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The Future of Unmanned Aircraft Systems
Published in R. Kurt Barnhart, Douglas M. Marshall, Eric J. Shappee, Introduction to Unmanned Aircraft Systems, 2021
Other material advances now and in the future will allow the shape of the air vehicle structure to become dynamic depending on the needs and conditions of the flight. Principles of aerodynamics dictate that the structural configuration of an aircraft is different when optimized for high-speed flight than for low-speed flight. Typically, altering the shape of a wing (the structural member most typically modified) has required the use of complex and heavy variable-sweep mechanisms. For example, a wing configured for high-speed flight will be very smooth (laminar) with low camber and will often sweep aft. Wings for low-speed flight are typically the opposite, with high camber, low sweep, and may have technology that disrupts laminar flow in favor of lift-enhancing technology such as vortex generators, flaps, and leading-edge slats. Military aircraft of the past have used complex variable wing sweep technology to assist in the transition from high-speed to low-speed flight. Materials in the future will allow the shape of the structure to be modified as the mission requires without the use of heavy, complex infrastructure. They may also be able to flex so that the requirement to have traditional control surfaces (flaps and ailerons) is eliminated.
The Future of Unmanned Aircraft Systems
Published in Douglas M. Marshall, R. Kurt Barnhart, Eric Shappee, Michael Most, Introduction to Unmanned Aircraft Systems, 2016
Other material advances now and in the future will allow for the shape of the air vehicle structure itself to become dynamic depending on the needs of the existing flight condition. Principles of aerodynamics dictate that the structural configuration of an aircraft is different when optimized for high-speed flight than for low-speed flight. Typically, altering the shape of a wing (the structural member most typically modified) has required the use of complex and heavy variable-sweep mechanisms to accomplish this task. For example, a wing configured for high-speed flight will be very smooth (laminar) with low camber and, often, aft sweep. Wings for low-speed flight are typically the opposite with high camber, low sweep, and often with technology that disrupts laminar flow in favor of lift-enhancing technology such as vortex generators, flaps, and leading-edge slats. Military aircraft of the past have used complex variable wing sweep technology as well to assist in the transition from high-speed to low-speed flight. Materials in the future will allow for the shape of the structure to be modified as the mission requires without the use of heavy, complex infrastructure even to the point of being able to flex so as to eliminate the requirement to have traditional control surfaces such as flaps and ailerons.
Compressible boundary-layer theory
Published in George Emanuel, Analytical Fluid Dynamics, 2017
Most boundary-layer flows are transitional or turbulent, except near their origin where they are laminar. Since the 1950s, however, high-speed flight at an elevated altitude has become a major concern of engineers. Under these conditions the boundary layer remains laminar much longer (Sternberg, 1952). Larger portions of the structure are thus covered by a laminar boundary layer because of surface cooling, boundary-layer acceleration, and a relatively low Reynolds number associated with the low density of the upper atmosphere. For instance, all three factors are present during atmospheric reentry of a missile or spacecraft.
A simple vortex approach to complex two-wing unsteady flapping problems in 2D applied to insect flight study
Published in European Journal of Computational Mechanics, 2018
Mitsunori Denda, Roberta Shapiro, Justin Wong
Notice that the order of the hind-wing and the fore-wing beating is relative in the cyclic beating situation and there is no absolute way to specify which beats first. Rather, this issue should be addressed in terms of the phase shift of the hind-wing beating relative to the fore-wing, which ranges from 0° to 360°. Therefore, we first investigate the effects of the phase shift in the two-wing flapping. The geometric (wing chord, wing length, fore and hind-wing separation distance), kinematic (stroke plane angle, stroke angles, pitch shift) and kinetic parameters (flapping frequency, flight speed, pitch rate) are determined from a sample dragonfly species and its high-speed flight video as listed in Table 1; the exact identification of the dragonfly is not important in this study since we are not investigating dragonfly flight itself.
Thermal Insulation Performance of Monolithic Silica Aerogel with Gas Permeation Effect at Pressure Gradients and Large Temperature Differences
Published in Nanoscale and Microscale Thermophysical Engineering, 2023
Hao-Qiang Pang, Sheng-Nan Zhang, Ting-Hui Fan, Xu Zhang, Tian-Yuan Liu, Yan-Feng Gao
Nevertheless, the air condition of aircraft changes from the atmospheric environment to the high vacuum environment during launch and high-speed flight [26]; heat transfer within thermal insulators (aerogel) should be further developed to investigate under different air pressure and transient pressure condition [27]. Spagnol et al. [28] tested the granular and monolithic silica thermal conductivity of aerogels at T = 300 ~ 315 K under variable atmospheric pressure, and the results showed that the gaseous λe accounted for over 60% of its total thermal conductivity. Zeng et al. [29] measured the gaseous thermal conductivity of silica aerogel with 94% porosity and 0.11 g·cm −3 density under a variable gas pressure at T = 296 K in air, and the results proved that the gaseous thermal conductivity was greatly affected by pressure and it decreased sharply to 0 when P < 10−2 bar. Swimm et al. [30], Bi et al. [31], and Zhang et al. [32] illustrated that the measured gaseous thermal conductivity was strongly affected by both gas pressure and porosity and the density of the aerogel. It can be concluded that heat transfer through a gas is the most important phase for aerogel. Thus, the internal pressure gradient can induce gas flow and affect gaseous thermal conductivity, eventually influencing the thermal insulation performance. Civan et al. [33] used the Hagen-Poiseuille equation [34] to calculate the gas permeability of tight porous media over Kn’s entire range. Liu et al. [35] studied the gas permeability within silica aerogel and its effect on thermal insulation performance. The results showed that the gas flow within silica aerogel significantly affected temperature response at specific gradient pressure conditions.
Boundary layer development over non-uniform sand rough bed channel
Published in ISH Journal of Hydraulic Engineering, 2019
Boundary layer is that portion of the flow zone where motion of fluid is affected by the solid boundary. The thickness of boundary layer is defined as the distance from the bed at which flow velocity is 99% of free stream velocity. The boundary layer may be laminar at the upstream end, and gradually thickens up to a certain point in the channel length at which the flow is in the stage of developing flow and beyond that point, the flow becomes fully developed. The flow details within the boundary layer are important for problems in aerodynamics together with the development of a wing stall, heat transfer occurs in high speed flight, the skin friction drag of a body and the performance of a high speed aircraft inlet (Schlichting 1968). The boundary layer flow can be laminar or turbulent, subsequent in radically different classes of profile shapes. Prandtl (1952), Batchelor (1967) and Schlichting and Truckenbrodt (1979) provide thorough descriptions of the boundary layer concept. The behaviour of a solid moving relative to the flow cannot be accurately described without an understanding of the boundary layer. When the bed particle moves relative to the flow, the boundary layer occurs very close to the bed surface as a result of no slip condition and viscosity (Prandtl 1904). Rohr et al. (1998) have suggested that the thickness of the boundary layer may be linked to the relative intensity of bioluminescence nearby a swimming dolphin. Allen (1961) achieved a qualitative description around the flow zone and possibly the growth of boundary layer by means of Schlieren technique. Anderson et al. (2001) investigated the normal and tangential velocity profiles of the flow boundary layer surrounding live swimming fish by means of digital particle tracking velocimetry, DPTV. Ardiclioglu and Ozturk (2006) investigated the boundary layer development in smooth surface open channel flow. In their study, boundary layer thickness is proposed both by numerical and experimental method. In-situ measurements have become sufficient to investigate turbulent quantities in the atmospheric boundary layer because of remote sensing technologies (Poulos et al. 2002; Kunkel and Marusic 2006; Drobinski et al. 2007). The boundary layer development is considered in the framework of a changeover of a well-developed boundary layer subject to abrupt change of wall roughness, also raised to as an internal boundary layer, (Cheng and Castro 2002; Belcher et al. 2003). Castro (2007) checked the development of boundary layer in a wind tunnel over different roughnesses at relatively high Reynolds numbers. Youen et al. (2009) studied boundary layer development in different geometric configuration and suggested some important areas of sediment transport modification. Tomas et al. (2011) experimentally investigated boundary layer development over a rough surface and proposed an empirical equation for boundary layer thickness. In a recent study, Devi et al. (2016) studied the boundary layer development in the vegetative channel under the application of downward seepage.