Chapter 5 Flashcards
What physical laws provide the foundation of control volume analysis?
- Conservation of Mass
- Newton’s Second Law of motion
- The first and second laws of thermodynamics.
Ref: Pg 145
How is the law of conservation of mass useful in solving fluid problems?
It allows us to create equations based on the fact that the amount of mass that enters the control volume must also exit it.
Ref: Pg 145
How does Newton’s second law help to solve fluid problems with respect to finite control volume analysis (FCVA)?
Newton’s second law of motion leads to the conclusion that forces can result from or cause changes in a flowing fluid’s velocity magnitude and/or direction.
Ref: Pg 145
The mechanical energy equation is derived from what two fundamental laws of nature?
The first and second laws of Thermodynamics.
Ref: Pg 145
What is the conservation of mass principle?
The time rate of change of the system mass = 0. In Ort her words, the mass of the system is constant with time.
Ref Pg 146
True or False
When a flow is steady, all field properties (i.e., properties at any specified point) including density remain constant with time and the rate of change of the mass of the contents of the control volume is zero.
True
Ref: Pg 147
True or False
The n^ direction is positive when it points out of a control volume and negative when it points into a control volume.
True
Ref: Pg 147
What is the net mass flow rate through a control surface?
The sum of the mass flow rates entering the control volume is equal to the sum of the mass flow rates leaving the control volume.
Ref: Pg 147
The continuity equation is a statement that mass _____.
Is conserved.
Ref: Pg 147
The mass flow rate can be calculated as … ?
m_dot = (rho)(Q) = (rho)(A)(V)
Ref: Pg 147
If velocity is assumed to be normally distributed over the section area A then_____.
The average value of the velocity is equal to the velocity V.
Ref: Pg 148
True or False
The dot product for V * n^ is positive for flow out of the control volume and negative for flow into the control volume.
True
Ref: Pg 153
True or False
For a fixed, non-deforming control volume; if the flow is steady then the mass flowrate in is equal to the mass flow rate out.
True
Ref: Pg 153
True or False
For a fixed, non-deforming control volume; if the flow is steady and incompressible then the volume flow rate entering the CV cannot be equal to the volume flow rate leaving.
False. If the given conditions are met (fixed, non-deforming control volume with steady, incompressible flow) then the volume flowrate in is equal to the volume flowrate out.
Ref: Pg 153
True or False
An unsteady but cyclical flow can be considered steady on a time-average basis.
True
Ref: Pg 153
When the velocity is not normally distributed over the opening in the control surface, the mass flowrate should be calculated with ___.
The average velocity.
Ref: Pg 154
With respect to a non-deformable moving control volume, describe the relative velocity vector W.
W is the fluid velocity seen by an observer moving with the control volume.
Ref: Pg 154
True or False
With respect to a moving control volume undergoing deformation, the velocity of the surface of a deforming control volume is not the same at all points on the surface.
True
Ref: Pg 157
In general, where fluid flows through the control surface, the control surface should be ______ to the flow.
Perpendicular
Ref: Pg 158
Newton’s second law of motion for a system can be described by …
The time rate of change of the linear momentum of the system = the sum of the external forces acting on the system.
Ref: Pg 159
Give examples of an “inertial” reference frame.
- A fixed coordinate system.
- A coordinate system that moves in a straight line with constant velocity and is thus without acceleration.
Ref: Pg 159
When a control volume is coincident with a system at an instant of time ________.
The forces acting on the system and the forces acting on the contents of the coincident control volume are instantly identical.
Ref: Pg 159
The time rate of change of a system’s linear momentum is expressed as ________.
The sum of the two control volume quantities: the time rate of change of the linear momentum of the contents of the control volume, and the net rate of linear momentum flow through the control surface.
Ref: Pg 159
True or False
As particles of mass move into or out of a control volume through the control surface, they carry linear momentum in or out. Thus, linear momentum flow should seem no more unusual than mass flow.
True
Ref: Pg 159
True or False
Linear momentum is non-directional.
False
Ref: Pg 164
True or False
The flow of positive or negative linear momentum into a control volume involves a negative dot product.
True
Ref: Pg 164
True or False
Momentum flow out of the control volume involves a negative V*n^ dot product.
False, it requires a positive dot product.
Ref: Pg 164
True or False
Momentum flux in non-uniform flow can not be evaluated using average velocity and momentum coefficient.
False, the average velocity can be used.
Ref: Pg 169
True or False
The linear momentum equation can not be written for a control volume that is moving.
False, the linear momentum equation applies to both moving and non-moving control volumes.
Ref: Pg 169
True or False
The linear momentum equation for a moving control volume involves the use of relative velocity.
True
Ref: Pg 172
True or False
The linear momentum equation can also be applied to problems involving torque.
True
Ref: Pg 174
What two quantities does the “moment-of-momentum” equation relate?
- Torques
- Angular Momentum
Ref: Pg 174
What fundamental physical law is the moment-of-momentum equation derived from?
Newton’s Second Law.
Ref: Pg 174
For a system, the rate of change of moment-of-momentum equals …?
The net torque.
Ref: Pg 175
What type of physical system is the moment-of-momentum equation useful for analyzing and give examples of such systems.
Machines that rotate around a single axis. Examples include rotary lawn sprinklers, ceiling fans, lawnmower blades, wind turbines, turbochargers, and gas turbines.
Extra Notes: As a class, these devices are often called “turbomachines.”
Ref: Pg 175
What happens when there is a change in the moment of a fluid’s momentum around an axis?
The result is torque and rotation around the same axis.
Ref: Pg 176
Give the definition of power with respect to angular velocity.
Power = (angular velocity)(torque)
Ref: Pg 178
When shaft torque and shaft rotation are in the same direction, power is _____.
Into the fluid.
Ref: Pg 180
When shaft torque and shaft rotation are in the opposite direction, power is _____.
Out of the fluid.
Ref: Pg 180
The first law of thermodynamics relates to which conservation law?
Energy
Ref: Pg 182
