Kinetic Energy Levels Research Articles

An experimental investigation on the aerodynamics of the Pseudaletia Unipuncta moth subjected to varying upstream flow conditions

The effects of varying flow conditions on the flight and wake of the Pseudaletia Unipuncta moth were investigated, for the first time, using Particle Image Velocimetry (PIV) measurements on a live tethered moth, subjected to three upstream flow conditions: no flow (hover), smooth flow and perturbed flow. High-Speed Imaging (HSI) was used to study the basic kinematics of the moth, from which the wingbeat frequency (F) of the moth was found to be approximately 45 Hz. In configurations with no upstream flow, a possible Leading Edge Vortex (LEV) was observed as well as counter-rotating vortices in the moth’s wake. Varying the upstream flow conditions significantly changed the mean turbulent statistics in the moth’s wake, with larger levels of mean turbulent kinetic energy being observed in configurations with upstream flow compared to configurations with no upstream flow. It was observed that the mean flow perturbation modified the wake generated by the moth.

Experimental modal analysis of RC beams strengthened with SHCC subjected to shear under impact strain rates

Strain Hardening Cement-based Composites (SHCC) holds great potential to be a remarkably effective material for shear strengthening of reinforced concrete beams in case of impact scenarios due to its great energy dissipation capability through the formation of multiple very fine cracks (∼100 μm of width). The effectiveness of this technique is investigated in the paper at hand through the observation of the changes in the dynamic properties in terms of natural frequencies, mode shapes, and damping ratios. Considering a non-destructive free vibration excitation of the specimens before the experiment (undamaged/intact state) to their condition after the destructive impact of an accelerated steel projectile. In this sense, two types of reinforcement configurations and two types of strengthening layers were investigated: reinforced concrete beams with and without stirrups were strengthened with SHCC made with polyvinyl alcohol (PVA) and ultra-high molecular weight polyethylene (UHMWPE) and tested under increasing levels of kinetic energy varying between 2.1 kJ and 6.4 kJ by using an accelerated impactor inside a drop tower facility. The parameter variation enabled conclusions with respect to the influence of the stirrup reinforcement and the SHCC strengthening contributions in the case of dynamic scenarios.

Wind Turbine Systematic Wake to Wake Analysis: Quantification of Turbulence and Statistical Analysis for Optimal Wind Farm Design

Through this work, we have analyzed wake to wake interactions between horizontal axis wind turbines located in line to each other. We have quantified wake interactions based on hot wire anemometric data and statistical correlations using a model wind farm fabricated inside a wind tunnel. The results of these measurements show that a high level of turbulent kinetic energy is generated at the region of intersection between rotor and stator wakes. Numerical models of wake interactions of wind turbines under varying turbulent inflow conditions using the actuator line technique and EllipSys3D Navier-Stokes solver were carried out to resolve dynamics of wake interactions. Our results showed good agreement with field measurements of wake deficit and turbulence intensity for inline and eccentric turbine arrangements.

Investigation on Jet Characteristics of Turbofan Exhaust System under Take-off Condition with High Angle of Attack

To understand the jet flow characteristics of turbofan separate exhaust system, a parametric design method based on the initial Class Shape Transformation function was developed. HBPR and UHBPR turbofan separate exhaust systems were designed. Furthermore, the jet flow characteristics of the HBPR turbofan exhaust system under take-off condition with zero angle of attack were studied based on numerical simulation. The jet flow characteristics of the HBPR and UHBPR turbofan exhaust system under take-off condition with high angle of attack were also simulated. The effects of angle of attack and bypass ratio on the jet flow characteristics were investigated and the related flow mechanisms were analyzed. Results show that the axisymmetric plumes of the HBPR turbofan exhaust system are distributed around the engine axis under take-off condition with zero angle of attack. With the plug wake as the center, the core flow, the fan/core shear layer, the fan flow, the fan/free stream shear layer and the free stream are wrapped around the plug wake from inside out. Vortexes appear in the lee area at the back of the cowl and jet flow under take-off condition with high angle of attack. These vortexes cause cross sectional secondary flow and expose the high-velocity core flow to the low-velocity free stream. The contact area and velocity gradient in the mixing region among the free stream, fan flow and core flow increase. Therefore, the mixture among jet flow and free stream strengthens. So the high-velocity region, the high-vorticity region, and the high turbulence kinetic energy region shorten by 55.1%, 47.7% and 50.9% respectively. The vorticity values and turbulence kinetic energy level peak on the upper side of the exhaust plumes increase by about 30% and 87% respectively. Relative to these parameters from the HBPR turbofan exhaust system, the jet velocity peak value of UHBPR turbofan decreases by 5.5% under take-off condition with high angle of attack. The vorticity values and turbulence kinetic energy level reduce due to decreased velocity gradient in shear layers downstream of the nozzle exit plane. The turbulence kinetic energy level peak on the upper side of the exhaust plumes decreases by 29.3%. The reasons are that the contact area between high-velocity core flow and the free stream decreases due to thicker fan flow and the velocity gradient in the core flow and free stream mixing region decreases because of the lower core flow velocity.

Effects of different mechanical and chemical surface post-treatments on mechanical and surface properties of as-built laser powder bed fusion AlSi10Mg

Laser powder bed fusion (LPBF) materials have various surface defects, which can detrimentally affect the mechanical properties and fatigue behavior of these materials. Surface post-processing can be crucial to reduce or eliminate these surface imperfections. In this study, the effects of different post-processing methods including shot peening, ultrasonic nanocrystalline surface modification and severe vibratory peening as well as electro-chemical polishing and other hybrid treatments obtained from their combination were surveyed on microstructure, surface and mechanical properties of LPBF AlSi10Mg specimens. Two different processes with varying levels of kinetic energy were considered for each mechanical surface treatment. The results indicate that mechanical treatments can effectively modify the surface morphology, improve surface hardness and induce compressive residual stresses. In addition, electro-chemical polishing was highly efficient in surface roughness reduction and increasing the wettability of the hybrid mechanically and chemically treated specimens.

Development of the wake shed by a system composed of a propeller and a rudder at incidence

Large Eddy Simulations are reported on a system consisting of a marine propeller and a hydrofoil, mimicking a rudder, for four orientations of the latter, modeling maneuvering conditions in marine propulsion. A cylindrical grid composed of almost 4 billion points was adopted. A detailed analysis of the overall wake downstream of the system is presented, with a focus on turbulent fluctuations. They are of interest, because of their importance in affecting the acoustic signature.In the near wake the streamwise evolution of turbulent kinetic energy is rising downstream of all configurations, due to the growing shear between the wakes shed by the propeller and the hydrofoil, respectively. Increasing the incidence of the hydrofoil results in a growing complexity of the wake topology and higher levels of turbulent kinetic energy. The latter are due to the adverse pressure gradient on the suction side of the hydrofoil and especially to the shear developed between the branch of the propeller wake coming from the pressure side of the hydrofoil and the shear layer from the trailing edge of the latter. In addition, the cross-stream distribution of turbulence undergoes an increasing deviation from the anti-symmetry developed at conditions of 0° incidence, because of the different cross-stream velocities gained by the two branches of the propeller wake while moving across the pressure and suction sides of the hydrofoil.

Numerical and experimental study of the ducted diffuser effect on improving the aerodynamic performance of a micro horizontal axis wind turbine

In the present study, the impact of utilizing an encompassing duct on the aerodynamic performance of a micro horizontal axis wind turbine (HAWT) was numerically and experimentally studied. A duct comprised of a diffuser and a flange was designed and constructed. A wind simulator facility and a micro scale wind turbine were employed to conduct the experimental investigations. Measurements of velocity and power output of the turbine have been carried out. The numerical results obtained at different stages of the research were validated separately and then compared with our experimental data and those obtained by other researchers. Consequently, the numerical predictions match well with the experimental measurements. In the first part of the study, fluid flow through the empty duct was analyzed. Then, by adding a nozzle at the diffuser entrance of the original duct, an improved design was proposed and numerically modeled to achieve higher wind speed at the throat position of the duct. The modified design was further enhanced by altering the converging and diverging section's angles (ranging from 0 to 90° with the angle increment step of 5°). Fifty-two different duct configurations have been considered to achieve the optimal angles of the nozzle, the diffuser, and the flange. Moreover, the aerodynamic performance of the micro HAWT was studied in three different conditions: without a duct, with the original duct, and with the improved duct. The results show that the improved duct increases the inlet wind speed from 5 m/s to 10.7 m/s (i.e., up to 2.14 times) at the throat position, which increases the wind speed up to 47% compared to the original duct. Furthermore, the power coefficient of the turbine increases from 0.33 to 1.2 (i.e., up to 3.64 times) in comparison to the case without duct, which increases by about 164% compared to the original duct. Also, employing the improved duct design causes a lower level of flow turbulence kinetic energy compared to the original duct design. This valuable outcome reduces the noise level generated by the system and the dynamics forced exerted by the rotor to other possible downstream structures.

Research on the Impacts of the Inertia and Droop Control Gains from a Variable-Speed Wind Turbine Generator on the Frequency Response

System frequency must be kept very close to its nominal range to ensure the stability of an electric power grid. Excessive system frequency variations are able to result in load shedding, frequency instability, and even generator damage. With increasing wind power penetration, there is rising concern about the reduction in inertia response and primary frequency control in the electric power grid. Converter-based wind generation is capable of providing inertia response and primary frequency response; nevertheless, the primary frequency and inertia responses of wind generation are different from those of conventional synchronous fleets; it is not completely understood how the primary frequency and inertia responses affect the given system under various disturbances and available kinetic energy levels. Simulations are used to investigate the influences of inertia and droop control strategies on the dynamic frequency responses, particularly the index of the second frequency drop under various disturbance and wind conditions. A quantitative analysis provides insight into setting of inertia and droop control coefficients for various wind and disturbance conditions to facilitate adequate dynamic frequency responses during frequency events.

Different mechanisms for the seasonal variations of the mesoscale eddy energy in the South China Sea

Using a recently developed functional analysis tool, Multiscale Window Transform (MWT) and the MWT-based theory of canonical transfer, this study investigates the dynamical processes responsible for the seasonal variability of the mesoscale eddies in the South China Sea (SCS). A three-scale energetics framework is employed, in which the original fields are decomposed into subfields on a nonstationary background flow window, a mesoscale eddy window and a high-frequency synoptic eddy window. Three local regions of high mesoscale eddy kinetic energy (EKE) levels, namely, the southwest of Taiwan Island (SWT), the southeast of Vietnam (SEV) and the northeast of the Natuna Island (NEN), are identified. The seasonal cycles of the mesoscale EKE in the three regions are not in phase, with peaks occurring around January, October and December, respectively. By diagnosing the canonical transfers between the background flow and the mesoscale eddies, we find that the seasonal cycle of the eddy variability in the SWT region is due to a mixed instability (baroclinic and barotropic instabilities), while barotropic instability plays a dominant role in determining the EKE seasonality in the SEV and NEN regions. This is quite different from the previous argument that baroclinic instability dominates the seasonal EKE in the SCS. The three-scale framework further reveals that high-frequency synoptic motions act to dissipate the lower-frequency mesoscale eddies through forward energy cascades. Besides the internal processes, external wind forcing is also found to influence the seasonal eddy variability, but is only limited to the NEN region. Our results highlight different mechanisms controlling the seasonal modulations of the mesoscale eddies in different regions of the SCS.

Scale interactions and anisotropy in Rayleigh–Taylor turbulence

We study energy scale transfer in Rayleigh–Taylor (RT) flows by coarse graining in physical space without Fourier transforms, allowing scale analysis along the vertical direction. Two processes are responsible for kinetic energy flux across scales: baropycnal work $\varLambda$ , due to large-scale pressure gradients acting on small scales of density and velocity; and deformation work $\varPi$ , due to multiscale velocity. Our coarse-graining analysis shows how these fluxes exhibit self-similar evolution that is quadratic-in-time, similar to the RT mixing layer. We find that $\varLambda$ is a conduit for potential energy, transferring energy non-locally from the largest scales to smaller scales in the inertial range where $\varPi$ takes over. In three dimensions, $\varPi$ continues a persistent cascade to smaller scales, whereas in two dimensions $\varPi$ rechannels the energy back to larger scales despite the lack of vorticity conservation in two-dimensional (2-D) variable density flows. This gives rise to a positive feedback loop in 2-D RT (absent in three dimensions) in which mixing layer growth and the associated potential energy release are enhanced relative to 3-D RT, explaining the oft-observed larger $\alpha$ values in 2-D simulations. Despite higher bulk kinetic energy levels in two dimensions, small inertial scales are weaker than in three dimensions. Moreover, the net upscale cascade in two dimensions tends to isotropize the large-scale flow, in stark contrast to three dimensions. Our findings indicate the absence of net upscale energy transfer in three-dimensional RT as is often claimed; growth of large-scale bubbles and spikes is not due to ‘mergers’ but solely due to baropycnal work $\varLambda$ .

Scale-Resolving Simulations of Steady and Pulsatile Flow Through Healthy and Stenotic Heart Valves.

Blood-flow downstream of stenotic and healthy aortic valves exhibits intermittent random fluctuations in the velocity field which are associated with turbulence. Such flows warrant the use of computationally demanding scale-resolving models. The aim of this work was to compute and quantify this turbulent flow in healthy and stenotic heart valves for steady and pulsatile flow conditions. Large eddy simulations (LESs) and Reynolds-averaged Navier-Stokes (RANS) simulations were used to compute the flow field at inlet Reynolds numbers of 2700 and 5400 for valves with an opening area of 70 mm2 and 175 mm2 and their projected orifice-plate type counterparts. Power spectra and turbulent kinetic energy were quantified on the centerline. Projected geometries exhibited an increased pressure-drop (>90%) and elevated turbulent kinetic energy levels (>147%). Turbulence production was an order of magnitude higher in stenotic heart valves compared to healthy valves. Pulsatile flow stabilizes flow in the acceleration phase, whereas onset of deceleration triggered (healthy valve) or amplified (stenotic valve) turbulence. Simplification of the aortic valve by projecting the orifice area should be avoided in computational fluid dynamics (CFD). RANS simulations may be used to predict the transvalvular pressure-drop, but scale-resolving models are recommended when detailed information of the flow field is required.

Intraseasonal Variability of the North Equatorial Current Bifurcation Off the Philippines

AbstractCharacteristics of the intraseasonal variability (ISV) of the North Equatorial Current bifurcation latitude (NBL) are analyzed by using both observations and reanalysis data. The maximum amplitude of the ISV of the NBL can reach about 6° of latitude, which almost triples its annual excursion. The NBL shows a broad spectral peak around 50–140 days with the highest peak occurring around 73 days. Both the Madden‐Julian oscillation and mesoscale eddies have a significant impact on its ISV. A case study for a cyclonic eddy indicates that it can transport about 1.5 Sv (1 Sv–106 m3 s−1) waters to the NBL, and the bifurcation shifts northward with an amplitude of about 8° of latitude in 1 month. Meanwhile, the influence of mesoscale eddies on the ISV of the NBL is highly modulated by the interannual variations of the eddy field. Active eddy activities associated with enhanced barotropic and baroclinic instabilities show a large influence on the NBL. The eddy kinetic energy level around the NBL showed a quite diverse character for the 1997–1998 and the 2015–2016 extreme El Niños, with the latter being double or triple that of the former due to the much stronger vertical and horizontal shears. Given the significant impact on the NBL by the individual eddies and the unevenly distributed cyclonic and anticyclonic eddies around the NBL, the effect of mesoscale eddies on the bifurcation latitude may not cancel each other on seasonal or interannual time scales.

Channel curvature improves water quality and nutrient filtering in an artificially deepened mesotidal idealized estuary

Estuarine ecology suffers from both physical aspects of human influence, such as dredging, and biogeochemical aspects, such as eutrophication. Apart from being dredged, modern estuaries often manifest rectified geometries deprived of meanders or other nonlinear topographic features. This study has two overarching aims, a theoretical and a practical one. The theoretical objective is to establish an understanding of the effect of physical dynamics induced by channel meanders on the biogeochemical dynamics in a typical estuarine oxygen minimum zone. The practical aim is to clarify whether and how channel curvature can mitigate the consequences of human intervention, such as dredging and eutrophication. To answer these questions, a coupled hydrodynamic and water quality model is applied to a pair of idealized funnel-shaped topographies with dimensions and axial depth distribution similar to the Elbe Estuary, Germany, serving as the prototype estuary in this study. The first topography is symmetric about the channel axis (straight channel), while the second topography contains a small section of sinusoidal meanders in the dredged limnic reach of the estuary. The setups are forced by an M-2 tide and daily salinity and temperature data at the seaward open boundary. Atmospheric and river forcings are based on regional operational and observational data to impose seasonal temperature variability and biogeochemical cycles. The model simulates tidally driven estuarine physics, as well as seasonal estuarine cycles and axial gradients of nutrients, oxygen and plankton that characterize alluvial human-shaped and eutrophied estuaries. The channel meanders in the lower limnic reaches lead to locally enhanced ebb dominance, vertical overturning and increased levels of turbulent kinetic energy. The curvature-induced dynamics decrease turbidity levels by up to 12.5% and increase oxygen concentrations by up to 14% in the area of the oxygen minimum zone, improving the ecological status of the eutrophied estuary. Finally, we assess the sustainability of the ecological benefits of the channel meanders in the face of global warming by applying a simple 2 °C warming scenario to the straight and meandering channel cases. We demonstrate that channel meanders potentially improve estuarine ecology even under increased pressure due to climate change.

RANS modeling of turbulent flow and heat transfer of non-Newtonian viscoplastic fluid in a pipe

A mathematical model of the movement and heat transfer of a turbulent non-isothermal non-Newtonian fluid through a pipe wall with a cold surrounding space has been developed and simulated numerically. Fluid turbulence is described in the framework of the isotropic two-parameter k– ε˜ model. The Newtonian properties of the fluid in the initial cross-sections of the pipe transformed gradually into a viscoplastic non-Newtonian Bingham-Schwedoff fluid state due to heat transfer through the pipe wall between the heated fluid and a cold environment. The value of its streamwise velocity in the axial zone increased significantly when the fluid moved along the pipe. On the contrary, it decreased in the near-wall zone and the height of the region with a zero fluid velocity increased. This occurred due to the viscoplastic properties of a non-Newtonian fluid. The height of the region with a zero fluid velocity in the pipe increased gradually as the non-Newtonian fluid (waxy crude oil) moved through the pipe. A noticeable increase in the level of turbulent kinetic energy in the axial zone of the pipe and its noticeable decrease in its near-wall region were observed. A significant increase in the average dynamic viscosity and yield stress in the near-wall part of the pipe was shown. The boundary of the area of existence of Newtonian properties of fluid was determined. The height of the region with a zero fluid velocity in the pipe increased gradually as waxy crude oil moved through the pipe and reached y/R ≈ 0.1 at x/D = 15.

Numerical Analysis of Natural Convection Driven Flow of a Non-Newtonian Power-Law Fluid in a Trapezoidal Enclosure with a U-Shaped Constructal

Placement of fins in enclosures has promising utilization in advanced technological processes due to their role as heat reducing/generating elements such as in conventional furnaces, economizers, gas turbines, heat exchangers, superconductive heaters and so forth. The advancement in technologies in power engineering and microelectronics requires the development of effective cooling systems. This evolution involves the utilization of fins of significantly variable geometries enclosed in cavities to increase the heat elimination from heat-generating mechanisms. Since fins are considered to play an effective role in the escalation of heat transmission, the current study is conducted to examine the transfer of heat in cavities embedding fins, as well as the effect of a range of several parameters upon the transmission of energy. The following research is supplemented with the interpretation of the thermo-physical aspects of a power-law liquid enclosed in a trapezoidal cavity embedding a U-shaped fin. The Boussinesq approximation is utilized to generate the mathematical attributes of factors describing natural convection, which are then used in the momentum equation. Furthermore, the Fourier law is applied to formulate the streaming heat inside the fluid flow region. The formulated system describing the problem is non-dimensionalized using similarity transformations. The geometry of the problem comprises a trapezoidal cavity with a non-uniformly heated U-shaped fin introduced at the center of the base of the enclosure. The boundaries of the cavity are at no-slip conditions. Non-uniform heating is provided at the walls (l1 and l2), curves (c1,c2 and c3) and surfaces (s1 and s2) of the fin; the upper wall is insulated whereas the base and sidewalls of the enclosure are kept cold. The solution of the non-dimensionalized equations is procured by the Galerkin finite element procedure. To acquire information regarding the change in displacement w.r.t time and temperature, supplementary quadratic interpolating functions are also observed. An amalgam meshing is constructed to elaborate the triangular and quadrilateral elements of the trapezoidal domain. Observation of significant variation in the flow configurations for a specified range of parameters is taken into consideration i.e., 0.5≤n≤1.5 and 104≤Ra≤106. Furthermore, flow structures in the form of velocity profiles, streamlines, and temperature contours are interpreted for the parameters taken into account. It is deduced from the study that ascending magnitude of (Ra) elevates level of kinetic energy and magnitude of heat flux; however, a contrary configuration is encapsulated for the power-law index. Navier–Stokes equations constituting the phenomenon are written with the help of non-dimensionalized stream function, temperature profiles, and vortices, and the solutions are acquired using the finite element method. Furthermore, the attained outcomes are accessible through velocity and temperature profiles. It is worth highlighting the fact that the following analysis enumerates the pseudo-plastic, viscous and dilatant behavior of the fluid for different values of (n). This study highlights that the momentum profile and the heat transportation increase by increasing (Ra) and decline as the viscosity of the fluid increases. Overall, it can be seen from the current study that heat transportation increases with the insertion of a fin in the cavity. The current communication signifies the phenomenon of a power-law fluid flow filling a trapezoidal cavity enclosing a U-shaped fin. Previously, researchers have studied such phenomena mostly in Newtonian fluids, hence the present effort presents novelty regarding consideration of a power-law liquid in a trapezoidal enclosure by the placement of a U-shaped fin.

Mathematical Modelling of the Fruit-Stone Culture Seeds Calibration Process Using Flat Sieves

Abstract The paper provided describes a mathematical model of calibration process of fruit-stone culture seeds of cherry, sweet cherry, cherry-plum, apricot and almond using flat sieves with impact shock ball cleaners oscillating in the horizontal plane. It has been defined that the mathematical expectation of time of knocking out the fruit-stone from the sieve opening T ⌢ \mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\frown$}}\over T} is the minimum value of ratio of average time of complete ball motion cycle in space under sieve to the probability of knocking out the stone by a ball with the kinetic energy level of 2 Mj. The dependences of energy distribution density of ball on impact on the sieve have been obtained, based on which the intervals of ball cleaner parameters have been determined, i.e. the ball diameter D belongs to the interval 25–35 mm; the space height H under sorting sieve belongs to the interval 1.2D–1.4D mm; the value range for distance between rods t belongs to the interval 0.5D–0.7D mm. Using the method of golden section, the following parameters of ball cleaner were obtained: D = 33 mm, t = 23 mm, H = 40 mm. The parameters obtained provide mathematical expectation of time of knocking out the fruit-stone from the sieve opening T ⌢ \mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\frown$}}\over T} = 0.03 s. Consequently, the average ball velocity v ⌢ \mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\frown$}}\over v} is = 0.4 m∙s-1, and the average ball path is L ⌢ \mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\frown$}}\over L} = 0.006 m.

Thermal ablation mechanism of polyimide reinforced with POSS under atomic oxygen bombardment

The thermal stability and reactive atomic oxygen (AO) resistance of polyhedral oligomeric silsesquioxane (POSS)-reinforced polymer nanocomposites have attracted significant interest from both experimental and theoretical researchers. However, understanding the damage-mitigation mechanism of inorganic core constituents in POSS hybrids is still limited due to the lack of empirical insights. In this study, the ablation resistance of polyimide (PI)/POSS nanocomposites was examined using reactive molecular dynamics and first-principles calculations in terms of physicochemical changes in the POSS cage structure. According to the AO collision kinetic energy and frequency level, the origin of ablation resistance in the system is classified. Each damage-mitigation behavior is strongly correlated with the normalized residue mass and byproduct distribution during the erosion of the surface. In particular, the results suggest that the kinematic collapse of POSS prevents the exfoliation of large carbonic molecules and plays a crucial role in the formation of a ceramic passivation layer, which significantly improves the damage-mitigation effect. Therefore, this study provides a new perspective on the design of ablative thermal protection systems by understanding the behavior of the inorganic nanocage constituting POSS hybrids at the molecular level.

Numerical Study on Natural Ventilation around a Burning Car inside a Tunnel

This study aimed to analyse the flow and temperature fields around a car on fire inside a medium-size tunnel under natural ventilation. The study used the fire dynamics simulator (FDS), which is an open-source software package, to simulate the thermo-fluidic characteristics inside the tunnel. A constant heat release rate (HRR) was assumed to ensure the release of consistent heat from the car while varying the speed of the mass flux natural ventilation. A medium-size tunnel and scaled car were modelled to simulate a realistic field environment; the car was assumed to have its primary components, including the body frame, tyres, seats, and other flammable materials. The constant HRR was set at 3.8 , which was measured in a field experiment using a cone calorimeter. The closed car windows were set to break when the temperature around them reached 600°C. Additionally, natural ventilation was one of the parameters used in the calculation; it was assigned as an inlet condition, referred to as the steady longitudinal inlet velocity, and ranged from 1.8 to 3.0 m/s. Based on the ventilation velocities, the variations of the flow characteristics, temperature field, turbulence kinetic energy levels, and vortex structure inside the tunnel were investigated. Additionally, the critical longitudinal velocity for natural ventilation was estimated by performing the current simulation under different ventilation velocities. Given that the thermo-fluidic parameters and geometry information are all similar, the FDS results were compared to those from three semi-empirical models for predicting critical ventilation velocities from previous studies.

Simulation of turbulence induced sound generation inside stenosed femoral artery models with different severities and eccentricities

Background and objectivesRecent developments of low-cost, compact acoustic sensors, advanced signal processing tools and powerful computational resources allow researchers design new scoring systems for acoustic detection of arterial stenoses. In this study, numerical simulations of blood flow inside stenosed arteries are performed to understand the effect of stenosis severity and eccentricity on the turbulence induced wall pressure fluctuations and the generated sound. MethodsAxisymmetric and eccentric elliptic stenoses of five different severities are generated inside a 6.4 mm diameter femoral artery model. Large eddy simulations of pulsatile, non-Newtonian blood flow are performed using the open source software OpenFOAM. ResultsPost-stenotic turbulence activity is found to be almost zero for 50 and 60% severities. For severities of 75% and more, turbulent kinetic energy rises significantly with increasing severity. The location of the highest turbulence activity on the vessel wall from the stenosis exit decreases with increasing severity. The maximum level of turbulent kinetic energy seen in 95% severity models is about 9 and 31 times higher than that of 87% and 75% models, respectively. Spectrum of wall pressure fluctuations show that 50 and 60% axisymmetric models are almost silent. The spectrum starts to get richer with 75% severity, and the fluctuation intensity increases with severity. Compared to the axisymmetric models, more activity is observed in the 0–150 Hz band for the 50 and 60% eccentric models. Axial extent of the acoustically active region is also longer in them. Converting wall pressure data into sound revealed that murmurs that can be considered as signs of vascular stenosis are obtained for models with 75% and higher severity. ConclusionsSound patterns generated from simulation results are similar to the typical sounds obtained by Doppler ultrasonography, and present distinct characters. Together with a sensor technology that can measure these sounds from within the stenosed artery, they can be processed and used for the purpose of non-invasive diagnosis. Computational fluid dynamics studies that simulate large number of cases with different stenosis severities and morphologies will play a critical role in developing the necessary sound databases, which can be used to train new diagnostic devices.

Effects of inflow boundary layer on the wake of a radially non-uniform porous disk

This study presents the results of an experimental investigation focusing on the effects of the inflow boundary layer on the wake characteristics of a 0.12 m diameter porous disk with radially non-uniform porosity in terms of mean flow, turbulence, and wake scaling. Two-dimensional two-component particle image velocimetry measurements within the wake are performed up to 7.5 diameters downstream as the disk is lowered deeper into a boundary layer that is representative of a neutral atmospheric boundary layer over a flat terrain. Results show that otherwise symmetrical wake velocity profiles that exist outside the boundary layer get skewed and sheared around the disk centerline in the boundary layer due to the inflow wind shear. The turbulent kinetic energy, its production, and Reynolds shear stress levels in the wake get asymmetrical around the centerline of the disk such that the production of turbulent kinetic energy is observed to be higher above centerline. Due to the inflow shear, the wake centerline gets shifted downwards (i.e., toward the wind tunnel wall), which is in contrast to the observations on real wind turbine wakes in the literature where the wake actually lifts up. The asymmetrical and skewed velocity profiles both in the streamwise and cross-stream directions can be collapsed onto a single function by using proper wake scaling parameters based on the ratio of local strain to average strain within the velocity profile calculated separately for either side of the wake.