Unsteady Velocity Research Articles

Identification of the heat transfer frequency response in pulsating laminar and subcritical flow across a cylinder

The steady-state heat transfer from a cylinder in cross-flow is a prototype problem in thermo-fluiddynamics. However, in many applications such as the Rijke tube, the flow may fluctuate. This work analyses the phenomenon combining numerical simulation with system identification. Direct numerical simulation of laminar flow and Large Eddy Simulation at subcritical flow at Reynolds number equal to 3900 are used, respectively. Fluctuations of the inlet velocity in the simulation are excited over a wide range of frequencies. Time series of unsteady heat release and velocity are post-processed to identify dynamic models, which may be represented as transfer functions. They accurately describe the dynamic behavior and can be used for further modeling.

The effect of nanoparticles added to heated micropolar fluid

This paper presents an analysis of momentum, angular momentum and heat transfer during the unsteady natural convection in micropolar nanofluids. Presented phenomena are modelled in the vicinity of a vertical plate and heat flux which rises suddenly at a given moment, using the boundary layer concept. Differential equations of angular momentum conservation are used according to the theory of micropolar fluids developed by Eringen. Finite difference method is used to solve the equations for conservation of mass, energy, momentum and angular momentum. Selected nanofluids treated as single phase fluids contain small particles with diameter size d = 10 nm and d = 38.4 nm. In particular, two ethylene glycol based nanofluids and one water-based nanofluid are analysed. Volume fraction of these solutions is 6%. First ethylene glycol solution contain Al2 O3 nanoparticles (d = 38.4 nm), and the second ethylene glycol solution contained Cu nanoparticles (d = 10 nm). Water based nanofluid contain Al2 O3 nanoparticles (d = 38.4 nm). As a result of solving conservation equations, unsteady velocity field (U, V), temperature (T), microrotation component normal to (x, y) plane (N), velocity gradient ∂U/∂Y and temperature gradient ∂T/∂Y are obtained. These results are compared to theoretical and experimental results presented in literature. At the end of this paper, heat transfer enhancement for analysed nanofluids is estimated.

Effects of prong-wire interferences in dual hot-wire probes on the measurements of unsteady flows and turbulence in low-speed axial fans

Support needles of Dual Hot Wire (DHW) anemometers induce significant inaccuracies for flow angle and turbulence measurements in the case of X-array probes with prongs perpendicular to the flow plane. At certain angular ranges of the incident flow, a wake interference is established between the sensors which leads to a practical limitation of the device. In the case of turbomachinery environments, this is even more critical due to the inherent unsteadiness of the flow direction rotor downstream.In the present work, the measurement deviation caused by hot-wire probes operated under interference effects has been studied and evaluated, in both steady and unsteady conditions, especially for turbomachinery flows. New designs of DHW probes without prong-wire interference effects in their operative angular ranges were developed for validation. In particular, both V-type and Z-type interference-free probes are compared to a classic X-type probe susceptible for prong-wire interferences. Firstly, a steady calibration is performed to show the baseline deviation of the X-array probe in the measurement of the velocity magnitude, the flow angle and the turbulence intensity. Typical errors up to 10–13% in velocity, 5.5–7deg in angle and 1.5–2.5 points overestimation in turbulence levels are observed. Also, unacceptable inaccuracies are found in the turbulence spectra of the measurements.Following, the impact of the interference for unsteady flow measurements is highlighted comparing the performance of the three probes within the single stage of a low-speed axial fan. The unsteady measurements of the X-array probe have revealed similar averaged discrepancies to those observed in the steady performance, but the instantaneous deviations can be as high as a 20% in velocity and 16–18deg in flow angle in those regions (rotor wakes) with large unsteady velocity gradients and turbulence generation. Turbulence intensity measured in the rotor wakes is also excessively higher.

Objective Eulerian coherent structures.

We define objective Eulerian Coherent Structures (OECSs) in two-dimensional, non-autonomous dynamical systems as the instantaneously most influential material curves. Specifically, OECSs are stationary curves of the averaged instantaneous material stretching-rate or material shearing-rate functionals. From these objective (frame-invariant) variational principles, we obtain explicit differential equations for hyperbolic, elliptic, and parabolic OECSs. As an illustration, we compute OECSs in an unsteady ocean velocity data set. In comparison to structures suggested by other common Eulerian diagnostic tools, we find OECSs to be the correct short-term cores of observed trajectory deformation patterns.

Study on the flow instability of a spray granulation tower

Abstract The hydrodynamics of a jet-driven swirling flow has a dramatic influence on the separation performance of a spray granulation tower. To understand the macro-instabilities of the flow patterns, this work performs a detailed study on the gas flow field in a spray granulation tower with an array of nozzles. The experimental results attained by a phase Doppler particle analyzer (PDPA) indicate that the flow field is in a fully turbulent state and that a low-frequency oscillation exists in the spray granulation tower. The velocity profiles and frequency distributions obtained using the full-scaled grid system and the Reynolds stress model are in good agreement with experimental data. Based on the observed flow field, a general flow pattern is summarized. Both the experimental and numerical results indicate that a quasi-periodic unsteady velocity flow is observed in the spray granulation tower. Compared with the original spray granulation tower, it is beneficial for reducing the flow oscillation and velocity fluctuation when the baffle is added. The flow stability criterion proposed in this study can characterize the flow state in the spray granulation tower.

Unsteady airloads on static airfoils through high angles of attack and in reverse flow

The present work is aimed at understanding the sources, magnitude, and frequency of unsteady airloads acting on airfoils at high angles of attack and in reverse flow in order to improve the design of rotor blades for high-speed helicopters and wind turbines. Four rotor blade airfoils were tested at angles of attack through 180° and at three Reynolds numbers, up to one million. The unsteady airloads acting on each airfoil were calculated by integrating time-resolved pressure measurements acquired along the midspan of the airfoil. Unsteady velocity fields for a NACA 0012 were calculated from time-resolved particle image velocimetry measurements. For all airfoils, the unsteady airloads were found to be large in magnitude near stall, when an unstable shear layer induces unsteady flow on the suction side of the airfoil. At post-stall angles of attack, the unsteady airloads generally decreased as a region of separated flow builds over the suction side. As the angle of attack was increased further, the airloads become periodic and the flow entered the deep stall vortex shedding regime. At high angles of attack (30°≤α⩽150°), the unsteady airloads were greatest for airfoils with a blunt aerodynamic trailing edge due to unsteady induced flow from trailing edge vortices. Aerodynamic hysteresis was shown to cause large unsteady airloads as the static angle of attack is decreased near stall. The unsteady airloads of a NACA 0012 in reverse flow were observed to be insensitive to Reynolds number due to flow separation at the sharp leading edge of this relatively thin airfoil.

A new semi-Lagrangian routing procedure for constituent transport in steady and unsteady flow velocity fields

Summary In this paper, a new numerical model for the simulation of constituent transport in both steady and unsteady flow conditions is presented. The transport model is a routing procedure in which the advection process is solved by means of the Lagrangian coordinate transformation, while the dispersion process is approximated within each time step by means of the convolution principle, exploiting a multilinear procedure. In order to facilitate the application of the Lagrangian coordinate transformation during unsteady flow conditions, the unsteady velocity field corresponding to the linearized parabolic approximation of the Saint Venant Equations is provided, taking into account appropriate boundary conditions. Finally, classic BOD–DO relationships are embedded into the routing procedure in order to perform water quality applications with reactive constituents. The model is first demonstrated with respect to a numerical water quality model in both steady and unsteady hydraulic conditions, and is then applied to two real-world cases. Because of its characteristics, the proposed model seems suitable for real time forecast of pollutant concentrations when an emergency event occurs, or for water quality management in real rivers.

Modeling 3D conjugate heat and mass transfer for turbulent air drying of Chilean papaya in a direct contact dryer

A 3D model considering heat and mass transfer for food dehydration inside a direct contact dryer is studied. The k– e model is used to describe turbulent air flow. The samples thermophysical properties as density, specific heat, and thermal conductivity are assumed to vary non-linearly with temperature. FVM, SIMPLE algorithm based on a FORTRAN code are used. Results unsteady velocity, temperature, moisture, kinetic energy and dissipation rate for the air flow are presented, whilst temperature and moisture values for the food also are presented. The validation procedure includes a comparison with experimental and numerical temperature and moisture content results obtained from experimental data, reaching a deviation 7–10 %. In addition, this turbulent k– e model provided a better understanding of the transport phenomenon inside the dryer and sample.

Measurement of unsteady velocity flow field of pulsating flow

This paper describes an investigation of the commercial synthetic jet actuator “SynJet ZFlow 90 Outdoor Cooler” (Nuventix). The velocity flow field was investigated using the thermoanemometry method in the constant temperature mode. The Eigen frequency of the used actuator was also calculated in this work.

CHARACTERISTICS OF BOUNDARY LAYER FLOW INDUCED BY SOLITARY WAVE PROPAGATING OVER HORIZONTAL BOTTOM

Experimental results on the flow characteristics of bottom boundary layer induced by a solitary wave propagating over a horizontal bottom are presented. Particle-trajectory flow visualization technique and high-speed particle image velocimetry (HSPIV) were used to elucidate detailed velocity fields underneath solitary waves with the ratios of wave height to water depth from 0.130 to 0.386. The results show that the velocity profiles can be classified into two classes with respect to the passage of the solitary wave-crest at the measuring section: ＂the pre-passing＂ and ＂post-passing phases＂. For the pre-passing phase, the velocity distributions can be deduced to a unique similarity profile with the use of unsteady free stream velocity and time-dependent boundary layer thickness as the characteristic velocity and length scales. On the other hand, the similarity profile for the flow reversal, acting like an unsteady wall jet, is obtained from the velocity distributions during the post-passing phase. The velocity deficit between the unsteady free stream velocity and the maximum negative velocity as well as the (time-dependent) thickness of reversal flow were identified as the characteristic velocity and length scales, respectively.

Study of He II boiling flow field around a heater

We studied boiling phenomena in He II based on the flow velocity measurement data by using a PIV (Particle Image Velocimeter). Noisy and silent film boiling modes together with non-boiling state were generated on/around a horizontal planar or a cylindrical heater. For PIV tracer particles, we used H2-D2 solid particles that were neutrally buoyant in He II. Video images showing the development and collapse of vapour bubble or film and the motions of tracer particles were PIV-analysed. We found the PIV velocity field was composed of AC and DC velocity components of the normal fluid. The AC component follows the dynamic behaviour of vapour, and the DC results primarily from the thermal counter flow and secondarily is induced by the asymmetric vapour bubble motion. We also investigated unsteady velocity component. The objective of this series of study is to compare the characteristic features of the flow field of He II film boiling states and peculiar He I boiling state in He II and to make clear the difference in the heat transfer performance of each boiling mode.

Mathematical modeling for solute transport in aquifer

Groundwater pollution may occur due to human activities, industrial effluents, cemeteries, mine spoils, etc. This paper deals with one-dimensional mathematical modeling of solute transport in finite aquifers. The governing equation for solute transport by unsteady groundwater flow is solved analytically by the Laplace transform technique. Initially, the aquifer is subjected to the spatially dependent source concentration with zero-order production. One end of the aquifer receives the source concentration and is represented by a mixed-type boundary condition in the splitting time domain. The concentration gradient at the other end of the porous media is assumed to be zero. The temporally dependent velocity and the dispersion coefficients are considered. A numerical solution is obtained by using an explicit finite difference scheme and compared with the analytical result. Accuracy of the solution is discussed by using the root mean square error method. Truncation error is also explored for the parameters like numerical dispersion and velocity terms. The impact of Peclet number is examined. For graphical interpretation, unsteady velocity expressions (i.e., such as exponential, sinusoidal, asymptotic, and algebraic sigmoid) are considered. The work may be used as a preliminary predictive tool for groundwater resource and management.

Numerical investigation of three-dimensional effects during dynamic stall

The numerical simulation of dynamic stall around a 3D finite-span oscillating wing of constant OA209 airfoil section is compared to experimental results obtained in the ONERA F2 wind-tunnel, assuming fully turbulent flow. A deep dynamic stall case is considered for a reduced frequency typical of helicopter problems. A detailed comparison with the experimental data available (unsteady pressure distribution and velocity field) shows that the main flow features are captured by the numerical simulation, more especially the large spanwise flow component induced by separation.

Dynamic Stall Analysis of S809 Pitching Airfoil in Unsteady Free Stream Velocity

AbstractIn this paper, the dynamic stall of S809 airfoil that widely used in horizontal axis wind turbine, in different reduced frequencies is investigated. The simulation was carried out numerically handling Navier-Stokes equations. For this purpose, the segregated solver with SIMPLE algorithm was chosen to solve the momentum equations. The effect of turbulence on the flow field is taken into account using Shear Stress Transport (SST)K-ωturbulence model. The obtained numerical results are compared with experimental and a few numerical results. The results indicate that theK-ωSST model is in good agreement with experimental results for both steady and unsteady conditions. Furthermore, a non-dimensional parameter, relating the acceleration of unsteady free stream velocity and acceleration of pitch motion (known as reduced frequency), is also investigated. In addition, the results show that any increase in the reduced frequency increases the instantaneous aerodynamic characteristics of oscillating airfoil.

Advances in Horizontal Axis Wind Turbine Blade Designs: Introduction of Slots and Tubercle

Despite being harvested thousands of years ago, wind energy was neglected during the industrial revolution because of the strong dependence on fossil fuels. However, after the alarming decrease in the fossil fuels reserves, many have drawn their attentions back to a renewable energy technology, especially the wind energy. This paper presents some of the new designs that are being tested, including slotted blades and tubercles design models. The experimental results are used to validate the numerical studies that are being conducted parallel to the experiments for better understanding and more detailed results. The new slotted blade design produced more power compared to the straight blade for lower wind speeds, while the tubercle blades showed better power performance in severe wind conditions and a more steady behavior under unsteady and higher wind velocities.

Direct numerical simulation of the compression stroke under engine-relevant conditions: Evolution of the velocity and thermal boundary layers

The unsteady velocity and thermal boundary layers during the compression stroke under engine-relevant conditions are investigated using direct numerical simulation (DNS). The initial conditions are derived by a precursor DNS of the intake stroke, avoiding the use of any artificial initial and boundary conditions. The results show decreasing velocity and thermal boundary-layer thicknesses towards Top Dead Center (TDC) as a result of the decreasing kinematic viscosity. Compared to the fluctuating flow field, the azimuthally-averaged velocities are found to have a minor effect on the boundary-layer profiles, and as a result the boundary-layer structures at all engine walls during compression are similar. The averaged velocity and thermal boundary-layer profiles deviate strongly from the law of the wall, which is the basis for many wall heat transfer models in internal combustion engines. In density wall-normal units, the averaged as well as the rms boundary-layer profiles for velocity and temperature show a collapse onto a single curve during the whole compression stroke. This finding can serve as base for the development of novel wall heat transfer models. Finally, the DNS data showed that between 60% and 80% of the total wall heat losses during the compression stroke are attributed to convective transport due to wall-normal velocity fluctuations.

Start-up slip flow in a microchannel with a rectangular cross section

The paper outlines results of the theoretical study of an incompressible fluid flow in a rectangular microchannel subject to a sudden time-dependent pressure drop. The momentum equation together with the independent and dependent variables was reduced to a self-similar form by means of the symmetry analysis. The problem was solved using two analytical approaches, the Fourier method and the method of eigenfunction decomposition, as well as numerically by means of the lattice Boltzmann method. The unsteady two-dimensional velocity profiles in the microchannel were predicted using the infinite series and validated against the numerical solution. As expected, the flow pattern asymptotically attains the fully developed state, which is reached more rapidly for smaller Knudsen numbers. The analytical solution yielded expressions for the calculation of the hydraulic resistance coefficient.

Human heart preservation analyses using convective cooling

– Currently, human hearts destined for transplantation can be used for 4.5 hours which is often insufficient to test the heart, the purpose of this paper is to find a compatible recipient and transport the heart to larger distances. Cooling systems with simultaneous internal and external liquid cooling were numerically simulated as a method to extend the usable life of human hearts. , – Coolant was pumped inside major veins and through the cardiac chambers and also between the heart and cooling container walls. In Case 1, two inlets and two outlets on the container walls steadily circulated the coolant. In the Case 2, an additional inlet was specified on the container wall thus creating a steady jet impinging one of the thickest parts of the heart. Laminar internal flow and turbulent external flow were used in both cases. Unsteady periodic inlet velocities at two frequencies were applied in Case 3 and Case 4 that had four inlets and four outlets on walls with turbulent flows used for internal and external circulations. , – Computational results show that the proposed cooling systems are able to reduce the heart temperature from +37°C to almost uniform +5°C within 25 min of cooling, thus reducing its metabolic rate of decay by 95 percent. Calculated combined thermal and hydrodynamic stresses were below the allowable threshold. Unsteady flows did not make any noticeable difference in the speed of cooling and uniformity of temperature field. , – This is the pioneering numerical study of conjugate convective cooling schemes capable of cooling organs much faster and more uniformly than currently practiced.

Unsteady Modes in Flowfield About Airfoil with Horn-Ice Shape

An experimental investigation was conducted on an NACA 0012 airfoil with a leading-edge horn-ice shape to identify and characterize flowfield unsteadiness related to ice-induced flow separation. This type of iced-airfoil flowfield was dominated by a leading-edge separation bubble, which is associated with several inherent modes of unsteadiness. Using unsteady surface pressure and wake velocity measurements, three distinct modes of unsteadiness were identified at various locations throughout the flowfield. These unsteady modes included a regular mode of vortical motion and a shear-layer flapping mode, which are both associated with separation bubbles. A low-frequency mode that was associated with the thin-airfoil stall type of the iced airfoil was also observed and was represented by a global oscillation of the airfoil circulation. The characteristic frequencies and flowfield locations corresponding to these unsteady modes were compared with those reported in the literature for iced-airfoil flowfields and flows about canonical geometries.

Centrifugal compressor surge detecting method based on wavelet analysis of unsteady pressure fluctuations in typical stages

Centrifugal compressors are complex energy equipment. Automotive control and protection system should meet the requirements: of operation reliability and durability. In turbocompressors there are at least two dangerous areas: surge and rotating stall. Antisurge protecting systems usually use parametric or feature methods. As a rule industrial system are parametric. The main disadvantages of anti-surge parametric systems are difficulties in mass flow measurements in natural gas pipeline compressor. The principal idea of feature method is based on the experimental fact: as a rule just before the onset of surge rotating or precursor stall established in compressor. In this case the problem consists in detecting of unsteady pressure or velocity fluctuations characteristic signals. Wavelet analysis is the best method for detecting onset of rotating stall in spite of high level of spurious signals (rotating wakes, turbulence, etc.). This method is compatible with state of the art DSP systems of industrial control. Examples of wavelet analysis application for detecting onset of rotating stall in typical stages centrifugal compressor are presented. Experimental investigations include unsteady pressure measurement and sophisticated data acquisition system. Wavelet transforms used biorthogonal wavelets in Mathlab systems.