Design of a Thermal Anemometer for a Titan Lander Colin F

Lunar and Planetary Science XLVIII (2017) 1859.pdf

Design of a Thermal Anemometer for a Titan Lander Colin F. Wilson1 Ralph D. Lorenz2, 1 Department of Physics, University of Oxford, Oxford, UK ([email protected]) 2 Space Exploration Sector, JHU Applied Physics Laboratory, Laurel, MD 20723, USA. ([email protected])

Introduction: Weather on Titan is of great inter- ExoMars EDM in 2016, with the same geometry and est: Titan has an active hydrological cycle just like only minor changes to film resistances to optimize Earth's (but different!) and its geomorphology reflects power efficiency [6]. modification not only in the wind-mediated distribution of liquid methane on the surface, but also by the vast dunefields that girdle Titan's equator. Any landed mission to Titan (e.g. [1]) is certain to demand wind meas- urement capability. Almost all planetary anemometers to date are those deployed on Mars, and most of these have been thermal anemometers due to their simplicity of construc- tion and lack of moving parts. Calibration of thermal wind sensors on Mars is notoriously tricky: neither Pathfinder [2] and MSL [3] anemometers have met their performance goals, with much of their data still uncalibrated. However, the thermophysical characteris- tics of Titan’s atmosphere [4] make it much better suit- Fig. 1 – Beagle 2 Wind Sensor (B2WS). Central cylin- ed for thermal anemometry than that of Mars, in sever- der is 10 mm in diameter x 18 mm high [5]. al ways: (1) its higher density leads to higher convective heat transfer coefficients, leading to better wind Flow regime for a Titan thermal wind sensor: speed sensitivity; (2) Temperature fluctuations in Ti- We consider first the heat transfer from a uniformly tan’s near surface atmosphere are < 0.1 K [4] (unlike heated cylinder with the same dimensions as B2WS, 10 Mars where air temperatures can change by 10 K in 10 mm in diameter x 18 mm high, in typical flow condi- seconds in daytime turbulence); (3) thermal IR radia- tions on Earth, Mars and Titan; these results are shown tive fluxes are very low due to the low environmental in Table 1. Table 1 includes calculations of Reynolds temperature of only 93 K, leading to reduced uncer- and Nusselt numbers, which are dimensionless repre- tainties. We therefore assess performance of a heritage thermal wind sensor for Titan, based on that flown on the Beagle 2 & Schiaparelli Mars landers.

Beagle 2 / Schiaparelli Wind Sensor (B2WS): A lightweight Mars wind sensor was designed for the exceptionally resource-constrained Beagle-2 lander which was delivered to Mars in 2003 [5]. This em- ployed the basic principle of thermal anemometry, as used previously on Viking and Pathfinder, but ensuring an adequate overheat to avoid the inaccuracies associ- ated with strongly-heated turbulent daytime conditions that challenged interpretation of the Pathfinder data. Electrical power is dissipated in three platinum film heaters which are distributed around a 1 cm Rohacell foam cylinder; the films’ resistances are measured, allowing their temperatures, and therefore the corre- sponding convective heat transfer coefficients, to be calculated. The combination of the three film temperatures allows a two-dimensional wind vector to be measured. The system was reflown as part of the DREAMS meteorology package on the Schiaparelli Lunar and Planetary Science XLVIII (2017) 1859.pdf

sentations of wind speed and convective heat transfer An RHU-powered Sensor: Electrical power is ex- coefficient. Earth atmospheric conditions, at 20°C, are pected to be severely constrained on any future Titan included to demonstrate the ease with which Titan in-situ mission (for which solar power is impractica- wind conditions can be simulated in a standard 0 – 20 ble). For high-duty-cycle meteorological measure- m/s wind tunnel on Earth, with 1:1 full scale models. ments, it may be desirable to avoid electrical heating The Titan flow around the sensor differs important- demands, and thus we consider a sensor heated by a ly from Mars, due to the higher air density. The Reyn- RadioIsotope Heater Unit (RHU). These units – of olds numbers of 8000 – 16000 for wind speeds of 1 – 2 which 35 were flown on Huygens – are cylindrical, m/s on Titan correspond to a subcritical flow regime: with a diameter of 26 mm and height of 32 mm, and flow on the upwind side will be laminar, and on the provide about 0.8W of thermal output depending on downstream a turbulent wake will develop. For Reyn- the fuel blend and age [8]. olds numbers < 20,000, the convective heat transfer One anemometry approach is to place three tem- efficiency is markedly higher on the upstream face of perature sensors on the cylindrical surface of the RHU. the cylinder than on the downstream face. This is im- The high thermal conductivity of the RHU shell (it is portant, because this difference allows wind direction made of a carbon-carbon composite) leads to small sensing. If the cylinder diameter were to be increased temperature differences across the RHU. To improve further, Reynolds numbers would increase prortionally, sensitivity to wind direction, an external layer with low and the downwind side would experience increased thermal conductivity, of 5 – 10 mm thickness, may be wake turbulence and correspondingly higher convec- applied around the RHU; this will be assessed during tive heat transfer, as shown in Figure 2; this would wind tunnel testing. The discussion above of Reynolds make wind direction sensing progressively more diffi- numbers shows that although this design would provide cult (although wind speed sensing would be unaffect- a good sensitivity to wind speed, its diameter of ≥26 ed). This leads us to a preliminary conclusion that a mm would put it in a Reynolds number regime with cylindrical anemometer should be 1 cm or less in di- reduced sensitivity to wind direction, particularly at ameter for optimum wind direction sensitivity. higher wind speeds, so an additional sensor such as a thermal wake sensor might be required. A second approach would be to use a 1 cm B2WS- like cylindrical sensor, with a thermally conductive rod in its centre, directly heated by an RHU, as a heat source. For example, this could be a 5 mm diameter rod in the centre of a 10 mm cylinder. Our preliminary calculations suggest that this would provide good sensitivity to both wind speed and direction.

Conclusion: The environment of Titan is particu- Fig. 2 – Distribution of local heat transfer coefficients larly well-suited for thermal anemometry. A compact (expressed as dimensionless Nusselt number Nu), for anemometer design similar to that used for Beagle 2 different Reynolds numbers Re. From Ref. [7]. and Schiaparelli landers would work well on Titan, whether heated electrically or by a radioisotopic heat Table 1 also includes calculations of mean convec- source. tive and radiative heat flows, with the assumption of uniform surface temperature. This illustrates that the References: radiative loads, which can provide a considerable [1] Turtle, E. P. et al., LPSC 2017. source of uncertainty on Mars, are negligle on Titan [2] J.R. Murphy et al., DPS 2002. due to the cold environmental temperature. The over- [3] M. de la Torre Juarez et al., AGU 2015. heat (Tfilm – Tair) of 80 K specified for the Mars case is [4] Lorenz, R. D. AIAA J. Thermophys. Heat Trans- typical of the B2WS; a far lower value of 20 K can be fer, 30, 257-265, 2016 used for the much more isothermal environment of [5] Towner et al., Planetary and Space Science 52, Titan. It should also be noted that, for a B2WS-like 1141-1156, 2004. design with localized heating, heat flows will be signif- [6] F. Esposito et al., Space Sci Rev 2017 under re- icantly less than those listed in this table because tem- view. peratures drop significantly between the heated films – [7] Lohrisch, W.: V.D.I.-Forschungsheft Nr. 322 nevertheless this table provides a useful starting point (1929). for design calculations. [8] https://solarsystem.nasa.gov/rps/rhu.cfm