Flow Distortion Research Articles

Evaluation of a passive method for determining particle penetration through protective clothing materials

ABSTRACTThe risk of workers' exposure to aerosolized particles has increased with the upsurge in the production of engineered nanomaterials. Currently, a whole-body standard test method for measuring particle penetration through protective clothing ensembles is not available. Those available for respirators neglect the most common challenges to ensembles, because they use active vacuum-based filtration, designed to simulate breathing, rather than the positive forces of wind experienced by workers. Thus, a passive method that measures wind-driven particle penetration through ensemble fabric has been developed and evaluated. The apparatus includes a multidomain magnetic passive aerosol sampler housed in a shrouded penetration cell. Performance evaluation was conducted in a recirculation aerosol wind tunnel using paramagnetic Fe3O4 (i.e., iron (II, III) oxide) particles for the challenge aerosol. The particles were collected on a PVC substrate and quantified using a computer-controlled scanning electron microscope. Particle penetration levels were determined by taking the ratio of the particle number collected on the substrate with a fabric (sample) to that without a fabric (control). Results for each fabric obtained by this passive method were compared to previous results from an automated vacuum-based active fractional efficiency tester (TSI 3160), which used sodium chloride particles as the challenge aerosol. Four nonwoven fabrics with a range of thicknesses, porosities, and air permeabilities were evaluated. Smoke tests and flow modeling showed the passive sampler shroud provided smooth (non-turbulent) air flow along the exterior of the sampler, such that disturbance of flow stream lines and distortion of the particle size distribution were reduced. Differences between the active and passive approaches were as high as 5.5-fold for the fabric with the lowest air permeability (0.00067 m/sec-Pa), suggesting the active method overestimated penetration in dense fabrics because the active method draws air at a constant flow rate regardless of the resistance of the test fabric. The passive method indicated greater sensitivity since penetration decreased in response to the increase in permeability.

Interaction of instability waves and a three-dimensional roughness element in a boundary layer

The influence of a single roughness element on the evolution of two-dimensional (2-D) Tollmien–Schlichting (TS) waves is investigated experimentally. Experiments are carried out in a region of zero pressure gradient of an airfoil section. Downstream from the disturbance source, TS waves interact with a cylindrical roughness element with a slowly oscillating height. The oscillation frequency of the roughness was approximately 1500 times lower than the wave frequency and approximately 250 times slower than the characteristic time of flow passing the region of transition development. Therefore, the roughness behaved as a quasi-steady disturbance. The set-up enabled us to perform hot-wire measurements phase locked to the waves and to the roughness movement. Experimental results show a scattering of the 2-D waves into oblique ones and a relatively weak distortion of the mean flow for roughness heights as large as 0.2 times the boundary layer displacement thickness ($\unicode[STIX]{x1D6FF}^{\ast }$). Transfer functions for TS wave scattering at the roughness are obtained. Results show an unexpected coincidence in shape with acoustic receptivity functions found in Würz et al. (J. Fluid Mech., vol. 478, 2003, pp. 135–163) for the problem of excitation of TS waves by scattering of acoustic waves at surface roughness. In the present work, the ratio between the incoming 2-D wave amplitude to the amplitude of the scattered oblique waves scaled linearly with the roughness height only for very shallow roughness. For roughness elements higher than $0.08\unicode[STIX]{x1D6FF}^{\ast }$ and below $0.2\unicode[STIX]{x1D6FF}^{\ast }$, the wave scattering exhibited a quadratic variation with respect to the roughness height. In addition, this feature did not vary significantly with respect to TS wave frequency. An analysis of the weakly nonlinear interactions triggered by the roughness element is also carried out, assisted by numerical solution of nonlinear parabolized stability equations, performed for a two-dimensional Blasius boundary layer. A comparison between experiments and simulations reveals that the weakly nonlinear interactions observed are not substantially affected by mean flow distortions that could be produced in the wake of the small and medium sized roughness elements ($h<0.2\unicode[STIX]{x1D6FF}^{\ast }$). From a practical perspective, results suggest that scattering coefficients might be employed to include the effect of isolated and medium sized roughness elements in transition prediction tools developed for smooth surfaces.

Acoustic analysis of two small axial-flow fans in series

The noise signatures of two identical small axial-flow cooling fans installed in series with a flow straightener between them are experimentally measured and analyzed. A technique of noise source component decomposition is used to decompose the raw sound signals into tonal and broadband components, both further attributed to either the upstream or the downstream fan. Two major inlet flow conditions (unobstructed inlet and a distorted inlet flow case by the simplified circuit board) and a range of operating conditions under which the tested fan works are investigated. The tonal and broadband noises radiated by the upstream fan are greatly increased by inlet flow distortion, especially in the low workload region, while the two noise components radiated by downstream fan are hardly affected. The upstream fan and the flow straightener flush mounted between the two fans both play a significant role in keeping the downstream fan free from the influence of inlet flow distortion. Themagnitude relations among all the noise source components under different inlet flow and operating conditions are compared, which would definitely offermuch help in designing a quiet two-stage or even multiple-stage fan and adopting an appropriate method to efficient fan noise control and reduction.

Stability assessment of an airflow distorted military engine’s FAN

Military aircraft are often subjected to severe flight maneuvers with high angles of attack and angles of sideslip. These flight attitudes induce non-uniformity in flow conditions to their gas turbine engines, which may include distortion of inlet total pressure and total temperature at the aerodynamic interface plane. Operation of the downstream engine’s compression system may suffer reduced aerodynamic performance and stall margin, and increased blade stress levels. The present study presents a methodology of evaluating the effect of inlet flow distortion on the engine’s fan stability. The flow distortion examined was induced to the aerodynamic interface plane by means of changing the aircraft’s flight attitude. The study is based on the steady-state flow results from 27 different flight scenarios that have been simulated in computational fluid dynamics. As a baseline model geometry, an airframe inspired by the General Dynamics/LMAERO F-16 aircraft was chosen, which has been exposed to subsonic incoming airflow with varying direction resembling thus different aircraft flight attitudes. The results are focused on the total pressure distribution on the engine’s (aerodynamic interface plane) face and how this is manifested at the operation of the fan. Based on the results, it was concluded that the distorted conditions cause a shift of the surge line on the fan map, with the amount of shift to be directly related to the severity of these distorted conditions. The most severe flight attitude in terms of total pressure distortion, among the tested ones, caused about 7% surge margin depletion comparing to the undistorted value.

Pressure Flowfield and Inlet Flow Distortion Metrics Reconstruction from Velocity Data

Complex engine intakes are susceptible to unsteady flow distortions that may compromise the propulsion system operability. Hence, the need for high spatial and temporal resolution flow information is essential to aid the development of distortion-tolerant closely coupled propulsion systems. Stereoscopic particle image velocimetry methods have been successfully applied to these flows, offering synchronous velocity datasets of high spatial resolution across the aerodynamic interface plane. However, the total pressure distortion measurements are still typically provided by low bandwidth intrusive total pressure rakes of low spatial resolution, which results in limited characterization of the total pressure distortion. This limitation can potentially be addressed by pressure field reconstruction from non-intrusive high resolution velocity data. A range of reconstruction methods is assessed based on representative data from steady and unsteady computational simulations of an S-duct configuration. In addition to the reconstructed total pressure field, the impact on the key distortion metrics is assessed. The effect of the Mach number is considered. Overall, the reconstruction methods show that the distortion metrics can be determined with sufficient accuracy to indicate that there is a potential benefit from exploiting high resolution velocity measurements in evaluating total pressure distortion.

Effect of flow distortion on fuel/air mixing and combustion in an upstream-fueled cavity flameholder for a supersonic combustor

This paper describes an experimental study of the effects of an incident shockwave on the flow field, fuel distribution and combustion within a cavity flameholder with upstream fuel injection. Two impingement locations are employed: (1) near the fuel injector (the so-called shock-on-jet case) and (2) on the cavity shear layer (the shock-on-cavity case). Shadowgraph is used to characterize the flow field. Air seeded with nitric oxide (NO) is used as the simulated fuel and the resulting planar laser-induced fluorescence (NO-PLIF) from NO molecules is used to characterize fuel/air mixing while planar laser-induced fluorescence of OH molecules to characterize the actual combustion process. The shadowgraph and NO-PLIF images are compared with a CFD (Computational Fluid Dynamics) solution of the Reynolds-averaged-Navier Stokes (RANS) for assessment and explanation of experimental results of non-reacting tests. The effect of the shock on the cavity shear layer is to control the fuel distribution within the cavity. The effect of the shock on the jet is to force the shear layer deep within the cavity, which results in higher fuel concentrations near the cavity centerline. The shock-on-cavity case causes the shear layer to separate upstream of the cavity. Mixing uniformity is enhanced by the increased breakup of the fuel plume. Combustion is stronger and more uniform with the shock impinging on the cavity, while it is limited to the edges of the cavity with shock impingement on the jet. The greater mixing afforded in the shock-on-cavity case reduces the fuel concentration near the centerline and allows stronger burning in the center of the cavity. Doubling the fuel injection momentum flux ratio does not strongly affect the pattern of fuel distribution in either case, but combustion in the shock-on-cavity case is reduced, because the fuel concentration at the centerline is high.

Evaluation of Probe-Induced Flow Distortion of Campbell CSAT3 Sonic Anemometers by Numerical Simulation

The Campbell CSAT3 sonic anemometer is one of the most popular instruments for turbulence measurements in basic micrometeorological research and ecological applications. While measurement uncertainty has been characterized by field experiments and wind-tunnel studies in the past, there are conflicting estimates, which motivated us to conduct a numerical experiment using large-eddy simulation to evaluate the probe-induced flow distortion of the CSAT3 anemometer under controlled conditions, and with exact knowledge of the undisturbed flow. As opposed to wind-tunnel studies, we imposed oscillations in both the vertical and horizontal velocity components at the distinct frequencies and amplitudes found in typical turbulence spectra in the surface layer. The resulting flow-distortion errors for the standard deviations of the vertical velocity component range from 3 to 7%, and from 1 to 3% for the horizontal velocity component, depending on the azimuth angle. The magnitude of these errors is almost independent of the frequency of wind speed fluctuations, provided the amplitude is typical for surface-layer turbulence. A comparison of the corrections for transducer shadowing proposed by both Kaimal et al. (Proc Dyn Flow Conf, 551–565, 1978) and Horst et al. (Boundary-Layer Meteorol 155:371–395, 2015) show that both methods compensate for a larger part of the observed error, but do not sufficiently account for the azimuth dependency. Further numerical simulations could be conducted in the future to characterize the flow distortion induced by other existing types of sonic anemometers for the purposes of optimizing their geometry.

Impact of inlet distortion on turbocharger compressor stage performance

In a turbocharger system, curved pipes are usually used to connect compressors with other parts due to limited packaging space. It is well known that an elbow can generate flow distortion to and interact with the compressor inducer and therefore deteriorate compressor stage performance. The compressor efficiency change may vary with relative positions of inlet elbow and volute. The compressor’s stable flow range is also possible to vary due to the shift of surge and/or chock points. In this paper, eight relative circumferential positions between the inlet elbow and the volute were studied by 3D numerical simulations with experimental validations. The numerical results confirm that adjusting the inlet bend from one circumferential orientation to others does cause an obvious deviation in the compressor aerodynamic efficiency. The result also indicates that the total pressure deficit and the vortices at the compressor inlet are the main contributors to the compressor operating range. With the numerical findings in this paper which was validated by experimental measurements, it is suggested that unfavorable orientations of an elbow relative to volute should be avoided in an intake system.

Convoluted Intake Distortion Measurements Using Stereo Particle Image Velocimetry

The unsteady distorted flowfields generated within convoluted aeroengine intakes can compromise the engine performance and operability. Therefore, there is a need for a better understanding of the complex characteristics of the distorted flow at the exit of S-shaped intakes. This work presents a detailed analysis of the unsteady swirl distortion based on synchronous, high-spatial-resolution measurements using stereoscopic particle image velocimetry. Two S-duct configurations with different centerline offsets are investigated. The high-offset duct shows greater levels of dynamic and steady swirl distortion and a notably greater tendency toward bulk swirl patterns associated with high swirl distortion. More discrete distortion patterns with locally high swirl levels and the potential to impact the engine operability are identified. The most energetic coherent structures of the flowfield are observed using proper orthogonal decomposition. A switching mode is identified that promotes the alternating swirl switching mechanism and is mostly associated with the occurrence of potent bulk swirl events. A vertical mode that characterizes a perturbation of the vertical velocity field promotes most of the twin swirl flow distortion topologies. It is postulated that it is associated with the unsteadiness of the centerline shear layer.

Delayed Detached-Eddy Simulation and Particle Image Velocimetry Investigation of S-Duct Flow Distortion

The dynamic flow distortion generated within convoluted aeroengine intakes can affect the performance and operability of the engine. There is a need for a better understanding of the main flow mechanisms that promote flow distortion at the exit of S-shaped intakes. This paper presents a detailed analysis of the main coherent structures in an S-duct flowfield based on a delayed detached-eddy simulation. The capability of this numerical approach to capture the characteristics of the highly unsteady flowfield is demonstrated against high-resolution, synchronous stereoscopic particle image velocimetry measurements at the aerodynamic interface plane. The flowfield mechanisms responsible for the main perturbations at the duct outlet are identified. Clockwise and counterclockwise streamwise vortices are alternately generated around the separation region at a frequency of , which promote the swirl switching at the duct outlet. Spanwise vortices are also shed from the separation region at a frequency of and convect downstream along the separated centerline shear layer. This results in a vertical modulation of the main loss region and a fluctuation of the velocity gradient between the high- and low-velocity flow at the aerodynamic interface plane.

Complex Aeroengine Intake Ducts and Dynamic Distortion

For many embedded engine systems, the intake duct geometry introduces flow distortion and unsteadiness, which must be understood when designing the turbomachinery components. The aim of this work is to investigate the capabilities of modern computational methods for these types of complex flows, to study the unsteady characteristics of the flowfield, and to explore the use of proper orthogonal decomposition methods to understand the nature of the unsteady flow distortion. The unsteady flows for a range of S-duct configurations have been simulated using a delayed detached-eddy simulation method. Analysis of the conventional distortion criteria highlights the main sensitivities to the S-duct configuration and quantifies the unsteady range of these parameters. The unsteady flowfield shows signature regions of unsteadiness, which are postulated to be related to the classical secondary flows as well as to the streamwise flow separation. A proper orthogonal decomposition of the total pressure field at the duct exit identifies the underpinning flow modes, which are associated with the overall total pressure unsteadiness distributions. Overall, the unsteady distortion metrics are not found to be solely linked to a particular proper orthogonal decomposition mode, but are dependent on a wider range of modes.

Analysis of Fan Stage Conceptual Design Attributes for Boundary Layer Ingestion

This paper describes a new conceptual framework for three-dimensional turbomachinery flow analysis and its use to assess fan stage attributes for mitigating adverse effects of inlet distortion due to boundary layer ingestion (BLI). A nonaxisymmetric throughflow analysis has been developed to define fan flow with inlet distortion. The turbomachinery is modeled using momentum and energy source distributions that are determined as a function of local flow conditions and specified blade camber surface geometry. Comparison with higher-fidelity computational and experimental results shows the analysis captures the principal flow redistribution and distortion transfer effects associated with BLI. Distortion response is assessed for a range of (i) design flow and stagnation enthalpy rise coefficients, (ii) rotor spanwise work profiles, (iii) rotor–stator spacings, and (iv) nonaxisymmetric stator geometries. Of the approaches examined, nonaxisymmetric stator geometry and increased stage flow and stagnation enthalpy rise coefficients provide the greatest reductions in rotor flow nonuniformity, and may offer the most potential for mitigating performance loss due to BLI inlet distortion.

End correction model for the transfer impedance of microperforated panels using viscothermal wave theory.

Microperforated panels (MPPs) are efficient sound absorbers featuring microperforations with low porosity. Sound absorption occurs inside the perforations and their vicinity as well, which is represented with an end correction in the transfer impedance of the MPPs. Many empirical models for the end correction were derived from experiment or numerical simulation data. In order to validate these models, this paper presents an analytical solution of the end correction for sharp-edged circular perforations using viscothermal wave theory. The perforations are assumed to be periodically distributed and each perforation is associated with a square duct resulting from a periodic spatial partition. The velocity profile and the temperature field in each perforation are derived from the low reduced frequency model, which pose the boundary conditions to determine the modal coefficients of the acoustic, entropy, and viscous waves in the duct. An impedance end correction model is derived from the asymptotic expansion of the modal solution. It improves the conventional model by introducing a static flow resistance term to describe the energy dissipation due to the acoustic flow distortion outside the perforations. Numerical and experimental examples validate that the proposed model offers better prediction for the transfer impedance and the sound absorption of the MPPs.

JET ENGINE INLET DISTORTION SCREEN AND DESCRIPTOR EVALUATION

Total pressure distortion is one of the three basic flow distortions (total pressure, total temperature and swirl distortion) that might appear at the inlet of a gas turbine engine (GTE) during operation. Different numerical parameters are used for assessing the total pressure distortion intensity and extent. These summary descriptors are based on the distribution of total pressure in the aerodynamic interface plane. There are two descriptors largely spread around the world, however, three or four others are still in use and can be found in current references. The staff at the University of Defence decided to compare the most common descriptors using basic flow distortion patterns in order to select the most appropriate descriptor for future department research. The most common descriptors were identified based on their prevalence in widely accessible publications. The construction and use of these descriptors are reviewed in the paper. Subsequently, they are applied to radial, angular, and combined distortion patterns of different intensities and with varied mass flow rates. The tests were performed on a specially designed test bench using an electrically driven standalone industrial centrifugal compressor, sucking air through the inlet of a TJ100 small turbojet engine. Distortion screens were placed into the inlet channel to create the desired total pressure distortions. Of the three basic distortions, only the total pressure distortion descriptors were evaluated. However, both total and static pressures were collected using a multi probe rotational measurement system.

Evaluation of eddy covariance latent heat fluxes with independent lysimeter and sapflow estimates in a Mediterranean savannah ecosystem

We evaluated the underlying causes of differences between latent heat (LE) fluxes measured with two enclosed-path eddy covariance systems (EC) at two measurement levels and independent estimates in an open oak-tree grass savannah over almost one year. Estimates of LE of the well-stablished underlying grass by replicated weighable tension-controlled lysimiters (LELys) provided a robust baseline against which to compare EC LE measured at 1.6m above ground (LE_1.6). Similarly and at the ecosystem level, LE up-scaled using independent measurements (LEupscaled=sap flow+lysimeter) was benchmarked with 3 EC-derived LE estimates: 1) LE measured by a EC tower at 15m above ground (LE_15), 2) LE_15 adjusted to close the energy balance by using the Bowen ratio method (LEBowen=(Rn −G)/(1+β)), and 3) LE derived from the energy budget residual (LEresidual=Rn −G−H_15). The sensitivity of EC LE to the correction method applied (i.e. corrections for low-pass filtering effects on water vapor fluctuations and the so-called angle-of-attack correction) and its impact on the energy balance closure (EBC) were also evaluated.Comparison of EC LE between 1.6m- and 15m-heights showed that grass dominated annual evaporative loss from 69 to 87% depending upon the spectral correction method applied. Results revealed substantial underestimation of LE_1.6 (up to 35%) compared to LELys, which mostly occurred during the growing season. However those differences were remarkably lower when likening LE_15 versus LEupscaled (14%) suggesting that the dampening of the water vapor fluctuations due to low-pass filtering effects is more pronounced near the surface. Interestingly, a diagnostic evaluation of the errors with a random forest model showed that differences followed quite structured patterns and were associated with certain atmospheric conditions: turbulent mixing deficiencies and or stable atmospheric stratification. In addition, the model showed that differences increased with increasing relative humidity (RH) and soil moisture. Our results revealed that the degree of EBC is highly sensitive to the flux correction method applied, in particular when correcting for flow distortion effects. Typically, turbulent fluxes fell below the measured available energy (slope 0.92) but the slope switched abruptly when the angle-of-attack correction was applied (slope 1.07). Consistent with the EBC, independent LE estimates matched well with LEBowen and the EBC gap decreased when LEupscaled was used (slope 0.96). The use of independent estimates of LE together with machine learning methods are proposed as a powerful means to diagnose the complexity behind LE errors and give insights into the energy imbalance problem. In addition to inherent randomness of EC LE data, accounting for uncertainties associated with the appropriateness of the correction method applied is highly recommended.

Combustor Dilution Hole Placement and Its Effect on the Turbine Inlet Flowfield

Dilution jets in a gas turbine combustor are used to oxidize remaining fuel from the main flame zone in the combustor and to homogenize the temperature field upstream of the turbine section through highly turbulent mixing. The high-momentum injection generates high levels of turbulence and very effective turbulent mixing. However, mean flow distortions and large-scale turbulence can persist into the turbine section. In this study, a dilution hole configuration was scaled from a rich-burn–quench–lean-burn combustor and used in conjunction with a linear vane cascade in a large-scale, low-speed wind tunnel. Mean and turbulent flowfield data were obtained at the vane leading edge with the use of high-speed particle image velocimetry to help quantify the effect of the dilution jets in the turbine section. The dilution hole pattern was shifted (clocked) for two positions such that a large dilution jet was located directly upstream of a vane or in between vanes. Time-averaged results show that the large dilution jets have a significant impact on the magnitude and orientation of the flow entering the turbine. Turbulence levels of 40% or greater were observed approaching the vane leading edge, with integral length scales of approximately 40% of the dilution jet diameter. Incidence angle and turbulence level were dependent on the position of the dilution jets relative to the vane.

A study on heat flow and thermal distortion in terms of air-carbon-arc gouging of steel plates

In groove design, a proper shape should be chosen during the preparation for the welding of thick plates of steel structures to minimize the potential for any welding deformations. While the effect of welding on both sides of the plate in terms of the deformation potential is a foremost consideration during the groove-design process, the effect of gouging is not included even though it involves the usage of a high-heat arc input that may significantly influence the welding-deformation potential. In this study, the effect of the air-carbon-arc gouging process on the deformation of steel plates was investigated using a numerical finite element method analysis. For this purpose, the heat input from the gouging torch was first modeled using an analytical solution that is based on the Rosenthal equation. The model was then used as a heat input for the thermo-mechanical steel-plate-gouging model that is proposed in this study to obtain deformation estimations during the gouging process. A series of numerical analyses were carried out to predict the deformations according to changes in the plate thickness. The results of the analyses were found to be in sound agreement with those of the experiments that were performed to verify the simulations. The comparison revealed that the models that are proposed in this study are feasible ways of estimating the deformation caused by the gouging on steel plates.

Performance and Adaptive Surge-Preventing Acceleration Prediction of a Turboshaft Engine under Inlet Flow Distortion

The intention of this paper is to research the inlet flow distortion influence on overall performance of turboshaft engine and put forward a method called Distortion Factor Item (DFI) to improve the fuel supply plan for surge-preventing acceleration when turboshaft engine suddenly encounters inlet flow distortion. Based on the parallel compressor theory, steady-state and transition-state numerical simulation model of turboshaft engine with sub-compressor model were established for researching the influence of inlet flow distortion on turboshaft engine. This paper made a detailed analysis on the compressor operation from the aspects of performance and stability, and then analyzed the overall performance and dynamic response of the whole engine under inlet flow distortion. Improved fuel supply plan with DFI method was applied to control the acceleration process adaptively when encountering different inlet flow distortion. Several simulation examples about extreme natural environments were calculated to testify DFI method’s environmental applicability. The result shows that the inlet flow distortion reduces the air inflow and decreases the surge margin of compressor, and increase the engine exhaust loss. Encountering inlet flow distortion has many adverse influences such as sudden rotor acceleration, turbine inlet temperature rise and power output reduction. By using improved fuel supply plan with DFI, turboshaft engine above-idle acceleration can avoid surge effectively under inlet flow distortion with environmental applicability.

Design optimization of a three-dimensional diffusing S-duct using a modified SST turbulent model

This paper examines design optimization of a three-dimensional diffusing S-duct using a modified k–ω shear stress transport (SST) turbulent model as a turbulence prediction method. According to based-Reynolds-stress and based-separation ideas, a robust solution procedure for the model is described and a grid convergence study is presented. An automated system employs this model to design an S-duct. The aerodynamic performances of the optimal duct are investigated in on-design and off-design conditions. It is shown that the sensitivity of the modified model with respect to shape variations allows its use in the design system. Using a multi-objective optimization strategy, this design system significantly improves aerodynamic performance of the S-duct and has low computation cost and excellent design efficiency. The centerline's curvature and the cross-sectional area ratio become reasonable to avoid overexpansion of the optimal duct. Compared with the original design, the flow distortion coefficient of the optimal duct is reduced by 16.3% and the total pressure recovery factor is increased by 1.1% in on-design condition.

Invariant solutions in a channel flow using a minimal restricted nonlinear model

Simulations using a Restricted Nonlinear (RNL) system, where mean flow distortion resulting from Reynolds stress feedback regenerates rolls, is applied in a channel flow under subcritical conditions. This quasi-linear restriction of the dynamics is used to study invariant solutions located in the bulk of the flow found recently by Rawat et al. (2016) [14]. It is shown that the RNL system truncated to a single streamwise mode for the perturbation supports invariant solutions that are found to bifurcate from a relative periodic orbit into a travelling wave solution when the spanwise size is increasing. In particular, the travelling wave solution exhibits a spanwise localized structure that remains unchanged for large values of the spanwise extent as the invariant solution lying on the lower branch found by Rawat et al. (2016) [14]. In addition, travelling wave solutions provided by this minimal RNL system are self-similar with respect to the Reynolds number based on the centreline velocity, and the half-channel height varying from 2000 to 5000.