Wall effect on Lift and Drag Forces

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:

  1. 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;
  2. 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 ?

This entry was posted in hydro, Matlab, numerical. Bookmark the permalink.

3 Responses to Wall effect on Lift and Drag Forces

  1. Matteo says:

    While working on incipent motion of logs in rivers I studied a little bit of drag and lift problems recently. As far as I’m concerned, it’s no surprise the walls have a strong influence on both forces, as the velocity field changes. Your explanation of the difference due to the velocity gradient around the body seems reasonable, anyway what do you intend by “physics-based proof”? Do you mean lab experiments? I don’t know if it can be helpful, as this paper deals with numerical simulations, but maybe you can find some useful references:
    Jasberg A, Koponen A, Kataja M, et al.
    Hydrodynamical forces acting on particles in a two-dimensional flow near a solid wall
    COMPUTER PHYSICS COMMUNICATIONS 129 (1-3): 196-206 JUL 2000

  2. Alvise says:

    I’d like to distinguish the wall effects on different drags: induced, skin, wake. Any idea?

  3. Urban Andersson says:

    A very late answer – and perhaps not relevant.
    I think that the increased effect for the thin channel could be caused be an instabillity in the flow pattern where the flow choses either the upper or the lower compartment. This is a common phenomena/risk(?) in e.g. guide vanes/split exhaust tubes.
    In your two pictures – the object mainly affects the flow close to the object in the wider channel, while in the thinner there is a substantial difference in the flow in the upper and lower compartments. Hence the case with a smaller channel acts more like a split channel than a purely blocked object.

Leave a Reply