1. Slip to No-slip in Viscous Fluid Flows
P M V Subbarao
Professor
Mechanical Engineering Department
I I T Delhi
An Unified Analysis of Macro & Micro Flow Systems…

2. Incident of A Single Fluid Particle on a Smooth Wall
When the particle hits a smooth wall of a solid body, its velocity abruptly changes.
This sudden change due to perfectly smooth surface (ideal surface) was defined by Max Well as Specular Reflection in 1879.
The Slip is unbounded ur vr ui vi Φi Φr

3. Incident of A Single Fluid Particle on a Rough Wall
When the particle hits a rough wall of a solid body, its velocity also changes.
This sudden change due to a rough surface (real surface) was defined by Max Well as Diffusive Reflection in 1879.
The Slip is finite. ur vr ui vi Φi Φr

4. The condition of fluid flow at the Wall
This is fundamentally different from the case of an isolated particle since a flow field has to be considered now.
The difference is that a fluid element in contact with a wall also interacts with the neighbouring fluid.
The problem of velocity boundary condition demands the recognition of this difference.
During the whole of 19th century extensive work was required to resolve the issue.
The idea is that the normal component of velocity at the solid wall should be zero to satisfy the no penetration condition.
What fraction of fluid particles will partially/totally lose their tangential velocity in a fluid flow?

5. Thermodynamic equilibrium
Thermodynamic equilibrium implies that the macroscopic quantities need have sufficient time to adjust to their changing surroundings.
In motion, exact thermodynamic equilibrium is impossible as each fluid particle is continuously having volume, momentum or energy added or removed.
Fluid flows heat transfer can at the most reach quasi-equilibrium.
The second law of thermodynamics imposes a tendency to revert to equilibrium state.
This also defines whether or not the flow quantities are adjusting fast enough.

6. Knudsen Layer
In the region of a fluid flow very close to a solid surface, the occurrence of quasi thermodynamic equlibrium is also doubtful.
This is because there are insufficient molecular-molecular and molecular-surface collisions over this very small scale.
This fails to justify the occurrence of quasi thermodynamic-equilibrium.
Two defining characteristics of this near-surface region of a gas flow are the following:
First, there is a finite velocity of the gas at the surface ( velocity slip).
Second, there exists a non-Newtonian stress/strain-rate relationship that extends a few molecular dimensions into the gas.
This region is known as the Knudsen layer or kinetic boundary layer.

7. Knudsen Layer
Surface at a distance of one mean free path/lattice spacing. us λ uw
Wall

8. Molecular Flow Dimensions
Mean Free Path is identified as the smallest dimensions of gaseous Flow.
MFP is the distance travelled by gaseous molecules between collisions.
Lattice Spacing is identified as the smallest dimensions of liquid Flow.
Mean free path :
Lattice Dimension:
d : diameter of the molecule
V is the molar volume
NA : Avogadro’s number.
n : molar density of the fluid, number molecules/m3

9. Velocity Extrapolation Theory
Knudsen defined a non-dimensional distance as the ratio of mean free path of the gas to the characteristic dimension of the system.
This is called “Knudsen number”

10. Boundary Conditions
Maxwell was the first to propose the boundary model that has been widely used in various modified forms.
Maxwell’s model is the most convenient and correct formulation.
Maxwell’s model assumes that the boundary surface is impenetrable.
The boundary model is constructed on the assumption that some fraction (1-ar) of the incident fluid molecules are reflected form the surface specularly.
The remaining fraction ar are reflected diffusely with a Maxwell distribution.
ar gives the fraction of the tangential momentum of the incident molecules transmitted to the surface by all molecules.
This parameter is called the tangential momentum accommodation coefficient.

11. Slip Boundary conditions
Maxwell proposed the first order slip boundary condition for a dilute monoatomic gas given by:
Where :
Tangential momentum accommodation coefficient ( TMAC ) -- --
Velocity of gas adjacent to the wall --
Velocity of wall --
Mean free path --
Velocity gradient normal to the surface

12. Modified Boundary Conditions
Non-dimensional form ( I order slip boundary condition)
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Using the Taylor series expansion of u about the wall, Maxwell proposed second order terms slip boundary condition given by:
Second order slip boundary condition
Maxwell second order slip condition
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13. Regimes of Engineering Fluid Flows
Conventional engineering flows: Kn < 0.001
Micro Fluidic Devices : Kn < 0.1
Ultra Micro Fluidic Devices : Kn <1.0

14. Flow Regimes
Based on the Knudsen number magnitude, flow regimes can be classified as follows :   Continuum Regime : Kn <   Slip Flow Regime : < Kn < 0.1   Transition Regime : 0.1 < Kn < 10   Free Molecular Regime : Kn > 10
In continuum regime no-slip conditions are valid.
In slip flow regime first order slip boundary conditions are applicable.
In transition regime (according to the literature present) higher order slip boundary conditions may be valid.
Transition regime with high Knudsen number and free molecular regime need molecular dynamics.