“A DETAILED ANALYSIS AND ESTABLISHMENT OF EMPIRICAL RELATIONS FOR ESTIMATION OF AFFLUX IN OPEN CHANNELS

CONSIDERING RECTANGULAR OBSTRUCTIONS”

AAKAAR 2013, IIT BOMBAY

PRESENTED BY:

KULDEEP KAUSHIK RITUKESH BHARALI

STUDENTS, 6TH SEMESTER, B.E, DEPARTMENT OF CIVIL ENGINEERING

ASSAM ENGINEERING COLLEGE, JALUKBARI, GUWAHATI, ASSAM

INTRODUCTION

Afflux - defined as the elevation of the water level upstream of a body due to the

presence of obstructions in its flow.

The local loss of energy due to fluid friction is compensated by an increase in stream

potential energy in the form of afflux upstream.

In the fig: Picture depicting river affluxDashed line : normal water flowDotted line : flow of water in the presence of structuresY4 - Y1 : Afflux height

OBJECTIVESAs the water level varies due to civil constructions and banks breached in

flood times, the study basically deals in estimation of the water level due to

obstructions and approach in preventing the breaching of water in flood times

The paper mainly focuses in developing an empirical relation for the

estimation of afflux upstream

It basically enhances upon establishment of the optimized model keeping in

view of the safety and the economic factor of design.

THEORY

Afflux – dependence on structural and flow variablesStructural variables: opening ratio, skew, eccentricity, etc.,Flow variables: Froude’s number, sediment and debris

transport, choking.Dimensional Analysis – Buckingham Pie Method and

Graphical AnalysisAnalysis of variables –

PROCEDURE

EXPERIMENTAL SETUP:1) Rectangular open channel (5000 mm x 76mm x

170mm) for the flow of water2) Rectangular piers (25mm x 6mm x 200mm) for

the construction of the structure3) Sheets of mica (1mm thickness) for the

variation of opening ratio accordingly

METHODOLOGY: After the set up water is allowed to flow

through the channel, keeping the blockage at 3.5 m

Afflux is measured accordingly at various points with the variation of opening ratio from 84% to 26%

The data is then analyzed for developing the suitable empirical formula for afflux estimation.

OBSERVATIONS Observations based on the experimental results:

Plotting of graph with water level in Y-Axis and distance of the open channel in X-Axis.

Decreasing of opening ratio is inversely proportional to the afflux.

COMPUTATION FROM GRAPHS

DATA ANALYSIS AND RESULTSDIMENSIONAL ANALYSIS

•Graph 1 provides the flow pattern of water for a variation of open ratio of 84% to 26% concluding O.R α 1/(Water Level Upstream)

•Graph 2 depicts the family of curves between the water level and open ratio for a specific L/B ratio signifying Afflux α 1/ (L/B)

•Dimensional analysis computes the relation between the estimation on of afflux with parameters L/B and O.R (constant Froude’s number 0.642) as follows-

dh/h = K. (0.642) (L/B) ̵ ͣ (δ) ̵ ᵇK, a, b are obtained from experimental data

From graph 2 , the relation is obtained for the best fit curve:

dh/h= -0.0065 δ + 0.795

From the experimental results of table 2 , the best suited results for K and a is obtained as:

  dh/h= [0.89(-0.0065 δ + 0.795)] /

COMPARISION OF COMPUTED AND MEASURED dh/h AND AVERAGE % ERROR

GRAPHICAL ANALYSIS

No significant change in the intercept

Variation of slope (m) with L/B ratio

For L/B < 2.6: c= 0.792

For L/B > or = 2.6 : c = 0.785

CONCLUSION

Experimental results proves afflux to be a function of δ, Fr and L/B ratio of the channel.

Dimensional and graphical analysis resulted in two empirical formulae. Maximum errors recorded were 6% and 8% respectively.

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