Unit 5 Drag and Lift in Fluid Mechanics Important Question and Answer

For Btech AKTU students, this blog contains crucial questions and answers on Unit 5 – Drag and Lift in Fluid Mechanics. To assist students prepare for their examinations, the questions include subjects such as lift and drag forces, flow past cylinders, and boundary layer separation, as well as their related answers.

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Important Questions For Fluid Mechanics:
*Unit-01     *Unit-02    
*Unit-03    *Unit-04 
*Unit-05    *Short-Q/Ans
*Question-Paper with solution 21-22 

Q1. Explain the force exerted by a flowing fluid on a stationary body. 

Ans.

• 1. As shown in Fig.  consider a body held stationary in a real fluid, which is flowing at a uniform velocity U.
• 2. The fluid exerts a force on the stationary body. 
• 3. The entire force (F) that the fluid applies to the body is perpendicular to its surface. The whole force is therefore angled in the direction of motion.

A. Drag: 

• 1. The component of the total force (F_R) in the direction of flow is called drag.
• 2. This component of force is denoted by F_P.

B. Lift:

• 1. The component of the total force (F_R) in the direction perpendicular to the direction of flow is known as lift. 
• 2. This is denoted by F_L.
• 3. Only when the body’s axis is inclined toward the direction of fluid flow does lift force exist.   

Q2. Discuss about the drag on a cylinder and coefficient of drag for cylinder. 

Ans.

• 1. Consider a cylinder with dimensions of D and L that is submerged in a fluid with kinematic viscosity of v and free flow velocity of u.
• 2. In comparison to the viscous force, inertia forces are insignificant if the flow’s Reynold’s number is less than 0.2.
• 3. The inertia force increases along with Reynold’s number, which causes the flow pattern to become asymmetrical with respect to the axis perpendicular to the flow direction. 
• 4. From experiment following observations are to be made 

Q3. Write a short note on Aerofoil/ airfoil. Draw pressure distribution, theoretical as well as experimental on an airfoil in the fluid flow. 

Ans.

• 1. A streamlined body that can be either symmetrical or asymmetrical is known as an aerofoil or airfoil. 
• 2. Following are the necessary and important definitions related to aerofoil:

A. Chord Line: 

• 1. It is the line joining the leading and trailing edges of the aerofoil. 
• 2. The length of the line is known as chord of aerofoil.

B. Profile Centreline:

1. It is the line joining the midpoints of the profile.

C. Angle of Attack: 

1. Angle of attack is the angle formed by the chord line and fluid stream direction.  

D. Camber: 

1. It is the curvature of an airfoil. 

E. Stall: 

1. This is the condition when angle of attack (∝) greater than angle of attack at maximum lift.

2 When an aircraft stalls, the air separates from the wing or airfoil, causing eddies that significantly raise the drag coefficient. 

F. Aspect Ratio (AR.): 

1. The ratio of span of the wing to its mean chord is called the aspect ratio of a wing. 

Q4. Write down short note on Magnus effect. 

Ans. 1. Magnus effect describes how a spinning cylinder creates lift in a fluid stream. 

2. Examples:

a. This effect has been successfully employed in the propulsion of ships. 

b. The Magnus effect may a!so be used with advantage in the games like table tennis, golf, cricket etc.  

Q5. Give importance of model testing. What are the advantages and disadvantages of distorted models? 

Ans. A. Importance: The following are the importance of model analysis 

• 1. The model tests are very affordable and practical (because the design. construction and operation of a model may be changed several times if necessary, without increasing much expenditure, till most suitable design is obtained).
• 2. Models can be used to anticipate how hydraulic structures and machinery will perform in the future.
• 3. Model testing makes it possible to learn about the safety and reliability of such parts, which is extremely required when constructing a specific component of the structure when a clear-cut analytical and trustworthy approach is not available.
• 4. To find and fix problems with an existing structure that isn’t working properly, model testing might be utilized.

B. Advantages: 

• 1. Due to increased scale ratio accurate measurements are possible. 
• 2. Surface tension can be reduced. 
• 3. The size of the model can be suitably decreased, which simplifies its operation and significantly decreases the crests. 
• 4. To produce better results, a model’s Reynolds number might be raised. 

C. Disadvantages: 

• 1. The distribution of pressure and velocity is not accurately replicated.
• 2. The model wave might or might not be the same as the prototype wave.
• 3. Slopes of river bends, earth cuts and dikes cannot be truly reproduced. 
• 4. It is challenging to extrapolate or interpolate the outcomes of distorted models.

Q6. What do you mean by dimensional analysis ? What are its uses ? What are advantages and limitations of dimensional analysis ?

Ans. A. Dimensional Analysis: 

• 1. It is a mathematical test or method that uses the investigation of dimensions to address various engineering issues.
• 2. It is based on the principle of dimensional homogeneity.

B. Uses of Dimensional Analysis: 

• 1. Dimensional analysis can be used to determine whether an equation is dimensionally homogeneous. 
• 2. Dimensional analysis can be used to derive a flow phenomenon formula. 
• 3. in order to demonstrate the relative importance of each parameter, an equation that can be stated in terms of non-dimensional parameters must be obtained. 
• 4. to organize model tests in order to get experimental data that may be presented in a methodical way.

C. Advantages of Dimensional Analysis: 

• 1. It provides the variables functional relationships in dimensionless words.
• 2. It reduces the number of variables involved in a physical phenomenon. 
• 3. Design curves can be created by the use of dimensional analysis utilizing experimental data or by solving the issue directly. 
• 4. Dimensionless parameters can be utilized to make specific logical inferences about the problem through the careful choice of variables. 

D. Limitations: 

• 1. There are no absolute or ideal guidelines for selecting the variables. 
• 2. Dimensional analysis does not provide coefficient or constant values; it merely suggests that there may be some relationship between parameters. 

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