Talk:Added mass

2602:30A:C0D7:70E0:5D77:80BE:30F5:FAC4 (talk) 04:31, 27 July 2013 (UTC)==Comments==

To clarify many of the below questions: Added mass shall NOT be thought of as mass. Any textbook stating this is misleading. Added mass shares the same unit as the mass term, hence the name added mass, but there ends any and all similarities. Whereas mass is just a single number (in this context), added mass is in general both shape and frequency dependent. To be clear:

F=m*a, Newtons famous Force = mass x acceleration

The objects mass is to the right, the "added mass" come from "stuff" inside the F(orce). That "stuff" is actually pressure forces between the body and the fluid. While tempting to think of it as the mass of the fluid following the body, it is not.

Added mass is not fudge engineering. The reason for all the coefficients is that through empirical testing, one has found good rules of thumb values for the added mass. We can also calculate analytic solutions for simple shapes, such as cylinders, spheres etc., provided we assume something regarding the frequency of the forces applied to the shape. For a general shape the, i.e. a ship, the added mass is commonly calculated using a panel model of the hull. 12pi (talk) 13:51, 13 March 2014 (UTC)

How does one determine how much of the surrounding mass to add? It sounds like added-mass is an egineering fudge factor rather than good science.134.117.137.20 02:50, 3 December 2005 (UTC)

As far as I can tell, Added mass = Volume * Density * Added Mass coefficient. The Added mass coefficient, like the Drag coeficient in teh drag equation, needs to be determined experimentally for an object. --Numsgil 00:39, 10 December 2005 (UTC)

One could interpret drag as an added mass effect, so how does one _experimentally_ distinguish between an added mass effect and a force like drag? 134.117.137.251 03:17, 15 December 2005 (UTC)

Drag is traditionally either quadratically or linearly related to relative velocity with the medium (depends on Reynolds number obviously). Added mass is independant of relative velocity. Simple as that. --Numsgil 23:21, 16 December 2005 (UTC)

Ok. Is buoyancy a special case of added mass? 134.117.137.211 04:38, 18 December 2005 (UTC)

I would say they're related, in that they're dependant of fluid displaced, but no, they're not quite the same thing. Added mass exists independant of any gravity forces. Bouyancy is entirely dependant on the weight (that is, mass * gravity) of the displaced fluid.

Added mass was first used to account for the effort required by an accelerating submerged object in non-accelerating water to move water. In this context, buoyancy is not added mass per se. 209.167.89.139 15:06, 27 April 2007 (UTC)

I think that the article is quite misleading at this point, and doesn't explain the concept of added mass properly. Firstly, it states that added mass forces act like drag forces, but this is wrong: they are proportional to acceleration, not velocity, and do not dissipate energy. Secondly, the article reinforces the misconception that the added mass is a physical amount of mass that moves with the body: this is wrong, as the whole fluid oscillates as a body moves through it, to some degree. For example, there isn't a volume of water that sticks to a ship and moves with it, giving an increased mass. It is much better to interpret added mass as a pressure force which acts in proportion to acceleration. Thirdly, the examples are imprecise and do not state which degree of freedom the added masses have been solved for, and the final example seems to suggest that the added mass should be calculated from the buoyancy force. Andy Ross 12:51, 1 December 2006 (UTC)

The article as stands is mostly lifted right from the MIT lab manual that it links to, but condensed of course. 1. The article only references drag forces as a method of example. As this talk page illustrates, they are not the same thing. The phrase should be read "added mass forces act like drag forces". Meaning a similie. Emphasis on like. To the lay reader, drag is the closest familiar concept to what added mass is and does. Feel free to replace with a more familiar property if you can think of one. 2. The particular phrase is again lifted from the MIT lab notes. In reality, as I understand it, added mass is the inertia of the fluid that must be moved as an object moves through the fluid. Depending on the reynolds number and a number of other factors, there might be some fluid that does actually follow the object, or there might not be. Feel free to change the wording if you feel it is misleading. 3. They are lifted from the MIT lab notes. Presumably they are along one degree of freedom, since the lab was dealing with dashpot systems. I do not see your point about the final equation indicating anything about the bouyancy forces. There's no mention of gravity anywhere in it.

Unfortunately, there is a shortage of sources on Added Mass on the web. If you feel you can find other useful sources, by all means add to the article. It's limited at the moment because I don't know much about it, and I'm the one that wrote it. --Numsgil 06:59, 2 December 2006 (UTC)

The Naval Civil Engineering Lab Review, which is referenced by the article, is much better written than the article. There are several issues still require treatment, in my opinion. After the added mass matrix is formed, how does one input it in a commercial finite element program? As lumped masses? Or as density? What if the added mass varies from mode to mode? Last but not least, how much added mass is to participate in the forcing term? According to ASME III, it is zero. 209.167.89.139 13:48, 25 April 2007 (UTC)

The question here is how in depth you want to get. Remember that this is meant as a short encyclopedic article and not a chapter on added mass in a physics textbook. If there are factual errors or oversights, please correct them. But let's be careful not to let the article be incomprehensible to everyone but the people who don't need the article. --Numsgil 03:25, 26 April 2007 (UTC)

Notwithstanding, the fact remains that the coverage is less than adequate. For instance, the article does not mention that there are two basic views of added mass. One is from a point of view of force, which results in F=(M+M_added)a. This relationship appeals to intuition and has been used to estimste M_added by experiments. However, it does not allow formulation of the added mass. The other view point considers the kinetic energy imparted to water by the accelerating submerged object. For a potential flow, the velocity field can be solved for some simple geometries and calculated numerically in some more complicated situations. The results are then repackaged to obtain a term that plays the role of mass, hence added mass. Also not mentioned is the distinction between accelerating and non-accelerating water in the far field. The results arrived therefrom are quite different. 209.167.89.139 15:06, 27 April 2007 (UTC)

That's good, you should add that to the article. Most of the article is my own writing, and I've come across the idea of added mass second hand. My physics background involved some entry level calc based physics courses at University and not much else, so please elaborate the article where you can. --Numsgil 03:01, 28 April 2007 (UTC)

I deleted the following statement "Since added mass is a virtual mass and not a real mass, it is not taken into account for structural designs"

First off, this is the first time I've ever commented/edited Wikipedia so I apologize if the does not somehow follow a proper protocol. Moving on...

While skimming through this page and starting to navigate away but the second to last sentence in the "Naval architecture" section caught my eye. The statement "added mass is a virtual mass and not a real mass, it is not taken into account for structural designs" is entirely incorrect. Added mass is a "real" mass added to some other mass to make more (or less) total mass. The statement is a fallacy as it seems to suggest that somehow "not real mass" could present itself in our universe in the first place. I believe the writer likely has misinterpreted the reference "virtual mass".

As I noted above, the statement about added mass not being considered in design is incorrect. In fact, in marine hydrodynamics added mass is very critical and in some situations a structure's response is dominated by the inertial influence of hydrodynamic added mass (due to the whole F=ma relationship). In the design of ships and floating structures added mass effects response differently depending on it being a product of steady flow, oscillating flow, or both (i.e., current and waves). In this case if the virtual mass was not considered in equilibrium conditions the hydrodynamic equation of motion would correctly describe a vessel and likely result in a unsafe design.

It is worth noting here that for AEROdynamic situations the fluid flow is considered inviscid and inertial terms vanish from the equations that describing the system. Considering the above I am going to delete the statement indicted in the title of this section . I invite someone knowledgeable to flesh out this page.

As a last comment, considering the statement I read above noting that there is little material on the web about added mass. I suggest reading "Recommended Practice DNV-RP-C205 - ENVIRONMENTAL CONDITIONS AND ENVIRONMENTAL LOADS" published by DNV for use by engineers in offshore engineering community. This guidance address added mass in some detail as well as my other marine hydrodynamic technical topics. It also happens to be free to download (http://exchange.dnv.com/publishing/codes/download.asp?url=2010-10/rp-c205.pdf) 2602:30A:C0D7:70E0:5D77:80BE:30F5:FAC4 (talk) 04:31, 27 July 2013 (UTC)

Renormalization

Re: “The concept of added mass is arguably the first example of renormalization in physics.”

According to the article on renormalization, it is “any of a collection of techniques used to treat infinities arising in calculated quantities.”

Can someone provide more detail on the infinities that arise in the calculation of added mass? Psalm 119:105 (talk) 00:05, 2 May 2012 (UTC)

I added a citation to Biesheuvel & Spoelstra (1989). -- Crowsnest (talk) 14:09, 3 May 2012 (UTC)