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Dependence of Boundary Layer on Rayleigh Number

Abstract

This article explores the influence of the Rayleigh number on the development and characteristics of boundary layers in natural convection. The Rayleigh number plays a crucial role in determining the onset of convection, boundary layer thickness, and flow patterns in various fluid systems. This article discusses the fundamental concepts of the Rayleigh number, boundary layer formation, and the impact of varying Rayleigh numbers on the stability and transition of flow regimes.

1. Introduction

In fluid dynamics and heat transfer, boundary layers represent regions where flow properties, such as velocity and temperature, vary drastically near a surface. In natural convection, the Rayleigh number (Ra) is a critical dimensionless quantity that governs the onset and strength of buoyancy-driven convection in a fluid, thus influencing the behavior of thermal and hydrodynamic boundary layers. The Rayleigh number is defined as:

$Ra={\frac {g\cdot \beta \cdot \Delta T\cdot L^{3}}{\nu \cdot \alpha }}$

where:

• g is the gravitational acceleration,
• β is the thermal expansion coefficient,
• ΔT is the temperature difference between the surface and the fluid,
• L is the characteristic length,
• ν is the kinematic viscosity, and
• α is the thermal diffusivity.

A higher Rayleigh number indicates stronger buoyancy forces relative to viscous forces, which leads to more vigorous convection currents and impacts the boundary layer structure.

2. Hydrodynamic and Thermal Boundary Layers

2.1 Hydrodynamic Boundary Layer

In natural convection, the hydrodynamic boundary layer forms as the fluid rises due to buoyancy forces generated by temperature differences. As the Rayleigh number increases, the boundary layer thickness decreases, and the flow becomes more turbulent.

2.2 Thermal Boundary Layer

The thermal boundary layer represents the region where heat is transferred between the surface and the fluid. Like the hydrodynamic boundary layer, the thickness of the thermal boundary layer decreases with an increase in the Rayleigh number, leading to higher heat transfer rates.

3. Influence of Rayleigh Number on Boundary Layers

The Rayleigh number significantly influences both the hydrodynamic and thermal boundary layers, determining whether the flow remains laminar or transitions to turbulence.

3.1 For Low Rayleigh Numbers (Ra < 10^3)

At low Rayleigh numbers, the fluid motion is primarily driven by diffusion, and the flow remains laminar. The boundary layers are thick, and the rate of heat transfer is relatively low. The hydrodynamic and thermal boundary layers develop slowly, with minimal interaction between fluid layers.

3.2 For Intermediate Rayleigh Numbers (10^3 < Ra < 10^6)

As the Rayleigh number increases, buoyancy forces become more dominant, leading to the formation of convection cells. The hydrodynamic and thermal boundary layers become thinner, and the flow transitions from laminar to mildly turbulent. In this regime, heat transfer increases significantly due to enhanced convection.

3.3 For High Rayleigh Numbers (Ra > 10^6)

At very high Rayleigh numbers, the boundary layers are extremely thin, and the flow becomes fully turbulent. The fluid exhibits complex convection patterns with strong mixing, leading to a substantial increase in heat transfer rates. The transition to turbulence in both the hydrodynamic and thermal boundary layers occurs more rapidly.

4. Implications of Rayleigh Number Variation

The variation in the Rayleigh number has significant implications for heat transfer and fluid dynamics:

4.1 Heat Transfer Rate

As the Rayleigh number increases, heat transfer becomes more efficient due to thinner thermal boundary layers and stronger convection currents.

4.2 Flow Instability

Higher Rayleigh numbers lead to greater instability in the flow, causing the hydrodynamic boundary layer to transition from laminar to turbulent. This instability affects the overall flow pattern and boundary layer separation.

4.3 Boundary Layer Separation

At high Rayleigh numbers, the thinning of the hydrodynamic boundary layer can lead to earlier boundary layer separation, affecting flow control and drag.

5. Conclusion

The Rayleigh number is a fundamental parameter in natural convection, directly influencing the thickness, stability, and behavior of boundary layers. As the Rayleigh number increases, both the hydrodynamic and thermal boundary layers become thinner, leading to more efficient heat transfer and turbulent flow. Understanding the dependence of boundary layers on the Rayleigh number is essential for applications ranging from engineering heat exchangers to predicting atmospheric and oceanic circulations.

References

• Incropera, F. P., DeWitt, D. P., Bergman, T. L., & Lavine, A. S. (2007). Introduction to Heat Transfer (5th ed.). Wiley.
• Kays, W. M., & Crawford, M. E. (1993). Convective Heat and Mass Transfer (3rd ed.). McGraw-Hill.
• Bejan, A. (2013). Convection Heat Transfer (4th ed.). Wiley.

Article prepared by

• Ayush Beniwal (Roll no.-21135038), IIT BHU Varanasi
• Ayush Mishra (Roll no.-211355039), IIT BHU Varanasi
• Ayushman Singh (Roll no.-21135040), IIT BHU Varanasi
• Mitanshu Chakrawarty (Roll no.-21135082), IIT BHU Varanasi
• Harshwardhan Singh Chauhan (Roll no.-21134035), IIT BHU Varanasi