Showing posts with label Aerospace. Show all posts
Tweet Share Share Share Share Share

December 21, 2021

 


A human-built spacecraft has swooped in and made contact with the Sun, marking a historic milestone.

NASA's Parker Solar Probe passed into and through the solar corona, the Sun's upper atmosphere, on April 28, 2021. Not only did it survive – showing the effectiveness of Parker's high-tech heat shielding – but it also took in situ measurements, providing us with a trove of never-before-seen data on our Solar System's core.

"The Parker Solar Probe 'touching the Sun' is a watershed moment for solar science and a really extraordinary achievement," said astrophysicist Thomas Zurbuchen, associate administrator for NASA Headquarters' Science Mission Directorate.

"Not only does this achievement provide us a better understanding of our Sun's evolution and its effects on our Solar System, but everything we learn about our own star tells us more about stars throughout the Universe."

The Parker Solar Probe was launched in 2018 with the primary goal of studying the solar corona. It should make a total of 26 close approaches, or perihelions, to the Sun over the course of its seven-year mission, employing a total of seven gravity assist manoeuvres from Venus to bring it closer. The perihelion in April was the seventh and first to penetrate the corona.

Parker recorded variations in the Sun's magnetic field and sampled particles during his nearly five-hour stay inside the solar atmosphere. Previously, we depended on external data to estimate these qualities.



"Flying so near to the Sun, the Parker Solar Probe now detects circumstances in the magnetically dominated layer of the solar atmosphere - the corona – that we've never been able to detect before," said astronomer Nour Raouafi of the Johns Hopkins Applied Physics Laboratory.

"Magnetic field measurements, solar wind data, and photos all provide evidence of being in the corona. The spacecraft can be seen travelling through coronal structures that can be seen during a total solar eclipse."

Above: Coronal streamers, which can only be viewed from Earth during an eclipse, are the brilliant features shown in these images. The Parker probe captured these images during the ninth perihelion in August of this year.

There is no solid surface on the Sun. Instead, the Alfvén critical surface, where gravity and the Sun's magnetic fields are too weak to hold the solar plasma, defines its border.

Above this point, the solar wind appears, sweeping powerfully through the Solar System and breaking away from the Sun in waves. The photosphere, which is made up of churning convection cells and plasma, is much below what we call the Sun's'surface.'

Parker's purpose was to learn more about the Alfvén critical surface, such as where it is and what its topography is like, because we didn't know anything about it. The Alfvén critical surface was estimated to be between 10 and 20 solar radii from the Sun's centre. Parker reached the corona at a distance of 19.7 solar radii and sank as low as 18.4 solar radii throughout its corona journey.

Surprisingly, the probe only encountered the corona's magnetic conditions on a sporadic basis, implying that the Alfvén critical surface is wrinkled. Parker came across a magnetic structure known as a pseudostreamer at a lower depth, which we can see arcing out from the Sun during solar eclipses. Parker's findings show that these structures are to blame for the Alfvén critical surface's deformation, albeit we don't know why.

Conditions were quieter inside the pseudostreamer than in the surrounding solar environment. Particles were no longer as chaotically buffeting the spaceship, and the magnetic field was more ordered.

Parker also looked on the occurrence of solar switchbacks. These are Z-shaped kinks in the solar wind's magnetic field, and it's unclear where or how they develop. Switchbacks have been around since the 1990s, but it wasn't until Parker examined them in 2019 that we discovered how ubiquitous they are. The data from the probe's sixth flyover revealed that switchbacks are caused by patches.

Parker has now discovered them within the solar atmosphere, indicating that at least some of the switchbacks originate in the lower corona.

"The structure of the switchback zones lines up with a small magnetic funnel structure at the base of the corona," astronomer Stuart Bale of the University of California, Berkeley, stated. "This is what some theories predict, and it identifies a source for the solar wind itself."

We still don't know how these strange structures came to be, but with dozens more perihelions on the way, some as close as 9.86 solar radii from the Sun's centre, we're sure to find out.

"

We've been studying the Sun and its corona for decades, and we know there's some fascinating physics at work to heat and accelerate the solar wind plasma. We still don't know exactly what that physics is "Raouafi explained.

"With the Parker Solar Probe now travelling towards the magnetically dominated corona, we will finally gain some answers about how this mysterious region works."


Read more »
Tweet Share Share Share Share Share

March 19, 2020
If astronauts reach the moon according to NASA’s Artemis project plan, one of their main goals is to mine ice in the crater near the moon’s South Pole. However, they will need to accurately navigate to the site. They already have equipment such as landing ships, lunar vehicles, drilling equipment, and supply vehicles. However, on the moon’s orbit or on a very strange surface of the moon, they need to know their position in real-time and accurately. Of course, they will need a GPS for this. Nevertheless, will a GPS work accurately? 


On Earth, global positioning systems (GPS) have changed our lives. A number of countries operate a large number of satellites that help people navigate in many ways. On Earth, GPS can pinpoint locations in centimeters. Can it help astronauts land on the moon?

YES – GPS WILL WORK ON THE MOON

Zhang Jiaming and Li Charles of the NASA Jet Propulsion Laboratory performed some mathematical calculations on the possibility. Mathematically and theoretically, YES, a GPS will work accurately in the moon. Signals from existing global navigation satellites near Earth can be used to navigate astronauts on the moon 385,000 kilometers away.


They mapped the orbits of navigation satellites from the US Global Positioning System and Galileo in Europe and GLONASS in Russia, for a total of 81 satellites. Most of them have directional antennas pointing to the surface of the earth, but their signals also radiate into space. The researchers said the signals were strong enough to be read by a spacecraft with a fairly compact receiver near the moon. They calculated that the spacecraft in lunar orbit would be able to “see” the signals of 5 to 13 satellites at any given time. At this distance, the positioning accuracy is about 200 to 300 meters. In computer simulation, they can continue to implement various methods to improve accuracy. NASA scientists believe that relay satellites in the lunar orbit as a positioning beacon.
Read more »
Tweet Share Share Share Share Share

January 04, 2020

Flutter, and aeroelasticity in general, is a topic that is often misunderstood or incorrectly applied due to its inherent complexity.



To start, let’s first consider what flutter is; an instability due to an interaction between aerodynamic, inertial, and elastic forces.
Although this type of instability is not unique to aircraft, the instability was initially investigated due to its prominence in aircraft. As with a normal modes (free-vibration) analysis, to carry out a flutter analysis we require a global mass matrix (inertial forces) and a global stiffness matrix (elastic forces), which can be seen in the normal modes equations of motion:
The additional piece we then require when conducting an aeroelastic analysis is the influence from the unsteady aerodynamics present in the problem. These are generated in NASTRAN by default using the Doublet-Lattice Method (DLM), which is the unsteady corollary to the Vortex-Lattice Method (VLM). When using this approach, a 2D planform surface is generated and discretized as seen below, and can be used to generate the Aerodynamic Influence Coefficients (AIC’s).
With these three distinct forces modeled, the flutter problem is often solved iteratively as an eigenvalue problem at all prescribed free-stream air speeds. The results include the damping and associated frequencies of each mode shape at each airspeed. When the damping of one of these modes becomes positive, the first flutter speed has been found.

1.What are Splines and how do they work?

When generating the AIC’s for a particular model, there is a slight problem that must be resolved. The structural model degrees of freedom (DOF) and the aerodynamic DOF are not the same.
The hallmark of aeroelasticity in general is the interaction between aerodynamics and structural forces, so there must be a way to freely pass information from one system to another (such as displacements and forces). This is most commonly achieved using splines:
The default spline (surface) used in NASTRAN is the infinite plate spline in which the differential equation of an infinite plate under a point load is solved at all of the structural DOF and then the N linear solutions are superimposed on one another. What this means is that when constructing splines to connect the aerodynamic and structural DOF, reducing regions connected by splines can drastically reduce the computational time required.
For example, take a straight rectangular wing with no control surfaces. In terms of accuracy, there is very little difference between using 1 spline to connect the aerodynamic and structural DOF, and dividing the wing into two regions (such as an outer wing and an inner wing section) and using two splines. Splitting it up into two regions and using two splines will drastically reduce the computational time required to generate the splines, and by proxy, the run of the aeroelastic analysis in general.

2. You can detect Divergence in a flutter analysis!

When conducting an aeroelastic analysis, it is actually possible to also detect the divergence speed as well. This might seem strange as divergence is a static aeroelastic instability and flutter analysis in inherently a dynamic stability analysis. When the AIC’s are generated using the DLM, the quasi-steady AIC terms (when the reduced frequency k=0) are actually generated using VLM. When conducting a divergence analysis, all that is needed is information about the elastic forces, and the quasi-steady aerodynamic forces.
Since this information is already included in the matrices required to run a classical flutter analysis, it stands to reason that we should be able to detect divergence as well. This explanation is hand waving at best from an academic perspective, but sufficient for the user interested in the application of flutter analysis.
In order to detect divergence, we must look at the vibration frequency of the modes. Notice in the figure below that mode 1 frequency goes to zero at some point. With the behavior of this mode becoming steady, you would then have to go to the damping plot to observe when the mode 1 damping became positive, signaling divergence.

3. What are the Mach Number and Reduced Frequency and why do they matter?

Two very important numbers that appear repeatedly in flutter analysis are the Mach number and reduced frequency. When implementing a flutter analysis, these are actually the only two numbers required to generate your AIC’s once the aerodynamic model has been generated. Many are familiar with the Mach number as the free-stream air speed over the speed of sound.
For subsonic flutter analysis, the Mach number is used to account for compressibility effect between 0.3<M<0.8, which is nicely illustrated in the following picture taken from a lecture series from the Mechanical and Aerospace Engineering Department of the Florida Institute of Technology:

As the Mach number increases, the aerodynamic forces are also scaled up. This in turn can greatly affect the results of a flutter analysis in the compressible subsonic flight regime.
The reduced frequency is a much subtler parameter. The reduced frequency is calculated using the following:
where ω is the circular frequency, b is the half chord, and U is the free-stream velocity. The reduced frequency is really an indication of how unsteady the behavior of the system is. As the reduced frequency approaches 0, the behavior of that corresponding mode approaches steady behavior (such as divergence). As one might expect as the reduced frequency grows larger, the behavior of a mode is more unsteady. As a rule of thumb, a reduced frequency larger than 1 corresponds to highly unstable behavior. In addition, as the reduced frequency increases indicating more unsteady behavior, the lift is reduced and the phase lag between the lift and the dynamic motion of the structure also increases.

4. Modeling Body Freedom Flutter is Easy!

When conducting flutter analysis on a wing, it is common to simply constrain the root of the wing to be fixed. When conducting a flutter analysis on a full aircraft however, it is very important to consider how the dynamics of the rigid body motion can also couple with the structural and aerodynamics of the model. What this means is that you can end up with a lower flutter speed than otherwise predicted if the aircraft were fixed in place.
This type of behavior is particularly apparent in the Air Force Research Lab (AFRL) Body Freedom Flutter (BFF) aircraft, X-56A, seen below in a picture from the NASA website:
In order to test control surface flutter suppression techniques, this blended wing aircraft was designed to exhibit body freedom flutter. For more conventional design aircraft, body freedom flutter tends to appear less frequently, however it should not be dismissed. Below is what this vehicle looks like when it reaches its flutter speed:
When using NASTRAN’s solution 145, body freedom flutter can be incorporated by simply running a model without any constraints.  This allows the six ~0 Hz (rigid body) modes to couple with the elastic modes.

5. What can you achieve with Aeroelastic Tailoring?

One of the benefits of using composite materials in an aircraft’s design is the possibility to aeroelastically tailor the loads and behavior of the aircraft. This can actually be done on traditional metallic aircraft structures such as by moving the shear center, however since stiffness of metals is uniform tailoring metallic aircraft structures is more difficult. When using composites, the stiffness of a structure can be manipulated by simply changing the ply angles within the structure, rather than changing the physical geometry.
Typically, aeroelastic tailoring is associated with the tailoring of a wings lift distribution or to allow for better wing gust response, however aeroelastic tailoring can also be used to manipulate the flutter speed and divergence speed of an aircraft. One of the simplest examples of this kind of tailoring was explored by Dr. Patil in 1997 using a composite box beam:
By changing the fiber angles of the four sides of the box beam, he was able to show how the flutter and divergence speed of a wing using this structure changes parametrically as a function of ply angle:
Read more »
Subscribe to: Posts (Atom)
Powered by Blogger.