Airflow Separation Research Articles

Boundary Layer Turbulence over Surface Waves in a Strongly Forced Condition: LES and Observation

AbstractThe impact of sea state on air–sea momentum flux (or wind stress) is a poorly understood component of wind–wave interactions, particularly in high wind conditions. The wind stress and mean wind profile over the ocean are influenced by the characteristics of boundary layer turbulence over surface waves, which are strongly modulated by transient airflow separation events; however, the features controlling their occurrence and intensity are not well known. A large-eddy simulation (LES) for wind over a sinusoidal wave train is employed to reproduce laboratory observations of phase-averaged airflow over waves in strongly forced conditions. The LES and observation both use a wave-following coordinate system with a decomposition of wind velocity into mean, wave-coherent, and turbulent fluctuation components. The LES results of the mean wind profile and structure of wave-induced and turbulent stress components agree reasonably well with observations. Both LES and observation show enhanced turbulent stress and mean wind shear at the height of the wave crest, signifying the impact of intermittent airflow separation events. Disparities exist particularly near the crest, suggesting that airflow separation and sheltering are affected by the nonlinearity and unsteadiness of laboratory waves. Our results also suggest that the intensity of airflow separation is most sensitive to wave steepness and the surface roughness parameterization near the crest. These results clarify how the characteristics of finite-amplitude waves can control the airflow dynamics, which may substantially influence the mean wind profile, equivalent surface roughness, and drag coefficient.

The stall of different ice accretion airfoil

Ice along the leading edge of upper surfaces have detrimental effects on performance and control of an aircraft. Various icing shapes airfoil have various influence on aerodynamic characteristics and stall phenomenon. The mechanics model, of which ice accretion zone is on the leading edge, is produced by the assembly of various ice shapes (rime ice and mixed ice) and NACA0012 airfoil, In order to analyse the cause of stall phenomenon. The Reynolds averaged Navier-Stokes (N-S) equations are solved using Spalart-Allmaras (S-A) turbulence models the entire flow field. The results show: 1) the stalling angle of attack is various in case of various ice shapes on the leading edge. The stalling angle of attack is 15.5° in case of none ice, about 9° in case of rime ice and about 7.8° in case of mixed ice. 2) The way of airflow separation is different in case of various ice shapes on the leading edge. The vortex zone is expanded from the trailing edge to the leading edge in case of rime ice. Both vortex zones at the leading edge and the trailing edge are expanding meanwhile into one vortex zone in case of mixed ice.

Experimental investigation of energy (temperature) separation of a high-velocity air flow in a cylindrical channel with a permeable wall

Energy (temperature) separation is the redistribution of the total enthalpy (stagnation temperature) in a gas flow without external work and heat exchange with the ambient medium. Some simple devices for obtaining “hot” and cold” flows of compressed gas can be developed on the basis of the temperature separation effect. The best known of these devices is, apparently, the Ranque-Hilsch vortex tube. In this study, a new method of the temperature separation of gas flows is described and the first systematic experimental data are obtained. The method is based on the well-known effect of energy (temperature) separation in the boundary layer of a compressible gas flow. The “cold” regions layers are separated from the “hot” ones in a device consisting of a nozzle and a cylindrical tube with a permeable wall. When compressed air flows out into the atmosphere through this device, a part of the flow seeps through the permeable wall due to a steady pressure gradient. Its stagnation temperature turns out to be smaller than the initial temperature, whereas the stagnation temperature of the other part of the flow is greater than the initial temperature. In the pressure and temperature ranges considered the cooling effect is by a factor of about five greater than that of Joule—Thomson. The initial pressure of the compressed air varied in the range from 1.2 to 8.0 bar, while the initial temperature was 295.6 K. Both convergent and convergent-divergent (supersonic) nozzles were used. The temperature separation increases with increase in the initial pressure up to a certain value but it remains almost constant after this value has been attained. The greatest temperature difference between the “hot” and “cold” flows is about 11 K.

Abnormal enhancement of separated turbulent air flow and heat transfer in inclined single-row oval-trench dimples at the narrow channel wall

New shapes of surface vortex generators are proposed which allow improving the energy performance of channels of propulsion devices and contribute towards the safety of space flights. Abnormal enhancement of heat transfer and separated turbulent air flow in oval-trench dimples of depth 0.3 inclined at an angle of 45о to the incoming flow and located in-line at the heated wall of the narrow channel is accompanied by a four-fold growth of relative negative friction and an almost five-fold increase of relative heat transfer in the separation zone. At the same time, a maximum secondary flow velocity in the vicinity of the dimple reaches a maximum flow velocity in the plane-parallel channel. A reason for the onset of this phenomenon (ultra intensification of transport processes) caused by the formation of a high pressure drop between closely spaced stagnation zones on the windward side of the dimple and a low pressure on the entrance spherical portion of the dimple is disclosed.

Application of the Bionic Concept in Reducing the Complexity Noise and Drag of the Mega High-Speed Train Based on Computer Simulation Technologies

Regarding the continuous development of high-speed trains and the increase of running speeds, the aerodynamic design of high-speed trains has become significantly important, while reduction of drag and noise comprises a significant challenge in order to optimize aerodynamic design of high-speed trains. The design form factor of a high-speed train is highly influenced by aerodynamic aspects including aerodynamic drag, lift force, and noise. With the high-speed train as the object, the paper aims to take bionic concept as the entry point, selecting the hummingbird as the bionic prototype and extracting bionic elements to establish a bionic train model. Then, the finite volume method was used for numerical simulation and analysis of the aerodynamic performance and aerodynamic noise of the bionic high-speed train. Computational results prove that drag and noise of the bionic head type were lower than those of the original train; drag of the head train of the bionic model was reduced by 2.21% in comparison with the original model, while the whole-train drag was reduced by 3.53%, indicating that drag reduction effects are available and implying that the bionic head type could reduce drag and noise. Noise sources of the bionic train are mainly located at positions with easy airflow separation and violent turbulence motion. Large turbulence energy is in bogie areas and mainly exists at the leeward side of the bogie area. Obviously, the bogie area is the major noise source of the train. Aerodynamic noise of the bionic train in far-field comprises a wide-frequency range. Noises were concentrated within 613 Hz~3150 Hz. When the bionic high-speed train ran at 350 km/h, through comparative analysis of total noise levels at observed points of the high-speed train, it is found that this position with the maximum noise level was 25 m away from the head train nose tip, with the maximum value of 88.4 dB (A). When the bionic train ran at 600 km/h, the maximum sound pressure level at the longitudinal point was 99.7 dB (A) and the average noise level was 96.6 dB (A). When the running speed increased from 350 km/h to 600 km/h, the maximum noise level increased by 11.3 dB (A) and the average noise level increased by 11.6 dB (A). Computation results of aerodynamic noise at the point which is 7.5 m away from the rail center show that the maximum aerodynamic noise level existed at the first-end bogie of the head train, while the noise level was larger at the position closer to the ground.

Analysis of lift, drag and CX polar graph for a 3D segment rigid sail using CFD analysis

ABSTRACTA rigid sail is an alternative source of propulsive power for marine vessels and they have been installed on ocean-going powered ships in the past. To determine the potential power that could be harnessed by a rigid sail its specific lift and drag characteristics must be known. For this purpose, a segment rigid sail was studied utilising computation fluid dynamics analysis and a virtual wind tunnel. This analysis enabled lift and drag force tables to be produced and based on these data a CX (thrust coefficient) polar graph for the sail was derived. Analysis of velocity magnitude and static pressure contours was also undertaken and the aerodynamic characteristics of the sail including airflow separation and wake size studied.

Experimental visualisation of wake flows induced by different shaped moving manikins

Traditional smoke visualisation is based on stationary objects with a moving fluid. This paper presents wake flow from moving manikins investigated by a new smoke visualisation technique. Three scaled models 1/5th of realistic manikin models were used, including two body shapes (thin and wide) and two gestures (standing and walking). Smoke visualisation was produced by a chemical reaction between acetic acid (CH3COOH) and cyclohexylamine (C6H13 N) to generate smoke. Highspeed photography and image processing techniques were used to determine the qualitative and quantitative data on the airflow patterns and separation points from the manikin motion. Detailed flow separation images showed that regular vortices were produced off the head and the shoulder, while flow separating at the hand swirled behind and inwards. Analysis of flow over the head revealed how the separation point shifted from the back of the head to the front as the velocity decreased. When the manikin came to a stop, the results also showed increased airflow activity around the face which increases the risk of inhaled particulate matter from the surroundings. Image processing analysis produced quantitative data which included the vortex shedding frequency coming off the shoulder, and the separation point from the manikin head. It is expected that the experimental results will provide validation data for future computational fluid dynamic (CFD) modelling results.

Aerodynamic properties of potato tubers to airflow separation from stones and clods

ABSTRACTSeparation of potato (Solanum tuberosum L.) tubers from clods and stones in a potato harvester is difficult. It is necessary to develop a better means of separation of tubers from other like size, shape, or weight objects. The study examined the aerodynamic properties of potato tubers terminal velocity, drag coefficient, Reynolds number, diameter of equivalent sphere, and volume shape factor to design a better air separator mechanism of varieties of potato tubers from stones and clods. The experimental and theoretical aerodynamic properties of potato tubers were determined and compared using a vertical wind tunnel. The tuber size class positively affected geometric mean diameter and mass of tubers. Variety affected tuber density with ‘Sante’ having a greater density than ‘Marfona’. The interaction of tuber size class and variety affected sphericity of tubers with increasing tuber size class causing reduced sphericity. In similar tuber size classes, ‘Marfona’ had greater sphericity compared to ‘Sante’ except for class 3 that opposite was true. Mean values of aerodynamic properties of varieties tested increased with increasing tubers size. Values of terminal velocity obtained for potato tubers were lower than expected for this product compared to other agricultural crops. Airflow can be used to separate tubers from stones and clods.

A survey of separated airflow patterns at inlet of circular exhaust hoods

In this paper our objectives are to perform a numerical and experimental study of the velocity field in the area affected by a circular exhaust duct, to define the outline of flow separation area at its inlet, to come up with a reliable mathematical simulation of the separated flow at exhaust duct inlet, and to determine the effect of the duct inclination angle its serviced area.The stationary discrete vortex method has enabled development of an adequate and reliable technique for computation of separated flows at inlet of a local exhaust hood. We have studied airflow patterns around the exhaust duct using the discrete vortex method, CFD and natural experiment. Characteristic dimensions of the flow separation area at exhaust hood inlet have been determined with relation to the length and inclination angle of the hood. Shaping the exhaust duct to fit the resulting separation zone outline will enable savings of energy that would otherwise be wasted on overcoming local drag forces; furthermore, this will prevent escape of contaminants circulating in the separation area to the environment. Determining airflow velocity distribution patterns will facilitate the choice of the most efficient exhaust duct design to minimize the cost of contaminant removal.

The turbulent airflow over wind generated surface waves

The airflow dynamics above the air–sea interface are believed to have a significant impact on the fluxes of momentum and scalars across the ocean surface. We present an experimental study of the turbulent structure of the airflow above wind generated surface waves. Measurements, taken at a fetch of 22.7 m in University of Delaware’s large wind-wave–current facility, are reported for wind wave experiments with 10-m extrapolated wind speeds spanning from 2.19 m s−1 to 16.63 m s−1. In order to complete this study, we developed a complex imaging system, combining particle image velocimetry with laser induced fluorescence techniques. High resolution two-dimensional (18.7 cm x 9.7 cm) velocity fields were measured as close as 100 μm above the air–water interface (on average). In addition, we acquired high spatial and temporal resolution wave field data simultaneously with the airflow measurements. Using this imaging system, we were able to perform phase averaging and separate the turbulent, mean and wave-induced velocity fields. We observe direct evidence of airflow separation events past the crests of wind waves, starting at low to moderate wind speeds (U10>2.19 m s−1). In general, the turbulent boundary layer in the air is characterized by numerous velocity sweeps and ejections, accompanied by intense downwind-tilted spanwise vorticity (shear) layers stemming from the surface. We were able to directly observe these turbulent events, and estimate their statistical significance using quadrant analysis. These events become phase-locked in the presence of waves, and, when U10≥5 m s−1, they are accompanied by intermittent airflow separation events past wave crests. The production of TKE also shows wave phase locked features indicating that further analysis of the wave-coherent contributions to the TKE balance is needed.

Effects of the wind on the breaking of modulated wave trains

The effect of wind on the wave breaking induced by the modulational instability is investigated numerically using the open source software Gerris. The two-phase flow is modelled by the two-dimensional Navier–Stokes equations for a single incompressible fluid and a Volume of Fluid technique is employed to capture the air–water interface. The flow is initialized as a fundamental wave component with two side-band perturbations so that the breaking is induced via the Benjamin–Feir instability mechanism. In order to investigate its effect on the wave evolution and on the breaking process, a uniform wind, twice the phase speed, is assigned in the air phase. The simulation covers the initial development of the wind profile, the growth of the modulational instability, the breaking and post breaking phases. Results show the occurrence of air flow separation from the wave crests shortly after the initial start. Pressure and tangential stress acting on the free surface are computed. It is shown that due to the flow separation there is a favourable pressure gradient about the wave crests whereas the tangential stresses are generally in favour of the wave propagation on the back of the wave but are opposed to the propagation along the forward face and in the wave trough. An initial growth of the energy content in water is observed, followed by a dissipation stage which is not related to the breaking process. In agreement with the experiments, the growth rate of the side-bands is reduced when compared to the corresponding no-wind solution. Because of the slower growth, the limiting condition for the onset of the breaking is reached with some delay. At the end of the breaking process, when the downshift is completed, the amplitude of the left side-band in the wind case is somewhat lower than that for the no-wind case. No substantial differences have been found in terms of the total energy dissipated by the whole breaking process although the dissipation rate for the wind case is noticeably higher. The higher dissipation rate observed in the wind case is found to be related to the larger amount of air entrained by the breaking process.

Numerical simulation on the impact of the bionic structure on aerodynamic noises of sidewindow regions in vehicles

The paper adopted a bionic hemispherical convex structure in the A pillar-rear view mirror regions according to actual requirements. Furthermore, impacts of the bionic structure on aerodynamic characteristics and noises in the region were studied. Friction resistance of airflows was greatly reduced, fluctuations and pulsation pressures of flow fields were also reduced, and characteristics of flow fields and sound fields were improved. The computational results were finally verified by the experimental test. Firstly, the aerodynamic lift force coefficient and drag force coefficient of the bionic model were computed, and they were obviously lower than those of the original model. The adhesive force between tires and ground during vehicle running was increased, and the danger degree of “waving” of high-speed vehicle running was weakened. In this way, stability of vehicle running could be improved. Secondly, flow fields of the bionic model were computed. Compared with the original model, an obvious vortex was behind the original model, while no vortexes were behind the bionic model. Therefore, convex structures of the bionic model had obvious impacts on flow fields behind the rear view mirror. Airflow separation situations were obvious improved at wheels, windshield and rear side windows of the bionic model. Due to blocking of convex structures of the A pillar and rear view mirror in the bionic model, airflows was hindered and obvious dragging phenomena were formed. Therefore, flow fields in the side window regions could be improved greatly. In addition, the flow field scope under the rear view mirror in the bionic model was also decreased. Ringed vortex structures appeared behind the rear view mirror in the bionic model. The ringed vortex structures were closely interlaced and then extended together backwards. Vortexes behind the rear view mirror in the original model were chaotic, where most of them were attached on the surface of side windows. In the original model, turbulent flows with certain strength were on the right upper corner of the side window region. In the bionic model, no turbulent flows were in the same regions. This result indicated that through using the bionic convex structures, airflows flowing through side windows could be combed and could move backwards towards upper and lower edges of the side windows. It could be predicted that pulsation pressures on the side window surface would surely decrease. Therefore, aerodynamic noises caused by pulsation pressures in side window regions would also be improved correspondingly. Especially in regions behind A pillar-rear view mirrors, the maximum noise reduction amplitude reached about 20 dB.

Turbulent Flow over Steep Steady and Unsteady Waves under Strong Wind Forcing

AbstractTurbulent flow over strongly forced steep steady and unsteady waves is simulated using large-eddy simulation (LES) with timetand spacexvarying wave heighth(x,t) imposed as a lower boundary condition. With steady waves,h(x,t) is based on measurements of incipient and active breaking waves collected in a wind-wave flume, while a numerical wave code is used to generate an unsteady evolving wave packet (group). Highly intermittent airflow separation is found in the simulations, and the results suggest separation near a wave crest occurs prior to the onset of wave breaking. The form (pressure) drag is most sensitive to the wave slope, and the form drag can contribute as much as 74% to the total stress. Wind and scalar profiles from the LES display log-linear variations above the wave surface; the LES wind profiles are in good agreement with the measurements. The momentum roughness increases as the water surface changes from wind ripples to incipient breaking to active breaking. However, the scalar roughness decreases as the wave surface becomes rougher. This highlights major differences in momentum and scalar transport over a rough wavy surface. For a rapidly evolving, strongly forced wave group, the form drag is highly correlated with the wave slope, and intermittent separation is found early in the packet evolution when the local wave slope −∂h/∂x(x,t) ≥ 0.22. The packet root-mean-square wave slope is 0.084, but the form drag fraction is 2.4 times larger than a comparably forced steady wave. Thus, a passing wave group can induce unsteadiness in the wind stress.

Impacts of wave age on turbulent flow and drag of steep waves

Turbulent flow over steep steady and unsteady wave trains with varying height h(x, t) and propagation speed c is simulated using large-eddy simulation (LES) in a wind-wave channel [17]. The imposed waveshape with steady wave trains is based on measurements of incipient and active breaking waves collected in a wind-wave tank, while a numerical wave code is used to generate an unsteady evolving wave train (or group) [3]. For the adopted waveshapes, process studies are carried out varying the wave age c/u* from ~ 1 to 10: the airflow friction velocity is u*. Under strong wind forcing or low wave age c/u* ~ 1, highly intermittent airflow separation is found in all simulations and the results suggest separation near a wave crest occurs prior to the onset of wave breaking. As wave age increases flow separation is delayed or erased for both steady and unsteady wave trains. Flow visualization shows that near the wave surface vertical velocity w and waveslope ∂h/∂x are positively correlated at c/u* ~ 1 but are negatively correlated at c/u* = 10. The vertical speed of the underlying wave oscillations depends on the local waveslope, increases with phase speed, and is a maximum on the leeward side of the wave. Vigorous boundary movement [8] appears to alter the unsteady flow separation patterns which leads to a reduction in form (pressure) drag as wave age increases. For example, the pressure contribution to the total drag of the active breaker wave train decreases from 74% at c/u* = 1.23 to less than 20% at c/u* = 10. Critical layer dynamics appears to play a secondary role in the air-wave coupling over steep waves, but requires further investigation. For all simulations, the form drag is found to be strongly dependent on both waveslope ∂h/∂x and wave age c/w*. The simulations are in good agreement with experimental results for turbulent flow over steep waves under strong wind forcing.

Numerical investigation on aerodynamic noises of the lateral window in vehicles

The paper firstly conducted a numerical simulation for flow fields and aerodynamic noises of the lateral window region in vehicles, and verified its correctness using the experimental test. Numerical simulation shows that: A pillar has a complicated shape and large corner, so that airflows will be separated here. An eddy structure is caused in the lateral window region and develops along the A pillar to generate serious pressure pulsations. A low pressure region is formed behind the A pillar. Obvious airflow separation regions are in the A pillar, rear view mirrors, wheels and wheel chambers. These airflow separation regions are typical positions causing aerodynamic noises. Additionally, large separated regions are located at the tail part of the vehicle, which is a main reason for causing the aerodynamic resistance. Intensity and velocity of eddies near the lateral window surface are relatively large, while its intensity near edges of the rear view mirror is weak. The shape of eddies extends along the flow direction to be an oval shape. The separated and broken eddies are sources for causing pressure pulsations. According to sound pressures of observation points, it can be also found that the separated eddy is a main reason for causing aerodynamic noises. Sound pressures are low at the right upper corner of lateral windows. In addition, noise distributions on the lateral window become gradually uniform with the increased frequency. In order to reduce flow noises, a bionic saw-tooth structure is applied to A pillars and rear view mirrors. After the bionic structure is introduced, some fluids are adhered to A pillars and rear view mirrors, so that the energy of fluids reaching the lateral window is reduced. In addition, fluids in rear regions of the rear view mirror presented a spiral shape, so that the possibility of fluid diffusion will be also reduced. In the original model, the maximum energy is 57.77, while that in this region with the bionic saw-tooth structures is 55.00. Obviously, the eddy energy is weakened. Compared with the original model, flow noises of all the observation points are reduced to different degrees, and the noise reduction effect is obvious. The results fully prove that this region with bionic saw-teeth in this paper has obvious advantages in noise reduction.

Numerical investigation of wave-induced vibrations and their effect on the fatigue damage of container ships

Wave-induced vibrations of the ship hull girder consist of springing when it is associated with a resonance phenomenon, and whipping when it is caused by a transient impact loading due to wave slamming. This paper presents a new approach to predict the wave-induced vibrations and their contributions to fatigue damage of container ships. The seakeeping, whipping, springing and elastic modes calculations are coupled in every time step. According to the design characteristics of large container ships the existing methods are modified, air cushion effect and flow separation are considered in the slamming calculation, symmetric and asymmetric vibration modes equations are applied to predict the springing response in head seas and oblique seas. Based on rain flow counting method, spectral-based analysis method is used to calculate the additional fatigue damage due to springing and whipping.

Biomass Gasification for District Heating

Latvia is required to increase the share of energy produced from renewable sources. This study is focused on the potential of biomass gasification in the production of energy for the purposes of district heating. This study describes an innovative technology for syngas production from wood chips. The sensible heat of the produced syngas was used to dry wood chips. Gasification plant was tested at various capacities with and without air flow separation at the primary and secondary flows. Syngas composition, temperature in the gasifier, syngas temperatures, char heating value and several other parameters are measured and analyzed. The energy balance of the gasification process is calculated and presented. Results show that air separation causes growth in the efficiency of the gasification process.

Mechanism and Fine Coal Beneficiation of a Pulsating Airflow Classifier

ABSTRACTTraditional air classifiers produce binary splits based on the terminal velocity rather than the density of the feed. In this study, a pulsating air with a sine-like shape wave was introduced into a vertical column air classifier to achieve the density dominant separation on −6+3 mm fine coal. A lab-scale separation device was used to investigate the effects of pulsating characteristics (amplitude and frequency) and particles volumetric concentration on pulsating airflow separation in a batch process. The optimum operating conditions of 34.1 cm pulsation amplitude, 2.45 Hz frequency and 500 g feed amount achieved 48.08% separation efficiency, 68.22% combustible matter recovery and produced a clean coal with 17.98% ash (from a feed containing 42.02% ash), 47.73% yield and Ep value of 0.17. The results showed that the pulsating air classifier has a potential for fine coal dry beneficiation. The motion of particles in a pulsating airflow field under a hindered settling condition has been presented.

A REVIEW ON DIGITAL PTV MEASUREMENTS OF A SEPARATED AIR FLOW BEHIND A BACKWARD-FACING STEP

A REVIEW ON DIGITAL PTV MEASUREMENTS OF A SEPARATED AIR FLOW BEHIND A BACKWARD-FACING STEP

Wind-break walls with optimized setting angles for natural draft dry cooling tower with vertical radiators

To get larger cooling performance enhancement for natural draft dry cooling tower with vertical cooling deltas under crosswind, setting angles of wind-break walls were optimized. Considering specific structure of each cooling delta, an efficient numerical model was established and validated by some published results. Aerodynamic fields around cooling deltas under various crosswind speeds were presented, and outlet water temperatures of the two columns of cooling delta were exported as well. It was found that for each cooling delta, there was a difference in cooling performance between the two columns, which is closely related to the characteristic of main airflow outside the tower. Using the present model, air inflow deviation angles at cooling deltas’ inlet were calculated, and the effects of air inflow deviation on outlet water temperatures of the two columns for corresponding cooling delta were explained in detail. Subsequently, at cooling deltas’ inlet along radial direction of the tower, setting angles of wind-break walls were optimized equal to air inflow deviation angles when no airflow separation appeared outside the tower, while equal to zero when outside airflow separation occurred. In addition, wind-break walls with optimized setting angles were verified to be extremely effective, compared to the previous radial walls.