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INDIAN INSTITUTE OF TECHNOLOGY – DELHI
COURSE – Experimental Methods in Thermal Engineering (MEL - 705)
------------------ FLUID DYNAMICS CYCLE --------------Title of study
Study of Mixing Layers
Date of submission
Experiment no. Date of experiment
#5
Course coordinator
Dr. PMV Subbarao (Professor)
Department of Mechanical Engg.
Objectives:
1. Creation of plane mixing layer with different velocity ratios.
2. Identification of transition region and self similarity region.
3. Identification of length of mixing layer.
4. Recognition of Laminar and Turbulent Mixing Layers.
Introduction
A celebrated contribution of Fluid Mechanics to Mathematics is Boundary Layer Mathematics,
which later sliced Mathematics into Theoretical Mathematics and Applied Mathematics. Boundary
layer Mathematics is one of the major subject in Applied Mathematics.
The credit goes to Prof. Ludwig Prandtl, whose pioneering explanation to frictional fluid mechanics
innovated new dimension to Aerodynamics. The boundary layer approximation can be applied to
many flows those occur in nature and engineering.
The Plane Mixing Layer
The plane mixing layer is characterized by the merging of two co-flowing fluid streams with
different velocities. Typically, the two streams are separated by an impermeable object upstream of
the confluence of these streams.
Downstream of the confluence, the two streams exchange momentum as they come into intimate
contact with each other. The mixing layer itself is defined by the region in which this merging
process is occurring. Being such a simple configuration, it stands to reason that the mixing layer is
one of the more common flows experienced in nature.
Mixing layers are encountered in many applications such as combustion furnaces, chemical lasers,
and the lip of an intake valve in an internal combustion engine and the trailing edge of a turbine blade
etc. Mixing layer studies have been popular over the years, not only because of their important role in
practical aerodynamics, but also because their asymptotic behavior is thought to be quite simple in
theory.
Townsend showed that the governing equations and boundary conditions for the plane turbulent
mixing layer can yield "self-similar" solutions for sufficiently high Reynolds number and
downstream distance. The necessary conditions for self-preservation are that the mixing layer grows
linearly and that the shapes of the mean velocity and turbulence profiles are independent of
downstream distance when scaled by the velocity difference and local mixing layer thickness. It is
widely believed that mixing layers achieve such a self-similar condition after a sufficient
development distance.
The governing equations are derived from the full incompressible Navier-Stocks equations. These are
solved in a domain which is finite in the stream-wise direction, x and doubly infinite in the cross
stream direction of y.
The mean component of the streamwise velocity at the inlet plane of the domain, termed "reference"
velocity also, is represented by:
U y 
The Test Rig

The experiments are to be
conducted in a specially
designed
Mixing
Layer
Wind Tunnel.

It
consists
bifurcated
a
into
tunnel
two
individually driven sections,
forcing
respective
draft
section/
through
duct.

1 U high  U low
 tanh 2 y 

2 U high  U low

Each leg is separately driven by a variable speed blower

The two streams are allowed to merge at the sharp edge of a tapered splitter plate which
divides the contraction into equal sections.

The flow speed in each leg can be varied separately.

A Pitot tube with separate static port with traversing mechanism is used to map the flow field.
Experimental Procedure





Draw a neat skecth with all the major dimensions of the test rig and details of all the
insturments available on the test rig.
Select various combinations of speeds for the blowers.
Set a speed combination for the blowers.
Measure velocity profiles at five locations along main flow direction.
Repeat the measurements for all the speed combinations.
Data Analysis:

Collection of instantaneous Pitot tube data at various x-y locations.

Computation of mean local dynamic pressure and local velocity.

Identification of transition nature of the velocity profiles.

Identification of Self similarity nature of velocity profiles.

Computation of mixing length.

Computation of similarity variable for mixing layer.