things you should know by now: aerodynamics senior

Things You Should Know By Now or Find Out Right Away

What is a fluid?
How are fluids different from solids?
What are Newton's Laws of Motion?
What is pressure? Why does it occur in a gas?
What is hydrostatic pressure?
What is dynamic pressure?
What is "shear stress"?
What is "Archimedes' Principle"?
What is the Avodagro Number? What does it indicate?
A Pitot tube is used to measure the stagnation pressure in an air stream where the flow speed is 10m/s. Given that the static temperature is 280K, and the static pressure is 101300 N/m2, find the stagnation pressure at the probe tip.
Why does the pressure of the atmosphere decrease as you go up?
Why does the density of the atmosphere decrease as you go up?
What is a vector? Is hydrostatic pressure a vector or a scalar? What about velocity? temperature? density?
Write down Bernoulli's equation for incompressible flow/
What is the ideal lift curve slope of a symmetric thin airfoil in low speed flow?
What is a perfect gas?
Plot the section lift coefficient as a function of angle of attack for a 2-D, low-speed, symmetric airfoil. Also plot the lift coefficient versus angle of attack for a 3-D rectangular wing with a symmetric section (incompressible flow). What is the slope of this line? Why, physically, are the two slopes similar / different? What happens when the angle of attack gets large?
What is induced drag? What does its magnitude depend on?
Prove that the speed for minimum drag occurs when the induced drag coefficient is equal to the coefficient of lift-independent drag.
Calculate the lift per unit span of a thin symmetrical airfoil of chord 1.2 m at 5 degrees angle of attack at a velocity of 30 m/s at 10 km altitude on a standard day above Earth.
What is the effect of increasing aspect ratio on the drag of a wing in low speed flow?
You are considering two wings, with the same airfoil shape and area. Neither has any twist. One has an average chord of 2m, and the other has an average chord of 3m. Which will give the better lift-to-drag ratio at low speeds? Why?
Air is made up on discrete molecules, yet we analyze aerodynamics assuming that air is a "continuum". Why? Give examples where this is not a good assumption.
The pressure of air at a given location at a given instant is 210,000 N/m² and the density is 1.2kg/m^3. Find the temperature.
Nitrogen has a molecular weight of 28. How many molecules of nitrogen are present in 1 cubic meter of nitrogen at standard conditions?
The lift curve slope of a modern airfoil is very close to the ideal value. Its zero-lift angle of attack is -3 deg. Estimate its lift coefficient at 10 degrees angle of attack.
How is the stress-strain relation for solids different from that for fluids?.
Write down, without looking in any book, the law of conservation of mass as applied to flow through a control volume, for unsteady compressible flow. Then write the form for steady incompressible flow.
What are Newton's 1st, 2nd and 3rd Laws of Motion?
In a closed circuit wind tunnel (such as the John J. Harper Wind Tunnel) air goes round and round through a big tube, getting whacked by the blades of a big fan, then going through an expanding duct, then turning 90 degrees twice through sets of turning vanes (which are themselves like wings, attached to the floor and ceiling), then through a honeycomb and a fine-mesh screen, going through a contracting passage into a straight, constant-area section, zipping by an ugly piece of metal with sharp and rounded edges, past a clutter of tubes, wires, camera lenses, laser beams and several grinning faces, then back into an expanding duct, turning another 90-degree bend and into the fan, and so on.Sketch this situation. Then plot the following quantities as functions of distance along the tunnel axis.
a) Stagnation enthalpy per unit mass of the air. Identify where heat is put in/taken out, and where work is put in/taken out.
b) Momentum of the air per unit mass.
c) Static pressure.
e) Velocity.
Write down the unsteady term in the momentum equation.
What are the unknowns that we typically try to solve in fluid dynamics? How about aerodynamics?
What are the basic facts that we use to generate enough equations to solve for these?
Is the lift on a wing equal to the airfoil lift-per-unit span times the span? If not, is it greater or less? Why?
What is a streamline? Write down equations describing a streamline.
What are "body forces"? Write down the body force term due to one type of body force in the momentum equation.
What is the Biot-Savart Law?
Why does the pressure term on the rhs of the momentum equation (and the energy equation) have a negative sign in front of it?
Define work in the context of fluid dynamics. As Georgia Tech's star Goal Line Defensive Tackler, you exert 2000 Newtons along the direction of the vector 2i + 0j + 3k, on an FSU Running Back who weighs 300lbs. The FSURB moves 5 feet along -2i + 0j - 10k as you continue to exert 2000 Newtons. Calculate the work done by you on the FSURB.
Define circulation. A wing is placed at negative angle of attack in a flow which goes from right to left. Is the circulation around a section of the wing positive or negative?
What is the direction of rotation of the tip vortex at the right-side wingtip of an aircraft that is flying straight and level, east to west? What is the direction of the velocity induced by this vortex, at a point 50m downstream (behind) the mid-semi-span of the right wing, if the wing span is 30meters?
Let us estimate the velocity here. Assume for the moment that the strength of this vortex is equal to the value of the Bound Circulation on the wing. Assume that this Bound Circulation can be found using the Kutta-Jowkowsky Theorem, where the lift of the whole wing is 100,000 N, and it is distributed uniformly along the span. Find the magnitude of the induced velocity at the point described above (50m behind, etc.)
Consider a rectangular box, which experiences pure shear strain along all edges. Sketch the shape of the box at a subsequent time.
Write down a vector expression in terms of velocity for dilatation.
How do you solve for the lift coefficient and pressure distribution of a thick airfoil?
What is Reynolds number?
A circular cylinder of radius 10mm is moved at a speed of 10m/s through air at standard sea-level conditions. What is the Reynolds number based on cylinder diameter? What do you think will happen if the cylinder is made to start spinning about its axis while it is moving?
What is induced drag?
Calculate the pressure coefficient at a point on a wing where the velocity is 2.5 times the freestream value
A two-dimensional source (of air) of strength 10 m^2/s is placed at a point P in a freestream of air that is moving from right to left at 30 m/s. Assuming that the density of the flow is the same everywhere at 1kg/m^3, find the location of the upstream stagnation point. Draw a line from the source to the stagnation point, and move 30 degrees clockwise along a circle that passes through the stagnation point. Find both components of velocity there. Is the flow tangent to the circle there?
A vortex sheet is used to represent a symmetric thin airfoil of chord 1m, at an angle of attack of 5 degrees in an incompressible steady flow of air at 30m/s, with a freestream at static sea level conditions. Which variables impact the vortex strength needed, at the mid-chord position? How (quantitative relationships)? If the freestream velocity is doubled, what happens to the vortex sheet strength needed? If the angle of attack is halved, what happens to the vortex strength needed? Why do the pressure and density affect/not affect the vortex strength - from a basic point of view, arguing using the conservation equations?
A doublet is used to represent a cylinder, in a freestream. How will you orient the doublet with respect to the freestream? Why not reverse it? Given freestream velocity U, you can find the doublet strength needed to get a cylinder of radius R, by satisfying the surface boundary condition: that the total velocity normal to the surface at any point, induced by all of the potential, should be zero. You can do this by writing out the full potential as the sum of the potential due to the freestream and the potential due to the doublet, and finding the partial derivatives of the total potential with respect to x and y to find the u and v components of velocity. Then you can find the component of the velocity perpendicular to the surface at any point on the cylinder, and set this to zero. The easiest point is the front stagnation point (180 degrees azimuth) - here the freestream velocity is cancelled head-on by the velocity induced by the doublet, at radius R. Question: find the pressure coefficient at the 135 degree azimuth (where 0 degrees is pointed downstream, and 180 degrees is pointed upstream).
What is Kelvin's theorem? Please look it up in your textbook, and compare with what was given in class. Can you reconcile the two?
What are Helmholtz's vortex theorems? How do these follow logically from Kelvin's theorem?
What happens to the total vorticity in a flow problem if the angle of attack of an airfoil in the flow starts increasing? How is this explained?
Sketch the vortex system of a finite wing, and indicate the sense of rotation of every vortical element there.
Look up the expression for the bound vorticity of a straight wing as given by Prandtl's lifting line theory, for an elliptical load distribution. How do you find its value at "mid-semi-span"?
What would be the value of the vortex sheet strength at a point 300 meters downstream of such a wing?
What is the expression for velocity induced at a point near a SEMI-infinite line vortex? (Look at how the expression for an INFINITE vortex was derived, and modify it).
How do you find the pitching moment coefficient of a cambered thin airfoil, whose camber line is an arc of a circle, and the maximum camber is 3% of chord, at an angle of attack of 2 degrees?