Effects Of Flow Distortion Research Articles

Effects of Flow Distortions as They Occur in S-Duct Inlets on the Performance and Stability of a Jet Engine

One of the research areas at the Institute of Jet Propulsion focuses on the design and optimization of s-shaped engine inlet configurations. The distortion being evoked within such inlet ducts should be limited to ensure an optimal performance, stability, and durability of the engine's compression system. Computational fluid dynamics (CFD) play a major role in the design process of bent engine inlet ducts. The flow within such ducts can be computed, distortion patterns can be visualized, and related distortion coefficients are easily calculated. The impact of a distortion on flow phenomena within the compressor system can, however, only be computed with major computational efforts and thus the quality of an s-duct design in development is usually assessed by analyzing the evoked distortion with suitable distortion coefficients without a true knowledge of the duct's influence on the downstream propulsion system. The influence of inlet distortion on both the performance and stability of the Larzac 04 jet engine was parameterized during experimental investigations at the engine test bed of the Institute of Jet Propulsion. Both pressure and swirl distortion patterns as they typically occur in s-duct inlet configurations were reproduced with distortion generators. Pressure distortion patterns were generated using seven types of distortion screens. The intensity of the distortion varies with the mesh size of the screen whereas the extension of the distortion is defined by the dimensions of the screen in radial and circumferential direction. A typical counter rotating twin-swirl was generated with a delta-wing installed upstream of the compressor system. First, the development of flow distortion was analyzed for several engine operating points (EOPs). A linear relation between the total pressure loss in the engine inlet and the EOPs was found. Second, the flow within the compressor system with an inlet distortion was analyzed and unsteady flow phenomena were detected for severe inlet distortions. Finally, the effect of both pressure and swirl distortion on the performance and stability of the test vehicle was parameterized. A loss in engine performance with increasing inlet distortion is observable. The limiting inlet distortion with respect to engine stability was found; and moreover, it was shown that pressure distortion has a stronger influence on the stability of the compressor system compared to a counter rotating twin-swirl distortion. The presented parameterization was essential for the s-duct design, which was under development for an experimental setup with the Larzac 04 jet engine.

Air-flow distortion and wave interactions on research vessels: An experimental and numerical comparison

Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES) are two schemes for modeling turbulent flows. Here they are compared for modeling flow distortion over the oceanographic research vessel R/V Knorr, which is important for correcting observations from sonic anemometers. Using the OpenFOAM RANS solver SimpleFOAM and the LES solver PisoFOAM, computations are compared with experimental data taken from various anemometer sites on-board the research vessel. The LES showed mean accuracy levels of ∼3% of the wind speed bias whereas the RANS simulations showed mean accuracies of ∼7%. A LES analysis of the wind speed vector pitch and yaw was also conducted. The dominant forcing was found to be the pitch, which gave a 7% increase to overall magnitude of the wind vector. It was also found that the pitch of the wind speed was the main component responsible for the horizontal flow distortions, found to be due to flow separation in the 10–20 ms−1 range. We also use the LES simulations over a range of orientations from −60° to +60°, in increments of 10°. The numerical analysis showed close agreement to experimental measurements with a 6% mean difference prediction due to flow distortion effects. We also explore two different methods to define a wave induced flow distortion correction and when finally added to the air-flow distortion correction, improved the overall accuracy of the models by 3%.

Optimization of orifice meter's energy consumption

Orifice meters are commonly used in many industrial facilities and pipelines. However, they increase the annual energy consumption and cost due to their high pressure loss. The present research introduces a new design that reduces this high pressure loss by inserting a ring downstream the standard orifice meter. Maximum reduction of pressure loss is achieved by optimizing the downstream ring geometry. Numerical optimization is implemented using CFD simulation together with a genetic algorithm. Accurate CFD simulation is performed to solve the flow field at different downstream ring geometries while the genetic algorithm is used to estimate the optimum ring geometry. Optimization results show 29.8–33.5% reduction of orifice meter pressure loss for a Reynolds number Re=1.84×104 to 8.69×104. An increase of the discharge coefficient by 17.7–22% is also obtained within the investigated operating range. Both the effect of upstream distance and inlet flow disturbance and distortion are investigated. This investigation shows that the downstream ring reduces the pressure loss of standard orifice meters by 31–33.2% even under high flow disturbance and short upstream length. The proposed design adds many new advantages to the well known standard orifice meters.

CFD Technique for Estimating Flow Distortion Effects on Lidar Measurements When Made in Complex Flow Fields

The effect of flow distortion on the measurements produced by a LiDAR or SoDAR in close proximity to either complex terrain or a structure creating localised flow distortion is difficult to determine by analytical means. Also, as LiDARs and SoDARs are not point measurement devices, the techniques they employ for velocity measurements leads to complexities in the estimation of the effect of flow distortion on the accuracy of the measurements they make. This paper presents a method by which the effect of flow distortion on measurements made by a LiDAR in a distorted flow field may be determined using computational fluid dynamics. The results show that the error created by the flow distortion will cause the vector measured by a LiDAR to differ significantly from an equivalent point measurement. However, the results of the simulation show that, if the LiDAR is being used to measure the undisturbed flow field above a structure which creates highly localised flow distortion, the LiDAR results are less affected by the distortion of the local flow field than data acquired by a point measurement technique such as a cup anemometer.

Direct Numerical Simulation on Transition of an Incompressible Boundary Layer on a Flat Plate

Direct Numerical Simulation(DNS) was carried out for laminar-turbulent transition of an incompressible boundary layer on a flat plate based on disturbance Navier-Stokes(N-S) equation in spatial mode with Massage Passing Interface(MPI) technology. Study on breakdown mechanism of laminar-turbulent transition was carried on. The effect of mean flow distortion on the process of breakdown in laminar-turbulent transition was investigated. Results indicate that change of instability characteristic of mean flow profile plays a key role during process of breakdown.

Study for the Effect of Combined Pressure and Temperature Distortion on a Turbojet Engine

This paper describes how the effect of inlet flow distortion to a turbojet can be evaluated by a distortion model of whole engine, which is based on mass, momentum and energy conservation, and has been proved to be a valid tool for evaluating the effects of inlet flow distortion on turbofan.In this model, pressure (or temperature) distortion is quantified with coefficient which associates the pressure (or temperature) in the spoiled sector with the value in the clean sector. Detailed computation and analysis under different overlap phase and distortion intensity are conducted on the stability of turbojet after the representation of combined distortion index. It can be concluded that there is a maximal loss of stall margin when total pressure distortion sector laps over total temperature distortion sector completely. The reduction of stall margin arising due to combined inlet distortions can be minimized through the optimization of a disturbance overlap phase.

Uncertainties in wind speed dependent CO<sub>2</sub> transfer velocities due to airflow distortion at anemometer sites on ships

Abstract. Data from platforms, research vessels and merchant ships are used to estimate ocean CO2 uptake via parameterisations of the gas transfer velocity (k) and measurements of the difference between the partial pressures of CO2 in the ocean (pCO2 sw) and atmosphere (pCO2 atm) and of wind speed. Gas transfer velocities estimated using wind speed dependent parameterisations may be in error due to air flow distortion by the ship's hull and superstructure introducing biases into the measured wind speed. The effect of airflow distortion on estimates of the transfer velocity was examined by modelling the airflow around the three-dimensional geometries of the research vessels Hakuho Maru and Mirai, using the Large Eddy Simulation code GERRIS. For airflows within ±45° of the bow the maximum bias was +16%. For wind speed of 10 m s−1 to 15 m s−1, a +16% bias in wind speed would cause an overestimate in the calculated value of k of 30% to 50%, depending on which k parameterisation is used. This is due to the propagation of errors when using quadratic or cubic parameterisations. Recommendations for suitable anemometer locations on research vessels are given. The errors in transfer velocity may be much larger for typical merchant ships, as the anemometers are generally not as well-exposed as those on research vessels. Flow distortion may also introduce biases in the wind speed dependent k parameterisations themselves, since these are obtained by relating measurements of the CO2 flux to measurements of the wind speed and the CO2 concentration difference. To investigate this, flow distortion effects were estimated for three different platforms from which wind speed dependent parameterisations are published. The estimates ranged from −4% to +14% and showed that flow distortion may have a significant impact on wind speed dependent parameterisations. However, the wind biases are not large enough to explain the differences at high wind speeds in parameterisations which are based on eddy covariance and deliberate tracer methods.

Advanced tilt correction from flow distortion effects on turbulent CO2 fluxes in complex environments using large eddy simulation

AbstractMeasurement of the turbulent fluxes of gases, momentum and heat can be biased by obstacles such as buildings or instrument platforms distorting the flow of air to the flux instruments. Standard methods have long been used to account for non‐horizontal mean flows. Here we demonstrate a novel approach to correct for the effects of flow distortion which combines numerical flow modelling with eddy covariance measurements of the fluxes. This approach applies a flow distortion correction to the data prior to the application of the standard planar‐fit and double‐rotation methods. This new direction‐dependent flow correction allows the determination of the correct orthogonal wind vector components and hence the vertical turbulent fluxes. We applied the method to a 10 Hz dataset of 3D wind components, temperature, and the concentrations of carbon dioxide and water vapour, as measured on top of a military tower above the city of Münster in northwest Germany during spring and summer 2007. Significant differences appeared between the fluxes that were calculated with the standard rotation methods alone and those that underwent flow distortion correction prior to the application of the rotation methods. The highest deviations of 27% were obtained for the momentum flux. Pronounced differences of 15% and 8% were found for the diurnal net fluxes of carbon dioxide and water vapour, respectively. The flow distortion correction for the carbon dioxide fluxes yielded the same magnitude as the WPL (Webb–Pearman–Leuning) correction for density fluctuations. Copyright © 2009 Royal Meteorological Society

Effects of Inlet Flow Distortion on the Performance of Aircraft Gas Turbines

This paper describes how the fundamental effects of inlet flow distortion on the performance of gas turbines can be evaluated with any engine performance program that employs an integrated parallel compressor model. In this simulation method, both pressure and temperature distortions are quantified with coefficients, which relate the pressure (respectively temperature) in the spoiled sector to the value in the clean sector. In single spool compressor engines, the static pressure at the exit of the clean sector equals that of the distorted sector. This hypothesis does not hold true with multispool compressor engines because the short intercompressor ducts, which often contain struts or vanes, do not allow the mass flow transfer over the sector borders, which would be required for balancing the static pressures. The degree of aerodynamic coupling of compressors in series can be described in the performance simulation program by the simple coupling factor introduced in this paper. There are two fundamentally different reasons for the change in engine performance: First, there is the impact of the flow distortion on the component efficiencies and thus the thermodynamic cycle and second there are performance changes due to the actions of the control system. From the engine system simulation results, it becomes clear why inlet flow distortion has only a minor impact on the thermodynamic cycle if the comparison of the two operating conditions (with clean and distorted inlet flow) is made at the properly averaged engine inlet conditions. For each compressor, the parallel compressor theory yields two operating points in the map, one for the clean sector and one for the spoiled sector. The performance loss due to the distortion is small since the efficiency values in the two sectors are only a bit lower than the efficiency at a comparable operating point with clean inlet flow. However, the control system of the engine can react to the inlet flow distortion in such a way that the thrust delivered changes significantly. This is particularly true if a compressor bleed valve or a variable area nozzle is opened to counteract compressor stability problems. Especially, using recirculating bleed air for increasing the surge margin of a compressor affects the performance of the engine negatively. Two examples show clearly that the pros and cons of recirculating bleed can only be judged with a full system simulation; looking at the surge line improvement alone can be misleading.

Effects of Inflow Distortion due to Hub Cap's Shape on the Performance of Axial Flow Fan

Performance characteristics of an axial flow fan having distorted inlet flow have been investigated using numerical analysis. Two kinds of hub-cap, rounded and right-angled front shape, are tested to investigate the effect of inlet flow distortion on the fan performance. In case of right-angled front shape, axisymmetric distorted inflow is induced by flow separation at the sharp edge of hub-cap, and the characteristics of the inflow depend on the distance between hub-cap and blade leading edge. Three-dimensional Reynolds-averaged Navier-Stokes equations are introduced to analyze the flow characteristics inside the blade passage. Numerical solutions are validated in comparison with experimental data measured by a five-hole probe downstream of the fan rotor. It is found from the numerical results that non-uniform axial inlet velocity profile near the hub results in the change of inlet flow angle. Large recirculation flow upstream the fan rotor for the right-angled hub-cap induces separated flow on the blade surfaces near the hub region, and thus deteriorates the performance of fan rotor. The effect of the distance between hub-cap and blade leading edge on the efficiency is also discussed.

Divergence and Vorticity from Aircraft Air Motion Measurements

Abstract This paper considers the accuracy of divergence estimates obtained from aircraft measurements of the horizontal velocity field and points out an error that appears in these estimates that has heretofore not been addressed. A procedure for eliminating this error is presented. The divergence and vorticity are estimated from the coefficients of a least squares fit to a wind field obtained from the Second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) circular flight legs. These estimates are compared with estimates from numerical models and satellites and with airplane estimates based on tracer budgets and the temporal changes in cloud-top height. The estimates are consistent with expectations and estimates using other methods, albeit somewhat high. Furthermore, significant differences occur among the cases, likely due to the large differences in the techniques. The results indicate that the wind field technique is a viable approach for estimating mesoscale divergence if the wind measurements are accurate. The largest source of wind field systematic error may be the result of flow distortion effects on the air velocity measurement and limitations of in-flight calibrations. Because of flow distortion, the only way the current systems can be calibrated is by flight maneuvers, which assume a steady-state homogeneous nonturbulent atmosphere. Analysis of the errors in this technique suggests that wind field measurements with minimal systematic errors should provide estimates of divergence with much greater accuracy than is now possible with other existing methods.

Continuing nonlinear growth of Tollmien-Schlichting waves

This brief paper is concerned with the effect of mean flow distortions on spatially growing Tollmien-Schlichting waves in the Blasius boundary layer. It is shown that the coherent Reynolds stresses arising from the waves affect the mean flow by tending to reverse the sign of the cross-stream derivative of vorticity in the critical layer. If the wave amplitude is high enough to initiate the reversal, it further contributes to the wave amplification. That, in turn, increases the region of the vorticity derivative with opposite sign.

허브 캡 형상에 따른 축류송풍기 성능특성

Performance characteristics of an axial flow fan having distorted inlet flow have been investigated using numerical analysis as well as experiment. Two kinds of hub-cap, rounded and right-angled front shape, are tested to investigate the effect of inlet flow distortion on the fan performance. Numerical solutions are validated in comparison with experimental data measured by a five-hole probe downstream of the fan rotor. It is found from the numerical results thatnon-uniform axial inlet velocity profile near the hub results in the change of inlet flow angle. Large recirculation flow upstream the fan rotor for the right-angled hub-cap induces a negative incidence, thus invokes separated flow on the blade surfaces and deteriorates the performance of fan rotor.

Quantifying the Airflow Distortion over Merchant Ships. Part II: Application of the Model Results

Abstract Wind speed measurements obtained from ship-mounted anemometers are biased by the presence of the ship, which distorts the airflow to the anemometer. Previous studies have simulated the flow over detailed models of individual research ships in order to quantify the effect of flow distortion at well-exposed anemometers, usually sited on a mast in the ship's bows. In contrast, little work has been undertaken to examine the effects of flow distortion at anemometers sited on other merchant ships participating in the voluntary observing ship (VOS) project. Anemometers are usually sited on a mast above the bridge of VOS where the effects of flow distortion may be severe. The several thousand VOS vary a great deal in shape and size and it would be impractical to study each individual ship. This study examines the airflow above the bridge of a typical, or generic, tanker/bulk carrier/general cargo ship using computational fluid dynamics models. The results show that the airflow separates at the upwind leading edge of the bridge and a region of severely decelerated flow exists close to the bridge top with a region of accelerated flow above. Large velocity gradients occur between the two regions. The wind speed bias is highly dependent upon the anemometer location and varies from accelerations of 10% or more to decelerations of 100%. The wind speed bias at particular locations also varies with the relative wind direction, that is, the angle of the ship to the wind. Wind speed biases for various anemometer positions are given for bow-on and beam-on flows.

Quantifying the Airflow Distortion over Merchant Ships. Part I: Validation of a CFD Model

Abstract The effects of flow distortion created by the ship’s hull and superstructure bias wind speed measurements made from anemometers located on ships. Flow distortion must be taken into account if accurate air–sea flux measurements are to be achieved. Little work has been undertaken to examine the wind speed bias due to flow distortion in wind speed reports from voluntary observing ships (VOS). In this first part of a two-part paper the accuracy of the computational fluid dynamics (CFD) code VECTIS in simulating the airflow over VOS is investigated. Simulations of the airflow over a representation of the bridge of a VOS are compared to in situ wind speed measurements made from six anemometers located above the bridge of the RRS Charles Darwin. The ship’s structure was ideal for reproducing the flow over VOS when the wind is blowing onto either beam. The comparisons showed VECTIS was accurate to within 4% in predicting the wind speed over ships, except in extreme cases such as wake regions or the region close to the bridge top where the flow may be stagnant or reverse direction. The study showed that there was little change in the numerically predicted flow pattern above the bridge with change in Reynolds number between 2 × 105 and 1 × 107. The findings showed that the CFD code VECTIS can reliably be used to determine the mean flow above typical VOS.

Numerical investigation of fan performance in a forced draft air-cooled steam condenser

Inlet air flow distortions in a large air-cooled steam condenser (ACSC), caused by structures, wind and other fans may result in a significant reduction in fan performance, or volumetric effectiveness, as well as fan blade vibration. This phenomenon has an adverse effect on the heat rejection capacity of an ACSC due to a decrease in air mass flow rate. In this study the effect of inlet flow distortions on the flow rate through the fans in an ACSC is numerically investigated by modelling the flow field in a section of such a system using the computational fluid dynamics code, FLUENT. The effect of platform height on the volumetric effectiveness of two different types of axial flow fans is considered. The two fans have the same diameter, number of blades and rotational speed, but feature different blade designs, and hub-tip-ratios of respectively 0.153 and 0.4. Numerical simulations show all-round superior performance in terms of volume flow rate for the fan with a hub-tip-ratio of 0.4. It is furthermore confirmed that the addition of a walkway can significantly increase the flow rate through the fans located near the edge of the fan platform.

Effect of background melt flow and interface distortion on the stability of Hall--Héroult cells

We use the method of linear stability analysis to study the effect of background melt flows and interface deformation on the stability characteristics of Hall-Heroult reduction cells. The linearized perturbation equations are based on a two-dimensional shallow water model. Two kinds of background states are investigated, one with a strong interface deformation and negligible melt flow, and another with strong vortical flows in aluminum and cryolite but a practically flat interface. It is found that the interface deformation with the length scale of the order of the cell size has a strong destabilizing effect. We also confirm earlier predictions that the unstable interfacial wave becomes wall-limited as the electromagnetic instability parameter increases. Introduction. Virtually all the primary aluminum is manufactured in the so-called Hall-Heroult (reduction) process nowadays. The description of the pro- cess can be found elsewhere (see, for example, (1-12)) so only a brief outline is provided here. From the hydrodynamic viewpoint, an idealized Hall-Heroult cell is an approximately rectangular bath filled with a layer of molten aluminum at the bottom and a layer of molten salt cryolite atop of it (see Fig. 1 for an illustration). The aluminum oxide dissolved in the cryolite is reduced to a pure metal under the action of a strong electric current (density sometimes exceeding 10 kA/m 2 ) passing from carbon anodes immersed in the cryolite to carbon cathodes that form the cell bottom. Strong magnetic fields (up to 0.01 T) are generated by currents flowing within the cells and conducting lines in the plant. The system is very shal- low, with typical horizontal dimensions of the cell being 10 m by 3 m and typical thicknesses of the aluminum and cryolite layers being, correspondingly, 20 cm and

An overview of the airflow distortion at anemometer sites on ships

AbstractWind speed measurements obtained from ship‐mounted anemometers are biased by the presence of the ship itself distorting the flow of air to the anemometer. Recent studies have used numerical models to simulate the flow around very detailed representations of individual research ships in order to quantify the effects of flow distortion at the anemometer locations. Such anemometers are generally in well‐exposed positions, typically on a mast in the bows of the ship. In contrast, very little is known about the possible effects of airflow distortion on the wind speed measurements from fixed anemometers on Voluntary Observing Ships (VOS). This is because (1) the several thousand or so merchant vessels vary significantly in shape and size and it would be impractical to study each individual ship, and (2) the anemometer location is not usually known.This paper describes initial results from a study of flow distortion over ‘typical’ tankers and bulk carriers. A method to describe the shapes of these VOS and container ships is presented. The influence of distortion on the flow above the bridges of VOS is shown to be significant, with possible biases in the measured wind speed of between +11% and −100%, depending on the anemometer location. Recommendations for the siting of anemometers are made. Copyright © 2005 Royal Meteorological Society

Numerical investigation of the effect of inlet flow distortions on forced draught air-cooled heat exchanger performance

Forced draught air-cooled heat exchangers (ACHEs) are often arranged into banks consisting of multiple rows of fan-heat exchanger combinations. Fans on the outer edge of the banks are subject to severe cross-flow conditions as the air is swept past en route to fans located deeper within the banks. The cross-flow conditions give rise to increased inlet flow losses. The current investigation seeks to develop a better understanding of the nature of these increased inlet flow losses through the numerical resolution of flow fields associated with systems comprising a single and two banks of ACHEs. For installations comprising two banks of ACHEs it is found that the inlet flow losses of the periphery fan are dominated by flow separation occurring around the inlet lip of the fan inlet section. These flow losses can be reduced by the installation of a walkway at the edge of the fan platform or by the removal of the periphery fan inlet section. For a system comprising of a single bank of ACHEs it is found that the numerical models used accurately reflect the experimentally determined differences in inlet flow losses experienced by systems using either bell-mouth, conical or cylindrical fan inlet sections.

Effects of Inlet Flow Distortion on Cavitation Instability and Blade Stress Fluctuation in an Inducer

To investigate the effect of inlet flow distortion on cavitation instability, unsteady pressure and blade stress fluctuation were observed in a 3-bladed inducer. The inlet flow distortions were generated by using screens installed upstream of the inducer. In addition, the effect of the frequency of the fluctuation of the angle of attack caused by the inlet flow distortion was examined with three screens. The following results were obtained : (1) The present inlet flow distortions have almost no effect both on the pressure and cavitation performances. (2) The conventional and higher order rotating cavitations were suppressed by the present inlet flow distortion. However, the occurrence of the conventional cavitation surge was enhanced at lower cavitation number. (3) Amplitude of the blade stress fluctuation caused by the inlet flow distortion decreased with increasing of the frequency of the fluctuation of the angle of attack. (4) The phase difference between the blade stress fluctuation due to the inlet flow distortion and the fluctuation of the angle of attack increased as decreasing of the cavitation number.