124 research outputs found
A direct unified wave-particle method for simulating non-equilibrium flows
In this work, the Navier-Stokes (NS) solver is combined with the Direct
simulation Monte Carlo (DSMC) solver in a direct way, under the wave-particle
formulation [J. Comput. Phys. 401, 108977 (2020)]. Different from the classical
domain decomposition method with buffer zone for overlap, in the proposed
direct unified wave-particle (DUWP) method, the NS solver is coupled with DSMC
solver on the level of algorithm. Automatically, in the rarefied flow regime,
the DSMC solver leads the simulation, while the NS solver leads the continuum
flow simulation. Thus advantages of accuracy and efficiency are both taken. At
internal flow regimes, like the transition flow regime, the method is accurate
as well because a kind of mesoscopic modeling is proposed in this work, which
gives the DUWP method the multi-scale property. Specifically, as to the
collision process, at , it is supposed that only single collision
happens, and the collision term of DSMC is just used. At , it is
derived that of particles should experience multiple
collisions, which will be absorbed into the wave part and calculated by the NS
solver. Then the DSMC and NS solver can be coupled in a direct and simple way,
bringing about multi-scale property. The governing equation is derived and
named as multi-scale Boltzmann equation. Different from the original
wave-particle method, in the proposed DUWP method, the wave-particle
formulation is no more restricted by the Boltzmann-BGK type model and the
enormous research findings of DSMC and NS solvers can be utilized into much
more complicated flows, like the thermochemical non-equilibrium flow. In this
work, one-dimensional cases in monatomic argon gas are preliminarily tested,
such as shock structures and Sod shock tubes
Shake-n-shack : enabling secure data exchange between Smart Wearables via handshakes
Since ancient Greece, handshaking has been commonly practiced between two people as a friendly gesture to express trust and respect, or form a mutual agreement. In this paper, we show that such physical contact can be used to bootstrap secure cyber contact between the smart devices worn by users. The key observation is that during handshaking, although belonged to two different users, the two hands involved in the shaking events are often rigidly connected, and therefore exhibit very similar motion patterns. We propose a novel Shake-n-Shack system, which harvests motion data during user handshaking from the wrist worn smart devices such as smartwatches or fitness bands, and exploits the matching motion patterns to generate symmetric keys on both parties. The generated keys can be then used to establish a secure communication channel for exchanging data between devices. This provides a much more natural and user-friendly alternative for many applications, e.g., exchanging/sharing contact details, friending on social networks, or even making payments, since it doesn't involve extra bespoke hardware, nor require the users to perform pre-defined gestures. We implement the proposed Shake-n-Shack 1 system on off-the-shelf smartwatches, and extensive evaluation shows that it can reliably generate 128-bit symmetric keys just after around 1s of handshaking (with success rate >99%), and is resilient to real-time mimicking attacks: in our experiments the Equal Error Rate (EER) is only 1.6% on average. We also show that the proposed Shake-n-Shack system can be extremely lightweight, and is able to run in-situ on the resource-constrained smartwatches without incurring excessive resource consumption
A gas-surface interaction algorithm for discrete velocity methods in predicting rarefied and multi-scale flows
The rarefied flow and multi-scale flow are crucial for the aerodynamic design
of spacecraft, ultra-low orbital vehicles and plumes. By introducing a discrete
velocity space, the discrete velocity method (DVM) and unified methods can
capture complex and non-equilibrium distribution functions and describe flow
behaviors exactly. The unified methods predict flows from continuum to rarefied
regimes by adopting unified modeling, and they can be further applied to other
multi-scale physics such as radiation heat transfer, phonon heat transfer and
plasma. In the flow field, the concrete dynamic process needs to describe the
gas-gas interaction and gas-surface interaction (GSI). However, in both DVM and
unified methods, only a simple but not accurate GSI is used, which can be
regarded as a Maxwell GSI with a fixed accommodation coefficient of 1 (full
accommodation) at the present stage. To overcome the bottleneck in extending
DVM and unified methods to the numerical experiment and investigate real
multi-scale flow physics, this paper realizes precise GSI in the DVM framework
by constructing the boundary conditions of a concrete Maxwell GSI with an
adjustable accommodation coefficient. In the constructing process, the problems
of macro-conservation and micro-consistency in the DVS at the boundary are well
solved by reflected macroscopic flux and interpolation distribution function
and interpolation error correction, respectively. Meanwhile, considering that
the multi-scale flows in the background of aeronautics and aerospace are often
at supersonic and hypersonic speeds, the unstructured velocity space (UVS) is
essential. From the perspective of generality, the GSI is forced on UVS.
Besides, by combined with the unified method (the unified gas-kinetic scheme in
the paper), the effectiveness and validity of the present GSI on the DVM
framework are verified by a series of simulations
A global adaptive velocity space for general discrete velocity framework in predictions of rarefied and multi-scale flows
The rarefied flow and multi-scale flow are crucial for the aerodynamic design
of spacecraft, ultra-low orbital vehicles and plumes. By introducing a discrete
velocity space, the Boltzmann method, such as the discrete velocity method and
unified methods, can capture complex and non-equilibrium velocity distribution
functions (VDFs) and describe flow behaviors exactly. However, the extremely
steep slope and high concentration of the gas VDFs in a local particle velocity
space make it very difficult for the Boltzmann method with structured velocity
space to describe high speed flow. Therefore, the adaptive velocity space (AVS)
is required for the Boltzmann solvers to be practical in complex rarefied flow
and multi-scale flow. This paper makes two improvements to the AVS approach,
which is then incorporated into a general discrete velocity framework, such as
the unified gas-kinetic scheme. Firstly, a global velocity mesh is used to
prevent the interpolation of the VDFs at the physical interface during the
calculation of the microscopic fluxes, maintaining the program's high level of
parallelism. Secondly, rather than utilizing costly interpolation, the VDFs on
a new velocity space were reconstruction using the ``consanguinity"
relationship. In other words, a split child node's VDF is the same as its
parent's VDF, and a merged parent's VDF is the average of its children's VDFs.
Additionally, the discrete deviation of the equilibrium distribution functions
is employed to maintain the proposed method's conservation. Moreover, an
appropriate set of adaptive parameters is established to enhance the automation
of the proposed method. Finally, a number of numerical tests are carried out to
validate the proposed method
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