[Praneeth]

How do we choose between the Free Slip and No Slip Boundary Conditions?

In most of the Marine Engineering problems, we have a Free Slip Boundary Condition. But up until now, I have learnt about No Slip Boundary Condition. I am not able to understand the reasons for choosing the Free Slip Boundary Condition

[LuckyTran]

No-slip or not depend on the detailed micro-molecular dynamics. No-slip occurs when the adhesive force between the fluid-solid boundary is stronger than the cohesive fluid-fluid forces, which is generally the case. Fluids are "fluids" because these cohesive forces are weak. The Knudsen number characterizes this relationship and for practical situations the Knudsen number is very low. Fluid slip doesn't generally occur under ordinary conditions unless you are working with MEMs and such.

I don't know where in Marine Engineering you are working with where you have slip condition as the norm. Even when flow cavitates on an impeller or any other surface, that puts you in the realm of multi-phase flows with those complications before the no-slip condition breaks down. Even hydrophobic surfaces don't break the slip condition (they introduce surface tension, i.e. multi-phase effects).

[CFDfan]

Originally Posted by LuckyTran

No-slip or not depend on the detailed micro-molecular dynamics. No-slip occurs when the adhesive force between the fluid-solid boundary is stronger than the cohesive fluid-fluid forces, which is generally the case. Fluids are "fluids" because these cohesive forces are weak. The Knudsen number characterizes this relationship and for practical situations the Knudsen number is very low. Fluid slip doesn't generally occur under ordinary conditions unless you are working with MEMs and such.

I don't know where in Marine Engineering you are working with where you have slip condition as the norm. Even when flow cavitates on an impeller or any other surface, that puts you in the realm of multi-phase flows with those complications before the no-slip condition breaks down. Even hydrophobic surfaces don't break the slip condition (they introduce surface tension, i.e. multi-phase effects).

Free Slip condition is normally applied to the walls of the computational domain. Are there situations where No slip condition would be more appropriate for the computational domain wall(s)?

[LuckyTran]

Are we talking about walls or not?

No-slip is (almost) always the physically correct boundary condition for a solid wall unless what you are simulating is not a solid wall or you don't care about correctly representing things because you are tuning your model to match other things. Using slip conditions in fluids is the equivalent of assuming the universe is made of frictionless spherical point bodies.

Now if you are talking about computational domain boundaries that are not solid walls but some type of farfield condition, then there are a number of ways that can be modeled. The crudest way is to use a symmetry or slip wall. Usually you do this only because the software you are using sucks and doesn't give you any options. Anyone that has ever done this knows that it introduces blockage and artificial acceleration and some implementation of a far field boundary condition (sometimes called freestream boundary condition) is used. If we are talking about a region where there is not a solid wall, you would never use a no-slip condition there because that is a BC for solid walls. If it ain't a wall then don't use a wall BC–and also, don't call it a wall. There are tons of viable alternatives (like a pressure inlet/outlet).

[calim_cfd]

"Usually you do this only because the software you are using sucks and doesn't give you any options" lol

[ngrube]

Originally Posted by Praneeth

How do we choose between the Free Slip and No Slip Boundary Conditions?

In most of the Marine Engineering problems, we have a Free Slip Boundary Condition. But up until now, I have learnt about No Slip Boundary Condition. I am not able to understand the reasons for choosing the Free Slip Boundary Condition

Sometimes you do not have the computational power to get a full solution but you still want partial information. Maybe you are designing a supersonic jet and you want to study the shockwave arrangement or the wave drag or something similar. You could ignore viscosity and use slip BCs on the plane's surface and solve the Euler equations to get the shock wave arrangement. As long as the BL is not causing major flow separation or somehow drastically changing the flow, you should be able to get useful information this way.

I am certainly not an expert in marine engineering, but perhaps the bow wave and some other useful quantities could be similarly computed without paying attention to the boundary layers? Or maybe you could design a bulbous bow without using no-slip BCs. If you use a no-slip BC, then a BL will form and resolving that will require a finer grid. If you don't care about the BL, then you can avoid the need for a finer grid by not making a BL form in the first place.