Journal of Fluid Mechanics | Year: 2010
Uniform suspensions of bottom-heavy, upswimming (gyrotactic) micro-organisms that are denser than water are unstable, through a gravitational mechanism first described by Pedley, Hill & Kessler (J. Fluid Mech., vol. 195, 1988, p. 223). Suspensions of downswimming, head-heavy cells do not experience this instability. In the absence of gravity, a uniform suspension of swimming micro-organisms may be unstable because of the particle stresses generated by the swimming cells themselves, each of which acts as a force-dipole or stresslet (Simha & Ramaswamy, Phys. Rev. Lett., vol. 89, 2002, p. 058101). The stresslet strength S is positive for pullers such as algae and negative for pushers such as bacteria or spermatozoa. In this paper, the combined problem is investigated, with attention being paid also to the effect of rotational diffusivity and to whether the probability density function f(e) for the cells' swimming direction e can be approximated as quasi-steady in calculations of the mean swimming direction, which arises in the cell conservation equation, and the particle stress tensor, which appears in the momentum equation. The existence of both the previous instabilities is confirmed at long wavelength. However, the non-quasi-steadiness of the orientation distribution means that the particle-stress-driven instability no longer arises for arbitrarily small |S|, in the Stokes limit, but requires that the dimensionless stresslet strength (proportional to the product of S and the basic state cell volume fraction no) exceed a critical value involving both viscosity and rotational diffusivity. In addition, a new mode of gravitational instability is found for head-heavy cells, even when they exert no particle stresses (S = 0), in the form of weakly growing waves. This is a consequence of unsteadiness in the mean swimming direction, together with non-zero fluid inertia. For realistic parameter values, however, viscosity is expected to suppress this instability. © 2010 Cambridge University Press.
Antonelli P.,CMS |
Marcati P.,University of LAquila
Archive for Rational Mechanics and Analysis | Year: 2012
In this paper we study global existence of weak solutions for the quantum hydrodynamics system in two-dimensional energy space. We do not require any additional regularity and/or smallness assumptions on the initial data. Our approach replaces the WKB formalism with a polar decomposition theory which is not limited by the presence of vacuum regions. In this way we set up a self consistent theory, based only on particle density and current density, which does not need to define velocity fields in the nodal regions. The mathematical techniques we use in this paper are based on uniform (with respect to the approximating parameter) Strichartz estimates and the local smoothing property. © 2011 Springer-Verlag.
News Article | March 22, 2016
“I’m a fan of supersymmetry, largely because it seems to be the only route by which gravity can be brought into the scheme. It’s probably not even enough, but it’s a way forward to get gravity involved. If you have supersymmetry, then there are more of these particles. That would be my favourite outcome.” -Peter Higgs The Standard Model, in its final form, came into being in the 1960s and 1970s. From the time it was proposed in 1964, it took approximately 50 years for all of the particles within to be discovered, culminating with the discovery of the Higgs Boson a few years ago. Yet a number of puzzles, including dark matter, neutrino masses and the matter-antimatter asymmetry, require new particles that aren’t found in the Standard Model to explain them. Scenarios like supersymmetry, extra dimensions and technicolor are perhaps the most interesting, because they might give rise to particles detectable at the LHC. Right now, both the CMS and ATLAS collaborations see excessive signals at ~750 GeV, about 5 times the mass of the Higgs boson. Could this be a new particle? Or, more likely, is it simply a fluctuation in our data given the limited statistics thus far? The smart money’s on the latter, but we won’t have to wait long: we’ll know for sure after the LHC restarts in May with just a few months of data! Go get the whole story on Forbes today.
News Article | March 20, 2016
Data from a Large Hadron Collider (LHC) experiment strengthens the possibility of discovering a new particle. In December 2015, experts were able to detect an excess gamma-ray photon pairs as a result of particle collisions at LHC. The said photons had a combined energy equivalent to about 750 gigaelectronvolts. Data about the mysterious particle from LHC's ATLAS and CMS had physicists talking and excited. New analysis presented by the CMS at a conference in Italy on Thursday made the new particle more statistically significant. However, data presented by ATLAS on the same day did not exhibit new information to the dismay of many. What more, the significance presented by ATLAS plummeted as the data was considered more conservative than the previous one. The data set used in the most recent CMS review is said to be 22 percent bigger than the one used in the December experiment. This is because the new one entailed collisions from the early part of LHC's 2015 run. Experts working at the CMS group repeated the regulation of the entire data set, which is something that experts perform at the end of each testing to see how radiation impacts measurements. With such interventions, the statistical significance of the CMS bump shoots up from 1.2 to 1.6 sigma. "The fact that their excess has gone up is a hopeful sign," says Marco Delmastro from the CNRS Theoretical Physics Laboratory in France. He, however, warns about taking necessary caution as statistics, he says is a "harsh mistress." Delmastro was one of ATLAS results presenters. More To Come More information is expected to come in the next few weeks as more discussions in La Thuile, Italy are yet to happen. If the excess will be proven to be a sign of a new particle, then it would most likely be a boson, which is a particle, connected with basic forces. ATLAS physicist Marumi Kado from the University of Paris says being able to confirm the existence of a new signal would be very interesting as it would pave the way for more knowledge and more questions. Then again, nothing is confirmed yet and as per previous experiments, the more data are collated, the more statistical bumps go away. For this, Kado says, caution is necessary. "Our job is to doubt — to always check for possible problems," he says.
It’s no secret, I love Markdown. But when it comes to collaborating with someone, Google Docs, Quip, Pages on iCloud.com and Word on Office.com are all rich-text editors. Meet Canvas, a real-time Markdown editor on the web and now on iOS. With this editor, I can see myself collaborating on stories with my coworkers and seamlessly put the result in our CMS. Markdown is a markup language for writers, especially those who write for the web. It lets you write and format text using plain text and easy-to-remember syntax. So instead of blindly relying on a rich-text editor and clutter your text with HTML syntax, Markdown is a nice alternative. In addition to that, many apps and services now support Markdown, making it a popular standard on the web. Canvas is a straightforward Markdown editor. When you start a document, you’re presented with a blank screen. You type the title of the document, hit enter and start writing. There’s no preview pane, no toolbar, nothing. This is what it looks like: I like this missing UI, it reminds me of many other Markdown editors I’ve been using. Canvas also automatically collapses the markup so that it doesn’t get in the way once you read back what you’ve written. For those not familiar with Markdown, a link in Markdown looks like this: . Canvas automatically collapses this link unless you click on the linked word. If you want to share a document with someone, simply copy and paste the URL in the address bar. By default, the other person will only be able to view the document, you can activate the edit features for guest by clicking on the gear icon in the corner. Canvas also added a few collaboration features. In addition to letting multiple people write at the same time, you can also add comments like in Google Docs. I already use Markdown for all my writing. On my Mac, iPhone and iPad, I use Ulysses and sync my articles between all these devices. With this setup, I can start writing a post on any device and pick it up on another. Ulysses recently added native WordPress exporting, making it even easier to publish my post on this website — TechCrunch runs on WordPress. But what if I want to write a story with a second person? I’ve relied on Google Docs until now. Start a document, share it with a fellow writer, write the article, then copy and paste the document into Ulysses, format it and export it to WordPress. With Canvas, I can replace Google Docs and skip a step. Canvas lets you copy and paste Markdown by selecting text in your document. You can also view your file as raw Markdown by adding at the end of the URL, or export it to HTML by adding . Unfortunately, Canvas renders the HTML in the browser so you have to view the source of the page to select the HTML. I hope they’ll add a way to copy and paste the HTML formatting directly on this export screen. WordPress supports Markdown and HTML, so I can just copy and paste the Canvas document into WordPress once I’m down writing it. As for the new mobile app, it works exactly the same way. You can view and edit your files in real time from your iPhone or iPad. You can then export them using the standard iOS share sheet. It’s a bit harder to write in Markdown on a phone as you have to hunt for some special characters, so Canvas should add a shortcut toolbar above the keyboard to make this easier. You can already format a paragraph by swiping left and write, but I couldn’t find any gesture to italicize or add links. I’m quite impressed with Canvas and I plan to use it. It’s a solid collaborative text editor and a good addition to my writing setup. Of course, many people don’t want to deal with Markdown. But for the nerdy writers like me who find plain text more powerful than a Word document, this one is for you.