Wet or dry mass in virtual mass of a towed object?

[floating]

I would like to calculate the virtual mass (actual mass plus added mass) of a towed object, including the lines used to tow it. I'm calculating the mass of each component then summing them up. Do I use the wet rather than dry mass? If so, does it matter that a section of line is buoyant, so its wet mass is negative? [I guess it still requires force to tow it because its added mass is positive, but the concept is confusing.]

[Ad Hoc]

It all depends upon the shape and also the axis that the shape is being pulled.

It is the "below" volume shape, and the "mass" is the buoyancy of this immersed volume.

Hoerner's "Fluid Dynmaic Drag" book is excellent and will give you basic added mass of shapes. A must book!

[Ad Hoc]

PS..should have added, as a very very VERY rough and crude rule of thumb, around 80% of mass, as added mass. BUT really depends upon the shape and direction of flow relative to the shape etc etc etc. It is a good starting position anyway.

[jehardiman]

The answer is you have to add both, the coefficient of added mass is dependent on your shape relative to the flow, and no, everything always has positive mass both in acceleration and deceleration (i.e. your tow body can ram you if contitions are correct).

The equation is Mefficitive = M + Ca *V*rho, where Ca is dependent on the direction of acceleration, and V is the envelope volume (especially important for "open" bodies), and rho is the fluid mass density.

Hoerner is good, but Blevins is better for Ca. Rule of thumb for long slender axisymetric bodies is Ca=0.1 along the axis and 1.0 accross it. For pure square or I-beam shapes Ca transverse =2. For other angles of attack a rough Ca = 0.1+.9*(sin^2(alpha)*Ca transverse). It is important to remember what Ca is; it is the intergral of pressure over the surface of the body caused by the acceleration of the steady state flow (which may already be accelerated relative to the free stream by the shape, but that is the pressure drag) over the body. It therefor works in both speeding up and slowing down. Many a tug has been rammed by it's tow.

[Ad Hoc]

Jehardiman

Not come across Blevins...what is the name of the book, i'll look it up.
I've alwasy used Hoerner, but lost my copy when i moved to Japan

[jehardiman]

http://www.amazon.com/Applied-Dynami.../dp/157524182X

[Ad Hoc]

J.Man

thanks...much cheaper than Hoerner's too!

[jehardiman]

Originally Posted by Ad Hoc

J.Man

thanks...much cheaper than Hoerner's too!

Blevins is a lot like Hoerner. Gives the theory, then gives the compiled test data (with references to the actual paper/data!) to show the difference. I recommend it to everyone who wants a balanced insight to fluid dynamics