Test 3 Concept Questions Flashcards
What does steady flow mean in terms of energy conservation?
The rate of change of energy within a CV is equal to 0.
What do we need “alpha (a)” for and how is it calculated?
Alpha is the kinetic energy flux coefficient and is used to convert u^3 to v^3 or non-uniform velocity in to uniform velocity. It is calculated by the (intgral over the area of u^3)/(V^3*A)
What is the physical meaning of the terms of the energy equation?
P/pg = Rate at which work is done on the control volume z = rate at which potential energy is gained/lost by CV av^2/2g = rate at which KE is entering/leaving CV hL = Rate of energy loss in CV ht = Rate of energy that is provided to the turbine by the flow hp = Rate at which energy is provided to flow by the pump
How do you convert the quantity of a given head into power and vice versa?
To convert head to power multiply head by “pgQ”
To convert power to head divide power by “pgQ”
What do the energy and hydraulic grade lines represent?
HGL = P/pg + z EGL = P/pg + z + av^2/2g
What term in the energy equation represents a change in heat/internal energy?
hL = Head Loss
Why is dimensional analysis important?
By grouping the variables pertinent to a study into dimensionless groups, we can extract max information from a minimum number of experiments or tests. It also provides meaningful results and a better understanding of the flow behavior.
What are the primary dimensions?
M,L,T
What are the rules for selecting repeating variables?
Nr <= Nd Nr = Nd
Repeating Variables must not form a dimensionless group
Do not choose the variable being investigated as a repeating variable
What is similitude and how do we achieve it?
The theory that allows us to predict the behavior of the prototype based on a geometric, kinematic, and dynamically similar model.
Knowledge of Reynolds number, Froude’s Number, Weber’s Number, and Machs Number?
Re = pvL/u = Fi/Fv Fr = V/sqrt(gL) = Fi/Fg Weber = sigma/v^2Lp = Fi/Fsigma Mach = V/sqrt(E/p) = Fi/Fe
Expressions for inertial, viscous, gravity, elastic, and surface tension in forces?
Inertial = pV^2L^2 Viscous = uVL Gravity = pgL^3 Elastic = EL^2 Surface = sigma(L)
How do we achieve geometrical, dynamic, and kinematic similarities.
Geometric = Same shape
Dynamic = Rep = Rem
= Fp = Fm
If geometric and dynamic are satisfied so is kinematic.
What is the Boundary Layer?
A zone close to a solid surface where velocity is less than Uo.
How does the wall shear stress vary along the length of a plate in a typical BL?
Look at the chart in notes
What is the difference between laminar and turbulent flow?
Laminar Flow = Particles move in layers without mixing
Turbulent Flow = Fluid particles move haphazardly in all directions
When does the LBL turn into a TBL?
When Re > 500,000 (Leading Edge)
When Re > 3000 (Pipe)
Which type of flow is more conducive for mixing?
Turbulent
The difference between the laminar and turbulent velocity profiles
Look at chart in notes.
Which type of boundary layer grows faster?
Turbulent grows much faster.
What are the components of Drag Force?
Total Drag Force = Friction Drag + Form Drag
For Cylinders and spheres what causes a sudden reduction in the drag coefficient?
A sudden reduction in Cd is associated with LBC changing to TBC
LBC
Laminar Boundary Condition
TBC
Turbulent Boundary Condition
Under what condition is the drag coefficient for cylinders and spheres independent of Re?
When the total drag is made up of form dray only. When friction drag’s contribution to the total drag is negligible
For bodies with sharp edges, why is Cd mostly independent of Re
Because flow separation points are determined by the geometry of the body. The flow always separates at the same point independent of Re.
