Lecture Solution Shallow water FD FV 1D 2D · equations in 1D and 2D were solved mostly using...
Transcript of Lecture Solution Shallow water FD FV 1D 2D · equations in 1D and 2D were solved mostly using...
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Overview of Finite Volume Methods for Solution of the Shallow Water Equations in 1D and 2D
Prof. Arturo S. Leon, Ph.D., P.E., D.WREFlorida International University
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Evolving from Finite Difference (FD) to Finite Volume (FV)• Over the last several decades, the shallow water
equations in 1D and 2D were solved mostly using Finite Difference (FD) techniques.
• Since about a decade ago (~2005), there is more emphasis on using Finite-Volume (FV) methods for the solution of the shallow water equations in 1D and 2D
• A FV solution approach, similar to what was added for 2D modeling will be available for 1D modeling in HEC-RAS version 5.1
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Preissmann Scheme (Finite Difference) This method has been widely used (e.g.,
HEC-RAS)
The advantage of this method is that variable spatial grid may be used
Steep wave fronts may be properly simulated by varying the weighting coefficient
1D HEC-RAS (< V. 5.1)
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1 11 1( ) ( )2
k k k ki i i if f f ff
t t
xff
xff
xf k
ik
ik
ik
i
))(1()( 1
111
))(1(21)(
21
111
1k
ik
ik
ik
i fffff
Preissmann Scheme cont…
Where α is a weighting coefficient By selecting a suitable value for α, the scheme may
be made totally explicit (α=0) or implicit (α=1) Usually, the scheme is stable if 0.6< α ≤1.
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Finite Volume Methods(1D-2D)
Adapted from Lecture Notes on Shallow-Water equations by Andrew Sleigh, Toro (1999,2001) and Leon et al. (2006, 2010)
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Ability to handle extreme flows Transitions between subcritical / supercritical
flows are easily handled– Other techniques have problems with trans-critical
flows Steep wave fronts can be accurately
simulated
Finite Volume Shock-Capturing Methods
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Dam break
Source: An unstructured node-centered finite volume scheme for shallow water flows with wet/dry fronts over complex topography, Nikolos and Delis
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Dam break (animation)http://www.youtube.com/watch?v=-QXUViTi_b0
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Shallow-water equations in 1D
USUFU xt
QA
U
1
2
gIA
UF
fo SSgAgI
US2
0
Governing equations in conservative form
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I1 and I2 Trapezoidal channel
– Base width B, Side slope SL= Y/Z
Rectangular, SL = 0
Source term
322
1LShBhI
dxdSh
dxdBhI L
3212
2
BABhI22
22
1 dxdB
BAI 2
2
2 2
)3/4(2
2
RAnQQ
S f
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Rectangular Prismatic
huh
U
22
21 ghhu
huUF
fo SSgh
US0
dUUSdtUFdxUVV
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Finite volume formulation For homogeneous form
– i.e. without source terms
rectangular control volume in x-t space
0V
dtUFdxU
xi-1/2 xi+1/2
tn+1
i
Fi-1/2Fi+1/2
xi-1/2 xi+1/2
tn+1
i
Fi-1/2Fi+1/2
t
x
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Finite Volume Formulation (Cont.)
Defining as integral averages
Finite volume formulation becomes
1/2
1/2
1 ,i
i
xni nx
U U x t dxx
1/2
1/2
11
1 ,i
i
xni nx
U U x t dxx
1
1/2 1/21 ,n
n
t
i itF F U x t dt
t
1
1/2 1/21 ,n
n
t
i itF F U x t dt
t
2/12/11
iini
ni FF
xtUU
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So far no approximation was made
The solution now depends on how the integral averages are estimated
In particular, the inter-cell fluxes Fi+1/2 and F1-1/2
need to be estimated.
Finite Volume Formulation (Cont.)
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Godunov method for flux comput. Uses information from the wave structure Assume piecewise linear data states
Flux calculation is solution of local Riemann problem
n
n+1
i-1 i i-1
Fi+1/2Fi-1/2Cells
Data states
n+1
n
U(0)i+1/2U(0)i-1/2
n
n+1
i-1 i i-1
Fi+1/2Fi-1/2Cells
Data states
n+1
n
U(0)i+1/2U(0)i-1/2
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Riemann Problem The Riemann problem is an initial value
problem defined by
The solution of this problem can be used to estimate the flux at xi+1/2
0 xt UFU
2/11
2/1,i
ni
ini
n xxifUxxifU
txU
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Riemann Problem (Cont.)The Riemann problem is a generalisation of the dam break problem
Dam wall
Deep water at rest
Shallow water at rest
Dam wall
Deep water at rest
Shallow water at rest
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Dam Break Solution
Evolution of solution
x
x
x
Water levels at time t=t*
t*t
Velocity at time t=t*
v
h
Shock
Rarefaction
xx
xx
xx
Water levels at time t=t*
t*t
Velocity at time t=t*
v
h
Shock
Rarefaction
Wave structure
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Exact Solution
Toro (1992) demonstrated an exact solution
Considering all possible wave structures a single non-linear algebraic equation gives solution.
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Exact Solution Consider the local Riemann problem
Wave structure
0 xt UFU
00
0,xifUxifU
xUR
L
x
t
Right wave, (u+c)Left wave (u-c)Star region, h*, u*
(u)
vL
tShear wave
vR
hL, uL, vL hR, uR, vR
0xx
t
Right wave, (u+c)Left wave (u-c)Star region, h*, u*
(u)
vL
tShear wave
vR
hL, uL, vL hR, uR, vR
0
hvhuh
U
huv
ghhuhu
UF 22
21
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PossibleWave structures
x
tRight Shock
Left Rarefaction Shear wave
x
t
Left Shock Right RarefactionShear wave
x
tRight Rarefacation
Left RarefactionShear wave
x
t
Right ShockLeft ShockShear wave
xx
tRight Shock
Left Rarefaction Shear wave
xx
t
Left Shock Right RarefactionShear wave
xx
tRight Rarefacation
Left RarefactionShear wave
xx
t
Right ShockLeft ShockShear wave
Across left and right wave h, u change v is constant
Across shear wave v changes, h, u constant
LL cudtdx /**/ cudtdx
LL cudtdx /
**/ cudtdx
* */dx dt u c
/ R Rdx dt u c
* */dx dt u c / R Rdx dt u c
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Conditions across each wave
Left Rarefaction wave
Smooth change as move in x-direction Bounded by two (backward) characteristics
Left bounding characteristic
x
t
hL, uL h*, u*
Right bounding characteristic
LL cudtdx / **/ cudtdx
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Crossing the rarefaction
We cross on a forward characteristic
States are linked by:
or
constant2 cu
** 22 cucu LL
** 2 ccuu LL
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Solution inside the left rarefaction
The backward characteristic equation is For any line in the direction of the rarefaction
Crossing this the following applies:
Solving gives
On the t axis dx/dt = 0
cucu LL 22
cudtdx
dtdxcuc LL 2
31 1 2 2
3 L Ldxu u cdt
LL cuc 231
LL cuu 231
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Right rarefaction
Bounded by forward characteristics Cross it on a backward characteristic
In rarefaction
On the t axis dx/dt = 0
RR cuc 231
** 22 cucu RR RR ccuu ** 2
dtdxcuc RR 2
31
dtdxcuu RR 22
31
cudtdx
RR cuu 231
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Shock waves
Two constant data states are separated by a discontinuity or jump
Shock moving at speed Si
Using Conservative flux for left shock
LL
LL uh
hU
**
** uh
hU
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Conditions across shock
Rankine-Hugoniot condition
Entropy condition
λ1,2 are equivalent to characteristics. They tend towards being parallel at shock
LiL UUSUFUF **
*USU iiLi
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Shock analysis
Change frame of reference, add SL
Rankine-Hugoniot (mass and momentum ) gives
LLL Suu ˆLSuu **ˆ
LL
LL uh
hU
ˆˆ
**
** ˆ
ˆuh
hU
222*
2**
**
21ˆ
21ˆ
ˆˆ
LLL
LL
ghuhghuh
uhuh
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Shock analysis
Mass flux conserved
From momentum eqn.
Using
also
LLL uhuhM ˆˆ**
L
LL uu
hhgMˆˆ2
1*
22*
** /ˆ hMu L LLL hMu /ˆ
LLL hhhhgM **21
LL uuuu ** ˆˆ
L
LL uu
hhgM*
22*
21
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Left Shock Equation
Equating gives
Also
LLL hhfuu ,**
L
LLLL hh
hhghhhhf*
*** 2
1,
L L L LS u c q
2
**
21
L
LL h
hhhq
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Right Shock Equation
Similar analysis gives
Also
RRR hhfuu ,**
R
RRRR hh
hhghhhhf*
*** 2
1,
R R R RS u c q
2
**
21
R
RR h
hhhq
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Complete equation Equating the left and right equations for u*
Which is the iterative of the function of Toro (2001)
* *,L L Lu u f h h RRR hhfuu ,**
* *, , 0R L L L R Ru u f h h f h h
0,, *** uhhfhhfhf LRLL
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Determine which wave
Which wave is present is determined by the change in data states thus:
– h* > hL left wave is a shock– h* ≤ hL left wave is a rarefaction
– h* > hR right wave is a shock– h* ≤ hR right wave is a rarefaction
Steps to determine exact solution
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Solution Procedure Construct this equation
And solve iteratively for h (=h*). – The functions may change in each iteration
, ,L L R Rf h f h h f h h u
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f(h) The function f(h) is defined as
And u*
, ,L L R Rf h f h h f h h u
)(21
)(2
shockhhifhh
hhghh
nrarefactiohhifghghf
LL
LL
LL
L
)(21
)(2
shockhhifhh
hhghh
nrarefactiohhifghghf
RR
RR
RR
R
LR uuu
* * *1 1 , ,2 2L R R R L Lu u u f h h f h h
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Iterative solution
The function is well behaved and solution by Newton-Raphson is fast – (2 or 3 iterations)
One problem – if negative depth calculated! This is a dry-bed problem. Check with depth positivity condition:
RLLR ccuuu 2
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Dry–Bed solution
Dry bed on one side of the Riemann problem
Dry bed evolves Wave structure is
different.
x
t
x
t
x
t
Wet bed
Dry bed
Wet bed
Wet bed
Wet bed
Dry bed
Dry bed
xx
t
xx
t
xx
t
Wet bed
Dry bed
Wet bed
Wet bed
Wet bed
Dry bed
Dry bed
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Dry-Bed Solution (Cont.) Solutions are explicit
– Need to identify which applies – (simple to do) Dry bed to right
Dry bed to left
Dry bed evolves h* = 0 and u* = 0– Fails depth positivity test
LL cuc 231
* LL cuu 231
*
RR cuc 231
* RR cuu 231
*
gch /2**
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Shear wave (discontinuities that arise from eigenmodal analysis)
The solution for the shear wave is straight forward.– If vL > 0 v* = vL
– Else v* = vR
Can now calculate inter-cell flux from h*, u* and v*– For any initial conditions
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Approximate Riemann Solvers –1D Model No need to use exact solution
Exact solutions require iterations and are computationally expensive
For some problems, exact solutions may not exist
Many Riemann solvers are available (Roe’s and HLL are most popular)
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Toro Two-Rarefaction Solver
Assume two rarefactions Take the left and right equations
Solving gives
For critical rarefaction use solution earlier
** 2 ccuu LL RR ccuu ** 2
24*RLRL ccuuc
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Toro Two-Shock Solver Assuming the two waves are shocks
Use two rarefaction solver to give h0
RL
RLRRLL
qquuhqhqh
*
LLRRRL qhhqhhuuu *** 21
21
Lo
LoL hh
hhgq2
Ro
RoR hh
hhgq2
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Roe’s Solver Governing equations are approximated as:
Where is obtained by Roe averaging
xtxtxt UAUAUUUFU ~
A~
L L R R
L R
u h u hu
h h
RLhhh ~
22
21~
RL ccc
Approximate Riemann Solvers – 1D
; L L R Rc gh c gh
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Roe’s Solver (Cont.) Properties of matrix
– Eigen values
– Right Eigen vectors
– Wave strengths
Flux is given by
cu ~~~1 2 u c
cu
R ~~1~ )1(
cu
R ~~1~ )2(
u
chh ~
~
21~
1
u
chh ~
~
21~
2
; R L R Lh h h u u u
2
112/1
~~~21
21
j
jjj
ni
nii FFF R
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HLL Solver Harten, Lax, Van Leer Assume wave speed Construct volume Integrate
Or,
t
URUL
U*
FRFL F*
t
xL xR
t
URUL
U*
FRFL F*
t
xL xR
0* LRRLRRLL tUtUUxxUxUx
txS L
L
t
xS RR
LRRLLLRR SSFFUSUSU /*
LRLRRLRLLR SSUUSSFSFSF /*
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HLL Solver What wave speeds to use?
– One option:
For dry bed (right)
Simple, but robust
TRTRLLL cucuS ,min
TRTRRRR cucuS ,min
LLL cuS
RRR cuS 2
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Higher-Order in Space Construct Riemann problem using surrounding cells May create oscillations Piecewise reconstruction
Need to use limiters
i-1 i i+1 i+2
i-1 i i+1 i+2
L
R
LR
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Limiters Obtain a gradient for variable in cell i, Δi
Gradient obtained from Limiter functions Provide gradients at cell faces
Limiter Δi =G(a,b)
iiL xUU 21
11 21
iiR xUU
ii
iii xx
uua
2/1
12/1
1
12/1
ii
iii xx
uub
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Limiters (Cont.) A general limiter
β=1 give MINMOD, β=2 give SUPERBEE
Van Leer
0,max,,max,0min0,min,,min,0max
,aforbabaaforbaba
baG
ba
bababaG
,
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Higher order in time Needs to advance half time step MUSCL-Hancock
– Primitive variable– Limit variable– Evolve the cell face values 1/2t:
Update as normal solving the Riemann problem using evolved WL, WR
0 xt WWAW
Ri
Li
ni
RLi
RLi x
t WWWAWW
21,,
2/12/11
iini
ni FF
xtUU
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Wet / Dry Fronts
Wet / Dry fronts are difficult– Source of error– Source of instability
Found in: Filling of storm-water and combined sewer
systems Flooding - inundation
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Dry front speed
Dry front is fast Can cause problem with time-step / Courant
number
*
*
2 20
2
L
L L
L L L
S u cu c u c
cS u c
t
Wet bed
x
Dry bed
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Solutions for wet/dry fronts
Most popular way is to artificially wet bed Provide a small water depth with zero
velocity Can drastically affect front speed Need to be very careful about dividing by
zero
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Boundary Conditions
Set flux on boundary– Directly– Ghost cell
Wall u, v = 0. Ghost cell un+1=-un
Transmissive Ghost cell hn+1 = hn
un+1 = un
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Source Terms
“Lumped” in to one term and integrated Attempts at “upwinding source” Current time-step
– Could use the half step value E.g.
xB
BKuu
xzghS
2
02
2
nRK
3/2
2/1CRK
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Main Problem is Slope Term Flat still water over uneven bed starts to
move. Problem with discretisation of
xzgh
xzzgh lr
i
i-1 i i+1
z, hx
datum level
bed level
water surface
iz1izlz1iz
rh
rz
1ihihlh1ih
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Discretisation
Discretised momentum eqn
For flat, still water
Require
lrirl
rlini zzgh
xtghghhuhu
xthuhu
22
22221
022
221
ri
rli
lni zhhzhh
xtghu
rir
lil zh
hzh
h
22
22
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A solution Assume a “datum” depth, measure down.For horizontal water surface:
Momentum eqn:– Flat surface
212
i ii i
h hzg h g h gx x x
x
zzhzzhg liirii
22
21
2222'
2 liiriirli zzhzzhhhxtghu
liil zzhh riir zzhh
ihzx x
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Shallow-water in two-dimensions
In 2-d we have an extra term:
Friction
hvhuh
U
yy
xx
fo
fo
SSghSSghUS
0
22)3/1(
2
vuuhnS
xf
USUGUFU yxt
huv
ghhuhu
UF 22
21
22
21 ghhv
huvhu
UG
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Finite Volume in 2-DIf nodes and sides are labelled as :
Solution is
Where Fns1 is normal flux for side 1 etc.
A1A3
A2
A4
V
L4L3
L2
L1
A1A3
A2
A4
V
L4L3
L2
L1
11 1 2 2 3 3 4 4
n ni i s s s s
tU U Fn L Fn L Fn L Fn LV
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FV 2-D Rectangular Grid
Solution is
Fi-1/2Fi+1/2
Gj-1/2
Gj+1/2
i+1/2,j-1/2
i-1/2,j+1/2
i-1/2,j-1/2
i+1/2,j+1/2
i, jFi-1/2Fi+1/2
Gj-1/2
Gj+1/2
i+1/2,j-1/2
i-1/2,j+1/2
i-1/2,j-1/2
i+1/2,j+1/2
i, j
2/1,2/1,,2/1,2/11
jijijijini
ni GG
ytFF
xtUU
y
x
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Eigen values
Right Eigen vectors
cu ~~~1 3 u c
(1)
1R u c
v
(3)
1R u c
v
2 u
(2)
001
R
Roe’s Solver:
Roe’s Solver is simple and one of the most popular. This solver will be used in this class.
Approximate Riemann Solvers – 2D
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L L R R
L R
u h u hu
h h
L L R R
L R
v h v hv
h h
22
21~
RL ccc
; L L R Rc gh c gh
RLhhh ~
Roe’s Solver (Cont.)
Where:
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Wave strengths1 2
1 2 3 1
1 23
( ) ; ; 2
( )2
u u c u u v uc
u u c uc
Roe’s Solver (Cont.)
1 2
3
; or ;
R L R R L L R L
R R R L
u h h u u h u h q qu h v h v
Where:
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Update of solution:
3
1/2 11
1 12 2
jn ni i i j j
jF F F
R
Roe’s Solver (Cont.)
Numerical Flux is given by (Toro 2001)
2/1,2/1,,2/1,2/11
jijijijini
ni GG
ytFF
xtUU
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Show Demo in MATLAB for Solution of 2D Shallow Water Equations using Roe’s solver (Droplets and Dam break problem)Download Matlab files from Canvas.
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References E.F. Toro. Riemann Solvers and Numerical Methods for
Fluid Dynamics. Springer Verlag (2nd Ed.) 1999.
E.F. Toro. Shock-Capturing Methods for Free-Surface Flows. Wiley (2001)
Lecture notes on Shallow-Water equations by Andrew Sleigh
Leon, A. S., Ghidaoui, M. S., Schmidt, A. R. and Garcia, M. H. (2010) “A robust two-equation model for transient mixed flows.” Journal of Hydraulic Research, 48(1), 44-56.
Leon, A. S., Ghidaoui, M. S., Schmidt, A. R. and Garcia, M. H. (2006) “Godunov-type solutions for transient flows in sewers”. Journal of Hydraulic Engineering, 132(8), 800-813.