User Forum of Software BASEMENT

Joshua

User

Registered: 2016-12-08

Posts: 3

stability issues when refining the mesh

Hello everyone,

right now I'm using Basement for my bachelor thesis in laterally coupled 1D/2D mode to simulate flood inundation events. I started to investigate the effect of mesh resolution and distance from one cross section to the next on model results. But when I try to refine the mesh or to shorten the distance of two cross sections below a specific threshold I get numerical problems. The program tells me that at some timestep the maximum velocity of 200 m/s is exceeded for a specific element. As a result the timestep is reduced below the minimum limit of 1e-12 and the simulation stops.

I create my mesh as Contrained Delaunay triangulation with the "Triangle" mesh generation software. The elements have a minimum angle of 20.87° degrees and a minimum area of 0.76 m².

My configurations look like this:

BASECHAIN_1D {
    HYDRAULICS {
        PARAMETER {
            minimum_water_depth   = 0.1
            riemann_solver        = roe
            stretch_boundary_elem = no
            SECTION_COMPUTATION {
                type         = table
                min_interval = 0.0001
                max_interval = 0.0002
        }
    }
    TIMESTEP {
        CFL               = 0.8
        total_run_time    = 194400
        initial_time_step = 0.1
        maximum_time_step = 30.0
        start_time        = 0.0
    }
}
BASEPLANE_2D {
    TIMESTEP {
        total_run_time    = 194400
        CFL               = 0.6
        initial_time_step = 1.0
        start_time        = 0.0
        maximum_time_step = 30.0
        minimum_time_step = 0.0001
    }
    HYDRAULICS {
        PARAMETER {
            minimum_water_depth     = 0.1
            riemann_solver          = exact
            simulation_scheme       = exp
            velocity_update_partial = volume_area
            riemann_tolerance       = 1.0e-2
            dynamic_depth_solver    = on
        }
    }
}
COUPLINGS {
    COUPLING_LATERAL {
        basechain                = Selke
        baseplane                = Selke_left
        weir_from_dyke1D         = no
        connections_filename     = connections_left.dat
        damping_factor           = 0.5
        weir_determination_nodes = yes
        side_weir_factor         = 0.95
        my_weir                  = 0.66
    }
}

The warning message shows this:

WARNING -> The velocity 'v= 200.674 m/s' in the element # 33725 exceeded the maximum
   velocity 'vmax=200 m/s' and is set to this maximum. No more messages are
   shown.

ERROR -> BASEchain: the new determined timestep 9.25061e-13 is smaller than the
   minimum limit 1e-12. This indicates that your simulation has a severe
   numerical problem. So we quit here.

Can someone give me a hint why the stability issues occur and what I can do to prevent the problems?

Thank you very much for your advice!

Greets, Joshua

Stephan Kammerer

Developer

Registered: 2015-03-31

Posts: 67

Re: stability issues when refining the mesh

In the "COUPLINGS" block you can define parameters and set the "max_time_level" to 1, which means that all sub domains run with the same minimum timestep. It's recommended to use this configuration for laterally coupled simulations.

COUPLINGS {
        PARAMETER {
            max_time_level = 1
        }