High Turbulence Intensity Research Articles

THE HYDRODYNAMIC CHARACTERISTICS FOR VEGETATIVE CHANNEL WITH GRAVEL BED DUNES

Aquatic plants are known to provide flow resistance and impact the turbulence intensity and turbulent kinetic energy within the vegetated area. This paper further investigates the impact of both vegetation and dunes in open channels to the hydrodynamic characteristic of flow. Emergent vegetations were built from rigid wooden rod in staggered arrangement with 0.5% vegetations density were applied in the flume. Experiments were conducted with flow rate of 0.0058 m3/s throughout the experiments. Dunes were constructed from gravel of 2 mm size diameter in the shape of standing waves of three different lee slope angles of 3⁰, 6⁰ and 9⁰. Flow velocities are measured by using a velocimeter to get the raw data for the three-dimensional flow velocity in the x, y, and z directions. The velocities data were then analysed to calculate the mean velocity, turbulence intensity and turbulent kinetic energy. Experimental results showed that, for all three lee slope angles presented higher flow velocity in the vegetated channel compared to the non-vegetated channel. It was also found that greater lee slope angle dunes generate higher velocity for both channels with and without vegetation. Higher turbulence intensity can be found near the bed area and greater turbulence intensity also shown in the positive slope of a dunes compared to negative slope area. Higher turbulent kinetic energy values were recorded within the vegetated channel compared to the non-vegetated channels.

Numerical analysis of vane endwall film cooling and heat transfer with different mainstream turbulence intensities and blowing ratios

In present study, effects of mainstream turbulence intensity and blowing ratio on the endwall film cooling and heat transfer are numerically investigated. Upstream double-row streamwise cylindrical holes with inclination angel (θ) of 30 deg is applied. Four blowing ratios (M = 0.5, 1.0 1.5, 2.0) and three turbulence intensities (Tu = 5%, 15%, 25%) are studied. Distributions of film cooling effectiveness and heat transfer coefficient are presented together with vortex structure. Results show that with the increasing of blowing ratio, the strength of horseshoe vortex is significantly weakened and the level of film cooling effectiveness is obviously improved. In addition, the low heat transfer zone downstream the holes and upstream the leading edge is enlarged because of the increased momentum, while the high heat transfer zone at the exit of passage gradually shrinks. With the increasing of turbulence intensity, the variation of film cooling effectiveness is very limited, but the level of heat transfer near the pressure side in the second half of the vane passage obviously decreases. Moreover, a more uniform film coverage and heat transfer distribution is obtained at high turbulence intensities.

Onset of wake meandering for a floating offshore wind turbine under side-to-side motion

Wind turbine wakes, being convectively unstable, may behave as an amplifier of upstream perturbations and make downstream turbines experience strong inflow fluctuations. In this work, we investigate the effects of the side-to-side motion of a floating offshore wind turbine (FOWT) on wake dynamics using large-eddy simulation and linear stability analysis (LSA). When the inflow turbulence intensity is low, simulation results reveal that the turbine motion with certain Strouhal numbers$St = fD/U_\infty \in (0.2,0.6)$(where$f$is the motion frequency,$D$is the rotor diameter, and$U_\infty$is the incoming wind speed), which overlap with the Strouhal numbers of wake meandering induced by the shear layer instability, can lead to wake meandering with amplitudes being one order of magnitude larger than the FOWT motion for the most unstable frequency. For high inflow turbulence intensity, on the other hand, the onset of wake meandering is dominated by the inflow turbulence. The probability density function of the spanwise instantaneous wake centres is observed being non-Gaussian and closely related to that of the side-to-side motion. This complements the existing wake meandering mechanisms, that the side-to-side motion of an FOWT can be a novel origin for the onset of wake meandering. It is also found that LSA can predict the least stable frequencies and the amplification factor with acceptable accuracy for motion amplitude$0.01D$. Effects of nonlinearity are observed when motion amplitude increases to$0.04D$, for which the most unstable turbine oscillations shift slightly to lower frequencies and the amplification factor decreases.

Characterization of Turbulent Flow Behind a Transcatheter Aortic Valve in Different Implantation Positions.

The development of turbulence after transcatheter aortic valve (TAV) implantation may have detrimental effects on the long-term performance and durability of the valves. The characterization of turbulent flow generated after TAV implantation can provide fundamental insights to enhance implantation techniques. A self-expandable TAV was tested in a pulse replicator and the three-dimensional flow field was extracted by means of tomographic particle image velocimetry. The valve was fixed inside a silicone phantom mimicking the aortic root and the flow field was studied for two different supra-annular axial positions at peak systole. Fluctuating velocities and turbulent kinetic energy were compared between the two implantations. Velocity spectra were derived at different spatial positions in the turbulent wakes to characterize the turbulent flow. The valve presented similar overall flow topology but approximately 8% higher turbulent intensity in the lower implantation. In this configuration, axial views of the valve revealed smaller opening area and more corrugated leaflets during systole, as well as more accentuated pinwheeling during diastole. The difference arose from a lower degree of expansion of the TAV's stent inside the aortic lumen. These results suggest that the degree of expansion of the TAV in-situ is related to the onset of turbulence and that a smaller and less regular opening area might introduce flow instabilities that could be detrimental for the long-term performance of the valve. The present study highlights how implantation mismatches may affect the structure and intensity of the turbulent flow in the aortic root.

SPTV sheds light on flow dynamics of fractal-induced turbulence over a plate-fin array forced convection

A 3D stationary particle tracking velocimetry (SPTV) with a unique recursive corrective algorithm has been successfully established to detect the instantaneous regional fluid flow characteristics. The veracity of SPTV is corroborated by conducting actual displacement measurement validation, which gives a maximum percentage deviation of about 0.8%. This supports the accuracy of the current SPTV system in 3D position detection. More importantly, the SPTV detected velocity fluctuations are highly repeatable. In this study, SPTV is proven to be able to express the nature of chaotic fractal grid-induced regional turbulence, namely: the high turbulence intensity attributed to multilength-scale wake interactions, the Kolmogorov’s −5/3 law decay, vortex shedding, and the Gaussian flow undulations immediately leeward of the grid followed by non-Gaussian behaviour further downstream. Moreover, by comparing the flow fields between control no-grid and fractal grid-generated turbulence of a plate-fin array, SPTV reveals vigorous turbulence intensity, smaller regional integral-length-scale, and energetic vortex shedding at higher frequency for the latter, particularly between fins. Thereupon, it allows the unravelling of detailed thermofluid interplays of plate-fin heat sink heat transfer augmentation. The novelty of SPTV lies in its simplicity, use of low-cost off-the-shelf components, and most remarkably, low computational complexity in detecting fundamental characteristics of turbulent fluid flow.

Flame structure and burning velocity of ammonia/air turbulent premixed flames at high Karlovitz number conditions

This paper presents experimental studies of the structures and burning velocities of premixed ammonia/air jet flames at high Karlovitz (Ka) number conditions. Simultaneous planar laser-induced fluorescence (PLIF) imaging of imidogen (NH) and hydroxyl (OH) radicals was performed to investigate the local flame structure and Laser Doppler Anemometry (LDA) measurements were employed for extracting complement relevant turbulent quantities from the flow field. All the selected cases are located in the regime of distributed reaction zones (DRZ) in the Borghi-Peters diagram, with a maximum Karlovitz (Ka) number and turbulent intensity (u′/SL) up to 1008 and 240, respectively. The OH- and NH-PLIF data were used to determine the flame surface density, flame-surface area ratio, and turbulent burning velocity (ST). The main findings include: (a) The NH layer remains thin and continuous over the investigated range of turbulent intensity and Karlovitz number, and the thickness keeps constant statistically without any significant broadening by turbulent eddies; (b) Spatial correlations of the NH and OH radicals show that overlap of NH and OH layers always exists in a thin region where OH has a weaker signal intensity; (c) The ratio of turbulent to laminar burning velocity (ST/SL) shows a nearly linear increase with turbulent intensity, while the ratio of wrinkled flame surface area to that of ensemble-averaged flame surface area increases only slightly with turbulent intensities. The slower increase of wrinkled flame surface area with turbulent intensity can be attributed to under-resolution in the current state-of-the-art PLIF experiments, the neglection of 3D flame wrinkles in 2D experiments, and the increase in flame stretch factor at high turbulent intensities.

An experimental study on base pressure behavior during bistability characterisation for a bluff body vehicle close to the ground

The present experimental investigation is concerned with the existence of Bi-stability behaviour at various ground clearances in the wake of Square Back Ahmed Body geometry (SBAB). The experiments were performed at a Reynolds number of 7.2 × 104 in a subsonic open-type wind tunnel facility. The raw readings from the pressure scanner alone were used to predict the existence of bistability character. The bistability behavior in the wake was explored in two different ways (i) The conventional base-pressure contours and pressure gradient plot, and (ii) The wavelet analysis. The minimum ground clearance for the wake to exhibit bistability was found to be 0.468H, much higher than the values determined in the previous experimental works. The discrepancy was due to relatively high turbulence intensity in the test section and experimental setup. We also found that the bi-stability switching rate was found to increase with a decrease in ground clearance. The wavelet analysis helped in determining the existence of long time-scale frequencies associated with the Bi-stability behavior.

Measurement of the convective heat transfer coefficient of the human body in the lift-up design

So far, the research on the convective heat transfer coefficient (hc) in outdoor thermal comfort has mainly employed CFD simulation and wind tunnel experiments, which are difficult to fully restore the complex microclimate wind environment. In the traditional thermal comfort model, the influence of turbulence intensity (TI) on the hc might be underestimated. This study aims to measure the hc of the human body surface in the outdoor environment. A thermal manikin was placed in a lift-up building. The ambient wind speed ranged from 0.5m/s to 4m/s, with the TI ranging from 4% to 55%. The experimental results show that under the same wind speed, the difference in hc between high and low TI can be up to 15%. Based on that, the regression formula for predicting hc related to wind speed and TI was proposed. This experimental study supplements the lack of field measurement of hc in outdoor thermal comfort research, which is helpful to improve the accuracy of the outdoor thermal comfort model.

Experimental investigation of the wake behind a cooled cylinder

An experimental channel intended for investigation of a wake behind a cooled cylinder was designed and constructed in this work. The setup is designed for measurements with air at low Reynolds numbers. The measuring techniques based on the standard visualization with a fog generator and the constant temperature anemometry (CTA) were used. The flow performance was investigated behind two cylinders placed inside a channel with square cross section. One cylinder was kept at a constant flow temperature, while the second cylinder could be cooled using a water circuit with a thermostatic bath. The dependency of the Strouhal number on the Reynolds number was evaluated and compared to the literature data and the empirical correlations. The concept of an effective temperature in a non-isothermal flow was also applied. Unfortunately, a considerable difference between the preliminary measurements and the literature data was detected. The air-flow was found to have unsatisfactory high turbulence intensity. Consequently, some modifications of the current channel design are proposed.

Investigation into Thermal-Fluid interaction of ammonia turbulent swirling flames under various Non-Premixed burner conditions

This study deals with flow and turbulence interaction of an ammonia flame in a non-premixed burner. Numerical investigations have been performed on the velocity field, thermal field and the turbulence parameters for various oxidizers. In order to validate the simulations and select the most favorable turbulence model, some modeling have first been performed under non-reacting conditions, and the predictions are compared with the measurements. Although the predicted results obtained by using all k-epsilon models are in acceptable consistency with the experimental values, the predictions acquired through RNG k-epsilon model have showed more consistent results with the experimental measurements since there are swirling flows in the flow-field. The further modeling results under the reacting conditions showed the main flow and the external recirculation regions occurred in the flow-field. However, a central recirculation zone was not observed for all cases studied. It is revealed that the predicted axial velocity values for ammonia-air combustion are higher than those of oxy-conditions in the reaction zone. The radial and the tangential velocity values are lower than axial velocity values for all cases. Besides, the tangential velocity component is much lower in case of the ammonia-oxy combustion. It was observed that high turbulence intensity values (about % 350 and % 290 for oxy- and air-combustion) occurred in the mixing zone due to mean velocity affecting velocity fluctuations. Moreover, a consistency between the temperature and the velocity profiles has been predicted in the main flow zone.

Velocity field and diameter distribution of long-chain alcohol-aviation kerosene transverse jet

The velocity field and diameter distribution of long-chain alcohol-aviation kerosene transverse jets were measured and analyzed. Aviation kerosene was chosen as the base fuel. During the transverse jet process, the droplet velocity direction transformed from the vertical direction to the horizontal direction, i.e., to the crossflow direction. The highest turbulence intensity appeared in the liquid–air interaction field. Both the resultant velocities on XZ and XY planes, Uxz and Uxy, and the component velocities along X, Y and Z coordinates, Ux, Uz, and Uy, increased at first and then decreased with the increment of long-chain alcohol volume proportion while the corresponding proportions are different between that of n-butanol and that of n-pentanol. The existence of vortex region was determined in the liquid–air interaction field and the near-field region in the downstream of the transverse jet. The particle size field analysis showed that the droplet size and shape factor and number density were mainly affected by Weg (Weber number of cross gas flow). The characteristics were different under different breakup modes. With the increase in the volume fraction of long-chain alcohol, the droplet size and Sf (shape factor) first increased and then decreased. The addition of long-chain alcohol also reduced the range of particle size with the largest number density. The maximum size of the droplets in the particle size field of n-pentanol-aviation kerosene was larger than that of n-butanol-aviation kerosene.

Heat transfer in shallow caves: influence of turbulent convection

The enhancement of the conservation conditions of cave paintings requires a detailed understanding of heat transfer in such cavities. This article presents a numerical investigation of turbulent free convection in a parallelepipedic cavity. Non-homogeneous wall temperatures are prescribed from a large-scale model taking into account external temperature fluctuations damped by heat diffusion in the rock massif above the cavity. Large Eddy Simulation is performed to solve the turbulent flow fields for a given wall temperature field corresponding to a Rayleigh number of 8.5 x 109. The outcomes of the model are analysed in terms of statistical mean. Results show complex large scale flow patterns with regions of high turbulent intensity. The Q-criterion is used to identify turbulent structures for an instantaneous flow field. Then we analyse the spatial distribution of the conductive heat flux at the walls to locate the regions with intense convection. We show that the conductive flux smaller than the wall-to-wall radiative flux in the major part of the cavity, and close to its value at some spots.

The Effects of Aerodynamic Interference on the Aerodynamic Characteristics of a Twin-Box Girder

To investigate the aerodynamic characteristics of a twin-box girder in turbulent incoming flow, we carried out wind tunnel tests, including two aerodynamic interferences: leading body-height grid, and leading circular cylinder. In this study, the pressure distribution and the mean and fluctuating aerodynamic forces with the two interferences are compared with bare deck in detail to investigate the relationship between aerodynamic characteristics and the incoming flow characteristics (including Reynolds number and turbulence intensity). The experimental results reveal that, owing to the body-height flow characteristics around the deck interfered with by the body-height grid, the disturbed aerodynamic characteristics of the twin-box girder differ considerably from those of the bare twin-box girder. At the upstream girder, due to the vortex emerging from the body-height grid breaking the separation bubble, pressure plateaus in the upper and lower surface are eliminated. In addition, the turbulence generated by the body-height grid reduces the Reynolds number sensitivity of the twin-box girder. At a relatively high Reynolds number, the fluctuating forces are mainly dominated by turbulence intensity, and the time-averaged forces show almost no change under high turbulence intensity. At a low Reynolds number, the time-averaged forces change significantly with the turbulence intensity. Moreover, at a low Reynolds number, the wake of the leading cylinder effectively forces the boundary layer to transition to turbulence, which reduces the Reynolds number sensitivity of the mean aerodynamic forces and breaks the separation bubbles. Additionally, the fluctuating drag force and the fluctuating lift force are insensitive to the diameter and the spacing ratio.

Effect of freestream turbulence on the structure of boundary-layer flames

This study investigates the role of turbulence induced instabilities on three-dimensional boundary-layer flames. Boundary-layer diffusion flames under 0.99–2.43 m/s crossflow velocities were produced over a gaseous propane line burner. Incoming flows with turbulence intensities ranging from 0.5% to 16.8% were generated in a wind tunnel with fine wire meshes and perforated grids. It was observed that freestream turbulence initiated an earlier onset of visible coherent flame streaks, enlarged the initial streak spacing, and accelerated the growth of the streak spacing along the streamwise direction. Both spacing and fluctuation frequency of the flame streaks showed a nearly quadratic growth at high turbulence intensities. This instability promoted the transition of flames to a turbulent state, which ultimately modified the overall flame heating dynamics. The forward attachment length of the flame was found to be negatively correlated to the turbulence intensity; a dimensionless relationship was proposed to correlate the flame attachment length based on the Froude number, heat-release rate, and freestream turbulence intensity. Two heating modes, a momentum-dominated and a plume mode, were observed and found to be segregated by a critical Richardson number. The downstream heat flux was found to increase from 30 to 40 kW/m2 in the momentum-dominated regime, when flow turbulence intensity changed from less than 1% to a level of 14.9–16.8%. Finally, it was observed that placing a bar upstream of the burner tripped the flow to the point where the downstream flame structure closely resembled flames under the highest turbulence intensity investigated, suggesting a simplistic configuration for future study. Ultimately, these results may improve our understanding of the structure and heating dynamics of boundary-layer flames as they transition to turbulence.

A New Experimental Approach for Heat Transfer Coefficient and Adiabatic Wall Temperature Measurements on a Nozzle Guide Vane With Inlet Temperature Distortions

AbstractModern gas turbines lean combustors allow to limit NOx pollutant emissions by controlling the flame temperature, while maintaining high turbine inlet temperatures. On the other hand, their adoption presents other challenges, especially concerning the combustor–turbine interaction. Turbine inlet conditions are generally characterized by severe temperature distortions and swirl degree, which, in turn, is responsible for very high turbulence intensities. Several past studies have focused on the description of the effects of these phenomena on the behavior of the high pressure stages of the turbine, both considering them as separated aspects, and, in very recent years, accounting for their combined impact. Nevertheless, very limited experimental results are available when it comes to evaluate the heat transfer coefficient (HTC) on the nozzle guide vane (NGV) external surface, since relevant temperature distortions present a severe challenge for the commonly adopted measurement techniques. The work presented in this paper was carried out on a non-reactive, annular, three-sector test rig, made by a combustor simulator and a NGV cascade. Making use of three real hardware burners of a Baker Hughes heavy-duty gas turbine, operated in similitude conditions, it can reproduce a representative swirling flow, with temperature distortions at the combustor–turbine interface plane. This test apparatus was exploited to develop an experimental approach to retrieve reliable HTC and adiabatic wall temperature distributions simultaneously, in order to overcome the known limitations imposed by temperature gradients on state-of-the-art methods for HTC calculation from transient tests. A non-cooled mockup of a NGV doublet, manufactured using low thermal diffusivity plastic material, was used for the tests, carried out using infra-red thermography with a transient approach. In the authors’ knowledge, this presents the first experimental attempt of measuring a NGV HTC in the presence of relevant temperature distortions and swirl.

Chemical modeling for methane oxy-combustion in Liquid Rocket Engines

Methane–oxygen burning is considered for many future rocket engines for practicality and cost reasons. As this combustion is slower than hydrogen–oxygen, flame ignition and stability may be more difficult to obtain. To address these questions, numerical simulation with realistic chemistry is appropriate. However the high pressure and turbulence intensity encountered in rocket engines enhance drastically the stiffness of methane oxy-combustion. In this work, Analytically Reduced Chemistry (ARC) is proposed for accurate chemistry description at a reasonable computational cost. An ARC scheme is specifically derived for typical rocket engine conditions. It is validated by comparison with its parent skeletal mechanism on a series of laminar flames. Then the numerical stiffness of chemistry is overcome with an original approach for time integration, allowing to run simulations close to the acoustic time step whatever the chemical stiffness. It is demonstrated on laminar cases that the flame structure is well preserved, and that numerical stability is ensured while decreasing significantly the computational cost. The performance of ARC with the fast time integration method is finally demonstrated in a 3D Large-Eddy Simulation of a lab-scale Liquid Rocket Engine combustion chamber, where a detailed flame analysis is conducted.

A novel local-variable-based Reynolds-averaged Navier–Stokes closure model for bypass and laminar separation induced transition

A one-equation Reynolds-averaged Navier–Stokes closure model is established for bypass transition in this paper. A new local indicator is proposed to describe the variation of turbulence intensities and pressure gradients. Based on this new indicator, a novel and efficient transition criterion is formulated. For laminar separation bubble induced transition, a reasonable modified intermittency factor is developed to complete the reattachment process and control the size of separation bubbles. Incorporated with Menter's k−ω shear stress transport turbulence model, the new transition-turbulence model is built for a high turbulence intensity environment. Several classical flow cases, including the ERCOFTAC (European Research Community on Flow, Turbulence and Combustion) series flat plates with various pressure gradients, the Pratt and Whitney low pressure turbine cascade, and a highly loaded linear compressor cascade, are all employed for the model verifications. Decent agreement with the experimental data and direct numerical simulation data can be obtained in a wide range of incoming flow conditions.

Study on heat transfer characteristics of NGVs influenced by non-reacting lean burn combustor simulator flow

Modern lean burn combustors are developed to meet environmental requirements of low NOx emissions. Comparing with the rich quench lean combustor, lean burn combustor can produce aggressive swirl fields, higher turbulence intensities and different hot streak patterns at the combustor and turbine interface. The flow structures and heat transfer characteristics of high pressure turbine, especially the nozzle guide vanes (NGVs), are significantly affected by the flow patterns from the upstream combustor. In this paper, unsteady integrated calculations are performed for the full geometry contained two NGVs and a non-reacting swirl combustor. The results of three cases of the full geometry, including positive swirler aligned with passage (Case PSW1), positive swirler aligned with leading edge (Case PSW2) and negative swirler aligned with passage (Case NSW), are compared with a baseline case of only NGV simulations with uniform condition (Case UNF). The results indicate that the core of the swirl migrates near the right NGV pressure side in Case PSW1, and near the left NGV suction side in Cases PSW2 and NSW. Higher intensity of the horse shoe vortices and passage vortices is generated in the three non-uniform cases. Hot streak is closer to the hub in Case NSW than Cases PSW1 and PSW2 at NGV exit. Adiabatic wall temperature distributions are influenced by the hot streak and cold fluid migrations. Nusselt number distributions are influenced by the boundary layer accumulation on the NGV surfaces, and by the secondary flow variations on the endwalls. The flow structure and Nusselt number show the similar distribution patterns in Cases PSW1 and PSW2, especially near the endwalls. While, the Nusselt number and the adiabatic wall temperature are obviously changed by the swirl orientation.

Leading-Edge Suction Parameter in Turbulent Flow at a Low Reynolds Number

Low Reynolds numbers and high turbulence intensities are inherent in the flight of small unmanned aerial vehicles (UAVs). A significant aspect of this unsteady motion is the pitching component of the flow, as perceived by the UAV, and the inherent risk of flow separation. The leading-edge suction parameter (LESP) has been proposed in the literature as a separation criterion. This paper trials the LESP in the erratic high-pitch-rate flow of onset turbulence. Two airfoils, a NACA0012 and NACA4412, have been tested at a Reynolds number of 100,000 and in turbulence intensities from 1.3 to 15%, replicating small UAV flight conditions. It is seen that, although the peaks in the LESP can indicate vortex shedding, the LESP in itself is not a sufficient criterion. Notably, there is not a critical value corresponding to vortex activity. Furthermore, the LESP does show sensitivity to camber, which is primarily apparent through the angle of the leading-edge suction vector.

Experimental investigation of wind loads on wind turbine blade under various turbulent flows

For investigation of the effects of turbulence intensity (TI) on the wind loads on wind turbine blade, a 1:20 scaled model of a typical 3D wind turbine blade is designed and used for the pressure measurement test in a wind tunnel. Five uniform flows with different turbulence intensities are simulated in the wind tunnel test. The mean and root-mean-square (RMS) wind pressure coefficients, base moment coefficients, and their power spectral densities are presented and discussed in detail. Combined with the dynamic properties of the blade structure, wind-induced displacements at the tip of the blade are calculated by the random vibration theory. The results show that the increasing of TI amplifies the aerodynamic loads on the blade in terms of RMS wind pressure coefficients and RMS bending moment coefficients. Large wind-induced displacement of the wind turbine blade may be stimulated by high TI even under the feathering condition. This article aims to further the understanding of wind loads on wind turbine blades and provide useful information for the wind-resistant design of wind farms established in regions with high turbulence levels.