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MAE 741 FUNDAMENTALS OF TURBULENCE

MAE 741 FUNDAMENTALS OF TURBULENCE. PRESENTATION ON ‘HOT-WIRE ANEMOMETRY’ BY:ASEEM NEVREKAR. OPERATING PRINCIPLE. USED TO MEASURE VARIABLES IN TURBULENT FLOWS SENSORS HEATED ELECTRICALLY AND COOLED BY FORCED CONVECTION INFORMATION DEDUCED FROM TEMPERATURE OR RESISTANCE. METHODS.

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MAE 741 FUNDAMENTALS OF TURBULENCE

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  1. MAE 741 FUNDAMENTALS OF TURBULENCE PRESENTATION ON ‘HOT-WIRE ANEMOMETRY’ BY:ASEEM NEVREKAR

  2. OPERATING PRINCIPLE • USED TO MEASURE VARIABLES IN TURBULENT FLOWS • SENSORS HEATED ELECTRICALLY AND COOLED BY FORCED CONVECTION • INFORMATION DEDUCED FROM TEMPERATURE OR RESISTANCE

  3. METHODS • CONSTANT CURRENT METHOD • CONSTANT RESISTANCE OR TEMPERATURE METHOD (WHEATSTONE BRIDGE)

  4. u’ AS A PERCENTAGE OF U • GOOD WIND TUNNELS:0.05 % • GRID TURBULENCE:0.2-2 % • WAKES:2-5 % • BOUNDARY LAYERS OR PIPE FLOWS:3-20 % • JETS:OVER 20 %

  5. TYPICAL HOT-WIRE ANEMOMETER • DIA:0.5 -5 MICRONS • LENGTH:0.1-1 MM • MATERIAL:PLATINUM OR TUNGSTEN

  6. DYNAMIC RESPONSE OF THE WIRES • R IN TERMS OF R* (M/R*-Ra) d(R-Ra)/dt + (R-Ra)/(R*-Ra)=1 Ra-RESISTANCE AT AMBIENT TEMP Ta FOR SMALL VELOCITY FLUCTUATIONS M dr/dt + r = r* M-TIME CONSTANT OF THE WIRE(DEPENDS ON DIA,MATRL ETC)

  7. ELECTRONIC CIRCUITRY CONSTANT CURRENT ANEMOMETER: • r* TO BE DEDUCED FROM r • COMPENSATIONG AMPLIFIER USES DIFFERENTIATOR • USEFUL FOR SMALL PERTURBATIONS • ERRORS FOR LARGE FLUCTUATIONS (TURBULENCE LEVELS LARGER THAN 5%) BECAUSE OF STATIC AND DYNAMIC NON-LINEARITIES

  8. ELECTRONIC CIRCUITRY CONSTANT TEMPERATURE ANEMOMETER: • WHEATSTONE BRIDGE • UNBALANCED VOLTAGE AMPLIFIED AND FED BACK • ALGEBRAIC RESPONSE dT/dt = 0

  9. SIGNAL PROCESSING • CALIBRATION OF THE ANEMOMETERS: SENSITIVITY CO-EFFICIENTS OF THE WIRES W.R.T. FLOW VARIABLES • SEPARATION OF THE FLUCTUATIONS: OFTEN DONE WITH A LINEAR THEORY (SMALL PERTURBATIONS),OP-AMPS,MEAN SQUARE VALUES OF SIGNALS CALCULATED BEFORE AVAILABILITY OF OP-AMPS (CROSS-CORRELATION CO-EFFICIENTS)

  10. SPECIAL APPLICATIONS OF HOT-WIRE ANEMOMETRY • MEASUREMENTS CLOSE TO A WALL • UNUSUAL WIRE ARRANGEMENTS • NON-NEWTONIAN FLUIDS • MEASUREMENTS IN LIQUID METALS • MEASUREMENTS IN TWO-PHASE FLOWS

  11. CONCLUSIONS • SIMPLE PRINCIPLE BUT CAREFUL INSIGHT INTO ASSOCIATED PHYSICS REQUIRED. • BOTH CONSTANT CURRENT AND CONSTANT TEMPERATURE ANEMOMETERS HAVE ROLES TO PLAY • USE OF DIGITAL TECHNIQUES TO IMPROVE ACCURACY • IMPORTANT CONTRIBUTION TO THE KNOWLEDGE OF TURBLULENT FLOWS

  12. REFERENCE ANNUAL REVIEW OF FLUID MECHANICS (VOL. VIII) 1976

  13. THANK YOU

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