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INDIAN INSTITUTE OF TECHNOLOGY – DELHI
COURSE – Experimental Methods in Thermal Engineering (MEL - 705)
------------------ FLUID DYNAMICS CYCLE --------------Title of study
Measurement of drag on a cylinder and an aerofoil.
Date of submission
Course coordinator
Experiment no. Date of experiment
Dr. PMV Subbarao (Professor)
Department of Mechanical Engg.
#7
Objectives
1) To do experimental study of flow past airfoil and cylinder
2) To plot static pressure distribution over both the geometries at different angle of attacks
(aerofoil)
3) Calculate Coefficient of drag (Cd) and Coefficient of drag (Cl)for both the sections at
different angle of attack (airfoil) . This can be done in two ways,
i)
By measuring surface pressure distribution (pressure integral method).
ii)
By momentum balance method (wake survey method)
4) Do uncertainty analysis of complete calculations done.
Introduction

Lift and Drag:
When anybody moves through a fluid, an interaction between the body and the fluid occurs; this
effect can be described in terms of the forces at the fluid–body interface.
It is often useful to know the detailed distribution of shear stress and pressure over the surface of the
body, although such information is difficult to obtain. Many times, however, only the integrated or
resultant effects of these distributions are needed. The resultant force in the direction of the upstream
velocity is termed as drag, and the resultant force normal to the upstream velocity is termed as lift.
Typical pressure distribution over an airfoil is given in Fig.1. Remember this pressure distribution
changes dramatically with angle of attack.
Fig.1 Pressure distribution over an airfoil.
Different regimes of flow over a cylinder is given in Fig.2. Here the flow characteristics are a strong
function of Reynolds number (Re).
Fig.2 Flow across a cylinder

Pressure Integral method
This method relates pressure and skin friction coefficient to lift and drag coefficient. Drag is found
over a small local element as a contribution of both pressure and shear stress; which integrated over
the entire perimeter and dividing it by the characteristic pressure gives drag coefficient of the
cylinder.

Wake survey method
An airfoil develops Lift due to lower pressures above the wing (suction side) and higher pressures
below the wing (pressure side w.r.t. the pressure of the approaching air. In addition, the Lift force
increases with higher angle of attack up to a critical angle. Beyond this critical angle the lift force
decreases significantly and the wing is said to have “stalled”. The overall pressure distribution on the
wing can be measured with small tubes embedded in the wing leading to a suitable pressure
transducer. Principle of momentum integral states that the drag force on an aerofoil in the flow
should be equal to the loss in linear momentum of the flow.
Experimental Precedure

Draw a neat skecth with all the major dimensions of the test rig and details of all the
insturments available on the test rig.

Note the positions of pressure tappings for both cylinder and aerofoil.

At a particular flow rate (Re), note down the pressure distribution around the cylinder.

Also measure velocity at upstraem and downstream of the section.

Repeat the same for three different Reynolds number (Re).

Now replace cylinder with the aerofoil.

Repeat all measurements at selected angles of attack (α) and Re.
Data Analysis




Draw pressure distibution profiles and wake velocity profile.
Obtain the formulae for computation of drag and lift.
Calculate drag and lift at various Reynolds numbers.
Plot coefficient of drag and lift Vs Reynolds number.