18 research outputs found
Computational study on aerodynamic characteristics of a flying wing MAV
In this paper an effective method is developed to study aerodynamic characteristics of a flying wing micro air vehicle (MAV). The method is based on momentum source method (MSM), low Mach number preconditioning and lower-upper symmetric Gauss-Seidel (LU-SGS) implicit dual time-stepping algorithm on hybrid dynamic meshes. The S-A turbulence model is also applied to capture flow separation. Momentum source items are utilized to replace the propeller in the numerical simulation by simplifying the unsteady flow into a steady one. Compared with wind tunnel experimental results, the computed results indicate that the method developed is capable of dealing with steady and unsteady aerodynamic characteristics of flying wing MAV
Parametric study on the water impacting of a free-falling symmetric wedge based on the extended von Karman's momentum theory
The present study is concerned with the peak acceleration azmax occurring
during the water impact of a symmetric wedge. This aspect can be important for
design considerations of safe marine vehicles. The water-entry problem is
firstly studied numerically using the finite-volume discretization of the
incompressible Navier-Stokes equations and the volume-of-fluid method to
capture the air-water interface. The choice of the mesh size and time-step is
validated by comparison with experimental data of a free fall water-entry of a
wedge. The key original contribution of the article concerns the derivation of
a relationship for azmax (as well as the correlated parameters when azmax
occurs), the initial velocity, the deadrise angle and the mass of the wedge
based on the transformation of von Karman momentum theory which is extended
with the inclusion of the pile-up effect. The pile-up coefficient, which has
been proven dependent on the deadrise angle in the case of water-entry with a
constant velocity, is then investigated for the free fall motion and the
dependence law derived from Dobrovol'skaya is still valid for varying deadrise
angle. Reasonable good theoretical estimates of the kinematic parameters are
provided for a relatively wide range of initial velocity, deadrise angle and
mass using the extended von Karman momentum theory which is the combination of
the original von Karman method and Dobrovol'skaya's solution and this
theoretical approach can be extended to predict the kinematic parameters during
the whole impacting phase.Comment: arXiv admin note: text overlap with arXiv:2207.1041
CPCM: Contextual Point Cloud Modeling for Weakly-supervised Point Cloud Semantic Segmentation
We study the task of weakly-supervised point cloud semantic segmentation with
sparse annotations (e.g., less than 0.1% points are labeled), aiming to reduce
the expensive cost of dense annotations. Unfortunately, with extremely sparse
annotated points, it is very difficult to extract both contextual and object
information for scene understanding such as semantic segmentation. Motivated by
masked modeling (e.g., MAE) in image and video representation learning, we seek
to endow the power of masked modeling to learn contextual information from
sparsely-annotated points. However, directly applying MAE to 3D point clouds
with sparse annotations may fail to work. First, it is nontrivial to
effectively mask out the informative visual context from 3D point clouds.
Second, how to fully exploit the sparse annotations for context modeling
remains an open question. In this paper, we propose a simple yet effective
Contextual Point Cloud Modeling (CPCM) method that consists of two parts: a
region-wise masking (RegionMask) strategy and a contextual masked training
(CMT) method. Specifically, RegionMask masks the point cloud continuously in
geometric space to construct a meaningful masked prediction task for subsequent
context learning. CMT disentangles the learning of supervised segmentation and
unsupervised masked context prediction for effectively learning the very
limited labeled points and mass unlabeled points, respectively. Extensive
experiments on the widely-tested ScanNet V2 and S3DIS benchmarks demonstrate
the superiority of CPCM over the state-of-the-art.Comment: Accepted by ICCV 202
Effects of wave parameters on load reduction performance for amphibious aircraft with V-hydrofoil
An investigation of the influence of the hydrofoil on load reduction
performance during an amphibious aircraft landing on still and wavy water is
conducted by solving the Unsteady Reynolds-Averaged Navier-Stokes equations
coupled with the standard turbulence model in this paper. During the
simulations, the numerical wave tank is realized by using the velocity-inlet
boundary wave maker coupled with damping wave elimination technique on the
outlet, while the volume of fluid model is employed to track the water-air
interface. Subsequently, the effects of geometric parameters of hydrofoil have
been first discussed on still water, which indicates the primary factor
influencing the load reduction is the static load coefficient of hydrofoil.
Furthermore, the effects of descent velocity, wave length and wave height on
load reduction are comprehensively investigated. The results show that the
vertical load reduces more than 55 at the early stage of landing on the
still water through assembling the hydrofoil for different descent velocity
cases. Meanwhile, for the amphibious aircraft with high forward velocity, the
bottom of the fuselage will come into close contact with the first wave when
landing on crest position, and then the forebody will impact the next wave
surface with extreme force. In this circumstance, the load reduction rate
decreases to around 30, which will entail a further decline with the
increase of wave length or wave height
Numerical Study of Wave Effect on Aircraft Water-Landing Performance
Aircraft, such as amphibious planes, airliners, helicopters and re-entry capsules, are frequently subject to impacting loads from water-landing/ditching on various free surfaces, especially under wave conditions. Understanding and quantifying the water-landing/ditching performance on wave surfaces are of fundamental important for the design and certification of crashworthiness in the field of aerospace engineering. This study aims to numerically assess the effect of wave surface on water-landing process of an amphibious aircraft. The numerical implementation is realized in Reynolds-averaged Navier–Stokes (RANS) framework by combining finite volume method (FVM), volume of fluid (VOF) approach and velocity-inlet wavemaker. The temporal-spatial characteristic of numerical wave and the accuracy of presented model are, respectively, validated by analytical wave and convergence studies. The aircraft landing simulations with different free surface conditions, i.e., calm water, regular wave with different wave heights are then performed and quantitatively compared through several physical parameters, including acceleration, velocity, pressure, pitch angle and free surface deformation. It was found that the aircraft regular wave-landing process experiences several unique stages comparing with the calm-water-landing case. The results clearly confirm that wave surface can influence the aircraft landing performance to a great extent. The fundamental mechanism is found to be that the wave surface slope and wave particle velocity remarkably change the impacting position and effective impacting velocity of the aircraft
Numerical study on the flow characteristics of micro air vehicle wings at low Reynolds numbers
The aerodynamic characteristics around a micro air vehicle wing with an inverse-Zimmerman configuration are numerically investigated by an in-house programmed solver particularly dedicated for aircrafts operating in low Reynolds number regime. The complex three-dimensional aerodynamic performance was investigated in terms of force generation and flow structures visualization. Results show that the flow around the low aspect ratio MAV wing is characterized by complex three-dimensional separation-dominated flow. The flow fields exhibit separation, reattachment, secondary separation, secondary reattachment, and strong interaction between the separated boundary layer and wingtip vortices. In addition, the effect of tip-attached vertical stabilizers on flow structure and aerodynamic forces is addressed in this paper. The stabilizers significantly influence both the flow structure and aerodynamic forces via reducing the strength of wingtip vortices and shedding and interacting of wingtip vortices. Eventually, the unsteadiness of the aerodynamics revealed that higher angle of attack will result in stronger unsteady phenomena as demonstrated by the oscillating forces.Aerodynamic