Flexible Mesh vs Rectangular Grid

[Anonymous]

I have a general Question, I appreciate that if someone can answer to my Question.
what is the difference between flexible Mesh and Rectangular grids? when should we use Flexible Mesh rather than Rectangular grids?

[cameron]

Flexible meshes – triangular/rectangular elements that can vary in size and shape throughout the model boundaries. These models are also known as finite element or finite volume. ADH, SRH-2D, RiverFlow2D, RMA2, FESWMS, TUFLOW FV and the DHI products are examples of this type of model. The elevations for the model are usually based on the nodes of the triangles.

Rectangular grids – these models can only have one cell size for the entire grid, but some models can have smaller grids placed inside the larger grids. These models are usually reffered to as finite difference models. These models include TUFLOW Classic (XP-2D), FLO-2D, and DHI products. The elevations for these models is usually an average over each cell and are represented in the cell center. Most models have added features such as levees/ridge lines, and 1D channels to allow them to have larger cell sizes and still pick up all of the detail.

Rectangular grid models have been known to be more stable than the flexible mesh models in the past due to wetting and drying, but more recent flexible mesh models can be just as stable. Another thing to look at is whether a model is uses an implicit or explicit solver which may control how fast a model will run.

HEC-RAS 2D is different as each element is like a mini storage area and has a elevation/volume relationship and that each face acts like a cross-section.

When to choose one over the other – it depends on the type of project. If you are trying to determine pier scour, flows around abutments, or mostly riverine then a flexible mesh model is probably best. I would also use a flexible mesh if I have a very large area and need fine detail only in certain parts as this can allow you to have a lot less number of total elements (speeds up runtime).

If you are trying to model overland flow and generate floodplains grid models may be better as they usually offer more features they may help model hydraulic structures such as culverts.

[Chris G.]

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Subject: Re: Flexible Mesh vs Rectangular Grid
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It depends on the model and how it solves the equations. In RAS 2D, a rectan=
gular works fine, as does unstructured. =46rom what I understand, flexible h=
as no advantage in RAS.=20

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> On Sep 22, 2014, at 3:38 PM, “Mike [via HEC-RAS Help]” wrote:
>=20
> I have a general Question, I appreciate that if someone can answer to my Q=
uestion.=20
> what is the difference between flexible Mesh and Rectangular grids? when s=
hould we use Flexible Mesh rather than Rectangular grids?=20
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It depends on the model and how it solves the equations. In RAS 2D, a rectangular works fine, as does unstructured. From what I understand, flexible has no advantage in RAS. 

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On Sep 22, 2014, at 3:38 PM, “Mike [via HEC-RAS Help]” <[email protected]> wrote:

I have a general Question, I appreciate that if someone can answer to my Question.

what is the difference between flexible Mesh and Rectangular grids? when should we use Flexible Mesh rather than Rectangular grids?

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[Anonymous]

Thanks for your response.
Mike

[Anonymous]

Thanks for your response

[Author]

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