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Why do we need a VTOL aircraft to measure the parameters of the Martian planetary boundary layer?




One of the scientific missions we envisage for our aircraft is the measurement of the Martian atmospheric boundary layer parameters.


By definition, the planetary boundary layer is the region of the lower atmosphere where the planet’s surface strongly influences the atmospheric temperature, moisture, and wind through turbulent transfer. For Mars, the planetary boundary layer may reach up to 10 km above the surface (as opposed to Earth, 1-3 km).


A clear and quantitative description of the way in which the atmosphere interacts with the surface on Mars is important for a better understanding of the past, present, and future Martian environment. Also, this will enable scientists to make reliable predictions of the environmental conditions encountered during spacecraft entries and operations for mission safety and efficient design.


In our design and analysis work related to the LEMFEV project, we extensively use the Martian Climate Database [1], which, according to the validation documentation [2], provides reliable data for a variety of parameters of the Martian atmosphere, including dust concentration in different scenarios, icing, and wind components. The figure below shows how accurately the MCD predicts even anomalies – in this case, wind velocities and direction observed by Insight during the period Ls=325°-0°-200° compared to the corresponding MCD6.1 predictions (climatology scenario) [2].



 

The question arises, why does one need to undertake an expensive and high-risk campaign to measure the parameters that may be modeled on the computer?


As far as we know, the existing Martian climate numerical models are based on the solution of the Navier-Stokes (NS) equations which express in mathematical form the flow continuity, as well as the conservation of momentum and energy. The NS equations completely resolve the aerodynamics of fluids (except for chemical-reaction effects at high temperatures); unfortunately, they cannot be analytically solved for any useful flow conditions. For this reason, they are sometimes described as ‘some of the nastiest differential equations in theoretical physics’ [3], primarily due to the difficulty in mathematically analyzing turbulence (which occurs at a molecular level and requires billions of molecule-sized grids).


The history of theoretical aerodynamics to date can largely be described as a quest for solvable simplifications of the NS equations, with the turbulence being handled with some type of separate statistically calibrated model apart from the NS solution. Unfortunately, any simplification introduces some errors in the simulation result.


In general, CFD codes tend to produce reasonable-looking flow fields and pressures, but sometimes the integration of the calculated pressures yields forces that do not match experience, especially when separated flow is present. Reproduction of experimental data requires extensive "calibration" (i.e., fudging!) of the turbulence model. For this reason, CFD results are always somewhat suspect until the code has been checked against experimental data.


Overall, the sources of errors in computational flow simulation codes include those related to how the flow is described mathematically (flow model), discretization errors, residual errors, as well as round off errors [4].


Currently, the prediction of turbulence remains the toughest challenge for CFD, and CFD does not replace experimentation yet.


As a consequence, we are always expected to verify the numerical results.


Returning to modeling the Marian environment, it seems additionally that some physical processes shaping the atmosphere and climate of Mars are still to be clarified [5]. The better we understand the physical phenomenon, the better we model it numerically.


Now, what about in situ measurements used to validate the MCD?


Currently, direct observational measurements within the Martian boundary layer remain relatively sparse, and the vast majority of in situ measurements on Mars have been obtained at altitudes slightly higher than 1 m.


However, for example, to accurately measure turbulence, one has to resolve the entirety of turbulent scales, which can range from the smallest dynamically important scales (in the order of millimeters) to the largest turbulent scales (in the order of the atmospheric boundary layer thickness) within the entire planetary boundary layer.


On Earth, turbulence data is frequently obtained in the form of temporal information through cup and sonic anemometers, which have a temporal response of 1–2 and 20 Hz respectively and a spatial resolution of centimeters [6]. In this way, we watch the eddies drift by the sensor.


To translate this temporal information into spatial information, Taylor’s frozen flow hypothesis is commonly invoked using some suitably selected convection velocity (typically the local mean velocity). Taylor’s hypothesis says that we can assume that the turbulent eddies do not significantly evolve or are "frozen" as they pass the sensor, and thus the change in flow parameters within each eddy is negligible. Taylor’s hypothesis has been found to work reasonably well for the smallest scales of turbulence, but it is generally accepted to be in error for larger-scale, long-wavelength motions.


Based on the above, for us, it seems that there is plenty of work for a VTOL UAV here. The use of such a probe to conduct measurements in the atmospheric boundary layer has the potential to address the need to obtain a spatial description of the structure and organization of turbulence, to mention just one atmospheric parameter. In addition, within, let’s say, a 15-minute period of quasi-stationarity flight within the atmospheric boundary layer, this UAV will be able to collect substantially more data than a fixed-point measurement, which requires the turbulence to pass the measurement point. Finally, a UAV also has an advantage over fixed towers (and orbiters, as well as surface-based probes on Mars!)  in terms of portability and the potential to measure in locations and altitudes where the use of other types of probes is prohibitive.


Therefore, future instrumental campaigns related to the study of the Martian boundary layer may need to focus on a series of objectives, including the following:

  • the expansion of the geographical and temporal coverage of measurements in the planetary boundary layer;

  • the vertical structure measurements that are required to follow strong temporal variations anticipated in this region of the atmosphere on diurnal, synoptic (2–20 sols), seasonal, and interannual timescales [7].


Does it sound persuasive?

 

[3] Anderson, John David. “Fundamentals of Aerodynamics.” (1984).

[4] AGARD, report 783

[5] Murray, J.; Tartabini, P. Development of a Mars airplane entry, descent, and flight trajectory. In Proceedings of the 39th Aerospace Sciences Meeting and Exhibit, Reno, NV, USA, 8–11 January 2001. https://doi.org/10.2514/6.2001-839.

[6] Witte, B., F.S., S., & Bailey, S. (2017). Development of an Unmanned Aerial Vehicle for the Measurement of Turbulence in the Atmospheric Boundary Layer. MDPI Atmosphere. DOI: 10.3390/atmos8100195

[7] Petrosyan, A.; Galperin, B.; Larsen, S.E.; Lewis, S.R.; Määttänen, A.; Read, P.L.; Rennó, N.; Rogberg, L.P.; Savijärvi, H.; Siili, T.; et al. The Martian atmospheric boundary layer. Rev. Geophys. 2011, 49, RG3005. https://doi.org/10.1029/2010RG000351.


 
 
 

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