g i i trj xdi Idn nha't h^j^^gj^.
Altunin (1975,1977) khdng true tiep tinh eUdng dd cua x o i y ban d i u , song sd dyng sd dd ddng e h i y khi
h d xdi dn djnh vdi g i i thiet \du lUdng do try cho4n bang luu IUpng dUdc phan phdi lgi dpc hai ben trg trong phgm vi hd xdi v i didn tfch m i t cat ngang ho xdi vuong gdc vdi hudng ddng c h i y tdi try de xac djnh xdi eye bd tUdng dd'i d try trdn,
h^/b = f(Reb/Reo,v/Vo) (ig.a) trong dd Ret, l i s d Ray ndn eda try, Reg l i sd RSy
non cua ddng c h i y . Cong thdc {16.a) dupc Belikov va Tsypin(1988) sd dung 150 kd't q u i thf nghidm v&
105 s d lieu do xdi thyc te k i l m tra, cho ket q u i {16.b}
vdi 5 = V 2 : A ' / « = 2 1 % ; A = (hct/hca-l). cd t h i sii dyng trong thiet kd.
h^/b = 0,9 Fr°'^ [vh I v^bf'^ (d^^ I hf'"^ {I6.b) Qadar (1981) do tdc dp quay v i kfch thudc cOa xoay trUde c h i n tru tren d i y phang khdng xdi, ket quji thf nghiem cho ban kinh xoay ban dau ro = 0,1b (hay Dy = 0,2b) cudng dp x o i y Cg = VQ rg = 0,1b VQ. Sau dd dat quan hd gida chieu sau xdi eye bd h^ trong d i y c i t vdi cUdng dp xoay d dang.
hc = KsC'' {I7.a) Ddi vdi d i y day cat cd d < 0,5mm th) n = 1,28; Kg
= 538 va Vo = 0,092b"-5v°'33 ^ho b > 0,025m, do do
h(,=538Co'>2« (17,b) hay hc= 1,332b°'eV'°s (i7.c) Cdng thdc nay tUdng tu nhu cdng thdc (6)-Shen
va ccs hay {7)-Latyshenkov va khdng chda chilu sSu ddng c h i y , khdng cung thd nguyen.
Juravlev{1979,1984) eho ranged che xdi r i t philc tgp va gdm 2 phan chinh l i i p lye mdt vao try v^
chuyen dpng rdi cua bdn cat. Ddng den try trong phgm vi 2/3 c h i l u sau td day ldn (2/3h), hinh thinh 2 x o i y lien tiep khdng t i c h rdi nhau tryc tiep xdi d^t trudc try. Vung trao ddi rdi manh liet cd t h i coi nhU x o i y dgng elip trdn xoay cd true nghidng vdi phudng ddng gde cp, cd t g ^ = v/Vy, trong dd Vy l i tdc dp xio Idn day trung binh trudc try,
Vy=Vyd(h/d)°.''« (18) V v d = V ^ (19) trong dd oj l i dp thd thuy lye bun cat; v la tdc dp CLia
ddng c h i y ddn try.
Juravlev d l nghj edng thUc x i c djnh xdi eye b6:
{1) Cho xdi nude trong
{v < Vy ) hp = 1,1 b°'Shf•" (v / v^ )"•" Kg^Ko (20.a) (2) Cho xdi nudc dye
( v > V y ) h c = 1 , V 6 ^ ( v / v „ ) ' " - ' ' ^ KshKe{20.b) trong do n-| = 1.0 khi v > Vy^j v i n2 = 0,67 khi v
<Vyj, he sd hinh dgng try v i hudng ddng e h i y den tru Ksh Ke theo nghien cdu cda ngUdi Nga.
Kothyari v i ccs(1992b,1998) bang thi nghiem cho dudng kinh cua xoay cudng bde d c h i n tru trdn:
Dy = 2ro = 0.28 bO'BshO''^ (21) va dd nghj cong thdc xae djnh xdi cue bp ed dgng:
{1) Cho xdi nudc trong
(2) Cho xdi nudc due
Agd J
- = 0,S8
" (22.a)
(22.b)
KHOA H O C - C 6 N G N G H E Tap Chl GTVT 10/2012 11
vdi tdc dd bat d i u xdi 6 tru
{v^j /Agd) =1,2{b/d)-°'"{h/d)°''s (23) A = ( P S - P V P « ' ' . 6 5
Muzzammil v i Gangadhanah (2003) bang thi nghiem trong phdng tren day khdng xdi cho thd'y dd'i vdi sd Reynolds cao {10''< Re < 1,4x10=) x o i y cudng bUc cd dgng elip vdi hai b i n trye la a va b, dUdng kfnh trung binh c u a xoay Dy = (a+b)/2 « 0,2b {dudng kinh try), tdc dp c u a x o i y ve = 7iNDy =^50% v { t d c d d ddng d i n try) vdi N l i t i n s d quay eda tam xoay, luu sd la cudng dd xoay
r = TiDyVe = (0,2Tib)^N (24) Thay ddi cd x o i y trong q u i trinh phat trien hd xdi
{Dy/D = 0,8{hc/D).
Vdi g i i thiet ed x o i y dde ldp vdi di c h u y i n cua bun eat va s d d y n g cac bien da neu de nghj chieu sau xdi eye bp tUdng ddi:
10,93
dt-'
trong dd Vi, Vc, Vec l l n lUpt la tde dp bat dau xdi tgi try theo (23), td'c dp tdi han xdi theo Shields, va tde dp
j ^ A g / v \
4. Gidi thidu tdm tat kd't q u i nghien cdu cua cua t i c g i i va cpng sU
T.D.Nghien (1988,1993) phan tieh k i t q u i thi nghiem trdn day khdng xdi ddi vdi try cd he s d Rdyndn try Re^, = {12,832,53)10^Fr - (0,230,7) cho thay dudng kinh cda xoay Dy = {0,22^0,32)b d mat day va Dy - (0,35-=-0,45)b each day dpe than try (Hinh 4), vi tri dudng tach ddng dau tien Xg = (0,9-^1,1 )b (Hinh 5), ddng thdi rut ra b i n kfnh cda x o i y cUdng bde gay xdi tai mat phang qua tim try rg phy thude vao chieu sau ddng c h i y v i chieu rpng try cd R = 0,915 (bd xung them sd lieu cda Qadar va Muzzam- mil) (Hinh 6)
(25) 'n
.
- 0 125b»"h»2
"^^t^^^^^j—
^^^^^^^^^P*
. . .,-.. .. =
(30)
-".r^rr
Hmh 4. Vung xody tf mo hinh try
Hmh 6. Bieu dd xac djnh rQ cua xody
x o i y t P i h a n , d* = d ' v ^ s ' •' . Khi h^ tang t h i r giam l i m luu lupng bun c i t g i i m . Ddi vdi nUde trong khi h^
t i n g lam cho r g i i m va va g i i m den gia trj nhd hdn gia tri dng s u i t t i l p di chuyen hgt trong hd xdi se ngdng xdi.
Unger va Hager(2007) bang ket q u i thi nghidm chT ra x o i y mdng ngya d i u tien thay doi theo thdi gian.
Hinh bao td'c dp cda xoay phu thudc v i o dUdng kinh try D, chieu sau ddng c h i y h, dUdng kinh hat eat tUdng ddi [D^hyVd^Q, v i thdng sd phd rdt cda hgt F^
= v/(g'd5o)"^, trong dd g'= [(pg- p)/p]g la gia tdc cda hat d trong nUdc.
C d x o i y va hinh dgng xoay la ham cda thdi gian.
Ty s d eua hai b i n trye eda x o i y dang elip d hd xdi cd dang ham mu a^/by ^ exp(hc/D) ^ (26)
Veerappadevaru va ccs (2011) tim h i l u cd c h d xdi cho try trdn mdng lien khdi, xac djnh quan he dudng kinh trung binh xoay dang ellp Dy vdi dUdng kinh mdng, ehidu s i u xdi hQ,
Va de nghi nang IUdng xoay
P „ = (iLlc xoayxtSc do) = p i ^ ^ D ^ N D ^ v ^ (28) Sii d y n g IUu IUdng ddn vj dong chay 6 ho xoi q = (h+h(,) va can bang nang luong xoay vdi nang luong xdi hat da't hinh thanh hS xoi de rut ra quan he chieu sau xoi tai trg tren mong Hen l<hoi (tU mat nudc d i n day hd' xdi) can cU vao l<et quli thi nghISm cua ho va 09 nhudi l<hac.
PhUdng trinh hoi quy vdi R ' = 0,94 cho:
[(h+h<,)/Dm) = 0,48FD.°'» (29.a) PhUdng trinh dudng bao cho
[(h+he)/Dn,l = 0 , 7 F D . (29-b) trong d d : F Q . = q Q . / ( A u . c ) " ' , u.,, = u.l.Jgd^
=,u. la td'c dd ddng li;c, A=(Ps-p)/p
Hinh 5. Vi Irl (3uilng tich Hinh 7. So d6 hd xdi Unh S^
dong
T.A.Tuan va T.D.Nghlen(2005) g\k thiet xoay cd dang elip chim vao trong ho xdi on dinh vdi dudng kinh trung binh D^ = Xh^ va can bang nang iUdng gay xdi vdi nang IUdng ngdng xdi trong hd xdi dn djnh TtDyVo = Xhf.KVf, d ^ rut ra chieu sau xdi cue bd
h^ = {Dyim v»/Vc) = (0,25/X)b''»h°'2( v./V(,) (31) Xac dinh V, theo odng thUc ve = 0,45v[1 +(n V ? Kti'"].
Lcl'y he sd Kakman K = 0,4 va he sd nham n theo Mamlng-Stncld n = d"»/6,76^/g hay
n\fg = d"«/6,75 thi V. se la Ve = 0,45v[1+(d/h)"/2,7]
(32) Thay (32) vao (31) dUOc
hc=(0,1125/X)b»»h»''(v/V;,) [1+(d/h)"»/2,7] (33) Bat K l = (0,1125a) va sap xep iai se dUdc (33) S dang khdng ddn vi
K, 1
2,7
g)
(34)"(o;")'"
Hd sd K-i va sd mu n dUdc xac djnh theo sd iieu thi nghiem va do xdi thuc te (xoi nudc trong: 30 s d lieu thuc do,17 sd lieu thi nghiem vdi he sd tuong quan R
= 0,8; xdi nudc due: 40 sd lieu thuc do chu ydu d song Amuddaria cua Lien Xd trudc day v d i he s d tuong quan R = 0,85). Cdng thUc kien nghi thiet ke cd dang:
he = K, KjKshK.b»»h°'2(v/Vc)" (35.a) trong dd K^ = [1+(d/h)"»/2,7], Kg^Ke lan ludt ia he sd hinh dang tru va hudng cua ddng c h i y den try.
m
KHOA HOC - CONG NGHE-4gh^[hldJ'
K i = 1,21; n = 0,75 khi v < Vj, (xdi nUdc trong) va
^ 1 = 0,93; n = 1,5 khi V > Vp {xdi nUdc dye).
T.D Nghien {2012) lay v^ = u-c5,75lg{5,53h/d5o);
31 s d lieu thye te neu trdn d tru vuong khdng ed xdi nude trong, ehl cd xdi nudc due (v > Vp) dUdc phUdng trinh tUdng quan tfnh h^ vdi he s d R = 0,73
hp = 0,41 KjjKflb''«h°-^(v/Vc)''--'^ (35.b) T.D Nghien (1988,1993.2000) k i l n nghj sU thay ddi
ap lyc mat try thanh thay ddi dpng nang giOa vung c h i y bao quanh tru vdi ddng ngoai can bang vdi ddng ndng xoay eudng bde tgi chan t r y , td dd x i c dmh dupe luu so xoay hay cudng dp xoay phy thude vao chieu rpng try, c h i l u sau ddng c h i y den try, he sd i p lyc vao try, he sd tang tde dp eua ddng c h i y trong ldp bidn bao try. Can bang cUdng dp x o i y gay xdi
r^ ={3,545/^jC^)^(k- - I ) / k ^ V (36) vdi cudng dp x o i y dn dinh trong hd xdi
r = Inr'Q}^^^ = 25,fi;,,,, = 45,.v„„ / /?, rut ra b i l u
(37) thdc eua md hinh ly thuyet /JC^,)K,M(v/v„^) (38) trong dd K-j =^ik- -])/k the hien phan phdi lai tdc dd trong idp bien bao try. G i i thiet x o i y chim trong h d xdi cd dudng kinh tUdng dUdng bang h^ thi Sj£/hc=7ihQ/4, thay g i i trj n i y v i o (38) dupc
hc='i.^3(^/y[C^)K^4bhiv/v,,^) hay {39.a)
h c / b = 1 , 1 3 { l / , y C ^ ) ^ , V ^ ( v / v „ g ) {39.b) B i l u thUe (39.b) cd dgng khdng ddn vj quan hd vdi chieu rpng try, la thdng sd ed b i n xdi eye bp try. Ddi vdi d i y cat tgi mat phang dpe tim tru h d xdi ed dang tam giic,cd dien tich S^ = h^Qeotg(ti/2 (Hinh 7). Ket qua cda t i c g i i v i n h i l u ngudi khac do dupc goc nghidng cda hd xdi xap xi 33°{(t) = SS^") do do Sj( = S f , do dd cdng thdc ly thuyet x i c dmh h^ se la
he =(3,544KiIn ^Jc^)4bh(v I v^^) hay {40.a)
h(, = K v ^ ' ^ t ^ ^ ^«-), vdi K = 3,544Ki/'^V*^/"" {40.b) hay d dang khdng ddn vi de x i e djnh he so v i s d mij thdng qua s d lidu thyc nghidm vdi Vpg da chpn
S d dung k i t q u i thi nghiem cda tac g i i , eua mpt sd ngudi k h i c k i t hpp vdi s d lieu thyc do xdi d cac c l u dang khai thac cda Viet Nam, Lidn Xd trUde day.
A n D d , Pakistan dUde b i l u thdc x i e dinh c h i l u sau xdi cye bd h^ d l nghi trong thiet k e .
h(,= K V ^ ( v / v „ ^ ) " K s h K e (41) trong d d Cpf^ l i he s d i p lyc mat tru C^^ ^ 1,0;
K = 1,24 ; n = 0,77 cho v < v ^ g (eoi nhu xdi nudc trong); va K = 1,11; n = 1 , 0 c h o v > v^g {xdi nUdcdue).
myhld^QJ ; co la dp thd thuy lyc eua ''ng=
hgt cat (42)
Cdng thdc ap dyng cho try cd c h i l u sdu td 3m din 18m, ehieu rdng try tdi 6m, tdc dp ddng e h i y d i n try tdi 2,5m/s v i e i t hat rdi ed dsQ < 1,53mm.
5. Ket ludn
- Hieu bidt nhieu hdn v l c d ehe x o i y , cudng dO x o i y , nang lupng x o i y 6 day edng va day xdi.
- He thdng x o i y eUdng bde gdy xdi trudc try rit phdc tap, chua cd s y t h i n g nhat gida cae nha nghifin cdu, song ban dau dudng kinh trung binh cua xoay cudng bde cd the la Dy = 0,2b {dudng kfnh try) khi so Reynolds eao (10''< R e < 1,4x10^), hayed dgng quan hd Dy =2ro^ 0,28 b'>-^^h°-''- (Kothyari va ccs1992b,1998); Dy = 0,25 b°s h^^ {T.B Nghidn1988,1993,2000). Trong hd xoi ay/by^exp(hc/D) Unger v i Hager(2007) d l i vdi trg ddn, A V A - , ^ 0 , 1 8 [ l + ( ; z ^ / D , „ ) ] Veerappadevaru va ccs (2011) ddi vdi try tren mdng lien khdi.
- X o i y cd the dang trdn hay elip trong hd xdi 6 dit khdng dinh, thay ddi kfch cd trong qua trinh xoi vio day c i t , song de tfnh da dUa v l dang trdn tUOng dUdng.
-Thdng nhat luu s d xoay l i cUdng dp x o i y , song e i c h tid'p can chUa thdng nhat.
- Cd nhieu eong thdc k h i c nhau rut ra td cd ch^
xdi cd t h i vi chua h i l u day dd ed che xdi, chua hilu rd tinh khdng dn djnh cda xoay mdng ngya trong ho xdi, tUdng t i c giua xoay mat va xoay day, tUdng tac cda xoay vdi cd hgt e i t khae nhau trong lid xdi, nong dp moi trUdng trong h d xdi va mdt sd y l u td khac.
- Rieng thdng so Reynolds try khong phu hpp cho tfnh xdi d d i t khdng dfnh.
- Chieu sdu xdi phy thupc va thay ddi theo cUdng dp dong ehay (hg/b ~V/VQ), ehieu rpng try b, hinh dgng try Sh, c h i l u s i u ddng ehay h, tde dd ddng chiy v, tinh chat dat day sdng d, la eac b i l n quan trpng xac dinh vdi cye bd hp, e i c thdng s d quan trong nay phin ldn deu ed mat trong cae edng thdc rut ra td cd che.
Thuc te moi bien n i y i n h hudng den xdi r i t phdc tgp vi t i e dung lUdng hd giOa chdng v i tu dieu chTnh trong dieu kien ddng lyc cua ddng nude v i bun c i t 6 thi thdng nhat khi eac dieu kien thay d l i d i l n ra •
Tai lieu t h a m k h i o
[1]. Tran Dinh Nghidn "Nghidn edu xdi eye bp try cau qua sdng" Luan i n t i l n sy,trudng DHGTVT,2000.
[2]. Tdng Anh Tuan " X i y dung b i l u thdc dy doan xdi eye bp try cau qua sdng" Lugn an thgc sy, TrUdng Dai hpc GTVT, 2005.
[3]. Baker, C.J.(1980). "The turbulent horseshoe vortex". J . Wind Eng.lnd. Aerodyn. 6{1-2), 9-23.
[4]. Dey, S.. Bose, S.K., Sastry, G.L.N.
(1995)."Clear water scour at circular piers: A model".
J. Hydr Eng. 121(12), 8 6 9 - 8 7 6 .
[5]. Garde and Kothary "Scour around bridge piers' PINSA 64,A,No.4,July 1988.
[6]. Kirkil, G., Constantinescu, S.G., Ettema, R.
(2008). "Coherent structures in the flow field around a circular cylinder with scour hole". J . Hydraulic Eng.
134{5), 5 7 2 - 5 8 7 .
[7]. Lee "Physical modeling of local scour around complex brirge pires" Ph.D.Dissertation May, 2006.
(Xem tiip trang 23)
KHOA HQC - C O N G N G H E
Tap Chi GTVT 10/2012 I Q
B^ng 1. Kich thu6c md hinh thi nghidm tri^ cau
Thiln i i v
^ ( m l
111)6 Um)
26 Be coc
him) 0.125
Km) 0.31
T(m) 0045
.Mlfimcoe n 2
m
"
(1 (m) 0.027
Qlm'W 1)04
Duflng kinh hat IXnun) 03
Chieu sau h,(m)
021 Sau khi lap dgt md hinh thf nghiem, thi nghidm dupc tidn h i n h vdi luu IUdng 401/s, chieu s i u m y c nudc l i 2 1 c m . Nude dung d l thi nghidm dUdc bdm b i n g bdm c d n g sua't Idn ehay tuan h o i n trong he thdng c d n g n g l m va m i n g thi nghiem. LUu lupng dupc do bang ddp tran thanh mdng hinh chO nhat d thupng luu va dupc k i l m chdng lai bang mpt dap tran thanh mdng mat cat hinh tam giac 6 trUdc p h i n m i n g thi n g h i d m . MUc nude trong m i n g thi nghiem dUdc khdng ehe va d i l u chinh bang he thdng van dieu tiet d hg lUu. T r u d c khi thi nghiem van dieu tiet hg lUu dupc ddng lai, nude d thupng luu dupe t h i o t d t d vao he thdng thf nghiem de d i m b i o cat d day m i n g khdng bj xao ddng, khi nudc vao m i n g du luu lupng c i n thiet thi van d i l u tiet d ha luu dUpc md d i n den myc nude 21 em, thi nghiem dupe b i t dau, ddng thdi t i l n h i n h quan sat q u i trinh xdi theo thdi gian nhd vao thudc g i n trye tiep trdn mo hinh. Sau 3 ddn 4 gid thi nghidm (xdi d tru c l u c h i m p h i t t r i l n ) , tien h i n h do IUu tdc va ap s u i t xung quanh khu v y c ddt tru. Thi nghigm ket thue khi quan s i t t h i y e i c hat c i t trong d i y h d xdi hau nhU khdng di c h u y i n vd c h i l u s i u hd xdi khdng t a n g qua 1mm trong thdi gian 30 phut {thudng keo dai t d 6 den 8 tieng). Ket q u i do chieu sau xdi Idn n h l t trong thf nghiem, chidu s i u xdi tfnh toan theo eac phUdng phap va hai thdng so thdng ke la sai sd tuyet ddi trung binh v i phUdng sai cho tdng phUdng p h i p ndu trong b i n g 2 va hinh 5. Cae sd in d i m chi ra eae gia trj thap cda tUng thdng so thdng kd.
PhUdng p h i p cda Sheppard va theo quy trinh Nga cho g i i tri c h i l u sau hd xdi tUdng ddi s i t vdi chieu sau thf nghidm, nhung phUdng p h i p cda Sheppard thi ed trj s d Idn hdn edn phUdng p h i p theo quy trinh Nga deu cd trj s d nhd hdn tri s d thf nghidm, phudng phap cda Melville va Coleman eho tri sd chieu sdu hd xdi Idn nha't (sai so tuydt dd'i v i phUdng sai l i 0,162m).
4. Ket luan
Bdn phUdng phap dupe s d d y n g d l d y doan chieu sdu xdi eye bd tgi try cau phUc tgp d i dupc s d d y n g d l ddi chieu vdi s d lidu thyc nghiem.
PhUdng phap do Sheppard de nghi la phUdng p h i p eho chieu sdu xdi eda try e l u bd eao gan vdi
kd't qua thi nghiem hdn, ed e i c g i i trj thd'p cda thdng sd thdng ke.
Bang 2. Kit qua TN xdi cue bd try ciu bg cao vd chieu sdu xdi tinh todn
T l , ™
i d i h . T ^ « « ]lu nehiSin
Tluo Ol N c i
CUeu cao lEdi b; nw Y(cnil
0 0 3 ' Q0J8 0<B5 n o i l
asM
1 ^ , - r r . . . . - „ .
\ —
Hfcm)
Hinh 5: Kit qua TN xdi eye bg try ciu bg cao va chiiu sdu xdi tinh todn
PhUdng p h i p theo quy trinh Nga eung eho cae thdng sd thdng kd thap, song phUdng phap n i y chUa dua ra day dd eac trudng hpp xdi eye bp try e l u bd cao {khi ehieu cao be cpc Idn hdn 0,3 lan chieu s i u ddng e h i y thi try c l u bd eao dupe tfnh nhu try l i l n khdi).
PhUdng p h i p cda Melville v i Coleman (2000) thidn ve gia trj idn trong dy doan chieu sau hd xdi eye bd tru c l u •
Tai li$u tham k h i o
[1]. Coleman, S. E. "Clearwater Local Scour at Complex Pier^' J. Hydr. Engrrg. April 2005.
[2]. Richarson, E. V., and Davis, S. R. "Evaluating Scour At Bridges • Fourth Editiori' U.S. Department of Transportation - FHWA, May 2 0 0 1 .
[3]. Sheppard, D. M., and Renna, R. "Bridge Scour Manual' Florida Department of Transportation - May 2005.
[4]. C 32 - 102 - 95 "CoopyMoHufi MoctoBbix nepexoAoB u noATonnpeMbix Haetine^' - 1996.
cd CHE xdi cue BO...
(TiS'p Iheo trang 16}
[8]. Melville, B.W., Raudkivi, A.J. (1977). "Fiov»
characteristics in local scour at bridge piers." J . Hy- draulic Res. 15(4), 3 7 3 - 3 8 0 .
(9]. IVIuzzammil, M., Gangadharaiah, T. (2003)."
T h e mean characteristics of horseshoe vortex at a cylindrical pier." J. Hydraulic Res. 41(3), 2 8 5 - 2 9 7
[10] .Ollveto, G., Hager, W.H. (2005)." Further re-
sults to timedependent local scour at bndge ele- ments." J . Hydraulic Eng.131(2), 9 7 - 1 0 5 .
[11]. Shen, H.W., Scheider, V.R., Karaki, S.
(1969)." Local scour around bridge piers". J . Hydraulic Divn. Proc. ASCE, Voi. 95, HY-6.
[12]. T.D.Nghien "Laboratory investigation of scour reduction near bridge pier by delta-wing lii<e passive device" M.tech.Departt.Clvil Engrg IIT.Kanpur,Janu- ary, 1988
[13]. Veerappadevaru va ccs "Vortex scouring process around bridge pier with a caisson"
J.Hydr.Rearch, 17 jun 2 0 1 1 , 49:3,378-383
