Nozzle Wake Research Articles

A Study on the Wake Evolution of a Set of RIM-Driven Thrusters

In the current paper, high-fidelity improved delayed detached-eddy simulations (IDDES) using the OpenFOAM library are employed to characterize the performances and the wake dynamics of RIM-driven thrusters. Despite their relatively old conception, this type of propulsors has gained attention only recently thanks to innovative manufacturing technologies and materials, which finally have made possible their practical implementation. Fostered by stringent regulations on radiated noise and emissions enforced in protected areas, they are also replacing conventional thrusters and main propulsors. By mitigating the tip vortex cavitation and, more in general, by reducing the strength of tip vortices, indeed, these propulsors may grant a reduction in the induced pressure pulses and of the radiated noise without excessively sacrificing the efficiency of the equivalent ducted propellers they replace. To provide proof of this, three different RIM-driven thrusters (one four-bladed, two six-bladed) are analyzed and compared to a reference decelerating ducted propeller delivering the same thrust at identical functioning conditions. The evolution of the trailing wakes of the propulsors, the role of the leakage vortex, and the nozzle wake destabilizing effects, are highlighted, and the superior performances of RIM-driven thrusters, in terms of less intense tip vortices, are discussed. Near-field pressure pulses, as a measure of the radiated noise, are compared, showing a reduction in the sound pressure levels of the selected RIM propulsors up to 15 dB (non-cavitating case) with respect to the reference ducted propeller.

A novel thrust vector nozzle with conical surface based on Coanda effect

In this paper, a novel thrust vector control method based on Coanda effect is proposed for conical surface. The effect of thrust vector control is studied by computation fluid dynamic (CFD) numerical simulation. When there is no side plate on both sides of Coanda surface, there are large vortex structures on both sides of nozzle wake, and the vector control cannot be realized. After adding the side plate, the comprehensive control of the pitch and yaw thrust vectors was realized. When the nozzle pressure ratio (NPR) is 2~2.4, the pitch and yaw vector angles decreased with the increase of NPR, and the maximum pitch vector angle is 8°. And the yaw vector angle is 8°.

Study of Air Pressure and Velocity for Solution Blow Spinning of Polyvinylidene Fluoride Nanofibres

Solution blow spinning (SBS) is gaining popularity for producing fibres for smart textiles and energy harvesting due to its operational simplicity and high throughput. The whole SBS process is significantly dependent on the characteristics of the attenuation force, i.e., compressed air. Although variation in the fibre morphology with varying air input pressure has been widely investigated, there is no available literature on the experimentally determined flow characteristics. Here, we have experimentally measured and calculated airflow parameters, namely, output air pressure and velocity in the nozzle wake at 12 different pressure values between 1 and 6 bar and 11 different positions (retracted 5 mm to 30 mm) along the centreline. The results obtained in this work will answer many critical questions about optimum protrusion length for the polymer solution syringe and approximate mean fibre diameter for polyvinylidene fluoride (PVDF) at given output air pressure and velocity. The highest output air pressure and velocity were achieved at a distance of 3–5 mm away from the nozzle wake and should be an ideal location for the apex of the polymer solution syringe. We achieved 250 nm PVDF fibres when output air pressure and velocity were 123 kPa and 387 m/s, respectively.

Unsteady Effects of Upstream Nozzle Wakes on Hub-Endwall Flow and Heat Transfer of an HPT Rotor

Nozzle wakes have significant effects on the heat transfer on the rotor blade and endwall surfaces. Numerical studies have been carried out in a subsonic high pressure turbine stage to investigate the rotor’s secondary flow field and endwall heat transfer. Both steady and unsteady RANS analyses were accomplished for the multiple blade rows using mixing-plane and domain-scaling techniques respectively. Special attention was focused on the particular nozzle wake structure of secondary passage vortex near the hub endwall and its effects on the endwall heat transfer characteristics. Unsteady solution indicates that the passage vortex near the rotor hub is transported toward the midspan due to the blade interaction and rotation effects. In the front passage region, the time-averaged result yields higher heat transfer up to 20% than a steady one, and the transient fluctuation amplitude reaches 40% of mean values along the passage vortex moving path. In the rear passage region, the difference between steady and unsteady solutions is negligible. Current study reveals that the major difference of wake effects between an actual turbine and a linear cascade with moving bars comes from the movement of the vortical endwall passage vortex in the incoming flow.

Instabilities on Nozzle-Wake Loads due to Axial Clearance and Throat Variations in a First Stage Turbine

A study of flow unsteadiness in a high pressure first stage turbine is presented. The study is based on numerical simulations of nozzle wake using a relative reference frame sliding mesh. Pressure and loads on first stage blades needed for stage efficiency calculation in turbine design are discussed. Excessive wear occurring at engine operating conditions affects the distance between rotating and non-rotating elements by varying throat size and axial clearance. Consequently, force unsteadiness produces changes on cyclic loads amplitude, which modifies the work produced by first stage. Compared against design, increments of throat minimum area lead to maximum total force, while throat area reductions increased pressure unsteadiness across the axial clearance. Also, the amplitude of load on blade increases as axial clearance length reduces. Harmonics of force on blade were used to calibrate a useful life prediction tool for turbine components through FFT analysis.

Investigation of the Inner Nozzle Wake on Kelvin Helmholtz Instabilities in a Coannular Jet

In the present work, we study stability characteristics of synthetic profiles prototypical of coannular jets, using a spatial, inviscid and compressible formulation of linear stability theory. The focus of our study is to understand the effect of the wake of the nozzle that separates the primary jet from the secondary jet, on the characteristics of different instability modes. Some of the parameters under focus are Mach number of the primary jet (M1), wake deficit (Wamp), and temperature ratio (T01/T∞). One of the other aims of this work is to compare our results with that of the 2-D planar mixing layers separated by a splitter plate - for which counteracting effect of compressibility and the wake of the splitter plate, on the instability characteristics, are known. Our results show that, for circular jets, unlike 2-D planar mixing layers, the vorticity introduced by the wake fails to overcome the stabilization effect caused by compressibility. Heating the jet, i.e., increasing the temperature ratio T01/T...

Infrared Thermography and Pitot Pressure Measurements of a Scramjet Nozzle Flowfield

An experimental study on single expansion ramp nozzle flows was carried out at a freestream Mach number of 7 in the hypersonic wind tunnel in Cologne, Germany. The Reynolds number in the tunnel flow was varied to study the performance of the scramjet nozzle at different flight altitudes. The effects of different nozzle pressure ratios were investigated and compared by pitot pressure measurements and schlieren photographs. The temperature distribution on the surface of the single expansion ramp was measured by using infrared thermography. The Stanton number and heat-flux distribution on the surface were determined from measured surface temperature history. Here, it can be seen that the external flow does not influence the temperature distribution. The temperature and Stanton number distribution, however, do depend on the nozzle pressure ratio. A pitot rake was used to measure the pitot pressure distribution in the nozzle wake and characterize the interaction between the nozzle and external flow, as well as the influence of the temperature on the nozzle flow. This showed that the nozzle core flow is independent from the external flow conditions, but the interactions between the two flows are highly dependent. The data obtained by the different measurement techniques gave insight into the properties of single expansion ramp nozzle flows.

Measurement of Flow Properties and Thrust on Scramjet Nozzle Using Pressure-Sensitive Paint

An experimental study of flows over a single expansion ramp nozzle with flush-mounted side walls was carried out at the DLR, German Aerospace Center. The tests were performed at a freestream Mach number of 7 in the hypersonic wind tunnel H2K in Cologne. The Reynolds number and nozzle pressure ratios in the wind-tunnel flow were varied to study the performance of the scramjet nozzle at different flight altitudes and run conditions. The static pressure distribution on the single expansion ramp was measured by both the pressure-sensitive paint method and by using pressure transducer taps located at discrete points on the surface. These two methods showed that the nozzle pressure ratio and the freestream Reynolds number do not substantially influence the static pressure ratio on the expansion ramp. A pitot rake was also used to measure the pressure distribution in the nozzle wake. Using this technique the nozzle flow in the wake could be visualized and the strong influence of the nozzle pressure ratio on the interaction between nozzle and external flows was revealed. The data obtained by the different measurement techniques permitted a characterization of the nozzle flow and gave insight into the properties of single expansion ramp nozzle flows. The thrust generated by the nozzle, the thrust efficiency coefficient, and the thrust vector angle were calculated by using the pressure data obtained by both pressure measurement techniques and the results compared. It was shown that thrust values obtained from pressure tap measurements were overestimated. Criteria were developed to aid in the design and optimization of the thrust-related performance criteria for this single ramp nozzle.

Experimental Study of a Scramjet Nozzle Flow Using the Pressure-Sensitive-Paint Method

An experimental study on scramjet nozzle flows was carried out at the German Aerospace Center. The tests were performed at a free-stream Mach number of 7 in the hypersonic wind tunnel H2K in Cologne. The Reynolds number in the tunnel flow was varied in order to study the performance of the scramjet nozzle at different flight altitudes. The effects of different nozzle pressure ratios were investigated and compared. The static pressure distribution on the single expansion ramp was measured by both the Pressure-Sensitive Paint (PSP) Method and PSI pressure transducers. The PSP method enables one to gain detailed field information about the pressure distribution on the entire nozzle surface. A Pitot rake was used to measure the Pitot pressure distribution in the nozzle wake. The data obtained by the different measurement techniques permitted a characterization of the nozzle flow and gave an insight into the properties of scramjet nozzle flows.

Unsteady Flow Field of an Axial-Flow Turbine Rotor at a Low Reynolds Number

The unsteady flow field of an annular turbine rotor was investigated experimentally using a laser Doppler velocimetry (LDV) system. Detailed measurements of the time-averaged and time-resolved distributions of the velocity, flow angle, turbulence intensity, etc., were carried out at a very low Reynolds number condition, Reout=3.5×104. The data obtained were analyzed from the viewpoints of both an absolute (stationary) frame of reference and a relative (rotating) frame of reference. The effect of the turbine nozzle wake and secondary vortices on the flow field inside the rotor passage was clearly captured. It was found that the nozzle wake and secondary vortices are suddenly distorted at the rotor inlet, because of the rotating potential field of the rotor. The nozzle flow (wake and passage vortices) and the rotor flow (boundary layer, wake, tip leakage vortex, and passage vortices) interact intensively inside the rotor passage.

Film Cooling Effectiveness on the Leading Edge Region of a Rotating Turbine Blade With Two Rows of Film Cooling Holes Using Pressure Sensitive Paint

Detailed film cooling effectiveness distributions are measured on the leading edge of a rotating gas turbine blade with two rows (pressure-side row and suction-side row from the stagnation line) of holes aligned to the radial axis using the pressure sensitive paint (PSP) technique. Film cooling effectiveness distributions are obtained by comparing the difference of the measured oxygen concentration distributions with air and nitrogen as film cooling gas respectively and by applying the mass transfer analogy. Measurements are conducted on the first-stage rotor blade of a three-stage axial turbine at 2400rpm (positive off-design), 2550rpm (design), and 3000rpm (negative off-design), respectively. The effect of three blowing ratios is also studied. The blade Reynolds number based on the axial chord length and the exit velocity is 200,000 and the total to exit pressure ratio was 1.12 for the first-stage rotor blade. The corresponding rotor blade inlet and outlet Mach numbers are 0.1 and 0.3, respectively. The film cooling effectiveness distributions are presented along with discussions on the influence of rotational speed (off design incidence angle), blowing ratio, and upstream nozzle wakes around the leading edge region. Results show that rotation has a significant impact on the leading edge film cooling distributions with the average film cooling effectiveness in the leading edge region decreasing with an increase in the rotational speed (negative incidence angle).

Investigation of Vortex Shedding and Wake-Wake Interaction in a Transonic Turbine Stage Using Laser-Doppler-Velocimetry and Particle-Image-Velocimetry

The current paper presents a time-resolved experimental flow investigation in a highly loaded transonic gas turbine stage operating continuously under engine representative conditions. The measurement was performed with a two-component laser-doppler-velocimeter (LDV) and a three-component stereoscopic particle-image-velocimeter (3C-PIV). Unsteady velocity data were obtained in axis perpendicular planes (LDV) and tangential planes (3C-PIV) between stator and rotor as well as downstream of the rotor. The results of the time-resolved investigation at several radii show the vortex shedding process from the trailing edges of nozzle guide vanes and rotor blades. This vortex shedding was found to be phase locked to higher harmonics of the blade passing frequency. Pressure waves evoked by reflection of the trailing edge shocks of the vanes on the passing rotor blades interact with the boundary layers on the rear suction side of the vanes and on the rotor blade surfaces while running upstream and downstream the flow. They are responsible for this phase-locking phenomenon of the shedding vortices. At midspan, the vortices shedding from stator and rotor blades were also observed by PIV. The in-plane vorticity distribution was used to discuss the wake-wake interaction indicating that wake segments from the nozzle guide vanes were chopped by the rotor blades. These chopped segments are still visible in the distributions as a pair of counter rotating vortices. The nozzle wake segments are transported through the rotor passages by the flow, influencing the vortex street of the rotor blades as they pass by with the higher velocity of the main flow. A comparison with a numerical simulation is also given.

B114 LDVによる軸流タービン動翼の非定常流れの可視化

The unsteady flow field within an annular turbine rotor was investigated experimentally using an LDV system. Detailed measurements of the time-averaged and time-dependent distributions of the velocity, the flow angle, and the turbulence intensity were carried out at a very low Reynolds number condition, Re, out=3.60×104 . The obtained data was analyzed from the viewpoints of both absolute (stationary) frame of reference and relative (rotating) frame of reference. The effect of the turbine nozzle wake and secondary vortices on the flow field inside the rotor passage was clearly captured. The nozzle wake and secondary vortices are suddenly distorted at the rotor inlet, because of the three-dimensional distributions of velocity and flow angle around the rotor. The nozzle flow (wake and secondary vortices) and rotor flow (separation, wake, secondary vortices, and tip leakage vortex) interact intensively near the rotor exit.

503 ノズル励振によるラジアルタービン翼の振動応答予測(翼振動・羽根車,OS-4 ロータダイナミクス)

A method for predicting radial inflow turbine forced response is presented. Minding to use easily in design routine, this method does not use unsteady technique (unsteady CFD, structural transient analysis) but only uses established technique (steady state CFD and structural eigenvalue analysis) these are usually used in actual blade design. This method is applied to radial inflow turbine blade of turbo-charger excited by multiple orders of shaft rotational frequencies. In this case, excitation source is assumed to be circumferential non-uniformity of flow caused by the existence of nozzle wake. Predicted responses are compared to the experimental data and predicted value based on unsteady CFD respectively. Result shows the effectiveness of this method for rough estimation of forced response level in design routine. But at same time, some discrepancies still exists and more improvements will be required for the precise prediction.

Unsteady Viscous Flow Causing Rotor-Stator Interaction in Turbines, Part 1: Data, Code Pressure

A coupled experimental and computational study of the effects of the nozzle wake-rotor interaction is carried out to understand the cause and effects of the unsteady e ow in turbine rotors. The paper is organized in two parts. Part 1 deals with the experimental and numerical program and interpretation of the blade pressure e eld. The experiment was carried out in a low-speed turbine at 22.6% of the nozzle chord spacing between the rotor and the nozzle. The laser Doppler velocimetry (LDV) data, acquired earlier, is integrated with the dynamic pressure measurements and numerical simulation to provide an integrated interpretation. A systematic study has been carried out to evaluatethe effect of the turbulence model and artie cialdissipation on the accuracy of thenumerical prediction. The steady e owe eld, unsteady pressure at design and off-design conditions on both blade surfaces, and the hub are presented, interpreted, and compared with the predictions from the Navier ‐Stokes code. The predictions are in a good agreement with the data. The second part deals with the integrated interpretation of the unsteady velocity and pressure e eld, as well as the e ow physics associated with the nozzle wake transport and decay.

Unsteady Viscous Flow Causing Rotor-Stator Interaction in Turbines, Part 2: Simulation, Integrated Flowfield, and Interpretation

Numerical simulation has been performed to investigate the unsteady e ow physics associated with the nozzle ‐ rotorwakeinteractioninturbines.Acomparison between thepredictedand theexperimentalunsteadypressureon the blade surface presented by Lakshminarayana et al. (Lakshminarayana, B., Chernobrovkin, A., and Ristic, D., “ Unsteady Viscous Flow Causing Rotor-Stator Interaction in Turbines, Part 1: Data, Code Pressure,” Journal of Propulsion and Power , Vol. 16, No. 5, 2000, pp. 744 ‐750) shows good agreement. The predicted e owe eld also correlates well with the blade-to-blade laser Doppler velocimetry data. Results of the numerical prediction in conjunction with the experimental data have been used to gain a better understanding of the unsteady e ow physics. An assessment of the viscous and the inviscid contribution to the nozzle wake decay and the unsteady loss distribution in the rotor passage reveals the dominant effect of the viscous decay upstream of the leading edge. Inside the passage, the inviscid effects have a signie cant ine uence. The numerical solver has been able to predict mostof the features associated with theunsteady transition on a turbineblade. The predicted e ow at theoff-design conditionhasbeeninterpreted tounderstandthenatureoftheunsteadye owe eldata high negativeincidenceangle.

Three-Dimensional Navier–Stokes Computation of Turbine Nozzle Flow With Advanced Turbulence Models

A three-dimensional Navier–Stokes procedure has been used to compute the three-dimensional viscous flow through the turbine nozzle passage of a single-stage turbine. A low-Reynolds-number k–ε model and a zonal k-ε/ARSM (algebraic Reynolds stress model) are utilized for turbulence closure. The algebraic Reynolds stress model is used only in the endwall region to represent the anisotropy of turbulence. A four-stage Runge–Kutta scheme is used for time integration of both the mean-flow and the turbulence transport equations. For the turbine nozzle flow, comprehensive comparisons between the predictions and the experimental data obtained at Penn State show that most features of the vortex-dominated endwall flow, as well as nozzle wake structure, have been captured well by the numerical procedure. An assessment of the performance of the turbulence models has been carried out. The two models are found to provide similar predictions for the mean flow parameters, although slight improvement in the prediction of some secondary flow quantities has been obtained by the ARSM model.

Unsteady Flow Field Due to Nozzle Wake Interaction With the Rotor in an Axial Flow Turbine: Part I—Rotor Passage Flow Field

The flow field in turbine rotor passages is complex with unsteadiness caused by the aerodynamic interaction of the nozzle and rotor flow fields. The two-dimensional steady and unsteady flow field at midspan in an axial flow turbine rotor has been investigated experimentally using an LDV with emphasis on the interaction of the nozzle wake with the rotor flow field. The flow field in the rotor passage is presented in Part I. while the flow field downstream of the rotor is presented in Part II. Measurements were acquired at 37 axial locations from just upstream of the rotor to one chord downstream of the rotor. The time-averaged flow field and the unsteadiness caused by the wake have been captured. As the nozzle wake travels through the rotor flow field, the nozzle wake becomes distorted with the region of the nozzle wake near the rotor suction surface moving faster than the region near the rotor pressure surface, resulting in a highly distorted wake. The wake is found to be spread out along the rotor pressure surface, as it convects downstream of midchord. The magnitude of the nozzle wake velocity defect grows until close to midchord, after which it decreases. High values of unresolved unsteadiness were observed at the rotor leading edge. This is due to the large flow gradients near the leading edge and the interaction of the nozzle wake with the rotor leading edge. High values of unresolved unsteadiness were also observed near the rotor pressure surface. This increase in unresolved unsteadiness is caused by the interaction of the nozzle wake with the flow near the rotor pressure surface.

Unsteady Flow Field Due to Nozzle Wake Interaction With the Rotor in an Axial Flow Turbine: Part II—Rotor Exit Flow Field

The two-dimensional steady and unsteady flow field at midspan in a turbine rotor has been investigated experimentally using an LDV with an emphasis on the interaction of the nozzle wake with the rotor flow field. The velocity measurements are decomposed into a time-averaged velocity, a periodic velocity component, and an unresolved velocity component. The results in the rotor passage were presented in Part I. The flow field downstream of the rotor is presented in this paper. The rotor wake profiles and their decay characteristics were analyzed. Correlations are presented that match the decay of the various wake properties. The rotor wake velocity defect decays rapidly in the trailing edge region, becoming less rapid in the near and far wake regions. The rotor wake semi-wake width increases rapidly in the trailing edge region and then grows more gradually in the near and far wake regions. The decay of the maximum unresolved unsteadiness and maximum unresolved velocity cross correlations is very rapid in the trailing edge region and this trend slows in the far wake region. In the trailing edge region, the maximum periodic velocity correlations are much larger than the maximum unresolved velocity correlations. But the periodic velocity correlations decay much faster than the unresolved velocity correlations. The interactions of the nozzle and rotor wakes are also studied. While the interaction of the nozzle wake with the rotor wake does not influence the decay rate of the various wake properties, it does change the magnitude of the properties. These and other results are presented in this paper.

Time-Accurate Euler Simulation of Interaction of Nozzle Wake and Secondary Flow With Rotor Blade in an Axial Turbine Stage Using Nonreflecting Boundary Conditions

The objective of this paper is to investigate the three-dimensional unsteady flow interactions in a turbomachine stage. A three-dimensional time-accurate Euler code has been developed using an explicit four-stage Runge–Kutta scheme. Three-dimensional unsteady nonreflecting boundary conditions are formulated at the inlet and the outlet of the computational domain to remove the spurious numerical reflections. The three-dimensional code is first validated for two-dimensional and three-dimensional cascades with harmonic vortical inlet distortions. The effectiveness of the nonreflecting boundary conditions is demonstrated. The unsteady Euler solver is then used to simulate the propagation of nozzle wake and secondary flow through the rotor and the resulting unsteady pressure field in an axial turbine stage. The three-dimensional and time-dependent propagation of nozzle wakes in the rotor blade row and the effects of nozzle secondary flow on the rotor unsteady surface pressure and passage flow field are studied. It was found that the unsteady flow field in the rotor is highly three dimensional and the nozzle secondary flow has significant contribution to the unsteady pressure on the blade surfaces. Even though the steady flow at the midspan is nearly two dimensional, the unsteady flow is three dimensional and the unsteady pressure distribution cannot be predicted by a two-dimensional analysis.