Experiment

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Experiment 6Velocity Field Measurements of a
Rectangular Jet
An Introduction and Setup

• Teaching Group 6
• Jose Cardenas
• Cedric Harris
• Salina Songha
• Wale Adewole
• Brian Moore
• Richard McNaron
• Steven Lydzinski
• Dwayne Tillman

EML 4304L-Thermal Fluids Lab
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Outline

• Jet Flow
• Calibration in Experiments
• Purpose
• Equipment and Setup
• Background for Calibrating the Hotwire
• Actual Performed Experimental Procedure

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What is a Jet?

• One Fluid flowing from a nozzle into an Ambient
  Fluid
• When the Ambient fluid is at rest it is referred
  to as a free jet
• When the ambient fluid is moving it is call a
  coflowing jet

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Applications of Jets

• Jet Engines
• Combustors
• Chemical Lasers
• Printer Head
• Painting
• Cleaning

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What is the Shear Layer

• A highly unstable area caused by the difference
  in velocity of the jet and ambient fluids
• Subject to unstable flows
• Forms large vortical structures
• Causes Fluctuation in the flow
• Enhances mixing

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Free Jet Flow
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What is the potential core?

• Central region of the jet
• Almost uniform mean velocity
• Disappears at about 4-6 diameters downstream from
  the nozzle.

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Equipment
This slide now appears before any part of the
procedure of the experiment appears, to enable
the viewers to familiarize with the equipment
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Description

• Air Pump- To force air through the jet nozzle
• Rectangular Jet- Directs the air blown by the
  pump through jet nozzle.
• Hot wire anemometer- Used to measure the field
  velocity because of their excellent dynamic
  response.

Moved slide from equipment section
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Description (contd)

• Pitot-static tube Collects stagnant air for the
  hot wire anemometer to measure fluid velocity.
• The software is used to move the tube to check
  for pressure differences at different positions.

Moved slide from equipment section
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Purpose

• To learn how instrument calibration works and how
  it is done.
• Voltage to measurement relationship
• ADC relationship
• To study the fluid dynamic properties of a
  rectangular air jet
• Jet growth
• Jet spreading
• Development in space

This purpose outline is now before calibration,
in order to have a more logical flow
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Calibration In Experiments

• Measurements may be misleading if proper
  calibration is not done.
• All measurement devices should be calibrated
  and/or zeroed.
• Learning about calibration gives more insight on
  what is actually happening on all aspects of the
  experiment.
• Watch out for the aliasing effect and the
  attenuation factor.

calibration works by having a reference
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• Many times instrument are calibrated for us,
  however, it should be known how the process was
  done
• In more advanced experiments, or when designing
  our own experiment, it will be up to us to
  calibrate all the instruments appropriately.
• Take into consideration the sampling frequency by
  changing it and seeing if the signal is distorted.

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Background for Calibrating the Hotwire (not used
in experiment)

• A hotwire is a VERY delicate device with fast
  time responses that measures fluid velocity.
• It works by relating the forced convection on a
  very thin wire (infinitely long cylinder) to a
  voltage/resistance output.
• There are two modes
• Constant Current
• Constant temperature

One time response is approximately 37 of the
difference between the initial and final
measurements
Annotation regarding the no use of the hotwire
was added
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Constant Current Mode

• A constant current is passed through the wire
  creating internal heating.
• An equilibrium temperature is reached depending
  on the heat lost to convection (related to fluid
  flow velocity)
• The resistance across the wire is recorded.
• The relationship between resistance change and
  temperature is known a priori.

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Constant Temperature Mode

• This is the mode used in this experiment.
• The temperature is maintained constant by using a
  feedback circuit to adjust the current.
• The increase in temperature of the wire, or
  current is proportional to the convection

I2R hA(Tf Ta) Using Kings Law for wires (E
Eo)2 Co C1(V)1/2
Energy Balance for a Hotwire
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The Pitot Static Tube

• Instead of a hotwire, a pitot static tube was
  used to measure velocity of the fluid.
• The pitot static tube is connected to a pressure
  transducer and compares the stagnation pressure
  with the atmospheric pressure
• This way one can get the velocity using the
  dynamic pressure formula

Added slide, since hotwire method was not used,
instead, a pitot static tube was used
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Pitot Tube Schematic
New slide to compliment Pitot static tube
explanation including a schematic and formula
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General Procedure

• Check the computer, digital readout, and the
  rectangular jet.
• Make sure the wire is centered prior to measuring
  the jet centerline velocity
• Turn on jet flow and set to 0.07psi
• Record pressure readings at distances away
  indicated.

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Velocity Profile

• Reset conditions.
• Return the wire back to 0cm from the centerline
  off the nozzle.
• Individually at four, ten, twenty, and thirty cm
  away from the nozzle record the pressure
  vertically.

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• Vertical movements are done by the computer,
  whereas the horizontal movements are done by
  manually.
• You can double check your accuracy on your
  readings.

This schematic, along with the comments, was
included in order to provide visual reference to
the procedure of the experiment
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The End