A short discussion on lateral wall effect on Lift/Drag Forces on a rigid body in a flow. And a question, too.
Based on my knowledge, forces on rigid bodies in a flow stream are given for an undefined flow field, from a Drag point of view, I mean. A practice problem I recently had was to evaluate the Lift/Drag forces on a body (a 60 m x 45 m rectangular body, in my case) close to two lateral walls.
At the moment, my only change was to do a numerical analysis. So, I performed the analysis by using Navier2d mathematical model, written in Matlab language (M-files and/and M-functions). I considered two cases:
- A 180 m wide channel with the obstacle 60 m large and 45 m long along the symmetry channel axes. The flow domain has been discredized with 23354 triangles and 11869 vertices, built by using Mesh2D toolbox;
- A 360 m wide channel with the obstacle 60 m large and 45 m long along the symmetry channel axes; The flow domain has been discredized with 11570 triangles and 5953 vertices, built by using Mesh2D toolbox.
The two geometries were forced with an uniform current of 1 m/s. The cinematic viscosity was set to 1.0e-6 m^2/s. No turbulence model for sub-grid analysis was used.
The following Figure shows an instant of motion (velocity magnitude) for both the cases. The vortex wake behind the body is well formed for either case.
I thought that the lateral walls influence on the forces determination could be negligible, given the same flow velocity. Viceversa, I obtained that the 360 m channel reduces the forces, both Lift (Fx) and Drag (Fy), on the body, as seen in Figure (quite noisy, but so far it is my best… anyway it is quite clear, isn’t it).
I guess the difference is due to the velocity gradient around the body, stronger for the 180 m channel. Does anyone have a physics-based proof ?
After sharing with my public my funny ideas about an electric-analogical model of the Venice lagoon, I can now proudly show you the first results:
using this tool I could make the first essays. It was kindly published by Israel A. Wagner and programmed by Leonid Kleyman & Evgeny Skarbovsky. I’d like to warmly thank them all for sharing their work on-line.
Hp: (for those who know a bit of the lagoon morphology)
- only simple (monoharmonic) sinusoidal potential oscillation at the three “mouths” of the circuit (same frequency for all, of course)
- three inlet channels reproduced via resistors
- blind (grounded) capacitors are for the reservoir effect offered by the mud flats, tidal shallows and salt marshes
- the three sub-domains communicate thanks to dissipative link (small channels), a little resistance
- the two “sensors” are placed near the wave generator (tidal boundary condition, in green) and near the main, central capacitor (the in-lagoon measurement, in yellow)
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This is not a post, this is a call: a call for help.
Long time ago I thought about modelling the Venice lagoon in a different way. You know, there are already many kinds of models, conceived for a better knowledge of this lagoon phenomenology:
- the physical ones, like the big one in Voltabarozzo, Padova (Italy) that reproduce the whole area (550 km²) with a planar geometric factor of 1:250
- the many numerical models, of all types (finite differences, finite elements, finite volumes et coetera…), that aim to reproduce the many different aspects of such a complex hydraulic, hydrodynamic, morphologic, biologic quest… or, in fewer words, of such a huge environmental problem.
The second ones bloomed and I can’t make a list of the models realised so far, even if I’ve seen some of them.
Waiting for an SPH model of the lagoon (see my SPH experiments), we still must deal with the imperfect reality of modelization.
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