Virtual Wall Research Articles

Wall-modelled large-eddy simulation of turbulent flow past airfoils

We present large-eddy simulation (LES) of flow past different airfoils with $Re_{c}$, based on the free-stream velocity and airfoil chord length, ranging from $10^{4}$ to $2.1\times 10^{6}$. To avoid the challenging resolution requirements of the near-wall region, we develop a virtual wall model in generalized curvilinear coordinates and incorporate the non-equilibrium effects via proper treatment of the momentum equations. It is demonstrated that the wall model dynamically captures the instantaneous skin-friction vector field on arbitrary curved surfaces at the resolved scale. By combining the present wall model with the stretched-vortex subgrid-scale model, we apply the wall-modelled LES approach to three different airfoil cases, spanning different geometrical parameters, different attack angles and low to high $Re_{c}$. The numerical results are verified with direct numerical simulation (DNS) at low $Re_{c}$, and validated with experiment data at higher $Re_{c}$, including typical aerodynamic properties such as pressure coefficient distributions, velocity components and also more challenging measurements such as skin-friction coefficient and Reynolds stresses. All comparisons show reasonable agreement, providing a measure of validity that enables us to further probe simulation results into aspects of flow physics that are not available from experiments. Two techniques to quantify hitherto unexplored physics of flows past airfoils are employed: one is the construction of the anisotropy invariant map, and the second is skin-friction portraits with emphasis on flow transition and unsteady separation along the airfoil surface. The anisotropy maps for all three $Re_{c}$ cases, show clearly that a portion of the flow field is aligned along the axisymmetric expansion line, corresponding to the turbulent boundary layer log-law behaviour and the appearance of turbulent transition. The instantaneous skin-friction portraits reveal a monotonic shrinking of the near wall structure scale. At $Re_{c}=10^{4}$, the interaction between the primary separation bubble and the secondary separation bubble contributes to turbulent transition, similar to the case of flow past a cylinder. At higher $Re_{c}=10^{5}$, the primary separation breaks into several small separation bubbles. At even higher $Re_{c}=2.1\times 10^{6}$, near the turbulent separation, the skin-friction lines show small-scale reversal flows that are similar to those observed in DNS of the flat plate turbulent separation. A notable feature of turbulent separation in flow past an airfoil is the appearance of turbulence structures and small-scale reversal flows in the spanwise direction due to the vortex shedding behaviour.

Towards OPM-MEG in a virtual reality environment

Virtual reality (VR) provides an immersive environment in which a participant can experience a feeling of presence in a virtual world. Such environments generate strong emotional and physical responses and have been used for wide-ranging applications. The ability to collect functional neuroimaging data whilst a participant is immersed in VR would represent a step change for experimental paradigms; unfortunately, traditional brain imaging requires participants to remain still, limiting the scope of naturalistic interaction within VR. Recently however, a new type of magnetoencephalography (MEG) device has been developed, that employs scalp-mounted optically-pumped magnetometers (OPMs) to measure brain electrophysiology. Lightweight OPMs, coupled with precise control of the background magnetic field, enables participant movement during data acquisition. Here, we exploit this technology to acquire MEG data whilst a participant uses a virtual reality head-mounted display (VRHMD). We show that, despite increased magnetic interference from the VRHMD, we were able to measure modulation of alpha-band oscillations, and the visual evoked field. Moreover, in a VR experiment in which a participant had to move their head to look around a virtual wall and view a visual stimulus, we showed that the measured MEG signals map spatially in accordance with the known organisation of primary visual cortex. This technique could transform the type of neuroscientific experiment that can be undertaken using functional neuroimaging.

Stokes-layer formation under absence of moving parts—A novel oscillatory plasma actuator design for turbulent drag reduction

A novel plasma actuator concept is proposed to mimic the effect of spanwise wall oscillations without mechanically moving parts, where four groups of electrodes and three independently operated high-voltage power supplies maintain a pulsatile dielectric barrier discharge (DBD) array. Time-resolved planar velocity fields are obtained with high-speed particle image velocimetry (PIV) in proximity of the discharge zones for quiescent ambient conditions. Resulting flow topologies and wall-normal velocity profiles indicate the Stokes-layer-like flow formation, which is elevated above the wall due to the no-slip condition. The underlying body forces are derived from the PIV data to provide further insight into cause-effect relations between pulsatile discharge and oscillatory flow. The momentum transfer domain is found to be only interrupted with the width of the exposed electrode, which is an important step toward homogeneous virtual wall oscillations. A comparison with earlier studies by Gatti et al. [“Experimental assessment of spanwise-oscillating dielectric electroactive surfaces for turbulent drag reduction in an air channel flow,” Exp. Fluids 56, 110 (2015)] leads to the hypothesis that DBD-based turbulent drag reduction might be a competing alternative to conventional active and passive shear-layer formation strategies, where the adjustability of both oscillation frequency and velocity amplitude might cover a wide range of Reynolds numbers.

Natural convection in a square enclosure with two hot inner cylinders, Part II: The effect of two elliptical cylinders with various aspect ratios in a vertical array

In this study, two-dimensional numerical simulations were conducted for the natural convection in a cold square enclosure with a vertical array of two hot elliptical cylinders at Rayleigh numbers of 104 ≤ Ra ≤ 106. We observed the effect of the aspect ratios of the elliptical cylinders in the range of 0.25 ≤ AR ≤ 4.00. The immersed boundary method (IBM), which is based on the finite volume method (FVM), was used to capture the virtual wall boundary of the cylinders. The transition of the flow regime from steady state to unsteady state depends on the aspect ratio at Ra = 106. At low Rayleigh numbers of Ra = 104 and Ra = 105, the time- and surface-averaged Nusselt numbers for the enclosure walls increase with the AR of the upper elliptical cylinder. At a relatively high Rayleigh number of Ra = 106, the time- and surface-averaged Nusselt numbers of the enclosure walls increase by 3.9% at the upper elliptical cylinder at AR = 0.50 and at the lower elliptical cylinder at AR = 2.00 compared to two circular cylinders.

Illustration Design Teaching Mode Based on Virtual Wall Painting Technology

With rapid development of 3D virtual scene, virtual reality (VR) technology starts to be applied in teaching practice. Because of its conception and innovation, VR technology has certain advantage in art creation learning and teaching. In traditional art course teaching, theoretical knowledge is more than design practice, and the depth of design cases is insufficient. Thus, students lack driving force of art creation. On this basis, wall painting technology and VR technology were combined and applied in the course of Illustration Design in this paper. According to the course requirement of Illustration Design – many design cases, Illustration Design teaching mode based on virtual wall painting technology was constructed. The study result showed that, students more approved the new teaching mode, their interest improved greatly and teaching efficiency boosted significantly. The teaching mode can make up for defects of traditional art design teaching mode and contributes to cultivating students’ art creation ability.

Rendering Stiff Virtual Walls Using Model Matching Based Haptic Controller.

This paper presents a model matching framework for design of haptic controllers. The controller design problem is formulated as a sequence of ${H_{\infty }}$ optimization problems in order to achieve maximal transparency bandwidth. This enables automatic synthesis of robust controllers to achieve a desired bandwidth while limiting actuator forces and guaranteeing stability. This method is applied to address the problem of accurate rendering of high-stiffness virtual walls. Simulation of virtual walls of various stiffnesses are tested on a PHANToM Premium 1.5, at a sampling rate of 1KHz. The sampled nature of the interface, sensor quantization and Coulomb friction impose limits on the maximum stiffness that can be stably achieved with the commonly used open loop impedance controller. Results presented here demonstrate that the proposed controllers can be used to accurately render virtual walls with stiffness up to 5000N/m in an oscillation free manner. This stiffness value is well beyond stability limits for the PHANToM under open loop impedance control. Relationships between the weighting functions and the transparency bandwidth are established to outline a procedure for selection and tuning of the weighting functions. Specifically, it is shown that through appropriate choice of weighting functions high-frequency oscillations observed during rendering of stiff surfaces can be eliminated. The passivity of the designed controllers over the frequency range of operation is verified.

A numerical study on the three-dimensional natural convection with a cylinder in a long rectangular enclosure. Part I: Size effect of a circular cylinder or an elliptical cylinder

This study conducted three-dimensional numerical simulations of the natural convection in a long, cold, rectangular enclosure containing a hot, circular cylinder or an elliptical cylinder. The immersed boundary method (IBM) was used to capture the virtual wall boundary of the cylinder based on the finite volume method (FVM). Four radii of inner cylinders were considered in the Rayleigh number range of 104 ≤ Ra ≤ 106 as key simulation parameters to investigate the size effect of the circular and elliptical cylinders. The detailed effects were analyzed by visualization of the three-dimensional iso-surfaces of the temperature and vortical structures, as well as the three-dimensionality in the system. The heat transfer characteristics from the surfaces of a circular cylinder or elliptical cylinder and the walls of enclosure were addressed. The heat transfer characteristics were affected by the radius of the circular and elliptical cylinders.

Polarization Reconfigurable Air-Filled Substrate Integrated Waveguide Cavity-Backed Slot Antenna

The Internet of Things requires highly efficient ultra-wideband antenna systems that yield high performance at low manufacturing cost. Therefore, a novel ultra-wideband circular air-filled substrate-integrated-waveguide (AFSIW) cavity-backed annular slot antenna is proposed that enables straightforward integration into general-purpose materials by means of standard manufacturing techniques. The cavity top plane, serving as antenna aperture, contains two concentric annular slots, both split into two by shorting tabs that create a virtual electric wall. This enables the generation of a TE11,slot even mode in both parts of each annular slot, giving rise to a conical radiation pattern. By exciting two such modes and judiciously positioning their resonance frequencies, all the unlicensed national information infrastructure (U-NII) [5.15-5.85] GHz radio bands are covered. The annular slot antenna is then made polarization reconfigurable through an innovative excitation of the slot modes by replacing the shorting tabs with four pairs of the PIN diodes. These dynamically switch between two orthogonal linear polarizations by changing the dc control current at the antenna RF port through an external bias tee. This simple, yet effective, bias network enables the integration of all polarization control electronics inside the antenna cavity to protect them from environmental effects. A low-cost antenna substrate was realized through standard additive manufacturing in a 3D-printed substrate, while a standard high-frequency laminate was used to implement the upper conducting plane containing the radiating elements and the polarization reconfiguration electronics. The antenna features an impedance bandwidth of 0.93 GHz, a front-to-back ratio of 14 dB, a total antenna efficiency higher than 95%, and 4.9 dBi gain for each polarization state.

The Virtual Wall of the Fourth Amendment

The evolution of computing cannot be fully explained without consideration of the legal and social friction that challenged individuals at the frontier of computer development. The incorporation of the personal computer into nearly every American household has made the technology central to modern social interaction and has expanded daily life into cyberspace. How do fundamental laws, such as the Fourth Amendment, expand alongside this new media? With the ever-increasing virtualization of our lives, we will be faced with a whole array of legal conundrums and we can learn a great deal about how to alleviate these issues by recognizing the challenges of the past.

Control of power-augmenting lower extremity exoskeleton while walking with heavy payload

This article presents a design and control framework for a prototype of lower extremity exoskeleton to enhance the human strength during locomotion. The hybrid control strategy is practically applied according to the two gait phases, that is, stance and swing. The weight shift method based on human’s weight shift information was proposed and implemented to acquire the detection of gait phase. In the stance phase, a stiff virtual wall method is applied to support the entire weight of the exoskeleton while carrying a heavy payload. Direct feedback and feed-forward torque control are used to reduce mechanical impedance in the swing phase. In experiments, to verify the performance of the proposed control strategy, a human subject wearing the prototype of power-augmenting lower extremity exoskeleton was able to walk with a 50-kg (110-lb) payload at a maximum speed of 1.67 m/s (6 km/h). Satisfactory results were obtained with regard to walking experiments with the heavy payload.

A numerical study on the three-dimensional natural convection with a cylinder in a long rectangular enclosure. Part II: Inclination angle effect of the elliptical cylinder

This study conducted three-dimensional numerical simulations of the natural convection in a cold, long, rectangular enclosure containing a hot elliptical cylinder. The immersed boundary method (IBM) was used to capture the virtual wall boundary of the inner cylinder based on the finite volume method (FVM). The effect of inclination angles of 0° ≤ ψ ≤ 90° were analyzed by a visualization technique with various radii of the cylinder at a relatively high Rayleigh number of Ra = 106. The variation in the flow and thermal structures and the corresponding heat transfer characteristics were investigated in regard to the transition of the flow regime from the steady state to the unsteady state. The heat transfer characteristics from the cylinder surface and enclosure walls were also addressed. As a result, the heat transfer characteristics were improved by the variation in the inclination angle in accordance with the mean radius of the elliptical cylinder.

Does Sharing Your Emotion Make You Feel Better? An Empirical Investigation on the Association Between Sharing Emotions on a Virtual Mood Wall and the Relief of Patients' Negative Emotions.

Background:Emotional management is very important for patients with chronic diseases, such as diabetes. Since these patients may experience strong negative emotions, it is highly desirable to develop an information technology solution for the relief of negative emotions. Recently, online health communities introduced a new design element called the virtual mood wall, which allows patients to express their emotions. We are interested in determining the association between the use of virtual mood wall and the relief of patients' negative emotions.Objective:The main purpose of this work is to evaluate whether sharing one's emotions on the virtual mood wall is associated with the relief of the patient's negative emotions.Methods:We used a secondary econometric analysis using transaction data from 2013 obtained from a leading online diabetes community in China, Sweet Home. A total of 572 patients were included. We compared the change in negative emotion from 1 month before to 1 month after the patient's initial post on the virtual mood wall. Multivariate regression and propensity score matching were used to estimate the effect of the virtual mood wall on the relief of patients' negative emotions.Results:Posting on the virtual mood wall is significantly associated with the decrease of patients' negative emotions (p < 0.001). The control variables included the initial negative emotion (p < 0.001), the summer (p < 0.05) and autumn (p < 0.05) season, and the volume of homepage visitors (p < 0.05), all of which are significantly associated with the relief of negative emotions.Conclusions:This study empirically validated the positive association between the online sharing of emotions and the relief of negative emotions. We demonstrated that information technology provided a channel for sharing emotions. The virtual mood wall is an effective design element to help patients with chronic disease to recover from their negative emotions.

Book Review: Franziska Lys and Michael Dreyer (eds): Virtual Walls? Political Unification and Cultural Difference in Contemporary Germany

Book Review: Franziska Lys and Michael Dreyer (eds): <i>Virtual Walls? Political Unification and Cultural Difference in Contemporary Germany</i>

High performance LCD for augmented reality and virtual reality displays

ABSTRACTFor augmented reality and virtual reality displays, high-resolution density, high luminance, and fast response time are critically needed. High-resolution density helps eliminate the screen-door effect; high luminance and fast response time enable low duty ratio operation, which plays a key role for suppressing image blurs. By using a low viscosity material and new diamond-shaped electrode configuration, we demonstrate a fringe field switching liquid crystal display, abbreviated as d-FFS LCD, with high transmittance, fast grey-to-grey response time, low operation voltage, wide viewing angle, and indistinguishable colour shift and gamma shift. We also investigate the rubbing angle (α) effects on transmittance and response time. When α = 0°, the virtual wall effect is strong, resulting in fast response time but compromised transmittance. When α ≥ 1.2°, the virtual walls disappear, as a result, the transmittance increases dramatically, but the trade-off is in slower response time.

Stability and performance of haptic simulation involving interaction with non-passive virtual environment

SUMMARYPrevious researches on analysis of stability of haptic simulation largely assume that the virtual environment is passive. But the virtual environment can become non-passive due to various reasons including discretization errors and interaction dynamics between virtual tools and objects. This paper provides an analysis of the stability and performance of the haptic simulation involving non-passive virtual environment. The dynamic interaction between the virtual tools and the objects is modeled using the two-port networks. The analysis is carried out using a velocity and force mapping matrix for six-DOF simulation. New stability condition resulting from the analysis is applied to an example simulation of a one-DOF virtual wall. Maximum stiffness satisfying the stability condition established in the previous literature, and the proposed condition is compared with the maximum stiffness experimentally determined with various time steps. The newly proposed stability condition manifests less standard deviation of errors than the widely applied absolute stability condition.

Author’s Response: The Third Revolution Is Real

Author’s Response: The Third Revolution Is Real Elizabeth C. Economy (bio) Let me begin by thanking David Shambaugh, Liselotte Odgaard, Yongjin Zhang, and Michael Auslin for providing such thoughtful and well-considered reviews of The Third Revolution: Xi Jinping and the New Chinese State. It is an honor to have four such distinguished scholars take the time to address the ideas and themes of the book, and I am delighted to have the chance to respond. Two broad issues are raised by more than one reviewer. The first is the essential question of whether the third revolution is a genuine “revolution.” Shambaugh suggests that much of the rhetoric associated with Xi Jinping’s China is not new, nor are the changes as progressive or transformative in nature as those undertaken by the leaders of the two previous revolutions, Deng Xiaoping and Mao Zedong. Zhang notes that revolution “by definition” is associated with “human emancipation” and with “progressive, sometimes violent struggle” that is directed against “subjugation and oppression.” In fact, Xi’s policy direction, he argues, is regressive or reactionary. Zhang further points to cooperation between the state and society on issues such as environmental protection as evidence that Xi has not ushered in a revolution. I did, in fact, grapple at length with the term revolution and at one point considered characterizing Xi’s leadership as a “reactionary revolution” to capture precisely the regressive nature of the policies that Zhang references. Ultimately, however, I understand both revolution and reform as political processes that are devoid of normative bias. Both signify change in political processes and institutions; the transformative effect on those processes and institutions, however, is far greater in revolution than in the case of reform. Moreover, if one accepts Deng’s own characterization of his leadership as the “second revolution,” it is certainly plausible to argue that Xi has ushered in a “third revolution”—a transformation of equal import that has reversed many of the political processes initiated by Deng. Under Xi’s leadership, China has moved away from collective decision-making back to one-man rule and eliminated the two-term presidency; reasserted the role of the Chinese Communist Party (CCP) in society and the economy, [End Page 162] reversing Deng’s trend toward withdrawing the party from everyday decision-making; rejected Deng’s welcoming of foreign ideas and capital and built a virtual wall of restrictions and regulations designed to constrain foreign competition and influence; and upended Deng’s low-profile foreign policy to adopt one that is far more ambitious and expansive. Moreover, the Chinese state increasingly is fusing technology with politically repressive laws and regulations, as evidenced by the social credit system, internet constraints, and a country-wide surveillance system. In this context, Odgaard’s suggestion that Xi’s authoritarianism may be veering into totalitarianism is particularly relevant and further underscores the revolutionary nature of Xi’s policies. To Zhang’s note that state-societal cooperation on environmental protection necessarily undermines the concept of revolution, I would offer two reflections: first, Xi’s regime has embraced environmental protection, alongside anticorruption, as a pillar issue of regime legitimacy within its revolution; second, the political processes of cooperation between the state and society around environmental protection have changed significantly from the era of Hu Jintao to Xi. For example, opportunities for open discourse or coordinated environmental activism via the internet, social protest, and NGOs are much diminished. Xi has also pushed for party committees to establish a presence in and actively guide NGOs, and he has sharply limited the ability of environmental NGOs to engage with their foreign counterparts through the Law on the Management of Foreign NGOs. Zhang and Odgaard further raise the important issue of whether China’s third revolution will exert a transformative impact on the international system. Zhang argues that if China is indeed experiencing a revolution, it could not be “caged” within Chinese borders, and he seeks evidence of China’s revolutionary agency and impact on the liberal international order. Odgaard wonders whether China will attempt to use coercive power abroad in order to recreate a Chinese sphere of influence in Asia—particularly in the event that the United States...

Acoustic impedance control for reproducing sound environment by constructing virtual wall

In order to obtain high presence, it is necessary to reproduce the sound environment of remote places. Conventionally, sound environments at remote places are reproduced by recreating a sound signal at that place. In this study, an acoustic technique that reproduces sound environments of remote places by recreating the dynamics of the environment at that place is proposed. In the proposed acoustic technique, the desired sound environment is attained by making listeners on the spot feel as if there were a wall that reflects and absorbs the sound waves. By controlling the acoustic impedance at the position where the wall is constructed, the wall is virtually built. In addition, the acoustic impedance including the virtual viscosity and elasticity that can be arbitrarily determined at the position where the wall constructed is set. As a result, by deciding the acoustic output from the secondary sound source so as to attain the desired acoustic impedance, the wall is virtually constructed to reproduce the desired sound environment.

Virtual Walls? Political Unification and Cultural Difference in Contemporary Germany

Virtual Walls? Political Unification and Cultural Difference in Contemporary Germany

INTERVIEW WITH GABRIEL SCHIVONE

In this interview, Gabriel M. Schivone, 2018 visiting scholar at the University of Arizona's Agnese Nelms Haury Program in Environment and Social Justice, talks to the Journal of Palestine Studies about the multi-billion-dollar surveillance technology industry and how U.S., Israeli, and Mexican state and corporate entities collaborate in the “laboratory” of the U.S.-Mexico borderlands. Schivone discusses how, in 2006 and 2014, the U.S. government subcontracted Israel's Elbit Systems to provide a “virtual wall” under President George W. Bush's Secure Fence Act. In 2014, Elbit's U.S. subsidiary was awarded a new $145 million contract to build the Integrated Fixed Towers project, a similar “virtual wall” concept to provide fifty-two Israeli-style surveillance towers along southern Arizona's border with Mexico.

Stochastic modelling of three-dimensional particle rebound from isotropic rough wall surface

This paper describes an extension of the two-dimensional approach to particle-rough wall collision modelling (Sommerfeld and Huber, 1999; Konan et al., 2009) to the case of three-dimensional particle rebound from an isotropic rough wall surface. The virtual three-dimensional rough wall is represented as a Gaussian correlated surface. Normal vector angle statistical distributions are investigated in detail for such virtual rough walls, and a statistical modelling approach for these angles is proposed and validated in the frame of the low roughness approximation. Next, deterministic simulations of fully elastic particle collisions with the three-dimensional virtual wall roughness structure are carried out for various particle incident angles. It is shown that the rebound angle, in the bouncing plane of the particle, obeys the distribution given by the two-dimensional modelling approach. However, the three-dimensional structure induces a transverse deviation bouncing angle that obeys a Gaussian distribution with a standard deviation that increases with increase in incident angle. A statistical modelling approach for the virtual wall normal vector seen by any particle for a given incident angle is proposed and validated from deterministic simulation results. The probability that particles make only one rebound is in agreement with the two-dimensional multiple-collision model assumption. A new stochastic procedure for particle-isotropic rough wall interactions in a Lagrangian framework is developed and verified by comparisons with deterministic simulations and available experimental results.