In fluid dynamics, the Oseen equations (or Oseen flow) describe the flow of a viscous and incompressible fluid at small Reynolds numbers, as formulated by Carl Wilhelm Oseen in 1910. Oseen flow is an improved description of these flows, as compared to Stokes flow, with the (partial) inclusion of convective acceleration.
Contents
- Equations
- Importance
- Bio engineering application
- Solution summary
- Calculations
- Error in Stokes solution
- Solution for a moving sphere in incompressible fluid
- Elaboration
- Modifications to Oseens approximation
- References
Oseen's work is based on the experiments of G.G. Stokes, who had studied the falling of a sphere through a viscous fluid. He developed a correction term, which included inertial factors, for the flow velocity used in Stokes' calculations, to solve the problem known as Stokes' paradox. His approximation leads to an improvement to Stokes' calculations.
Equations
The Oseen equations are, in case of an object moving with a steady flow velocity U through the fluid—which is at rest far from the object—and in a frame of reference attached to the object:
where
The boundary conditions for the Oseen flow around a rigid object are:
with r the distance from the object's center, and p∞ the undisturbed pressure far from the object.
Importance
The method and formulation for analysis of flow at a very low Reynolds number is important. The slow motion of small particles in a fluid is common in bio-engineering. Oseen's drag formulation can be used in connection with flow of fluids under various special conditions, such as: containing particles, sedimentation of particles, centrifugation or ultracentrifugation of suspensions, colloids, and blood through isolation of tumors and antigens. The fluid does not even have to be a liquid, and the particles do not need to be solid. It can be used in a number of applications, such as smog formation and atomization of liquids.
Bio-engineering application
Blood flow in small vessels, such as capillaries, is characterized by small Reynolds and Womersley numbers. A vessel of diameter of 10 µm with a flow of 1 millimetre/second, viscosity of 0.02 poise for blood, density of 1 g/cm3 and a heart rate of 2 Hz, will have a Reynolds number of 0.005 and a Womersley number of 0.0126. At these small Reynolds and Womersley numbers, the viscous effects of the fluid become predominant. Understanding the movement of these particles is essential for drug delivery and studying metastasis movements of cancers.
Solution summary
The fundamental solution due to a singular point force embedded in an Oseen flow is the Oseenlet. The closed-form fundamental solutions for the generalized unsteady Stokes and Oseen flows associated with arbitrary time-dependent translational and rotational motions have been derived for the Newtonian and micropolar fluids.
Using the Oseen equation, Horace Lamb was able to derive improved expressions for the viscous flow around a sphere in 1911, improving on Stokes law towards somewhat higher Reynolds numbers. Also, Lamb derived—for the first time—a solution for the viscous flow around a circular cylinder.
Calculations
Oseen considered the sphere to be stationary and the fluid to be flowing with a flow velocity (
Inserting these into the Navier-Stokes equations and neglecting the quadratic terms in the primed quantities leads to the derivation of Oseen’s approximation:
Since the motion is symmetric with respect to
the function
and by some integration the solution for
thus by letting
then by applying the three boundary conditions we obtain
the new improved drag coefficient now become:
and finally When Stokes' solution was solved on the basis of Oseen's approximation, it showed that the resultant drag force is given by
The force from Oseen's equation differs from that of Stokes by a factor of
Error in Stokes' solution
The Navier Stokes equations read:
but when the velocity field is:
In the far field
The inertia term is dominated by the term:
The error is then given by the ratio:
This becomes unbounded for
Solution for a moving sphere in incompressible fluid
Consider the case of a solid sphere moving in a stationary liquid with a constant velocity. The liquid is modeled as an incompressible fluid (i.e. with constant density), and being stationary means that its velocity tends towards zero as the distance from the sphere approaches infinity.
For a real body there will be a transient effect due to its acceleration as it begins its motion; however after enough time it will tend towards zero, so that the fluid velocity everywhere will approach the one obtained in the hypothetical case in which the body is already moving for infinite time.
Thus we assume a sphere of radius a moving at a constant velocity
Since these boundary conditions, as well as the equation of motions, are time invariant (i.e. they are unchanged by shifting the time
The equations of motion are the Navier-Stokes equations defined in the resting frame coordinates
where the derivative
Oseen's approximation sums up to neglecting the term non-linear in
for a fluid having density ρ and kinematic viscosity ν = μ/ρ (μ being the dynamic viscosity). p is the pressure.
Due to the continuity equation for incompressible fluid
where
Note that in some notations
Elaboration
where:
The vector laplacian of a vector of the type
It can thus be calculated that:
Therefore:
Thus the vorticity is:
where we have used the vanishing of the divergence of
The equation of motion's left hand side is the curl of the following:
We calculate the derivative separately for each term in
Note that:
And also:
We thus have:
Combining all the terms we have:
Taking the curl, we find an expression that is equal to
where
Also, the velocity is derived by taking the curl of
These p and u satisfy the equation of motion and thus constitute the solution to Oseen's approximation.
Modifications to Oseen's approximation
One may question, however, whether the correction term was chosen by chance, because in a frame of reference moving with the sphere, the fluid near the sphere is almost at rest, and in that region inertial force is negligible and Stokes' equation is well justified. Far away from the sphere, the flow velocity approaches u and Oseen's approximation is more accurate. But Oseen's equation was obtained applying the equation for the entire flow field. This question was answered by Proudman and Pearson in 1957, who solved the Navier-Stokes equations and gave an improved Stokes' solution in the neighborhood of the sphere and an improved Oseen’s solution at infinity, and matched the two solutions in a supposed common region of their validity. They obtained: