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Palo Alto, CA, United States

Most CFD tunnel fire simulations have so far focused on the thermal field and the critical velocity for suppression of the hot backlayering flow. However, there is a great need in understanding the characteristics of a real-scale tunnel fire in terms of the flame propagation and the toxic gas generation. In this study, an extension of the eddy dissipation concept incorporating two chemical reaction steps is integrated into an internationally recognised CFD fire simulation code. A full-scale over-ventilated tunnel fire is simulated with the model and the measured temperature profiles are correctly reproduced. The model is then used to investigate the characteristics of a tunnel fire in three aspects: the length of backlayering, the flame length and the effects of an object within the tunnel. An inverse dependence of the backlayering length with the wind velocity, and the levelling-off of a maximum of the backlayering length for a ventilation velocity of 2 m/s are predicted. Both the CFD prediction and an analytical correlation indicate that an increase of the heat generation rate enhances the flame length, and while, a growth of the ventilation velocity shortens the flame length. The effects of blockage in a tunnel on the propagation of smoke and fire are numerically investigated with the model. The results of this study have shown that the model is promising in qualitatively examining the correlation of the propagation of smoke, the production and the transport of carbon monoxide and soot with the ventilation rate and the fire heat release rate. © 2011 Elsevier Ltd. All rights reserved. Source


Longere P.,CNRS Clement Ader Institute | Dragon A.,Teleport
Engineering Fracture Mechanics | Year: 2013

Starting from experimental observations on hat shape samples of a Titanium alloy of Ti6Al4V family showing that the failure of ductile materials under low triaxiality may result (at least partially) from void growth, the present work develops an approach based on a damage related softening mechanism acting as a kinematic mean stress drop causing a shift of the yield locus centre. For micro-porous viscoplastic material described via micromechanics based elliptic potentials (and obeying the normality rule) this 'back mean stress' concept allows reproducing the inelastic dilatancy due to void growth, in addition to the isochoric plastic deformation due to dislocation glide, at zero and low negative stress triaxiality. The approach is applied to the well-known GTN model and the 3D constitutive equations are implemented as user material in the engineering finite element computation code Abaqus®. Numerical simulations are conducted considering a single finite element under simple shear loading and a hat shape specimen under shear-compression loading. Numerical results are compared with experimental ones. © 2012 Elsevier Ltd. Source


De Saxce G.,Lille Laboratory of Mechanics | Vallee C.,Teleport
Journal of Elasticity | Year: 2013

In this paper, we study the geometric structure of the 2D elasticity tensor space using the representation theory of linear groups. We use Kelvin's notation system in which O(2) acts on the 2D stress tensors as subgroup of O(3). We present the method in the simple case of the stress tensors and we recover Mohr's circle construction. Next, we apply it to the elasticity tensors. We explicitly provide a linear frame of the elastic tensor space in which the representation of the rotation group is decomposed into irreductible subspaces. Thanks to five independent invariants chosen among six, an elasticity tensor in 2D can be represented by a closed line or, in degenerated cases, by a circle or a point. The elasticity tensor space, parameterized with these invariants, consists in the union of a manifold of dimension 5, two volumes and a surface. The complete description requires five polynomial invariants, two linear, two quadratic and one cubic. We reveal the physical and geometrical meaning of these invariants and we propose a simple method to determine the elastic behaviour of an anisotropic material of which the symmetry is not known a priori, thanks to invariant measures of the lack of symmetry with respect to class of materials. © 2012 Springer Science+Business Media B.V. Source


It’s a big hairy vision — knowledge-workers freely migrate around the globe, and city and federal governments compete to lure them. Teleport, a startup founded by early Skype employees around the idea of supporting an increasingly global mobile workforce, is taking another step toward that reality today. It’s launching a set of iOS and Android apps that help tech workers find the best place to live out of 100 startup-friendly cities around the world. You articulate your preferences around the cost of living and other quality-of-life measures like traffic and pollution and the healthcare system, and it will help identify cities that best match your requirements. Right now, the app accounts for factors, such as housing and accommodation (especially markets with fast-moving rental markets), startup job availability and salary offers, cost of food and entertainment, the vibrancy of the startup scene and the investment climate, the time and flight distance from other places you need to visit often, climate preferences, local language requirements, safety and security, quality of Internet, the education system, and individual and corporate tax levels. Right now, the app is targeted at startup workers who can easily change their location, but the focus will expand later. It’s best for people moving alone or with families, although the company is building out features for groups and teams on the move. With this launch, Teleport will also have “Scouts” and partner governments that can help tech workers plan their moves. The idea over the long run is to have domestic and municipal governments compete with the best mix of incentives and regulations to attract knowledge workers. Teleport’s “Scouts” help users get in touch with governments and programs to facilitate quick and easy transitions. The first partner city governments include Tallinn, Estonia and Helsinki, Finland. Estonia is a small Baltic state that has been a pioneer in building tech-savvy government practices. They launched e-residency cards last year that let foreigners easily set up businesses in the country. Finland has also been aggressive in bolstering its local startup system and is attracting international knowledge-workers, especially after the decline of longstanding mobile phone giant Nokia. The startup is run by Sten Tamkivi who ran Skype’s first research and development lab and held a number of key engineering roles there over seven years. His co-founder, Silver Keskküla, was Skype’s first researcher and optimized the company’s peer-to-peer network topologies using machine learning and artificial intelligence. A digital nomad himself, he’s studied at Tsinghua University in Beijing along with Stanford and MIT. The last co-founder is Balaji Srinivasan, a board partner at Andreessen Horowitz who has written about the way the Internet could lead to the formation of cloud communities or even cloud countries as like-minded people freely migrate and congregate around the world. Mirroring its philosophy, the company operates in a distributed way with employees in Palo Alto, Tallinn, Munich, Bern and roaming employees in Colombia and the United Kingdom. They have raised $2.5 million in seed funding from Andreessen Horowitz, SV Angel and Seedcamp, as well as angel investors including Jeff Dean, Jaan Tallinn, Scott and Cyan Banister and Rain Rannu.


News Article | January 18, 2015
Site: techcrunch.com

Silicon Valley generally believes it’s the ideal place to be for all software developers. Against the backdrop of the Valley’s well documented success, Paul Graham wrote an essay calling for the US government to let the 95% of great software developers not in Silicon Valley join the 5% who already are. This immigration discussion has a tendency to become patronizing, to be about “allowing” some people in and “keeping” others out. In my experience from building Skype’s 10 offices across 16 timezones, clearly top tech talent needs to be chased and courted, not just “allowed” to move anywhere. So it was refreshing to read Graham nailing this nuance: “… this whole discussion has taken something for granted: that if we let more great programmers into the US, they’ll want to come.” With an increasingly global map of startup hubs, Silicon Valley needs to look not just at the factors bringing talent in, but also the negative factors diverting talent elsewhere. When a hub’s defining trait is optimism, is any time spent thinking of the darker side of foreign founders’ life in Silicon Valley? Reasons abound for international entrepreneurs and top technical talent to stay away from Silicon Valley and build their startup somewhere else. While the visa issues raised by Paul Graham and many other tech luminaries are real, there is more nuance to them than just raising quotas. As a foreign founder, just one look at this welcoming government website might scare you. It takes time to master this alphabet soup of H and O and E visa options. What is much harder to describe is the sense of suspicion still ahead. When you’re trying to get to the Land of the Free with an explanation about creating products and jobs, be prepared for questioning looks all the way through your visa interviews, from the border guard and that other, frequent and unexplained secondary inspection borderguard. It doesn’t feel like the people guarding the Valley know that inside the Valley “entrepreneur” is not just a French word for someone unemployed. There is no question that as a guest immigrants must always follow the rules of the house. By being careful, patient, and polite for half a year an incoming tech founder can eventually make it, jumping through bureaucratic hoops and mailing stacks of paper around (“because the electronic filing is less reliable”). Yet, the uncertainty, the sense of being at the mercy of an anonymous official never leaves. This alone can be enough for the US to start losing against some other nation states that are working hard to make it more than easy, but actually pleasant for founders to come. Sometimes the stars align. If you’ve got $50k to invest personally and another tenth of that to share with lawyers, co-founders from your home country who have majority control of your new corporation, and/or all your passports happen to be on the friendly list (which does not include the top engineering talent sources, China & India) your fresh E2 visa might eventually bring a rare smile on the border guard’s face. But if you’re a 21-year-old geek looking to build… a lot could be built with $55k and 6 months anywhere in the software world. That’s the thing about being a courteous guest: sometimes when you feel belittled by the host for a while, maybe it is just a hint to go back home or visit somewhere more welcoming? Now assume you finally get in. And, despite your love for Airbnb, you’re ready to settle down. If you really haven’t followed the dimensions of what renting (let alone buying) a place around Silicon Valley means today, take an hour to think along with Kim-Mai Cutler’s epic reporting on the systemic constraints behind San Francisco Bay Area housing. As a result, every day brings new stories of climbing through an open window to gain an edge at an open house; competitors for an overpriced two-bedroom bragging loudly about their Ivy League educations or a bunk bed in a hacker house costing $1500 per month. The situation is actually insane for the financial and emotional toll on entrepreneurs who are in Silicon Valley to build software, not apartments. There is also a less obvious time bomb for founders that is haunting me: how will all this play out for early stage funding sources. As Balaji Srinivasan of a16z has observed, roughly 50%+ of the capital allocated for early stage tech investments is actually flowing into Bay Area real estate, directly through office rentals and indirectly via home rentals as a primary driver of skyrocketing salaries. In a weird way early stage investors are shorting the booming real estate for themselves, i.e. as the rental prices rise they get even less of intended return on the tech investment they originally made (= less tech done per $). To buy a middle class house in the Bay Area you need to have already won with your startup. And frankly, you need to be winning something already to rent. It’s only natural that people in the Valley expect to make enough to be able to live here. Especially for an early stage software startup, the cost of people shadows everything else in their budget. For international founders, the mandatory intro on the expected burn is Danielle Morrill‘s Medium post. Bottom line: a million dollars puts a team of five people to work for a year. Fine on Facebook or Google margins, I guess, but I’ll leave it for you to calculate how many engineers you could convince to join your startup on TopTal or back home with a cool million in cash on the table. It’s not just the cost. Being the most vibrant and exciting startup ecosystem in the world has changed the cultural norms around corporate tenure. In most of the scene it seems to be OK to jump at every opportunity. To “get on the next rocket ship”, not when you’re proud of your contributions to the previous one (or when it fails) – but rather on the metronome beat of 1 year option vesting cliffs. Arguably it’s good for employees in some ways (though beware “Silicon Valley Syndrome”); inarguably bad for founders. So Silicon Valley is an extraordinarily competitive and expensive place to hire; the risks are high for you as a founder – and yet you have to hire faster than you ever have. Candidates are gone if you don’t catch them in a day. A data scientist (you know, “a statistician living in Silicon Valley“) candidate told me bluntly she would never spend three days of her life on our test task, if she could walk through 7 hours of interviews at LinkedIn or Google and have a great job by the evening. Most countries have a pretty clear threshold for how long you have to be physically present to become a subject to different laws and regulations. Take taxes for example. In Silicon Valley, beware: the American presence test rules that might be boolean in other places (“have you been here for more than 180 days this year?”) are instead continuous, rolling tests across multiple years with different state and federal standards. For instance I’ve been in a situation where the United States did not consider me a tax resident yet, but California happily claimed their share of my past global income. If your country has a double-taxation avoidance agreement with the US, then the Republic of California proudly doesn’t care – and couldn’t even sign an international treaty if they wanted to, because it’s not a country. All of this is rarely fun to figure out, especially if you’d actually like to spend time building a company. And you’ll get used to all this in the US, and such friction is immediately measurable in a hefty lawyer bill, too. My other pet peeve (maybe because I’m a digitally pampered citizen of Estonia), is the bureaucracy one has to live with in Silicon Valley just to be a good citizen or an operating business. You will actually use physical checks for payment. My driver’s licence is still being processed by the DMV over a year after passing my exams, because as I was told in a  voicemail, “the fax came out black and you should try to find another copy machine and send us a proper fax”. Even though I just mentioned three technological wonders much of the world considers a property of the 80s, it’s less amusing when you realize this is happening within a 30 minute drive from the home of the world’s best consumer software. I posed this question in a speech at Stanford a few years ago: what would the DMV look like if someone from Apple designed the retail experience? Why are these things from different planets? On a practical level as an entrepreneur, you have to be acutely aware of more than the laws and modus operandi of the country, but those of your state, county, town and sometimes even your zip code. If you have to be in the US for some reason, because of clients or partners, don’t default to the Valley presuming it’s the most innovation friendly locale. Take the time to understand what it means for you to choose  the Bay, over Austin or Seattle or Portland or New York. Just ponder the progress of self-driving cars being allowed in the streets or where 23andme will finally be allowed to innovate… and think twice before you hope to legally operate your bitcoins-by-drones startup from Los Altos. The competition for future-friendly regulatory environments is just beginning. And to finish on a vaguer critique on the quality of life, as Benedict Evans – my favourite Authentic Brit of Sand Hill Road – recently quipped: You’re unlikely to find someone ever complaining about the weather in Silicon Valley (except for drought), but recent immigrants often point out the monofunctional focus on the tech industry. From healthcare (to avoid going into a separate 5-page rant, just look at these numbers and know it is true, it will impact you and you will be seeing $20k bills for checking in on your headache if you’re not careful) to education – do not presume global competitiveness from any feature of society in Silicon Valley, save those which are directly related to building great tech companies. For some this might be perceived as a “lazy European” thing, for others the personal experience of actually burning out on 100 hour weeks a few startup cycles ago — in every entrepreneurial grind there’s the yearning for great architecture, theatres, music or a classical art museum to find inspiration and balance in life. It is true that San Francisco can offer a colorful lifestyle for those with the means (which is part of the reason it has drawn a new generation of startup people from the orchards of South Bay up to the city) but it’s no London, New York, Hong Kong or Paris when comparing many of the features these places are known for. I know, when starting up a new venture, there has to be a singular focus. But be honest with yourself about what you expect from the lifestyle for you and your family in five years too. Switching from problems to solutions, what you can do depends on your current relationship with Silicon Valley. If you’re an American citizen and a voter who cares about Silicon Valley being prosperous in a decade from now – if you have faith in the political system, maybe there is something in the above that you’d like to pick up with your Congressman, Governor, or an NGO like FWD.US. If you lack such faith, you might pick one such problem and try to address it through technology. Some of these issues are informational problems and amenable to such an approach. If you’re a foreign government dreaming of attracting talent to your own startup hub, stop talking all this nonsense of building “your own Silicon Valley”. Take the laundry list of the weaknesses above and start being 10X better than Silicon Valley in a specific area by changing antiquated e-governance, healthcare, creative arts, education, affordable housing, or immigration rules. Or, if you already are 10X better, find a way to communicate this advantage to the global community of mobile workers and startup entrepreneurs. We’re heading for a world where all governments will be competing for every citizen. The places that win the front line techies are positioned to win anyone else down the line. If you’re among the international 95% of amazing software developers and product designers, there is probably an implicit suggestion in this piece that Silicon Valley is your dream, but try to avoid crossing the bridge that takes you here alone. Have someone invite you. Many of these issues become someone else’s problem when you’re heavily recruited and handled by a human resources team at an influential tech corporation. Lone founders will envy you for having someone sponsor your visas and write checks for hotels during transition. It might well be worth to a few years of service at a “stable” corporation for that, before you get in the startup game. Which means a whole bunch of startups are going to be born somewhere else, by founders who don’t want to settle for a salary job first. And for the international founders who keep pondering if they should come to Silicon Valley or pick one of the other 100 startup hubs on Earth to win, you have three basic choices: Save the ticket money, and pick your own hub for success. Just look at Atomico’s Unicorn research: a full 61% of billion dollar tech companies come from outside the Valley. Why can’t you? This is a scenario reserved mostly for startups who don’t build software businesses or don’t care about the US as a market for their products. Of course, as software eats the world, you can argue every company would benefit from some Silicon Valley network connections as they grow, but they can still time the development of a physical presence. The US is not the top market for internet usage any more and competition is increasing. Europe has it’s own 580M clients to serve in an increasingly borderless digital market. Chinese e-commerce Unicorns, or African feature phone based mobile payment providers show how companies can grow without being in the US. As I wrote about Silicon Valley vs Europe last year, too many founders can’t even verbalize what is it that they need from the Valley. It probably makes sense to stay put before you know what you want to achieve. Sure, come for all the good things just be conscious of and prepared to spend real time & money on all the unpleasantries enumerated above. You can’t just brush off the hassles and pretend they don’t exist. The downside of Silicon Valley will potentially take more than 25% of your waking hours or 50% of your seed funding. As a founder it’s your call if your nascent startup can survive those distractions. As a person (or family) you can turn to software to optimize where you live, buty it’s your call if you can manage frugally enough.. As the structural issues of Silicon Valley intensify, I predict a mixed model of operating internationally will become a much more widespread practice among startups. That’s eventually the only way to solve the early stage startup paradox: you can’t afford to be in the Valley, but you can’t afford to not be in the Valley either. Startups should be in the Valley to network, raise money, meet partners, get incubated or accelerated. Personally, I would never trade my studies at Stanford or EIR period at Andreessen Horowitz for anything. Silicon Valley is the perfect place to learn from the best for any entrepreneur. That said, I have now picked other places to hire a team, write code, and build product. Teleport’s first 8 people are in 4 countries and my co-founder Silver is moving from Mountain View to Colombia this week. We are by no means special – you can see (and add to!) a growing list of multi-site startups on this crowdsourced spreadsheet. Startups with global ambitions need to accept that there is no single best place in the world to win, not even the almighty Silicon Valley. If you learn this early on you have the advantage of building your culture, your processes and systems to be able to handle the fluid, distributed and mobile work to come. As Matt Mullenweg responded to Paul Graham, that’s how you really get to the 95% of world’s tech talent, not by asking them to move to Silicon Valley. Editor’s note: Sten Tamkivi is the CEO and co-founder of Teleport. He was previously an early executive at Skype and an Entrepreneur in Residence at Andreessen Horowitz. Follow him on Twitter @seikatsu.

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