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Fluid Mechanics Qualifying, Exams of Physics

Sample Exam 2 Questions with Answers.

Typology: Exams

2021/2022

Uploaded on 02/24/2022

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Fluid Mechanics Qualifying
Examination
Sample Exam 2
Allotted Time: 3 Hours
The exam is closed book and closed notes. Students are allowed one (double-sided) formula sheet.
There are five questions on this exam. Answer any four (each for 25 points).
If you answer all five, the best four will be considered.
State all your assumptions, and explain your reasoning clearly.
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Fluid Mechanics Qualifying

Examination

Sample Exam 2

Allotted Time: 3 Hours

The exam is closed book and closed notes. Students are allowed one (double-sided) formula sheet.

There are five questions on this exam. Answer any four (each for 25 points). If you answer all five, the best four will be considered.

State all your assumptions, and explain your reasoning clearly.

a) Show that for an irrotational flow that is also incompressible, the velocity potential  also satisfies the Laplace equation. b) Furthermore, in two-dimensional, incompressible flows, a stream-function (x,y) exists such that 𝜕  𝜕𝑥 = −𝑣^

𝜕  𝜕𝑦 = 𝑢 so that  and  satisfy the Cauchy-Riemann equations:

where w(z) is a complex analytic function of z = x+iy. Interpret any four of the following flows, write down expressions for the velocity components, and draw streamlines where possible: I. w(z) =z II. w(z) = z^2 III. w(z) = m log(z) IV. w(z) = i log(z) V. w(z) = U a (z/a + a/z), with real positive constants U and a.

Hint: Feel free to use polar coordinates z=r exp(i) for some of the above cases to simplify analyses.

  1. Consider steady adiabatic 1𝐷 compressible flow of air through a variable area duct. At a certain section of the duct, the flow area is 0.2 m^2 , the pressure is 80 kPa, the temperature is 5^0 C and the velocity is 200 m/s. If, at this section, the duct area is changing at a rate of 0.3 m^2 /m (i.e., 𝑑𝐴/𝑑𝑥 = 0.3 m^2 /m), find 𝑑𝑝/𝑑𝑥, 𝑑𝑉/𝑑𝑥, and 𝑑𝜌/𝑑𝑥. Assume a frictionless flow. Assume, for air, 𝑅 = 0. 287 kJ/kg∙K and 𝐶𝑝 = 1.004 kJ/kg∙K.
  1. Air is induced in a smooth insulated tube of 7.16 mm diameter by a vacuum pump. Air is drawn from a room, where the pressure and temperature are 101 kPa (abs) and 23^0 C respectively, through a smoothly contoured converging nozzle. At section 1, where the nozzle joins the constant-area tube, the static pressure is 98.5 kPa (abs). At section 2, located some distance downstream in the constant-area tube, the temperature is 14^0 C. Assume 𝑘 = 1.4. Determine the following quantities at section 2: a. the mass flow rate

b. the local isentropic stagnation pressure c. the friction force on the duct wall between sections 1 and 2

What additional information and assumptions would you need if you are asked to determine the distance between the sections 1 and 2? Where would you find these additional quantities?

Show the steps you would have followed to find the distance between the sections 1 and 2. Use either a symbol or a logical value for any missing quantity (for this question only).