AERODYNAMICS Flashcards
“the property of fluid to resist shearing stress”
VISCOSITY
“the sticky or adhesive characteristic of a fluid”
VISCOSITY
It can also be described in terms of a fluid’s thickness.
VISCOSITY
In real life there is a drag force on the sphere tending to retard the motion of the sphere.
REAL FLOW
The flow separates on the rear surface of the sphere, setting up a complicated flow in the wake and causing the pressure on the rear surface to be less than that on the forward surface. Hence, a drag force is exerted on the sphere
REAL FLOW
This is a concept used in physics and engineering to study the behavior of fluids under idealized conditions.
FRICTIONLESS FLOW
In reality, all fluids experience some degree of friction and viscosity, which affects their flow and energy transfer.
FRICTIONLESS FLOW
useful as a starting point to understand the basic principles of fluid mechanics, but it is not applicable to real-world situations.
FRICTIONLESS FLOW
Another region of __________ (sometimes called potential flow) outside the boundary layer.
FRICTIONLESS FLOW
Fluid that experiences no friction or viscosity, meaning that there is no resistance to the flow of the fluid.
FRICTIONLESS FLOW
The region of viscous flow which has been retarded owing to friction at the surface.
BOUNDARY LAYER
Friction is important. It is the layer near the surface.
BOUNDARY LAYER
Thickness grows as the flow moves over the body
BOUNDARY LAYER
More and more of the flow is affected by friction as the distance along the surface increases.
BOUNDARY LAYER
The presence of friction creates a shear stress at the surface “τw”.
BOUNDARY LAYER
This shear stress has dimensions of force/area and acts in a direction tangential to the surface.
BOUNDARY LAYER
Τw gives rise to a drag force called “skin friction drag”.
BOUNDARY LAYER
Where μ is called the ____________ (viscosity of the gas). It has dimensions of mass/(length)(time).
Absolute viscosity coefficient
For liquids, μ decreases as T increases. “Oil gets thin when Temp Increases.
Absolute viscosity coefficient
But for gases, μ increases as T increases (Air gets “thicker” when temperature is increased).
Absolute viscosity coefficient
Constant value of absolute viscosity coefficient IN METRIC SYSTEM
𝜇 = 1.7894x10^−5 𝑘𝑔/𝑚. 𝑠
Constant value of absolute viscosity coefficient IN ENGLISH SYSTEM
𝜇 = 3.7373x10^−7𝑠𝑙𝑢𝑔/𝑓𝑡. s
TWO BASIC TYPES OF VISCOUS FLOWS
LAMINAR FLOW
TURBULENT FLOW
Flow in which the streamlines are smooth and regular and the fluid element moves smoothly along the streamline.
LAMINAR FLOW
-Smooth motion fluid elements in a laminar flow.
LAMINAR FLOW
- Flow in which the streamlines break up and a fluid element moves in a random, irregular, and tortuous fashion.
TURBULENT FLOW
Tortuous, irregular motion of fluid elements
TURBULENT FLOW
Less skin friction drag; pressure drag predominates.
LAMINAR FLOW
Unstable, tends to favor flow separation
LAMINAR FLOW
Transition region is closer to leading edge
LAMINAR FLOW
Thinner boundary layer
LAMINAR FLOW
Greater skin friction drag.
TURBULENT FLOW
More stable; disorder is favored by nature
TURBULENT FLOW
Transition region is farther aft
TURBULENT FLOW
Thicker boundary layer
TURBULENT FLOW
It is a measure of the ratio of inertia forces to viscous forces.
REYNOLD’S NUMBER
Helps determine whether the flows in a body and its scaled version are aerodynamically similar.
REYNOLD’S NUMBER
Can be applied in determining whether all, or portion of boundary layer is laminar or turbulent.
REYNOLD’S NUMBER
Dimensionless, It varies linearly with X. Sometimes called a local Reynolds number, because it is based on the local coordinate X.
REYNOLD’S NUMBER
REYNOLD’S NUMBER
FORMULA
𝑅𝑒𝑥 = ρ∞V∞x/µ∞
The flow always starts out from the leading edge as laminar. Then, becomes unstable and small “bursts” of turbulent flow begin to grow in the flow.
TRANSITION
Over a certain region called the ________ region, the boundary layer becomes completely turbulent.
TRANSITION
Transition is said to take place is the critical value xcr. In turn, xcr allows the definition of a critical Reynolds number for transition as
Rexcr = ρ∞V∞cr/µ∞
FACTORS AFFECTING TRANSITION FROM LAMINAR TO TURBULENT
- Increased surface roughness
- Increased turbulence in the free stream
- Adverse pressure gradients
4.Heating of the fluid by the surface - Similarity parameters of the flow (Mach Number and Reynold’s Number)
Friction also causes another phenomenon, called ___________, which, in turn, creates another source of aerodynamic drag, called pressure drag due to separation (Form Drag).
FLOW SEPARATION
Two Major Consequences of Separated Flow Over an Airfoil
- A drastic loss of lift (stalling).
- A major increase in drag, caused by pressure drag due to separation.
A condition wherein the angle of attack increases beyond a certain point such that the lift begins to decrease.
STALL
Occurs when the critical angle of attack of the airfoil is exceeded.
STALL
Typically about 15 degrees, but it may vary significantly depending on the fluid, and Reynolds number.
STALL
An ideal type of flow in which there is no friction.
Frictionless flow
The Karman Vortex Street was named after aerodynamicist ________________.
Theodor Von Karman
A ____________ is a repeating pattern of swirling vortices caused by the unsteady separation of a fluid around blunt bodies.
Karman vortex street
An _______ is a streamlined body which when set at a suitable angle of attack, produces more lift than drag while also producing a manageable pitching moment.
Airfoil
An airfoil is said to be _________ if the upper and lower cambers are equal in shape.
SYMMETRICAL
An airfoil is said to be ___________ if the upper and lower cambers are not equal in shape.
ASYMMETRICAL
The Precise distance from the Leading edge measured along the chord line.
Chord
A straight line connecting the leading and trailing edges of an airfoil.
Chord Line
Locus of all points equidistant from the top and bottom of the airfoil.
Mean Camber Line
The maximum distance between the chord line and the mean camber line.
Camber
Maximum distance between the top and bottom surfaces of the wing.
Thickness
Is the maximum thickness-to-chord ratio.
Thickness Ratio
The most forward points of the mean camber line.
Leading Edge
The most rearward points of the mean camber line.
Trailing Edge
The distance from wing tip to wing tip.
Wingspan
The area of the projection of the actual outline on the plane of the chord.
Wing Area
The ratio of the span to the chord. If the wing is not rectangular in shape, it is the square of the spane to the area.
Aspect Ratio –
Angle between the relative wind and the chord line.
Angle of Attack
Defined as the component of aerodynamic force perpendicular to the relative wind.
Lift
Defined as the component of the aerodynamic force parallel to the relative wind.
Drag
The air far upstream of the airfoil.
Relative Wind
Developed by the National Advisory Committee for Aeronautics
NACA AIRFOIL
These are airfoil shapes for aircraft wings.
NACA AIRFOIL
The shape is described by a series of digits followed by the word _____. Can either be 4, 5, or 6 – digits.
NACA
