Non-dimensional Time Research Articles

Multiple solutions for the modified Fourier and Fick’s theories for Carreau nanofluid

The present paper numerically investigated the dual solutions of Carreau nanofluids in the presence of Cattaneo–Christov double diffusion with focus on heat and mass transfer which contains the effects of Brownian motion and thermophoresis parameter. A nonlinearly shrinking sheet has been utilized to create the flow. The thermal and concentration diffusions are considered by introducing Cattaneo–Christov fluxes. This paper provides information about the energy and concentration equations which are constructed with the help of Cattaneo–Christov double-diffusion theory in the existence of Brownian motion parameter and thermophoresis parameter. The study showed the local similarity variables are used to renovate the governing equations into a set of nonlinear ordinary differential equations. The ascending differential system which is a collection of momentum, temperature and concentration equations is preserved through a numerical approach called the Runge–Kutta–Fehlberg integration technique. The study reveals that the multiple solutions occur for the different vital physical parameters, for example, suction parameter s, Weissenberg number We, Prandtl number Pr, velocity slip parameter $$\delta $$ , viscosity ratio parameter $$\beta ^{*}$$ , non-dimensional thermal relaxation time $$\delta _{e}$$ , Brownian motion parameter Nb and thermophoresis parameter Nt. Moreover, higher values of thermal relaxation time $$\delta _{e}$$ decrease the temperature profile.

Effects on oily sand jet evolution from impact momentum and channelization of particles through an immiscible interface

The evolution of sand jets passing through an immiscible layer and entering water was experimentally investigated to understand the effects of controlling parameters on the formation of particle clusters. In order to increase the impact momentum, sand particles were released from different heights from the surface of the immiscible interface. The channelization of particles through the immiscible layer was found to be correlated with the ratio of the nozzle diameter to the diameter of sand particles. For relatively large nozzle diameters, dry particles directly impinged into the ambient water with minimum contact with the immiscible layer. The channelization of particles significantly reduced the particle momentum transfer and the energy dissipation within the immiscible layer. It was found that the channelization effect begins when the nozzle diameter is approximately forty times larger than the mean particle diameter. The dynamic characteristics of oily sand jets – penetration length, width, and frontal velocity – were measured and results were analyzed in both time and space frames. It was observed that the space domain is suitable for regime classification while the time domain is suitable for developing semi-empirical correlations. A non-dimensional time instance t* was defined for flow visualization and to study the transitional effects of the jets’ evolution where at t*=1 all particles enter into the ambient. The average shear stress in the immiscible layer and in the early stages of evolution (t*<1) was calculated from the measurements: the normalised shear stress between sand particles and the immiscible layer was found to linearly increase with the impact momentum. The average drag coefficient of sand jet front was calculated and results were compared with the classical drag models. The average drag coefficient of oily sand jets was found to be smaller than the drag coefficient of individual sand particles in a steady-state condition.

Regime transitions and energetics of sustained stratified shear flows

We describe the long-term dynamics of sustained stratified shear flows in the laboratory. The stratified inclined duct (SID) experiment sets up a two-layer exchange flow in an inclined duct connecting two reservoirs containing salt solutions of different densities. This flow is primarily characterised by two non-dimensional parameters: the tilt angle of the duct with respect to the horizontal, (a few degrees at most), and the Reynolds number , an input parameter based on the density difference driving the flow. The flow can be sustained with constant forcing over arbitrarily long times and exhibits a wealth of dynamical behaviours representative of geophysically relevant sustained stratified shear flows. Varying and leads to four qualitatively different regimes: laminar flow; mostly laminar flow with finite-amplitude, travelling Holmboe waves; spatio-temporally intermittent turbulence with substantial interfacial mixing; and sustained, vigorous interfacial turbulence (Meyer & Linden, J. Fluid Mech., vol. 753, 2014, pp. 242–253). We seek to explain the scaling of the transitions between flow regimes in the two-dimensional plane of input parameters . We improve upon previous studies of this problem by providing a firm physical basis and non-dimensional scaling laws that are mutually consistent and in good agreement with the empirical transition curves we inferred from 360 experiments spanning and . To do so, we employ state-of-the-art simultaneous volumetric measurements of the density field and the three-component velocity field, and analyse these experimental data using time- and volume-averaged potential and kinetic energy budgets. We show that regime transitions are caused by an increase in the non-dimensional time- and volume-averaged kinetic energy dissipation within the duct, which scales with at high enough angles. As the power input scaling with is increased above zero, the two-dimensional, parallel-flow dissipation (power output) increases to close the budget through an increase in the magnitude of the exchange flow, incidentally triggering Holmboe waves above a certain threshold in interfacial shear. However, once the hydraulic limit of two-layer exchange flows is reached, two-dimensional dissipation plateaus and three-dimensional dissipation at small scales (turbulence) takes over, at first intermittently, and then steadily, in order to close the budget and follow the scaling. This general understanding of regime transitions and energetics in the SID experiment may serve as a basis for the study of more complex sustained stratified shear flows found in the natural environment.

Analysis of melting behavior of PCMs in a cavity subject to a non-uniform magnetic field using a moving grid technique

Melting flow and heat transfer of electrically conductive phase change materials subjecting to a non-uniform magnetic field are addressed in a square enclosure. The top and bottom walls of the cavity are adiabatic, and the sidewalls are isothermal at different temperatures. The temperature of the hot wall is higher than the fusion temperature of PCM (Tf), and the cold wall is at the fusion temperature or lower. At the initial time, the cavity is filled with a solid saturated PCM. In the vicinity to the hot wall, there is an external line-source magnet, inducing a magnetic field. The location of the magnetic source (Y0) can be changed along the hot wall. The cavity domain is divided into two parts of the liquid domain and the solid domain. The moving grid method is utilized to track the phase change interface at the exact fusion temperature of Tf. The governing equations for continuity, flow and heat transfer associated with the Arbitrary Lagrangian–Eulerian (ALE) moving mesh technique are solved using the finite element method. The results are investigated for the melting behavior of PCM by the study of Hartmann number (0 ≤ Ha ≤ 50) and the location of the magnetic source (0 ≤ Y0 ≤ 1). Outcomes show that the effect of the magnetic field on the melting behavior of PCM is negligible at the initial stages of the melting (Fo < 1.15). However, after the initial stages of the melting, the effect of the presence of a magnetic field becomes significant. Moreover, the location of the magnetic source induces a feeble effect on the melting front at the initial melting stages, but its effect on the shape of the melting front increases by the increase of the non-dimensional time. The location of the magnetic source also significantly affects the streamlines patterns. Changing the position of the magnetic source from the bottom of the cavity (Y0 = 0.2) to the almost middle of the cavity (Y0 = 0.6) would decrease the required non-dimensional time of full melting from Fo = 10.4 to Fo = 9.0.

Effect of Prandtl number on leading edge accretion and ablation: A numerical study of unsteady boundary layer flow over a flat plate

AbstractAn efficient numerical method, namely, the Runge‐Kutta fourth order integration scheme with shooting technique is employed to give a suitable solution for the unsteady magnetohydrodynamic boundary layer flow of viscous incompressible fluid with accretion or ablation effects over a flat plate under the influence of homogenous first order chemical reaction. When compared to the other numerical techniques such as perturbation methods, this approach provides the accurate numerical results valid uniformly for all nondimensional time. The unsteady behavior of chemically reacting magnetohydrodynamic boundary layer flow is investigated by analyzing the nature of buoyancy and magnetic parameters in the momentum equation. Also, results are extended to the energy and concentration equations by considering the viscous dissipation, Joule heating and chemical reaction effects. With the help of suitable similarity transformations, the highly nonlinear, coupled, time‐dependent partial differential equations are reduced to ordinary differential equations. Furthermore, the numerical solutions in terms of velocity, temperature and concentration profiles within the boundary layer are presented for the various values of control parameters. Also, the impact of physical parameters on the flow, heat and mass transfer characteristics are examined thoroughly. The present investigation reports that, the increasing magnetic parameter increases the temperature field and decreases the velocity field. Also, Eckert number enhance the thermal field whereas, the chemical reaction parameter decays the concentration field. Before concluding the considered problem, present results are validated with the previous results and are found to be in good agreement.

Influence of the surface temperature on the spreading and receding dynamics of an impacting biodiesel drop

The paper presents an experimental study of the impact of a Jatropha biodiesel drop on a heated flat stainless steel surface at various temperatures and Weber numbers (We). The effect of the surface temperature (TS) on the spreading and the receding of the drop is explored. Entire impact process is recorded using a high-speed camera and the captured images are processed to obtain the evolution of the drop profile and impact morphology at various surface temperatures. The analysis of the morphology provides physical changes in the drop during impact. The maximum spreading factor, βmax (maximum spreading diameter normalized by pre-impact diameter), increases with both We and TS. The receding is found to be dependent on the surface temperature. The average velocity of the receding increases with TS, because of the decrease in surface free energy at higher temperatures. The decrease is explained from the measurement of the temperature-dependent static contact angles. The non-dimensional time taken to reach βmax increases with TS and this increment is more pronounced at higher We. βmax is found to be highly sensitive to We and shows a sharp rise at higher surface temperatures.

Hydrodynamic Features of an Undular Bore Traveling on a 1:20 Sloping Beach

The hydrodynamic characteristics, including local and convective accelerations as well as pressure gradient in the horizontal direction, of the external stream of an undular bore propagating on a 1:20 sloping beach are experimentally studied. A bore with the water depth ratio of 1.70 was generated downstream of a suddenly lifted gate. A high-speed particle image velocimetry was employed to measure the velocity fields during the run-up and run-down motions. The time series of free surface elevation and velocity field/profile of the generated bore, comprising a pure bore accompanied by a series of dispersive leading waves, are first demonstrated. Based on the fast Fourier transform (FFT) and inverse FFT (IFFT) techniques, the free surface elevation of leading waves and the counterpart of pure bore are acquired separately at a specified measuring section (SMS), together with the uniform horizontal velocity of the pure bore. The effect of leading-wave-induced velocity shift on the velocity profiles of the generated bore are then evaluated at the SMS. To understand the calculation procedure of accelerations and pressure gradient, three tabulated forms are provided as illustrative examples. Accordingly, the relationships among the partially depth-averaged values of the non-dimensional local acceleration, convective acceleration, total acceleration and pressure gradient of the generated/pure bore acquired at the SMS versus the non-dimensional time are elucidated. The trends in the non-dimensional accelerations and pressure gradient of the external stream of generated bore are compared with those of the pure bore. During the run-up motion from the instant of arrival of the bore front to the moment of the peak level at the SMS, continuous decrease in the onshore uniform horizontal velocity, and successive deceleration of the pure bore in the onshore direction are evidenced, exhibiting the pure bore under the adverse pressure gradient with decreasing magnitude. However, the pure bore once ridden by the leading waves is decelerated/accelerated spatially and accelerated/decelerated temporally in the onshore direction during the rising/descending free surface of each leading wave. This fact highlights the effect of pre-passing/post-passing of the leading wave crest on the velocity distribution of generated bore. It is also found that, although the leading waves have minor contribution on the power spectrum of the free surface elevation as compared with that of the pure bore, the leading waves do play an important role on the magnitudes of both accelerations and pressure gradient. The largest magnitude of the acceleration contributed by the leading waves is around 26 times the counterpart contributed by the pure bore. Further, during the run-down motion right after the moment for the peak level of the bore, a linear increase in the magnitude of the offshore uniform horizontal velocity and a constant local acceleration with increasing time are both identified. The partially depth-averaged value of the non-dimensional pressure gradient is equal to a small negative constant (−0.0115) in the offshore direction, indicating that the bore is subject to a constant favorable pressure gradient.

A half-analytical correlation of total melting time for shell-and-tube latent-heat thermal energy storage unit

This study proposes a new concept of dimensionless total melting time in view of the nonlinear dynamic characteristic of latent-heat thermal energy storage unit (LHTESU). This dimensionless total melting time has a mild nonlinear relationship with its influence parameters, which makes it is possible to develop a unified correlation formula of total melting time. The melting process of a vertical single-pass shell-and-tube LHTESU is numerically studied to build a half-analytical correlation of total melting time proposed by this paper. Boussinesq approximation is used to investigate the effects of natural convection on the melting characteristics. The proposed correlation has an extensible format and includes five dimensionless parameters: Stefan number, dimensionless initial temperature, Reynolds number, diameter ratio, and length-to-diameter ratio. The results show that the non-dimensional total melting time has a lower-order nonlinear-relationship with its influence parameters compared with the existing concept of total melting time. The common reason for these five concerned parameters affecting the dimensionless total time is that they all have effects on the inhomogeneous temperature distribution in LHTESU. The half-analytical correlation of dimensionless total time developed in this study has acceptable accuracy. The deviation between the total melting time predicted by empirical correlation and the numerical result is less than 5%.

Regime transitions and energetics of sustained stratified shear flows

We describe the long-term dynamics of sustained stratified shear flows in the laboratory. The stratified inclined duct (SID) experiment sets up a two-layer exchange flow in an inclined duct connecting two reservoirs containing salt solutions of different densities. This flow is primarily characterised by two non-dimensional parameters: the tilt angle of the duct with respect to the horizontal, $\unicode[STIX]{x1D703}$ (a few degrees at most), and the Reynolds number $Re$, an input parameter based on the density difference driving the flow. The flow can be sustained with constant forcing over arbitrarily long times and exhibits a wealth of dynamical behaviours representative of geophysically relevant sustained stratified shear flows. Varying $\unicode[STIX]{x1D703}$ and $Re$ leads to four qualitatively different regimes: laminar flow; mostly laminar flow with finite-amplitude, travelling Holmboe waves; spatio-temporally intermittent turbulence with substantial interfacial mixing; and sustained, vigorous interfacial turbulence (Meyer &amp; Linden, J. Fluid Mech., vol. 753, 2014, pp. 242–253). We seek to explain the scaling of the transitions between flow regimes in the two-dimensional plane of input parameters $(\unicode[STIX]{x1D703},Re)$. We improve upon previous studies of this problem by providing a firm physical basis and non-dimensional scaling laws that are mutually consistent and in good agreement with the empirical transition curves we inferred from 360 experiments spanning $\unicode[STIX]{x1D703}\in [-1^{\circ },6^{\circ }]$ and $Re\in [300,5000]$. To do so, we employ state-of-the-art simultaneous volumetric measurements of the density field and the three-component velocity field, and analyse these experimental data using time- and volume-averaged potential and kinetic energy budgets. We show that regime transitions are caused by an increase in the non-dimensional time- and volume-averaged kinetic energy dissipation within the duct, which scales with $\unicode[STIX]{x1D703}Re$ at high enough angles. As the power input scaling with $\unicode[STIX]{x1D703}Re$ is increased above zero, the two-dimensional, parallel-flow dissipation (power output) increases to close the budget through an increase in the magnitude of the exchange flow, incidentally triggering Holmboe waves above a certain threshold in interfacial shear. However, once the hydraulic limit of two-layer exchange flows is reached, two-dimensional dissipation plateaus and three-dimensional dissipation at small scales (turbulence) takes over, at first intermittently, and then steadily, in order to close the budget and follow the $\unicode[STIX]{x1D703}Re$ scaling. This general understanding of regime transitions and energetics in the SID experiment may serve as a basis for the study of more complex sustained stratified shear flows found in the natural environment.

Evolution of water column pulled by partially submerged spheres with different velocities and submergence depths

Abstract The evolution of a water column pulled by a partially submerged sphere, known as the water exit problem, is evaluated by numerical simulation. The lattice Boltzmann method was combined with a mass-tracking algorithm. The accuracy of the numerical method was demonstrated by comparisons with previous experimental results. Moreover, the evolution processes of the water column with different Froude and Weber numbers (0.4134 ≤ Fr ≤ 8.2479, 5.15 × 102 ≤ We ≤ 2.06 × 105), as well as different initial submergence depths (Hs/D = 1/2, 1/4, 1/8), were discussed. Spontaneous breakup of water column was observed for higher Weber numbers. Moreover, vertical stretching of the water column approximately followed a power law in which the power exponent depended on the initial setup of the sphere and its time scales. However, the horizontal evolution of the water column followed an approximately linear rule. Variation in the water column diameter appeared to be irregular at different physical times or in non-dimensional time. Furthermore, the initial submergence depth of the sphere was also a significant factor in the pinch-off dynamics and water column evolution. This study of water exit is of a great importance in designing marine vehicles that take off from a free water surface.

Time-Scale Transformed Iterative Learning Control for a Class of Nonlinear Systems With Uncertain Trial Duration

Iterative learning control (ILC) is a powerful tool for improving the tracking performance in systems characterized by trial-repetitive behavior, including exogenous signals such as references and disturbances, through repeated update of control signals based on previous error histories. The application of ILC algorithms to physical systems typically requires trial-to-trial invariance that is expressed as a set of assumptions on the operation of the process. Among these assumptions is that the duration of the system trajectory is trial-invariant. In some physical processes, however, the trial duration may be a function of system outputs or states that may force the trial to be either shorter or longer than expected. This results in situations in which the trial duration is unknown a priori , breaking the typical invariance assumptions, but in which updating the control input is still desired. In order to address this class of processes, the approach proposed here uses a time-scale transformation to nondimensionalize the trial duration. An ILC algorithm is then proposed on the nondimensional time scale, and it is shown that the output error converges asymptotically toward the origin. A learning operator design is proposed, which provides a direct tradeoff between convergence rate and steady-state error. Simulation results on a benchmark nonlinear system demonstrate the efficacy of the proposed approach.

Base heat flux measurement techniques in impulse hypersonic test facilities

AbstractSafe and efficient hypersonic missions require exhaustive testing to capture the surface heat flux used in optimizing thermal protection systems. Though low, base heat transfer rates are still relevant and their estimation is of the utmost importance. Impulse hypersonic test facilities like the shock tunnel are commonly used for such estimation tests. The present paper compares contemporary heat flux measurement techniques, namely, E‐type coaxial thermocouples, Pt‐thin films, and atomic layer thermopiles in a hypersonic shock tunnel at the base of a scaled‐down re‐entry capsule. Base flow establishment is ensured using pressure measurements. The measurements indicated that the base‐stagnation point heat flux was approximately 2% to 3% of the forebody stagnation heat flux and it stabilized at an approximate nondimensional establishment time of 43.2 ± 2.4.

Temporal and Topographic Source Effects on Tsunami Generation

AbstractWe present a systematic study of the influence on tsunami waves generated by the uplift of a rectangular plug, of the rise time of the deformation, and of its topographic details (e.g., the presence of a sill). We are motivated by the fact that most simulation codes use an instantaneous deformation of a flat ocean floor as an initial condition of the problem, although Hammack (1973, http://resolver.caltech.edu/CaltechAUTHORS:HAMjfm73) performed pioneering laboratory studies as well as analytical computations featuring variable rise times. Here, we consider three 2‐D source shapes, including a flat seafloor, a simple elevated piston, and additional trapezoidal sill on top of it, all with variable rise times, and simulate the resulting waves using the fully nonlinear smooth particle hydrodynamic model graphics processing unit smooth particle hydrodynamic. We validate our results against Hammack's (1973) laboratory measurements and analytical results. We find that a relatively large sill, with height and width of more than half of the local depth and width of the source, has a profound effect on the spatiotemporal structure of the generated free surface wavefield. Specifically, we show that the maximum water surface elevation over the source region is not always the same as the bottom displacement, as assumed in most tsunami propagation models. Next, we obtain simple scaling relationships to predict the maximum height of the generated tsunami over and outside the source, based on the geometry of the sill and the nondimensional bed rise time. Last, we show that inertial effects may lead to an initial free surface displacement over the generation region greater than the maximum vertical displacement of the displaced seabed.

Numerical simulation of flow with large eddy simulation at Re = 3900

PurposeOver the past few decades, the flow around circular cylinders has been one of the highly researched topics in the field of offshore engineering and fluid-structure interaction (FSI). In the current study, numerical simulations for flow around a fixed circular cylinder are performed at Reynolds number (Re) = 3900 with the LES method using the ICEM-CFD and ANSYS Fluent tool for meshing and analysis, respectively. Previously, similar studies have been conducted at the same Reynolds number, but there have been discrepancies in the results, particularly in calculating the recirculation length and angle of separation. In addition, the purpose of this study is to address the impact of time interval averaging to obtain the fully converged solution.Design/methodology/approachThis study presents the LES method, using the ICEM-CFD and ANSYS fluent tool for meshing and analysis.FindingsIn the current study, turbulence statistics are sampled for 25, 50, 75 and 100 vortex-shedding cycles with the CFL value O (1). The recirculation length, angle of separation, hydrodynamic coefficients and the wake behind the cylinder are investigated up to ten diameters. The drag coefficient and Strouhal number are observed to be less sensitive, whereas the recirculation length appeared to be highly dependent on the average time statistics and the non-dimensional time step. Similarly, the mean streamwise and cross-flow velocity are observed to be sensitive to the average time statistics and non-dimensional time step in the wake region near the cylinder.Originality/valueIn the current investigation, turbulence statistics are sampled for 25, 50, 75 and 100 vortex-shedding cycles with the CFL value O (1), using large eddy simulation method at Re = 3900 around a circular cylinder. The impact of time interval averaging to obtain the fully converged mean flow field is addressed. No such consideration is yet published in the literature.

Droplet impingement on nano-textured superhydrophobic surface: Experimental and numerical study

Abstract The impact characteristics of liquid droplet on the nano-textured superhydrophobic surface have been investigated experimentally and numerically to understand the underlying mechanism and select appropriate models to describe them. The evolution of impact process with droplet impact velocity varied form 0.11–3.9 m/s (or Weber number ranging from 0.3 to 421.4) was recorded and analyzed. Besides, a conservative level set method coupled with dynamic contact angle models was developed to track the phase interface and made a comparison with the experiment. During the spreading and receding phases, the numerical simulation results showed good agreement with the experiment. The impact of droplet satisfied the energy conservation between kinetic and surface energy, and the maximum spreading factor (βmax) can be well correlated by a scaling law of βmax ~ We0.52. Both of the contact time and non-dimensional contact time of bouncing droplets were independent of Weber number in the range of 1.5 to 121. The predicted non-dimensional contact times were in good agreement with the reference value of 2.6 ± 0.1 at low and medium Weber numbers. The dynamic contact angle models derived from the hydrodynamic theory and molecular-kinetic theory, respectively, can mainly apply to the advancing and receding phases of droplet impact on nano-textured superhydrophobic surfaces.

Mixture theory modeling for characterizing solute transport in breast tumor tissues

BackgroundTumor numerical models have been used to quantify solute transport with a single capillary embedded in an infinite tumor expanse, but measurements from different mammalian tumors suggest that a tissue containing a single capillary with an infinite intercapillary distance assumption is not physiological. The present study aims to investigate the limits of the intercapillary distance within which nanoparticle transport resembles solute extravasation in a breast tumor model as a function of the solute size, the intercapillary separation, and the flow direction in microvessels.MethodsSolute transport is modeled in a breast tumor for different vascular configurations using mixture theory. A comparison of a single capillary configuration (SBC) with two parallel cylindrical blood vessels (2 BC) and a lymph vessel parallel to a blood vessel (BC_LC) embedded in the tissue cylinder is performed for five solute molecular weights between 0.1 kDa and 70 kDa. The effects of counter flow (CN) versus co-current flow (CO) on the solute accumulation were also investigated and the scaling of solute accumulation-decay time and concentration was explored.ResultsWe found that the presence of a second capillary reduces the extravascular concentration compared to a single capillary and this reduction is enhanced by the presence of a lymph vessel. Varying the intercapillary distance with respect to vessel diameter shows a deviation of 10–30% concentration for 2 BC and 45–60% concentration for BC_LC configuration compared to the reference SBC configuration. Finally, we introduce a non-dimensional time scale that captures the concentration as a function of the transport and geometric parameters. We find that the peak solute concentration in the tissue space occurs at a non-dimensional time, {T}_{peak}^{ast } = 0.027 ± 0.018, irrespective of the solute size, tissue architecture, and microvessel flow direction.ConclusionsThis work suggests that the knowledge of such a unique non-dimensional time would allow estimation of the time window at which solute concentration in tissue peaks. Hence this can aid in the design of future therapeutic efficacy studies as an example for triggering drug release or laser excitation in the case of photothermal therapies.

A Large-Eddy Simulation-Based Assessment of the Risk of Wind Turbine Failures Due to Terrain-Induced Turbulence over a Wind Farm in Complex Terrain

The first part of the present study investigated the relationship among the number of yaw gear and motor failures and turbulence intensity (TI) at all the wind turbines under investigation with the use of in situ data. The investigation revealed that wind turbine #7 (T7), which experienced a large number of failures, was affected by terrain-induced turbulence with TI that exceeded the TI presumed for the wind turbine design class to which T7 belongs. Subsequently, a computational fluid dynamics (CFD) simulation was performed to examine if the abovementioned observed wind flow characteristics could be successfully simulated. The CFD software package that was used in the present study was RIAM-COMPACT, which was developed by the first author of the present paper. RIAM-COMPACT is a nonlinear, unsteady wind prediction model that uses large-eddy simulation (LES) for the turbulence model. RIAM-COMPACT is capable of simulating flow collision, separation, and reattachment and also various unsteady turbulence–eddy phenomena that are caused by flow collision, separation, and reattachment. A close examination of computer animations of the streamwise (x) wind velocity revealed the following findings: As we predicted, wind flow that was separated from a micro-topographical feature (micro-scale terrain undulations) upstream of T7 generated large vortices. These vortices were shed downstream in a nearly periodic manner, which in turn generated terrain-induced turbulence, affecting T7 directly. Finally, the temporal change of the streamwise (x) wind velocity (a non-dimensional quantity) at the hub-height of T7 in the period from 600 to 800 in non-dimensional time was re-scaled in such a way that the average value of the streamwise (x) wind velocity for this period was 8.0 m/s, and the results of the analysis of the re-scaled data were discussed. With the re-scaled full-scale streamwise wind velocity (m/s) data (total number of data points: approximately 50,000; time interval: 0.3 s), the time-averaged streamwise (x) wind velocity and TI were evaluated using a common statistical processing procedure adopted for in situ data. Specifically, 10-min moving averaging (number of sample data points: 1932) was performed on the re-scaled data. Comparisons of the evaluated TI values to the TI values from the normal turbulence model in IEC61400-1 Ed.3 (2005) revealed the following: Although the evaluated TI values were not as large as those observed in situ, some of the evaluated TI values exceeded the values for turbulence class A, suggesting that the influence of terrain-induced turbulence on the wind turbine was well simulated.

The deposition characteristics of micron particles in heat exchange pipelines

Comprehensive understanding of particles deposition in heat exchange pipelines is significant for environment and energy engineering applications. In this study, a numerical simulation method was used to investigate the deposition characteristics of micron particles in heat exchange pipelines based on Reynolds stress model and discrete particle model. Based on previous research, an improved particle deposition model combined with particle rebound was presented. Moreover, the improved model was used to study the effect of thermophoretic force (hot wall and cold wall), flow velocity, and particle diameter on the non-dimensional deposition rate of particles. The results showed that the non-dimensional deposition rate increased with the increase of non-dimensional relaxation time and then approached a certain value. After introducing the particle rebound, the non-dimensional deposition rate exhibited different trend, i.e., first increasing and then decreasing. When the gas had significant temperature gradient in the boundary layer, the effect of thermophoretic force on small particles was found to be significantly stronger than that of large particles. Moreover, the higher the temperature gradient, the more obvious the thermophoretic force of the particles. The results indicated that the thermophoretic forces prevented the deposition of particles on the hot wall, and promoted this process on the cold wall. Furthermore, the flow velocity also significantly affected the particle deposition rate. At the same flow velocity, the particles non-dimensional deposition rate first increased and then decreased with the increase in the particle diameter, and it showed a different maximum at different flow velocities.

An Efficient Optimization Algorithm to Study the Stochastic Responses of Vehicle-Bridge Coupling Systems

An efficient algorithm is proposed for the stochastic responses of vehicle–bridge systems. The pseudo-excitation method is combined with the self-adaptive Gauss integration to handle the stochastic responses of vehicle–bridge systems, for which the statistic characteristics are calculated. The applicability and accuracy of the proposed hybrid numerical method are demonstrated by comparing the results with those from Monte Carlo simulation. Besides, nondimensional time is introduced to evaluate the stochastic responses of vehicles.

Stability analysis of real-time hybrid simulation in consideration of time delays of actuator and shake table using delay differential equations

Real-time hybrid simulation evaluates the response of a structure in real time. In this study, a building with multi-story structure is divided into numerical and experimental substructures, and the vibration behavior of the experimental story is studied among the real-time simulation of the other stories. For applying the effect of static and inertial forces produced by the other stories to the experimental story, an electrohydraulic actuator and a shake table are used, respectively. The dynamics of the electrohydraulic actuator and the shake table can be estimated entirely by time delays, and these delays in the loop of simulation can reduce accuracy and cause system instability. Therefore, a delayed differential equation is used to determine the critical time delays depending on the system parameters. Results of simulation show the effect of non-dimensional parameters and time delays on the stability margin of hybrid simulation.