19.9
Écoulement laminaire stable dans des tubes circulaires
L'écoulement de Hagen-Poiseuille décrit l'écoulement stable et incompressible d'un fluide visqueux à travers un tube cylindrique de rayon constant R.…
Considérons un écoulement stable et incompressible à travers un tube cylindrique droit de rayon constant R, connu sous le nom d’écoulement de Hagen-Poiseuille.
La géométrie cylindrique permet d’utiliser les coordonnées r, θ et z pour les mesures radiales, angulaires et axiales.
Le fluide s'écoulant parallèlement à l'axe du tube a une composante de vitesse purement axiale et ne varie qu'avec la distance radiale.
Ces conditions simplifient les équations de Navier-Stokes, révélant un profil de vitesse parabolique montrant que la vitesse est la plus élevée au centre et diminue vers les parois, atteignant zéro à la frontière.
Cela démontre l’écoulement laminaire, où les particules de fluide se déplacent en douceur en couches organisées.
L'intégration de ce profil sur la section transversale du tube donne la loi de Poiseuille, qui exprime le débit Q comme dépendant de la quatrième puissance du rayon du tube, de la viscosité du fluide et du gradient de pression.
La vitesse moyenne V, calculée à partir de Q, est la moitié de la vitesse maximale observée au centre du tube.
Cette distribution de vitesse confirme que l’écoulement est le plus élevé au centre et diminue paraboliquement jusqu’à zéro au niveau de la paroi.
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Q1: What is the parabolic velocity profile in Hagen-Poiseuille flow?
In Hagen-Poiseuille flow, the velocity profile is parabolic, with maximum velocity at the tube's center and decreasing to zero at the wall due to viscous drag. This parabolic distribution arises from solving the simplified Navier-Stokes equations under steady, incompressible flow conditions. The no-slip condition at the boundary ensures zero velocity at the tube wall, creating the characteristic smooth, layered velocity distribution.
Q2: How does Poiseuille's law relate tube radius to flow rate?
Poiseuille's law expresses volumetric flow rate Q as dependent on the fourth power of the tube's radius R, the pressure gradient, and fluid viscosity. This fourth-power relationship means even small increases in radius dramatically boost flow rate. The law demonstrates why narrow channels like capillaries have significantly restricted flow compared to larger tubes under identical pressure conditions.
Q3: What role do cylindrical coordinates play in analyzing Hagen-Poiseuille flow?
Cylindrical coordinates (r, θ, z) align naturally with the tube's geometry, representing radial, angular, and axial dimensions. This coordinate system simplifies the Navier-Stokes equations because the velocity varies only with radial distance r and points purely along the z-axis. The symmetry of cylindrical geometry eliminates angular velocity components, reducing complexity and revealing the parabolic velocity profile.
Q4: Why is the mean velocity exactly half the maximum velocity in laminar tube flow?
The mean velocity V equals half the maximum velocity at the tube center because of the parabolic velocity distribution. Integrating the parabolic profile across the tube's cross-section yields this precise relationship. This predictable ratio confirms the layered, organized nature of laminar flow and allows engineers to calculate average transport rates from maximum velocity measurements.
Q5: What is the no-slip condition and how does it affect laminar flow?
The no-slip condition requires fluid velocity to be zero at the tube wall due to friction between fluid and the solid boundary. This boundary constraint forces the velocity to decrease parabolically from the center to the wall. The no-slip condition is fundamental to laminar flow, ensuring fluid particles move in parallel layers without lateral mixing or turbulence.
Q6: How do the Navier-Stokes equations simplify for Hagen-Poiseuille flow?
For Hagen-Poiseuille flow, the Navier-Stokes equations simplify because velocity has only an axial component and varies solely with radial distance. Angular and radial velocity components vanish, eliminating multiple terms. This reduction transforms the full three-dimensional equations into a simpler form, yielding the parabolic velocity profile and enabling direct calculation of flow rate and pressure relationships.
Q7: Why is Hagen-Poiseuille flow considered a foundational example of laminar flow?
Hagen-Poiseuille flow demonstrates laminar flow principles in their simplest form: steady, incompressible flow through a constant-radius tube with organized, parallel fluid layers. The analytical solution provides exact velocity and flow rate relationships, making it ideal for teaching fluid mechanics fundamentals. Its applicability to narrow channels like capillaries illustrates how laminar flow governs transport in biological and industrial systems.