Slip - Boundary Conditions

Slip is a boundary condition allows fluid for stress-free sliding along the wall, enforcing zero mass flux through the wall. This condition can be applied to the wall type boundaries.

This condition is applied at solid boundaries and operates under the premise that there is no diffusion (i.e., zero viscosity) of the velocity field normal to the boundary, which implies no resistance to the fluid’s parallel (tangential) motion. In simpler terms, it posits that fluid particles do not adhere to the walls but can slide along them, in contrast to the No-Slip condition where fluid particles stick to the wall, resulting in zero velocity at the wall.

Slip effectively is a Symmetry condition applied to a velocity vector field. It physically enforces zero mass flux through the wall by setting zero value to the normal velocity component. The stress-free sliding is achieved by enforcing zero-gradient condition on the tangential velocity components. It is worth noting that the zero-gradient condition enforces the value of derivatives in the normal direction to the boundary to be zero. In this case, it means that the derivatives, in the direction perpendicular to the wall, of tangential components are set to zero. For example, for the velocity field \(U=(U_x, U_y, U_z)\) with a wall perpendicular to the Z-axis we have:

Slip might be utilized in simulations where surface roughness or the interaction level between the fluid and the wall is minimal, or when the focus is more on observing trends and qualitative outcomes rather than on precise boundary layer predictions.

The Slip boundary condition manipulates the velocity field at boundary patches to enforce a zero normal gradient. This is accomplished using the surface normal gradient, which calculates the gradient perpendicular to the boundary face. This gradient is then used to adjust the field values in the boundary condition.

It is important to note that while Slip implies a zero gradient for the velocity normal to the boundary, it does not enforce a zero-velocity condition overall. The tangential components of the velocity remain non-zero, ensuring that the fluid can move parallel to the boundary without penetration through it.

In contrast, the No-Slip boundary condition sets the fluid velocity to zero at the wall, completely restricting any motion of the fluid at the boundary. Graphically, the difference between the two boundary conditions can be illustrated as shown in Figure 1.

Please note that No-Slip imposes zero velocity on the wall. Slip, in contrast, imposes non-zero velocity on the wall.

The Slip boundary condition is a specific type of boundary condition used to model situations where there is no viscous shear stress transmitted across the boundary, but normal flow into the boundary is still prohibited. This condition is particularly useful in simulating flow around objects where the boundary layer effects are negligible.

Very often a Symmetry boundary condition is used to define a boundary that lies far away from the obstacle that is the main focus of the simulation. Similarly, Slip can be used as a replacement for the Symmetry. If for the pressure the Zero Gradient is used, the result would be exactly the same as applying the Symmetry condition to both fields.

The definition of boundary conditions in SimFlow is both simple and intuitive. To define Slip condition on the wall, the proper option must be selected from the drop-down menu in Boundary Conditions tab - Figure 2

In this section, we propose boundary conditions that are alternative to Slip. While they may fulfill similar purposes, they might be better suited for a specific application and provide a better approximation of physical world conditions.