Laminar Region Research Articles

Contactless Localization of Premature Laminar–Turbulent Flow Transitions on Wind Turbine Rotor Blades in Operation

Thermographic flow visualization enables a noninvasive detection of the laminar–turbulent flow transition and allows a measurement of the impact of surface erosion and contamination due to insects, rain, dust, or hail by quantifying the amount of laminar flow reduction. The state-of-the-art image processing is designed to localize the natural flow transition as occurring on an undisturbed blade surface by use of a one-dimensional gradient evaluation. However, the occurrence of premature flow transitions leads to a high measurement uncertainty of the localized transition line or to a completely missed flow transition detection. For this reason, regions with turbulent flow are incorrectly assigned to the laminar flow region, which leads to a systematic deviation in the subsequent quantification of the spatial distribution of the boundary layer flow regimes. Therefore, a novel image processing method for the localization of the laminar–turbulent flow transition is introduced, which provides a reduced measurement uncertainty for sections with premature flow transitions. By the use of a two-dimensional image evaluation, local maximal temperature gradients are identified in order to locate the flow transition with a reduced uncertainty compared to the state-of-the-art method. The transition position can be used to quantify the reduction of the laminar flow regime surface area due to occurrences of premature flow transitions in order to measure the influence of surface contamination on the boundary layer flow. The image processing is applied to the thermographic measurement on a wind turbine of the type GE 1.5 sl in operation. In 11 blade segments with occurring premature flow transitions and a high enough contrast of the developed turbulence wedge, the introduced evaluation was able to locate the flow transition line correctly. The laminar flow reduction based on the evaluated flow transition position located with a significantly reduced systematic deviation amounts to 22% for the given measurement and can be used to estimate the reduction of the aerodynamic lift. Therefore, the image processing method introduced allows a more accurate estimation of the effects of real environmental conditions on the efficiency of wind turbines in operation.

Full-Aircraft Energy-Based Force Decomposition Applied to Boundary-Layer Ingestion

This Paper introduces a generic force decomposition method derived from mechanical energy conservation. A transformation from relative to absolute reference frame captures the power transfer from pressure and skin-friction forces on aircraft surfaces to mechanisms in the flowfield. A unique flow-feature extraction procedure isolates these mechanisms into different regions including the jet-plume substructures as well as shocks and shear layers located externally to the jet. Featured is a novel shear-layer identification metric that captures both laminar and turbulent regions. The resulting energy balance is rearranged into a force decomposition formulation with contributions attributed to shocks, jets, lift-induced vortices, and the remaining wake. Boundary-layer ingestion is used to demonstrate the method where a potential for energy recovery factor is introduced and defines the amount of energy available at the trailing edge of an unpowered body. Computational fluid dynamics results of a fuselage suggest 10% of its drag power is available for reuse. Computational fluid dynamics studies of a boundary-layer ingesting propulsor show local minima in power consumption at a given thrust split for particular combinations of fan pressure ratio and amount of boundary layer ingested. A noteworthy finding reveals significant contributions of volumetric pressure work, a term often neglected in previous work.

A new modeling approach for mixture fraction statistics based on dissipation elements

The probabilty density function (PDF) of the mixture fraction is of integral importance to a large number of combustion models. Here, a novel modelling approach for the PDF of the mixture fraction is proposed which employs dissipation elements. While being restricted to the commonly used mean and variance of the mixture fraction, this model approach individually considers contributions of the laminar regions as well as the turbulent core and the turbulent/non-turbulent interface region. The later region poses a highly intermittent part of the flow which is of high relevance to the non-premixed combustion of pure hydrocarbon fuels. The model assumptions are justified by means of the gradient trajectory based analysis of high fidelity direct numerical simulation (DNS) datasets of two turbulent inert configurations and a turbulent non-premixed jet flame. The new dissipation element based model is validated against the DNS datasets and a comparison with the beta PDF is presented.

EFFECT OF HEAT SUPPLY TO A NARROW BAND OF THE BOUNDARY LAYER ON ITS STABILITY

Stability of a subsonic boundary layer with heat supply into its narrow band is studied. The maximum length of the stable boundary layer in the case of heat supply is found to be approximately three times smaller as compared to the case of a thermally insulated wall without heat supply. The frequency range of disturbances and their growth rates are approximately doubled. In contrast to the case without heat supply, three-dimensional disturbances are the most rapidly growing disturbances. In the case of a heated wall, the growth rate of disturbances decreases after heat supply into the boundary layer, which may lead to an increase in the length of the laminar region of the boundary layer.

Transition model based design method investigation on laminar flow nacelle

Aerodynamic shape optimization of natural-laminar-flow (NLF) nacelle for lange-range wide-body transport aircraft was investigated. Perturbed class function/shape function transformation (CST) method was used to parameter the three typical generatrix of the nacelle combined with the perturbation based transfinite interpolation method to achieve surface and space grid update. The k-ω SST (shear stress transport) turbulence model and ϒ – Re θ transition models was use to predict the transition location.The gradient based optimization method called Sequential Quadratic Programming was selected for the high efficiency. Natural laminar flow design was performed on three dimensional nacelles and about 45%∼55% laminar flow region was achieved by the optimization design.

Quantitative analysis of astrocyte and axonal density relationships: Glia to neuron ratio in the optic nerve laminar regions

Astrocytes are critical for the maintenance of retinal ganglion cell (RGC) axonal function and viability, and form a key component of the functional neurovascular unit. Recently, we described the quantitative properties of astrocytes in relation to the capillary distributions in optic nerve laminar regions. Here, we provide a quantitative analysis of astrocytes and RGC axons in longitudinal sections of optic nerve tissue. Histological and immunocytochemical techniques are used to demonstrate the density of astrocytes, RGC axons and glia-neuron ratios across the pre laminar, lamina cribrosa and post laminar compartments of the optic nerve head (ONH). A study of human, pig, horse and rat optic nerves was performed and comparisons are made between species. This study demonstrates that the distribution of astrocytes correlates closely with the density of axonal processes, in accordance with the functional requirement of different regions of the ganglion cell axon. There was a consistency of glia-neuron ratios in the majority of laminar compartments, except for the human and rat prelaminar regions, which demonstrated lower ratios of astrocyte to axonal processes. The distribution of astrocytes may reflect a functional susceptibility to development of disease in the prelaminar region of the optic nerve. Interspecies comparison at the lamina cribrosa showed strikingly consistent glia-neuron ratios. Collectively, our findings suggest there may be a critical ratio of glia to neuron needed to maintain healthy cellular physiology across different laminar compartments of the optic nerve, with particular importance for the health of the lamina cribrosa region. It is possible that, in disease processes, the glia-neuron relationships across the different laminar compartments may be perturbed and this may be relevant for the development of glaucoma. Emerging technologies may further aid our understanding in how the physiology of optic nerve tissue cellular structure may be affected by changes to ONH characteristics and elevated intraocular pressure induced damage. Such findings may also permit the early identification of RGC axonal injury by identifying quantifiable changes in structural tissue architecture when pathophysiological pathways predominate.

Computational Fluid Dynamics Investigation of Pitch Variations on Helically Coiled Pipe in Laminar Flow Region

Abstract Experimental investigations of the flows inside helically coiled pipe are difficult and may also be expensive, particularly for small diameters. Computational fluid dynamics (CFD) packages, which can easily construct the geometry and change the dimensions with 100% of accuracy, provide an alternative solution for the experimental difficulties and uncertainties. Therefore, a CFD study was conducted to analyze the flow structure and the effect of varying the coil pitch on the coil friction factor, through utilizing different models' configurations. Two coils were tested, all of them sharing the same pipe and coil diameter: 0.005 m and 0.04 m, respectively. Pitch variations began with 0.01 and 0.05 m for the first and second model, respectively. In this study, the velocity was analyzed, and the effects of this reduction on coil friction factor were also examined using laminar flow. The results were validated by Ito's equation for the laminar flow.

Experimental study of flame heat transfer in a vertical turbulent wall fire

This paper presents experimental measurements in a vertical turbulent wall fire, with a primary goal of developing a deep understating and database for heat transfer on solid surfaces where fuel pyrolyzes. An enhanced porous-plate vertical wall-fire burner was developed, and experiments were conducted with various mass transfer rates of ethylene to measure flame temperature, radiative power, spectral emission, radiative and convective heat flux. The maximum mean flame temperature is found to decrease with height, and the temperature profile in the outer flame boundary layer to be self-similar. The radiant power and heat flux show a linear increase vertically in the pyrolysis region and the peak shifts to higher evaluation with a greater fuel flow rate. Soot emission contributes a dominant fraction to thermal radiation. On the other hand, the convective heat flux promptly reduces from the flame leading edge in the laminar region at the bottom of the wall-burner and remains relatively steady in the downstream turbulent region. Convective heat flux dominates the heat transfer in the lower height while radiative heat transfer is more prominent in the upper region of the wall.

Numerical investigation of molten salt-based nanofluid laminar heat transfer in a circular tube using Eulerian-Lagrangian method

Molten salt-based nanofluid in a laminar region of a circular tube with constant wall heat flux was numerically investigated. An Eulerian-Lagrangian method, discrete phase model, was used to predict the heat transfer performance of nanofluid, considering the factors of inlet Reynolds number, the mass concentration of the nanoparticles, and nanoparticles diameter. Validation results were found in a good match with experimental results obtained from the literature. Numerical results showed that the heat transfer performance of nanofluid was considerably better than that of pure molten salt. The local heat transfer coefficient and Nusselt number of nanofluid are about 30% higher than these of pure molten salt and increase with an increase of Reynolds number and nanoparticles concentration. Moreover, the heat transfer performance of nanofluid with the small size of the nanoparticles (10~100 nm) is improved significantly.

Thermal Performance Analysis in Sinusoidal-Cavities-Ribs Microchannel Heat Sink with Secondary Channel Geometry for Low Pumping Power Application

The increases in the demand for high performance electronic device has led cooling technology from air cooling to liquid cooling method in order to prevent the operating temperature in the electronic device goes beyond the junction temperature of the electronic device. Liquid as a heat transportation in a micro-cooling technology is an effective method to maintain the operating temperature. Among the micro-cooling technology, microchannel heat sink appears as a promising method that provides high heat transfer rate with minimal pressure drop. However, the conventional microchannel heat sink which has straight channels (CR MCHS) inadequate to remove high heat flux generated by the current devices due to high thermal resistance in laminar region. High heat transfer rate just can be obtained in the microchannel heat sink by using a large mass flowrate which affects the pumping power consumption. The method is not suitable for the low pumping power application. So instead of increases the mass flowrate, in this paper, a hybrid microchannel heat sink was proposed in order to satisfy the cooling demand with low pumping power consumption. The effect of sinusoidal cavities, rectangular ribs and secondary channel geometry on thermal resistance and pumping power in the hybrid microchannel sink (SD-RR-SC MCHS) was studied numerically. The result showed the SD-RR-SC MCHS and RR MCHS had the lowest thermal resistance and temperature variation for the mass flowrate that less than 6.78×10−5 kg/s. Despite both designs had a similar thermal performance with the average deviation of 5%, SD-RR-SC MCHS required only 11.3% of CR MCHS pumping power, while RR MCHS required 31.3% of CR MCHS pumping power in order to achieve the optimum thermal performance of CR MCHS. Means, SD-RR-SC MCHS is the most suitable design which can fulfil cooling demand for low pumping power application.

EXPERIMENTAL STUDIES ON PRESSURE DROP CHARACTERIZATION OF CURVED TUBE SECTIONS IN LAMINAR FLOW REGIME

Flow measurement is an important task in many engineering applications. There are various methods available for flow measurement like differential pressure flow meters, displacement flow meters, and velocity flow meters etc., which indirectly measure the mass flow rate of a fluid. Whereas direct mass flow measurement devices like Coriolis Mass Flow Meter (CMFM) are, nowadays, gaining attention due to high accuracy and reliability. In practice, most of the times, flowing fluid is quite often turbulent and hence, the studies on flow field had been around turbulent flow regime. Generally, higher velocities have been found in the turbulent regime. However, as the fluid viscosity increases there can be increase in the mean velocity in the proportion such that the flow still remains laminar. The performance of CMFM is found to be deviated in this flow regime leading to underestimation of the flow rate as reported by Kumar and Anklin [1]. Bobovnik et al. [2] carried out numerical simulations to predict the performance of shell type Coriolis flow meter for a range of Re with the tube of two different aspect ratios. The velocity profile effect was presented in terms of anti-symmetric fluid forces which resulted in a loss of sensitivity at low flow rates. Kutin [3] presented a numerical study on the velocity profile effects for two straight tube configurations, one operating in a beam-type mode and other in a shell-type mode and reported that in both the cases, the meter sensitivity was affected. The author highlighted the intensive deviations in flow meter sensitivity at low Re. This can lead to a substantial economic loss in Petroleum industries. The literature review of CMFM in the laminar region has highlighted the development of secondary flow phenomenon in curved tube sections leading to under-read the meter in this region. The issue of under reading in case of CMFM needs attention and thereby remedial solution as an outcome of research.

Investigate the Effect of Pitch Variations on Helically Coiled Pipe for Laminar Flow Region Using CFD

Investigate the Effect of Pitch Variations on Helically Coiled Pipe for Laminar Flow Region Using CFD

Circular Scramjet Combustor Transition Lengths at Mach 8 and 12.8 Flight Conditions

This paper presents an experimental study of laminar-to-turbulent boundary-layer transition in a circular duct at high supersonic Mach number. The experiments were conducted as an essential precursor for scramjet drag reduction studies using a direct-connect scramjet combustor. The experiments were carried out at the University of Queensland T4 shock tunnel using a 33.2-mm-diam and 500-mm-long circular duct at flow stagnation enthalpies of 3.2 and . The experimental work was supported with two-dimensional and three-dimensional Reynolds-averaged Navier–Stokes simulations. Comparisons between numerical and experimental results show initial rise in heat transfer at the first shock-wave boundary-layer interaction, followed by an unsteady laminar region interspersed with possible turbulent spots. Heat transfer measurements along the wall that show sustained transition to turbulence did not occur within the full length of the duct. Transition to turbulence within the circular duct is significantly delayed when compared with estimations based on flat-plate results. These results indicate that there is value in revisiting the assumption of a fully turbulent boundary layer for previous semi-direct-connect scramjet combustion experiments that have used transition length correlations derived from flat-plate experiments within the same facility. The findings from this investigation have implications for conclusions drawn from experimental investigations of boundary-layer-combustion-induced drag reduction in circular ducts.

Flow and heat transfer measurements in the laminar wake region of semi-circular cylinder embedded within a rectangular channel

Flow and heat transfer in the wake region of a semi-circular cylinder embedded within a rectangular channel have been studied experimentally using non-intrusive techniques. The semi-circular cylinder embedded within the channel with a blockage ratio of 0.45 (d/H) acts as an obstacle to the incoming fully developed laminar flow and creates a wake region in its downstream side. The velocity field in the downstream wake region is mapped using PIV (particle image velocimetry) technique with water as the working medium. The fluctuating shear layers and the vortices shed by the cylinder interact with the bottom heated wall of the channel. The influence of the wake region of the cylinder manifests on the heat transfer characteristics of the bottom wall of the channel and acts in a way to passively improve the overall heat transfer characteristics. The resulting whole field temperature values are captured using interferometry and augmentation in heat transfer rates vis-à-vis a channel without cylinder are compared. The dominant frequencies of the downstream unsteady flow in the wake of the cylinder are captured using schlieren technique. Heat transfer enhancement up to 42 percentage is observed in the case of the presence of the semi-circular cylinder in the channel in comparison with the flow in a plain channel for the Reynolds number range of 75–200 under which the experiments are performed.

Analysis of the blade boundary-layer flow of a marine propeller using a RANS solver

In this paper a comparison between RANS simulations carried out with the k−ω SST turbulence model and γ−R̃eθt transition model, and experimental measurements for marine propeller P4119 is made. The experiments were conducted at the David Taylor Model Basin and comprehended three-dimensional velocity components measurements of the blade boundary-layer using a LDV system in uniform conditions. The present work includes an estimation of the numerical errors that occur in the simulations, analysis of the propeller blade flow, chordwise and radial components of the boundary-layer velocity profiles and boundary-layer characteristics. From this comparison and depending on the selected inlet turbulence quantities, we conclude that the transition model is able to predict the extent of laminar and turbulent regions observed in the experiments. Moreover, the predicted velocity profiles show good agreement with the experimental measurements and verified the location of transition.

Numerical study of forced convection over phase change material capsules in a traditional spherical shape and a biomimetic shape

Abstract The use of biomimetic red blood cell (RBC) shaped capsules for latent heat thermal storage is a novel concept. The RBC shape provides a much higher heat transfer rate than that of other shapes. However, the heat transfer characteristics of external flows around RBC-shaped capsules and the optimal attack angle are unknown. In the present work, forced convection of external flows over an RBC-shaped PCM capsule was numerically studied in a steady-state laminar flow region with Reynolds number in the range of 20–200, and compared to that of a conventional spherical capsule. Moreover, the local Nusselt number, surface-average Nusselt number and drag coefficient at various Reynolds numbers and attack angles of RBC-shaped capsules were investigated. The surface-average Nusselt number of the RBC-shaped capsule was found slightly smaller than that of the spherical capsule with a maximum disparity of 12.04%. The drag coefficient of the RBC and spherical capsule both decreased with the increase of Reynolds number, and the disparity between them was very small. 0° attack angle was recommended for the RBC-shaped capsule layout, as it had the lowest drag force with a relatively high average Nusselt number.

Hydrodynamics Simulation for Air Flow Past Over an SD 2030 Airfoil Using Two Dimensional Model

In the old new present work cares with change of Reynolds number effect on the hydrodynamic characteristics. The different pitch angle is taken to analysis drag, lift, and pressure coefficients. The goal of these parameters is to understand the nature of the flow over the upper surface of the airfoil since the long laminar boundary layer and transition region have a light point for aerodynamics performance assessment. The results indicated that indicated the improvement in performance is parallel in path with the increasing length of the laminar flow region.

Promotion of uniform radial temperature distribution using buoyancy effect in inclination pipes

AbstractA model case was formulated to examine the buoyancy effect on radial temperature distribution in the heating section of thermal sterilization for high water content juices. Computational Fluid Dynamics (CFD) was employed in the short isothermal pipe to study the effect of Reynolds numbers (50, 300, and 2000), Rayleigh numbers (1 × 106 and 4.29 × 106), and pipe inclined angles (0 ≤ θ ≤ 60°). The simulation results were validated with the experimental and numerical data obtained from literature. When the buoyancy effect was dominant, the velocity and temperature were found to be unevenly distributed in the horizontal pipe. Besides, the coldest spot fluid reached maximum velocity, which does not allow adequate thermal sterilization. However, the significant effect of the inclined pipe angle was observed upon the enhancement of radial temperature uniformity. Moreover, the local Nusselt number increased with escalating pipe angles when compared with the horizontal pipe.Practical applicationsRadial temperature uniformity is important in continuous thermal sterilization for safety and retention of nutritional properties. When the fluid enters the heating section of thermal sterilization, it operates in a laminar or low turbulent region and gets heated up. Hence, the buoyancy effect may be dominant in this part. A method is proposed to exploit the buoyancy effect for promoting the radial mixing in the heating section of thermal sterilization. The present study asserts that the use of an upward inclined pipe can provide uniform radial temperature distribution and a high heat transfer coefficient. This approach may be applicable for fruit juices of low‐to‐moderate viscosity.

Trailing-edge broadband noise prediction of an airfoil with boundary-layer tripping

Hybrid methods for trailing edge noise prediction are applied to a NACA 6512-63 airfoil at zero angle of attack embedded in the actual narrow stream of the Siegen open-jet anechoic wind-tunnel. Incompressible large-eddy simulations (LES) of the airfoil trailing-edge flow yield the noise sources, and different analytical acoustic analogies then predict the acoustic far-field pressure. The chord-based Reynolds number of 1.9 × 105 triggers long laminar flow regions along the airfoil, leading to instability waves with an associated broadband noise radiation. To avoid this laminar-instability noise, the airfoil boundary layer is experimentally tripped on both sides. The actual serration tripping is included in the numerical grid as well as a simplified stair-step trip. Aerodynamic and acoustic results of the performed LES are compared with measurements and compressible direct numerical simulation (DNS) results. No noticeable influence of the trip geometry is found whereas the numerical schemes play a more important role. LES predictions with boundary layer trip compare well with experimental measurements and DNS cases. If the boundary-layer trip is not sufficiently thick in the numerical model, or not present at all, the far-field acoustic pressure is overpredicted and resembles more closely the sound radiation from the untripped airfoil with laminar instability noise.

Heat transfer and power consumption of Newtonian and non-Newtonian liquids in stirred tanks with vertical tube baffles

Agitation and heating of Newtonian and non-Newtonian fluids are commonly used processes in various industrial sectors. Traditionally, heating is mediated using jackets and helical coils. Compared to these conventional techniques, a vertical tube baffles offers geometrical advantages that favor mixture quality and heat-transfer efficiency. However, the available literature on this subject is scarce. Therefore, this study aims to determining expressions for calculating the Nusselt number. To this end, plots of the power number were obtained as functions of the Reynolds number during the agitation and heating of Newtonian and non-Newtonian fluids in batch-operated tanks with vertical tube baffles. Two tanks (10 and 50 L) were used along with pitched blade turbine (PBT) and Rushton turbine (RT) mechanical impellers. The Newtonian (water and aqueous sucrose solutions) and non-Newtonian (carboxymethyl cellulose and Carbopol solutions) fluids were heated at different rotational speeds using the transient technique for heat transfer. The power consumed by the turbine was measured experimentally in terms of the torque generated during motor agitation. Non-linear regression analyses were used to develop models to plot Nusselt as a function of the Reynolds, Prandtl, and the corrective factor of viscosity. The models demonstrated good fit with the experimental results. The power number, a function of Reynolds, was found to vary in the range of 2–5 for the PBT and 5–9 for the RT. The developed heat-transfer models were found to be valid across a wide Reynolds range of 20–200,000 for both impellers in the laminar and turbulent regions, which were delimited at a critical Reynolds of 10,000.