Boundary Layer Theory | Mechanical Engineering SSC JE (Technical) PDF Download

BOUNDARY LAYER THEORY

  •  The concept of boundary layer was first introduced by L. Prandt in 1904. 
  • Boundary layer is a region in the immediate vicinity of the boundary surface in which the velocity of flowing fluid increases gradually from zero at the boundary surface to the velocity of the main stream.

Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)

  • Boundary Conditions of Boundary Layer Region 
    (i) at y = 0, v = 0
    (ii) at y = d, v = V0 (d = boundary layer thickness V0 = free stream velocity)
    (iii) at y = d, Boundary Layer Theory | Mechanical Engineering SSC JE (Technical) = 0 [∵ Q v = V0 = constant]
    (iv) 
    Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)
     
  • Boundary layer thickness (d) It is defined as the distance from the boundary surface in which the velocity reaches the 99% of the velocity of the main stream.
    y =d
    for v = 0.99 V
  • Displacement Thickness (d*) It is the distance measured normal to the boundary, by which the free stream is displaced on account of the formation of boundary layer.
    Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)
    The quantity (V0 – v) is known as the velocity defect.
  •  Momentum Thickness (q)

Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)

The ratio of displacement thickness to momentum thickness is called the shape factor (H)

Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)

  • Its value should always greater than 1 
     
  • Energy Thickness
    Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)

Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)

  •  Von-Korman momentum integral equation
    Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)
  •  Blassius experimental results/ when  
    • Laminar Conditions : Velocity distribution is parabolic
      (i) Boundary Layer thickness

      Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)
      (ii) Local coefficient of drag C*D
      Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)

      (iii) Avg. Coefficient CD
      Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)
  •  Turbulent Conditions : Velocity distributions is logarithmic
    (i) 
    Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)
    (ii)
    Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)
    (iii)
    Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)
    Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)

where
d = Boundary layer thickness
t = Shear stress at solid surface
x = Distance from where solid surface starts 

  • Boundary Layer Separation It is caused by adverse pressure gradient

Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)
Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)

  • Location of Separation point : The separation point S is determined from the condition
    Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)
    For a given velocity it can be determined whether the B.L.(Boundary layer) has separated or on the verge of separation or will not separate from the following conditions :

1.
Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)
< 0 : Flow has separated.

2.
Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)
= 0 : Flow is on verge of separation.

3.
Boundary Layer Theory | Mechanical Engineering SSC JE (Technical)
0 : Flow is attached with the surface

 

  •  Methods of Preventing Separation
    (i) Rotating the boundary in the direction of flow.
    (ii) Suction of the slow moving fluid by a suction slot.
    (iii) Supplying additional energy from a blower.
    (iv) Providing a bypass in the slotted wing.
    (v) Providing guide blades in a bend.
    (vi) Injecting fluid into boundary layer.
    (vii)Streamlining of body shapes.
The document Boundary Layer Theory | Mechanical Engineering SSC JE (Technical) is a part of the Mechanical Engineering Course Mechanical Engineering SSC JE (Technical).
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FAQs on Boundary Layer Theory - Mechanical Engineering SSC JE (Technical)

1. What is boundary layer theory in mechanical engineering?
Ans. Boundary layer theory is a concept in mechanical engineering that describes the behavior of fluid flow near a solid surface. It focuses on the thin layer of fluid adjacent to the surface, known as the boundary layer, and how it affects the overall flow characteristics, such as velocity, pressure, and heat transfer.
2. How is the boundary layer formed in fluid flow?
Ans. The boundary layer is formed due to the no-slip condition, which states that the fluid velocity at the solid surface is zero. As the fluid flows over the surface, the velocity gradually increases from zero at the surface to the free stream velocity away from the surface. This velocity gradient creates the boundary layer, which can be laminar or turbulent depending on the flow conditions.
3. What are the applications of boundary layer theory in mechanical engineering?
Ans. The boundary layer theory has various applications in mechanical engineering, including: - Aerodynamics: Understanding the boundary layer is crucial in designing efficient aircraft wings, airfoils, and other aerodynamic components. It helps in reducing drag, improving lift, and enhancing overall performance. - Heat transfer: Boundary layer theory plays a significant role in analyzing heat transfer between a solid surface and surrounding fluid. It helps in designing heat exchangers, cooling systems, and thermal management in various engineering applications. - Fluid flow analysis: By studying the boundary layer, engineers can predict the flow characteristics, such as pressure distribution, flow separation, and drag forces. This information is useful in designing efficient pipelines, ducts, and other fluid-carrying systems.
4. What are the main challenges in boundary layer analysis?
Ans. Boundary layer analysis can be complex due to various factors, including: - Transition from laminar to turbulent flow: Determining when and where the transition from laminar to turbulent flow occurs is challenging. It significantly affects the flow behavior and requires accurate modeling and experimental techniques. - Non-uniform surfaces: Real-world surfaces are rarely smooth and uniform. Surface roughness, irregularities, and obstacles can disrupt the boundary layer and introduce additional complexities in the analysis. - Three-dimensional flow: Boundary layer theory is primarily developed for two-dimensional flow. Analyzing three-dimensional flows requires additional mathematical models and computational techniques, making it more challenging.
5. How can engineers optimize boundary layer effects in design?
Ans. Engineers can optimize boundary layer effects in design through various strategies, including: - Streamlining: By carefully shaping the surface geometry, engineers can reduce drag and minimize flow separation, leading to improved overall performance. - Surface treatments: Applying special coatings or textures to the surface can alter the boundary layer characteristics, reducing drag and improving heat transfer. - Active flow control: Using techniques such as suction, blowing, or synthetic jets, engineers can actively manipulate the boundary layer to enhance flow characteristics and achieve desired performance. - Computational fluid dynamics (CFD): Advanced numerical simulations using CFD allow engineers to analyze and optimize boundary layer effects in a virtual environment. This enables rapid design iterations and cost-effective optimization before physical prototyping.
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