Highly resolved turbulence budgets over a desert playa
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1 Highly resolved turbulence budgets over a desert playa Vigneshwaran Kulandaivelu Derek Jenson & Eric Pardyjak Department of Mechanical Engineering University of Utah Gilad Arwatz & Marcus Hultmark Princeton University
2 Motivation Measurements of shear production, buoyancy production, turbulent transport and the dissipation rate of TKE terms in the budget of turbulent kinetic energy and the terms in the scalar (Temperature) variance budget were made during the MATERHORN experimental campaign at the SLTEST site of the US army s Dugway Proving Grounds in the Utah s west desert. The velocity and temperature fluctuations were obtained with two co-located x-wire and cold wires that are very close to the playa surface (~ 2cm to 50cm).
3 Important Components : Experimental Setup Two sets of X-wires and cold wires (Co-located) An array of five fine wires at different heights Hot and Cold wire calibrations were done at the site Known for : Measuring high frequency fluctuations (for both velocity and temperature.) Issues : Calibration needs to be done for every few hours Response of the system is sensitive to atmospheric temperature changes (drifting).
4 Experimental Setup
5 Hot-Wire Anemometer Principle of operation : Electrical current passes through a thin wire (5 micron) Wire resistance changes with temperature Variation in resistance is monitored Can measure velocity or temperature Velocity measurement : The wire is heated by electrical current and cooled down by forced convection Temperature measurement : The wire is kept at temperature close to the ambient temperature Resistance is sensitive to temperature changes
6 Anemometer operation modes Constant temperature CTA (velocity) : Voltage difference across the bridge is amplified and used to balance the bridge through a feedback loop. Wire resistance and temperature are nearly constant Constant current CCA (temperature) : Low constant current is fed to the wire The voltage drop over the wire is measured and amplified Amplifier
7 Temperature measurement (CCA) Serves as a constant current source with zero offset, R1=R3>>R2 I x ~0.2 ma, V x =20 mv Hot-Wire
8 Dynamic calibration and modeling of a cold wire for temperature measurements This plot shows the Bode diagram Bode Diagram for the frequency response of a 0 Cold wire system (measuring temperature fluctuations). -10 Magnitude (db) Phase Magnitude (deg) (db) Bode Diagram Frequency (Hz)
9 Dynamic calibration and modeling of a cold wire for temperature measurements The blue line indicates the actual signal 0 Bode Diagram -10 Magnitude (db) Phase Magnitude (deg) (db) Bode Diagram Frequency (Hz)
10 Dynamic calibration and modeling of a cold wire for temperature measurements The blue line indicates the FFT of Bode the Diagram actual temperature time series. The red line indicates the amount of attenuation for different frequency ranges. Magnitude (db) Phase Magnitude (deg) (db) Bode Diagram Frequency (Hz)
11 Dynamic calibration and modeling of a cold wire for temperature measurements The blue line indicates the actual signal The red line indicates the amount of attenuation for different frequency ranges. Magnitude (db) almost no attenuation from 1/100 Hz to 1/10 Hz Phase Magnitude (deg) (db) Bode Diagram Bode Diagram Frequency (Hz)
12 Dynamic calibration and modeling of a cold wire for temperature measurements The blue line indicates the actual signal The red line indicates the amount of attenuation for different frequency ranges. Magnitude (db) Phase Magnitude (deg) (db) Bode Diagram 10 db attenuation from 1Hz to 200Hz -40 Bode Diagram Frequency (Hz)
13 Dynamic calibration and modeling of a cold wire for temperature measurements The blue line indicates the actual signal The red line indicates the amount of attenuation for different frequency ranges. Magnitude (db) Phase Magnitude (deg) (db) Bode Diagram Bode Diagram Roll off frequency Frequency (Hz)
14 Dynamic calibration set up Laser as heat source Optical chopper to vary the frequency (Gilad and Hultmark)
15 Modeling the cold wire Considerations in constructing the model (Gilad et. al. 2013): Three elements: wire filament, stubs, prongs Heat transfer to each element through laser source End conduction between adjacent elements Holder acts as a heat sink Stubs Prongs Holder Wire filament
16 Cold wire model (Gilad et. al. 2013)
17 Cold wire model (Gilad et. al. 2013) Stubs Prongs Wire filament
18 Cold wire model (Gilad et. al. 2013) Stubs Prongs Wire filament Heat accumulated in wire Heat accumulated in stubs Heat accumulated in prongs
19 Cold wire model (Gilad et. al. 2013) Heat transfer by laser Stubs Prongs Wire filament Heat accumulated in wire Heat accumulated in stubs Heat accumulated in prongs
20 Cold wire model (Gilad et. al. 2013) Heat transfer by laser Heat conducted from prongs to holder Stubs Prongs Wire filament Heat accumulated in wire Heat accumulated in stubs Heat accumulated in prongs
21 Cold wire model (Gilad et. al. 2013) Heat transfer by laser Conduction among elements Heat conducted from prongs to holder Stubs Prongs Wire filament Heat accumulated in wire Heat accumulated in stubs Heat accumulated in prongs
22 Results and the model Wire filament length: Less attenuation for longer wire Lower roll-off frequency for longer wire (Gilad et. al. 2013)
23 Results and the model Wire filament diameter: Lower roll-off frequency for thicker wire (Gilad et. al. 2013)
24 Oscillating jet of hot air Comparison of fine wire with cold wire Experimental validation
25 Oscillating jet of hot air Comparison of fine wire with cold wire Experimental validation
26 Oscillating jet of hot air Comparison of fine wire with cold wire Experimental validation
27 Normalized intensity-1 Experimental validation
28 Normalized intensity Experimental validation
29 Normalized intensity Experimental validation
30 Closer look Experimental validation
31 X wire Calibration X-wire calibration is done for a range of velocities for a range of angles.
32 Blue Square Post Calibration X wire Calibration
33 X wire Calibration Blue Square Post Calibration Green Circle Pre Calibration
34 X wire Calibration and Temperature Compensation Blue Square Post Calibration Green Circle Pre Calibration E1 & E2 X wire voltages
35 X wire Calibration and Temperature Compensation Calibration curves for CTA operation at different temperatures for the same wire. [Hultmark and Smits (2010)]
36 X wire Calibration and Temperature Compensation Calibration curves for CTA operation at different temperatures for the same wire. C [Hultmark and Smits (2010)]
37 X wire Calibration and Temperature Compensation Calibration curves for CTA operation at different temperatures for the same wire. C E wire voltage & U - velocity [Hultmark and Smits (2010)]
38 X wire Calibration and Temperature Compensation Substantial drift in the voltage for the same velocity range when the temperature changes from 48 to 33 degree Celsius. [Hultmark and Smits (2010)]
39 X wire Calibration and Temperature Compensation Replotted calibration curves using the similarity variables of Hultmark and Smits (2010). [Hultmark and Smits (2010)]
40 Preliminary results
41 Turbulence Energy Budget terms The hot wire calibration drift is taken care of.
42 Turbulence Energy Budget terms The hot wire drift is taken care of. And the cold wire attenuation is also taken care of.
43 Turbulence Energy Budget terms The hot wire drift is taken care of. And the cold wire attenuation is also taken care of. Now we can calculate the terms like Sensible heat flux, Momentum flux, TKE dissipation and turbulent transport of TKE as well the temperature variance budget terms from the Hot and Cold wires.
44 Sensible heat flux comparison of Sonic and X- wire data sets (Spring 2013 IOP 5)
45 Sensible heat flux comparison for Sonic and X- wire data sets (Spring 2013 IOP 5)
46 Momentum flux comparison for Sonic and X- wire data sets (Spring 2013 IOP 5)
47 TKE Budget (correlation) terms
48 TKE Budget (correlation) terms
49 Normalized TKE budget terms
50 Heat flux budget (correlation) terms
51 Heat flux budget (correlation) terms
52 Sensible heat flux comparison for Sonic and X- wire data sets (Spring 2013 IOP 9)
53 Momentum flux comparison for Sonic and X- wire data sets (Spring 2013 IOP 9)
54 TKE Budget (correlation) terms
55 TKE Budget (correlation) terms
56 Normalized TKE budget terms
57 Normalized TKE budget terms
58 Heat flux budget (correlation) terms
59 Heat flux budget (correlation) terms
60 Streamwise energy spectra (u-spectra)
61 Summary & Conclusion Based on these two IOPs, very close to the surface, the contribution of buoyancy term is very minimal to the TKE energy budget followed by the turbulent transport of TKE term. Very close to the surface, the dissipation rate of TKE is dominating the other terms of the TKE budget where the shear production term is dominant away from the surface. The techniques and the issues associated with measuring the velocity and temperature fluctuations were addressed. An extensive quality control mechanism should be administered in order to obtain high fidelity data set. Its still a work in progress.
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