RNG K-epsilon Turbulence Model Research Articles

Air pollutant dispersion around high-rise building cluster forms: the case of Port City, Colombo, Sri Lanka.

Air quality in dense urban environments is a growing concern, especially in rapidly developing cities. In the face of growing traffic associated with urbanisation, there is evidence for high levels of pollutant concentration at street level which is influenced by building forms. In this paper, we examine the potential effects of high-rise, cluster developments permitted by the local planning authorities in the newly established Port City development in Colombo, Sri Lanka. We designed possible building forms based on specific guidelines for the development in terms of plot coverage, floor area ratio, and maximum height. The three-dimensional building clusters were simulated using the RANS RNG k-epsilon turbulence model, to determine pollutant dispersion of a complex street formation in a high-dense high-rise building cluster, within the development and the surrounding context (existing Colombo). Results show that while increased porosity within the built fabric facilitates better pollution dispersion, a low correlation was seen between wind velocity and pollution concentration, especially in deep narrow high-rise canyons. Dispersion patterns at street level and at the urban canopy differed with each built form and are dependent on each canyon geometry. Thus, the study highlights the need for building regulations to take a holistic approach to capture the various elements of a complex urban cluster rather than the current two-dimensional parameters proposed for Port City, Colombo.

Thrust Force for Drone Propeller with Normal and Serrated Trailing Edge

The drone becomes more recognized in the civilian sector; the drone's popularity becomes increases as time goes by. Nevertheless, despite the excitement of flying drones, several types of issues occur caused by the drone. In some circumstances, the aeroacoustics noise is a big concern, and quiet drone propellers would be more environmentally friendly to the surrounding area. Moreover, the noise from the drone can be a nuisance for the surrounding population and animals. Therefore, a solution needs to be proposed to reduce the sound level produced by the drone so that drone can be piloted in a surrounding area without breaking any noise level limit set by the government. Hence, the propeller's serrated trailing edge type is the proposed solution to this problem. The serrated trailing edge propeller can reduce several drone noise decibels based on past research. Thus, an investigation is conducted to study the thrust force between the normal propeller and the serrated propeller. The aerodynamic performance of the serrated propeller is analysed using computational fluid dynamic simulation and compared to that of the normal propeller. Ansys Fluent 2021 is used to solve the dependable RNG k-epsilon turbulence model. The thrust force, thrust coefficient, and lift coefficient operating on both propellers were all simulated. The results obtained by the transient approach for propellers have been validated by earlier experimental studies.

Design and analysis of a window-integrated passive system (WIPS) with the use of solar heat gains

As one of the passive building design strategies to decrease ventilation energy consumption, several window-integrated passive systems (WIPS) have been developed and implemented into buildings. This study proposed a new WIPS inspired by a double skin façade (DSF) design that provides pre-heating and ventilation by utilizing solar heat gains collected in the air cavities. The performance of the proposed WIPS was analyzed and developed based on different design parameters such as material, width, height, shape, depth, and opening area, for the wintertime in Seoul, Korea. Furthermore, the computational fluid dynamics model, Fluent 2021 R2 with the RNG k-epsilon turbulence model, was used for the simulation. To focus on the buoyancy effect occurring in the air cavities, the influence of wind was excluded from the CFD modelling and analysis. Therefore, 0 Pa pressure differences between the inlet and outlet of the room were applied. The results showed that the buoyancy effect increased in the WIPS with a wider and higher inflow cavity than a case with narrower and shorter cavity, also utilizing glass material for the cavity surfaces exposed to the outside resulted in absorbing more solar radiation and more buoyancy effect. Moreover, the converged shape cavity increased the volume flow rate of the cavity due to the increased air velocity from the Venturi effect. Overall, all design parameters can impact the performance of WIPS either by hindering or assisting the airflow of the WIPS.

Neighborhood, Topography, and Wind Direction Effects on Wind Pressure Distribution on the Low-rise Building Roof

Low-rise buildings are the majority of the houses that are constructed all over the world. Experiments of the wind loads acting on these buildings provide vital information to design secure structures and adverse weather conditions resistants, considering the basic parameters as roof slopes and the wind direction. This study estimated the distribution of wind pressures around the contour of buildings with gable roofs, considering diverse neighborhood conditions, namely the number and geometric configuration of buildings on the ground, in conjunction with the different angles of wind incidence and topography. The simulations took place with Ansys Workbench software, and the RNG K-Epsilon turbulence model and tetrahedral mesh were employed. The application validation of the CFD technique occurred in the double-sloped pitched roof structure. The results showed good concordance with the literature. The pressure coefficients were analyzed, as well, in the flow visualization, highlighting the attachment points and the recirculation zones.

Comparative analysis and manufacturing of airfoil structures suitable for use at low speeds

An aerodynamic technique to calculating lift and drag coefficients is one of the required instruments in the wing design process. During the last decades, several tools and software have been developed according to aero-dynamics and numerical methods. Nowadays, aeronautical architecture requires many calculations. Today’s techno-logists use a variety of simulation techniques to avoid a expensive model testing. This paper explains how wing profiles can be modelled using ANSYS Fluent and tested by low-speed tests considering experimental literature re-sults. With the selected wing profile, the geometry is shaped in two dimensions and designed in three dimensions. Computational fluid dynamics (CFD) was adopted as the method for studying wing profiles. Wing profiles created at 0 to 20-degree attack angles are calculated in the simulation area equal to the actual wind tunnel scale, and equations are solved using the RNG k-Epsilon turbulence model. The process of developing the grids was realized with Ansys Mesher software. The solution stage and the result show operations were carried out with the CFD Post software. The study of the low velocity and high transport wing profiles, the drag coefficient, the lift coefficient, and the effect on the lift-drag ratio were studied using a numerical procedure. After determining the high efficiency of wing profi-les, production of a selected profile began with a static examination.

Controlling infectious airborne particle dispersion during surgical procedures: Why mobile air supply units matter?

The ventilation system in an operating room (OR) plays a vital role in reducing the risk of patients contracting an infection while undergoing a surgical procedure. The clean air supplied from the ceiling-mounted diffuser removes the airborne particles from the surgical site. The clean air, however, is often obstructed by the medical staff and other objects. Hence, some sterile instruments might remain outside the protected area. The present study aims to examine the effectiveness of a mobile air supply (MAS) unit in reducing the particle settlement on a patient under different airflow velocities supplied from the MAS unit. A simplified computational fluid dynamics (CFD) model of the OR was developed and validated based on published data. An RNG k-epsilon turbulence model, based on the Reynolds-Averaged Navier-Stokes (RANS) equations, was used to simulate the airflow, while a discrete phase model (DPM) was used to simulate the movement of the infectious airborne particles. The MAS unit was evaluated as an extension of unidirectional airflow ventilation. Results showed that the MAS unit successfully reduced the settlement of airborne particles by 78% from 45 particles/m 3 to 10 particles/m 3 . However, the operation of the MAS unit showed a reverse effect on the particle settlement (∼7 particles/m 3 ) on the patient when the MAS unit supplied air at a velocity of 0.6 m/s. The present study showed that air supply at a velocity of 0.5 m/s provided an optimum wiping effect that removed the airborne particles from the surgical zone. • Mobile air supply (MAS) unit evaluated as extended ceiling-mounted unidirectional airflow ventilation. • MAS successfully reduced airborne particles settlement from 45 to 10 particles/m 3 (or by 78%). • Air velocity of 0.5 m/s optimally removed (wiping effect) airborne particle from surgical zone.

Numerical Study of Unsteady Pressure Fluctuation at Impeller Outlet of a Centrifugal Pump

Intense fluid-dynamic interaction at the impeller outlet strongly affects the unsteady flow and pressure stability within the centrifugal pump. In order to have a better understanding of the pressure fluctuation of centrifugal pumps, a numerical calculation is carried out by using the RNG k-epsilon turbulence model under various flow rates. The numerical calculation results are compared with the experimental results in order to verify the reliability of the calculation model. The amplitude and frequency distribution of pressure fluctuation at the impeller outlet is obtained and analyzed in the time and frequency domain. The research results show that the blade passing frequency is the dominant frequency of the pressure fluctuation. And the pressure fluctuation is a periodic fluctuation. As the flow rate decreases, the periodicity of the pressure fluctuation decreases. Besides, the amplitude and intensity of pressure fluctuation are closely related to flow rate and spatial location. At the low flow rate, the amplitude of pressure fluctuation in the time domain and frequency domain is enlarged greatly, especially near the tongue region. The pressure difference distribution on both sides of the blade surface is extremely uneven, and the pressure changes significantly.

Influence of Wind Angle Incidence and Architectural Elements on the External Pressure Coefficient of Hyperbolic Paraboloid Roofs

In the study of wind loads in buildings, the aerodynamics of roofs with parabolic shapes, which cause complex pressure distributions due to their sensitivity to wind, are often omitted and neglected by several codes and norms. In this way, computer simulations are a viable and reliable alternative. Here, wind action was considered in an innovative project composed of parabolic and circumferential generatrices: the Church of Saint Francis of Assisi. Designed by Brazilian architect Oscar Niemeyer in Belo Horizonte, Brazil, two paraboloid vaults and three circular arches of reinforced concrete composed its structure. This work generated great international recognition for the architect after 1943, as the design of the roofs did not require walls. For geometry modeling, Autodesk AutoCAD software was adopted, and the models were considered in a control volume. The simulations were performed using Ansys Workbench software and the RNG K-Epsilon turbulence model. The wind speed at different heights was calculated using the Power-law approximation. A basic wind speed of 30 m/s was adopted, and the mesh used was composed of tetrahedrons. To validate the methodology, different models with hyperbolic-paraboloid roofs from the literature were considered. In addition, the visualization of the flow around the geometry from the streamlines, the wind profile, and the analysis of the isobaric lines of the external pressure coefficients for different directions of incidence and architectural elements that make up the building were presented.

Flow and Heat Transfer Characteristics of S-CO2 in a Vertically Rising Y-Tube

The supercritical carbon dioxide Brayton cycle has gradually become a research focus, but we also see a deficiency in research related to the flow and heat transfer characteristics of S-CO2 boiler staves with high parameters. In this paper, the flow and heat transfer of supercritical carbon dioxide is investigated in a 1000 MW supercritical boiler cooled wall tube in the parameters of a pressure of 30.42 MPa, a mass flux of 1592~2207 kg/(m2·s), and a heat flux of 39.8~71.2 kw/m2; a three-dimensional model of supercritical CO2 fluid in the cooling wall tube is established with the RNG k-epsilon turbulence model. Numerical simulations are carried out according to the following boundary conditions: an adiabatic half side, a heated half side, and a Y-type three-way two-to-one. The effects of the mass flux, inlet temperature, and heat flux on the flow and heat transfer characteristics in the Y-tube are analyzed, which exerts great influence on the research of S-CO2 boiler stave thermodynamics.

Effects of novel hybrid nanofluid (TiO2–Cu/EG) and geometrical parameters of triangular rib mounted in a duct on heat transfer and flow characteristics

Heat transfer and flow characteristics have been numerically analyzed by using four different fluids [pure ethylene glycol (EG), TiO2/EG and Cu/EG nanofluids and 50%:50% TiO2–Cu/EG hybrid nanofluid] and changing the position, length and height of a triangular rib placed in a two-dimensional duct under forced convection and turbulent flow conditions using RNG k-epsilon turbulence model. While forming the hybrid nanofluid, all nanoparticles are added at 50–50%. A constant heat flux of 500 W m−2 is applied to the bottom wall of the duct. The Reynolds number is considered in the range of 50,000 and 100,000. Two different nanoparticle volume fractions, 1.0% and 4.0%, are used. The parameters of the rib are dimensionlized, and three different rib positions (X/H = 1, 3 and 5), lengths (S/H = 0.5, 1 and 1.5) and heights (h/H = 0.1, 0.2 and 0.3) are used. Governing equations are determined with the finite volume method. In the study, analysis of Nusselt number, Darcy friction factor (f), PEC number and velocity streamlines is performed in detail. Generally, heat transfer performance increases with increasing Reynolds number and nanoparticle volume fraction, but the increase in nanoparticle volume fraction also increases the Darcy friction factor. TiO2–Cu/EG hybrid nanofluid having the 4.0% nanoparticle volume fraction is the best heat transfer fluid. When using TiO2–Cu/EG hybrid nanofluid, the increase in the PEC number varies between 8 and 30% in the smooth duct. From the results obtained by using hybrid nanofluid in the duct with the triangular rib, it is understood that the rate of heat transfer enhances with increasing dimensionless rib position, length and height. But the most effective parameter of these is change in the rib height. The most suitable fluid for the study is TiO2–Cu/EG hybrid nanofluid with 4.0% nanoparticle volume fraction, and the most favorable rib parameters are rib height of h/H = 0.3, rib length of S/H = 1.5 and rib position of X/H = 5.

Conjugate forced convective heat transfer in a sandwich panel with a Kagome truss core: The effects of strut length and diameter

Heat exchangers with a cellular metal core have become an alternative to those with louvered fins due to their high heat transfer performance and mechanical strength. Because of the multi-functionality, they are mostly used in systems such as rockets and space vehicles or in others in need of both high heat transfer performance and mechanical strength. The geometrical parameters of the unit cell of cellular metals have important effects on both fluid and heat transfer characteristics and these effects need to be studied. Accordingly, conjugate forced convective heat transfer in a sandwich panel with a Kagome truss core both with a constant heat flux (CHF) and a constant temperature (CT) boundary condition was studied numerically in this study for various values of the Reynolds number, strut length, and strut diameter. The 3D turbulent flow and heat transfer was modeled by the RNG k-epsilon turbulence model. The near-wall behavior of the flow was modelled by enhanced wall treatment. The channel was extended before and after the Kagome core to avoid entrance and exit effects. The computational results were obtained for the Reynolds numbers in the range of about 6000–17000, for the strut lengths of 3, 5 and 7 mm, and for the strut diameters of 1.3 and 1.6 mm. A nonlinear regression analysis was also performed for the pressure loss coefficient and average Nusselt number. The numerical results show that complex flow structures develop in the flow field due to the presence of unit cells. The results also show that the pressure drop along the Kagome core increases considerably with decreasing strut length and increasing strut diameter. The average Nusselt number increases significantly when the strut length is decreased and shows a small increase with an increase in the strut diameter. The effect of strut diameter on average Nusselt number becomes marginal for strut length of 3 mm. The average heat transfer rate for a fixed pumping power shows a considerable increase with a decrease in the strut length and a small increase with the strut diameter for high values of strut length. On the other hand, for the strut length of 3 mm, it shows a small decrease with an increase in the strut diameter for the CT case and remains almost constant for the CHF case.

Numerical simulation of pollutant dispersion in the residential areas with continuous grass barriers

In this paper, numerical modeling of the movement and dispersion of pollutant emissions between houses as well as the effect of a continuous barrier on the spread of pollutants was considered. To solve this problem, a system of Reynolds-averaged Navier–Stokes equations was used, and various turbulent models were applied to close this equation system. To test the mathematical model, the numerical algorithm and the choice of the optimal turbulent model, the test problem was solved numerically. The obtained numerical results were compared with experimental data and the results of modeling by other authors. After checking the mathematical model, the numerical algorithm and the choice of the optimal turbulent model, the main problem describing the process of pollutant emissions between houses using continuous grass barriers was solved. In this problem, the distribution of pollutants was considered when using herbal barriers and the optimal height for these barriers was chosen. For the simulation, the RNG k-epsilon turbulence model was applied, which was chosen from the obtained results of the test problem. The numerical simulation results were compared with the obtained calculated data using different heights of solid grass barriers. And it was found that when using a solid grass barrier with a height of 0.1 m, the concentration value drops by more than 1.5 times compared with that without using a barrier, and further increasing the height of the barrier does not give the desired result.

Simulation Analysis on Noise Reduction Effect of Sound Barriers with Different Geometric Shapes for High-Speed Train

In order to study the noise reduction effect of different geometrical sound barriers on high-speed train aerodynamic noise, simulation studies were carried out on three shapes of sound barriers. The RNG k-epsilon turbulence model was used to calculate the aerodynamic drag of the high-speed train. Based on the large eddy simulation LES method and the FW-H acoustic analogy equation, the pulsating pressure of the train was extracted as the far-field noise source, and the noise reduction capabilities of the upright, semi-circular, and quarter-circular sound barriers were compared. The results show that the semi-circular sound barrier has the largest average insertion loss in far field, and its frequency response characteristics present the best match to the aerodynamic noise of high-speed train. Therefore, the semicircular sound barrier has the best noise reduction effect.

Efficiency of Different Roof Vent Designs on Natural Ventilation of Single-Span Plastic Greenhouse

In the summer season, natural ventilation is commonly used to reduce the inside air temperature of greenhouse when it rises above the optimal level. The greenhouse shape, vent design, and position play a critical role in the effectiveness of natural ventilation. In this study, computational fluid dynamics (CFD) was employed to investigate the effect of different roof vent designs along with side vents on the buoyancy-driven natural ventilation. The boussinesq hypothesis was used to simulate the buoyancy effect to the whole computational domain. RNG K-epsilon turbulence model was utilized, and a discrete originates (DO) radiation model was used with solar ray tracing to simulate the effect of solar radiation. The CFD model was validated using the experimentally obtained greenhouse internal temperature, and the experimental and computed results agreed well. Furthermore, this model was adopted to compare the internal greenhouse air temperature and ventilation rate for seven different roof vent designs. The results revealed that the inside-to-outside air temperature differences of the greenhouse varied from 3.2 to 9.6oC depending on the different studied roof vent types. Moreover, the ventilation rate was within the range from 0.33 to 0.49 min-1. Our findings show that the conical type roof ventilation has minimum inside-to-outside air temperature difference of 3.2oC and a maximum ventilation rate of 0.49 min-1.

Improving of Film Cooling Effectiveness Using a Reverse Injection

Film cooling is a cooling technique widely used in high performance gas turbines to protect the turbine's airfoils from damage by hot gases. Film injection holes are placed in the blade body to allow the coolant to pass from the inner cavity to the outer surface. The cooling injection gas results a layer film cooling gas flowing along the outer surface of the airfoil. This study consists of a 3D numerical film cooling simulation. The CFD code used ANSYS CFX is based on the finite volume method for the resolution of the averaged Navier-Stokes equations. The objective of the study is to test a geometrical configuration with an injection hole in the reverse direction to a mainstream flow. The goal is to modify the flow structure to destroy the two kidney-shaped secondary vortexes (Kidney Vorticies) that are primarily responsible for the deterioration of cooling effectiveness. The geometric configuration is combined with different injection rates that have been simulated to evaluate the influence of the reverse injection. The computational domain is discredited using a structured mesh with highly refined multi-blocks including the region of the distribution box (plenum). The turbulence is modeled by the RNG k-epsilon turbulence model. The coolant was fed by a film cooling hole with an inclination angle of 30 ° in both directions. Three blowing ratio are tested respectively (0.5, 1 and 2). It has been found that the reverse injection hole improves the cooling performance across the entire computational field compared to the forward injection hole. The numerical results of forward and reverse holes are compared with the experimental study of Singh et al.

Assessment of different turbulence models on the large scale internal heated water pool natural convection simulation

In vessel retention (IVR) is treated as one of the strategies in the severe accident management aspect for the safety of nuclear reactor. The investigation of internal heated melt pool natural convection behavior is important for providing the reference for the IVR strategy. Various researchers conducted internal heated melt pool simulation with focusing on the different aspects and conditions. Based on the previous research, the large scale internal heated melt pool flow behavior located in the turbulence flow region and turbulence models used could affect the simulation result of the internal heated melt pool. Therefore, it is important to conduct the simulation of comprehensive assessment of different turbulence models on the melt pool simulation. In this work, the simulation of large scale internal heated water pool, which is one of the COPRA (COrium Pool Research Apparatus) water tests, is conducted by using ANSYS FLUENT 18.2. The large scale internal heated water pool transient behavior is investigated. The effect of various turbulence models on the internal heated water pool simulation is investigated. This work could provide better understanding of temperature distribution and velocity distribution of large scale internal heated water pool natural convection behavior from the numerical aspect. Based on the current simulation, Realizable k-epsilon turbulence model with the consideration of full buoyancy effects, RNG k-epsilon turbulence model with the consideration of full buoyancy effects, DES (Spalart-Allmaras) turbulence model and DES (SST k-omega) turbulence model are recommended for the simulation of large scale internal heated water pool. This work could bring the benefits for the further study on the numerical investigation of large scale internal heated water pool natural convection behavior.

Pressure fluctuation generated by the interaction of blade and tongue

Pressure fluctuation around the tongue has large effect on the stable operation of a centrifugal pump. In this paper, the Reynolds averaged Navier-Stokes equations (RANS) and the RNG k-epsilon turbulence model is employed to simulate the flow in a pump. The flow field in the centrifugal pump is computed for a range of flow rate. The simulation results have been compared with the experimental data and good agreement has been achieved. In order to study the interaction of the tongue with the impeller, fifteen monitor probes are evenly distributed circumferentially at three radii around the tongue. Pressure distribution is investigated at various blade positions while the blade approaches to and leaves the tongue region. Results show that pressure signal fluctuates largely around the tongue, and it is more intense near the tongue surface. At design condition, standard deviation of pressure fluctuation is the minimum. At large flow rate, the increased low pressure region at the blade trailing edge results in the increases of pressure fluctuation amplitude and pressure spectra at the monitor probes. Minimum pressure is obtained when the blade is facing to the tongue. It is found that the amplitude of pressure fluctuation strongly depends on the blade positions at large flow rate, and pressure fluctuation is caused by the relative movement between blades and tongue. At small flow rate, the rule of pressure fluctuation is mainly depending on the structure of vortex flow at blade passage exit besides the influence from the relative position between the blade and the tongue.

Effect of inlet guide vanes on the performance of small axial flow fan

Effects of the inlet guide vanes on the static characteristics, aerodynamic noise and internal flow characteristics of a small axial flow fan are studied in this work. The inlet guide vanes with different outlet angle are designed, which are mounted on the casing and located at the upstream of the impeller of the prototype fan. Both steady and unsteady flow simulations are performed. The steady flow is simulated by the calculations of Navier-Stokes equations coupled with RNG k-epsilon turbulence model, while the unsteady flow is computed with large eddy simulation. According to the theoretical analysis, the inlet guide vanes with outlet angle of 60° are regarded as the optimal inlet guide vanes. The static characteristic experiment is carried out in a standard test rig and the aerodynamic noise is tested in a semi-anechoic room. Then, performances of the fan with optimal inlet guide vanes are compared with those of the prototype fan. The results show that there is reasonable agreement between the simulation results and the experimental data. It is found that the static characteristics of small axial flow fan is improved obviously after installing the optimal inlet guide vanes. Meanwhile, the optimal inlet guide vanes have effect on reducing noise at the near field, but have little effect on the noise at the far field.

Effect of the uneven circumferential blade space on the performance of small axial flow fan

Effects of the uneven circumferential blade space on static characteristics and aerodynamic noise of a small axial flow fan are studied in this work. The blade angle modulation is adopted to design a series of unequally spaced fans, which have different maximum of modulation angular displacement. The steady flow is simulated by the calculations of Navier-Stokes equations coupled with RNG k-epsilon turbulence model, while the unsteady flow is computed with large eddy simulation. According to theoretical analysis, a fan with a maximum of modulation angular displacement of 6° is regarded as the optimal unequally spaced fan. The experiment of static characteristic is carried out in a standard wind tunnel and the aerodynamic noise of both fans is tested in a semi-anechoic room. Then, performances of the optimal unequally spaced fan are compared with those of the prototype fan. The results show that there is reasonable agreement between the simulation results and the experimental data. It is found that the discrete noise of the optimal unequally spaced fan is lower than that of the prototype fan at the near field monitoring point. This can be explained that the total pressure fluctuation of the optimal unequally spaced fan is much more regular than that of the prototype fan.

Numerical Simulation of Pressure Fluctuation around the Tongue Region in a Centrifugal Pump

Pressure fluctuation near the tongue is one of the primary sources of pump vibration and noise. In order to investigate the effect of pressure fluctuation near the tongue, the RANS equations and the RNG k-epsilon turbulence model are employed to simulate the flow in the pump. The SIMPLE algorithm is applied to couple the solutions of the system of equations. Flow field within the centrifugal pump under different flow rates are obtained by simulation. The simulation results are compared with the experimental data to verify the reliability of the calculation model. It is found that the pressure fluctuation at each monitor point is a periodic wave but non-uniform under small flow rate. When the flow rate is larger than the design flow rate, average pressure and standard deviation at monitor points is relative uniform. The dominate frequency of pressure fluctuation is the blade passing frequency and the amplitude of pressure fluctuation is regular. At small flow rate, complex unstable flow makes average pressure and standard deviation at monitor points increasing obviously. Amplitude of pressure fluctuation is larger than that of design flow rate conditions and the maximum amplitude of pressure fluctuation in frequency domain exists at the monitor point just behind the tongue along the impeller rotation direction.