Low Speed True/False Flashcards
In considering the incompressible flow of fluid through a tube, the average velocity is always less than or equal to the maximum velocity.
True
A fluid is a substance that deforms continuously under the influence of a normal stress.
False, A fluid is a substance that deforms continuously under the influence of SHEAR stress.
A “stagnation streamline” and a “separation streamline” mean the same thing.
False
A streamtube cannot be defined in a viscous fluid.
False, A streamtube is a control volume bounded by streamlines
Laminar flow is always steady.
False
The maximum lift coefficient for a simple airfoil is about 10.
False
If a flow velocity is given as V = axy i + bz j, the flow is two dimensional and steady.
False, there are there dimensions (x,y,z) in the equation
A streamline is the same as a streakline in steady flow.
True
P/roh + (V^2)/2 + gz = constant along a streamline if flow is incompressible, inviscid and steady.
True
The no-slip boundary condition is the appropriate boundary condition for viscous flow.
True
P = rhoRT for simple liquids such as water
False
Pgage = Pabs - Patm
True
The buoyant force acting on a body fully submerged in water is equal to the weight of the water displaced. This is true whether the body is made of Styrofoam or steel.
True
The Principle of Dimensional Homogeneity states that all additive terms in a single equation must have the same dimensions is the equation is to have physical meaning.
True
The dimensions of abs viscosity, mu, can be written as a force/(area*time).
False, mu is equal to force*time/area
If I want to study the drag on a Cessna 172, I can build a 1/10th scale model and simply test it in an atmospheric pressure/temperature wind (air) tunnel at 10 times flight speed. Since I will thus match Reynolds numbers between flight and model, T will be assured that the drag coefficients will also match.
False
The exponential scale height of Earth’s lower atmosphere is about 9km (“about” here means within 20%).
True
The area integral of the pressure obtained from the hydrostatics equation P - Pnaut = rhogH around a closed body fully submerged in a single component liquid produces no net force.
False
While the absolute viscosity of a gas increases with an increase in temperature, that of a liquid decreases with an increase in temperature.
True
Flow induced chemical changes should only be expected in airflow over a vehicle if the flow is hypersonic. Similarly, for practical purposes, the flow is virtually incompressible if the Mach number if 0.1.
True
We can expect boundary layer transition on a smooth flat plate in a zero pressure gradient flow at Re of about 500,000.
True
The lift curve for an airfoil (Lift coef vs. alpha) is linear for small angles of attack.
True
At a modest angle of attack of several degrees, the lift coefficient and the drag coefficient on a simple thin airfoil are both of similar size of order one.
False
In a steady flow a streamline, pathline, and streakline through a given point all follow the same path. In an unsteady flow, these three lines may also follow the same path. (think about 1-D flow, for example).
True
The area integral term in the continuity equation represents the net rate of mass flux through a control surface.
True
V = ay i + by^2 j represents a one dimensional steady flow.
True
All of the additive terms in the integral form of the momentum equation have the same dimensions, a force.
True
A Newtonian fluid will continue to deform under the influence of normal stress.
False, A Newtonian fluid will continue to deform under the influence of SHEAR stess
P = rhoRT applies to gasses away from their critical point.
True
The drag coefficient, Cd, represents a ratio of forces - the drag force divided by a force which would be created by the dynamic pressure times a suitable normalizing area.
True
If dimensional analysis yields the result that there is only one appropriate Pi group, that group must be equal to zero.
False, that Pi group must be equal to a constant.
We derived an expression for hydrostatics in class. If we had instead assumed that the fluid was undergoing a uniform acceleration, a, (e.g., think of doing an experiment in a railroad car undergoing acceleration a), then the equation would have been -delP + rho*(g-a) = 0.
True
g*R/T is the atmospheric scale height. Here, g is the gravitational acceleration, R the gas constant and T the abs temperature.
False, atmospheric scale height is R*T/g
A ship was floating, and then a leak developed and the ship sank. The ship displaced the same weight of water whether is was floating or sunk.
False
The viscosity of a gas increases with increasing temperature.
True
Consider a Couette flow experiment. On a plot of shear stress vs. strain rate, the slope of the line is the absolute viscosity. (Strain rate is along the x-axis). Moreover, for a Newtonian fluid, that slope is a constant everywhere along the line.
True
If we consider a simple laminar boundary layer developing on a thin flat plate in a uniform flow, the shear stress in the fluid is greatest at the plate surface and approaches zero at the boundary layer edge. Also, at the same height above the plate but still within the boundary layer, the shear stress will decrease with increasing distance from the plate leading edge.
True
The speed of a capillary wave should be proportional to the square root of the liquid surface tension.
True
Reynolds number represents the ratio of gravitational to viscous forces in a flow.
False, the Reynolds number is the ratio of inertial forces to viscous forces.
Dynamic similarity should apply to the flow about two objects if they are goemetrically similar and if all of the Pi-groups (like Reynolds number, Mach number,…) are the same.
True
A wing stalls when the flow on the lower surface separates and becomes turbulent.
False, Stall occurs when the flow on the UPPER surface separates and becomes turbulent.
If I am swimming at the bottom of a small pool (like a hot tub) and a huge person enters the tub and floats while no water is lost from the tub, the pressure I feel will rise.
True
If I am again swimming at the bottom of a small pool (like a hot tub) and a huge person enters the tub but sinks while no water is lost from the tub, the pressure I feel will not rise.
False, the water’s weight is still in the tub. The added weight of the fat swimmer will increase the pressure.
A boundary layer is the thin layer of fluid near a body moving through the flow in which shear stresses are important.
True
The boundary condition for a real fluid beside a real solid is the there is both no-through-flow and no-slip of the fluid with respect to the solid.
True
An ordinary airplane would commonly cruise at a lift to drag ratio of about one.
False, lift would have to be significantly higher than drag.
The viscosity of a gas decreases as its temperature increases. Similarly, the thermal conductivity which represents heat diffused across a temperature gradient by the thermal motion of molecules also decreases with an increase in temperature.
False, viscosity of a gas increases as its temperature increases.
The center of pressure is the location where the resultant force of a distributed load acts on a body.
False
V = ay i = bx^2 j represents a one dimensional steady flow.
False, there are more than one coordinate axis.
The absolute pressure of a gas must always be positive while the gauge pressure may be positive or negative.
True
A liquid is not compressible under any circumstances.
False, the Tate equations shows how it is possible for liquids to be compressible under extremely heavy loads.
The Principal of Dimensional Homogeneity says that all additive terms in an expression representing physical reality must be dimensionless.
False, not dimensionless but have the same dimensions
In an isothermal liquid as in an isothermal gas the hydrostatic pressure varies exponentially with distance above of below a fixed height.
False
We should expect a laminar boundary layer over a flat plate to transition to turbulence at an Re of about 500,000.
True
Since both an airfoil and a circular cylinder in a flow have a smooth and round leading edge, they both have about the same drag coefficient, something around one.
False, the streamlining at the end of an airfoil greatly reduces drag.
The ratio of the lift coefficient to the drag coefficient of an airfoil placed at a few degrees angle of attack could reasonably be 1000.
False
A streamline and a streakline, both passing through the same point in space, are the same line even in an unsteady flow.
True
V = atx i + aty j represents a two dimensional unsteady flow.
True
If we consider water emanating horizontally from a converging nozzle must be greater than atmospheric pressure.
Ture
Since we derived Bernoulli’s equation in class using a differential sized control volume whose periphery was made up of streamlines, we can use the resulting equation for either viscous of inviscid flow problems.
False
The dimension(s) of a momentum flux is a Force.
True
It is thought that the drag, F, on a block submerged in a flow depends on the block’s length, width and depth as well as on the free steam velocity, freesteam density and the freestream viscosity. This physical relationship can be expected to be able to be represented with two dimensionless Pi-groups.
False
The buoyancy force exerted be the atmosphere on an empty one liter soda bottle (with its cap on) is about one newton.
False
Shock waves can’t occur if the flow well upstream of an ordinary airfoil is subsonic. Also, in a hypersonic flow over some vehical one should expect there to be shock waves and the oxygen and nitrogen in the air may become hot enough to dissociate into their constituent atoms.
False, new chemical reactions will occur at hypersonic flight.
