# Macroscopic hydrodynamics, is there a fluid “vacuum” force?

### Background

I was looking at the following simulated fish movement article and how the physics could be improved (ie, made more realistic) with out a huge impact on performance. It looks like they are simplifying the effect of water resistance based upon water displacement, and then simplifying that by calculating the displacement simply based upon where a facet/triangle moved based on the resultant orientation and the centroid of the triangle, so rotations and morphing aren't taken into account.

One can calculate near the exact volume that was displaced based upon the previous triangle's position and current position. But in thinking about this, I became confused as to what to do with yet another simplification the article makes.

### Question

The equation for force used in the article is:

min(0, -triangle_area * dot(normal, velocity)) * normal


which means that movement inward of a triangle will result in zero force on the object. Am I correct that this inward movement creates some force from the water rushing in to fill where there was previously fish, some sort of "vacuum force"?

Here is a picture illustrating what I'm talking about.

Is there some sort of macroscopic force on the facet caused by this inward movement? I cannot seem to find anything that talks about the effects of this at the macroscopic level, but maybe I should be looking at the force in terms of what the pressure of the water will do instead?