ANNA UNIVERSITY, CHENNAI
DEPARTMENT OF AERONAUTICAL ENGINEERING
AE6801-WIND TUNNEL TECHNIQUES
TWO MARK QUESTIONS
UNIT-I
1. Define Mach’s number?
It’s defined as the square root of the ratio of the inertia force of a flowing fluid to the
elastic force. Mathematically, it is defined as
𝐼𝑛𝑒𝑟𝑡𝑖𝑎 𝑓𝑜𝑟𝑐𝑒 𝐹𝑖 𝑉
M =√ =√ =
𝐸𝑙𝑎𝑠𝑡𝑖𝑐 𝑓𝑜𝑟𝑐𝑒 𝐹𝑒 𝐶
2. Define Reynold’s number?
It’s defined as the ratio of the inertia force of a flowing fluid to the viscous force of the
fluid. The expression for Reynold’s number is obtained as
𝐼𝑛𝑒𝑟𝑡𝑖𝑎 𝑓𝑜𝑟𝑐𝑒 𝐹𝑖
Re = 𝑉𝑖𝑠𝑐𝑜𝑢𝑠 𝑓𝑜𝑟𝑐𝑒 = 𝐹𝑣
3. Define Euler’s number?
It’s defined as the square root of the ratio of the inertia force of the flowing fluid to the
pressure force. Mathematically, it is expressed as
𝐼𝑛𝑒𝑟𝑡𝑖𝑎 𝑓𝑜𝑟𝑐𝑒 𝐹
Eu =√ = √ 𝑖
𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑓𝑜𝑟𝑐𝑒 𝐹 𝑝
4. Define Weber’s number?
It’s defined as the square root of the ratio of the inertia force of the flowing fluid to the
surface tension force. Mathematically, it is expressed as
𝐼𝑛𝑒𝑟𝑡𝑖𝑎 𝑓𝑜𝑟𝑐𝑒 𝐹
We = √𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝑡𝑒𝑛𝑠𝑖𝑜𝑛 𝑓𝑜𝑟𝑐𝑒 = √𝐹𝑖
𝑠
5. Define Froude’s number?
It’s defined as the square root of the ratio of the inertia force of the flowing fluid to the
gravity force. Mathematically, it is expressed as
𝐼𝑛𝑒𝑟𝑡𝑖𝑎 𝑓𝑜𝑟𝑐𝑒 𝐹
Fe = √ = √ 𝑖
𝐺𝑟𝑎𝑣𝑖𝑡𝑦 𝑓𝑜𝑟𝑐𝑒 𝐹 𝑔
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.
Lm,bm,Dm,Am,∀𝑚 = corresponding values of the model
Lp ,bp ,Dp ,Ap ,∀𝑝 = corresponding values of the prototype
For geometric similarity between model and prototype, we must have the relation,
𝐿𝑝 𝑏 𝐷
= 𝑏 𝑝 = 𝐷 𝑝 = Lr, where Lr is called scale ratio.
𝐿𝑚 𝑚 𝑚
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.
Vp1,Vp2,ap1,ap2 = 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
𝑉𝑝1 𝑉 𝑎𝑝1 𝑎
= 𝑉𝑝2 = Vr ; = 𝑎 𝑝2 = ar
𝑉𝑚1 𝑚2 𝑎𝑚1 𝑚2
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.
(𝐹𝑖 )𝑝 ( 𝐹𝑣 ) 𝑝 (𝐹𝑔 )𝑝
=( =( ….. = Fr, Where Fr is the force ratio.
(𝐹𝑖 )𝑚 𝐹𝑣 ) 𝑚 𝐹𝑔 ) 𝑚
11.what is scale effect?
It is the correction necessary to apply to measurements made on a model in a wind tunnel
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 = √2𝑔𝐻 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 emphasis.
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 turbulence.
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 tunnels
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.
𝐾𝑖𝑛𝑒𝑡𝑖𝑐 𝑒𝑛𝑒𝑟𝑔𝑦 𝑜𝑓 𝑗𝑒𝑡 1
ER = = ∑ 𝐾0
𝐸𝑛𝑒𝑟𝑔𝑦 𝑙𝑜𝑠𝑠
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 driving
unit is a measure of the efficiency of a wind tunnel. It is nearly always greater than unity.
