Hydraulic Transients - Exercise # 2

Hydraulic Transients – Exercise session # 2: analysis of engineering hydraulic software packages

This exercise session contributes to your final mark. You shall form groups of 2 persons

and write a report under WordTM. It shall be sent in PDF format to

carole.delenne@umontpellier.fr, vincent.guinot@umontpellier.fr.

Please use the following file name format: "Report1_Name1_Name2.pdf", where Name1

and Name2 are the names of the 2 persons who wrote the report.

1. The HEC-Ras free surface flow software

The HEC-RAS software is a freely downloadable package. Since it is available for free, it

is used in a large number of engineering companies. As many packages for free surface

flow modelling, it has problems with handling critical flow. Near critical conditions, the

model becomes unstable. The screen capture taken from G.W. Brunner's report illustrates

what is meant by stability (Appendix A, Figure 1).

The technique proposed by the developers to overcome stability problems is the so-called

Local Partial Inertia (LPI) technique. It is briefly described in G.W. Brunner's report

(Appendix B, Figure 2). The LPI technique refers to a "numerical filter", but what this

notion means remains unclear at this stage.

After searching the WWWeb, one finds the text in Figure 3, that seems to belong to the

reference manual of HEC-RAS. It seems that the continuity equation is left unchanged,

while the momentum equation is modified.

Remark. Please be aware that the notation in this text is not exactly the same as that used

in the lectures (h does not stand for the water depth).

2. The Mike 11 package

Appendix 2 describes the governing equations and the numerical solution scheme used by

Mike 11 (developed by DHI – Danish Hydraulic Institute). The equations are solved

using the Abbott-Ionescu numerical scheme. The cross-sectional area A and the discharge

Q are computed at two different sets of points. The grid is said to be staggered.

3. Assignment

Assuming that the momentum distribution coefficient is unity ( = 1 in equation (16-1)),

1) Derive the non-conservation form of the LPI equations and for the Mike 11 solution.

Derive the expression for the Jacobian matrix A in the two packages.

2) Derive the expressions for the wave propagation speeds in HEC-RAS and in Mike 11.

3) How do they compare to the exact wave propagation speeds of the Saint Venant

equations ?

4) Use ExcelTM to plot the wave propagation speeds in the HEC-RAS and Mike 11

software packages, as well as the exact wave propagation speeds of the Saint Venant

equations. It is advised that you plot /c as a function of the Froude number Fr = u/c.

5) Solve the backwater curve equation under ExcelTM. You shall work on a flow

configuration that involves a hydraulic jump. Implement the HEC-RAS and Mike 11

solutions. How do they compare?

Hydraulic Transients - Exercise # 2

Appendix A: The LPI technique – taken from the HEC-RAS user manual

Figure 1. Screenshot from GWB's report. Definition of "model stability".

Figure 2. Screenshot from GWB's report. The LPI technique.

Hydraulic Transients - Exercise # 2

Figure 3. Screenshot from Chapter 16 in the reference manual.

Hydraulic Transients - Exercise # 2

Appendix B – The Mike 11 package (taken from the reference manual)

Hydraulic Transients - Exercise # 2

Hydraulic Transients - Exercise # 2

Hydraulic Transients - Exercise # 2

Hydraulic Transients - Exercise # 2

Hydraulic Transients - Exercise # 2

Hydraulic Transients - Exercise # 2

Hydraulic Transients - Exercise # 2

Hydraulic Transients - Exercise # 2