Drag Force at Liquid-Gas Interface

In summary, the conversation discusses the calculation of drag force on objects as they pass from a liquid medium (water) into a gas medium (air). The question is raised about how the drag force changes as the object crosses the liquid-gas interface, and whether the medium into which the object is moving affects the drag force. The suggestion is made to simplify the problem by using a high-low estimate and assuming the object is either fully in or out of the water at the interface. The possibility of using a weighted average of the densities is also mentioned. The conversation concludes with a discussion about the importance of analyzing the time spent at the interface and the possibility of numerical simulations.
  • #1
mrjeffy321
Science Advisor
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I am interested in calculating the drag force acting on simple objects (cube and sphere) as they passes from a liquid (water) into a gas (air). I can calculate the drag force as the objects travel through a uniform medium, but I am wondering how the drag force on an object changes as it passes through the liquid-gas interface.
 
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  • #2
Your question doesn't make any sense. If a cube falls from atmosphere into a body of water, that impact force isn't what we call drag (It's an impulsive force). Then it will sink (if the density is higher than water) and experience a net drag as it falls in the water. Drag occurs before and after impact, not during.
 
  • #3
The situation I am asking about is the other way around, a body going from water into air, not falling from the air and splashing into the water.

For example: Let's say one has a basket ball which one holds under water. After letting the ball go, it will rise to the surface and eventually cross the water-air boundary. As the ball crosses this boundary, where part of the ball is sticking up out of the water and part of it is still submerged, what drag force is acting on it?
Perhaps the situation is not as simple as I would like it to be for my purposes, as I would imagine that that there is still a “splashing”, of sorts, as the ball pops out of the water.

What about if the medium which the object is moving into is not a gas but just another liquid? For example from water into a thick layer of oil?
 
  • #4
What does it matter, it will last so short a time you can just ignore it.

You need to stop thinking like a physicist and start thinking like an engineer. When its at that point, what do you think is the dominant resistance force? I'd say its the water. In fact, the drag from the water is going to be orders of magnitude larger than the drag from the air (Just take the ratio of the densities). So, just assume its in water (worst case) until its completely out of the water OR, use the drag coefficient with an average density, and weight the density based on the ratio of the exposed surface to air/water.

Stop making this too complicated. :wink:

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http://www.royalnavy.mod.uk/upload/img_400/tomahawk.jpg [Broken]

I would say the "splashing" you see is after the missile leaves the water and the water falls into itself to fill the void left behind. I.e, that shouldn't change your drag - I wouldn't suspect it to.
 
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  • #5
In the case of small / short objects, the time it spends at the liquid-gas interface is, indeed, small.

However, I am also interested in the case of “big” objects, such as a really long cylinder (or missile, as in the case of your picture) where the time it spends at the interface is considerably longer, ignoring any rocket thrusters of course.

I can, as you suggest, simplify the problem by making a high-low estimate of what the drag force will be using the assumption that the object is still fully in, or out of, the water as it passes the interface.
As a physicists who rarely complicates things by taking into account drag, when I saw a discontinuity in my medium I thought, “hey, this might be an issue those engineers have to deal with sometimes”, but it would seem that it is me who is adding unnecessary complications.
 
  • #6
mrjeffy321 said:
In the case of small / short objects, the time it spends at the liquid-gas interface is, indeed, small.

However, I am also interested in the case of “big” objects, such as a really long cylinder (or missile, as in the case of your picture) where the time it spends at the interface is considerably longer, ignoring any rocket thrusters of course.

I can, as you suggest, simplify the problem by making a high-low estimate of what the drag force will be using the assumption that the object is still fully in, or out of, the water as it passes the interface.
As a physicists who rarely complicates things by taking into account drag, when I saw a discontinuity in my medium I thought, “hey, this might be an issue those engineers have to deal with sometimes”, but it would seem that it is me who is adding unnecessary complications.

I would strongly try using weighted density as a more 'educated' guess, as in:

[tex]\sigma_{air}C_{D_{air}}\rho_{air} + \sigma_{water}C_{D_{water}}\rho_{water}[/tex]

Where the sigmas are weighting factors that add up to 1. Each value of sigma is the exposed percentage of the body in each fluid medium. The pair of sigmas will start as (0,1) go to (.5,.5) and end at (1,0). It will make a quarter circle arc in the first quadrant.
 
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  • #7
The first thing I would ask myself is "is the time that there is a crossing between media really an important analysis?"

What do you hope to learn between the two extreme states of fully in the water and fully out of the water? If it were something along the lines of a control issue for a seal launched missile, then I would say it's important. If it is for something along the lines of looking at a power requirement for propulsion, then I'd say it's not worth the time.

Either way, for someone that has the time, it would be a cool problem to work.

My first pass would be a weighted average like Cyrus mentioned. I am sure there are other ways to make this much more complicated. I would expect the real experts would do this numerically over very short time spans.
 
  • #8
The problem I first started considering was similar to the example I gave above, where a basketball is held underwater and then released. While performing this 'experiment' the other day, it seemed like the basketball was not popping out of the water as far I might have expected from the amount of force required to hold it down , and the depth of release. I attributed this to (what I later calculated to be) a low terminal velocity of the basketball in the water, meaning that when it popped up at the surface it was not going very fast and could thus not travel very high into the air.

But this got me thinking about other objects and more general situations, so I decided that I wanted to try to describe the position of the object as a function of time after release from a given depth. This is relatively simple when the object is fully in the water or fully in the air, even taking into account drag, but since I know virtually nothing fluid dynamics I was unsure if I was justified in taking a weighted average for the drag force as the objected crossed the liquid-gas interface.
 
  • #9
Well, not you're going to have to include a bouyancy force that changes as it emerges from the water.

You really are trying to make your life miserable for no reason, lol. This solution is going to require some numerical integration methods. I don't see why you want to do all this work for little to no gain in knowing the answer.
 

1. What is drag force at the liquid-gas interface and how does it occur?

Drag force at the liquid-gas interface is the force that resists the motion of an object moving through a liquid-gas interface, such as a bubble rising in water or a boat moving through the ocean. It occurs due to the difference in the velocities of the liquid and gas molecules near the interface, causing a shear stress that acts against the motion of the object.

2. How is drag force at the liquid-gas interface calculated?

The drag force at the liquid-gas interface can be calculated using the drag equation, which takes into account the velocity of the object, the density and viscosity of the liquid and gas, and the surface area of the object. Alternatively, it can also be determined experimentally by measuring the force required to move the object through the interface at a constant velocity.

3. What factors affect the magnitude of drag force at the liquid-gas interface?

The magnitude of drag force at the liquid-gas interface is affected by several factors, including the velocity of the object, the density and viscosity of the liquid and gas, the surface area of the object, and the shape and roughness of the object's surface. Additionally, the presence of surface tension and turbulence can also influence the drag force.

4. How does the surface tension of the liquid affect drag force at the liquid-gas interface?

The surface tension of the liquid can affect drag force at the liquid-gas interface by creating a cohesive force that holds the liquid together and resists the motion of the object. This can increase the magnitude of drag force and make it more difficult for the object to move through the interface.

5. What are some real-life applications of understanding drag force at the liquid-gas interface?

Understanding drag force at the liquid-gas interface is important in various fields such as marine engineering, aerodynamics, and chemical engineering. It can be used to design more efficient and streamlined objects, such as ships and airplanes, and to optimize processes involving liquid-gas interactions, such as distillation and evaporation. It also plays a role in the study of ocean currents and weather patterns.

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