Higher Free Stream Velocity Research Articles

Bifurcation Analysis and Sticking Phenomenon for Unmanned Rotor-Nacelle Systems with the Presence of Multi-Segmented Structural Nonlinearity

Whirl flutter is a phenomenon caused by an aeroelastic instability, causing oscillations to propagate in manned or unmanned rotor-nacelle type aircraft. Under the conditions where multi-segmented freeplay are present, complex behaviors can dominate these oscillations and can lead to disastrous consequences. This study investigates a rotor-nacelle system with multi-segmented stiffnesses with a freeplay gap to encompass the real-world influences of aircraft. The mathematical aerodynamics model considers a quasi-steady application of strip theory along each blade to outline the external forces being applied. A free-body diagram is then used to incorporate the structural stiffness and damping terms with multi-segmented freeplay considered in the structural stiffness matrix. Multiple structural responses of the defined system are investigated and characterized to determine the influence of varying symmetric and asymmetric multi-segmented stiffnesses with varying gap parameters, including a route to impact investigation. The findings are characterized using phase portraits, Poincaré maps, time histories, and basins of attraction. It is found that under these conditions, the structural influences can lead to aperiodic oscillations with the existence of grazing bifurcations. Furthermore, these results unveil that under certain conditions and high freestream velocities, the sticking phenomenon becomes apparent which is strongly dependent on the strength of the multi-segmented representation, its gap sizes, and its symmetry. Lastly, a route to impact study shows the strong coupled influence between pitch and yaw when asymmetric conditions are applied and the possible presence of grazing-sliding bifurcations. The numerical simulations performed in this study can form a basis for drone designers to create reliable rotor-nacelle systems resistant to whirl flutter caused by freeplay effects.

Experimental and Numerical Analyses of a Novel Wing-In-Ground Vehicle

The AeroCity is a new form of transportation concept that has been developed to provide high-speed ground transportation at a much lower cost than the existing high-speed railway. Utilizing the Wing-in-Ground (WIG) effect, the AeroCity vehicle does not require complex infrastructures like other contemporary concepts, such as the Hyperloop or Maglev trains. In the current work, the aerodynamic characteristics of the AeroCity vehicle are examined through a Computational Fluid Dynamics (CFD) analysis. The results from the CFD analysis qualitatively match with the findings of wind tunnel experiments. Surface streamlines and boundary layer measurements correspond well with the numerical data. However, the force measurements show a discrepancy. It is found that the separation bubble over the side plates is not captured by the CFD, and this is responsible for an under-prediction of the drag at higher free-stream velocities. The Transition SST model improved the matching between the experiments and numerical simulations. The influence of the moving ground is numerically investigated, and the effect of non-moving ground on the vehicle aerodynamics was found not to be significant. Finally, the inclusion of the track wall is examined. It is found that the merging of the wingtip vortices is responsible for a significant drag increase and, therefore, an alternative track geometry should be investigated.

Investigation of ventilation demand variation in unsteady supercavitation

Understanding the air injection strategy of a ventilated supercavity is important for designing high-speed underwater vehicles wherein an artificial gas pocket is created behind a flow separation device to reduce skin friction. Our study systematically investigates the effect of flow unsteadiness on the ventilation requirements to form (CQf) and collapse (CQc) a supercavity. Imposing flow unsteadiness on the incoming flow has shown an increment in higher CQf at low free stream velocity and lower CQf at high free stream velocity. High-speed imaging reveals distinctly different behaviors in the recirculation region for low and high freestream velocity under unsteady flows. At low free stream velocities, the recirculation region formed downstream of a cavitator shifted vertically with flow unsteadiness, resulting in lower bubble collision and coalescence probability, which is critical for the supercavity formation process. The recirculation region negligibly changed with flow unsteadiness at high free stream velocity and less ventilation is required to form a supercavity compared to that of the steady incoming flow. Such a difference is attributed to the increased transverse Reynolds stress that aids bubble collision in a confined space of the recirculation region. CQc is found to heavily rely on the vertical component of the flow unsteadiness and the free stream velocity. Interfacial instability located upper rear of the supercavity develops noticeably with flow unsteadiness and additional bubbles formed by the distorted interface shed from the supercavity, resulting in an increased CQc. Further analysis on the quantification of such additional bubble leakage rate indicates that the development and amplitude of the interfacial instability accounts for the variation of CQc under a wide range of flow unsteadiness. Our study provides some insights on the design of a ventilation strategy for supercavitating vehicles in practice.

Accurate Boundary Layer Measurements Using Hot-Wire Anemometry—Improvements and Error Analysis

Abstract In this article, two approaches are presented dealing with common challenges of two-dimensional boundary layer measurements with hot wire anemometry under challenging test conditions. Novel procedures for accurate determination of the sensor position and correction of the wall heat effect were developed and tested at high freestream velocities of about M1 = 0.3 and thin boundary layers (δ99 = 0.7 − 3.5 mm) of different transitional state in a low-density environment. First, a novel procedure for automatized determination of the accurate hot wire sensor position relative to the wall is presented. The quantification and correction of possible subminiature sensor misalignments is achieved by taking advantage of the linear nature of the laminar sublayer of each boundary layer. The statistical approaches for identification and verification of the linear sublayer demonstrate satisfying results of minimized position uncertainties of about 24 μm. Second, a highly adaptable method for correction of the well-known wall heat effect is presented. In contrary to a series of static correction approaches, the biased velocity information is corrected by optimizing the parameters of an exponential approach, where the correction term is optimized for each boundary layer individually. This novel approach resolves the problem of the limited applicability of static correction methods, caused by system inherent measurement uncertainties.

Inhalability of micro-particles through the human nose breathing at high free-stream airflow velocities

In this study, the inhalability of microparticles through a human nasal passage under various ambient airflow conditions is calculated. As megacities are becoming polluted and being exposed to the polluted windy outdoor environments is inevitable nowadays, from the toxicological point of view, it would be wise to investigate the impact of air pollution on human health. Though, as few studies have attempted to address this concern, especially for high free-stream velocities comprehensively, the main emphasis was placed on assessing the impact of both inhalation and Total Deposition Fraction (TDF) of particles. Several free stream airflow velocities from 1 to 70 km/h are simulated to cover almost all stormy outdoor conditions that could typically happen. RANS models and stochastic models for turbulent particle dispersion were examined to simulate the particle motions properly in such turbulent conditions. The impact of high ambient airflow velocities on the aspiration efficiency and the TDF of micro-particles were assessed for a wide range of particle sizes. The results reveal that TDF of the integrated airway is mostly higher than that of the detached airway system due to the non-uniformity of both airflow field and particle distribution which happens at the nostrils, similar to the work of Naseri et al. (Anderson and Anthony, 2013) [24]. Yet, contrary to that study in which the flow field was laminar, and there were no notable differences between the TDF of these two cases at different breathing flow rates, the differences become more notable when the breathing regime becomes fully turbulent.

MHD heat flux mitigation in hypersonic flow around a blunt body with ablating surface

One of the possible applications of magnetohydrodynamic flow control is considered. Namely, the surface heat flux mitigation by means of magnetohydrodynamic (MHD) interaction in hypersonic flow around a blunt body. The 2D computational model realizes a coupled solution of chemically non-equilibrium ionized airflow in magnetic field. Heat- and mass-transfer due to the ablation of materials from the body surface is taken into account.Two cases of free-stream flow conditions are considered: moderate free-stream velocity (7500 m s−1) case and high free-stream velocity (11 000 m s−1) case. It is shown that the first flow case results in moderate ionization in the shock layer, while the second flow case results in high ionization. In the first case, the Hall effect is significant, and effective electrical conductivity in the shock layer is rather low. In the second case, the Hall effect reduces, and effective conductivity is high. Even if the Hall effect is strong, as in the first case, intensive MHD deceleration of the flow behind the shock is provided due to the presence of insulating boundaries, the bow shock front and non-conductive wall of the blunt body. In the second case, high effective conductivity provides a high intensity of MHD flow deceleration. In both cases, a strong effect of MHD interaction on the flow structure is observed. As a consequence, a noticeable reduction of the surface heat flux is revealed for reasonable values of magnetic induction. The new treatment of mechanism for the surface heat flux reduction is proposed, which is different from commonly used one assuming that MHD interaction increases the bow shock stand-off distance, and, consequently results in a decrease of the mean temperature drop across the shock layer. The new effect of ‘saturation of heat flux’ is discussed.

Multidisciplinary Aerodynamic Design of a Rotor Blade for an Optimum Rotor Speed Helicopter

The aerodynamic design of rotor blades is challenging, and is crucial for the development of helicopter technology. Previous aerodynamic optimizations that focused only on limited design points find it difficult to balance flight performance across the entire flight envelope. This study develops a global optimum envelope (GOE) method for determining blade parameters—blade twist, taper ratio, tip sweep—for optimum rotor speed helicopters (ORS-helicopters), balancing performance improvements in hover and various freestream velocities. The GOE method implements aerodynamic blade design by a bi-level optimization, composed of a global optimization step and a secondary optimization step. Power loss as a measure of rotor performance is chosen as the objective function, referred to as direct power loss (DPL) in this study. A rotorcraft comprehensive code for trim simulation with a prescribed wake method is developed. With the application of the GOE method, a DPL reduction of as high as 16.7% can be achieved in hover, and 24% at high freestream velocity.

Modeling of flow-induced stress on helical Savonius hydrokinetic turbine with the effect of augmentation technique at different operating conditions

Conventional Savonius rotor experiences negative and high static torque coefficient at several rotor angles. To prevail over this issue Helical Savonius hydrokinetic turbine (HSHKT) twisted with 90° is proposed, which is able to avoid excessive shaking due to hydrodynamic forces on blades. Usually, HSHKT have a low efficiency, Therefore to enhance the performance coefficient of the rotor some augmentation techniques are required to be used. In this study modeling of flow-induced stresses on HSHKT are performed through fluid-structure interaction analysis. The maximum hydraulic load and von-Mises stress on rotor are found as 0.79 Mpa and 145.45 Mpa respectively in case of duct with single deflector augmentation techniques. Computation results show that edge of shaft and joint of end plate and shaft rotor experiences zone of high von-Mises stress which leads to fatigue crack in this area. In consequence to that augmentation techniques creates adverse effect on stability, smooth operation and service life of rotor at the higher free stream velocity, which opposes the fact that the rotor performance is enhanced by employing augmentation techniques in certain conditions. In this study, it is found that the possibility of fatigue crack increases at higher free stream velocity while using augmentation techniques.

Blackout Analysis of Small Cone-Shaped Reentry Vehicles

The high temperatures associated with the hypersonic reentry process lead to an increase in the collisions between molecules, which may result in the disruption of the electronic structure, producing free electrons and ions. This production of free electrons and ions creates a plasma or ionized flowfield around the vehicle that is known to degrade the quality of radio-wave signal propagation, leading to a loss of communication or “blackout.” This study involves performing hypersonic computational fluid dynamics simulations in the commercial software CFD++ with a blunt-nosed cone geometry at different flight conditions to predict how and when ground communication can be achieved to aid in the design of an alert transmitter, which is an extension of aircraft collision-avoidance system technology. Computational results show that a small blunt-nosed cone geometry has decreased ionization regions as the cone angle decreases due to a shifting of the reaction zone further downstream. As a result, higher freestream velocities have less of an impact in determining the location for an antenna, and communicating along the stagnation line is seen to be independent of cone angle.

Numerical modeling of the conditions for realization of flow regimes in supersonic axisymmetric conical inlets of internal compression

The results of the numerical investigation of flow regimes in the axisymmetric inlets of internal compression at a supersonic flow around them are presented in the work. The main attention is paid to the determination of the ranges of the duct geometric convergence, in which a supersonic inflow in the inlet realizes. The investigation has been carried out at high supersonic freestream velocities corresponding to the Mach numbers М = 2−8 by the example of the frontal conical (funnel-shaped) inlets with the angles of the internal cone wall inclination δw = 7.5−15° under the variation of the relative area of the throat cross section. The flow structure alteration was studied and the critical relative areas of the inlet throat were determined, at which either there is no starting of the inlet in the process of flow steadying at the initial subsonic flow in it or a flow breakdown occurs in the process of flow steadying at an initial supersonic inflow. Numerical computations of the axisymmetric flow were done on the basis of the solution of the Navier–Stokes equations and the k-ω SST turbulence model.

Performance of a ducted propeller designed for UAV applications at zero angle of attack flight: An experimental study

Performance characteristics and velocity field of a 16 inch diameter ducted propeller are investigated experimentally using five different duct shapes. Experiments are conducted at zero angle of attack which simulates takeoff and forward flight modes of a vertical takeoff and landing (VTOL) tilt ducted propeller UAV. Effect of duct geometry is studied by means of force, torque, velocity field and surface pressure measurements under various flow conditions. Force and torque transducers are embedded into motor hub so that they do not disturb the flow. Thrust components acting on duct and propeller are measured individually. Velocity profiles at the inlet and exit sections are measured with hot-wire anemometer. Experimental results obtained for open and ducted propellers are compared. It is shown that power coefficients obtained for all ducted propeller arrangements are lower than that of open propeller which indicates that propeller operates more efficiently inside a duct. However, thrust obtained from the duct decreases and reaches negative values with increasing advance ratio which makes duct unfavorable at high freestream velocities. Pressure measurements show that, the origin of the duct thrust is suction effect induced by the propeller in converging part (forebody) of the duct, and magnitudes of pressure coefficients inside forebody diminish with increasing advance ratio. Optimization of inlet shape for all advance ratio range of the vehicle is essential for a better design.

Transverse jet-cavity interactions with the influence of an impinging shock

For high-speed air breathing engines, fuel injection and subsequent mixing with air is paramount for combustion. The high freestream velocity poses a great challenge to efficient mixing both in macroscale and microscale. Utilising cavities downstream of fuel injection locations, as a means to hold the flow and stabilise the combustion, is one mechanism which has attracted much attention, requiring further research to study the unsteady flow features and interactions occurring within the cavity. In this study we combine the transverse jet injection upstream of a cavity with an impinging shock to see how this interaction influences the cavity flow, since impinging shocks have been shown to enhance mixing of transverse jets. Utilising qualitative and quantitative methods: schlieren, oilflow, PIV, and PSP the induced flowfield is analysed. The impinging shock lifts the shear layer over the cavity and combined with the instabilities generated by the transverse jet creates a highly complicated flowfield with numerous vertical structures. The interaction between the oblique shock and the jet leads to a relatively uniform velocity distribution within the cavity.

Simulation of Wave-Flow-Cavitation Interaction Using a Compressible Homogenous Flow Method

AbstractA numerical method based on a homogeneous single-phase flow model is presented to simulate the interaction between pressure wave and flow cavitation. To account for compressibility effects of liquid water, cavitating flow is assumed to be compressible and governed by time-dependent Euler equations with proper equation of state (EOS). The isentropic one-fluid formulation is employed to model the cavitation inception and evolution, while pure liquid phase is modeled by Tait equation of state. Because of large stiffness of Tait EOS and great variation of sound speed in flow field, some of conventional compressible gasdynamics solvers are unstable and even not applicable when extended to calculation of flow cavitation. To overcome the difficulties, a Godunov-type, cell-centered finite volume method is generalized to numerically integrate the governing equations on triangular mesh. The boundary is treated specially to ensure stability of the approach. The method proves to be stable, robust, accurate, time-efficient and oscillation-free.Novel numerical experiments are designed to investigate unsteady dynamics of the cavitating flow impacted by pressure wave, which is of great interest in engineering applications but has not been studied systematically so far. Numerical simulation indicates that cavity over cylinder can be induced to collapse if the object is accelerated suddenly and extremely high pressure pulse results almost instantaneously. This, however, may be avoided by changing the traveling speed smoothly. The accompanying huge pressure increase may damage underwater devices. However, cavity formed at relatively high upstream speed may be less distorted or affected by shock wave and can recover fully from the initial deformation. It is observed that the cavitating flow starting from a higher freestream velocity is more stable and more resilient with respect to perturbation than the flow with lower background speed. These findings may shed some light on how to control cavitation development to avoid possible damage to operating devices.

Delay of natural transition with dielectric barrier discharges

Delay of boundary-layer transition along a flat plate with adverse pressure gradient is achieved via dielectric barrier discharges. In contrast to earlier investigations, transition is initiated by naturally occurring Tollmien–Schlichting waves. A single dielectric barrier discharge actuator is used to create a body force that locally alters the flow stability leading to an attenuation of broadband disturbances. Hot-wire measurements characterize the resulting transition delay, and particle image velocimetry clarifies the local influence on the velocity profiles. Linear stability theory is applied to analyze a numerical solution of this boundary-layer flow, showing good agreement with the experimentally measured data. The stability analysis shows that disturbances are locally attenuated and transition is correctly predicted to occur at Reynolds numbers increased by 10 %. In contrast to previous investigations, the experiments are conducted at a reasonably high free-stream velocity of 20 m/s paving the way for planned in-flight experiments.

Modeling of quasi-steady sodium droplet combustion in convective environment

In the event of accidental leakage of sodium from the systems of a Liquid Metal Fast Breeder Reactor (LMFBR), a spray of liquid sodium droplets may be formed which will burn by reacting with the surrounding atmospheric oxygen. In order to understand the burning characteristics of the complete spray, combustion of an individual sodium droplet forms the basis and this has been investigated in the present study. A comprehensive numerical model has been developed to analyze the isolated sodium droplet combustion in a mixed convective environment. The governing equations for mass, momentum, species and energy conservation have been solved in axisymmetric cylindrical coordinates using the Finite Volume Method (FVM). Finite rate kinetic mechanisms have been incorporated to simulate droplet burning, using available kinetics data for basic sodium oxidation reactions. Salient features of the numerical model include a global single-step reaction for sodium oxidation in air and the incorporation of property variations with temperature and concentration. An equilibrium mixture of sodium peroxide, sodium monoxide and sodium vapor is considered as the final reaction product, taking into account the effects of dissociation reactions. The numerical model has been validated with experimental results available in literature. Results for the fuel mass burning rates and flame shapes are presented for different sizes of droplets burning under different free-stream conditions. The model predicts the occurrence of envelope flame around a sodium droplet even at fairly high free-stream velocities.

Laminar flame propagation on a horizontal fuel surface: Verification of classical Emmons solution

This work analyses the classical Emmons (1956) solution of flat plate laminar flame combustion on a film of liquid fuel. A two-dimensional (2D) numerical model developed for this purpose has been benchmarked with experimental results available in the literature for methanol. In the parametric study, numerical predictions have been compared with Emmons classical solution. The study shows that the Emmons solution is valid in a range of Reynolds numbers where flame anchors near the leading edge of the methanol pool and the combustion zone is confined around the hydrodynamic and thermal boundary layers. However, in cases of low free stream velocities the combustion zone is beyond the boundary layer zone and the Emmons solution deviates. In cases of very high free stream velocities, the flame moves away from the leading edge and anchors at a location downstream. The Emmons solution is not applicable in this case as well. For the fuel considered in this study (methanol), accounting for thermal radiation, employing an optically thin radiation model, allows better agreement between experimental and numerical temperature profiles but does not affect the mass burning rates.

Wind-tunnel Modelling of Dispersion from a Scalar Area Source in Urban-Like Roughness

A wind-tunnel study was conducted to investigate ventilation of scalars from urban-like geometries at neighbourhood scale by exploring two different geometries a uniform height roughness and a non-uniform height roughness, both with an equal plan and frontal density of λ p = λ f = 25%. In both configurations a sub-unit of the idealized urban surface was coated with a thin layer of naphthalene to represent area sources. The naphthalene sublimation method was used to measure directly total area-averaged transport of scalars out of the complex geometries. At the same time, naphthalene vapour concentrations controlled by the turbulent fluxes were detected using a fast Flame Ionisation Detection (FID) technique. This paper describes the novel use of a naphthalene coated surface as an area source in dispersion studies. Particular emphasis was also given to testing whether the concentration measurements were independent of Reynolds number. For low wind speeds, transfer from the naphthalene surface is determined by a combination of forced and natural convection. Compared with a propane point source release, a 25% higher free stream velocity was needed for the naphthalene area source to yield Reynolds-number-independent concentration fields. Ventilation transfer coefficients w T /U derived from the naphthalene sublimation method showed that, whilst there was enhanced vertical momentum exchange due to obstacle height variability, advection was reduced and dispersion from the source area was not enhanced. Thus, the height variability of a canopy is an important parameter when generalising urban dispersion. Fine resolution concentration measurements in the canopy showed the effect of height variability on dispersion at street scale. Rapid vertical transport in the wake of individual high-rise obstacles was found to generate elevated point-like sources. A Gaussian plume model was used to analyse differences in the downstream plumes. Intensified lateral and vertical plume spread and plume dilution with height was found for the non-uniform height roughness.

Subcritical and supercritical droplet evaporation within a zero-gravity environment: Low Weber number relative motion

A validated comprehensive axisymmetric numerical model, which includes the high pressure transient effects, variable thermo-physical properties and inert species solubility in the liquid phase, has been employed to study the evaporation of moving n-heptane droplets within a zero-gravity nitrogen environment, for a wide range of ambient pressures and initial freestream velocities. At the high ambient temperature considered (1000 K), the evaporation constant increases with the ambient pressure. At low ambient pressure, the evaporation constant becomes almost a constant during the end of the lifetime. At high ambient pressures, the transient behavior is present throughout the droplet lifetime. The final penetration distance of a moving droplet decreases exponentially with increasing ambient pressure. The average evaporation constant increases with ambient pressure. The variation is almost linear for reduced ambient pressures smaller than approximately 2. For higher values, depending on the initial freestream velocity, the average evaporation constant either becomes a constant (at low initial freestream velocities) or it non-linearly increases (at high initial freestream velocities) with the ambient pressure. Droplet lifetime decreases with increasing ambient pressure and/or increasing initial freestream velocity.

Laminar flow over a steadily rotating circular cylinder under the influence of uniform shear

The present study has numerically investigated two-dimensional laminar flow over a steadily rotating circular cylinder with uniform planar shear, where the free-stream velocity varies linearly across the cylinder diameter. It aims to find the combined effects of shear and rotation on the flow. Numerical simulations using the immersed-boundary method are performed on the flow in the ranges of mainly ∣α∣⩽2.5 (low rotational speeds) and additionally 2.5<∣α∣⩽5.5 (higher rotational speeds) for 0⩽K⩽0.2 at a fixed Reynolds number of Re=100, where K and α are, respectively, the dimensionless shear rate and rotational speed. For low rotational speeds, results show that the positive shear, with the upper side having the higher free-stream velocity than the lower, favors the effect of the counterclockwise rotation (α>0) but countervails that of the clockwise rotation (α<0). Accordingly, the absolute critical rotational speed (∣αL∣), below which von Kármán vortex shedding occurs, decreases with increasing K for α>0, but increases for α<0. The vortex-shedding frequency (St) rises with increasing α (including the negative) and the increment slope (dSt∕dα) becomes steeper with increasing K. The mean lift slightly decreases with increasing K regardless of the rotational direction. However, the variations of the amplitudes of the lift- and drag-fluctuations as well as the mean drag with the shear rate depend strongly on the rotational direction. They all decrease with increasing K for α>0, but rise for α<0. For higher rotational speeds, on the other hand, the flow characteristics also depend significantly on the shear rate and rotational speed (including the rotational direction). Flow statistics as well as instantaneous flow fields are presented to identify the characteristics of the flow and then to understand the underlying mechanism.

Aerodynamic Entropy Generation Rate in a Boundary Layer With High Free Stream Turbulence

Aerodynamic Entropy Generation Rate in a Boundary Layer With High Free Stream Turbulence