Boundary Layer Suction Research Articles

Sweep Optimization to Reduce Aerodynamic Loss in a Transonic Axial Compressor with Upstream Boundary Layer Ingestion

Sweep Optimization to Reduce Aerodynamic Loss in a Transonic Axial Compressor with Upstream Boundary Layer Ingestion

Forced Vibration Induced by Dynamic Response Under Different Inlet Distortion Intensities

Boundary layer ingestion propulsion systems have attracted much attention due to their significant potential to reduce the fuel consumption of future commercial aircraft. However, the aeroelastic stability of the fan blade is affected by the continuous non-uniform incoming flow induced by the ingestion of the boundary layer. When the fan blades rotate in the junction area between the distorted area and the clean area, blade pressure fluctuations occur. This phenomenon triggers a dynamic response process in the blade. Previous numerical investigations explored the influence of the distorted inflow on the blade vibration amplitude, and found that there are two sources of low-order excitation to the blades: the distorted inflow and the dynamic response of the blade. The results show that the low-order excitation existing in the distorted inflow varies sinusoidally with the distortion extent. However, as a new source of excitation, the key influence mechanism of dynamic response is still unclear. To explore this issue, calculations and analyses were conducted for different distorted inflow intensities. The results show that the blade vibration amplitude increases with the rise in distortion intensity. The total pressure at the leading and trailing edge of the rotor blade was extracted for analysis. It was found that when the blade enters or leaves the distorted area, there is a consistent lag in the change in total pressure at the trailing edge compared to the leading edge. This lag leads to an abrupt variation in the total pressure ratio, which constitutes the dynamic response process of the rotor blade. This periodic change generates a second-order excitation that causes the blade to vibrate.

Optimizing the Landing Stability of Blended-Wing-Body Aircraft with Distributed Electric Boundary-Layer Ingestion Propulsors through a Novel Thrust Control Configuration

The imperative for energy conservation and environmental protection has led to the development of innovative aircraft designs. This study explored a novel thrust control configuration for blended-wing-body (BWB) aircraft with distributed electric boundary-layer ingestion (BLI) propulsors, addressing the issues of sagging and altitude loss during landing. The research focused on a small-scale BWB demonstrator equipped with six BLI fans, each with a 90 mm diameter. Various thrust control configurations were evaluated to achieve significant thrust reduction while maintaining lift, including dual-layer sleeve, separate flap-type, single-stage linkage flap-type, and dual-stage linkage flap-type configurations. The separate flap-type configuration was tested through ground experiments. Control experiments were conducted under three different experimental conditions as follows: deflection of the upper cascades only, deflection of the lower cascades only, and symmetrical deflection of both cascades. For each condition, the deflection angles tested were 0°, 10°, 20°, 30°, 40°, 50°, and 60°. The thrust reductions observed for these three conditions were 0%, 37.5%, and 27.5% of the maximum thrust, respectively, without additional changes in the pitch moment. A combined thrust adjustment method maintaining a zero pitch moment demonstrated a linear thrust reduction to 20% of its initial value. The experiment concluded that the novel thrust control configuration effectively adjusted thrust without altering the BLI fans’ rotation speed, solving the coupled lift–thrust problem and enhancing BWB landing stability.

Multidisciplinary Optimization of Aircraft Aerodynamics for Distributed Propulsion Configurations

The combination of different aerodynamic configurations and propulsion systems, namely, aero-propulsion, affects flight performance differently. These effects are closely related to multidisciplinary collaborative aspects (aerodynamic configuration, propulsion, energy, control systems, etc.) and determine the overall energy consumption of an aircraft. The potential benefits of distributed propulsion (DP) involve propulsive efficiency, energy-saving, and emissions reduction. In particular, wake filling is maximized when the trailing edge of a blended wing body (BWB) is fully covered by propulsion systems that employ boundary layer ingestion (BLI). Nonetheless, the thrust–drag imbalance that frequently arises at the trailing edge, excessive energy consumption, and flow distortions during propulsion remain unsolved challenges. These after-effects imply the complexity of DP systems in multidisciplinary optimization (MDO). To coordinate the different functions of the aero-propulsive configuration, the application of MDO is essential for intellectualized modulate layout, thrust manipulation, and energy efficiency. This paper presents the research challenges of ultra-high-dimensional optimization objectives and design variables in the current literature in aerodynamic configuration integrated DP. The benefits and defects of various coupled conditions and feasible proposals have been listed. Contemporary advanced energy systems, propulsion control, and influential technologies that are energy-saving are discussed. Based on the proposed technical benchmarks and the algorithm of MDO, the propulsive configuration that might affect energy efficiency is summarized. Moreover, suggestions are drawn for forthcoming exploitation and studies.

Multiple boundary layer suction slots technique for performance improvement of vertical-axis wind turbines: Conceptual design and parametric analysis

Vertical-axis wind turbines (VAWTs) are gaining attention for urban and offshore applications. However, their development is hindered by suboptimal power performance, primarily attributable to the complex aerodynamic characteristics of the blades. Flow control techniques are expected to regulate the flow on the blade surface and improve blade aerodynamics. In the present study, an effective active flow control technique, multiple boundary layer suction slots (MBLSS), is designed for VAWTs performance improvement. The impact of MBLSS on the aerodynamic performance of VAWTs is examined using high-fidelity computational fluid dynamics simulations. The response surface methodology is employed to identify the relatively optimal configuration of MBLSS. Three key parameters are considered, i.e., number of slots (n), distance between slots (d), and slot length (l), which vary from 2 to 4, 0.025c to 0.125c, and 0.025c to 0.075c, respectively. The results show that MBLSS positively affects the power performance and aerodynamics of VAWTs. Parameter n has the most significant effect on VAWT power performance and the importance of d and l is determined by tip speed ratios (TSRs). Tight and loose slot arrangements are recommended for high and low TSRs, respectively. The relatively optimal configuration (n = 2, d = 0.025c, l = 0.05c) results in a remarkable 31.02% increase in the average net power output of the studied TSRs. The flow control mechanism of MBLSS for VAWT blade boundary layer flow has also been further complemented. MBLSS can prevent the bursting of laminar separation bubbles and avoid the formation of dynamic stall vortices. This increases the blade lift-to-drag ratio and mitigates aerodynamic load fluctuations. The wake profiles of VAWTs with MBLSS are also investigated. This study would add value to the application of active flow control techniques for VAWTs.

Unsteady Aerodynamic Forcing Due to Distortion in a Boundary Layer Ingesting Fan

Abstract Boundary Layer Ingestion (BLI) is a concept with the potential to reduce energy consumption needed for aircraft propulsion. For the fan this results in a need to work well in an environment of increased continuous distortion. The objective of this study is to increase the understanding of unsteady aerodynamic forces due to total pressure distortion in a BLI fan. This is done by using computational fluid dynamics to analyze a fan design intended for a BLI installation. Results include low subsonic to transonic operating speeds and distortion in a wide range of wave numbers. The forcing exhibits a significant dependency on the aeroacoustic cut-on/cut-off condition. This is in particular true at part speed where the normalized unsteady force rises sharply before dropping suddenly as the corrected speed or engine order increases. Unsteady pressure in the blade passage is observed to exhibit the same increase in level followed by a sudden drop once the cut-on limit is passed and undamped pressure waves propagating out from the blade row appear. The unsteady forces on the first three modes exhibit different sensitivities to the distortion wavelength but all are affected by the acoustic condition. By comparing results for the reference fan to a similar fan design with a higher blade count the cascade properties of the blade row are found to dominate the interaction. A result of this is that a higher blade count fan may be less affected by the distortion, and be less prone to propagate noise due to low engine order distortion.

Receptivity of Mack modes to localized unsteady blowing and suction in a chemical non-equilibrium hypersonic boundary layer

Receptivity of Mack modes to localized unsteady blowing and suction in a chemical non-equilibrium hypersonic boundary layer

Effects of distortion on a BLI fan

Abstract The BLI (boundary layer ingestion) concept for propulsion seeks to improve the energy efficiency of aircraft propulsion. This is achieved by accelerating low momentum flow ingested from boundary layers and wakes developed over the fuselage through the fan. A major challenge that needs to be overcome to realise the benefits is that the fan needs to work efficiently in distorted flow. Understanding the effects of distortion on the aerodynamic performance and the distortion transfer through the fan is therefore essential to future designs. A BLI fan, designed at reduced scale, is used for analytic modelling and experiments in a rig designed for this purpose. The test rig replicates BLI conditions for a fan installed at the aircraft tail cone. An unsteady model that includes all blades and vanes of the fan, as well as the nacelle and the by-pass duct of the test rig is used for CFD (computational fluid dynamics) simulations. Test results are used to confirm that the CFD model is representative of the aerodynamics of the fan. The tests are conducted using varying fan operating conditions but also tests with an added distortion screen. Analysis results are then used to investigate the effects of distortion on the fan efficiency, as well as on the overall efficiency. The fan efficiency is found to be moderately decreased depending on the level of and extent of inlet circumferential distortion. In terms of overall energy efficiency, a net improvement over a similar fan in clean inlet flow is found.

Stability of plane Couette flow with constant wall transpiration

Plane Couette flow with constant wall transpiration, i.e., constant blowing from below and constant suction from above, is a solution of the Navier-Stokes equations in terms of exponentials. It is characterized by two Reynolds numbers Re and ReV, where Re parametrizes the moving wall and ReV describes the influence of the transpiration. For this flow the modified Orr-Sommerfeld equation admits one of the very few exact solutions in terms of hypergeometric functions. Based on this, we solve the stability problem, though for the main part focus on temporal stability. For small wall-transpiration rates up to ReV≃6.71, the flow remains unconditionally stable, analogous to the classical Couette flow for arbitrary Reynolds numbers Re. By further increasing ReV an instability sets in and a minimum critical Reynolds number of Re=668350.491 is reached at ReV=9.799. Hence, around this point, the destabilizing effect of blowing outweighs the stabilizing effect of suction. By further increasing the transpiration rate beyond this point, the corresponding critical Reynolds number Re increases again and continues to grow. This limiting case is accompanied by the development of a strong boundary-layer-like velocity profile near the upper wall and the flow transitions to the asymptotic suction boundary layer (ASBL). Thus, the present analysis comprises the whole range from classical Couette flows extended by transpiration to the ASBL, which is known to have a strongly stabilizing effect. Published by the American Physical Society 2024

Characteristics and evolution of ground vortex under heaving ground conditions: Insights from large eddy simulation (LES) analysis

When an aircraft engine operates near the ground, the ambient vortices generated by the freestream velocity boundary layer and engine suction will gather and lift, forming ground vortices which may result in intake distortion, consequently affecting the engine operation stability. To develop a shipborne test platform, this study conducted large eddy simulation calculations using the ANSYS Fluent and based on the assumption of incompressible flow. The 3-D unsteady Navier-Stokes (N-S) equations were solved using the WALE sub-grid model. The analysis focused on the characteristics and evolution of ground vortices under heaving ground conditions within 1 s. The results indicate that the ground vortex exhibits strong unsteady characteristics. During the descending phase of the platform, the quantity of vortex formed beneath the intake duct would decrease while the trailing vortex becomes more pronounced. However, the structure of the inlet vortex remains relatively unchanged at each time step. The intensity of the vortex and pressure distortion coefficient exhibit significant fluctuations over time, and their temporal variations approximately follow a sinusoidal function, which is also the heaving motion of the platform. Generally, when the velocity of the platform reaches its maximum downward vertical value, the average vortex intensity of the ground vortex tends to be minimal while the pressure distortion coefficient approaches its maximum value. Conversely, when the velocity of the platform reaches its maximum upward vertical value, the opposite trend can be observed.

Aeroacoustics of a ducted fan ingesting an adverse pressure gradient boundary layer

The aeroacoustics of a boundary layer ingesting (BLI) ducted fan is investigated experimentally. The study examines a ducted fan immersed in an adverse streamwise pressure gradient turbulent boundary layer developed over a curved wall. Aeroacoustics measurements indicate that the noise from the BLI ducted fan results from a complex interaction among the fan, duct and the incoming boundary layer. The fundamental mechanisms of noise generation are explained using a general source separation strategy. A detailed noise comparison is made at varying fan rotational speeds and across a wide range of axial inflow velocities. In a low thrust regime, the noise is found to be driven by the fan loading, coupled with duct acoustics and the haystacking phenomenon. In a high thrust regime, the contribution from duct acoustics diminishes, and the noise is predominantly driven by the fan loading coupled with the haystacking phenomenon.

Experimental investigation on a lightweight, efficient, counter-rotating fan with and without boundary layer ingestion

AbstractThe experimental investigations on the counter-rotating CRISPmulti fan with boundary layer ingestion are presented in this paper. The Counter Rotating Integrated Shrouded Propfan (CRISP) was designed, aerodynamically optimized and manufactured in lightweight material by the German Aerospace Center (DLR). The main task was to maximize the aerodynamic efficiency under restricted static, dynamic and acoustic properties of the blade designed with lightweight carbon-fiber material and to apply it as a demonstrator for the development of a new innovative blade manufacturing technology. The rotors were manufactured at the DLR Institute of Structures and Design and tested on the Multi-stage 2-shaft Compressor Test Facility (M2VP) at the DLR Institute of Propulsion Technology. The instrumentation enabled the measurement of the aerodynamic, static, dynamic and acoustic behavior of the fan. In general, the targets of the experimental investigation were to validate the optimization results, the numerical calculational methods and the acoustic tools. Additionally, inlet distortion measurements were carried out on the fan rig to investigate the effect of the boundary layer ingestion on a real counter-rotating propfan. A radially traversable disturbance body, designed to simulate the boundary layer of the aircraft fuselage on an aircraft architecture with embedded engines, enables a comprehensive measurement program to be carried out for both undisturbed and disturbed inflow.

Flowfield of an S-Shaped Inlet with High Boundary-Layer Ingestion Fraction

The efficiency of boundary-layer ingestion (BLI) propulsion systems is closely related to the amount of ingested boundary layer. A high BLI fraction requires low propulsor flow power. In this paper, the internal flow characteristics and performance of an S-shaped inlet with a high BLI, up to 83.8% of the inlet capture height, are explored in detail under a freestream Mach number of 0.75. The results reveal that the middle and lower parts of the aerodynamic interface plane (AIP) section are packed with low-energy flows dominated by a pair of vortices, formed by the coalescence of side and separation vortices. At MAIP=0.46, the side vortex, originating near the first inlet bend, is caused by a spanwise pressure gradient, while the side vortex is connected to the horseshoe vortex at the root of the inlet lip at MAIP=0.33. The vortex coalescence results in a sudden increase in the strength of the vortex core and a stepwise increase in circulation (ΓRx). The flow vortex intensity in the inlet is divided into three stages from the distribution of ΓRx. In addition, a reduction in MAIP increases the intensity of secondary flow along the section, particularly the intensity at the flow separation zone. With increasing MAIP, the swirl intensity decreases, whereas the total pressure distortion index DC60 rises, reaching 0.54, which is beyond the requirement of turbomachinery component flow uniformity.

A Review of Novel and Non-Conventional Propulsion Integrations for Next-Generation Aircraft

The aim of this review paper is to collect and discuss the most relevant and updated contributions in the literature regarding studies on new or non-conventional technologies for propulsion–airframe integration. Specifically, the focus is given to both evolutionary technologies, such as ultra-high bypass ratio turbofan engines, and breakthrough propulsive concepts, represented in this frame by boundary layer ingestion engines and distributed propulsion architectures. The discussion focuses mainly on the integration effects of these propulsion technologies, with the aim of defining performance interactions with the overall aircraft, in terms of aerodynamic, propulsive, operating and mission performance. Hence, this work aims to analyse these technologies from a general perspective, related to the effects they have on overall aircraft design and performance, primarily considering the fuel consumption as a main metric. Potential advantages but also possible drawbacks or detected showstoppers are proposed and discussed with the aim of providing as broad a framework as possible for the aircraft design development roadmap for these emerging propulsive technologies.

Conceptual Design of Layered Distributed Propulsion System to Improve Power-Saving Benefit of Boundary-Layer Ingestion

DPS (distributed propulsion system) utilizing BLI (boundary-layer ingestion) has shown great potential for reducing the power consumption of sustainable AAM (advanced air mobility), such as BWB (blended-wing body) aircraft. However, the ingesting boundary layer makes it easier for flow separation to occur within the S-shaped duct, and the consequent distortion due to flow separation can dramatically reduce the aerodynamic performance of the fan, which offsets the power-saving benefit of BLI. By analyzing the source of power saving and power loss of BLI, this paper presents the LDPS (layered distributed propulsion system) concept, in which the freestream flow and boundary-layer flow are ingested separately to improve the power-saving benefit of BLI. In order to preliminarily verify the feasibility of LDPS, an existing DPS is modified. The design parameters and the system performances of LDPS are studied using a 1D engine model. The results show that there is an optimal ratio of the FPR (fan pressure ratio) for the FSE (freestream engine) to the BLE (boundary-layer engine) that maximizes the PSC (power-saving coefficient) of LDPS. This optimal ratio of FPR for the two fans can be obtained when the exit velocities of FSE and BLE are the same. Under the optimal ratio of FPR for the two fans, the PSC of LDPS is improved by 5.83% compared to conventional DPS.

Behavior of flow distortion within a boundary layer ingestion inlet

One of the critical problems restricting the development of boundary layer ingestion (BLI) propulsion systems is the strong distortion and swirling flow in the inlet, which will threaten the stable operation of the fan. In this study, the effects of the Exit Mach number (MAIP), the ingested boundary layer thickness (δ) on the internal flow and flow distortion at the aerodynamic interface plane (AIP), are investigated under a freestream Mach number of 0.75. The experimental results indicate that a low total pressure region persistently located at the lower part of the AIP accompanied by a pair of vortices with a maximum swirl angle of 23° With the decrease of exit Mach number, these vortices move toward the center of the AIP, which decreases the total pressure distortion while increases the swirl intensity. However, as the ratio of the ingested boundary layer thickness to the inlet height (δ/h) increases from 18.6 % to 83.8 %, the length-scale of these vortices at AIP increases, leading to a descending trend of the total-pressure recovery coefficient from 0.942 to 0.871. The total pressure distortion gets intensified from 0.34 to 0.42, with the mean swirl intensity (SImean) dropping from 8° to 6.7° The influence of the δ/h on SImean could be attributed to the enhancement of SI in the outer ring of AIP. Meanwhile, the validated numerical results of δ/h = 83.8 % and δ/h = 43.3 % show that the vortices at AIP are originated from the coalescence of two vortices. MAIP has a substantial influence on the origin of the vortex, separation positions, and pressure distributions of the inlet. The flow separation position is associated with total pressure distortion at AIP. Further optimization design can be considered by controlling the flow separation position.

Control of separated flow at low Reynolds number around NACA0012 airfoil by boundary layer suction

The separated flow at low Reynolds number around the NACA0012 airfoil is numerically studied by large-eddy simulation. Strategies of boundary layer suction to control flow separation are investigated. A method of using two-zone suctions, near the leading edge and near the trailing edge, are calculated. Based on verification with direct numerical simulation (DNS) and experimental data, the results of the lift and the drag, the vortices, and the strength of near-field pressure fluctuations, are checked. The results show that the two-zone suctions can supress flow transition and separation, thereby increase the lift and reduce the drag. The shedding of vortices is weakened, and the near-field pressure fluctuations are attenuated. For comparison with the two-zone suctions, the strategies of suction near the leading edge only and suction near the trailing edge only are also studied. It is found that suction near the leading edge only may suppress transition and delay separation when the suction zone is large enough, but the flow property deteriorates due to shedding vortices in the wake. The suction near the trailing edge only may improve the flow performance by reducing the size of the vortices in the rear section of the airfoil and in the wake region, but it has little effect on the separation bubble and transition.

Potential for Energy Recovery from Boundary-Layer Ingesting Actuator Disk Propulsion

The theoretical benefits of highly integrated propulsion systems are highlighted herein by assessing the potential for energy recovery utilization using actuator disk propulsion. Decomposing aerodynamic forces into thrust and drag for closely integrated bodies, particularly those employing boundary-layer ingestion, becomes challenging. In this work, a mechanical energy-based approach was taken using the power balance method. This allowed the performance to be analyzed through the mechanical flow power in the fluid domain, disregarding the need for any explicit definition of thrust and drag. Through this, the benefit of boundary-layer ingestion was observed from a wake energy perspective as a decrease in the downstream mechanical energy deposition and associated viscous dissipation. From a propulsion perspective, the reduction in power demand necessary to produce propulsive force indicated the possibility of power savings by utilizing the energy contained within the ingested boundary-layer flow.

PROPELLER INFLUENCE ON SURFACE PRESSURE FLUCTUATION UNDER BOUNDARY LAYER INGESTION CONDITION

Research on propeller noise under boundary layer ingestion condition is essential for designing new aircraft concepts. Measurements of pressure spectra on a flat plate below a propeller give important information about the noise coming from the blades. These also are important for designing surface noise mitigation structures, such as acoustic black holes, meta-materials, and liners. This paper investigates the effects of a propeller on the flat plate surface pressure under boundary layer ingestion, using bottom of the wind tunnel channel as flat plate. Tripping devices on the wind tunnel bottom wall control the boundary layer thicknesses. The surface pressure is measured using an instrumented plate positioned underneath the propeller. The plate has 87 pressure taps to measure the unsteady and steady pressure using the remote microphone probe (RMP) technique. The results show the influence of the propeller inflow, the advance ratio, and the number of blades on the different regions underneath the propeller, and on coherent turbulent structures of the incoming flow. The boundary layer thicknesses affects the static pressure upstream of the propeller - creating high pressure regions. The propeller affects the turbulent structures downstream of the blades, increasing its correlation length.

A region-segmentation combinational loss model based on data-driven machine learning for a boundary layer ingestion fan

The boundary layer ingesting (BLI) fan is continuously working under distorted inflows. The rotor blade moves across the distorted and undistorted regions and the resultant off-design condition makes the loss present a notable non-uniform distribution feature. Accurately capturing the loss is very important for evaluations of BLI fan performances in preliminary design stage. However, unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations are extremely computational expensive even though they can well predict the loss, and the traditional empirical or physics-based loss models are usually fade when predicting loss variation trends and absolute magnitudes under complex inflow conditions. This paper develops a data-driven machine learning based region-segmentation combinational loss model to realize a fast and accurate prediction of the aerodynamic loss in a BLI fan rotor. According to the distinctions of loss sources in different spanwise fractions, a region segmentation idea is first used to create local loss model in each sub-region, and then a full-span combinational prediction model is constructed through a data-driven based neural network approach. Results show that the data-driven based region-segmentation combinational loss model presented in this work performs better than the traditional empirical or physics-based loss models in terms of predicting both the loss variation trends and absolute magnitudes. Compared with the high-fidelity URANS simulations, the averaged loss prediction errors in a BLI fan rotor are kept within 10 % under different blade load and inflow distortion conditions.