ANNA UNIVERSITY, CHENNAI DEPARTMENT DEPARTME NT OF AERONAUTICA AERONAUTICAL L ENGINEERI ENGINEE RING NG AE6801-WIND AE6801 -WIND TUNNEL TECHNIQUES TWO MARK QUESTIONS
UNIT-I
1. Define Define Mach’s Mach’s number? It’s defined defined as the the square root ro ot of the ratio ra tio of the the inertia inertia force of a flowin flowing g flui fluid d to the elastic force. Mathematicall Mathematica lly, y, it is defi de fined ned as
= =
M= 2. Define Reynold’s number?
It’s defined defined as the ratio of the inertia inertia force of o f a flowin flowing g flui fluid d to the the viscous viscous force of o f the flui fluid. d. The expression expre ssion for Reynold’s numbe numberr is obtained as R e =
=
3. Define Define Euler’s Euler’s numbe number? r? It’s defined defined as the square root roo t of the ratio of the the inertia inertia force of o f the flowin flowing g flui fluid d to the the pressu pressure force. Mathem Mathemati atically, cally, it is express expressed ed as
=
Eu = 4. Define Define Weber’s Web er’s number? number?
It’s defined defined as the square root roo t of the ratio of the the inertia inertia force of o f the flowin flowing g flui fluid d to the the surface surface tension tension force. Mathem M athemat atiically, ca lly, it is is expressed as as We =
=
5. Define Define Froude’s Fro ude’s number? number? It’s defined defined as the square root roo t of the ratio of the the inertia inertia force of o f the flowin flowing g flui fluid d to the the gravi gravity force. orce . Mathemat Mathematiically, ca lly, it is is expressed as as Fe =
=
6. Define Buckingham’s π-Theorem? It states that “If there are n variables (independent or dependent) in a physical phenomenon and if these variables contain m fundamental dimensions (M, L, T), then the variables are arranged into (n-m) dimensionless terms. Each term is called π-term” . 7. What is meant by similitude and types of similarities? Similitude is defined as the similarity between the model and its prototype in every respect, which means that the model and prototype have similar properties or model and prototype are completely similar. Three types of similarities must exist between the model and prototype. They are, ❖ ❖ ❖
Geometric similarity Kinematic similarity Dynamic similarity
8.What is meant by geometric similarity? It is said to exist between the model and the prototype. The ratio of all corresponding linear dimension in the model and prototype are equal.
∀ = corresponding values of the model
Lm,bm,Dm,Am,
∀
L p,b p,D p,A p,
= corresponding values of the prototype
For geometric similarity between model and prototype, we must have the relation,
= = = Lr, where Lr is called scale ratio. 9.What is meant by kinematic similarity? It is said to exist between the model and the prototype if the ratios of the velocity and acceleration at the corresponding points in the model and at the corresponding points in the prototype are the same. V p1, V p2, a p1,a p2 = corresponding values at the corresponding points of the fluid velocity and acceleration in the prototype. Vm1, Vm2, am1,am2 = corresponding values at the corresponding points of the fluid velocity and acceleration in the model. For kinematic similarity in velocity and acceleration, we must have
= = Vr ; = = ar Where, Vr is the velocity ratio and ar is the acceleration ratio. 10.What is meant by dynamic similarity? It is said to exist between the model and the prototype if the ratio of the corresponding forces acting at the corresponding points are equal.
= = ( ) ….. = Fr , Where Fr is the force ratio. ( ) 11.what is scale effect? It is the correction necessary to apply to measurements made on a model in a wind tunne l in order to deduce corresponding values for the full-sized object. Scale effect in various field we can say as, (aerospace engineering) The necessary corrections applied to measurements of a model in a wind tunnel to ascerta in corresponding values for a full-sized object. (fluid mechanics) An effect in fluid flow that results from changing the scale, but not the shape, of a body a round which the flow passes; this effect is relevant to wind tunnel experiments. 12.What is dimensional homogeneity? It means the dimensions of each terms in an equation on both sides are equal. Thus if the dimensions of each term on both sides of an equation are the same the equation is known as dimensionally homogeneous equation. For example: V =
√ 2 is dimensionally homogeneous.
13.What is the importance of Non-Dimensional Numbers in Dimensional analysis? The importance of experiments in fluid mechanics needs no additional emphas is. Experiments are required in design and testing of vehicles such as aeroplanes, ships, pumps, automobiles, turbines, fans and other equipment. We also have experiments which are carried out from the point of view of understanding a flow and fundamental phenomena such as turbulenc e. Needless to say that the experiments have to be planned and executed methodically.
UNIT-II 1.What is wind tunnel and its classification? Wind tunnels are devices that provide air streams flowing under controlled condition so that models of interest can be tested using them. From the operational point of view, wind tunne ls are generally classified as, ❖ ❖ ❖
Low-speed High-speed Special-purpose
2.What are the functions of effuser and diffuser? Effuser converts available pressure energy into kinetic energy and it is located in the upstream of the test section. Diffuser converts kinetic energy into pressure energy in downstream of the test section.
3.Define energy ratio of a wind tunnel It is defined as the ratio between the total kinetic energy of jet to the energy loss. ER =
= 1 ∑ 0
4.What is meant by energy ratio? The ratio of the energy of the airstream at the test-section to the input energy to the drivin g unit is a measure of the efficiency of a wind tunnel. It is nearly always greater than unity.
