3D CFX Modelling of Meandering Channel Flows (River Blackwater) By Xiaonan Tang Mark Sterling Donald W Knight School of Civil Engineering The University of Birmingham Contents 1) Background to the Project 2) 3D CFX Package 3) CFX modelling for 4 FCF Cases - Inbank flow [1] - Overbank flow [3] 10/09/2009, Warrington, UK (2009 IAHR UK-Section AGM ) 1 2 Project overview Combine new approaches ( PIV, ADCP,ADV & DP) to obtain a reliable estimation of the discharge and reconstruct the flow within a 300 m reach of a two-stage, double meandering channel Data collection: ADCP PIV & DP ADV EPSRC Research Grant: New Approaches to Estimating Flood Flows via Surface Videography and 2D & 3D Modelling University of Birmingham Loughborough University Centre of Ecology and Hydrology River Blackwater Basic facts: The River is man made Inbank flow capacity = 1.5 m 3 /s Overbank flow capacity = 4.3 m 3 /s Main channel width ~ 5 m Bankfull depth ~ 0.75 m Catchments area at the site ~ 35 km 2 Flow direction Farnborough Modelling: 3D Birmingham (CFX), UNESCO-IHE (Delft 3D), CEH (Phoenics) 2D Loughborough University (TeleMAC) Quasi-2D SKM method (analytical solution) 3 Reach valley slope is 1.0 x 10-3 4 1
River Blackwater Research Reach (~ 300 m) Seasonal variations CROSS-SECTIONS SECTIONS 2. STRAIGHT 3. Apex 4. CROSSOVER 5. APEX Flow direction 5 23/6/2007 6/3/2007 4 5 2 4 3 Winter 63.10 63.00 62.90 h [m] 62.80 Q = 0.24 m 3 /s Q = 3.3 m 3 /s 100 m 62.70 Q [m 3 /s] 62.60 0 0.5 1 1.5 2 2.5 5 6 Winter Flood (recent) Blackwater (FCF 1:5) 10 Feb 09 Winter Inbank flow (I): 132mm Overbank flow (H,B,C) (H=200mm, 187mm) Y CFX model xy coordinate X Case I H MC (m) 0.132 H FP (m) - Q (m 3 /s) 0.043 MC Rough FP Smooth H 0.200 0.050 0.146 Smooth Smooth B 0.187 0.050 0.125 d 50 =8mm Smooth 7 C 0.187 0.038 0.084 d 50 =8mm d 50 =8mm 8 2
ANSYS CFX Version 11.0 SP1 THE UNIVERSITY OF BIRMINGHAM Inbank flows THE UNIVERSITY OF BIRMINGHAM A high performance, general purpose CFD program Pre-processing (Workbench, ICEM-CFD) - Build Geometry and mesh generation H = 0.132 m, Q=0.043 m 3 /s (rough bed, smooth side wall) CFX-Pre (set-up both standard and highly complex fluid dynamics analyses) CFX-Solver (coupled algebraic multi-grid techniques - FVM) Post-processing (CFX-Post: data analysis and visualization) 0.85 m 1:1 9 10 Inbank (1) Mesh Ks=6 mm for bed (Ks =0.75mm for all side walls) So= 1 Inlet velocity = 0.328 m/s (average) Free slip B.C. for water surface Model: k- turbulence model Structure mesh (250 x 30 x 12) 11 12 3
Geometry Mesh (plan view) CFX model 13 14 Velocity vectors on surface Streamlines on surface 15 16 4
Stream lines between CS3 and CS5 Wall shear stress (inbank( inbank) 17 18 Velocity Contour Plot @CS3 (x=23m( x=23m) Velocity Contours and Vectors Depth (mm) 132 99 66 33 30 27.5 32.5 35 37.5 40 42.5 0 0 230 460 690 920 1150 Distance across Channel (mm) CS5 CS4 19 20 5
Inbank (2) Flow development around bend Rough (Ks=43mm for bed, 0.75mm for side wall) So = 1 Inlet velocity = 0.328 m/s (average) Free slip B.C. for water surface Model: k- turbulence model Structure mesh (250 x 30 x 12) 21 22 Wall shear Streamlines on the surface Ks = 43 mm Ks = 43 mm Ks = 6 mm 23 Ks = 6 mm 24 6
Velocity Contours @CS3 Inbank flow (B132) actual = 0.043 m 3 /s) (Qactual Ks = 43 mm Ks = 6 mm Ks = 43mm (bed) @CS3 (x=23m) Q =0.0452 m 3 /s Umin = 0.2056 m/s Umax = 0.4347 m/s Umean = 0.3332 m/s Tau (ave)=0.7488 N/m 2 Ks = 6 mm (bed) @CS3 (x=23m) Q =0.0452 m 3/s Umin = 0.1379 m/s Umax = 0.4616 m/s Umean = 0.3336 m/s Tau (ave)=0.4915 N/m 2 25 26 0.50 Depth averaged velocities and bed shear stress 0.50 Depth averaged velocities 0.40 0.30 0.20 0.10 CFX_CS3 (Ks=6mm) CFX_CS3 (Ks=23mm) CFX_CS3 (Ks=43mm) 0.0 0.2 0.4 0.6 0.8 1.0 Y (m) 1.2 0.40 0.30 0.20 0.10 CFX_CS4 (Ks=6 mm) CFX_CS4 (Ks=43mm) 0.0 0.2 0.4 0.6 0.8 1.0 Y (m) 1.2 CS4 1.60 0.50 Bed shear (N/m 2 ) 1.20 0.80 0.40 CFX_CS3 CS3 CS5 0.40 0.30 0.20 0.10 CFX_CS5 (Ks=6 mm) CFX_CS5 (Ks= 43mm) 0.0 0.2 0.4 0.6 0.8 1.0 Y (m) 1.2 0.0 0.2 0.4 0.6 0.8 1.0 Y (m) 1.2 27 28 7
Overbank Flows Case H (H=0.20m, h=0.15m) (smooth MC and FP) Case B (H=0.187m, h=0.137m) (rough MC bed only) THE UNIVERSITY OF BIRMINGHAM Investigation of Inlet B.C. Mesh (Inlet) : [1] Mesh (Inlet) : [2] Case C (H=0.187m, h=0.149m) (rough both MC and FP beds) 29 30 Overbank (H=0.187m) Case B (h=0.137m, Q=0.125 m 3 /s) Ks=23mm for MC bed, Ks=0.75mm for other walls) So = 1 Inlet velocity = 0.478 m/s (average) Free slip B.C. for water surface Model: k- turbulence model [2.1] BW187Lm2_001.res: (Umc = Ufp =0.478 m/s) Different Inlet B.C. [1] BW187Lm_005.res (only one plane as inlet) U=0.478 m/s (average) [2] BW187Lm2.def (Inlet plane split into 3 separate planes) [Amc =76.5%, Afp (L+R) =23.5%] (%Qmc = 76.5 %, %Qfp = 23.5 % ) [2.2] BW187Lm2_002.res: Umc = 0.531 m/s (%Q = 85 %) Ufp = 0.305 m/s (%Q = 15%) Structure mesh (200 x 107 x 21) 31 32 8
Vector plots @ CS3 Key data comparison @CS3 [1] [1] BW187Lm_005.res: U (ave( ave) ) =0.4067 m/s (Umc = 0.3938 m/s, Ufp = 0.4308 m/s) Q = 0.125 m 3 /s (Qmc( =0.0802 m 3 /s, Qfp = 0.04645 m 3 /s) Tau = 0.7885 N/m 2 [Tau (mc)=0.8886 N/m 2, Tau (fp)=0.7401 N/m 2 ] => No impact on the results [2.2] [2.1] BW187Lm2_001.res: U (ave) = 0.4070 m/s (Umc =0.3944 m/s, Ufp =0.431 m/s) Q = 0.125 m 3 /s (64.3%, 36.7%) (Qmc =0.0804 m 3 /s, Qfp = 0.0463 m 3 /s) Tau = 0.7914 N/m 2 ( -mc =0.8950 N/m 2, -fp = 0.7409 N/m 2 ) [2.2] BW187Lm2_002.res: U (ave) = 0.4070 m/s (Umc =0.3942 m/s, Ufp =0.431 m/s) Q = 0.125 m 3 /s (64.3%, 36.7%) (Qmc =0.0804 m 3 /s, Qfp = 0.0463 m 3 /s) Tau = 0.7919 N/m 2 ( -mc =0.8959 N/m 2, -fp = 0.7412 N/m 2 ) 33 34 Overbank (1) (H=0.20m) Case H (Q=0.146 m 3 /s) Smooth (Ks=0.75mm for all walls) So= 1 Inlet velocity = 0.4928 m/s (average) Free slip B.C. for water surface Model: k- turbulence model Structure mesh (200 x 107 x 21) THE UNIVERSITY OF BIRMINGHAM Mesh (Inlet) 35 36 9
Velocity vectors on surface (Case H) Velocity Streamlines on surface (Case H) Z=0.145m Z=0.145m 37 38 Flow stream lines on surface (Case H) CS5 Vector plots @ CS3, CS4, CS5 CS4 3D stream lines CS3 39 40 10
Vector plots @ all CS and Stream lines (z=0.145m) (Case H) Plan of CS3, CS4 and CS5 flow y x Y CFX Model X u [ U sin( ) V cos( )] 41 42 1.00 Wall shear & Ud~y @ CS3 (Case H) 1.80 Wall shear & Ud~y @ CS4 (Case H) Bed shear (N/m 2 ) 0.50 CFX_CS3 Bed shear (N/m 2 ) 1.30 0.80 CFX_CS4 0.30 0.0 1.0 2.0 3.0 4.0 5.0 Y (m) -0.20 0.0 1.0 2.0 3.0 Y (m) 4.0 0.50 0.60 0.40 0.50 0.30 0.20 CFX_CS3 0.40 0.30 0.20 CFX_CS4 0.10 0.10 0.0 1.0 2.0 3.0 4.0 5.0 Y (m) 43-0.10 0.0 1.0 2.0 3.0 Y (m) 4.0 44 11
0.50 0.40 0.30 0.20 0.10 Bed shear (N/m 2 ) 0.50 Wall shear & Ud~y @ CS5 (Case H) 1.00 CFX_CS5 (x) 0.0 1.0 2.0 3.0 4.0 Y (m) 5.0 CFX_CS5 (x) Overbank (2) (H=0.187m) Case B (Q=0.125 m 3 /s) Ks= 12 mm (MC bed), Ks=0.75mm for all others) So = 1 Inlet velocity = 0.4392 m/s (average) Free slip B.C. for water surface Model: k- turbulence model Structure mesh (200 x 107 x 21) 0.0 1.0 2.0 3.0 4.0 Y (m) 5.0 45 46 Velocity vectors on surface (Case B) Flow stream lines on surface (Case B) Z=0.145m Z=0.145m 47 48 12
Flow stream lines on surface (Case B) Flow stream lines on surface (Case B) 3D stream lines Z = 0.13m 49 50 CS3 Velocity pattern @CS3/4/5 (Case B) CS5 Vector plots @ CS3/4/5 (Case B) CS4 CS4 CS5 CS3 51 52 13
Vector plots @ all CS and Stream lines (Top) (Case B) 0.5 0.4 Ud~y @ CS3 (Case B) CFX_CS3 0.3 0.2 0.1 0.0 0.0 1.0 2.0 3.0 4.0 Y (m) 5.0 Meander cross section (CS3) 0.6 0.5 0.4 0.3 0.2 0.1 0-3.5-3 -2.5-2 -1.5-1 -0.5 0 0.5 1 1.5 2-0.1 y (m) 53 Z Exp data Telemac SKM Delft3D CFX 54 Ud~y @ CS4 and CS5 (Case B) Cross Over Section (CS4) 0.6 0.5 0.4 0.3 0.2 0.1 0-1.5-1 -0.5 0 0.5 1 1.5 2 2.5 3-0.1 y (m) Z Exp data Telemac SKM Delft3D CFX Overbank (3) (H=0.187m) Case C (Q=0.084 m 3 /s) Ks= 12 mm (MC bed), Ks= 10mm (FP bed) Ks=0.75mm for all other walls So= 1 Cross Section (CS5) Inlet velocity = 0.3163 m/s (average) 0.500 0.400 Free slip B.C. for water surface 0.300 0.200 0.100 0-1.00 1.00 2.00 3.00 4.00 5.00 y (m) -0.100 Bed Z Exp data Telemac SKM Delft3D CFX 55 Model: k- turbulence model Structure mesh (200 x 107 x 21) 56 14
Velocity vectors on surface (Case C) Flow stream lines on surface (Case C) Case B Case B Case C Case C Z=0.145m 57 58 Velocity vectors on surface (Case C) 3D flow stream lines CS3-CS5 CS5 (Case C) Case B Case B Case C Z=0.145m Case C 59 60 15
Flow stream lines on surface Vector plots @ CS3 Case B Case B Case C Case C 61 62 Vector plots @ CS4 Vector plots @ CS5 Case B Case B Case C Case C 63 64 16
Key data comparison @CS3 [2] Case B: B187Lm_002.res: U (ave) =0.3593 m/s (Umc =0.3248 m/s, Ufp =0.4080 m/s) Q = 0.125 m 3 /s (54%, 46%) (Qmc =0.0675 m 3 /s, Qfp = 0.0594 m 3 /s) Tau = 0.3128 N/m 2 ( -mc =0.3068 N/m 2, -fp = 0.3160 N/m 2 ) [1] Case H (H=0.20m) H200Lm_007.res: U (ave) =0.4072 m/s (Umc = 0.3872 m/s, Ufp = 0.4368 m/s) Q = 0.146 m 3 /s (58.2%, 41.8%) (Qmc =0.0853 m 3 /s, Qfp = 0.0630 m 3 /s) Tau = 0.6167 N/m 2 [Tau(mc)=0.4418 N/m 2, Tau(fp)=0.7010 N/m 2 ] [3] Case C: C187Lm_005.res: U (ave) =0.2683 m/s (Umc =0.2970 m/s, Ufp =0.2146 m/s) Q = 0.084 m 3 /s (73%, 27%) (Qmc =0.0612 m 3 /s, Qfp = 0.0237 m 3 /s) Tau = 0.2987 N/m 2 ( -mc =0.4093 N/m 2, -fp = 0.2246 N/m 2 ) Conclusions It shows the feasibility of CFX used for open channel flows (rivers) The depth-averaged velocities are predicted reasonably well Secondary flows are reproduced fairly well in terms of the flow pattern Some discrepancy exists for 3D velocity field near the surface 65 66 Future works CFX modelling for field river channel geometry (more complex) Generate good mesh (structure mesh) for complex geometry? Using high-order turbulence models (such as SST, RSM etc.) 67 17