KHOA H O C - C O N G N G H l
M6 phong dong chay xung quanh tru cau
bang phuong phap thuy dpng lire hpc hat min SPH (smoothed particle hydrodynamics)
« TS. N G U Y I N H I J U T H U A N - Trudng Dai hgc Giao thdng van tdi
TOM TAT. Cac d^c trung dong ch^y lu xung quanh than tru cau co Inh hudng rat Ion den hien tuong x6i nen mong cua tru cau, ^nh hudng den do ben va tuoi tho ci!ia tru cau va cong trinh cau n6i chung Bai bao thuc hien mo phing so 3D bang phuong phap dgng luc hoc hgt mm SPH (Smoothed Particle Hydrodynamics) doi voi dong chSy qua tru cau Trong do, cac d^c trung dong chay nhu van toe, ap luc nuoc khu vuc xung quanh than tru dupe xac dmh trong thdi gian dong chay qua tru, Ngoai ra, tuong tac giua dong ch^y vai than tru cung duoc mo ph6ng de danh gia luc tac dong cua dong chay len than tru
TL/ KHOA: Thuy dong luc hpc hgt mm, tai trpng va ch^m, tru cau, hgt, ap luc chat long
ABSTRACT: Parameters characterising the flood flows around the bridges piers are the mam vanables affecting the pier scour phenomenon In this study, a numerical model is applied to study the interaction of solitary wave and uniform current and their forces acting on a horizontal cylinder near the free surface. SPH model of a bridge pier IS established including the effects of water flow impact as well as the water circumferential motion around the pier The characteristisc of flood flows (velocity, press) around the cylinder are investigated and predited Then, the results show that the water flow force calculated using the formulas provided by the codes to account for the effect of flood impact on the bridge pier
KEYWORDS: Smoothed particle hydrodynamics (SPH), impact load, pier bridge, particles, fluid press.
I.OATVANDI
Do vi tri dia ly, Viet Nam la nUdc thudng xuyen chiu anh hudng eua 10, lut. Mot trong nhdng hau qua Idn mang lai la mUa, lu anh hudng nghiem trpng den eae cdng trinh xay dung noi ehung va cdng trinh eau ndi rieng, dae biet la tren dia ban eae vung nui d Viet Nam, Hang nam, d Viet Nam ed hang chye eay eau bi huhdng hoac sap do gay ra do mUa lu (Hinh 1.1). Dong chay vdi van toe Idn do lu gay ra hien tUOng xdi mdn nen mdng
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eong trinh cau, gay nghleng, lech ket cau nhjp, m6, try.
Hu hdng do mUa lu gay ra trd thanh mdt trong sd nhdng nguyen nhan hang dau gay hu hdng doi vdi cdng trinh eau d Viet Nam.
Hinh 1.1: Cau sap dddo mua lu 6 Vidt Nam: fa) • Cau Ngdi Thla, Yen Bii 10/2017; (b) - Cau Bin Ma, Thanh Hda, 10/2017
Viee phan tfeh, tinh t o i n va dU b i o I n h hudng ciia ddng chly den ket eau mo, tru eau da dugc nhieu tac g i l tren the gldl thuc hien t d r i t lau. Cac nghien cdu nay deu tap trung du bao elc dae trUng ddng ehly xung quanh than md, try eau nhU van tdc, dong chly, ap lUc nUdc, lUc tuong tae eua ddng ehly len than md, try..., t d do du bao anh hudng den toe do xdi tai khu vUe nen mdng cua md, try eau. Tuy vay, viee danh gia day du tUong tac dgng cua ddng ehly chat Idng len t h i n try clu cung nhU p h i n dng ddng eua ket eau dudi I n h hUdng nay van eon rat han ehe. Khi ddng ehay lu tae ddng vao t h i n tru, tae dgng eua dong ehly vao tru eau cd the phan thanh ba thanh phin; Phan tae dgng vao mat tiep xOc vdi try eau theo hudng ddng ehay ngay tai thdi diem va cham vao va phan ddng ehay xung quanh than tru sau thdi gian va cham ban dau, ngoai ra la t i e ddng tinh len than try cua edt nude. Van toe eua ddng chly lu se nhanh chdng bi giam khi va cham vdi than try, mdt phan nude sau khi va eham se bi can trd bdi than tru tao nen ddng chly ngUgc ngay mat trude cua than try, trong khi phan Idn ddng chly se re nhanh sang hai phfa than try va tiep tue chly ve phfa trudc (Hinh 1.2).
Hinh 1.2: So do co ehe tic ddng cua ddng chiy idn thdn tru cau tiet dien trdn [3]
Nhieu tae gia da tap trung md phdng thay ddi dong ehay khi gap ket eau vat ein va tUOng t i e gida ddng ehly vdi ket eau, vdi viec sd dyng nhieu phuang phap khac nhau nhu phan t d hdu han (FEM) [1-3]; phuong p h i p
KHOA H O C - C 6 N G N G H 6
VOF (Volume of fluid method) [4,5], hay phuong p h i p thuc nghiem [6]. Cac md hinh thue nghiem cung dugc thue hien vdi kich thudc thu nhd d ^ kiem chdng cac ket q u i phan tieh, dU bao t d md hinh phan tieh sd. Cae md hinh phan tich nay da dat dugc ket qua kha ehinh xae so vdi ket qua thf nghiem [1-6], tuy nhien cac phuong p h i p nay ed han che la gap khd khan trong viec md phdng dieu kien bien tiep xiic gida chat Idng vdi mat ket cau hoac d i y va mat ben eua kenh ddng ehly, dac biet vdi phuang p h i p phan t d hdu han gap khd khan khi chia ludi p h i n t d lai vdi phan t d chat long trong thdi gian md phong vi ddng ehly thay doi hinh dang lien tue, lam tang thdi gian tfnh toan va giam do ehinh xac cua ket qua md phdng. Cic khd khan nay cd the dUOc han ehe nhd sd dung phuong phap thuy ddng hat min hay phuang p h i p khdng ludi SPH (smoothed particles hydrodynamics).
Phuang phap SPH la phuang phap tinh t o i n ket cau duge gay dung doe lap vao nam 1977 bdi Monaghan Gingold [7] va Lucy [8], ban d i u de giai quyet cae bai toan trong linh vUe thien van hge [7]. Tuy nhien, ngay nay no da dugc sd dung rdng rai trong eae linh vUe tfnh toan khac nhu ea hge chat Idng, va cham ket cau [9,10]...
Phuang p h i p nay da duge sd dung trong mdt sd nam g i n day de md phdng tuong tac gida ddng chly sdng bien vdi ket cau edng trinh nhu ket eau nhip eau, tudng chan sdng, de bien [11 -15], Phuang phap nay cung dugc sd dyng cho ddng chay tuong tac vdi try eau trong nghien cdu Z.P. Wei ef al [16] va Gede Pringgana et al (17].Trongdd, Z.R Wei efo/[16] da md phong ddng ehly tUOng t i c vdi try elu eho mo hinh thu nhd vdi kfeh thUdc try 120mm; Gede Pringgana eta/[17] da nghien edu lUc tac dgng eua sdng than len tru c l u tren can gan bdbien.
dViet Nam, mdt so nghien edu eung da phan tfeh, mo phdng de du b i o elc dae trUng ddng ehay ed van toe Idn xung quanh try eau nam mye dfch la d i n h gia xoi Id tru cau. Cac nghien cdu gan day eua N.V. Thanh ef al [18-20], V.M.Tuan ef o/ [21] da thue hien phUOng phap md phdng so de nghien edu sU thay doi ddng chay va xdi cue bd xung quanh tru cau sd dyng phuang phap vl phan hdu han g i l l phuang trinh Navier-Stokes trung binh Reynolds ddi vdi ddng chly 3 chieu. Ket q u i m6 phdng cung dugc so sanh vdi ket qua thi nghiem md hinh vat ly trong phdng. Tuy nhien, cac nghien edu nay mdi chi tap trung vao mo phdng eae dac trung ddng chly v^ I n h hudng eua no den xdi cue bd quanh try c l u ma chua xem xet den tuong tac gida ddng chly vdi ket eau tru eau giy ra bdi ap suat dgng va va cham eua ddng chly vao try eau.
Trong nghien cdu nay, mdt md hinh SPH ba ehieu se dugc thuc hien de md phdng hinh thai ddng chay va elc tuong t i e gida ddng ehly vdi try cau tiet dien trdn cd kfeh thude gan gidng thUe te (dudng kfnh 01 m), qua dd p h i n tfeh eac dac trUng eua ddng ehay tai mdt sd vi tri xung quanh try c l u eung nhu cac tac dgng cua ddng chly len than tru.
2.PHao(NGPHAPSPH
Phuong phap khdng ludi SPH chia ket cau tinh toan
So 11/2018
thanh mdt nhdm cac nut phan t d (hat). Moi nut phan t d dai dien bdi vi tri, khdi lugng, van tdc, dng suat... Mol hat cd mot vung Snh hUdng eua nd len elc hat khac. Anh hudng nay cua hat len eae hat khac dugc dien ta bang mot ham t o i n hoc hinh ehuong. Mdc do I n h hudng phu thuge vao khoing each tU hat do den hat dang xet. Anh hudng la Idn nhat tai trung tam va b i n g khong tai bien cua vung I n h hUdng. Co sd eua phuong phap la g i l i bai toan tfeh phan cua ham so Idi W(r), vdi vi tri r trong mien tinh toan W.
f(.r) = jfO-)fF(r-r'.,h)dr' (1) n
De dan glcin trong tfnh toan, ngUdi ta da dUa ra cdng thde gan dung de tinh eae gia tri trj vat ly eho moi hat, dua tren tong eae gia tri cua cae hat khac trong vung anh hudng:
Gradien eua ham sd fduoc tinh toan bdi edng thde:
W = Y—fj'^^, (3) Cdccdng thdctodn hgc trong phuang phdp SPH
Chieu dai anh hirdng cua phan ni hat - T ' = S''/^^*''i ~ S ' ^ " ^ i Trpng lir(Tng rieng phan ni hgi ~T~^2^'"/''.^'','^"'.,
-z..,
(5) (6)
(7) Naag luQfng hai
Trong do.- r- Khoing each gida hai hat i va j ; /i - Chieu dai anh hUdng eua phan t d hat; p^ - Trong luong rieng euahati;m -Khoi luong eua hatj; v va v -Lan lugt la v i n tdc eua hati va j ; o va o - Lfng suat cua hati v a j ; e - N a n g lugng eua hati.
3.GIAIQUYgTVAND^
3.1. Md hinh phan tich
Mdt mo hinh phan tfeh cd kich thude nhu Hinh 3.7. Trong do, kich thUde theo ba chieu la 15x4x3m.Cdt nudc ban dau de tao ddng chly co kfeh thude la 3x4x2m.
Nude day kenh ban d I u eo chieu eao 0.2m. Try eau ed tiet dien trdn, dUdng kfnh 0.75m, eao 02m, n i m tai gida ddng ehay. Phuang phap SPH duoc sddyng trong trudng hop nay dugc viet tren phan mem ma ngudn md DualSPHysies ket hgp vdi Matlab de phan tfeh va xd ly ket qua. Trong dd, kich thUde hat phan t d dUOc sd dyng la dp - 0.1 m, tuong dng vdi so p h i n t d chat long la 32.600 (hat). Dieu kien bien dUOc sd dung vdi hai Idp phan t d vdi cae mat tiep xuc eua chat long, tong so phan t d cho dieu kien bien I I 14.164 (hat). Thdi gian thUe hien md phdng la 6s. Thdi gian nay dugc thuc tinh toan vdi CPU: [email protected], trong thdi gian 3 gid.
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KHOA H p c - C 6 N G NGHf
Hinh 3.1: Sa do pham vi va kich thudc mo hinh SPH (kich thudc-m)
3.2. Ket q u i va phSn tich
Ket q u i md phdng ddng ehay tae dgng vao than try eau dugc the hien tren Hinh 3.2 - 3.3, trong dd cho thay hinh thai ddng chay va trUdng van toe cua ddng chay tai mot sd thdi diem dae trUng: 0.8s, 1.1s, 1.8s, 3.0s va 3.5s.
Tai thdi diem t = 0.8s, ddng ehay bat dau tiep xuc vdi mat try cau (Hinh 3.2a). Sau thdi gian nay, ddng chay va eham vao try, hinh thanh mot khdi chat Idng eye bo ehay nguge len cao va ve phfa thugng lUu (Hinh 3.4), van tdc mgtsd phan t d chat Idng nay se giam; sau dd ddng chly chia ddi thanh hai hUdng hai ben than try va tiep tue di chuyen ve eudi pham vi md phdng (Hinh 3.2b - 3.2d). Sau thdi gian t - 3.5s, ddng chly trd ve trang thIi on dinh,
Hinh 3.3 the hien trUdng van toe ddng chay eye bd tai khu vUe xung quanh than tru. Ngoai ra, md phdng nay eung cho phep xuat ket q u i ve van tdc, ap lye nUdc tai bat cd vj tri nao trong pham vi md ph6ng de theo ddi suthay doi eua eae dai luong nay trong thdi gian md phdng. Hinh 3.5 vd 3.6 the hien van toe va ap suit ddng chay tai mot so vi tri diem co trude mat than tru (A, B v l C). Van toe Idn nhat theo thdi gian tai cac diem A, B va C lan lugt la 1.18; 1.19 va 1.27 m/s tai thdi diem ddng ehly di qua (~1s}.
Ket q u i phan tich ap luc ddng chay cho thay, d giai doan thdi gian ban dau (khoing 0.5s) ap suit chat Idng tai cae diem A, B va C ed gia trj bang ap lUe ngang tinh cua cot nude day kenh, do dong chay t d cdt nUdc cao ehua di chuyen tdi try, sau thdi diem 0.5s... dong ehly bat dau di chuyen qua eae diem A, B va C, va eham vao than try, gay ra hien tUOng tang ap suit chit Idng va dat gia tri Idn nhat tai thdi diem ~1 s. Day la thdi diem ddng ehay va vao mat tru, sau do b i t dau giam dan do hien tuong t i c nghen dong chly va gan ve g i l trj on djnh euoi md phdng. Ap suat chat Idng Idn nhat tai cac diem A, B, C lan lugt la: 7.96,10.07 va 10.89 kPa; tai eae thdi diem t ^ 1,01s, 1.10s va 1.04s. Sau thdi diem nay ap suit chat Idng tai cac diem tren giam nhanh va trd ve gia tri thuy tTnh ban dau. Sau khi dat gia trj dinh do va eham vao mat tru, viec giam nhanh ap suat chit Idng la do ehieu eao egt nUdetaodongchlyban dau (nang lugng) giam dan, gia tri ap suit chat Idng se dugc duy tri neu nang chieu cao egt nude tiep tuc dugc duy tri. Day la dieu kien phan anh dong ehay lu tae ddng vao try eau trong thUe te, se dien ra lien tyc trong khoang thdi gian dai. Tuy nhien, trong dieu kien md phong so. thdi gian tinh toan khdng eho phep nen cd the dU bao dUOc gia tri Idn nhat cua ap suat chat Idng len mat try cau trong thdi gian ngan.
Hinh 3.2: Trang thai ddng chiy vi trudng van toe tai mdt so ti diem md phdng SPH
Hinh 3.3: Trudng van tdc ddng chiy xung quanh than tru trong md phdng SPH
Hinh 3.4: Hinh thanh ddng chiy nguot tai vi tri va cham vdi mat tru
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KHOA H O C - C O N G NGHfi
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Hinh 3.5: Van toe ddng chiy tai mdt sddidm dai dipn trudc mat thin tru
Hinh 3.6: Ap suat dong chay tai mdt so diem dai dien trude mat than tru
3. KfT L U A N V A K I E N NGHj
Mdt md hinh ba chieu (3D) de p h i n tich ddng ehay tuang t i c vdi try cau da dugc thue hien bang phuang phap SPH vdi kich thudc tUOng duong vdi thuc te dUa tren code dugc lap trinh tren ma ngudn md. Ket qua md hinh eho thay cic che do dong ehay va eac dac trUng cua nd trong sudt thdi gian md phdng. Dac biet, md phdng cho phep tfnh t o i n ap suat dgng cua chat long g i n be mat try eau do tuang t i e dong chly vdi try cau. De hoan thien md hinh phan tieh cae nghien cdu tiep theo can tang sd lugng phan t d trong mo phdng sd; nghien cdu Inh hudng cua eac tham so den eae dac IrUng ddng chly quanh tru, ngoai ra nghien cdu thuc nghiem eung can duac tien hanh de so sanh vdi ket q u i md phdng sd Q
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Ngay nhSn bdi: 10/10/2018 Ngdy chap nhan dang: 30/10/2018 tigUiA phSn bien:TS. Ngd Van Minh
TS. Le Ba Anh